Hydrogen assisted cracking of type 304 stainless steel
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This paper reports a study of hydrogen assisted cracking in type 304 stainless steel. It shows that the most detrimental effect in increasing the susceptibility of the material to hydrogen cracking is the formation of martensite upon deformation. This is particularly damaging if the martensite is localized at the g r a i n boundaries. With martensite present intergranular impurities such as phosphorus play a secondary r o l e . As martensite bec o m e s more difficult t o f o r m , the importance of impurities increases.
T Y P E 304 stainless s t e e l is one of the most commonly used austenitic steels. Its uses would be even wider were it not for the fact that it b e c o m e s quite susceptible t o intergranular corrosion a f t e r heat treatment in the temperature r a n g e of 600 to 800°C, for t i m e s even as short as 15 min. This phenomenon, often r e f e r r e d to as sensitization, is usually a t t r i buted t o the formation of chromium carbides at the g r a i n boundaries and the resulting depletion of c h r o mium around the precipitates.1 Since hydrogen is a byproduct of most corrosion reactions, it is important to know if sensitization to corrosion also makes the m a t e r i a l more susceptible t o hydrogen cracking. More generally, it is important to know if 304 stainless s t e e l in either the sensitized or unsensitized condition is susceptible t o hydrogen cracking, as this could limit its use in aqueous or other hydrogen b e a r i n g environments. Many investigators have studied hydrogen assisted cracking in austenitic stainless steels. 2-~1 E a r l y work suggested that the formation of deformation induced martensite was essential for the embrittlement t o be observed. 2'8 This view a r o s e because steels which partially transform to martensite upon straining, such as 304L, were found to be susceptible to hydrogen cracking whereas those with stable austenite phases, such as 310, were difficult to embrittle. Also the fracture mode in 304L often appeared to be quasicleavage a l o n g martensite laths. However, hydrogen assisted cracking has been reported in 310 stainless steel, s'6 and Thompson could not correlate hydrogen embrittlement w i t h martensite formation in 309 stainl e s s .14 Recently, it has been suggested that low stacking fault energy and coplanar dislocation motion greatly enhance hydrogen cracking in stainless steels. 10'17'2°'21 Odegard, Brooks, and West1° found that the susceptibility to hydrogen c r a c k i n g in steels which did not transform t o martensite upon deformation correlated very well with t h e s e parameters, and that the fracture mode in hydrogen changed from ductile rupture t o intergranular fracture as the embrittlement b e c a m e more s e v e r e . They also found that martensite formation increased the susceptibility t o hydrogen fracture over that expected from stacking fault energy measurements. All of t h e s e results indicate that if martensite is present it increases the susceptiC. L. BRIANT is Staff Metallurgist, General Electric Researchand Deve
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