Crack Initiation and Propagation Analyses of Hydrogen-Related Fracture Surfaces of Tempered Martensitic Steel

Hydrogen-related crack initiation and propagation in tempered martensitic steel were investigated by fracture surface topography analysis (FRASTA) and crystallographic orientation analysis. Hydrogen-related fracture morphologies of tempered martensitic steel were characterized by intergranular and quasi-cleavage transgranular fractures. FRASTA results suggested that hydrogen-related crack initiation sites were inclusions and cracks propagated from quasi-cleavage fracture to intergranular fracture near the crack initiation site. Crystallographic orientation analysis results suggested that intergranular fracture propagated on prior austenite grain boundaries, whereas quasi-cleavage fracture propagated along {011} planes near the crack initiation site. However, quasi-cleavage fracture consisted of not only {011} planes but also various other planes. In a previous study, hydrogen-related fracture morphologies of tempered martensitic steel tended to change from quasi-cleavage to intergranular with an increase in strength or an increase in hydrogen content, and quasi-cleavage fractures propagated along {011} planes. However, the results of the present study indicate that the fracture propagation path changed from quasi-cleavage fracture along {011} planes and other various planes within the prior austenite grains to intergranular fracture on the prior austenite grain boundaries caused by the influence of an inclusion.