Hydrogen-induced cracking in 4340-type steel: Effects of composition, yield strength, and H₂ pressure

Abstract

Measurements of the threshold stress intensity for hydrogen-induced crack extension,Kth at room temperature were made on bolt-loaded WOL specimens of a commercial 4340 steel and of laboratory heats in which the bulk concentrations of manganese, silicon, phosphorus, and sulfur were varied. The hydrogen pressure was varied from 200 to 1600 torr (~0.03 to 0.22 MPa), and the yield strengths were varied from ~170 to 270 ksi (~1200 to 1900 MPa). Measurements ofKIc in air were also made as a function of composition and yield strength. Significant differences betweenKIc in air andKth in H₂ were found only in steels containing added Mn or Si; these elements are believed to promote segregation of phosphorus and sulfur to austenite grain boundaries. TheKth values were uniquely related to the percentage of intergranular fracture and also to a parameter containing the calculated maximum hydrogen concentration and the bulk concentrations of manganese, silicon, phosphorus, and sulfur. In a high purity steel free of manganese and silicon theKth was lower thanKIc only at yield strengths greater than 200 ksi (1400 MPa). The results are consistent with an additive reduction in cohesive strength by hydrogen and metalloid impurities. It is shown that theKth depends on hydrogen fugacity, yield strength, and grain boundary purity(i.e., cohesive strength).