Chiang Mai Journal of Science

Isothermal phase transformation behaviour in a Fe-22wt%Cr-3.2wt%Mo-6.2wt%Ni-0.037wt%C cast duplex stainless steel has been studied. The steel was solution heat-treated at 1,175^oC for 1 hour and isothermally heat-treated in the temperature range of  500-1,000^oC for 8 to 32 hours, followed by water quenching. Microstructure characterisation was performed by scanning electron microscopy and electron probe microanalysis. The typical microstructure before heat treatment consists of  primary austenite (γ₁) and δ-ferrite. Four secondary phases were observed after transformation including carbide, chi (χ), sigma (σ) and secondary austenite (γ₂). These phases can be distinguished by the bright contrast level in backscattered electron images in the order of χ > σ > γ₁ or γ₂  > δ > carbide. The transition temperature range where the σ phase took precedence over the  χ phase as a major transformation product is approximately above 700^oC. Features of  the isothermal phase transformation sequence of the cast material are discussed in comparison to that of wrought duplex stainless steel.