メカニカルミリングにより強加工した24 mass%Cr-8 mass%Ni 2相ステンレス鋼粉の相分離

Abstract

The phase decomposition behavior of (δ+γ) 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 (δ+γ) two phases at elevated temperatures.The strain stored within the δ phase powder through milling treatment increases with inceasing milling time and has a large effect on refining the (δ+γ) two-phase structure obtained after annealing. For example, the 360 ks milled powder has a fine (δ+γ) two-phase structure of less than 2 μm in the mean grain size after 1173 K-3.6 ks annealing. During heating to a given annealing temperature, the δ phase powder milled sufficiently transforms allotropically to the almost full γ structure because of the increased driving force for the γ phase nucleation, and then the δ phase again precipitates from the γ phase during isothermal annealing reaching the equilibrium structure and composition of the (δ+γ) two-phase.Those grains, however, easily coarsen during annealing at temperatures higher than 1273 K. Addition of alumina fine powder(0.06 μm) is quite effective in refining the (δ+γ) two-phase structure, because alumina particles not only suppress the grain growth at such a high temperature but also promote γ grain nucleation through the retardation of recovery in the deformed δ phase.