de executie.

3.DEFINITII

3.1. In conformitate cu SR EN-urile in vigoare. Defectoscopie cu pulberi magnetice. Terminologie1.SCOP

1.1. Prezenta procedura stabileste cerintele si responsabilitatile pentru examinarea nedistructiva cu pulberi magnetice a materialelor feromagnetice.

2.DOMENIUL DE APLICARE

2.1. Procedura se aplica semifabricatelor, pieselor turnate, forjate, placate, sudurilor si reparatiilor, in conformitate cu documentatia

3.2. Indicatiile liniare sunt indicatiile a caror lungime depaseste latimea de 3 (trei) ori.

3.3. Indicatiile rotunjite sunt indicatiile a caror lungime nu depasesc de 3 (trei) ori        latimea.

3.4. PM – pulberi magnetice.

3.5. CNCAN – Comisia nationala de control al activitatilor nucleare

3.6. ISCIR – Inspectia de Stat pentru controlul cazanelor, recipientilor sub presiune, instalatiilor de ridicat si a aparatelor consumatoare de combustibili de uz industrial.

4.DOCUMENTE DE REFERINTA

SR EN 1330-1-2002. Examinarea cu pulberi magnetice. Terminologie.

SR EN 1290-2000. Examinarea cu pulberi magnetice a imbinarilor sudate.

SR EN ISO 9934-1. Examinarea cu pulberi magnetice.

SR EN 1291-2002. Niveluri de acceptare suduri. Niveluri de acceptare.

SR EN 1369-1998. Examinarea cu pulberi magnetice turnate.

SR EN 5817/2003, SR EN ISO 9934-1/2002, EN 12062/1997

SR EN 473-2003. Calificarea si certificarea personalului pentru examinari nedistructive.

CR11. Autorizarea personalului care executa examinari nedistructive la instalatiile mecanice sub presiune si instalatiile de ridicat.

CR 8-2003. Colectia Prescriptii tehnice ISCIR.

CODUL ASME. Sectiunile V, editia 1998.

Manualul Calitatii

Prescriptii tehnice, colectia ISCIR pentru domeniul nuclear.

SR EN 5817 Imbinari sudate. Ghid de acceptare a defectelor.

5.RESPONSABILITATI

5.1. Societatile care solicita examinarea cu pulberi magnetice sunt responsabile de asigurarea conditiilor cerute de tehnicile de examinare mentionate in procedura si anume: starea suprafetei, temperatura piesei, a zonei etc.

5.2. Pesonalul care efectueaza examinari nedistructive cu pulberi magnetice trebuie sa fie calificat in conformitate cu standardul SR EN 473-2003 si/sau cu prescriptiile tehnice CR 11, colectia ISCIR.

5.3. Pentru personalul care executa examinarea, responsabilitatile sunt mentionate in SR EN 473-2003 sau in prescriptiile tehnice ISCIR, CR 11.

5.4. Operatorul de examinari nedistructive are obligatia ca inainte de a incepe activitatea propriu-zisa, sa examineze vizual fiecare componenta, pe intreaga zona de examinare, atât din punct de vedere al curatirii de impuritati, cât si din punctul de vedere al rugozitatii sau al existentei eventualelor discontinuitati vizuale cu ochiul liber.

5.4.1.In cazul in care starea suprafetei nu e conforma cu tehnologiile aplicabile, componentele sunt trimise in zona corespunzatoare pentru o noua curatire si eventual obtinerea unei noi rugozitati sau stare a suprafetei.

5.4.2.In cazul existentei unor discontinuitati, operatorul le va mentiona pe buletinul de examinare si pe harta cu discontinuitati, in cazul când acestea nu sunt acceptate, componenta se respinge.

5.5. Seful de laborator raspunde de modul de efectuare si conducere al examinarilor nedistructive conform procedurilor avizate; de formarea si indrumarea personalului din subordine; de structurarea si redactarea rapoartelor de examinari nedistructive.

5.6.Laboratorul CND are obligatia sa documenteze valabilitatea informatiilor referitoare la fiecare specialist in examinari nedistructive, inclusiv atestatele privind educatia, formarea si experienta acestor persoane, conform pct. 5.2.4. si 6.3. din SR EN 473-2003 si/sau CR 11, fara a se implica in procedura de certificare si autorizare.

5.6.1.Conducerea societatii va fi responsabila cu:

a)     obtinerea autorizatiei de lucru (daca e cazul);

b)     trimiterea personalului la medic pentru verificarea acuitatii vizuale, in mod special si a starii de sanatate in general.

6.PROCEDURA

6.1. Starea suprafetelor supuse examinarilor cu pulberi magnetice

6.1.1.In general rezultate satisfacatoare se pot obtine si pentru cazul când suprafata de examinare este asa cum rezulta din turnare, forjare, laminare sau sudare. In cazul in care neregularitatile suprafetei pot masca indicatiile provenite de la discontinuitati neacceptabile, se impune prelucrarea suprafetei prin polizare, aschiere, sablare etc.

6.1.2.Suprafata de examinare, impreuna cu o zona adiacenta cu o latime de minim 25 mm, trebuie curatata de impuritati, cum ar fi zgura, nisip, rugina, grasimi, ulei etc., impuritati ce ar putea sa impiedice examinarea corecta cu PM.

6.1.3.Pentru punerea in evidenta a discontinuitatilor fine, suprafata trebuie prelucrata la o rugozitate de cel mult 6.3 μm.

6.1.4.Curatirea suprafetei poate fi efectuata cu ajutorul solutiilor de decapare, degresare cu vapori, sablare, alicare etc.

6.1.5.Pentru degresarea suprafetelor supuse examinarii se vor utiliza solventi organici.

6.1.6.In caz ca benificiarul echipamentelor impune limitarea continutului de halogeni si sulf in substantele utilizate la examinare, restrictia se aplica si solventilor organici utilizati ca degresanti.

6.1.7.Se pot utiliza pentru curatire urmatorii solventi:

a)     acetona;

b)     white spirt;

c)      degresant folosit pentru lichide penetrante.

6.1.8.Dupa degresare este obligatorie operatia de uscare. Timpul de uscare este de minim 5 min. Uscarea se poate efectua fie prin evaporare naturala, fie cu aer comprimat filtrat.

6.1.9.Examinarea cu pulberi magnetice se poate efectua si pe suprafete pe care exista straturi de vopsea sau acoperiri de protectie aderente cu conditia ca grosimea acestora sa nu depaseasca 50μm.

6.2. Metoda de examinare cu PM

6.2.1.Prin metoda de examinare cu PM se pun in evidenta discontinuitati de suprafata sau in imediata apropiere a suprafetei, in materiale cu proprietati magnetice.

6.2.2.Deoarece aceasta metoda se bazeaza pe orientarea liniilor de forta ale câmpului magnetic, sensibilitatea sa va depinde de orientarea acestora fata de orientarea discontinuitatilor. Sensibilitatea maxima se obtine atunci când discontinuitatile sunt orientate perpendicular pe liniile de forta. Pentru detectarea tuturor discontinuitatilor, suprafata examinata se va magnetiza in cel putin doua directii perpendiculare (examinari succesive).

6.3. Tehnici de examinare

6.3.1.Liniile de forta ale câmpului magnetic pot fi puse in evidenta cu ajutorul pulberilor magnetice ce pot fi folosite fie sub forma de pulberi uscate (tehnica uscata), fie sub forma de suspensie intr-un lichid purtator (tehnica umeda).

6.3.2.Pulberile sunt de doua feluri:

pulberi colorate; pulberi fluorescente.

6.3.3.Tehnicile de examinare uscata-pulberi colorate.

a)     Utilizarea pulberilor colorate impune existenta unui contrast pronuntat de culoare intre pulbere si suprafata materialului examinat.

b)     Culorile cele mai folosite pentru pulberile magnetice sunt:

negru rosu

gri deschis

galben

c)      Pulberile magnetice trebuie sa aiba o permeabilitate magnetica mare, astfel incât sa fie magnetizate cu usurinta si o remanenta mica pentru a nu produce aglomerari de pulberi din cauza atractiei dintre ele.

d)     Pulberea se aplica pe suprafata de examinare prin prafuire usoara, având grija ca depunerea sa fie uniforma.

e)     Excesul de pulbere se indeparteaza inainte de interpretarea indicatiilor, cu ajutorul unui jet de aer, nu prea puternic, astfel incât sa nu distruga eventualele indicatii.

f)        Temperatura piesei pe care se plica PM uscata nu va depasi valoarea de 570C; daca instructiunile furnizorului de PM recomanda un anumit interval de temperatura in timpul examinarii, operatorul le va respecta pe acestea.

g)     Examinarea se face in spectrul vizibil (lumina alba), cu conditia ca pe suprafata de examinat sa fie 350 lx (pentru produsele speciale, de exemplu: nucleare, se respecta valoarea din documentatie).

h)      Temperatura piesei pe care se aplica PM umeda nu va depasi valoarea de 570C.

ATENTIE: Se interzice refolosirea pulberii uscate. Pulberea magnetica se poate

impurifica in timpul examinarii cu praf, nisip, pilitura, impurificare care îi    altereaza proprietatile.

6.3.4.Tehnica de examinare uscata – pulberi fluorescente

a)     Se vor respecta afirmatiile de la pct. 6.3.3.c pâna la pct. 6.3.3.f inclusiv, de la pulberi colorate si in cazul folosirii pulberilor fluorescente. Acestea au o stralucire galben verzui.

b)     Examinarea se face in spectrul ultraviolet (lumina neagra).

c)      Masurarea intensitatii luminii ultraviolete de pe suprafata de examinat se face cu instrumentul centrat pe lungimea de unda de 3650 Å la o distanta de 380 mm fata de suprafata de examinat.

d)     Prima masuratoare se face fara filtru, a doua cu filtru de absorbtie asezat peste elementul sensibil al instrumentului. Diferenta dintre cele doua citiri trebuie sa fie minim 800 μmW/cm2. Valorile masurate vor fi monitorizate.

e)     Examinarea propriu-zisa, precum si masuratorile de la pct. 6.3.4.c la pct. 6.3.4.d inclusiv, se vor face intr-un spatiu intunecos al carui fond luminos nu va depasi 1000 lux/metru patrat.

f)        Intensitatea luminii ultraviolete de pe suprafata de examinare trebuie masurata cel putin la 4 (patru) ore, ori de câte ori se schimba locul de lucru sau in cazul când se considera necesar.

6.3.5.Tehnica de examinare umeda

a)     Si aceasta tehnica, ca si tehnica uscata, foloseste atât pulberi colorate cât si pulberi fluorescente.

b)     Mediul de suspensie poate fi apa sau kerosenul (petrol lampant).

c)      Afirmatiile de la “Tehnica de examinare uscata-pulberi colorate” sunt valabile si in cazul “Tehnici de examinare umeda cu pulberi colorate”; la fel si in cazul pulberilor fluorescente. Face exceptie pct.6.3.3.f. pentru pulberi colorate si in plus 6.3.3.g. pentru pulberi fluorescente.

d)     Aplicarea pulberilor magnetice umede pe suprafetele de examinare ale piesei se poate face fie prin stropire, fie prin sprayere.

e)     Pulberile magnetice colorate sau fluorescente, folosite la tehnica umeda sunt livrate de fabricanti sub forma de pulbere, pasta concentrata sau spray.

f)        Amestecul pulberii magnetice cu mediul de suspensie, la concentratia recomandata va fi monitorizata de laboratorul de examinari nedistructive.

g)     In cazul utilizarii buteliilor cu aerosoli pentru produse la care se impun anumite limitari privind halogenii si sulful, se va avea grija ca furnizorul de butelii sa prezinte un certificat privind continutul de halogeni si sulf.

h)      Se impune ca lichidele de suspensie sa aiba o tensiune superficiala mica si sa nu faca spuma; se pot utiliza agenti antispumanti.

i)        Concentratia suspensiei se verifica o data pe zi, respectând urmatoarele etape:

se agita câteva minute intreaga masa a suspensiei; se toarna intr-un tub centrifugal gradat, in forma de pana, 100ml de suspensie;

se centrifugheaza tubul mentinând nivelul amestecului la diviziunea 100ml;

se aseaza tubul pe un stativ bine fixat, fara vibratii, mentinându-l 30 minute, timp in care pulberea se va depune pe fundul tubului;

dupa scurgerea celor 30 minute se va citi si nota nivelul pulberii depuse.

j)        Se recomanda pentru pulberea colorata ca nivelul depunerii sa fie cuprins intre 1,2-2,4 ml; pentru pulberea fluorescenta sa fie 0,4-0,8 ml.

ATENTIE: Daca furnizorul de pulberi recomanda alte valori, atunci laboratorul le va

respecta intocmai.

6.4. Tehnici de magnetizare

6.4.1.Tehnica jugului.

a)     Tehnica jugului se aplica numai pentru detectarea discontinuitatilor de la suprafata sau in imediata apropiere a suprafetei de examinare.

b)     Se pot utiliza juguri electromagnetice cu curent alternativ sau cu curent continuu, sau magneti permanenti.

6.4.2.Tehnica magnetizarii circulare cu conductor central.

a)     Se foloseste un conductor central (sub forma de tija, bara, cablu) pentru a examina suprafetele interioare ale pieselor de forma inelara sau cilindrica.

b)     Pentru cilindrii cu diametre mari, conductorul se va pozitiona aproape de suprafata sa. In acest caz conductorul nefiind centrat, circumferinta cilindrului va fi examinata pe portiuni; indicatorul de câmp magnetic va permite determinarea zonei de examinare.

c)      Daca este necesar un curent de 600 A pentru examinare, in cazul utilizarii unui conductor, pentru doi conductori avem nevoie de 300 A, iar pentru 5 (cinci) conductori avem nevoie de 120 A pe conductor.

6.4.3.Tehnica magnetizarii cu electrozi.

a)     Se utilizeaza electrozi de contact portabili care se preseaza pe suprafata in zona examinata.

b)     Trecerea curentului va fi permisa numai dupa ce electrozii vor fi pozitionati corect; acest lucru se face cu ajutorul unui comutator care are si rolul de a evita producerea arcului electric.

c)      Distanta dintre electrozi nu va depasi 200 mm. In cazul in care unele zone nu permit o astfel de distanta sau in cazul in care avem o sensibilitate mai mare, putem micsora distanta dintre electrozi pâna la 80 mm.

ATENTIE: Distanta dintre electrozi nu trebuie sa fie mai mica de 80 mm; la distante mai

mici pulberea magnetica se aseaza in jurul electrozilor.

d)     Zonele de contact ale electrozilor trebuie sa fie curate si acoperite cu plumb, otel sau aluminiu pentru a evita depuneri de cupru pe piesa examinata in cazul in care tensiunea in circuitul deschis este mai mare de 25 V.

e)     Se foloseste curent continuu sau redresat, cu valori cuprinse intre 100 A si 125 A pentru fiecare inch de distanta dintre electrozi, pentru sectiuni ale grosimii de ¾ inch (20mm) sau mai mari. Pentru sectiuni ale grosimii mai mici de ¾ inch, curentul va avea valori cuprinse intre 90 -110 A pentru fiecare inch de distanta dintre electrozi (1 inch=25,4mm).

6.4.4.Tehnica magnetizarii longitudinale.

a)     Magnetizarea se realizeaza fie cu ajutorul unei bobine, cu diametru, lungimea si numarul de spire fixate, fie cu ajutorul unui cablu infasurat in jurul piesei sau a unei sectiuni din piesa.

ATENTIE: Daca bobina are diametrul interior mai mare de 10 ori decât sectiunea sau

diametrul piesei, atunci piesa se va plasa nu in centrul bobinei, ci lânga peretele bobinei, pentru a fi examinata.

b)     Bobina fixa sau realizata cu ajutorul unui cablu infasurat in jurul piesei produce un câmp magnetic longitudinal paralel cu axa bobinei.

c)      Piesele lungi vor fi examinate pe sectiuni, ce nu vor depasi lungimea de L= 460mm. Diametrul exterior al piesei il notam cu D.

d)     Valoarea curentului necesar magnetizarii pieselor, pentru aceasta tehnica, se calculeaza astfel:

Piese cu raportul L/D egal sau mai mare ca 4 (patru).

ATENTIE: Lungimea L nu va depasi valoarea de 460 mm. Curentul de magnetizare

va avea valoarea amperi spira egala cu:

(± 10%)

Piese cu raportul L/D cuprins intre 2 si 4. Curentul de magnetizare va avea valoarea amperi spira egala cu:

(± 10%)

Piese cu raportul L/D mai mic ca 2. Se va folosi o alta tehnica de magnetizare.

e)     Curentul de magnetizare se va determina prin impartirea valorii amperi spira obtinuta cu una din cele doua formule de mai sus, la numarul de spire utilizat, adica:

6.4.5.Tehnica magnetizarii circulare prin contact direct.

a)     Magnetizarea se realizeaza prin trecerea curentului prin piesa de examinat. Se obtine un câmp magnetic circular, perpendicular pe directia curentului.

b)     Curentul de magnetizare poate fi cuntinuu sau redresat (semialternativ sau complet)

c)      Valoarea curentului va fi determinata dupa urmatoarele criterii:

-       Piese cu diametrul exterior pâna la 125 mm. Curentul va avea valoarea cuprinsa intre 700 si 900 A/inch de diametru (27,5 A/mm si 35,5 A/mm).

-       Piese cu diametrul exterior cuprins intre 125 mm si 250 mm. Curentul va avea valoarea 500 si 700 A/inch; diametru (20 A/mm si 27,5 A/mm).

-       Piese cu diametrul exterior cuprins intre 250 mm si 380 mm. Curentul va avea valoarea cuprinsa intre 300 si 500 A/inch; diametru (12 A/mm si 20 A/mm).

-       Piese cu diametrul exterior mai mare de 380 mm. Curentul va avea valoarea cuprinsa intre 100 si 300 A/inch; diametru (4 A/mm si 12 A/mm).

-       Piese cu marimi diferite de forma cilindrica; se va lua in considerare diagonala celei mai mari sectiuni intr-un plan perpendicular pe directia curentului. In functie de marimea diagonalei se va alege valoarea curentului data de criteriile  mai sus mentionate.

ATENTIE: Se poate folosi indicatorul de câmp magnetic pentru a determina

amperajul necesar magnetizarii, ca o alternativa, dar numai piesele necilindrice.

6.4.6.Tehnica de magnetizare multidirectionala.

a)     Magnetizarea se realizeaza prin impulsuri de mare amperaj, pe trei circuite, folosite alternativ in succesiune rapida.

b)     Se obtine o magnetizare completa pe directiile celor trei circuite, si anume câmpuri magnetice circulare, cât si longitudinale, in orice combinatie, daca se folosesc tehnicile de magnetizare longitudinala (pct.6.4.4.) si/sau tehnica de magnetizare circulara (pct.6.4.2. si 6.4.5.).

c)      Se va folosi curent trifazat, complet redresat. Curentul de magnetizare, pentru fiecare circuit, se va stabili conform pct.6.4.4., 6.4.2. si 6.4.5.

d)     Cu ajutorul indicatorului de câmp magnetic se va verifica daca se obtin câmpuri pe cel putin doua directii perpendiculare. In caz ca sunt zone unde nu se obtin intensitati adecvate ale câmpului magnetic sa se foloseasca tehnici suplimentare pentru doua directii perpendiculare.

6.4.7.Magnetizarea cu curent alternativ.

a)     Se poate realiza magnetizarea pieselor si cu ajutorul curentului alternativ.

b)     O astfel de magnetizare permite detectarea discontinuitatilor de suprafata.

6.5. Aparatura, echipamente, instalatii

6.5.1.Intensitatea câmpului magnetic se va verifica cu indicatorul de câmp magnetic (prezentat in codul ASME, sectiunea V, SE 709).

6.5.2.Daca liniile formate de pulberea magnetica formeaza o imagine bine definita pe suprafata de cupru a indicatorului, rezulta ca intensitatea câmpului magnetic a fost bine calculata.

6.5.3.Verificarea si etalonarea echipamentelor

a)     Aparatura, echipamentele etc. de magnetizare trebuie verificate cel putin o data pe an, sau ori de câte ori este necesar (reparatii, neutralizare un timp de peste 6 luni etc.).

b)     Se verifica aparatura electrica (ampermetre, voltmetre etc.) in conformitate cu Normele Metrologiei Nationale.

c)      Forta de magnetizare a jugului se verifica prin determinarea puterii de ridicare:

Jugul cu curent alternativ trebuie sa posede o forta portanta de cel putin 4,5 kg, la distanta maxima intre poli.

Jugul cu curent continuu sau cu magnet natural trebuie sa posede o forta portanta de cel putin 18,2 kg la distanta maxima intre poli.

d)     In cazul in care piesa se magnetizeaza prin tehnica trecerii curentului direct, elementele de contact sau electrozii vor asigura o presiune suficienta a suprafetelor de contact astfel incât sa nu se produca arsuri pe suprafata piesei.

e)     In cazul tehnicii de magnetizare cu electrozi, tensiunea din circuit nu va depasi 42V.

f)        Daca tensiunea in circuit depaseste valoarea de 5V, se vor utiliza la electrozi vârfuri din otel, plumb sau aluminiu. Pentru tensiuni cuprinse intre 5V si 20V, se pot utiliza si vârfuri cu plasa de cupru.

g)     Pentru iluminarea suprafetelor de examinare se poate folosi:

bec cu incandescenta de 100W asezat la o distanta de 0,2m; tub fluorescent de 80W asezat la o distanta de 1m;

la examinarea cu pulberi fluorescente se va utiliza o lampa de lumina fluorescenta (ce functioneaza in domeniul 3300-3900 Å) care sa asigure pe suprafata de examinat o intensitate de 800 μW/cm2.

h)      Laboratorul de examinari nedistructive trebuie sa fie dotat cu o trusa cu anexe, cum ar fi indicatorul de câmp magnetic (comform ASME, sectiunea V), etaloane cu fisuri si cu gauri, pulverizator, instrument de masura a câmpului remanent, avertizor de tensiune, agitator pentru solutii, cilindru gradat pentru determinarea concentratiilor solutiilor, lampa ultravioleta, instrument de masura in UV etc.

i)        Echipamentele de protectie pentru operatori, ochelari de protectie, cizme de cauciuc, manusi de cauciuc. Se vor lua masuri de protectie in conformitate cu NTSM pentru utilizarea instalatiilor sub tensiune.ATENTIE:In cazul in care se lucreaza in spatii inchise, este necesar ca lucrarile echipeide operatori (minim 2 operatori) sa fie supravegheata de o persoana dinexterior care sa poata intrerupe energia electrica si a interveni in caz denecesitate in sprijinul operatorilor.6.6. Demagnetizarea

6.6.1.Demagnetizarea pieselor examinate se efectueaza numai in cazul in care este impusa de proiect sau de beneficiarul pieselor.

ATENTIE:In cazul in care produsele examinate cu pulberi magnetice sunt supuse ulterior unui tratament termic, demagnetizarea nu mai este necesara.

6.6.2.Tehnici de demagnetizare.

a)     Piesa se introduce intr-o bobina prin care circula un curent alternativ de intensitate mare; piesa se scoate incet din interiorul bobinei.

b)     Se reduce curentul alternativ de magnetizare in pasi mici, pâna la valoarea zero. Sunt necesari aproximativ 25 de pasi de demagnetizare.

c)      Se trece prin piesa un curent continuu de magnetizare, reducând marimea acestuia in pasi consecutivi si totodata schimbând sensul curentului pentru fiecare pas.

d)     Magnetizarea remanenta a piesei nu trebuie sa depaseasca valoarea de 2 Öe.

6.7. Curatirea produselor examinate

6.7.1.Dupa examinarea nedistructiva se impune curatirea suprafetelor examinate folosind diverse tehnici, ca de exemplu:

a)     cu un jet de aer comprimat

b)     cu ajutorul unor perii confectionate din par de animale; in caz ca nu exista restrictii de halogeni si sulf se pot folosi si perii cu fire din plastic.

c)      prin spalare cu substante care sa se incadreze cu continutul de halogeni si sulf in limitele prevazute de proiectant sau beneficiar.

6.7.2.Dupa ce produsele au fost curatate vor fi examinate vizual astfel incât sa nu prezinte urme de pulberi.

7.MENTIUNI SI INREGISTRARI

7.1.Rezultatele examinarii nedistructive cu PM vor fi mentionate in buletinele de examinare cu PM (vezi Anexa 1) care constituie inregistrari ale sistemului calitatii.

7.2.Tehnica de examinare utilizata uzual este tehnica cu puberi fluorescente umede si jug magnetic de curent continuu tip PARKER INSTRUMENTS U.S.A. alimentat de la acumulatori portabili de 12V sau cu alimentare de le retea 200V, cu deschiderea polilor reglabila functie de complexitatea suprafetei.

7.3.Calibrarea echipamentelor se va face in conformitate cu art.7, pct.T 780, sect.V, codul ASME.

8.CRITERII DE ACCEPTARE / RESPINGERE

Criteriile de acceptare/respingere vor fi cele solicitate de client si/sau proiectant.

Exemple:

8.1. Criteriile de acceptare/respingere, dupa SR EN 1291-2002 (PT CR8-2003) sunt:

Nr.crt. Tipul indicatiilor Nivel de acceptare

1 2 3

1 Indicatii liniare L=lungimea indicatiilor

L<1,5mm L<3mm L<6mm

2 Indicatii neliniare D=axa cu dimensiunea maxima

D<2mm D<3mm D<4mm

8.2.Criteriile de acceptare/respingere, dupa codul ASME, sectiunea III, ale materialelor si reparatiilor prin sudura (NB-2545), inclusiv pentru turnate, sunt urmatoarele:

a)     Orice indicatie cu dimensiunea majora mai mare de 1,6 mm se considera relevanta.

b)     Urmatoarele indicatii relevante se considera neacceptabile:

orice indicatie liniara cu dimensiunea majora mai mare decât cele prezentate in tabelul 1.

Tabel 1

Lungimea indicatiei mm Grosimea materialului examinat t mm

> 1,6 t < 16

> 3,2 16 < t < 51

= 4,8 51 < t

Indicatiile liniare care sunt interpretate ca fisuri nu se accepta.

orice indicatie rotunjita cu dimensiunea majora mai mare decât cele prezentate in tabelul 2.

Tabel 2

DIMENSIUNEA MAJORA A INDICATIEI [mm]

GROSIMEA MATERIALULUI (t) EXAMINAT [mm]

> 3,2 t < 16

> 4,8 t > 16

c)      Patru sau mai multe indicatii in linie, separate printr-un spatiu de 1,6 mm sau mai putin, masurat margine la margine.

d)     Zece sau mai multe indicatii incadrate intr-o zona de 3870 mm2 cu dimensiunea majora a zonei de maxim 152 mm, amplasate in zona cea mai nefavorabila pentru evaluarea indicatiilor.

8.3.Criteriile de acceptare/respingere (conform SA-614) ale organelor de asamblare (suruburi, bolturi, prezoane, piulite) cu dimensiunea nominala peste 51 mm.

a)     Nu se admit discontinuitati liniare neaxiale.

b)     Discontinuitatile axiale mai mici de 25 mm sunt acceptate.

8.4.Criterii de acceptare/respingere in conformitate cu codul ASME, sectiunea VIII, pentru turnate.

a)     Indicatiile de suprafata se vor compara cu indicatiile din ASTM E125-1971 “Fotografii standard de referinta pentru indicatiile puse in evidenta cu PM pe turnate feroase”.

Nu vor fi acceptate cele ce depasesc limitele din tabelul 3.

Tabel 3

Tip Grad

1. Discontinuitati liniare (fisuri sau crapaturi termice)

orice indicatie

2. Retasuri 2

3. Incluziuni 3

4. Picaturi datorate lipsei de topire sau depuneri reci

1

5. Porozitate 1

8.5.Criterii de acceptare/respingere pentru sanfrene si suduri.

a)     La sanfrenele pentru suduri ale materialelor de peste 51 mm se accepta discontinuitati de tip laminare cu o lungime de pâna la 25 mm. Extinderea lor in material va fi determinata cu ajutorul metodei cu ultrasunete.

b)     Daca lungimea depaseste 25 mm, aceasta se va repara prin sudura pe adâncimea indicatiei dar nu mai mult de 10 mm (NB-5130).

c)      Sunt neacceptate urmatoarele indicatii:

fisurile si orice indicatie liniara; indicatiile rotunjite cu dimensiunea majora mai mare de 4,8 mm;

patru sau mai multe indicatii rotunjite, in linie separate printr-un spatiu de 1,6 mm sau mai putin, masurat de la margine la margine;

zece sau mai multe indicatii rotunjite incadrate intr-o zona de 3870 mm2, cu dimensiunea majora a zonei de maxim 152 mm, amplasate in zona cae mai nefavorabila pentru evaluarea indicatiilor.

8.6. Criterii de acceptare/respingere conform SR EN 5817/2006.

Examinarea cu particule   magnetice

 

1.Domeniul de aplicare

Se aplica semifabricatelor, pieselor turnate, forjate, placate, sudurilor si reparatiilor, in conformitate cu documentatia de executie. 2. Controlul nedistructiv cu pulberi magnetic Prin controlul nedistructiv cu pulberi magnetice sunt descoperite defecte de suprafaţă şi sub suprafaţă în materiale feromagnetice. în materialul magnetizat în prealabil. Aeează fluxuri de dispersie, detectabile cu ajutorul particulelor feromagnetice fine aplicate pe suprafaţă. De-a lungul defectului aceste particule se dispun în aşa fel încît săi marcheze o urmă vizibilă, care indică locul, aria, forma şi adîncimea acestuia.. Amprenta defectului este influenţată de direcţia şi intensitatea eîinpului magnetic, de metoda de magnetizare întrebuinţată, de aria, forma şi direcţia discontinuităţii, de caracteristicile pulberii magnetice şi metoda de aplicare, de caracteristicile magnetice ale corpului încercat, de starea suprafeţei şi forma obiectului. 3. Avantajele metodei  Avantajele metodei sînt numeroase şi printre ele putem cita :

 

 

 

— rapiditatea şi simplitatea modului de operare;— excelenta sensibilitate pentru defecte de suprafaţă;— formarea operatorilor nu e dificilă;— se pot încerca simultan suprafeţe mari din corp;— se pot descoperi goluri umplute cu alte materiale ;— nu e necesară pregătirea specială a suprafeţelor;— metoda nu e prea costisitoare';— utilizează echipamente electrotehnice nepretenţioase şi rezistente. 

Peste 50 de ani in NDT,

mai mult de 50 in sisteme NDT!

Englezul Saxby folosea inca din 1868 indicare cimpului magnetic de dispersie cu ajutorul busolei pentru detectarea fisurilor tevilor de tun. Ca in multe alte domenii ale dezvoltarii tehnologice, tot problemele de tehnica militara au reprezentat motivul pentru care americanul Hoke imagina metode de detectare a fisurilor cu ajutorul piliturii de fier. Doi ani mai tirziu a fost acordat un patent, pe baza caruia de Forest, intemeietorul firmei MAGNAFLUX din Chicago, a obtinut licenta, inainte ca sa se foloseasca, prima oara in 1929, metoda de magnetizare cu ajutorul fluxului de curent prin piesa. Primul aparat european de control al fisurilor a fost construit in anul 1937 de italianul Giraudi sub denumirea de Metalloscopio.

Istoria controlului fisurilor cu pulbere magnetica - la KARL DEUTSCH

Karl si Volker DEUTSCH au inceput in anul 1960 productia paratelor pentru controlul fisurilor prin metoda "recipientului cu virtej", trecind in 1965 la aparatele universale cu alimentare cu curenti alternativi defazati.

 

Astazi, tehnica de control PM este statuata in nemumarate specificatii, norme si directive, incit noutatile se impun greu, chiar daca se pot obtine avantaje tehnice clare. Necesitatile dezvoltarii tehnice se refera, in primul rind, la prelucrarea automata a imaginilor indicatiilor de fisuri oferite de controlul PM. in prezent exista instalatii pilot ale fabficantilor renumiti de aparatura PM. Ponderea in industrie a acestora s-ar putea realiza in ultima decada a mileniului .

Control nedistructivDe la Wikipedia, enciclopedia liberă

Salt la: Navigare, căutare

Control NDT în procesul de producţie

Controlul nedistructiv (engleză nondestructive testing, prescurtat NDT) reprezintă modalitatea de control al rezistenței unei structuri, piese etc fără a fi necesară demontarea, ori distrugerea acestora.

Este un ansamblu de metode ce permite caracterizarea stării de integritate a pieselor, structurilor industriale, fără a le degrada, fie în decursul producției, fie pe parcursul utilizării prin efectuarea de teste nedistructive în mod regulat pentru a detecta defecte ce prin alte metode este fie mai dificil, fie mai costisitor.

Cuprins[ascunde]

1 Domenii de aplicare 2 Scurt istoric

3 Metode de control nedistructiv

o 3.1 Radia ț ii penetrante

3.1.1 Examinare cu raze X

3.1.2 Examinare cu raze gamma (gammagrafie)

o 3.2 Magnetoscopie

o 3.3 Curen ț i turbionari

o 3.4 Ultrasunete

o 3.5 Lichide penetrante

o 3.6 Controlul visual

4 Alte metode

5 Simbolizare

6 Standarde ș i norme

7 Vezi ș i 8 Legături externe

[modifică] Domenii de aplicare

Domeniile de aplicare ale controlului nedestructiv sunt cele mai diverse sectoare ale industriei:

industria automobilelor (diferite piese) industria navală (controlul corpului navei și a structurilor sudate)

conducte îngropate sau submerse sub apă supuse coroziunii

platforme marine

aeronautică (aripile avioanelor, diferite piese de motor, etc)

industria energetică (reactoare, turbine, cazane de încălzire, tubulatură, etc)

industria aerospaţială și militară

arheologie

structuri feroviare

industria petrochimică

construcţii de mașini (piese turnate sau forjate, ansamble și subansamble)

Se poate afirma că metodele NDT se aplică în toate sectoarele de producție.

[modifică] Scurt istoric

În timpurile trecute, clopotarii și făurarii ascultau sunetele pe care le produceau obiectele create, astfel că fiecărui material îi corespundea un sunet.

1854 - în Hartford, Connecticut explozia unui boiler la firma Fales and Gay Gray Car, se soldează cu moartea a 21 de lucrători și rănind alţi 50. De atunci, s-a impus o verificare anuală a boilerelor

1895 - Wilhelm Conrad Röntgen a descoperit prezenţa razelor X. În prima sa lucrare arată despre posibilitatea detectării unui defect de structură

1920 - Dr. H. H. Lester concepe dezvoltarea radiografiei industriale a metalelor, apoi în 1924 folosește metoda pentru detectarea de fisuri în unele piese turnate la o termocentrală

1926 - este realizat primul aparat electromagnetic cu curenţi turbionari

1927 - 1928 - Elmer Sperry și H.C. Drake concep un sistem cu inducţie magnetică pentru detectarea defectelor din șinele de cale ferată

1929 - A.V. DeForest și F.B. Doaneeste realizează primul aparat și metoda de testare cu particule magnetice

1930 - Robert F. Mehl demonstrează realizarea de imagini radiografice folosind radiaţiile gamma din izotopi de radiu, ceeace permite examinarea de elemente cu grosimi mai mari

1940 - 1944 - Dr. Floyd Firestone dezvoltă în S.U.A. metoda de testare cu ultrasunet

1950 - J. Kaiser a introdus emisia acustică în metoda NDT

[modifică] Metode de control nedistructiv

Alegerea metodei de control nedistructiv utilizată se face în funcție de diferite criterii legate de utilitatea piesei de controlat, materialul din care este fabricată piesa, amplasament, tipul de structură, costuri etc. Cele mai utilizate metode de control nedistructiv sunt:

[modifică] Radiații penetrante

Metoda de examinare cu radiații penetrante sau radiografică constă din interacțiunea radiațiilor penetrante cu pelicule fotosensibile. Se poate efectua cu raze X sau raze gamma.

[modifică] Examinare cu raze X

Generator de raze X

Examinarea cu raze X constă în bombardarea piesei supuse controlului cu radiații X, obținându-se pe filmul radiografic imaginea structurii macroscopice interne a piesei.

Generatoarele de raze X, în funcție de energia ce o furnizează și de domeniul lor de utilizare pot fi:

generatoare de energii mici (tensiuni < 300 kV) pentru controlul pieselor din oţel de grosime mică (< 70 mm),

generatoare de energii medii (tensiuni de 300...400 kV) pentru controlul pieselor din oţel de grosime mijlocie (100...125 mm)

generatoare de energii mari (tensiuni de peste 1...2 MV și betatroane de 15...30 MV) pentru controlul pieselor din oţel de grosime mare (200...300 mm).

[modifică] Examinare cu raze gamma (gammagrafie)

Gammagrafia constă în iradierea piesei supuse controlului cu radiații gamma, după care se obține pe filmul radiografic imaginea structurii macroscopice interne a piesei respective, prin acționarea asupra emulsiei fotogafice.

Creșterea permanentă a parametrilor funcționali ai instalațiilor industriale moderne (presiune, temperatură, solicitări mecanice, rezistență la coroziune), au impus examinarea cu raze gamma ca o metodă modernă de control cu grad ridicat de certitudine.Elementul de bază al gammagrafiei este sursa de radiații gamma care datorită proprietăților sale (energie ridicată, masă de repaus nulă, sarcină electrică nulă), o fac deosebit de penetrantă.

Principala sursă de radiații folosită în gammagrafie o constituie izotopii radioactivi de Cobalt-60, Iridiu-192, Cesiu-137, Cesiu-134, Tuliu-170 și Seleniu-75, obținuți prin activare deoarece au un preț de cost mai scăzut și avantajul obținerii unor activități mari.

Acești izotopi sunt utilizați astfel: Cobalt-60 pentru oțeluri cu grosime mare (>80 mm), Iridiu-

192 pentru oțeluri cu grosime mijlocie (10-80 mm), iar Tuliu-170 pentru oțeluri cu grosime mică (<10 mm).

[modifică] Magnetoscopie

Metoda permite detectarea defectelor materialelor feromagnetice. Un material este considerat ca fiind feromagnetic atata timp cat este supus la un camp continuu de 2400 A/m si prezinta o inductie de cel putin 1 tesla.

Poate fi efectuată cu pulberi magnetice sau bandă magnetografică.

controlul cu pulberi (particule) magnetice constă în supunerea zonei de controlat la acţiunea unui câmp magnetic continuu sau alternativ. Se crează astfel un flux magnetic intens în interiorul materialului feromagnetic. Defectele întâlnite în calea sa determină devierea fluxului magnetic generând un câmp magnetic de dispersie la suprafaţa piesei. Câmpul de dispersie generat este materializat prin intermediul unei pulberi feromagnetice (particule colorate sau fluorescente) uscate sau în suspensie liuchidă foarte fine pulverizate pe suprafaţa de examinare și atrasă în dreptul defectelor de către forţele magnetice. Aceasta furnizează o semnatură particulară ce caracterizează defectul. Principalul avantaj al acestei metode este obţinerea de rezultate immediate.

metoda magnetografică utilizează o bandă feromagnetică flexibilă care se așează peste sudura ce trebuie examinată. Prin aplicarea unui scurt puls magnetic de aproximativ 15 ms, prin intermediul unui acumulator ce magnetizează un jug, câmpurile de distorsiuni sunt puse în evidenţă prin imprimarea lor pe bandă. Banda este examinată cu ajutorul unui traductor magneto-electric, după forma indicaţiilor putându-se aprecia natura defectelor din îmbinarea sudată. Echipamentul constă din jugurile pentru diferite geometrii ale îmbinărilor sudate, sursa de curent, banda feromagnetică, magneţii de fixare ai benzii și traductorii magnetoelectrici.

[modifică] Curenți turbionari

Metoda curenților turbionari este folosită ca o alternativă sau extensie a controlului nedistructiv cu particule magnetice, fiind utilizată, în special, pentru controlul țevilor cu diametrul exterior de maximum 140 mm. Sensibilitatea metodei este maximă la grosimi de perete de până la 5 mm. O dată cu creșterea grosimii pereților, scade eficiența metodei de evidențiere a defectelor interne, ea rămânând eficace pentru evidențierea defectelor de suprafață și din imediata apropiere a acesteia.Metoda constă în inducerea unor curenți turbionari în pereții țevii controlate. Câmpul magnetic al curenților turbionari induși, datorită prezenței unor discontinuități și neomogenități în material, modifică impedanța bobinei de măsurare, ceea ce afectează amplitudinea și faza curenților turbionari. Amplitudinea, defazajul și adâncimea de pătrundere a curenților turbionari, depind de amplitudinea și frecvența curentului de excitație, de conductibilitatea electrică, de permeabilitatea magnetică a materialului, de forma piesei controlate, de poziția relativă a bobinelor față de piesă, precum și de omogenitatea materialului controlat.Metoda mai este denumită și a curenților Foucault după numele fizicianului francez Léon Foucault care a descoperit fenomenul în anul 1851.

[modifică] Ultrasunete

Aparat cu ultrasunet

Metoda este bazată pe undele mecanice (ultrasunetele) generate de un element piezo-magnetic excitat la o frecvență cuprinsă de regulă între 2 și 5 Mhz. Controlul presupune transmiterea, reflexia, absorbția unei unde ultrasonore ce se propagă în piesa de controlat. Fasciculul de unde emis se reflectă în interiorul piesei și pe defecte, după care revine către defectoscop ce poate fi în același timp emițător și receptor. Poziționarea defectului se face prin interpretarea semnalelor.

Metoda prezintă avantajul de a găsi defectele în profunzime datorită unei rezoluții ridicate, însă este lentă datorită necesității de scanare multiplă a piesei. Uneori este necesară executarea controlului pe mai multe suprafețe ale piesei. Metoda de control prin ultrasunete este foarte sensibilă la detectarea defectelor netede.

[modifică] Lichide penetrante

Examinare cu lichide penetrante. 1-Defect nevizibil; 2-Aplicarea penetrantului; 3-Îndepărtarea excesului de penetrant; 4-Defect vizibil

Constă în aplicarea unui lichid capilar activ penetrant pe suprafața de examinat, îndepărtarea penetrantului rămas în afara discontinuităților și aplicarea unui material absorbant, ce absoarbe

penetrantul aflat în discontinuități punând astfel în evidență, prin contrast, defectele existente; această metodă se aplică pentru depistarea defectelor de suprafață. Se pot pune de asemenea în evidență fisurile de oboseală și de coroziune. Pentru control trebuie curătată și pregatită suprafața de examinare.

Metoda este aplicată cu success îmbinărilor sudate, dar se poate face și înainte de sudură (pentru efectuarea unui control al tuturor suprafețelor înainte de a fi sudate).

[modifică] Controlul visual

Orice tip de investigare trebuie să fie precedată de o examinare vizuală a supafeței. Procedeul este simplu dar indispensabil, examinarea vizuală presupune respectarea condițiilor de claritate satisfăcătoare a suprafețelor materialelor, echipamentelor și sudurilor luând în considerare caracteristicile și proprietățile acestora.Pentru control vizual se folosesc diferite ustensile optice cum ar fi endoscop, lupe, lămpi etc. Prin control visual sunt furnizate o serie de indicii legate de aspectul suprafeței metalului precum și estimarea unor defecte interne (recipiente metalice, butelii de gaze, conducte, tuburi etc)

Odată cu controlul visual se pot determina și dimensiuile defectelor de îmbinare, grosimile recipientului sudat, dimensiunile cordonului sudat etc.

[modifică] Alte metode Metoda radioscopică sau fluoroscopică, se bazează pe interacţiunea radiaţiilor penetrante cu

substanţe fluorescente. Metoda radiografică în timp real, combină tehnica fluoroscopică cu posibilităţile de

microfocalizare a radiaţiei X.

Metoda sondelor de potențial, funcţionează pe principiul variaţiei reluctanţei magnetice.

Metoda ferosondelor, discriminează variaţiile de inductanţă din piesă.

Metodele imagineriei procesate.

[modifică] Simbolizare

Metodelor uzuale de control nedistructiv le corespunde o serie de simboluri reglementate de norma europeană EN 473 și EN 4179 examinare nedistructivă END.

Metoda END Simbol

Emisie acustică AT

Curenți Foucault ET

Etanșeitate LT

Magnetoscopie MT

Lichide penetrante PT

Radiografie RT

Ultrasunet UT

Examen vizual VT

Speckle ST

Termografie IRT

[modifică] Standarde i normeș

[modifică] Vezi iș Sudare Sudare subacvatică

[modifică] Legături externe

Wikimedia Commons conţine materiale multimedia legate de Control nedistructiv

en NDT.org en NDT Encyclopedia

en British Institute of Non-Destructive Testing

In-Situ Metallography In-situ metallography is performed as NDT on actual site with a team comprising of expert metallographers and

metallurgists. It is used to find out in-service degradation of critical components of process plants operating under high temperature/ high pressure/ corrosive atmosphere. It provide damage assessments of fire affected equipment of plants. Microstructure survey for critical components viz., Boilers, Pipelines, Reactors and Vessels for condition monitoring and health assessments. TCR can also develop a data bank of critical components of equipment of process plant by periodical monitoring for preventive maintenance and planning for inventory control. TCR can provide suggestions on repair welding of used components of process plants. In-situ metallography checks the microstructure of component for intended service prior to being put in use. In-situ metallography is used for remaining life assessment studies.

Ultrasonic Inspection Ultrasonic methods of NDT use beams of sound waves (vibrations) of short wavelength and high frequency,

transmitted from a probe and detected by the same or other probes. Usually, pulsed beams of ultrasound are used and in the simplest instruments a single probe, hand held, is placed on the specimen surface. An oscilloscope display with a time base shows the time it takes for an ultrasonic pulse to travel to a reflector (a flaw, the back surface or other free surface) in terms of distance traveled across the oscilloscope screen. The height of the reflected pulse is related to the flaw size as seen from the transmitter probe. The relationship of flaw size, distance and reflectivity are complex, and a considerable skill is required to interpret the display. Complex mutiprobe

systems are also used with mechanical probe movement and digitization of signals, followed by computer interpretation are developing rapidly.

Ultrasonic examinations are performed for the detection and sizing of internal defects, flaws or discontinuities in piping, castings, forgings, weldments or other components. Exact sizing techniques have been developed to detect and monitor progressive cracking in a variety of equipment.

TCR can also undertake Automated UT using Time of Flight Diffraction technique (ToFD) in India as per code

case 181 for piping, code case 2235 for pressure vessels and API 650 appendix U for storage tanks. Dye Penetrant This method employs a penetrating liquid, which is applied over the surface of the component and enters the

discontinuity or crack. Subsequently, after the excess penetrant has been cleared from the surface, the penetrant exudes or is drawn back out of the crack is observed. Liquid penetrant testing can be applied to any non-porous clean material, metallic or non-metallic, but is unsuitable for dirty or very rough surfaces. Penetrants can contain a dye to make the indication visible under white light, or a fluorescent material that fluoresces under suitable ultra-violet light. Fluorescent penetrants are usually used when the maximum flaw sensitivity is required. Cracks as narrow as 150 nanometers can be detected.

Magnetic Particle Testing The Magnetic Particle Inspection method of Non-Destructive testing is a method for locating surface and sub-

surface discontinuities in ferromagnetic material. It depends for its operation on the face that when the material or part under test is magnetized, discontinuities that lie in a direction generally transverse to the direction of the magnetic field, will cause a leakage field, and therefore, the presence of the discontinuity, is detected by use of finely divided ferromagnetic particles applied over the surface, some of these particles being gathered and held by the leakage field, this magnetically held collection of particles forms an outline of the discontinuity and indicates its location, size, shape and extent.

Dry magnetic particle examinations and wet fluorescent magnetic particle examinations are performed on ferromagnetic materials to detect surface and slight subsurface discontinuities. Specialized wet fluorescent magnetic particle techniques are available for black light internal examinations of equipment through borescopes.

Liquid Penetrant Examination (PT) Various types of liquid penetrant examination methods are utilized to detect open or surface cracks or defects in

materials. Red dye or fluorescent penetrants are utilized as well as various types of wet and dry developers. Eddy Current Testing (ET) Eddy current testing is a rapid and accurate technique used to detect discontinuities in tubing, heat exchangers,

condensers, wires, plates, etc. Eddy current testing is also performed for alloy separation and for the determination of treatment conditions. The location of repair welds, girth welds and seam welds may also be detected on ground machined surfaces.

Leak Testing

Helium Leak testing in India is performed to detect and locate leaks in pressure containment parts and

structures. This includes welded, brazed, adhesion bonded and other assemblies. Certified Weld Inspectors (CWI)

TCR's team of Certified Welding Inspectors in India (CWI) can pinpoint exactly what testing is necessary to

qualify a weld, weld procedure, or individual welders. Each welding code follows three main categories of Welding Qualification; Welding Procedure Specification (WPS), Welding Procedure Qualification Record (WPQR), and Welder Performance Qualification (WPQ).

Visual Inspection Services Non-Destructive visual inspections can be preformed on-site or at the laboratory facility, and are based upon the

requirements of the client or specification. Industries utilizing this service include Fabrication, Construction, Automotive, Power Generation and Transportation. Inspections can be performed at the laboratory facility or onsite. These inspections are performed to IS, BS, ASTM, AWS, ASME (American Society for Mechanical Engineers) and many others.

Radiography With TCR's state-of-the-art equipment such as laser alignment devices, microprocessor controlled x-ray machines

and automatic film processors, we are able to increase the speed, quality and efficiency of our radiographic services.

Portable Hardness Per ASTM E110, this testing is normally used for on-site applications or on very large samples. The TCR portable

hardness unit performs the hardness testing by applying a 5 kg. Vickers load indenter and electronically converts the values in the preferred scale.

Magnetic-particle inspectionFrom Wikipedia, the free encyclopedia

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Magnetic particle inspection (MPI) is a non-destructive testing (NDT) process for detecting surface and subsurface discontinuities in ferrous materials. The process puts a magnetic field into the part. The piece can be magnetized by direct or indirect magnetization. Direct magnetization occurs when the electric current is passed through the test object and a magnetic field is formed in the material. Indirect magnetization occurs when no electric current is passed through the test object, but a magnetic field is applied from an outside source. The magnetic lines of force are perpendicular to the direction of the electric current which may be either alternating current (AC) or some form of direct current (DC) (rectified AC).

The presence of a surface or subsurface discontinuity in the material allows the magnetic flux to leak. Ferrous iron particles are applied to the part. The particles may be dry or in a wet suspension. If an area of flux leakage is present the particles will be attracted to this area. The particles will build up at the area of leakage and form what is known as an indication. The indication can then be evaluated to determine what it is, what may have caused it, and what action should be taken if any.

Contents[hide]

1 Types of electrical currents used 2 Equipment

3 Demagnetizing parts

4 Magnetic particle powder

o 4.1 Magnetic particle carriers

5 Inspection

6 Standards

7 References

[edit] Types of electrical currents used

There are several types of electrical currents used in MPI. For a proper current to be selected one needs to consider the part geometry, material, the type of discontinuity you're looking for, and how far the magnetic field needs to penetrate into the part.

Alternating current (AC) commonly used to detect surface discontinuities. Using AC to detect subsurface discontinuities is limited due to what is known as the skin effect, where the current runs along the surface of the part. Because the current alternates in polarity at 50 to 60 cycles per second it does not penetrate much past the surface of the test object. This means the magnetic domains will only be aligned equal to the distance AC current penetration into the part. The Frequency of the Alternating Current decides how deep the penetration.

Direct current (DC, full wave DC) Used to detect sub surface discontinuities where AC can not penetrate deep enough to magnetize the part at the depth needed. The amount of magnetic penetration depends on the amount of current passed through the part.[1] DC is also limited on very large cross sectional parts how effective it will magnetize the part.

Half wave DC (HWDC, pulsating DC) work similar to full wave DC with sightly more magnetic penetration into the part. HWDC is known to have the most penetrating ability in magnetic particle testing.[1] HWDC is advantageous for inspection process because it actually helps move the magnetic particles over the test object so that they have the opportunity to come in contact with areas of magnetic flux leakage. The increase in particle mobility is caused by the pulsating current which vibrates the test piece and particles.

Each method of magnetizing has its pros and cons. AC is generally always best for discontinuities open to the surface and some form of DC for subsurface.

[edit] Equipment

A wet horizontal MPI machine with a 36 in (910 mm) coil

An automatic wet horizontal MPI machine with an external power supply, conveyor, and demagnetizing system; its used to inspect engine cranks.

A wet horizontal MPI machine is the most commonly used mass production inspection machine. The machine has a head and tail stock where the part is placed to magnetize it. In between the head and tail stock is typically an induction coil, which is used to change the orientation of the magnetic field by 90° from head stock. Most of the equipment is customized and built for a specific application.

Mobile power packs: Are custom built magnetizing power supplies used in wire wrapping applications.

Magnetic yoke: is a hand held devices that induces a magnetic field between two poles. Common applications are for outdoor use, remote locations, and weld inspection. The draw back of magnetic yokes are they only induce a magnetic field between the poles so inspection is time consuming are on large parts. For proper inspection the yoke needs to be rotated 90 degrees for every inspection area to detect horizontal and vertical discontinuities. Yokes subsurface detection is limited. These systems used dry magnetic powders, wet powders, or aerosol cans.

[edit] Demagnetizing parts

A pull through AC demagnetizing unit

After the part has been magnetized its needs to be demagnetized. This requires special equipment that works the opposite of magnetizing equipment. Magnetizing is normally done with high current pulse that very quickly reaches a peak current and instantaneously turns off leaving the part magnetized. To demagnetize a part the current or magnetic field needed, has to be equal or greater than the current or magnetic field used to magnetized the part, the current or magnetic field then is slowly reduced to zero leaving the part demagnetized.

AC demagnetizing o Pull through AC demagnetizing coils: seen in Fig 3 are AC powered devices that generate

a high magnetic field where the part is slowly pulled through by hand or on a conveyor. The act of pulling the part through and away from coil's magnetic field slows drops the magnetic field in the part. Note many AC demagnetizing coils have power cycles of several seconds so the part must be passed through the coil and be several feet (meters) away before the demagnetizing cycle finishes or the part will have residue magnetism.

o AC step down demagnetizing: This is built in only a few MPI equipment, the process is where the part is subjected to equal or greater AC current, the current is reduced by X amps in several sequential pulses till zero current is reached. The number of steps required to demagnetizing a part is a function of amount current to magnetize the part.

Reversing DC demagnetizing: The simply reverses the current flow of magnetizing pulse canceling the magnetic flow. Note: This is built in the MPI equipment by the manufacturer.

[edit] Magnetic particle powder

The particles used to detect cracks is commonly iron oxide for both dry and wet systems.

Wet system particle range in size from <0.5 to 10 micrometres for use with water or oil carriers. Particles used in wet systems have pigments applied that fluoresce at 365 nm ( Ultraviolet A)

requiring 1000 µW/cm2 (10 W/m2) at the surface of the part for proper inspection. If the particles do not have the correct light applied in a Dark Room the particles can not be detected/seen. Its industry practices to use UV goggles/glasses to filter the UV light and amplify the visible light spectrum normally Green and Yellow created by the fluorescing particles. Green and Yellow fluorescence was chosen because the human eye reacts best to these colors.

Dry particle powders range in size from 5 to 170 micrometres, designed to be seen in white light conditions. The particles are not designed to be used in wet environments. Dry powders are normally applied using hand operated air powder applicators

Aerosol applied particles are similar to wet systems, sold in premixed aerosol cans similar to hair spray.

[edit] Magnetic particle carriers

It is common industry practices to use specifically designed oil and water-based carriers for magnetic particles. Deodorized kerosene, and mineral spirits have not been commonly used in the industry for 40 years. It is very dangerous to use kerosene or mineral spirits as a carrier to due to their low flash points, and inhalation of fumes by the operators.

[edit] Inspection

The following are general steps for inspecting on a wet horizontal machine:

1. Part is cleaned of oil and other contaminants2. Necessary calculations done to know the amount of current required to magnetize the part. See

ASTM E1444-05 for formulas.

3. The magnetizing pulse is applied for 5 seconds during which the operator washes the part with the particle, stopping before the magnetic pulse is completed. Failure to Stop prior to end of the magnetic pulse will wash away indications.

4. UV light is applied the operator looks for indications of defects that are 0 to +/- 45 degrees from path the current flowed through the part. Defects only appear that are 45 to 90 degrees the magnetic field. The easiest way to quickly figure out which way the magnetic field is running is grab the part with either hand between the head stocks laying your thumb against the part (do not wrap your thumb around the part) this is called either left or right thumb rule or right hand grip rule. The direction thumb points tell us the direction current is flowing, the Magnetic field will be running 90 degrees from the current path. On complex geometry like a engine crank the operator needs to visualize the changing direction of the current and magnetic field created. The current starts at 0 degrees then 45 degrees to 90 degree back to 45 degrees to 0 then -45 to -90 to -45 to 0 and repeats this for crankpin. So inspection can be time consuming to carefully look for indications that are only 45 to 90 degrees from the magnetic field.

5. The part is either accepted or rejected based on pre-defined accept and reject criteria

6. The part is demagnetized

7. Depending on requirements the orientation of the magnetic field may need to be changed 90 degrees to inspect for defects that can not be detected from steps 3 to 5. The most common way is change magnetic field orientation is to a use Coil Shot. in Fig 1 a 36 inch Coil can be seen then steps 4, 5, and 6 are repeated

[edit] StandardsInternational Organization for Standardization (ISO)

ISO 3059, Non-destructive testing - Penetrant testing and magnetic particle testing - Viewing conditions

ISO 9934-1, Non-destructive testing - Magnetic particle testing - Part 1: General principles

ISO 9934-2, Non-destructive testing - Magnetic particle testing - Part 2: Detection media

ISO 9934-3, Non-destructive testing - Magnetic particle testing - Part 3: Equipment

ISO 17638, Non-destructive testing of welds - Magnetic particle testing

ISO 23279, Non-destructive testing of welds - Magnetic particle testing of welds - Acceptance levels

European Committee for Standardization (CEN) EN 1330-7, Non-destructive testing - Terminology - Part 7: Terms used in magnetic particle

testing EN 1369, Founding - Magnetic particle inspection

EN 10228-1, Non-destructive testing of steel forgings - Part 1: Magnetic particle inspection

EN 10246-12, Non-destructive testing of steel tubes - Part 12: Magnetic particle inspection of seamless and welded ferromagnetic steel tubes for the detection of surface imperfections

EN 10246-18, Non-destructive testing of steel tubes - Part 18: Magnetic particle inspection of the tube ends of seamless and welded ferromagnetic steel tubes for the detection of laminar imperfections

American Society of Testing and Materials (ASTM) ASTM E1444-05 ASTM A 275/A 275M Test Method for Magnetic Particle Examination of Steel Forgings

ASTM A456 Specification for Magnetic Particle Inspection of Large Crankshaft Forgings

ASTM E543 Practice Standard Specification for Evaluating Agencies that Performing Nondestructive Testing

ASTM E 709 Guide for Magnetic Particle Testing Examination

ASTM E 1316 Terminology for Nondestructive Examinations

ASTM E 2297 Standard Guide for Use of UV-A and Visible Light Sources and Meters used in the Liquid Penetrant and Magnetic Particle Methods

Canadian Standards Association (CSA) CSA W59

Society of Automotive Engineers (SAE) AMS 2641 Magnetic Particle Inspection Vehicle AMS 3040 Magnetic Particles, Nonfluorescent, Dry Method

AMS 3041 Magnetic Particles, Nonfluorescent,Wet Method, Oil Vehicle, Ready-To-Use

AMS 3042 Magnetic Particles, Nonfluorescent, Wet Method, Dry Powder

AMS 3043 Magnetic Particles, Nonfluorescent, Wet Method, Oil Vehicle, Aerosol Packaged

AMS 044 Magnetic Particles, Fluorescent, Wet Method, Dry Powder

AMS 3045 Magnetic Particles, Fluorescent, Wet Method, Oil Vehicle, Ready-To-Use

AMS 3046 Magnetic Particles, Fluorescent, Wet Method, Oil Vehicle, Aerosol Packaged5

AMS 5062 Steel, Low Carbon Bars, Forgings, Tubing, Sheet, Strip, and Plate 0.25 Carbon, Maximum

AMS 5355 Investment Castings

AMS I-83387 Inspection Process, Magnetic Rubber

AMS-STD-2175 Castings, Classification and Inspection of AS 4792 Water Conditioning Agents for Aqueous Magnetic Particle Inspection AS 5282 Tool Steel Ring Standard for Magnetic Particle Inspection AS5371 Reference Standards Notched Shims for Magnetic Particle Inspection

United States Military Standard A-A-59230 Fluid, Magnetic Particle Inspection, Suspension

[edit] References1. ^ a b Betz, C. E. (1985), Principles of Magnetic Particle Testing, American Society for

Nondestructive Testing, p. 234, ISBN 9780318214856, http://wiki.magwerks.com/wiki/images/c/c6/Waveform_to_Depth_Comparison.pdf.

2. Introduction to Magnetic Particle Inspection3. Magnetic particle inspection (MPI) is a nondestructive testing method used for defect

detection. MPI is fast and relatively easy to apply, and part surface preparation is not as critical as it is for some other NDT methods. These characteristics make MPI one of the most widely utilized nondestructive testing methods.

4. MPI uses magnetic fields and small magnetic particles (i.e.iron filings) to detect flaws in components. The only requirement from an inspectability standpoint is that the component being inspected must be made of a ferromagnetic material such as iron, nickel, cobalt, or some of their alloys. Ferromagnetic materials are materials that can be magnetized to a level that will allow the inspection to be effective.

5. The method is used to inspect a variety of product forms including castings, forgings, and weldments. Many different industries use magnetic particle inspection for determining a

component's fitness-for-use. Some examples of industries that use magnetic particle inspection are the structural steel, automotive, petrochemical, power generation, and aerospace industries. Underwater inspection is another area where magnetic particle inspection may be used to test items such as offshore structures and underwater pipelines.

6.7. Basic Principles

8. In theory, magnetic particle inspection (MPI) is a relatively simple concept. It can be considered as a combination of two nondestructive testing methods: magnetic flux leakage testing and visual testing. Consider the case of a bar magnet. It has a magnetic field in and around the magnet. Any place that a magnetic line of force exits or enters the magnet is called a pole. A pole where a magnetic line of force exits the magnet is called a north pole and a pole where a line of force enters the magnet is called a south pole.

9. When a bar magnet is broken in the center of its length, two complete bar magnets with magnetic poles on each end of each piece will result. If the magnet is just cracked but not broken completely in two, a north and south pole will form at each edge of the crack. The magnetic field exits the north pole and reenters at the south pole. The magnetic field spreads out when it encounters the small air gap created by the crack because the air cannot support as much magnetic field per unit volume as the magnet can. When the field spreads out, it appears to leak out of the material and, thus is called a flux leakage field.

10. If iron particles are sprinkled on a cracked magnet, the particles will be attracted to and cluster not only at the poles at the ends of the magnet, but also at the poles at the edges of the crack. This cluster of particles is much easier to see than the actual crack and this is the basis for magnetic particle inspection.

11.12. The first step in a magnetic particle inspection is to magnetize the component that is to be

inspected. If any defects on or near the surface are present, the defects will create a leakage field. After the component has been magnetized, iron particles, either in a dry or wet suspended form, are applied to the surface of the magnetized part. The particles will be attracted and cluster at the flux leakage fields, thus forming a visible indication that the inspector can detect.

History of Magnetic Particle Inspection

Magnetism is the ability of matter to attract other matter to itself. The ancient Greeks were the first to discover this phenomenon in a mineral they named magnetite. Later on Bergmann, Becquerel, and Faraday discovered that all matter including liquids and gasses were affected by magnetism, but only a few responded to a noticeable extent.

The earliest known use of magnetism to inspect an object took place as early as 1868. Cannon barrels were checked for defects by magnetizing the barrel then sliding a magnetic compass along the barrel's length. These early inspectors were able to locate flaws in the barrels by monitoring the needle of the compass. This was a form of nondestructive testing but the term was not commonly used until some time after World War I.

In the early 1920’s, William Hoke realized that magnetic particles (colored metal shavings) could be used with magnetism as a means of locating defects. Hoke discovered that a surface or subsurface flaw in a magnetized material caused the magnetic field to distort and extend beyond the part. This discovery was brought to his attention in the machine shop. He noticed that the metallic grindings from hard steel parts (held by a magnetic chuck while being ground) formed patterns on the face of the parts which corresponded to the cracks in the surface. Applying a fine ferromagnetic powder to the parts caused a build up of powder over flaws and formed a visible indication. The image shows a 1928 Electyro-Magnetic Steel Testing Device (MPI) made by the Equipment and Engineering Company Ltd. (ECO) of Strand,

England.

In the early 1930’s, magnetic particle inspection was quickly replacing the oil-and-whiting method (an early form of the liquid penetrant inspection) as the method of choice by the railroad industry to inspect steam engine boilers, wheels, axles, and tracks. Today, the MPI inspection method is used extensively to check for flaws in a large variety of manufactured materials and components. MPI is used to check materials such as steel bar stock for seams and other flaws prior to investing machining time during the manufacturing of a component. Critical automotive components are inspected for flaws after fabrication to ensure that defective parts are not placed into service. MPI is used to inspect some highly loaded components that have been in-service for a period of time. For example, many components of high performance racecars are inspected whenever the engine, drive train or another system undergoes an overhaul. MPI is also used to evaluate the integrity of structural welds on bridges, storage tanks, and other safety critical structures.

Magnetism

Magnets are very common items in the workplace and household. Uses of magnets range from holding pictures on the refrigerator to causing torque in electric motors. Most people are familiar with the general properties of magnets but are less familiar with the source of magnetism. The traditional concept of magnetism centers around the magnetic field and what is know as a dipole. The term "magnetic field" simply describes a volume of space where there is a change in energy within that volume. This change in energy can be detected and measured. The location where a magnetic field can be detected exiting or entering a material is called a magnetic pole. Magnetic poles have never been detected in isolation but always occur in pairs, hence the name dipole. Therefore, a dipole is an object that has a magnetic pole on one end and a second, equal but opposite, magnetic pole on the other.

A bar magnet can be considered a dipole with a north pole at one end and south pole at the other. A magnetic field can be measured leaving the dipole at the north pole and returning the magnet at the south pole. If a magnet is cut in two, two magnets or dipoles are created out of one. This sectioning and creation of dipoles can continue to the atomic level. Therefore, the source of magnetism lies in the basic building block of all matter...the atom.

The Source of Magnetism

All matter is composed of atoms, and atoms are composed of protons, neutrons and electrons. The protons and neutrons are located in the atom's nucleus and the electrons are in constant motion around the nucleus. Electrons carry a negative electrical charge and produce a magnetic field as they move through

space. A magnetic field is produced whenever an electrical charge is in motion. The strength of this field is called the magnetic moment.

This may be hard to visualize on a subatomic scale but consider electric current flowing through a conductor. When the electrons (electric current) are flowing through the conductor, a magnetic field forms around the conductor. The magnetic field can be detected using a compass. The magnetic field will place a force on the compass needle, which is another example of a dipole.

Since all matter is comprised of atoms, all materials are affected in some way by a magnetic field. However, not all materials react the same way. This will be explored more in the next section.

Diamagnetic, Paramagnetic, and Ferromagnetic Materials

When a material is placed within a magnetic field, the magnetic forces of the material's electrons will be affected. This effect is known as Faraday's Law of Magnetic Induction. However, materials can react quite differently to the presence of an external magnetic field. This reaction is dependent on a number of factors, such as the atomic and molecular structure of the material, and the net magnetic field associated with the atoms. The magnetic moments associated with atoms have three origins. These are the electron motion, the change in motion caused by an external magnetic field, and the spin of the electrons.

In most atoms, electrons occur in pairs. Electrons in a pair spin in opposite directions. So, when electrons are paired together, their opposite spins cause their magnetic fields to cancel each other. Therefore, no net magnetic field exists. Alternately, materials with some unpaired electrons will have a net magnetic field and will react more to an external field. Most materials can be classified as diamagnetic, paramagnetic or ferromagnetic.

Diamagnetic materials have a weak, negative susceptibility to magnetic fields. Diamagnetic materials are slightly repelled by a magnetic field and the material does not retain the magnetic properties when the external field is removed. In diamagnetic materials all the electron are paired so there is no permanent net magnetic moment per atom. Diamagnetic properties arise from the realignment of the electron paths under the influence of an external magnetic field. Most elements in the periodic table, including copper, silver, and gold, are diamagnetic.

Paramagnetic materials have a small, positive susceptibility to magnetic fields. These materials are slightly attracted by a magnetic field and the material does not retain the magnetic properties when the external field is removed. Paramagnetic properties are due to the presence of some unpaired electrons, and from the realignment of the electron paths caused by the external magnetic field. Paramagnetic materials include magnesium, molybdenum, lithium, and tantalum.

Ferromagnetic materials have a large, positive susceptibility to an external magnetic field. They exhibit a strong attraction to magnetic fields and are able to retain their magnetic properties after the external field has been removed. Ferromagnetic materials have some unpaired electrons so their atoms have a net magnetic moment. They get their strong magnetic properties due to the presence of magnetic domains. In these domains, large numbers of atom's moments (1012 to 1015) are aligned parallel so that the magnetic force within the domain is strong. When a ferromagnetic material is in the unmagnitized state, the domains are nearly randomly organized and the net magnetic field for the part as a whole is zero. When a magnetizing force is applied, the domains become aligned to produce a strong magnetic field within the part. Iron, nickel, and cobalt are examples of ferromagnetic materials. Components with these materials are commonly inspected using the magnetic particle method.

Magnetic Domains

Ferromagnetic materials get their magnetic properties not only because their atoms carry a magnetic moment but also because the material is made up of small regions known as magnetic domains. In each domain, all of the atomic dipoles are coupled together in a preferential direction. This alignment develops as the material develops its crystalline structure during solidification from the molten state. Magnetic domains can be detected using Magnetic Force Microscopy (MFM) and images of the domains like the one shown below can be constructed.

Magnetic Force Microscopy (MFM) image showing the magnetic domains in a piece of heat treated carbon steel.

During solidification, a trillion or more atom moments are aligned parallel so that the magnetic force within the domain is strong in one direction. Ferromagnetic materials are said to be characterized by "spontaneous magnetization" since they obtain saturation magnetization in each of the domains without an external magnetic field being applied. Even though the domains are magnetically saturated, the bulk material may not show any signs of magnetism because the domains develop themselves and are randomly oriented relative to each other.

Ferromagnetic materials become magnetized when the magnetic domains within the material are aligned. This can be done by placing the material in a strong external magnetic field or by passing electrical current through the material. Some or all of the domains can become aligned. The more domains that are aligned, the stronger the magnetic field in the material. When all of the domains are aligned, the material is said to be magnetically saturated. When a material is magnetically saturated, no

additional amount of external magnetization force will cause an increase in its internal level of magnetization.

 

Unmagnetized Material   Magnetized Material

Magnetic Field Characteristics

Magnetic Field In and Around a Bar Magnet

As discussed previously, a magnetic field is a change in energy within a volume of space. The magnetic field surrounding a bar magnet can be seen in the magnetograph below. A magnetograph can be created by placing a piece of paper over a magnet and sprinkling the paper with iron filings. The particles align themselves with the lines of magnetic force produced by the magnet. The magnetic lines of force show where the magnetic field exits the material at one pole and reenters the material at another pole along the length of the magnet. It should be noted that the magnetic lines of force exist in three dimensions but are only seen in two dimensions in the image.

It can be seen in the magnetograph that there are poles all along the length of the magnet but that the poles are concentrated at the ends of the magnet. The area where the exit poles are concentrated is called the magnet's north pole and the area where the entrance poles are concentrated is called the magnet's south pole.

Magnetic Fields in and around Horseshoe and Ring Magnets

Magnets come in a variety of shapes and one of the more common is the horseshoe (U) magnet. The horseshoe magnet has north and south poles just like a bar magnet but the magnet is curved so the poles lie in the same

plane. The magnetic lines of force flow from pole to pole just like in the bar magnet. However, since the poles are located closer together and a more direct path exists for the lines of flux to travel between the poles, the magnetic field is concentrated between the poles.

 

If a bar magnet was placed across the end of a horseshoe magnet or if a magnet was formed in the shape of a ring, the lines of magnetic force would not even need to enter the air. The value of such a magnet where the magnetic field is completely contained with the material probably has limited use. However, it is important to understand that the magnetic field can flow in loop within a material. (See section on circular magnetism for more information).

 

General Properties of Magnetic Lines of Force

Magnetic lines of force have a number of important properties, which include:

They seek the path of least resistance between opposite magnetic poles. In a single bar magnet as shown to the right, they attempt to form closed loops from pole to pole.

They never cross one another.

They all have the same strength.

Their density decreases (they spread out) when they move from an area of higher permeability to an area of lower permeability.

Their density decreases with increasing distance from the poles.

They are considered to have direction as if flowing, though no actual movement occurs.

They flow from the south pole to the north pole within a material and north pole to south pole in air.

 

 

Electromagnetic Fields

Magnets are not the only source of magnetic fields. In 1820, Hans Christian Oersted discovered that an electric current flowing through a wire caused a nearby compass to deflect. This indicated that the current in the wire was generating a magnetic field. Oersted studied the nature of the magnetic field around the long straight wire. He found that the magnetic field existed in circular form around the wire and that the intensity of the field was directly proportional to the amount of current carried by the wire. He also found that the strength of the field was strongest next to the wire and diminished with distance from the conductor until it could no longer be detected. In most conductors, the magnetic field exists only as long as the current is flowing (i.e. an electrical charge is in motion). However, in ferromagnetic materials the electric current will cause some or all of the magnetic domains to align and a residual magnetic field will remain.

Oersted also noticed that the direction of the magnetic field was dependent on the direction of the electrical current in the wire. A three-dimensional representation of the magnetic field is shown below. There is a simple rule for remembering the direction of the magnetic field around a conductor. It is called the right-hand clasp rule. If a person grasps a conductor in one's right hand with the thumb pointing in the direction of the current, the fingers will circle the conductor in the direction of the magnetic field.

A word of caution about the right-hand clasp ruleFor the right-hand rule to work, one important thing that must be remembered about the direction of current flow. Standard convention has current flowing from the positive terminal to the

negative terminal. This convention is credited to Benjamin Franklin who theorized that electric current was due to a positive charge moving from the positive terminal to the negative terminal. However, it was later discovered that it is the movement of the negatively charged electron that is responsible for electrical current. Rather than changing several centuries of theory and equations, Franklin's convention is still used today.

Magnetic Field Produced by a Coil

When a current carrying conductor is formed into a loop or several loops to form a coil, a magnetic field develops that flows through the center of the loop or coil along its longitudinal axis and circles back around the outside of the loop or coil. The magnetic field circling each loop of wire combines with the fields from the other loops to produce a concentrated field down the center of the coil. A loosely wound coil is illustrated below to show the interaction of the magnetic field. The magnetic field is essentially uniform down the length of the coil when it is wound tighter.

The strength of a coil's magnetic field increases not only with increasing current but also with each loop that is added to the coil. A long, straight coil of wire is called a solenoid and can be used to generate a nearly uniform magnetic field similar to that of a bar magnet. The concentrated magnetic field inside a coil is very useful in magnetizing ferromagnetic materials for inspection using the magnetic particle testing method. Please be aware that the field outside the coil is weak and is not suitable for magnetizing ferromagnetic materials.

Quantifying Magnetic Properties(Magnetic Field Strength, Flux Density, Total Flux and

Magnetization)

Until now, only the qualitative features of the magnetic field have been discussed. However, it is necessary to be able to measure and express quantitatively the various characteristics of magnetism. Unfortunately, a number of unit conventions are used (as shown in the table below). SI units will be used in this material. The advantage of using SI units is that they are traceable back to an agreed set of four base units - meter, kilogram, second, and Ampere.

Quantity  SI Units

(Sommerfeld)SI Units

(Kennelly)CGS Units(Gaussian)

Field H A/m A/m oerstedsFlux Density(Magnetic Induction)

tesla tesla gauss

Flux weber weber maxwellMagnetization M A/m - erg/Oe-cm3

The units for magnetic field strength H are ampere/meter. A magnetic field strength of 1 ampere/meter is produced at the center of a single circular conductor with a one meter diameter carrying a steady current of 1 ampere.

The number of magnetic lines of force cutting through a plane of a given area at a right angle is known as the magnetic flux density, B. The flux density or magnetic induction has the tesla as its unit. One tesla is equal to 1 Newton/(A/m). From these units, it can be seen that the flux density is a measure of the force applied to a particle by the magnetic field. The Gauss is the CGS unit for flux density and is commonly used by US industry. One gauss represents one line of flux passing through one square centimeter of air oriented 90 degrees to the flux flow.

The total number of lines of magnetic force in a material is called magnetic flux,. The strength of the flux is determined by the number of magnetic domains that are aligned within a material. The total flux is simply the flux density applied over an area. Flux carries the unit of a weber, which is simply a tesla- meter2.

The magnetization is a measure of the extent to which an object is magnetized. It is a measure of the magnetic dipole moment per unit volume of the object. Magnetization carries the same units as a magnetic field: amperes/meter.

Conversion between CGS and SI magnetic units.  

The Hysteresis Loop and Magnetic Properties

A great deal of information can be learned about the magnetic properties of a material by studying its hysteresis loop. A hysteresis loop shows the relationship between the induced magnetic flux density (B) and the magnetizing force (H). It is often referred to as the B-H loop. An example hysteresis loop is shown below.

The loop is generated by measuring the magnetic flux of a ferromagnetic material while the magnetizing force is changed. A ferromagnetic material that has never been previously magnetized or has been thoroughly demagnetized will follow the dashed line as H is increased. As the line demonstrates, the greater the amount of current applied (H+), the stronger the magnetic field in the component (B+). At point "a" almost all of the magnetic domains are

aligned and an additional increase in the magnetizing force will produce very little increase in magnetic flux. The material has reached the point of magnetic saturation. When H is reduced to zero, the curve will move from point "a" to point "b." At this point, it can be seen that some magnetic flux remains in the material even though the magnetizing force is zero. This is referred to as the point of retentivity on the graph and indicates the remanence or level of residual magnetism in the material. (Some of the magnetic domains remain aligned but some have lost their alignment.) As the magnetizing force is reversed, the curve moves to point "c", where the flux has been reduced to zero. This is called the point of coercivity on the curve. (The reversed magnetizing force has flipped enough of the domains so that the net flux within the material is zero.) The force required to remove the residual magnetism from the material is called the coercive force or coercivity of the material.

As the magnetizing force is increased in the negative direction, the material will again become magnetically saturated but in the opposite direction (point "d"). Reducing H to zero brings the curve to point "e." It will have a level of residual magnetism equal to that achieved in the other direction. Increasing H back in the positive direction will return B to zero. Notice that the curve did not return to the origin of the graph because some force is required to remove the residual magnetism. The curve will take a different path from point "f" back to the saturation point where it with complete the loop.

From the hysteresis loop, a number of primary magnetic properties of a material can be determined.

1. Retentivity - A measure of the residual flux density corresponding to the saturation induction of a magnetic material. In other words, it is a material's ability to retain a certain amount of residual magnetic field when the magnetizing force is removed after achieving saturation. (The value of B at point b on the hysteresis curve.)

2. Residual Magnetism or Residual Flux - the magnetic flux density that remains in a material when the magnetizing force is zero. Note that residual magnetism and retentivity are the same when the material has been magnetized to the saturation point. However, the level of residual magnetism may be lower than the retentivity value when the magnetizing force did not reach the saturation level.

3. Coercive Force - The amount of reverse magnetic field which must be applied to a magnetic material to make the magnetic flux return to zero. (The value of H at point c on the hysteresis curve.)

4. Permeability, - A property of a material that describes the ease with which a magnetic flux is established in the component.

5. Reluctance - Is the opposition that a ferromagnetic material shows to the establishment of a magnetic field. Reluctance is analogous to the resistance in an electrical circuit.

Permeability

As previously mentioned, permeability is a material property that describes the ease with which a magnetic flux is established in a component. It is the ratio of the flux density to the magnetizing force and is represented by the following equation:

It is clear that this equation describes the slope of the curve at any point on the hysteresis loop. The permeability value given in papers and reference materials is usually the maximum permeability or the maximum relative permeability. The maximum permeability is the point where the slope of the B/H curve for the unmagnetized material is the greatest. This point is often taken as the point where a straight line from the origin is tangent to the B/H curve.

The relative permeability is arrived at by taking the ratio of the material's permeability to the permeability in free space (air).

(relative) = (material) /air

where: air = 1.256 x 10-6 H/m

The shape of the hysteresis loop tells a great deal about the material being magnetized. The hysteresis curves of two different materials are shown in the graph.

Relative to other materials, a material with a wider hysteresis loop has:

Lower Permeability Higher Retentivity

Higher Coercivity

Higher Reluctance

Higher Residual Magnetism

Relative to other materials, a material with the narrower hysteresis loop has:

Higher Permeability

Lower Retentivity

Lower Coercivity

Lower Reluctance

Lower Residual Magnetism.

In magnetic particle testing, the level of residual magnetism is important. Residual magnetic fields are affected by the permeability, which can be related to the carbon content and alloying of the material. A component with high carbon content will have low permeability and will retain more magnetic flux than a material with low carbon content.

In the two B-H loops above, which one would indicative of a low carbon steel? Answer

 

Magnetic Field Orientation and Flaw Detectability

To properly inspect a component for cracks or other defects, it is important to understand that the orientation between the magnetic lines of force and the flaw is very important. There are two general types of magnetic fields that can be established within a component.

A longitudinal magnetic field has magnetic lines of force that run parallel to the long axis of the part. Longitudinal magnetization of a component can be accomplished using the longitudinal field set up by a coil or solenoid. It can also be accomplished using permanent magnets or electromagnets.

A circular magnetic field has magnetic lines of force that run circumferentially around the perimeter of a part. A circular magnetic field is induced in an article by either passing current through the component or by passing current through a conductor surrounded by the component.

The type of magnetic field established is determined by the method used to magnetize the specimen. Being able to magnetize the part in two directions is important because the best detection of defects occurs when the lines of magnetic force are established at right angles to the longest dimension of the defect. This orientation creates the largest disruption of the magnetic field within the part and the greatest flux leakage at the surface of the part. As can be seen in the image below, if the magnetic field is parallel to the defect, the field will see little disruption and no flux leakage field will be produced.

An orientation of 45 to 90 degrees between the magnetic field and the defect is necessary to form an indication. Since defects may occur in various and unknown directions, each part is normally magnetized in two directions at right angles to each other. If the component below is considered, it is known that passing current through the part from end to end will establish a circular magnetic field that will be 90 degrees to the direction of the current. Therefore, defects that have a significant dimension in the direction of the current (longitudinal defects) should be detectable. Alternately, transverse-type defects will not be detectable with circular magnetization.

Watch this short movie showing the effect of field direction on indication visibility. (775 KB mov)

 

Magnetization of Ferromagnetic Materials

There are a variety of methods that can be used to establish a magnetic field in a component for evaluation using magnetic particle inspection. It is common to classify the magnetizing methods as either direct or indirect.

Magnetization Using Direct Induction (Direct Magnetization)

With direct magnetization, current is passed directly through the component. Recall that whenever current flows, a magnetic field is produced. Using the right-hand rule, which was introduced earlier, it is known that the magnetic lines of flux form normal to the direction of the current and form a circular field in and around the conductor. When using the direct magnetization method, care must be taken to ensure that good electrical contact is established and maintained between the test equipment and the test component. Improper contact can result in arcing that may damage the component. It is also possible to overheat components in areas of high resistance such as the contact points and in areas of small cross-sectional area.

There are several ways that direct magnetization is commonly accomplished. One way involves clamping the component between two electrical contacts in a special piece of equipment. Current is passed through the component and a circular magnetic field is established in and around the component. When the

magnetizing current is stopped, a residual magnetic field will remain within the component. The strength of the induced magnetic field is proportional to the amount of current passed through the component.

 

A second technique involves using clamps or prods, which are attached or placed in contact with the component. Electrical current flows through the component from contact to contact. The current sets up a circular magnetic field around the path of the current.

 

Magnetization Using Indirect Induction (Indirect Magnetization)

Indirect magnetization is accomplished by using a strong external magnetic field to establish a magnetic field within the component. As with direct magnetization, there are several ways that indirect magnetization can be accomplished.

The use of permanent magnets is a low cost method of establishing a magnetic field. However, their use is limited due to lack of control of the field strength and the difficulty of placing and removing strong permanent magnets from the component.

Electromagnets in the form of an adjustable horseshoe magnet (called a yoke) eliminate the problems associated with permanent magnets and are used extensively in industry. Electromagnets only exhibit a magnetic flux when electric current is flowing around the soft iron core. When the magnet is placed on the component, a magnetic field is established between the north and south poles of the magnet.

Another way of indirectly inducting a magnetic field in a material is by using the magnetic field of a current carrying conductor. A circular magnetic field can be established in cylindrical components by using a central conductor. Typically, one or more cylindrical components are hung from a solid copper bar running through the inside diameter. Current is passed through the copper bar and the resulting circular magnetic field establishes a magnetic field within the test components.

The use of coils and solenoids is a third method of indirect magnetization. When the length of a component is several times larger than its diameter, a longitudinal magnetic field can be established in the component. The component is placed longitudinally in the concentrated magnetic field that fills the center of a coil or solenoid. This magnetization technique is often referred to as a "coil shot."

Magnetizing Current

As seen in the previous pages, electric current is often used to establish the magnetic field in components during magnetic particle inspection. Alternating current and direct current are the two basic types of current commonly used. Current from single phase 110 volts, to three phase 440 volts, are used when generating an electric field in a component. Current flow is often modified to provide the appropriate field within the part. The type of current used can have an effect on the inspection results, so the types of currents commonly used will be briefly reviewed.

Direct CurrentDirect current (DC) flows continuously in one direction at a constant voltage. A battery is the most common source of direct current. As previously mentioned, current is said to flow from the positive to the negative terminal. In actuality, the electrons flow in the opposite direction. DC is

very desirable when inspecting for subsurface defects because DC generates a magnetic field that penetrates deeper into the material. In ferromagnetic materials, the magnetic field produced by DC generally penetrates the entire cross-section of the component.  Conversely, the field produced using alternating current is concentrated in a thin layer at the surface of the component.

Alternating CurrentAlternating current (AC) reverses in direction at a rate of 50 or 60 cycles per second. In the United States, 60 cycle current is the commercial norm but 50 cycle current is common in many countries. Since AC is readily available in most facilities, it is convenient to make use of it for magnetic particle inspection. However, when AC is used to induce a magnetic field in ferromagnetic materials, the magnetic field will be limited to narrow region at the surface of the component. This phenomenon is known as the "skin effect" and occurs because the changing magnetic field generates eddy currents in the test object. The eddy currents produce a magnetic field that opposes the primary field, thus reducing the net magnetic flux below the surface. Therefore, it is recommended that AC be used only when the inspection is limited to surface defects.

View a short video on the AC versus DC. (195 KB mov)

Rectified Alternating CurrentClearly, the skin effect limits the use of AC since many inspection applications call for the detection of subsurface defects. However, the convenient access to AC, drives its use beyond surface flaw inspections. Luckily, AC can be converted to current that is very much like DC through the process of rectification. With the use of rectifiers, the reversing AC can be converted to a one directional current. The three commonly used types of rectified current are described below.

Half Wave Rectified Alternating Current (HWAC)When single phase alternating current is passed through a rectifier, current is allowed to flow in only one direction. The reverse half of each cycle is blocked out so that a one directional, pulsating current is produced. The current rises from zero to a maximum and then returns to zero. No current flows during the time when the reverse cycle is blocked out. The HWAC repeats at same rate as the unrectified current (60 hertz typical). Since half of the current is blocked out, the amperage is half of the unaltered AC.

This type of current is often referred to as half wave DC or pulsating DC. The pulsation of the HWAC helps magnetic particle indications form by vibrating the particles and giving them added mobility. This added mobility is especially important when using dry particles. The pulsation is reported to significantly improve inspection sensitivity. HWAC is most often used to power electromagnetic yokes.

Full Wave Rectified Alternating Current (FWAC) (Single Phase)Full wave rectification inverts the negative current to positive current rather than blocking it out. This produces a pulsating DC with no interval between the pulses. Filtering is usually performed to soften the sharp polarity switching in the rectified current. While particle mobility is not as good as half-wave AC due to the reduction in pulsation, the depth of the subsurface magnetic field is improved.

Three Phase Full Wave Rectified Alternating CurrentThree phase current is often used to power industrial equipment because it has more favorable power transmission and line loading characteristics. This type of electrical current is also highly

desirable for magnetic particle testing because when it is rectified and filtered, the resulting current very closely resembles direct current. Stationary magnetic particle equipment wired with three phase AC will usually have the ability to magnetize with AC or DC (three phase full wave rectified), providing the inspector with the advantages of each current form.

 

Longitudinal Magnetic FieldsDistribution and Intensity

When the length of a component is several times larger than its diameter, a longitudinal magnetic field can be established in the component. The component is often placed longitudinally in the concentrated magnetic field that fills the center of a coil or solenoid. This magnetization technique is often referred to as a "coil shot."

The magnetic field travels through the component from end to end with some flux loss along its length as shown in the image to the right. Keep in mind that the magnetic lines of flux occur in three dimensions and are only shown in 2D in the image. The magnetic lines of flux are much more dense inside the ferromagnetic material than in air because ferromagnetic materials have much higher permeability than does air. When the concentrated flux within the material comes to the air at the end of the component, it must spread out since the air can not support as many lines of flux per unit volume. To keep from crossing as they spread out, some of the magnetic lines of flux are forced out the side of the component.

When a component is magnetized along its complete length, the flux loss is small along its length. Therefore, when a component is uniform in cross section and magnetic permeability, the flux density will be relatively uniform throughout the component. Flaws that run normal to the magnetic lines of flux will disturb the flux lines and often cause a leakage field at the surface of the component.

When a component with considerable length is magnetized using a solenoid, it is possible to magnetize only a portion of the component. Only the material within the solenoid and about the same width on each side of the solenoid will be strongly magnetized. At some distance from the solenoid, the magnetic lines of force will abandon their longitudinal direction, leave the part at a pole on one side of the solenoid and return to the part at a opposite pole on the other side of the solenoid. This occurs because the magnetizing force diminishes with increasing distance from the solenoid.  As a result, the magnetizing force may only be strong enough to align the magnetic domains within and very near the solenoid. The unmagnetized portion of the component will not support as much magnetic flux as the magnetized portion and some of the flux will be forced out of the part as illustrated in the image below. Therefore, a long component must be magnetized and inspected at several locations along its length for complete inspection coverage.

Circular Magnetic FieldsDistribution and Intensity

As discussed previously, when current is passed through a solid conductor, a magnetic field forms in and around the conductor. The following statements can be made about the distribution and intensity of the magnetic field.

The field strength varies from zero at the center of the component to a maximum at the surface.

The field strength at the surface of the conductor decreases as the radius of the conductor increases when the current strength is held constant. (However, a larger conductor is capable of carrying more current.)

The field strength outside the conductor is directly proportional to the current strength. Inside the conductor, the field strength is dependent on the current strength, magnetic permeability of the material, and if magnetic, the location on the B-H curve.

The field strength outside the conductor decreases with distance from the conductor.

In the images below, the magnetic field strength is graphed versus distance from the center of the conductor. It can be seen that in a nonmagnetic conductor carrying DC, the internal field strength rises from zero at the center to a maximum value at the surface of the conductor. The external field strength decrease with distance from the surface of the conductor. When the conductor is a magnetic material, the field strength within the conductor is much greater than it is in the nonmagnetic conductor. This is due to the permeability of the magnetic material. The external field is exactly the same for the two materials provided the current level and conductor radius are the same.

The magnetic field distribution in and around a solid conductor of a nonmagnetic material carrying direct current.

The magnetic field distribution in and around a solid conductor of a magnetic material carrying direct current.

 

When the conductor is carrying alternating current, the internal magnetic field strength rises from zero at the center to a maximum at the surface. However, the field is concentrated in a thin layer near the surface of the conductor. This is known as the "skin effect." The skin effect is evident in the field strength versus distance graph for a magnetic conductor shown to the right. The external field decreases with increasing distance from the surface as it does with DC. It should be remembered that with AC the field is constantly varying in strength and direction.

 

The magnetic field distribution in and around a solid conductor of a magnetic material carrying alternating current.

 

 

In a hollow circular conductor there is no magnetic field in the void area. The magnetic field is zero at the inside wall surface and rises until it reaches a maximum at the outside wall surface. As with a solid conductor, when the conductor is a magnetic material, the field strength within the conductor is much greater than it was in the nonmagnetic conductor due to the permeability of the magnetic material. The external field strength decreases with distance from the surface of the conductor. The external field is exactly the same for the two materials provided the current level and conductor radius are the same.

The magnetic field distribution in and around a hollow conductor of a nonmagnetic material carrying direct current.

The magnetic field distribution in and around a hollow conductor of a magnetic material carrying direct current.

When AC is passed through a hollow circular conductor, the skin effect concentrates the magnetic field at the outside diameter of the component.

As can be learned from these three field distribution images, the field strength at the inside surface of hollow conductor is very low when a circular magnetic field was established by direct magnetization. Therefore, the direct method of magnetization is not recommended when inspecting the inside diameter wall of a hollow component for shallow defects. The field strength increases rapidly as one moves out (into the material) from the ID, so if the defect has significant depth, it may be detectable.

The magnetic field distribution in and around a hollow conductor of a magnetic material carrying alternating current.

However, a much better method of magnetizing hollow components for inspection of the ID and OD surfaces is with the use of a central conductor. As can be seen in the field distribution image to the right, when current is passed through a nonmagnetic central conductor (copper bar), the magnetic field produced on the inside diameter surface of a magnetic tube is much greater and the field is still strong enough for defect detection on the OD surface.

 

Demagnetization

After conducting a magnetic particle inspection, it is usually necessary to demagnetize the component. Remanent magnetic fields can:

affect machining by causing cuttings to cling to a component. interfere with electronic equipment such as a compass.

create a condition known as "arc blow" in the welding process. Arc blow may cause the weld arc to wonder or filler metal to be repelled from the weld.

cause abrasive particles to cling to bearing or faying surfaces and increase wear.

Removal of a field may be accomplished in several ways. This random orientation of the magnetic domains can be achieved most effectively by heating the material above its curie temperature. The curie temperature for a low carbon steel is 770oC or 1390oF. When steel is heated above its curie temperature, it will become austenitic and loses its magnetic properties. When it is cooled back down, it will go through a reverse transformation and will contain no residual magnetic field. The material should also be placed with it long axis in an east-west orientation to avoid any influence of the Earth's magnetic field.

It is often inconvenient to heat a material above its curie temperature to demagnetize it, so another method that returns the material to a nearly unmagnetized state is commonly used. Subjecting the component to a reversing and decreasing magnetic field will return the dipoles to a nearly random orientation throughout the material. This

The magnetic field distribution in and around a nonmagnetic central conductor carrying DC inside a hollow conductor of a magnetic material .

can be accomplished by pulling a component out and away from a coil with AC passing through it. The same can also be accomplished using an electromagnetic yoke with AC selected. Also, many stationary magnetic particle inspection units come with a demagnetization feature that slowly reduces the AC in a coil in which the component is placed.

A field meter is often used to verify that the residual flux has been removed from a component. Industry standards usually require that the magnetic flux be reduced to less than 3 gauss after completing a magnetic particle inspection.

 

Solenoid - An electrically energized coil of insulated wire, which produces a magnetic field within the coil.

Measuring Magnetic Fields

When performing a magnetic particle inspection, it is very important to be able to determine the direction and intensity of the magnetic field. As discussed previously, the direction of the magnetic field should be between 45 and 90 degrees to the longest dimension of the flaw for best detectability. The field intensity must be high enough to cause an indication to form, but not too high to cause nonrelevant indications to mask relevant indications. To cause an indication to form, the field strength in the object must produce a flux leakage field that is strong enough to hold the magnetic particles in place over a discontinuity. Flux measurement devices can provide important information about the field strength.

Since it is impractical to measure the actual field strength within the material, all the devices measure the magnetic field that is outside of the material. There are a number of different devices that can be used to detect and measure an external magnetic field. The two devices commonly used in magnetic particle inspection are the field indicator and the Hall-effect meter, which is also called a gauss meter. Pie gauges and shims are devices that are often used to

provide an indication of the field direction and strength but do not actually yield a quantitative measure. They will be discussed in a later section.

Field Indicators Field indicators are small mechanical devices that utilize a soft iron vane that is deflected by a magnetic field. The X-ray image below shows the inside working of a field meter looking in from the side. The vane is attached to a needle that rotates and moves the pointer for the scale. Field indicators can be adjusted and calibrated so that quantitative information can be obtained. However, the measurement range of field indicators is usually small due to the mechanics of the device. The one shown to the right has a range from plus 20 gauss to minus 20 gauss. This limited range makes them best suited for measuring the residual magnetic field after demagnetization.

A field indicator is shown checking for residual magnetism in this movie. (194 MB mov)

 

Hall-Effect (Gauss/Tesla) MeterA Hall-effect meter is an electronic device that provides a digital readout of the magnetic field strength in gauss or tesla units. The meters use a very small conductor or semiconductor element at the tip of the probe. Electric current is passed through the conductor. In a magnetic field, a force is exerted on the moving electrons which tends to push them to one side of the conductor. A buildup of charge at the sides of the conductors will balance this magnetic influence, producing a measurable voltage between the two sides of the conductor. The presence of this measurable transverse voltage is called the Hall-effect after Edwin H. Hall, who discovered it in 1879.

The voltage generated Vh can be related to the external magnetic field by the following equation.

Vh = I B Rh / b

Where:Vh is the voltage generated.I is the applied direct current.B is the component of the magnetic field that is at a right angle to the direct current in the Hall element.Rh is the Hall Coefficient of the Hall element.b is the thickness of the Hall element.

Probes are available with either tangential (transverse) or axial sensing elements. Probes can be purchased in a wide variety of sizes and configurations and with different measurement ranges. The probe is placed in the magnetic field such that the magnetic lines of force intersect the major dimensions of the sensing element at a right angle. Placement and orientation of the probe is very important and will be discussed in a later section.