| As the best strengthening method for metallic materials,grain refinement can simultaneously improve the various mechanical properties of steel,such as the strength,hardness,plastic,and wear-resistance.It is the preferred process for optimizing the comprehensive mechanical properties of materials.Compared with other grain refinement processes,the cyclic phase transformation grain ultrafine technology can refine the grain size to the micron level while ensuring that the chemical composition and external dimensions of the material remain unchanged,which is of great significance for grain refinement and performance improvement of steel parts.There is a problem that 65 Mn low alloy steel clutch disc of tracked vehicle usually failure and fracture due to insufficient strength and insufficient wear-resistance when they are performed.Thus,the 65 Mn low alloy steel clutch disc was taken as the research object in this paper.Then,cyclic phase transformation technology was selected to refine the grain of 65 Mn alloy.Besides,a serious of 65 Mn low alloy steels with different grain sizes were prepared.Finally,the influence of grain refinement on the comprehensive mechanical properties of 65 Mn low alloy steel was analyzed through isothermal stretching,isothermal compression,Rockwell hardness,and friction-wear experiments,which provides important guidance for the optimization of the comprehensive mechanical properties of 65 Mn steel friction disks.The main contents and conclusions of this paper were as follows:(1)Detailly discussed the effects of cyclic phase transformation process parameters,such as cycle temperature,cycles number and cycle mode,on the degree of grain refinement,and a reasonable experimental scheme of cyclic phase transformation was designed.(2)According to the austenite grain size characterization experiments,the microstructure and grain size grades under different cyclic phase transformation process paths were obtained.The influence of cyclic phase transformation process parameters on the evolution of microstructure was analyzed,and the mapping relationship between the cyclic phase transformation process path and the grain size grades was established.(3)Based on the mapping relationship mentioned above,a seious of 65 Mn low alloy specimens with different grain sizes were prepared,and the isothermal stretching and isothermal compression exprimentment were carried out.The experimental results proved that the tensile strength,compressive strength and fracture elongation of 65 Mn low alloy steel at high and low temperatures increase with the refinement of the grain size.Under the deformation temperature of 400 ?,600 ? and 800 ?,the balance of strength and ductility of 65 Mn low alloy steel is improved from 22.4 GPa%,19.1 GPa%,7.5 GPa% to28.9 GPa,25.5 GPa%,9.5 GPa% after the grain size is refined from 65 ?m to 6.4 ?m %.(4)Rockwell hardness exprimentment were performed on 65 Mn low alloy steels with different grain sizes.Under the same quenching temperature,the Rockwell hardness of 65 Mn low alloy after quenching increased as the grain size is refined.65 Mn low alloy steel was cyclically quenched from 1 to 4 times at quenching temperature of 810 ?.Consequently,the grain size was refined from 13.4 ?m to 6.4 ?m and its hardness was increased from 38.4 HRC to 62.9 HRC.(5)Friction-wear exprimentment were performed on 65 Mn low alloy steels with different grain sizes to analyze the effect of grain refinement on friction coefficient and wear-resistance.The experimental results show that grain refinement can shorten the time of "fluctuation period" of the friction coefficient during the friction-wear test process,and keep the friction coefficient constant during the "stability period".After the average grain size was refined from 65 ?m to 6.4 ?m,the cumulative wear of 65 Mn low alloy steel decreased from 52 mg to 9 mg.The grain refinement improved the wear-resistance of65 Mn low alloy steel. |
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