Effect Of Pressure On Solidification Defects Of High Nitrogen Austenitic Stainless Steel

As one of the important production technology for high nitrogen austenitic stainless steel,pressurized metallurgy technology has the advantages of improving the microstructure and eliminating the defects of ingot.It has broad application prospects in the development and production process of high nitrogen austenitic stainless steel.At present,research on production process of high nitrogen austenitic steel by compression technology is mainly focused on smelting.However,solidification process is seldom mentioned.As one of the important parameters,pressure can change the thermodynamic and kinetic parameters during the solidification process.In this paper,ProCAST software,Thermo-Calc and DICTRA software were used to analyze the thermodynamics and kinetics of 19Cr-18Mn-2Mo-0.9N high nitrogen austenitic stainless steel during solidification.Combining the detection of solidification structure and ingredient analysis,the effects of pressure on the solidification structure,defects and segregation were studied,which provided theoretical support for the pressurized technology in the industrial production of high nitrogen austenitic stainless steel.The details are shown as follows.1.The calculation results of Thermo-Calc and DICTRA software are as follows.Pressure has effects on equilibrium phase diagram of 19Cr-18Mn-2Mo-0.9N steel,such as the region of each phase,precipitation temperature of each phase,precipitation order of each phase and precipitation content of each phase.With the increase of pressure,equilibrium partition coefficient of C,Si,Cr and N increases,equilibrium partition coefficient of Mn and Mo decreases,diffusion coefficient of C,Si,Cr and Mn increases,diffusion coefficient of Mo and N decreases,system volume decreases and system density increases.In addition,increasing pressure is helpful to reduce the critical nucleation radius,increase the nucleation rate,increase the nitrogen solubility,avoid the appearance of "ferrite trap" and inhibit the nitrogen release.2.By reasonably matching the addition amount of the nitriding alloy,smelting pressure and casting pressure,three ingots of 19Cr-18Mn-2Mo-0.9N under different pressures(0.15 MPa,1.0 MPa and 2.0 MPa)were obtained by 25 kg pressurized induction melting furnace.This research developed a new method of smelting high nitrogen austenitic stainless steel through controlling pressure by stages,and it realized the control of nitrogen content in steel.Combining the cooling curve of ingot and mold obtained by temperature measurement,this paper expounded the mechanism which pressure enhanced cooling rate.3.It can be seen that the formation of gas pore and porosity decreases with the increase of pressure by structure detection and simulation result.And the gas pores disappear when pressure is higher than 1.0 MPa.The maximum value of nitrogen critical nucleation pressure is between 0.702 MPa and 1.347 MPa by calculating in equilibrium solidification and Scheil solidification model.The simulation results by ProCAST show that the increase of pressure can reduce the range of porosity by improving the heat transfer coefficient between mold and ingot.In addition,the increase of pressure results in the decrease of second dendritic arm spacing and grain size,refines the dendritic structure.4.Based on the analysis of composition,Thermo-Calc and DICTRA simulation,the change regulation of segregation with the increase of solidification pressure was discussed.With the scope of experimental pressure,N,Cr and Mn have no obvious macrosegregation,Mo has macrosegregation to a certain extent.Simulation calculation shows that increasing pressure has little effect on the dendrite segregation of C,Si,Cr and Mn,but it increases the dendrite segregation of N and Mo.