Sulfide stress corrosion cracking and fatigue crack growth of welded TMCP API 5L X65 pipe-line steel

Abstract

Slow strain rate tensile test and fatigue crack growth (FCG) test were performed to evaluate the fracture behavior of API 5L X65 steel weldments after hydrogen-charging. Regardless of the testing environments, tensile fracture of all welds was located at the weld metal (WM), which had the lowest hardness as compared to that of the other parts of the weld. When hydrogen-charged in an H 2S-saturated solution, all specimens suffered a small drop in tensile strength as compared to those tested in air; however, the loss in ductility was more significant. The susceptibility to hydrogen embrittlement of the welds could be reduced significantly by subjecting the welds to 600°C/2 h tempering treatment. With the tempering treatment, the number of surface cracks in the WM after hydrogen-charging decreased and the tensile properties improved. Results of FCG test demonstrated that in a higher stress intensity factor range (Δ K), tempered steel plates with a severely banding structure had the higher fatigue crack growth rates (FCGRs) than those of as-received ones. The WM of an uncharged weld had the lower FCGRs as compared to the parent metal within the same Δ K range. However, the enhancement of crack growth in the WM was very pronounced after hydrogen-charging. SEM fractographs of tensile and fatigue-fractured surfaces revealed a quasi-cleavage fracture in the embrittled region.