PHASE-DECOMPOSITION IN 24 MASS PERCENT CR-8 MASS PERCENT NI 2-PHASE STAINLESS-STEEL POWDER HEAVILY COLD-WORKED THROUGH MECHANICAL MILLING

The phase decomposition behavior of (delta + gamma) two phase stainless steel powder at elevated temperatures has been investigated in association with the amount of strain given through mechanical milling treatment. The powder with a composition of Fe-24 mass%Cr-8 mass%Ni was prepared by N2 gas atomization. The structure of the as-received powder is fully ferritic at room temperature but transforms to (delta + gamma) two phases at elevated temperatures. The strain stored within the delta phase powder through milling treatment increases with inceasing milling time and has a large effect on refining the (delta + gamma) two-phase structure obtained after annealing. For example, the 360 ks milled powder has a fine (delta + gamma) two-phase structure of less than 2 mum in the mean grain size after 1173 K-3.6 ks annealing. During heating to a given annealing temperature, the delta phase powder milled sufficiently transforms allotropically to the almost full gamma structure because of the increased driving force for the gamma phase nucleation, and then the delta phase again precipitates from the gamma phase during isothermal annealing reaching the equilibrium structure and composition of the (delta + gamma) two-phase. Those grains, however, easily coarsen during annealing at temperatures higher than 1273 K. Addition of alumina fine powder (0.06 mum) is quite effective in refining the (delta + gamma) two-phase structure, because alumina particles not only suppress the grain growth at such a high temperature but also promote gamma grain nucleation through the retardation of recovery in the deformed delta phase.