CE V S PHG 250 0910 Cop yright 2010 b y Wur th 12 0001 Wur t h Canadas Quality S t at ement Wur th Canada s t ands behind the quality of its fas tener pr oducts and aligns itself with suppliers who maintain s t rict ISO90012000 quality assur ance pr ogr ams F ull lo t tr aceability allo w s us t o tr ack back t o the r a w material heat tr eat and plating pr ocesses used in pr oduction and ensur es that all fas teners supplied b y Wur th comply with es tablished IFI DIN ANSI ASME and ASTM fas tener s t andar ds Wh y some S t ainless S t eels are magnetic Stainless s t eel fas teners being nonmagnetic is one of the lar ges t misconceptions amongs t fas tener users This document will e xplain wh y mos t s t ainless s t eel fas teners ar e at leas t slightly magnetic and wh y man y ar e so magnetic that the y ar e attr acted t o e v en w eak household magnets Ther e ar e v e classes of s t ainless s t eel ferritic aus tenitic mar tensitic duple x and pr ecipitate har dened and only one is deemed nonmagnetic aus tenitic The aus tenitic class jus t happens t o include the mos t common and univ ersally used types of s t ainless s t eels on the mark et The k e y t o the magnetic pr oper ties of an aus tenitic fas tener is found in its micr os tructur e and the char acteris tics of that micr os tructur e can be alter ed in the manufacturing pr ocess t o cr eate a degr ee of magnetism Ho w Does an Aus t enitic S t ainless S t eel Become Magnetic As s t ated befor e the micr os tructur e of the metal is what giv es the s t eel its magnetic pr oper ties If the s t ainless chosen w as aus tenitic for e x ample type 316 and a por tion of the micr os tructur e w e r e changed t o an y one of the o t her four classes then the material w ould hav e some magnetic permeability magnetism built int o the s t eel The micr os tructur e of aus tenitic s t ainless s t eel can be changed b y a pr ocess called mar tensitic s t r ess induced tr ansformation MSIT This is a micr os tructur al change fr om aus tenite t o mar tensite and the tr ansformation can occur due t o cold w o rking the pr ocess b y which man y fas teners ar e made as w ell as slo w cooling fr om aus tenitizing temper atur es Af ter cold w o rking or slo w cooling an aus tenitic s t eel will hav e an appr eciable le v el of mar tensitic micr os tructur e Due t o mar tensite being magnetic the once non magnetic aus tenitic s t ainless s t eel will no w hav e a degr ee of magnetism Cold W o rking Mos t fas teners pr oduced carbon s t ainless s t eels can go thr ough quite a bit of cold w orking prior t o seeing ser vice in the eld Cold w o rking fas teners occurs in the wir e dr awing forming and thr ead r olling pr ocesses Each of these pr ocesses will typically cr eate enough mar tesite t o pr oduce a measur able degr ee of magnetism Suscep tible Allo y s Lo w allo y content s t ainless s t eel par ticularly that of lo w nick el carbon andor nitr ogen ar e mor e susceptible t o MSIT than s t ainless s t eel with higher allo y elements T ype 304 is an e x ample of a s t ainless s t eel that is quite susceptible t o forming mar tensite af ter cold w orking but in fact MSIT a ects all aus tenitic s t ainless s t eel t o some degr ee The char t belo w will help t o sho w case permeability magnetism r elativ e t o the per cent of cold w o rking Controlling Magnetism in S t ainless S t eel Magnetism is sometimes a fact or in deciding which fas tener t o use If a 100 nonmagnetic s t ainless s t eel fas tener is r e quir ed s t eps can be tak en t o r educe the amount of cold w o rking done t o the par t thr ead cutting v s thr ead r olling If thr ead cutting is no t an option annealing the par t af ter cold w orking may also ser v e t o eliminate an y permeability Without a complete annealing tr eatment a thr ead r olled par t will ne v er hav e zer o magnetism Ultimately a balance of allo w able magnetism and economic r e quir ements mus t be r eached in accor dance with the end user