Fatigue Crack Propagation Behavior And Wear Performance Of Laser Remelted 40Cr Steel

40Cr steel has good comprehensive mechanical properties after quenching and tempering treatment,and is commonly used in machining shafts,connecting rods and gears.However,during the service process,these components would be subjected to dynamic loads such as alternating,shock and vibration,and the faults caused by fatigue and wear are greatly increased,which will cause unpredictable losses and safety hazards.In this paper,the microstructure and mechanical properties of the specimens after laser surface remelting?LSR?treatment were obtained by characterizing the surface microstructure and stress state of the treated specimens.Based on the data of crack propagation rate,friction coefficient and wear amount,the fatigue crack propagation and friction and wear mechanisms were established.Meanwhile,the feasibility of LSR treatment for improving the service life of quenched and tempered 40Cr steel were explored.After LSR treatment,the microstructure of the melting zone was composed of fine martensite and carbide particles,and the microstructure of HAZ was mostly composed of the structure of the melting zone and the substrate zone.The microstructure of the overlap zone was composed of secondary quenching zone and different degrees of martensite tempering zone.The surface average hardness after LSR treatment was about 378 HV that was increased by 45%compared with the hardness of the substrate.After LSR treatment,the residual tensile stress was distributed in the transverse direction,and the maximum value of the residual tensile stress was 310 MPa.In the thickness direction,the residual tensile stress was rapidly reduced to zero and converted to residual compressive stress with the increase of the depth,the maximum residual tensile stress and residual compressive stress were 310 MPa and 133 MPa,respectively.The Paris constants C and m of the LSR specimens are 7.00874×10^-8and 3.02707,respectively,and the C and m of the untreated samples are 2.64345×10^-77 and 2.71917,respectively.According to the expression of fatigue crack propagation rate,the fatigue crack propagation rate of quenched and tempered 40Cr steel after LSR treatment decreased significantly.It was found that the refinement of the microstructure and the generation of residual compressive stress on the secondary surface after LSR treatment were the main reasons for the decrease of crack propagation rate.According to the friction coefficient and the wear amount data,the wear amount and friction coefficient of the specimens after LSR treatment were significantly lower than that of the untreated specimens,indicating that the LSR treatment can improve the wear performance of the quenched and tempered 40Cr steel.For untreated specimens,the wear mechanism gradually evolves from adhesive wear and abrasive wear to fatigue wear as the load increases.For untreated specimens,the wear mechanism evolved from adhesive wear and abrasive wear to fatigue wear with the increase of load.For LSR specimens,abrasive wear was the main wear mechanism of the whole wear process,accompanied by different degrees of oxidative wear and slight adhesive wear.