Study On Microstructure,properties And Heat Treatment Process Optimization Of Martensitic Stainless Cast Billet After Rolling Process

The X6CrNiMoVNb11-2 martensitic stainless steel as a special type of stainless steel is widely used in the production of gas turbine discs in liquid rocket engines and compressor disks in aero engines.In order to further meet the service requirements of the material at high temperature and under high pressure,the mechanical properties of the cast-rolled X6CrNiMoVNb11-2 martensitic stainless steel disk were specially adjusted by optimizing the parameters of the heat-treatment process.In order to meet the performance requirements of both shape controlling and property controlling during the advanced casting-rolling composite forming process,the quenching and tempering treatment process in the special practical application is adopted and then optimized.In this paper,through the calculation of thermo-physical parameters,heat treatment experiments,metallographic analysis,performance testing,the relationship between microstructure and mechanical properties of X6CrNiMoVNb11-2 rolled billet after heat treatment（1040℃quenching and 300-670℃tempering）was studied.The effects of two cooling methods（oil cooling and air cooling）and different tempering temperatures on hardness were considered,then the mathematical expressions of tempering hardness equation between tempering temperature,tempering holding time and tempering hardness under different cooling modes were established by polynomial fitting method,the reliability and accuracy of the tempering hardness equation are evaluated by R value and standard deviation S.At the same time,the relationship between microstructure and mechanical properties is analyzed and discussed.The results of mechanical properties under different cooling processes are compared and analyzed.Finally,the optimized heat treatment process was determined.The yield strength,tensile strength,cross-area reduction ratio,elongation and impact toughness under different cooling conditions were obtained by testing mechanical properties.The results show that the martensite microstructure becomes fine and uniform after oil quenching and air cooling.With tempering temperature rising from 550℃to 650℃,the tensile strength decreases and the elongation increases.Metallographic analysis shows that the grain size after tempering at 650℃for 1.5h is more uniform and smaller than that after tempering at 650℃for 2h,and the martensite grain size obtained by oil-quenching is smaller than that by air-quenching,and the optimum heat treatment process（1040℃oil-quenching,650℃tempering with air cooling for1.5h）has the best comprehensive performance.The fracture morphology after quenching is characterized by a large number of micro-cracks,dimples and second-phase particles.The fracture is prior to appears through the second-phase particles,which leads to the decrease of plasticity.After tempering,the fracture neck shrinks obviously,the proportion of radiation area is large,and the proportion of fiber area is small,which shows the characteristics of ductile fracture.In addition,a certain amount of high-density nanophase precipitation is found in the martensite phase transformation through the heat treatment involved in the quenching and tempering processes,where M₂₃C₆carbides are dispersed in lamellar martensite,with the close-packed Ni₃Mo and Ni₃Nb phases of high-density co-lattice nanocrystalline precipitation created during the tempering process,which significantly improves the performance of the martensitic stainless steel.The quenched structure is lath martensite with a large number of dislocations,a small amount of austenite and carbide,and the quenched and tempered structure is tempered martensite+residual austenite+cementite,the toughness is obviously improved.The optimum heat treatment parameters are:1040℃quenched oil cooling,650℃tempered air cooling,the final average hardness is 313HV,the elongation is 17.9%,the cross-area reduction ratio is 52%,the impact toughness is 65J,this study can provide a theoretical basis for the strengthening mechanism and performance control of the martensitic stainless steel during the composite casting-rolling process.The quenched microstructure is lath martensite with a large number of dislocations,a small amount of austenite and carbide,while the quenched and tempered structure is tempered martensite+residual austenite+cementite.After tempering,the hardness significantly decreases and the toughness increases obviously.The ideal process parameters are to quench at 1040℃in an oil-cooling medium and to temper at 650℃by air-cooling,where final hardness is averaged about 313HV,with an elongation of 17.9%,the cross-area reduction ratio is52%,and the impact toughness is about 65J,respectively.The study above provides a theoretical basis for the strengthening mechanism and performance controlling of the martensitic stainless steel during the composite casting-rolling process.