| 40CrNiMoA steel is a material widely used in mechanical key load-bearing structural parts and equipment.Mechanical structural components cannot avoid fretting fatigue under surface contact,fatigue load and vibration excitation conditions.Under fretting fatigue service conditions,the fatigue life of mechanical components is significantly reduced due to surface fretting damage,and the resulting fracture damage may cause catastrophic mechanical failures.Therefore,how to slow down the fretting fatigue damage and significantly prolong the fretting fatigue life is an important scientific problem.By using severe plastic deformation(SSPD)on the surface of metal materials,induced gradient nanostructure(GNS),obtained compressive residual stress(CRS)and surface work hardening(SWH)can greatly improve the surface wear resistance,resistance of metal materials.Corrosion(oxidation resistance)and fatigue resistance.These treatments have significant advantages in improving the fretting fatigue properties of metallic materials.Based on this,this paper uses ultrasonic surface rolling,abrasive water jet peening and ultrasonic surface rolling high-energy composite modification technology to modify the surface of 40 CrNiMoA steel,and conduct fretting on a self-designed fretting fatigue experimental device.Fatigue experiment.By combining confocal laser scanning microscopy(CLSM),X-ray diffraction(XRD),scanning electron microscopy(SEM),electron backscatter diffraction(EBSD),focused ion beam(FIB),transmission electron microscopy(TEM)and energy dispersive spectroscopy(EDS)and other detection and analysis methods to explore and compare the surface morphology,surface microstructure,residual stress,hardness and other surface integrity parameters of 40 CrNiMoA steel obtained under different surface modification treatments.The influence of the surface integrity parameters of40 CrNiMoA steel on the fretting fatigue properties was analyzed in depth,and the fretting fatigue mechanism of the material under different surface modification conditions was clarified.The main research contents and conclusions are:(1)The layer depth,surface morphology,surface microstructure,residual compressive stress and hardness of 40 CrNiMoA steel without surface treatment and ultrasonic surface rolling treatment with different process parameters were systematically studied.The changing law of related parameters.It is found that the surface integrity of 40 CrNiMoA steel after 9 passes and 900 N static pressure ultrasonic surface rolling treatment is better.The SSPD layer of the sample is 450 μm deep,and the surface residual compressive stress and maximum residual compressive stress are 919 MPa and 1251 MPa,respectively,which are increased by 554 MPa and886 MPa,respectively,compared with the untreated sample;its surface microhardness is 790.3 HV,compared with 481.5 HV of the untreated sample,an increase of 64.1%.(2)The microstructure of the surface layer of the 40 CrNiMoA steel treated by ultrasonic surface rolling was deeply studied,and it was found that the microstructure of the SSPD layer was equiaxed nanocrystalline,lamellar nanocrystalline,curved martensitic structure,and crystal grains.The size changes in a gradient pattern.Among them,in the outermost layer of the sample,the martensite grain size is refined to the nanometer level,the carbides are also significantly refined and dissolved,and the dislocation proliferation is obvious.The gradient structural strength of the material is jointly contributed by grain refinement strengthening,dislocation strengthening and precipitation strengthening.(3)The surface wear mechanism,fretting fatigue limit and fatigue mechanism of unmodified 40 CrNiMoA steel samples were systematically studied.The fretting fatigue limit of the unmodified sample was 330 MPa under the condition of 110 Hz and a positive pressure of 200 N.The research shows that the contact fretting wear surface of unmodified samples has comprehensive characteristics of oxidative wear,abrasive wear,adhesive wear and fatigue delamination.At this time,the fatigue crack source is initiated at the bottom of the contact edge wear micro-pit with more obvious stress concentration and more serious surface fretting wear.The initial crack propagation is at an acute angle of 50°to the surface parallel to the direction and expands to a certain depth,and then turns perpendicular to the test.Such rapid expansion to eventual buckling fracture.(4)The surface wear mechanism,fretting fatigue limit and fatigue mechanism of unmodified and different ultrasonic surface rolling 40 CrNiMoA steel specimens were studied.The results show that the fretting fatigue limit of the sample with 9 passes ultrasonic surface rolling is the highest(550MPa).Compared with the unmodified sample,the value increased by 66.7%.The GNS,induced CRS and SWH obtained by ultrasonic surface rolling can prevent the premature nucleation of fretting fatigue source,inhibit the rapid crack propagation and change the crack propagation path.At the initial stage of crack propagation,the Angle between the crack propagation direction and the surface parallel direction is about 20°,and the crack propagation path is serrated.It is found that the significant increase of fretting fatigue limit is related to the change of surface wear mechanism and the optimization of surface integrity.Compared with the untreated sample,the surface wear mechanism of40 CrNiMoA steel sample after ultrasonic surface rolling treatment is changed into adhesive wear,fatigue wear and slight oxidation wear.The surface wear degree is improved,the number of fretting fatigue crack sources is reduced,the crack initiation Angle is reduced,the fatigue strip edge boundary in the crack propagation zone is clear,the strip spacing is reduced,and the crack propagation rate is reduced.(5)The surface wear mechanism,fretting fatigue limit and fatigue mechanism of40 CrNiMoA steel samples treated with unmodified and abrasive water jet shot peening and ultrasonic surface rolling with high-energy composite surface modification were comparatively studied.Compared with the unmodified sample,the fretting fatigue limit of the composite modified sample increased by 59.1% to 525 MPa.The surface fretting wear mechanism is mainly characterized by adhesive wear,oxidative wear and delamination.The composite modification obtains obvious work hardening effect,reasonable residual compressive stress gradient,significantly increased number of grain boundaries,and improved surface morphology.Their synergistic effect affects the initiation and propagation of fretting fatigue cracks and improves the fatigue resistance of the material.However,due to the existence of a small number of folded micro-defects on the surface of the composite surface modified by abrasive water jet shot peening and ultrasonic surface rolling,the fretting fatigue performance of the composite modified sample is slightly lower than that of the ultrasonic surface rolling modified sample. |
PDF Full Text Request