Pressure Effect On Solidification Structure And Segregation In High Nitrogen Austenitic Stainless Steel

With the development of pressurized metallurgy and deeper understanding of nitrogen alloying mechanism,high intensity nitrogen stainless steels P2000,Cronidur 30 and M340 have been developed abroad,which can be selected in the manufacture of retaining rings of large military gas turbines in aircrafts carriers and submarines,main bearings of aircraft turbine engines and corrosion resistant plastic mould,etc.As one of the fundamental thermodynamic and kinetic parameter,pressure can effectively change thermodynamic and kinetic behaviors of solidification process,refine solidification structure,and even change the sequence of phase transition.However,pressurization is relatively hard due to high melting points of steels.Presently,pressurization research in metallurgy is forced on high nitrogen steels,especially in the preparation of high nitrogen stainless steels.But few researches on pressurized solidification process are reported.In this paper,embedded thermocouple experiment and self-programming is combined to analyze pressure effect on ingot-mould interfacial heat transfer condition.On the basis of above experiment,P2000 high nitrogen austenitic stainless steel is taken in this paper to analyze pressure effect on solidification structure and macrosegregation.The details are shown as follows.1.Inverse procedures for the calculation of interfacial hear transfer coefficient is completed by Fortran.ProCast is used to verify the accuracy of procedure,where the error is within 5%.Through embedded thermocouple experiments and temperature measurement in casting and mould under different solidification pressure(0.5MPa,0.85MPa and 1.2MPa),inverse procedure can further calculated interfacial heart transfer coefficient.The results show that temperature increment between measuring point and temperature gradient in casting and mould and interfacial heart transfer coefficient is evaluated with the increasing pressure.Pressure can significantly improve interfacial heat transfer conditions and intensify cooling effect.2.Macrostructure inspection is carried out on ingots solidified under different pressure.According to macroscopic observation,porosity in the ingots is decreased,compactness of macrostructure is improved,length of columnar crystals along radius is enhanced,the angle between columnar crystals and horizontal direction and angle between intersecting lines of columnar crystals at the ingot top and horizontal direction are decreased.Analyze upon microstructure under different pressure shows that secondary dendrite arm spacing and grain size decrease with pressure increase.The microstructure is refined to some extent.3.Sulfur print test,optical emission spectrometer(OES)and electron probe micro-analyzer(EPMA)are taken to observe pressure effect on segregation.For ingots of small sizes,no clear trend appears that macrosegregation changes with pressure,which calls for further study.Variations of microsegregation ratio of Si,Mn and Mo with pressure increment at different positions of ingot decrease,but N shows uncertain rules.4.Segregation of major alloy elements are simulated during equilibrium solidification and Scheil solidification of high nitrogen austenitic stainless steel P2000 by Thermo-Calc.Nucleation pressure for nitrogen gas pores is calculated.Experiments are carried out to verify the effect of cooling rate on gas pores.It is showed that pressurization and cooling rate are both beneficial to decrease and diminish gas pores during solidification of high nitrogen austenitic stainless steel P2000.