Effects of solution treatment temperatures on microstructure, tensile and fracture properties of Co‐free 18 Ni maraging steel

Abstract

The study presents the influences of solution treatment temperatures (S.T.T.) on microstructure, tensile and fracture properties of 18 Ni cobalt free maraging steel. The results are thoroughly discussed in light of the prevailing microstructural conditions as revealed by optical and transmission electron microscopy, and detailed scanning electron fractography. It is found that tensile strength in both unaged and aged conditions is independent of solutioning temperature and, consequently, grain size. However, the tensile ductility, CVN and K IC in aged condition get severely affected with the increase in S.T.T. because of a change occurring in the martensitic lath morphology from blocky to stringer type. This transition in the martensitic morphology starts at the prior austenitic grain size of 35‐40 μm and is completed at 120‐130μm. Solutioning at 1323 K followed by air cooling, however, results in thermal embrittlement of the steel due to grain boundary precipitation of TiCN. The prior austenitic grain size in this reaction plays only an indirect role. Based on these results, it has been suggested that the existing controversies on the effect of S.T.T. grain size on tensile ductility and fracture behaviour of maraging steels can be rationalized if the changes in martensitic lath morphology are considered as an important variable.