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 sigma phase in 19Cr-9Ni-2Mn and 18Cr-0.75Si stainless steels as hot-rolled at 800 degrees C have been investigated in this study. The morphology, quantity and chemical composition of the U 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 sigma phase showed a dendrite-like morphology in the as-received materials. A hot-rolling process changed the morphology of the sigma phase from dendrite-like to globular, especially at higher reduction ratio. The amounts of a phase in the stainless steels also increased gradually at 800 degrees C with increasing the reduction ratio from 0 to 75%. The XRD analyses showed that a higher reduction ratio enhanced the conversion of delta-ferrite (110) to or phase (542). The sigma 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 delta-ferrite in stainless steels as hot-rolled at 800 degrees C, these elements partition to the a phase at high temperature. (c) 2007 Elsevier B.V. All rights reserved.