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, att din punct de vedere al curatirii de impuritati, ct 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 cnd 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 cnd 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 m.

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 50m.

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 cmpului magnetic, sensibilitatea sa va depinde de orientarea acestora fata de orientarea discontinuitatilor. Sensibilitatea maxima se obtine atunci cnd 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 cmpului 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 inct 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, avnd 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 inct 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 i altereaza proprietatile.

6.3.4.Tehnica de examinare uscata pulberi fluorescente

a) Se vor respecta afirmatiile de la pct. 6.3.3.c pna 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 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 mW/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 cte ori se schimba locul de lucru sau in cazul cnd se considera necesar.

6.3.5.Tehnica de examinare umeda

a) Si aceasta tehnica, ca si tehnica uscata, foloseste att pulberi colorate ct 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, respectnd urmatoarele etape:

se agita cteva minute intreaga masa a suspensiei; se toarna intr-un tub centrifugal gradat, in forma de pana, 100ml de suspensie; se centrifugheaza tubul mentinnd nivelul amestecului la diviziunea 100ml; se aseaza tubul pe un stativ bine fixat, fara vibratii, mentinndu-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 cmp 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 pna 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 inch (20mm) sau mai mari. Pentru sectiuni ale grosimii mai mici de 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 dect sectiunea sau

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

b) Bobina fixa sau realizata cu ajutorul unui cablu infasurat in jurul piesei produce un cmp 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 cmp 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 pna 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 cmp 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 cmpuri magnetice circulare, ct 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 cmp magnetic se va verifica daca se obtin cmpuri pe cel putin doua directii perpendiculare. In caz ca sunt zone unde nu se obtin intensitati adecvate ale cmpului 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 cmpului magnetic se va verifica cu indicatorul de cmp 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 cmpului 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 cte 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 inct 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 vrfuri din otel, plumb sau aluminiu. Pentru tensiuni cuprinse intre 5V si 20V, se pot utiliza si vrfuri 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 ) care sa asigure pe suprafata de examinat o intensitate de 800 W/cm2.

h) Laboratorul de examinari nedistructive trebuie sa fie dotat cu o trusa cu anexe, cum ar fi indicatorul de cmp magnetic (comform ASME, sectiunea V), etaloane cu fisuri si cu gauri, pulverizator, instrument de masura a cmpului 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, pna la valoarea zero. Sunt necesari aproximativ 25 de pasi de demagnetizare.

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

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

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 inct 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 acceptare1 2 3

1 Indicatii liniare

L=lungimea indicatiilor

L

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 Grad1. Discontinuitati liniare (fisuri sau crapaturi termice)

orice indicatie

2. Retasuri 23. Incluziuni 34. 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 pna 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 adncimea 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 suprafa i sub suprafa n materiale feromagnetice. n materialul magnetizat n prealabil. Aeeaz fluxuri de dispersie, detectabile cu ajutorul particulelor feromagnetice fine aplicate pe suprafa. De-a lungul defectului aceste particule se dispun n aa fel nct si marcheze o urm vizibil, care indic locul, aria, forma i adncimea acestuia.. Amprenta defectului este influenat de direcia i intensitatea einpului magnetic, de metoda de magnetizare ntrebuinat, de aria, forma i direcia discontinuitii, de caracteristicile pulberii magnetice i metoda de aplicare, de caracteristicile magnetice ale corpului ncercat, de starea suprafeei i forma obiectului. 3. Avantajele metodei Avantajele metodei snt numeroase i printre ele putem cita :

rapiditatea i simplitatea modului de operare; excelenta sensibilitate pentru defecte de suprafa; formarea operatorilor nu e dificil; se pot ncerca simultan suprafee mari din corp; se pot descoperi goluri umplute cu alte

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 liber

Salt la: Navigare, cutare

Control NDT n procesul de producie

Controlul nedistructiv (englez nondestructive testing, prescurtat NDT) reprezint modalitatea de control al rezistenei unei structuri, piese etc fr a fi necesar demontarea, ori distrugerea acestora.

Este un ansamblu de metode ce permite caracterizarea strii de integritate a pieselor, structurilor industriale, fr a le degrada, fie n decursul produciei, fie pe parcursul utilizrii prin efectuarea de teste nedistructive n 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 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 i turbionari o 3.4 Ultrasunete o 3.5 Lichide penetrante o 3.6 Controlul visual

4 Alte metode 5 Simbolizare 6 Standarde i norme 7 Vezi i

8 Legturi externe

[modific] Domenii de aplicare

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

industria automobilelor (diferite piese) industria naval (controlul corpului navei i a structurilor sudate) conducte ngropate sau submerse sub ap supuse coroziunii p latforme marine aeronautic (aripile avioanelor, diferite piese de motor, etc) industria energetic (reactoare, turbine, cazane de nclzire, tubulatur, etc) industria aerospaial i militar arheologie structuri feroviare industria petrochimic construcii de maini (piese turnate sau forjate, ansamble i subansamble)

Se poate afirma c metodele NDT se aplic n toate sectoarele de producie.

[modific] Scurt istoric

n timpurile trecute, clopotarii i furarii ascultau sunetele pe care le produceau obiectele create, astfel c fiecrui material i corespundea un sunet.

1854 - n Hartford, Connecticut explozia unui boiler la firma Fales and Gay Gray Car, se soldeaz cu moartea a 21 de lucrtori i rnind ali 50. De atunci, s-a impus o verificare anual a boilerelor

1895 - Wilhelm Conrad Rntgen a descoperit prezena razelor X. n prima sa lucrare arat despre posibilitatea detectrii unui defect de structur

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

1926 - este realizat primul aparat electromagnetic cu cureni turbionari 1927 - 1928 - Elmer Sperry i H.C. Drake concep un sistem cu inducie

magnetic pentru detectarea defectelor din inele de cale ferat 1929 - A.V. DeForest i F.B. Doaneeste realizeaz primul aparat i metoda de

testare cu particule magnetice 1930 - Robert F. Mehl demonstreaz realizarea de imagini radiografice

folosind radiaiile gamma din izotopi de radiu, ceeace permite examinarea de elemente cu grosimi mai mari

1940 - 1944 - Dr. Floyd Firestone dezvolt n S.U.A. metoda de testare cu ultrasunet

1950 - J. Kaiser a introdus emisia acustic n metoda NDT

[modific] Metode de control nedistructiv

Alegerea metodei de control nedistructiv utilizat se face n funcie de diferite criterii legate de utilitatea piesei de controlat, materialul din care este fabricat piesa, amplasament, tipul de structur, costuri etc. Cele mai utilizate metode de control nedistructiv sunt:

[modific] Radiaii penetrante

Metoda de examinare cu radiaii penetrante sau radiografic const din interaciunea radiaiilor penetrante cu pelicule fotosensibile. Se poate efectua cu raze X sau raze gamma.

[modific] Examinare cu raze X

Generator de raze X

Examinarea cu raze X const n bombardarea piesei supuse controlului cu radiaii X, obinndu-se pe filmul radiografic imaginea structurii macroscopice interne a piesei.

Generatoarele de raze X, n funcie de energia ce o furnizeaz i de domeniul lor de utilizare pot fi:

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

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

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

[modific] Examinare cu raze gamma (gammagrafie)

Gammagrafia const n iradierea piesei supuse controlului cu radiaii gamma, dup care se obine pe filmul radiografic imaginea structurii macroscopice interne a piesei respective, prin acionarea asupra emulsiei fotogafice.

Creterea permanent a parametrilor funcionali ai instalaiilor industriale moderne (presiune, temperatur, solicitri mecanice, rezisten la coroziune), au impus examinarea cu raze gamma ca o metod modern de control cu grad ridicat de certitudine.Elementul de baz al gammagrafiei este sursa de radiaii gamma care datorit proprietilor sale (energie ridicat, mas de repaus nul, sarcin electric nul), o fac deosebit de penetrant.

Principala surs de radiaii folosit n gammagrafie o constituie izotopii radioactivi de Cobalt-60, Iridiu-192, Cesiu-137, Cesiu-134, Tuliu-170 i Seleniu-75, obinui prin activare deoarece au un pre de cost mai sczut i avantajul obinerii unor activiti mari.

Aceti izotopi sunt utilizai astfel: Cobalt-60 pentru oeluri cu grosime mare (>80 mm), Iridiu-192 pentru oeluri cu grosime mijlocie (10-80 mm), iar Tuliu-170 pentru oeluri cu grosime mic (

caracterizeaz defectul. Principalul avantaj al acestei metode este obinerea de rezultate immediate.

metoda magnetografic utilizeaz o band feromagnetic flexibil care se aeaz peste sudura ce trebuie examinat. Prin aplicarea unui scurt puls magnetic de aproximativ 15 ms, prin intermediul unui acumulator ce magnetizeaz un jug, cmpurile de distorsiuni sunt puse n eviden prin imprimarea lor pe band. Banda este examinat cu ajutorul unui traductor magneto-electric, dup forma indicaiilor putndu-se aprecia natura defectelor din mbinarea sudat. Echipamentul const din jugurile pentru diferite geometrii ale mbinrilor sudate, sursa de curent, banda feromagnetic, magneii de fixare ai benzii i traductorii magnetoelectrici.

[modific] Cureni turbionari

Metoda curenilor turbionari este folosit ca o alternativ sau extensie a controlului nedistructiv cu particule magnetice, fiind utilizat, n special, pentru controlul evilor cu diametrul exterior de maximum 140 mm. Sensibilitatea metodei este maxim la grosimi de perete de pn la 5 mm. O dat cu creterea grosimii pereilor, scade eficiena metodei de evideniere a defectelor interne, ea rmnnd eficace pentru evidenierea defectelor de suprafa i din imediata apropiere a acesteia.Metoda const n inducerea unor cureni turbionari n pereii evii controlate. Cmpul magnetic al curenilor turbionari indui, datorit prezenei unor discontinuiti i neomogeniti n material, modific impedana bobinei de msurare, ceea ce afecteaz amplitudinea i faza curenilor turbionari. Amplitudinea, defazajul i adncimea de ptrundere a curenilor turbionari, depind de amplitudinea i frecvena curentului de excitaie, de conductibilitatea electric, de permeabilitatea magnetic a materialului, de forma piesei controlate, de poziia relativ a bobinelor fa de pies, precum i de omogenitatea materialului controlat.Metoda mai este denumit i a curenilor Foucault dup numele fizicianului francez Lon Foucault care a descoperit fenomenul n anul 1851.

[modific] Ultrasunete

Aparat cu ultrasunet

Metoda este bazat pe undele mecanice (ultrasunetele) generate de un element piezo-magnetic excitat la o frecven cuprins de regul ntre 2 i 5 Mhz. Controlul presupune transmiterea,

reflexia, absorbia unei unde ultrasonore ce se propag n piesa de controlat. Fasciculul de unde emis se reflect n interiorul piesei i pe defecte, dup care revine ctre defectoscop ce poate fi n acelai timp emitor i receptor. Poziionarea defectului se face prin interpretarea semnalelor.

Metoda prezint avantajul de a gsi defectele n profunzime datorit unei rezoluii ridicate, ns este lent datorit necesitii de scanare multipl a piesei. Uneori este necesar executarea controlului pe mai multe suprafee ale piesei. Metoda de control prin ultrasunete este foarte sensibil la detectarea defectelor netede.

[modific] Lichide penetrante

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

Const n aplicarea unui lichid capilar activ penetrant pe suprafaa de examinat, ndeprtarea penetrantului rmas n afara discontinuitilor i aplicarea unui material absorbant, ce absoarbe penetrantul aflat n discontinuiti punnd astfel n eviden, prin contrast, defectele existente; aceast metod se aplic pentru depistarea defectelor de suprafa. Se pot pune de asemenea n eviden fisurile de oboseal i de coroziune. Pentru control trebuie curtat i pregatit suprafaa de examinare.

Metoda este aplicat cu success mbinrilor sudate, dar se poate face i nainte de sudur (pentru efectuarea unui control al tuturor suprafeelor nainte de a fi sudate).

[modific] Controlul visual

Orice tip de investigare trebuie s fie precedat de o examinare vizual a supafeei. Procedeul este simplu dar indispensabil, examinarea vizual presupune respectarea condiiilor de claritate satisfctoare a suprafeelor materialelor, echipamentelor i sudurilor lund n considerare caracteristicile i proprietile acestora.Pentru control vizual se folosesc diferite ustensile optice cum ar fi endoscop, lupe, lmpi etc. Prin control visual sunt furnizate o serie de indicii legate de aspectul suprafeei metalului precum i estimarea unor defecte interne (recipiente metalice, butelii de gaze, conducte, tuburi etc)

Odat cu controlul visual se pot determina i dimensiuile defectelor de mbinare, grosimile recipientului sudat, dimensiunile cordonului sudat etc.

[modific] Alte metode

Metoda radioscopic sau fluoroscopic, se bazeaz pe interaciunea radiaiilor penetrante cu substane fluorescente.

Metoda radiografic n timp real, combin tehnica fluoroscopic cu posibilitile de microfocalizare a radiaiei X.

Metoda sondelor de potenial, funcioneaz pe principiul variaiei reluctanei magnetice.

Metoda ferosondelor, discrimineaz variaiile de inductan din pies. Metodele imagineriei procesate.

[modific] Simbolizare

Metodelor uzuale de control nedistructiv le corespunde o serie de simboluri reglementate de norma european EN 473 i EN 4179 examinare nedistructiv END.

Metoda END Simbol

Emisie acustic AT

Cureni Foucault ET

Etaneitate LT

Magnetoscopie MT

Lichide penetrante PT

Radiografie RT

Ultrasunet UT

Examen vizual VT

Speckle ST

Termografie IRT

[modific] Standarde i norme

[modific] Vezi i

Sudare

Sudare subacvatic

[modific] Legturi externe

Wikimedia Commons conine 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

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

6. The part is demagnetized7. 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] References

1. ^ 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 1920s, 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 1930s, 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.