Microfracture model for hydrogen embrittlement of austenitic steels
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Microfracture Model for Hydrogen Embrittlement of Austenitic Steels R.E. STOLTZ, N. R. MOODY, and M. W. PERRA Microfracture models which relate microscopic fracture features to measured thresholds for crack advance have been recently proposed by Gerberich and co-workers. 1-4 These models have treated both static and fatigue thresholds for titanium alloys and martensitic steels. The models are predicated on the observation of a partially cracked (semicohesive) zone which, at a quasi-static cracking threshold, separates fully failed and unfailed regions of a cracked body. The ligaments in the partially cracked region provide tractions across the crack plane which reduce the local stress intensity and plastic zone in the unfailed material. This model has been used with success in titanium to account for the area (volume) fraction of ligaments in the zone as a function of temperature 2 and for the variation in crack advance threshold with temperature and hydrogen content. 3 This communication describes the results of fracture surface analysis of selected austenitic steels which exhibit cracking in high pressure hydrogen gas. In the alloys studied, evidence of a partially cracked region was observed in samples which exhibited a crack arrest threshold in hydrogen. By modifying the Gerberich approach, the length of this partially cracked region was related to the alloy yield strength and the arrest threshold stress intensity. Experimental details are described fully elsewhere. 5"6"7 Briefly, bolt loaded wedge-open-load samples were exposed to 200 MPa hydrogen at 298 K. Crack advance was monitored by measuring load decreases in situ. Once a crack arrest threshold had been established the samples were fractured in air and the threshold region of the fracture surface was examined. R. E. STOLTZ, Group Head, is with Surface Metallurgy Group, Exxon Corporate Research Laboratory, Linden, NJ 07036. N.R. MOODY, Member Technical Staff, and M.W. PERRA, Supervisor, Materials Development Division, are both with Sandia National Laboratories, Livermore, CA 94550. Manuscript submitted December 6, 1982. 1528--VOLUME 14A, JULY 1983
Table I, taken from Reference 7, gives results of crack arrest threshold vs yield strength for alloys with nominally clean grain boundaries (see References 5 and 6 for detailed discussion of grain boundary impurity effects in these materials). As seen from the table, alloys with yield strengths above 779 MPa exhibited hydrogen-induced cracking while those with strengths below 717 MPa did not. The two materials indicated by the asterisk (*) were studied in detail. Table II gives the composition and heat treatment of these alloys while Figure 1 shows fracture surface features in the threshold region. Hydrogen-assisted crack growth in these materials is intergranular while overload fracture following crack arrest is nominally by ductile void coalescence. The lines on the micrograph separate the partially cracked region from the fully cracked and uncracked regions. The measured widths, Ira,a,, of the partially crac
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