Strain controlled vs stress controlled hydrogen induced fracture in a quenched and tempered steel

Abstract

Experiments on commercial and laboratory heats of a 5 pct Ni steel have delineated a wide range of hydrogen-induced cracking behavior. Both precracked and notched specimens were tested in 0.21 MPa H₂ gas at room temperature; the microstructure and hardness were held essentially constant. One extreme of behavior was exhibited by specimens with negligible amounts of intergranular weakening due to impurity segregation; here, strain-controlled, plasticity-related cracking occurred along surfaces of maximum shear stress within the prior austenitic grains. As impurity segregation increased (due to 480 °C aging of samples with Mn and Si) increasing amounts of stress-controlled cracking occurred along prior austenite grain boundaries. The latter produced a steady decrease in the stress intensity for crack extension and the local stress for fracture of the notched bars. The mechanisms involved and the practical implications of these phenomena are discussed.