Precipitation behavior of σ phase in 19Cr–9Ni–2Mn and 18Cr–0.75Si stainless steels hot-rolled at 800 °C with various reduction ratios

The effects of various reduction ratios on the precipitation behavior of the σ phase in 19Cr–9Ni–2Mn and 18Cr–0.75Si stainless steels as hot-rolled at 800 °C have been investigated in this study. The morphology, quantity and chemical composition of the σ phase were analyzed by using optical microscopy (OM), X-ray diffractometry (XRD), image analysis, scanning electron microscopy (SEM) and wavelength dispersive spectrometer (WDS), respectively. The σ phase showed a dendrite-like morphology in the as-received materials. A hot-rolling process changed the morphology of the σ phase from dendrite-like to globular, especially at higher reduction ratio. The amounts of σ phase in the stainless steels also increased gradually at 800 °C with increasing the reduction ratio from 0 to 75%. The XRD analyses showed that a higher reduction ratio enhanced the conversion of δ-ferrite (1 1 0) to σ phase (5 4 2). The σ phase was identified to be a Cr- and Mo-rich phase by WDS analysis, which indicated that although Cr and Mo elements are stabilizers of δ-ferrite in stainless steels as hot-rolled at 800 °C, these elements partition to the σ phase at high temperature.