Effect of Cu addition on phase transformation and microstructure in 9 mass% Cr martensitic steels

Synopsis : Effect of Cu addition on microstructure was investigated in Fe-C r-Cu martensitic alloys by means of optical and transmission electron micro scopy, and X-ray diffractometry. Lath-martensitic single structure can be o btained in 9mass%-Cu steels water-quenched from the solution treatment temp erature of 1273K. The morphology of martensite-laths is independent of Cu c ontent in the steels with 4 mass% Cu or less. Dislocation density in marten sitic structure, however, is enhanced with increasing Cu content. On the so lution treatment, Cu atoms markedly segregate at austenite grain boundary, and the Cu content increases to about 50 at% (53 mass%) in the case of 9mas s%Cr-4mass%Cu steel. This results in the retardation of grain growth of aus tenite at the solution treatment temperature probably due to the grain boun dary dragging effect by Cu atoms and/or the decrease in grain boundary ener gy. The austenite grain refining by Cu addition also causes the refining of substructure within prior austenite grains; martensite-block and -packet s tructure.