Comparison of hydrogen gas embrittlement of austenitic and ferritic stainless steels
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I.
INTRODUCTION
AUSTENITIC stainless steels are generally less susceptible to hydrogen embrittlement than ferritic steels, and stable austenitic stainless steels such as AISI 310 are more resistant to hydrogen embrittlement than unstable ones such as AISI 301 and 304. No detailed observations have been reported on ferritic stainless steels in hydrogen gas, but when type 301 and 304 alloys were tested in the presence of hydrogen gas at atmospheric pressure, slow crack growth (SCG) was observed and c~' martensite (bcc) was found to be localized at the crack tip and fracture surface. ~-~No SCG was observed for AISI 310. A number of theories have been proposed to explain hydrogen-induced SCG behavior in steels and other structural materials. It is frequently postulated that in order for a crack to initiate and propagate, hydrogen has to be transported to and accumulated in an "embrittlement region" at or near the crack tip. By some unknown mechanism hydrogen then reduces the stress required to nucleate and propagate the crack. The rate of crack advance is controlled by the rate of supply and accumulation of hydrogen in that region. 6-13 If hydrogen transport is important in controlling the kinetics of crack growth, the stress-induced martensite in unstable austenitic stainless steels at the crack tip may therefore act as a suitable path for hydrogen to enter the matrix and cause severe embrittlement. In AISI 301 a 67 pct deformation, which resulted in 86 pct a ' , caused the effective diffusivity and permeability of hydrogen to be increased by two orders of magnitude for T = 120 to 320 ~ 14 The enhancement would be even larger at room temperature. For severely deformed A1SI 310, however, the effective hydrogen diffusivity was slightly reduced, while the permeability was essentially unchanged. No martensite was formed in AISI 310 at up to 80 pct deformation. ~4The enhanced hydrogen transport rate in AISI 301 was ascribed to the presence of martensite. 14In
T. P. PERNG, formerly Research Associate of the University of Illinois, is Associate Professor of Materials Science, National Tsing Hua University, Hsinchu, Taiwan. C.J. ALTSTETTER is Professor of Physical Metallurgy, University of Illinois at Urbana-Champaign, Urbana, IL 61801. Manuscript submitted October 3, 1985. METALLURGICALTRANSACTIONS A
the present research, hydrogen-induced cracking in austenitic stainless steels AISI 301 and 310 and a highly alloyed ferritic stainless steel AL 29-4-2 were studied. It was expected that a comparison of SCG in AL 29-4-2 with that in the austenitic alloys would be helpful in verifying the SCG rate-controlling process and increase our understanding of the effect of stress-induced martensite on the hydrogen embrittlement or hydrogen-induced cracking in the commercially important unstable austenitic stainless steels.
II.
EXPERIMENTAL METHODS
AISI 301 and 310 austenitic and AL 29-4-2 ferritic stainless steel alloys were the same ones on which hydrogen transport parameters had already been measured. 14 The chemical compositions
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