Stress State And Properties Of Fe-Cr-Ni Alloy By Laser Cladding

High strength martensitic steels have good comprehensive mechanical properties and are widely used in heavy duty and high temperature parts under harsh service environment.Parts are prone to wear,cracks and other local defects in the service process,which affect the stability of the service.Therefore,it is of great significance to repair the non-seriously damaged parts with high quality.Laser cladding repair technology has the advantages of high energy density,fine solidification structure and accurate repair.However,the residual tensile stress generated during the repair process will cause deformation or even cracking of parts so as to reduce fatigue life and seriously affect the reliability of parts repair.In response to the above problems,this article reduces the martensitic transformation temperature by adjusting the chemical composition of the steel,and uses the volume expansion effect of the martensitic transformation at low temperatures to reduce the residual stress level in the cladding layer.The solidification process and crystal growth mechanism of the cladding layer with different martensitic transformation temperatures and residual stress states were studied.Based on the reduction of residual stress,the martensitic steel was simulated repaired and the mechanical properties were tested.In this paper,the composition of low-temperature transformation alloys was designed according to the influence of alloying elements on martensitic transformation.The powders with various martensitic transformation temperatures were prepared and deposited by laser cladding and tested for residual stress.The influence of C,Cr and Ni elements on the residual stress of cladding layer was studied.When the carbon content is 0.1%(wt.%)and the mass fraction of Cr and Ni is 9%?18%,residual compressive stress can be formed in the cladding layer.The effect of martensite start temperature(Ms)on the residual stress of cladding layer was summarized.The residual compressive stress comes into being in the cladding layer when the Ms temperature is between 250?and 360?.The microstructure of cladding layer with different phase transition temperature and residual stress states were analyzed by metallographic microscope and scanning electron microscope.The results show that the martensitic transformation in the1Fe9Cr7 Ni cladding layer is incomplete,and the clear dendritic boundary of the original austenite is preserved in the microstructure at room temperature,in which the phase composition consists of lath martensite,carb-chromium compound and residual austenite.Relatively,the martensitic transformation of 1Fe4Cr2 Ni cladding layer is more complete,and the microstructure at room temperature mainly consists of lath martensite and a small amount of acicular martensite.The crystal orientation,grain size and microdeformation inside the cladding layer were investigated by electron backscatter diffraction(EBSD)technique.The results show that the grain orientation in the 1Fe4Cr2 Ni cladding layer is randomly distributed,while the 1Fe9Cr7 Ni cladding layer has obvious texture characteristics along the deposition direction.The average martensite grain size of the two cladding layers is 1.48?m and 1.51?m,respectively.The non-uniform shape variable in the 1Fe4Cr2 Ni cladding layer is larger,which indicates that the residual stress is larger,which is in accordance with the results obtained by the blind hole method.By simulating the repair of QP980 steel,the process of laser cladding repair of martensitic steel and the evolution of the residual stress in the repair area were studied under the constraint conditions.The results show that the phase change stress generated at low temperature can greatly reduce the residual stress in the repaired area.Based on the reduction of the residual stress in the repaired area,the mechanical properties of the repaired samples were tested.The tensile strength of the repaired samples and the base metal are 1073 MPa and 1026 MPa,respectively,while the elongation is 19.9% and15.5%,respectively.The fracture of the repaired samples occurs in the base metal region,indicating that the strength of the repaired samples is not lower than that of the base metal.The hardness of the repaired zone is 42.6HRC,which is higher than that of the base metal,but the hardness of the heat-affected zone is 32.3HRC.