Effects of water vapor on hydrogen induced slow crack growth in stainless steels
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INTRODUCTION
R E C E N T papers 1'2 and earlier work 3'4 on the kinetics of gaseous hydrogen-induced slow crack growth (SCG) in stainless steels have demonstrated that when ferrite is present, pure hydrogen gas at atmospheric pressure and below is sufficient to cause SCG in stainless steels. Three types of stainless steels were investigated: austenitic, ferritic, and duplex (a/7). In an effort to examine whether hydrogen transport is the rate controlling process for SCG, hydrogen permeation and effective diffusion coefficients were measured for some of the same alloys in pure H 2 gas atmospheres as a function of temperature and plastic deformation. 5 It was found that in an unstable austenitic alloy (AISI 301), deformation-induced a' martensite increased these hydrogen transport parameters by orders of magnitude, whereas they were decreased slightly by deformation of a stable austenitic alloy (AISI 310). This was held to be responsible for the fact that SCG and ductility loss were observed in unstable austenite but not in stable austenite tested in hydrogen gas at 100 kPa (1 atm) pressure. 1-4 In subsequent permeation experiments it was found that when H20 or H2S was mixed with hydrogen, the effective permeation and diffusion constants were drastically decreased due to a change in surface conditions. 6 For example, H20 reduced both quantities by a factor of two to three in AISI 302 at T = 200 to 320 ~ whereas for AL 29-4-2, a ferritic stainless steel alloy, there was a decrease of two to three orders of magnitude at T -> 280 ~ and an even larger decrease at T < 280 ~ 6 Thus, the relative reduction in hydrogen permeation was dependent on the crystal structure. It was proposed that the reduction was caused by interference with the adsorption process at the metal surface or by compound function or a combination of these two effects. 6 Since hydrogen transport and microstructure have been shown to be important in controlling the kinetics of SCG in stainless steels, the effects of admixture of H20 in H 2 gas on the SCG behavior should be understandable in terms of the known effects of moisture on permeation T-P. PERNG, formerly Postdoctoral Research Associate, Department of Metallurgy, University of Illinois in Urbana-Champaign, is Associate Professor, Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan. C. J. ALTSTETTER is Professor of Physical Metallurgy, Department of Materials Science and Engineering and the Materials Research Laboratory, University of Illinois at UrbanaChampaign, 1304 West Green Street, Urbana, IL 61801. Manuscript submitted May 27, 1987.
METALLURGICALTRANSACTIONS A
parameters. Specifically, since H20 reduces hydrogen permeation, addition of H20 to H2 gas during mechanical testing should result in a lower hydrogen flux in the metal and thus a lower crack propagation rate. Furthermore, since H20 affects only the surface process, not the bulk properties, the cracking behavior or mechanism should be the same as that in pure H2, but a lower cracking ra
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