Microstructural influences on hydrogen delayed fracture of high strength steels

This study was aimed at investigating the effect of the microstructural constituents of high strength steels on their hydrogen delayed fracture properties. For this purpose, a series of constant loading tests, slow strain rate tests, cyclic corrosion tests, and thermal desorption spectrometry analysis was conducted on the hydrogen pre-charged specimens with tempered martensite or full pearlite showing a similar tensile strength level of 1600 MPa. Constant loading tests and slow strain rate tests revealed that the tempered martensitic steel was more susceptible to hydrogen delayed fracture than the fully pearlitic steel. In slow strain rate tests, the maximum tensile strength decreased with increasing diffusible hydrogen content in a power-law manner. The content of hydrogen inflowing from environment was also simulated by cyclic corrosion tests. It was found that the fully pearlitic steel has the higher equilibrium hydrogen content than the tempered martensitic steel. The primary trapping sites were prior austenite grain boundaries for the tempered martensitic steel, and ferrite/cementite interfaces and dislocations for the fully pearlitic steel. SEM fractographs revealed that the cracks induced by hydrogen propagated along the prior austenite grain boundaries resulting in brittle intergranular fracture for the tempered martensitic steel while the fully pearlitic steel was fractured in a ductile manner.