Analysis of hydrogen-stress field interactions have led to kinetic criteria for slow crack growth. Using both elastic and plastic stress fields under opening-mode loading, criteria for stage I, II, III growth are developed in terms of the pressure tensor gradient at the crack tip. It is proposed that stage I (stress-intensity dependent) growth kinetics are predominantly controlled by the elastic stress field while stage II (nearly stress-intensity independent) kinetics are controlled by the plastic stress field. Measurements of slow crack growth in cathodically-charged AISI 4340 steel verify the overall aspects of the correlation. Detailed measurement and analysis of the increase in crack-tip radius with increasing applied stress intensity have led to a proposed decrease in crack growth rate during stage II growth. Some experimental evidence corroborates this later hypothesis and is consistent with long range diffusional flow of hydrogen as the controlling mechanism for crack growth kinetics.

A short-time diffusion correlation for hydrogen-induced crack growth kinetics

There is conflicting evidence in the literature with respect to the effect of grain size on hydrogen embrittlement. Differences may arise because of the degree of segregation in different grain size materials, because of different structures obtained in the effort to produce varying grain sizes, or because of the grain-size dependency of diffusion and growth processes. An extremely dirty heat of 4340 steel with 0.07 S and 0.015 P was investigated so that any tramp element segregation or hydrogen recombination poison effects would be present. Measurements were obtained on cathodically-charged samples with average grain sizes of 20, 50, 90 and 140 μm. In general, tramp element effects were not controlling. For those cases where the grain diameter was significantly larger than the plastic zone, increased grain size improved resistance. This was reflected by a slight increase in threshold stress intensity and an inverse grain-size squared dependence of crack velocity. Although the data are consistent with a pressure tensor hydrogen-assisted migration model, they could also be interpreted in terms of high austenitizing temperatures promoting retained austenite.

Grain Size Effects in Hydrogen-Assisted Cracking

The mechanisms of stress corrosion cracking (SCC) and hydrogen embrittlement were recently reviewed by Lynch in this journal. The present review, in contrast, focuses on the rate-limiting step of the SCC of low-alloy high-strength steels in water and particularly focuses on the influence of the applied stress rate on the SCC of lowalloy high-strength steels. Linearly increasing stress tests of low-alloy high-strength steels in distilled water indicated that the stress corrosion crack velocity increased with increasing applied stress rate until the maximum crack velocity, corresponding to v in fracture mechanics tests in distilled water. Moreover, the crack velocity was dependent only on the applied stress rate and was not influenced by the steel composition. The rate-limiting step could be the rupture of a surface film, which would control the rate of metal dissolution and/or the production and transport of hydrogen to the crack tip or to the regions ahead of the crack tip.

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C.S. Kortovich

Papers

A comparison of hydrogen embrittlement and stress corrosion cracking in high-strength steels