Improvement of delayed fracture strength of high strength steels by intergranular ferrite precipitation

Abstract

A new method for improving delayed fracture strength by the precipitation of ferrite along prior austenite grain boundaries has been demonstrated with high strength steels of 1300 MPa in tensile strength. Intergranular fracture which is characteristic to the delayed fracture of tempered martensitic steels is suppressed and is changed to quasicleavage by the intergranular ferrite precipitation. Hydrogen thermal desorption analysis has revealed that the amount of diffusive hydrogen increases under loading. The mean hydrogen occlusion rate rather than the hydrogen absorption under no loading or the amounts of hydrogen at the time of failure corresponds to the delayed fracture strength levels. The mechanism of the effect of intergranular ferrite on the hydrogen embrittlement based on the accumulation of lattice defects has been discussed.