Fracture Mechanics Consideration of Hydrogen Sulfide Cracking in High Strength Steels

Abstract

Hydrogen sulfide (H2S) stress corrosion cracking studies were conducted within the framework of fracture mechanics for several high strength steels (AISI 4340, 4140, HY-80, and HY-130). For all the steels and strength levels investigated (σys = 80 to 150 ksi), H2S stress corrosion cracking was found to exist. For each of the alloys investigated, a valid plane strain KIscc (which indicates the demarcation between detectable rates of crack extension, Δa/Δt ⩾ 10-5 in./min and those below these rates) was measured and found to depend significantly on yield stress with decreasing KIscc values reported for increasing yield stress. A limited investigation of crack growth kinetics found crack growth rates to accelerate most rapidly from presharpened fatigue cracks when loaded to K levels just beyond the KIscc threshold. In several instances, especially with the highest strength alloys, stress corrosion crack velocities attained peak values before being “damped” to some steady state velocity at increased K levels. The crack velocity damping might in part be attributed to crack division or plasticity effects associated with increasing plastic zone size to thickness ratio at higher K levels.