Solid Phase Sintering and Densification Behaviors of MnS Inclusions in 416 Stainless Steel

Abstract

MnS is an important strengthening phase for AISI 416 stainless steel with excellent processability, in order to explore the formation mechanism of long strip MnS inclusions. In this study, the solid phase diffusion, grain boundary diffusion, and sintering densification of MnS in 416 stainless steels at hot rolling temperature were investigated by confocal laser scanning microscopy (CLSM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and theoretical analysis. The MnS inclusions are precipitated at 1717 K (1444 °C). The sintering behavior of MnS includes two stages: diffusion and sintering. Diffusion includes volume diffusion, surface diffusion, and grain boundary diffusion. Sintering occurs through the coupling of grain boundary diffusion and surface diffusion. When the temperature is lower than 1343 K (1070 °C), J_S/J_Mn < 1, where S is the diffusion controlling element. On the contrary, Mn is the diffusion controlling element. The diffusion of MnS in the δ + γ-Fe matrix is caused by volume diffusion and surface diffusion together, and the diffusion rate of Mn in the matrix phase D_C is 2.90 × 10^–13 m²/s and the volume diffusion rate D_V,Mn is 1.89 × 10^–13 m²/s. At the beginning of holding at 1443 K (1170 °C), the grain boundary diffusion between Mn and S belongs to interstitial diffusion. The grain boundary width, 1.42 μm, does not reach the barrier limit hindering diffusion, and the grain boundary diffusion coefficient D_gb is 4.29 × 10^–14 m²/s. At the later stage of holding and cooling at 1443 K (1170 °C), the sintering of MnS is promoted by the coupling of grain boundary diffusion and surface diffusion. The effective sintering stress F^S increases with the equilibrium angle Ψ, and the sintering force becomes zero when the equilibrium configuration is reached. It is the existence of the interstitial phase that can make densification happen normally, and then make MnS particles reach equilibrium state.