| Springs used in high-speed train bogie always work under larger dynamic and impact load, it is one of the key parts in the train to ensure the train running safely. However, they are generally faced with the problems of short life and poor reliability, so, the influence of microstructure and surface integrity on fatigue properties was studied by controlling the microstructure of 60Si2 CrVAT spring steel and adopting different surface strengthening. With reference to the actual rolling process of the spring, the relationships among the microstructure, the surface integrity and fatigue performance of 60Si2 CrVAT spring steel were studied from three aspects of the hot deformation process, the quenching process and surface treatment and the mechanism of impact fatigue was also explored in this paper. The objectives are to build the anti-fatigue manufacturing technology of the spring and then optimize the spring production technology, provide theoretical basis and technical guidance for spring produce in future. The following main conclusions are obtained:1. The stress-strain curves of 60Si2 CrVAT spring steel during high temperature tensile deformation exhibited characteristics of dynamic recrystallization(DRX), established the equation of DRX, thus obtained parameters for the spring hot rolling process. The modified Arrhenius model, which is a hyperbolic sine function including the material parameter function of α(ε)、 n(ε)、 Q(ε) and A(ε), is used to describe the hot tension constitutive equations of 60Si2 CrVAT spring steel. The hot processing map shows that the instability area is easily occurred in high strain rate deformation zone, so as to establish the safety processing zone.2. The maximum cooling rate of 13%PAG is greater than that of oil about 90℃/s, and the comprehensive mechanical properties of 60Si2 CrVAT spring steel quenched by 13%PAG are superior to those of oil quenching.The fatigue limit of 13%PAG quenching(781.5 MPa) is higher than the oil quenching(714.0 MPa). From microstructural results, it suggests that the beneficial effect on the fatigue property of the 13%PAG quenched steels is caused by the sample of 13%PAG distribute many nano-twins,the thickness of the individual lath and block is less than that of oil quenched and fine carbide uniformly distributes in martensitic matrix, while many coarse carbides of oil quenched sample distributes in martensite lath and prior austenite grain boundaries.3. The fatigue life of PAG quenching increases after shot peening, decreases after ultrasonic rolling, while the fatigue life of oil quenching is improved obviously by shot peening and rolling. The fatigue limit of oil quenched polished specimen was 714 MPa, and raised to 771 MPa after shot peening; the fatigue limit of PAG quenched polished specimen was 782 MPa, and raised to 812 MPa after shot peening. Fractographs show that most fatigue damage originates from the internal inclusions, shot peening and rolling push fatigue source to the inside of the specimen, especially the rolling sample of oil quenching fatigue source close to the center of the specimen, the fracture surface is in a step shape, while the PAG quench rolling fracture is very smooth.4. The surface of all the states are residual compressive stress, rolling more than 1000 MPa, and the residual stress of PAG is higher than that of oil, the maximum residual stress is located at the subsurface layer of about 80μm, residual stress increases slowly with the increase of depth to the maximum compressive stress, and then slowly decreases to zero, the residual stress layer depth is about 300μm after shot peening, and the rolling depth is about 700μm. The surface microstructure of shot peening and ultrasonic rolling sample was refined, and the thickness of shot peening layer was about 140μm, while the thickness of rolling layer was about 280μm. Surface microstructure showed gradually coarsening gradient distribution from the surface to the center, it is difficult to distinguish surface layer microstructure of ultrasonic rolling sample, and the degree of refinement and the layer depth were significantly larger than shot peening.5. The improvement of the fatigue properties of oil quenched shot peening, rolling and PAG shot peening can be attributed to the surface residual compressive stress and formation of the gradient structure and a high structural homogeneity. While the reason for reduction fatigue life of PAG rolling is due to high hardness of PAG quenching specimen, thus, it is easy to form a micro crack because of high frequency shock induced of fragmentation during rolling process, meanwhile, release of residual compressive stress promotes rapid crack propagation under external load. |
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