Analysis of the various processes downstream cathodic hydrogen charging, IV: detailed mechanism of sulfide stress cracking

Abstract

The diabolic subtlety of the SSC mechanism is that for its occurrence on a smooth surface, crack initiation actually requires a superficial compressive strain, in order to get exactly the same combination of shear strain, dislocation glide and hydrogen drag as what is explained by fracture mechanics for a notch. This compressive strain is produced by the universal contraction induced by any tensile stress in the perpendicular direction, expressed by Poisson’s ratio, but never accounted for in simplified 1D or 2D calculations, whereas the hydrogen drag is much more powerful when it is directly fed by the raw charging flux in the few cropping out grains with the appropriate orientation. This new “evidence” results from a holistic approach analysing all the numerous questions remained unanswered since 70 years, and previously listed in Part III. Consequently, they were all explained, and to begin with, the basic relationship between SSC sensitivity and high strength or high hardness. It also occurs that the actually four stages of crack nucleation, crack growth, crack propagation and final instability have already been experimentally confirmed by acoustic emission. A new SSC severity diagram is finally proposed, both safer and less conservative than that of ISO 15156.