Precipitation in 18 wt% Ni maraging steel of grade 350

Abstract

The evolution of precipitates in maraging steel of grade 350 was studied using the complementary techniques of small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). These investigations revealed that ageing the steel at 703K involved a rhombohedral distortion of the supersaturated b.c.c. martensite accompanied by the appearance of diffuse ω-like structures. This was followed by the appearance of well-defined ω particles containing chemical order. At the ageing temperature of 783K, Ni₃(Ti,Mo) precipitates were the first to appear with a growth exponent of 1/3. The values of the Porod exponent obtained from the SAXS profiles indicated that the ω particles, formed below 723K, had diffuse interfaces up to an ageing time of 48h. On the other hand, Ni₃(Ti,Mo) precipitates, formed above 723K, developed sharp interfaces in just about an hour. Also, the steel exhibited scaling in phase separation both at 703 and 783K, but only during the early stages. Through this study it was established that, at temperatures of ageing less than 723K, evolution of ω particles takes place through the collapse of the unstable b.c.c. lattice and, at temperatures above 723K, precipitation of A₃B type of phases through the mechanism of clustering and ordering of atomic species. Sharp interfaces develop rather quickly when the mechanism of precipitation involves development and amplification of a concentration wave alone as in the nucleation of Ni₃(Ti,Mo) at 783K than when an interplay of both the displacement and concentration waves is required as in the evolution of ω at 703K. These results indicate towards the possibility of existence of two separate time-temperature-transformation (TTT) curves, one for the evolution of ω-phase and another for nucleation and growth of Ni₃(Ti,Mo).