Effects of Nitrogen Segregation and Solubility on the Formation of Nitrogen Gas Pores in 21.5Cr-1.5Ni Duplex Stainless Steel

The nitrogen gas pore-formation mechanism was discussed with regard to the solidification of 21.5Cr-1.5Ni duplex stainless steels (DSSs) by considering nitrogen segregation and solubility. The segregation behavior of nitrogen was investigated with phase transformation using experimental detection methods and Thermo-Calc software calculations. The process associated with the formation of gas pores was illustrated clearly. The factors that influenced the formation of gas pores, including shrinkage, nitrogen content, solidification pressure, and alloying elements (Mn and Cr), were discussed in detail. The formation of nitrogen-rich phases [austenite phase (FCC), AlN, and hexagonal close packed] is beneficial to eliminate nitrogen segregation and suppressing gas pore formation. The nitrogen-depleted phase (ferrite phase (BCC)) exhibits an opposite effect. Regular gas pores are initially formed in locations consisting of the austenite phase. As the gas pores lengthen, ferrite and austenite phases alternately form around the gas pores. Solidification shrinkage can promote the formation of irregular gas pores at the centerline of the ingots. Increasing the nitrogen content is favorable to the formation of gas pores. Increasing solidification pressure is effective with regard to suppressing the formation of gas pore defects in DSSs. Increasing the Mn content can reduce the likelihood of gas pore formation; this can be attributed to the increased nitrogen solubility in the residual liquid surrounding the dendrites and the formation tendency of the nitrogen-rich phase. Increasing the Cr content exhibits a dual effect on gas pore formation, which is caused by the increased nitrogen solubility and segregation in the residual liquid.