Distribution of ferrite and phases in low-Nickel 316L austenitic stainless steel billet

The low nickel content of 316L austenitic stainless steel leads to an increase in residual ferrite in the billet, and the residual ferrite and precipitated phases directly affect the hot workability and surface quality of austenitic stainless steel. This study takes 316L austenitic stainless steel billets with Ni content of about 10% as the research object, and analyzes the distribution of ferrite and σ phase in the billets. This study characterized the features of ferrite and precipitated phases in 316L billets through optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and electron probe microanalysis (EPMA), and analyzed the equilibrium solidification process of 316L billets in conjunction with Thermo-Calc thermodynamic calculations.The results showed that the ferrite content of 316L billets distributed in an "M" shape along the thickness direction, with the ferrite content increasing from 4.2% to 10.6% at the surface to 85mm, and then decreasing to 7.8% towards the center. The solidification mode of the billet is the FA mode in which ferrite solidifies and precipitates first. The ferrite shape changes from granular, skeletal to lath and short rod shape from the surface to the center in the thickness direction of the billet. The ferrite on the surface of the billet did not transform into other phases. The high-temperature δ ferrite inside the billet decomposes into the χ phase and σ phase during the solid-state phase transformation stage after solidification. From the surface to the center of the billet, the transformation ratio of ferrite to the χ phase and σ phase increases: the transformation ratio of the χ phase increases from 7.9% at a distance of 75mm from the surface to 13.4% at the center, and the transformation ratio of the σ phase at the center is 51%. The research results reveal the distribution pattern of δ-ferrite and its decomposition products (χ phase, σ phase) in low-nickel 316L square billets and their strong correlation with cooling rate.