Relationship Between Applied Stress and Hydrogen-Related Fracture Behavior in Martensitic Steel

In the present study, a relationship between the applied stress and the hydrogen-related fracture behavior in an 8Ni-0.1C martensitic steel was investigated by means of constant loading tensile tests under various applied stresses ranging from 400 to 1000 MPa. The time to fracture in hydrogen charged specimens increased from 0.5 to 30.5 s with decreasing the applied stress from 1000 to 400 MPa. Area fractions of brittle fracture surfaces, especially that of intergranular fracture surface, increased as the applied stress decreased. Orientation analysis using EBSD showed that micro-cracks observed in the vicinity of the main crack initiated along prior austenite grain boundaries regardless of the applied stress. The hydrogen-related fracture processes under various applied stresses were reconstructed by fracture surface topography analysis and the results indicated that intergranular fracture was dominant under the lower applied stress, while the fracture mode changed from intergranular fracture to quasi-cleavage fracture with progress of crack propagation under the higher applied stress.