EFFECTS OF SOLUTION TREATMENT TEMPERATURES ON MICROSTRUCTURE, TENSILEAND FRACTURE PROPERTIES OF CO-FREE 18 NI MARAGING-STEEL

The study presents the influences of solution treatment temperatures ( S.T.T.) on microstructure, tensile and fracture properties of 18 Ni co balt free maraging steel. The results are thoroughly discussed in ligh t of the prevailing microstructural conditions as revealed by optical and transmission electron microscopy, and detailed scanning electron f ractography. It is found that tensile strength in both unaged and aged conditions is independent of solutioning temperature and, consequentl y, grain size. However, the tensile ductility, CVN and K(IC) in aged c ondition get severely affected with the increase in S.T.T. because of a change occurring in the martensitic lath morphology from blocky to s tringer type. This transition in the martensitic morphology starts at the prior austenitic grain size of 35-40 mum and is completed at 120-1 30 mum. Solutioning at 1323 K followed by air cooling, however, result s in thermal embrittlement of the steel due to grain boundary precipit ation of TiCN. The prior austenitic grain size in this reaction plays only an indirect role. Based on these results, it has been suggested t hat the existing controversies on the effect of S.T.T. grain size on t ensile ductility and fracture behaviour of maraging steels can be rati onalized if the changes in martensitic lath morphology are considered as an important variable.