| 45CrNiMoVA steel is widely used in vehicles,bridges,buildings,and aerospace due to its high strength,high toughness,and high hardenability.In practical service conditions,the parts mainly endure high-frequency alternating loads,and the demand for their fatigue performance is increasing.Therefore,the requirements for the fatigue performance of the material are becoming higher.Surface mechanical strengthening technology is a processing method that utilizes tools to induce plastic deformation on the material’s surface,effectively improving its fatigue performance.Currently,research on surface mechanical strengthening mainly focuses on lowstrength model materials such as pure copper and 316 L stainless steel,while there is limited research on fatigue resistance technology for high-strength metal alloy materials with strong engineering applications,especially regarding torsional fatigue.This study optimized the heat treatment process of 45 CrNiMoVA steel and conducted surface mechanical rolling and ultrasonic rolling treatments using a 6mm diameter tungsten carbide ball.The surface roughness and morphology of the material were measured using an optical three-dimensional profilometer.Microstructures were observed using an optical microscope and scanning electron microscope,and residual stresses,mechanical properties,and fatigue properties were tested using an X-ray residual stress analyzer,MTS universal testing machine,and hydraulic servo torsional fatigue testing machine,respectively.This study analyzed the impact of rolling strengthening on the torsional fatigue life of the material and revealed the fatigue fracture mechanism of the specimens treated with rolling strengthening,providing reference for the application of surface rolling strengthening technology in industrial production.Firstly,this study investigated the effects of different cooling methods,including furnace cooling,air cooling,fan cooling,mist cooling,water cooling,and corresponding tempering,on the microstructure and mechanical properties of 45 CrNiMoVA high-strength steel.The results showed that with the increasing cooling rate,the microhardness and tensile strength of the material increased.The microstructure changed from ferrite and banded pearlite at low cooling rates to acicular bainite and martensite,with the microstructure under water cooling consisting almost entirely of martensite.After tempering,although the microhardness and tensile strength of the material decreased to some extent,they still showed an overall increasing trend with the increase of cooling rate,and there was no significant change in the microstructure before and after tempering.Secondly,this study investigated the microstructures and mechanical properties of45 CrNiMoVA steel treated with heat treatment combined with surface mechanical rolling and ultrasonic rolling.The study found that the surface of the samples exhibited obvious grain refinement,forming a gradient structure zone with a thickness of about 400 μm.The crosssection microhardness of the material showed a gradient distribution,with the highest hardness on the surface of the samples,which increased by 11% compared to the matrix and decreased to the same level as the matrix at a distance of about 400 μm from the surface.Tensile tests of the5 mm diameter cylindrical samples before and after rolling showed that the yield strength of the surface mechanically rolled samples increased by 9% compared to the original samples,the tensile strength increased by 10.4%,and the fracture elongation was 16.14%.The yield strength of the ultrasonic rolled samples increased by 15.86%,the tensile strength increased by 17.25%,and the fracture elongation was 14.52%.Finally,this study investigated the surface integrity,surface residual stress,and torsional fatigue properties of the 45 CrNiMoVA steel after rolling treatments.The results showed that the surface quality of the samples was significantly improved after rolling treatments,and the roughness was optimized by 177%.Rolling treatments introduced significant residual compressive stress on the sample surface,with the surface rolled samples showing an increase of 449% and the ultrasonic rolled samples showing an increase of 555% compared to the original samples.Torsional fatigue tests of 45 CrNiMoVA steel before and after rolling treatments under the same shear stress amplitude of 1020 MPa showed that the fatigue life of the original samples was 169,758 cycles,while the torsional fatigue life of the surface rolled samples increased to1,011,857 cycles,an improvement of approximately 496%,and the torsional fatigue life of the ultrasonic rolled samples increased to 2,044,689 cycles,an improvement of approximately1,104%.Analysis of the fatigue fracture surfaces revealed that the crack initiation in the original samples occurred on the surface,whereas in the surface rolled and ultrasonic rolled samples,the crack initiation shifted to the interior.Obvious plastic deformation was observed in the crack propagation zone,and cleavage planes and cleavage steps were observed in the instantaneous fracture zone.This indicates that the improved surface roughness,grain refinement,and introduced surface residual stress resulting from rolling strengthening have a positive effect on the fatigue performance of the material.They prevent fatigue cracks from initiating on the surface,enhance resistance to crack initiation,decrease the rate of crack propagation,and significantly improve the torsional fatigue life. |
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