Effect of heat treatment on delayed fracture resistance of structural steel 42CrMo

To investigate the effect of heat treatment on delayed fracture resistance of high strength structural steel, the steel 42CrMo was quenched and tempered and isothermally quenched, respectively. Tensile results of notch specimens in acid solution show that compared with quenching and tempering, isothermal quenching brought 42CrMo has higher delayed fracture resistance when it was treated to a high tensile strength of about 1500 MPa. Microstructural. observation shows that the isothermally quenched 42CrMo is mainly composed of bainitic laths with epsilon-carbides distributed in them and without carbides along the prior austenite grain boundaries, which is suggested to cause the improvement in delayed fracture resistance. With the decrease of isothermal temperature, the bainitic lath becomes finer, resulting in an increase of strength and delayed fracture resistance. It can be concluded that isothermal quenching is an effective way to overcome delayed fracture of 1500 MPa level high strength steel.