Mechanism of Stress Corrosion Cracking of Steels in H2S

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Abstract
For four low alloy steels with a wide range of tensile strength, the dynamical processes of nucleation and propagation of SCC in H2S were traced with metallographic shadowing technique. Specimens were polished WOL type constant deflection ones. Results showed that if KI >KISCC, hydrogen induced delayed plasticity would certainly occur in the front of the crack tip and then the nucleation and propagation of SCC would follow, i.e., the size and deformation degree of the plastic zone in front of the crack was enlarged continuously with time. For ultra-high strength steels after the closure of the delayed plastic zones, the SCC were nucleated at their tips. These discontinuous cracks would grow and join one another with the progress of the delayed plasticity. For lower strength steels, the SCC propagated continuously along the border of delayed plastic/elastic region with the enlargement of the delayed plasticity. The variation of KISCC and da/dt in H2S and in water with the strength of steels were investigated.

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