Cracking kinetics of two-phase stainless steel alloys in hydrogen gas
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INTRODUCTION
THEseverity of hydrogen embrittlement of stainless steels depends on their crystal structure. Austenitic stainless steels, like many ferritic steels, can suffer a loss in tensile strength and ductility in the presence of either external or intemal hydrogen. 1.2 It has been known for some time that stress-induced bcc martensite (designated a ') is a sufficient, but not necessary, condition for reduced ductility in hydrogen.3 Alloys which do not form martensite are also affected by hydrogen, but with much less severity. The reasons for this are not clear. In many practical applications the main concern is delayed failure rather than rapid failure modes characterized by fracture toughness and tensile properties. This form of hydrogen embrittlement occurs by stable propagation of a crack under constant load at a stress intensity below Kzc. Recent studies have shown that slow crack growth (SCG) takes place in unstable austenitic stainless steels when they are subjected to static tensile loads in hydrogen gas or after uniform precharging with hydrogen. Furthermore, a ' martensite was observed on the fracture surfaces. 3-7 In contrast, no SCG and no a ' were observed for the stable austenitic stainless steel, AISI 310. Moreover, it was found that the SCG velocity in an unstable alloy, AISI 301, was even higher than in a highly alloyed ferritic stainless steel, AL 29-4-2. *8 *AL 29-4-2 is a product of the Allegheny-Ludlum Steel Corporation.
There are various hypotheses to explain the different hydrogen embrittlement tendencies of different alloys. One suggestion is that it is not the martensite per se which promotes embrittlement, but the planar slip mode due to a low stacking fault energy which makes the unstable austenitic alloys more susceptible. 9'1~ In such alloys dislocations cross slip with difficulty, and dislocation pile-ups cause higher stress concentrations. If hydrogen moves with the dislocations, high hydrogen concentrations might build T-P. PERNG, formerly Postdoctoral Associate, University of Illinois, is Associate Professor, Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan. C.J. ALTSTETTER is Professor, Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, 1304 West Green Street, Urbana, IL 61801. Manuscript submitted September 30, 1986.
METALLURGICALTRANSACTIONS A
up at dislocation pile-ups. Furthermore, if hydrogen were to decrease the stacking fault energy, the alloy would be more likely to deform in a planar slip mode in the presence of hydrogen. Another view is that the greater susceptibility to hydrogen embrittlement of unstable austenites is due to the formation of stress-induced martensite in the stress field of the crack tip. 3 This phase could be intrinsically more susceptible to embrittlement. Regardless, since hydrogen diffuses several orders of magnitude faster in martensite than in austenite, the martensite acts as a medium for fast transport of hydrogen into the crack tip region. The unt
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