AUSTENITE REVERSION IN 18 NI CO-FREE MARAGING-STEEL

The mechanism of austenite reversion in 18 Ni Go-free maraging steel ( 250 grade) has been established by conducting extensive X-ray diffract ion (XRD) and transmission electron microscopy (TEM) under differently aged conditions. it has been proposed that contrary to the precipitat e dissolution mechanism suggested for the initiation of austenite reve rsion in 18Ni-8Co-5Mo type maraging steels, the initiation of transfor mation of martensite to austenite in this type of maraging steel is du e to the diffusion of Ni from matrix to the dislocations and other def ect structures on prolonged/high temperature ageing. This results in l ocal enrichment of Ni which lowers both A, and M(s) temperatures of th e region. Lowering of these transformation temperatures is responsible for the early reversion of martensite to Ni-enriched stable austenite which, on subsequent cooling to room temperature, does not transform back to martensite.