Study On The Dissolution Behavior Of Nitrogen In Stainless Steel Under Normal And High Pressure

Nitrogen-containing stainless steel has become a research hotspot in the field of stainless steel due to its excellent mechanical properties and corrosion resistance.Since the 1970s,research on gas phase nitrogen alloying process with nitrogen as the nitriding source has been gradually carried out at home and abroad.The resource-rich and cheap nitrogen gas is used as a raw material to replace or partially replace the nitride alloy,thereby greatly reducing the production cost of nitrogen-containing stainless steel,and reducing the pollution brought by the nitride alloy.C-HRA-5 austenitic stainless steel has high nitrogen solubility due to its alloy composition,and can be prepared by gas phase nitriding process under normal pressure to achieve its target nitrogen content.Cronidur 30 high-nitrogen martensitic stainless steel needs to be prepared through a pressurized gas phase nitriding process due to its high nitrogen content requirements.Therefore,the study on thermodynamics and kinetics of alloy melt gas phase nitriding under normal pressure and pressure is of great significance to the design and optimization of nitrogen-containing stainless steel smelting process.In this paper,the nitrogen dissolution behavior of molten Fe-Cr-Ni-Nb alloy system(C-HRA-5 Nb-containing austenitic stainless steel simplified system)was studied by gas phase equilibrium experiment under normal pressure.For molten Fe-Nb,Fe-Cr-Nb and Fe-Ni-Nb alloy systems,eNNb=-377/T+0.134,rNCr,Nb=41.0/T-0.01987 and rNNi,Nb=4.78/T-0.00197 were measured by regression analysis.Combined with the existing thermodynamic parameters and the interaction coefficients obtained in this study,the nitrogen solubility models of molten Fe-Cr-Ni-Nb alloy system and molten C-HRA-5 Nb-containing austenitic stainless steel under normal pressure were established respectively.The predicted value of the model was in good agreement with the measured value.The solubility of nitrogen decreased with the increase of smelting temperature,and increased with the increase of nitrogen pressure,which conforms to Sieverts’ law.Under the experimental conditions,the gas-phase nitriding rate was controlled by the interface chemical reaction.The kinetic model of gas-phase nitriding of molten C-HRA-5 Nb-containing austenitic stainless steel was established,and the predicted value of the model is in good agreement with the measured value.The rate constant(k)of the gas-phase nitriding reaction increased with the increase of temperature,and the nitrogen pressure had no obvious effect on it.The gas phase nitriding reaction rate constant of molten C-HRA-5 Nb-containing austenitic stainless steel was expressed as lgk=2.1-7889/T.The nitrogen dissolution behavior of molten Fe-Cr-Mo-C alloy system(Cronidur 30 high-nitrogen martensitic stainless steel simplified system)was studied by gas phase equilibrium experiment under pressure.The solubility of nitrogen under high pressure deviated from Sieverts’ law,through regression analysis,the correction coefficient of nitrogen pressure to nitrogen activity coefficient of molten Fe-Cr-Mo-C alloy system under high nitrogen pressure was measured δNP=0.06,and the nitrogen solubility model of the alloy system was established.The predicted value of the model was in good agreement with the measured value.The control link of the nitriding rate was the mass transfer of nitrogen in the liquid phase.The apparent mass transfer coefficient(kN)of the liquid phase has little dependence on nitrogen pressure.The relationship between kN and the temperature of the Fe-15Cr-1Mo-0.3C alloy system was expressed as:lgkN=2.493-7709/T.kN decreased with the increase of Cr and C content,and increased with the increase of Mo content.Considering the influence of pressure change during the pressurization stage on the nitriding rate,a new type of pressurized gas nitriding kinetic model was established,and the predicted value of the model was in good agreement with the measured value.