Thermodynamic Study on the Solubility of N in High Cr, Ni and Mo Content Fe-Cr-Ni-Mo-O Melts

The aim of the present study is to investigate the solubility of nitrogen in super or hyper duplex stainless steel, which is characterized by a very high Cr content, as well as the activity interaction parameters between N and other alloy elements. The chemical equilibrium method was employed in the present experiment. High Cr, Ni, and Mo content Fe−Cr−N−O and Fe−Cr−Ni−Mo−N−O melt are equilibrated at 1873 K under atmospheres of pure nitrogen and Ar/N2 gas mixture. The melts were placed in Al2O3 crucibles and coated with graphite crucibles. The experimental results showed that the solubility of N significantly increased with increasing Cr content, reaching over 1 wt pct at a Cr content of about 40 wt pct. In addition, the solubility of Cr increased slightly with a decrease in Ni content and an increase in Mo content. The activity interaction parameters were fitted using WIPF (Wagner’s Interaction Parameter Formalism), as shown as follows: eNCr=−0.07083, rNCr=+0.0005888, rNN=−0.00926, eNNi=+0.30885, rNNi=−0.03963, eNMo=−0.05882, rNMo=+0.00616; the comprehensive set of thermodynamic basic parameters obtained in this study can be effectively used to assess the N solubility in USSD with a Cr content exceeding 30 wt pct.