Study On The Properties And Strengthening Mechanismes Of A New Axle Steel Surface Combined Treated By Discrete Laser Hardening And Ultrasonic Rolling

High power density is one of the most important development directions in vehicle engineering field.By miniaturizing the structure of key components,increasing torque density and rotational speed,higher energy utilization and lower emissions can be achieved in high power density vehicles;however,it also deteriorates the working condition of part surface.Currently,apart from continuous development and application of new materials,surface strengthening has become the main technique means to improve the material surface properties.25 Cr Ni2Mo V steel is a new medium carbon alloy steel developed for the transmission system of high-speed and heavy-duty vehicles,which makes it has higher requirements in surface quality and precision.For this purpose,a novel surface strengthing method was proposed in this work by combing the discrete laser surface hardening(DLSH)and ultrasonic surface rolling(USR)treatment,to further improve the surface service performance of 25 Cr Ni2Mo V steel.The proposed process not only keeps the excellent wear resistance and small thermal deformation of laser hardened surface,but also eliminates the adverse effects of self-tempered soft zone and residual tensile stress outside the laser hardened zone.The discretization laser processing method effectively minimizes surface deformation and thermal stress while enhancing strengthening efficiency;thus,it is suitable for the surface strengthening of parts with high surface accuracy requirements and a large hot cracking tendency.In this paper,firstly,the DLSH treatment and further combined USR treatment were subjected to the 25 Cr Ni2Mo V steel surface.Combined with the characterization of microstructures,phases,hardness,and the finite element simulation results,the process parameters of combined strengthening were optimized,and the strengthening mechanisms of material were clarified.Finally,the anti-sliding wear,mechanical tensile,and contact fatigue properties of strengthened surfaces under optimized process parameters were experimentally evaluated,and the improving mechanisms of its service performance were discussed.The main research and conclusions are as follows:By studying the surface roughness,microstructures,phases,and microhardness distribution of DLSH treated and combined USR treated samples,the optimum strengthening parameters and corresponding material strengthening mechanisms were summarized,respectively.It is found that the laser hardened spot with high surface quality(Ra=0.75～1.5 μm)and sufficient hardening depth(≥0.2 mm)can be obtained when the laser power is controlled in the range of400～500 W.USR treatment can significantly improve the surface quality of samples,and further increase the surface hardness of DLSH treated sample by introducing a large number of deformed dislocations and twins to form a surface fine grain layer with gradient structure.Based on the theoretical analysis of physical fields during laser hardening and ultrasonic rolling,the finite element models of the DLSH and USR treatment were established,respectively.After that,the predictive validity of established models for the temperature,phase,hardness and residual stress during combined strengthening was verified by experiments,and the evolution of material properties under different processing parameters was also further explored.The results show that the finite element model accurately predicted the changes in main material properties during DLSH and USR treatment.With the increase of laser power and irradiation time,the residual stress in the laser hardened zone will gradually change from tensile stress to compressive stress,and then the compressive stress decreases again.During combined USR treatment,the surface deformation displacement and residual compressive stress increase with the increase of static pressure and ultrasonic amplitude.Under excessive ultrasonic frequency,the increased amplitude of surface deformation displacement and residual compressive stress decreases due to the Bauschinger effect.Based on different optimized process parameters,the sliding wear tests,room temperature tensile tests,and contact fatigue tests were carried out,and the strengthening mechanisms in the process of wear and failure were analyzed,respectively.The results show that DLSH treatment significantly delayed the occurrence time of severe surface scuffing failure under starved lubrication conditions.With reasonable optimization of laser parameters and hardened spot density,DLSH treated surfaces can obtain a similar or even lower surface wear rate than that of fully hardened surface.After further combined USR treatment,the plastic deformation strengthening of surface makes the sample exhibited better friction and wear resistance.The strengthening of the self-tempered soft zone after USR treatment increased the threshold of the crack initiation in the subsurface layer during the tensile test,and the mechanical properties of DLSH treated sample were significantly improved.The surface melting is not conducive to the contact fatigue resistance of the DLSH treated surface,and the combined USR treatment can significantly increase the contact fatigue life of the DLSH treated surface by inhibiting the initiation of surface fatigue cracks.