Study On Shot Peening And XRD Characterization Of High-strength Dual Phase Steel

High-strength dual phase steels containing martensite and retained austenite have high strength and hardness; and are widely used in the fields of heavy-duty or high-speed bearings and gears due to their superior properties. Shot peening(SP) is a well-known and effective surface modification technology to improve the surface properties of components by improving the stress distribution and microstructures. In this work, the effects of different SP parameters on the residual stress field, phase transformation and microstructures in martensite and retained austenite were discussed, and the residual stess distribution in and between the martensite and retained sustenite were also investigated by the finite element modeling(FEM) analysis. The mechanism for the change of residual stress and microstructure during SP were also discussed in detail. The residual stress relaxation and microstructure change for the shot peened surface of high-strength dual phase steels under the condition of annealing or loading were analyzed. In addition, the mechanical properties of the high-strength dual phase steels after SP and the strengthening mechanism were also discussed in detail.Experiments were conducted to investigate the effects of different SP techniques on the residual stress for the martensite and austenite of high-strength dual phase steels by X-ray diffraction(XRD) method. The results showed that SP induced the compressive residual stress(CRS) within the deformed layer. With the increase of SP intensity or step, the CRS increased. After tiple-step SP, the maximum CRS of-1463 MPa and-1039 MPa were obtained for the martensite and austenite of 18 Cr Ni Mo7-6 steel at the depth of 20 μm respectively, while the surface CRS for martensite and austenite attained-1256 MPa and-766 MPa, respectively. Moreover, with the increase of SP times, the residual stress distribution was more uniform at the surface.In order to obtain the residual stress distribution in the phase and phase boundary after SP, FEM was utilized to analyze the residual stress distribution after SP. According to the result of single shot impact model, martensite and austenite all had the CRS, and CRS for martensite was higher than that for austenite,stress concentration occurred at the phase boundary. According to the result of 8-layers impact model, CRS existed in two phases and their interface. CRS induced by SP increase first and then decrease with increasing depth. The simulation values are coincided well with the experiment values.The thermal relaxation of CRS after SP was investigated. At the starting point of annealing, the CRS was relaxed more dramatically. The higher temperature and longer time resulted in the faster relaxation rate. The process of thermal relaxation can be described via Zener-Wert-Avrami function. According to the regression analysis, the activation energies of CRS relaxation for 18 Cr Ni Mo7-6 steel and GCr15 steel were 108 and 115 k J/mol, respectively.The residual stress relaxation of high strength dual-phases steel after SP occurred under the loading. Under static loads, the larger applied loading could induce the more obvious CRS relaxation. Under cyclic loads, the CRS relaxation occured and the CRS relaxation rate was faster in the initial loading stage, the CRS became stable when the number of cycle reached a certain value. The larger cyclic loading resulted in the more obvious CRS relaxation, and the corresponding stable CRS was smaller. Under the cyclic loads of 500, 600 and 700 MPa, the CRS decreased more drastically after 10, 5 and 3 cycles, and the corresponding CRS decreased by 26.3 %, 52.3 % and 71.2 %, respectively.The effect of SP on FWHM for martensite and austenite was studied. The result showed that FWHM for martensite decreased first, and then increased while FWHM for austenite decreased gradually with increasing depth. The domain size and microstrain were obtained using various XRD line profile analysis procedures. Domain size for martensite and austenite phases all attained the minimum values at the top surface after SP, and then increased with the increase of depth. For martensite, microstrain decreased first, and then increased while microstrain for austenite decreased gradually with increasing depth. During SP, the change of microstrain for martensite and austenite is different; the martensite generated the cyclic softening as well as cyclic softening and hardening, while the austenite only generated the cyclic hardening. With increasing SP intensities or times, the variability of structure increased, and the distribution of domain size was more uniform.During the heating process, the microstructure was changed, the domain size of the high-strength dual phase steels increased while the microstrain reduced gradually with both increasing temperatures and times. For 18 Cr Ni Mo7-6 and GCr15 steel, the activation energy of grain boundary migration were 153 and 166 k J/ mol, respectively; and the relaxation activation energies of microstrain restoration were 131 and 136 k J/mol, respectively.The microhardness of the high-strengh dual phase steels after SP decreased first, and then increased with an increase of depth, the maximum hardness appeared on the top surface. After triple-step SP, the microhardness of 18 Cr Ni Mo7-6 steel attained 937 HV, which was improved 56 % compared to the unpeened smaple. With the increase of depth, the microhardness after SP decreased first, and then increased, corresponding to cyclic hardening and softening. The change of microhardness after SP can be ascribed to the changes of compressive residual stress and microstructure during SP. According to the contributions of stress and structure to the hardness, the relational expression between microhardness and residual stress and FWHM has been estimated; the contribution of stress to the hardness cannot be ingored. With the increase of SP times, the hardness distribution after SP was more uniform.