Study On Microstructure And Mechanical Properties Of 40Cr Steel Diffusion Welded Joints

40Cr steel has excellent comprehensive mechanical properties,is widely used in the manufacturing field.Its usual welding methods is difficult to avoid the fusion zone and the heat affected zone,thereby greatly shortening the life of welded components.Diffusion welding can be welded in the solid state,which needs lower temperature and has little effect on base metal.This paper focuses on the compositions,microstructures and mechanical properties of 40Cr steel diffusion welding joint in welded and quenched and tempered states.The main results are shown as following:(1)The metallographic observation showed that no bonding interface was observed in the welded joints.The microstructure of the bonding in welded joint was ferrite and pearlite,which transformed tempered sorbite after quenching and tempering.Furthermore,the microstructure morphology of the two states were the same with their base metals.(2)Analysis of composition by point scanning,line scanning and surface scanning showed that the elements were evenly distributed in the welded joints,no element segregation and composition gradient were along the bonding.(3)The basic mechanical property tests showed that tensile specimens of joints in welded and quenched and tempered states fractured in base metal,indicating the strength of the joints were not less than the base metal.The tensile strength in two states were 748.6MP and 835.2MPa,the yield strength were 360.6MPa and 669.2MPa,elongation were 14.9%and 11.7%.The tensile fracture in two states exhibited many dimples,so the fracture mode was ductile fracture.The hardness of the two was almost a line perpendicular to the bonding and without fluctuations on the whole.The impact energy of base metal,welded joints before and after quenching and tempering were 35.0J、34.7J and 46.0J、45.7J.(4)High cycle fatigue test results indicated that fatigue specimens of joints in welded and quenched and tempered states fractured in base metal,which showed the strength of the joints were not less than the base metal.The fatigue strength of welded joints with 50%and 95%survival probability were 261.382MPa and 253.205MPa,which were 381.939MPa and 366.488MPa after quenching and tempering.The crack initiation site,the fatigue crack propagation area and the fatigue fracture area could be observed in fatigue fracture.Fatigue cracks started at specimen surface.Fatigue crack propagation area existed fatigue striations,fatigue steps and tearing ridge,secondary cracks and dimples were observed on some of the areas.(5)40Cr steel diffusion welded joints have a good bond in interface,exhibiting integration of joint and base metal in this paper.