Precipitation Behavior of σ Phase in a Duplex Phase Stainless Steel

Abstract

Precipitation of σ phase in an Fe-25Cr-7Ni-3Mo alloy with a duplex structure of α and γ phases, has been studied systematically by means of optical microscopy, X-ray diffraction, hardness measurement, tensile test, SEM observation and dilatometric measurement. The results obtained in this study are as follows:
(1) TTT diagram for the precipitation of σ phase was a C-type with a nose at about 820°C and 2030 min.
(2) σ phase nucleated on the α/γ boundaries and grew into α grains, and thereafter the nucleation of σ and new γ phases began also in α grains. Volume fraction of α phase decreased and that of γ phase increased with increasing σ phase. Precipitation of σ phase occurred by the reaction, α→σ+γ. It was found that Cr, Mo and W were concentrated into α phase.
(3) Increase of hardness with precipitation of α phase was not so large, on the contrary, impact value decreased drastically even at a few percent precipitation of α phase. α phase was also found to be harmful for hot ductility.
(4) The precipitation range during cooling were about 700900°C and a phase precipitation could be avoided if cooling rate were larger than about 20°C/min after solution treatment.
It was clarified from the above results that α embrittlement during production of duplex phase stainless steels could be avoided by selecting adequate manufacturing conditions.