Effect of Microsegregation on Porosity Formation in High Cr Steel under Pressurized Atmosphere

Abstract

The effect of microsegregation on porosity formation in high nitrogen steels containing high chromium under pressurized atmosphere was investigated using pressurized directional solidification furnace. In this study, it was successful to add over 0.70 mass% nitrogen in the ingot with no porosity. Nitrogen distributions in unidirectional solidification ingots were almost homogeneous regardless of the presence of porosities. In the case of low nitrogen content and peritectic solidification, porosities were observed inside the ingot solidified under nitrogen–argon gas mixture in the total pressure below certain critical value. In the case of high nitrogen content and austenitic solidification, porosity was not observed inside the ingot solidified under nitrogen gas condition. Liquidus and solidus temperature, equilibrium partition coefficient, solid fraction and temperature at peritectic transition, the relationship between solid fraction and temperature during solidification were calculated with Thermo-calc. The microsegregation tendency of nitrogen and alloying elements was simulated using Scheil equation and the modified method of Ohnaka's microsegregation model in this study. As a result of solidification simulation, in high chromium steel, it is suggested that supersaturated solution of nitrogen caused by bubble formation during solidification is likely to occur near the peritectic transition. Defined critical pressure of porosity formation in this study would be effective to determine the pressure conditions to manufacture high nitrogen steel.