A new model for hydrogen-assisted cracking (hydrogen “embrittlement”)

A new model is presented for hydrogen-assisted cracking (HAC) which explains the observations of decreasing microscopic plasticity and changes of fracture modes with decreasing stress intensities at crack tips during stress-corrosion cracking and HAC of quenched-and- tempered steels. The model suggests that the presence of sufficiently concentrated hydrogen dissolved in the lattice just ahead of the crack tip aids whatever deformation processes the microstructure will allow. Intergranular, quasicleavage, or microvoid coalescence fracture modes operate depending upon the microstructure, the crack-tip stress intensity, and the concentration of hydrogen. The model unifies several theories but shows how the stress-sorption and lattice embrittlement models are unnecessary. The model shows that planar pressure effects are necessary at low stress intensities and are necessary only to augment the driving force from the applied loads. The basic hydrogen-steel interaction appears to be an easing of dislocation motion or generation, or both.

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