Grain Refinement of Austenite through Reverse Transformation from Martensite in Fe-15Ni-Co-Mo-Ti Alloys

Abstract

The effects of 5% Mo, 15% Co, or 2% Ti addition to Fe-15%Ni-10%Co-5%Mo base alloy on grain refinement of reversed austenite subjected to single austenitizing were studied at heating rates between 0.083 and 100°C/s. Grain refinement was promoted by Co or Ti addition and was suppressed by Mo addition. Grain refinement was promoted in the base, (base+15% Go) and (base+2% Ti) alloys at the lower heating rates, whereas it was suppressed in the (base+5% Mo) alloy. Refining processes depended on chemical compositions and heating rates, and were classified into the following two groups in terms of the differences in the initial stage of formation of austenitic grains:
(1) After completion of reverse transformation, austenitic grains were formed due to recrystallization of austenite.
(2) During reverse transformation, austenitic grains were formed probably by the nucleation and growth of diffusionally transformed austenite.
It was shown that the mechanisms of reverse transformation associated with chemical compositions and heating rates played an important role in these refining processes.