Research On The Heat Treatment Process And Microstructure Properties Of 28Mnb5 Steel For High-speed Plow

At present,the boron steel represented with 28MnB5 steel is gradually replacing the traditional 65Mn steel as the key material in the field of high-speed plow due to its excellent mechanical properties,quenching and cracking resistance,high hardenability and hot forming.However,the existing 28MnB5 steel treated by the Quenching-Tempering（QT）process still suffers from insufficient hardness and toughness,which has become an important technical bottleneck restricting the development of the high-speed plow industry.In order to solve this problem,the Normalizing-Quenching-Tempering（NQT）process was adopted to obtain the NQT28MnB5 steel with both fine grain size and high strength and toughness,and the Normalizing-Carburizing-Quenching-Tempering（NCQT）process was used to further improve the surface hardness and wear resistance of the NCQT 28MnB5 steel.On this basis,the strengthening and toughening mechanism of NQT 28MnB5 steel was discussed,and the abrasive wear failure mechanism of NCQT and NQT 28MnB5 steel was compared and analyzed.The main research contents and results are as follows:The highest strength and toughness of NQT 28MnB5 steel were achieved when the normalizing,quenching and tempering process parameters were selected as920℃×1.0 h,880℃×0.5 h and 200℃×2.0 h,respectively.Compared with the QT28MnB5 steel,the yield strength,tensile strength and impact absorbed work of the NQT 28MnB5 steel treated by the above process were increased by 5.2%,5.2%and5.5%,separately.This is mainly attributed to the effective refinement of both the lath martensite and the martensite subgrain structure of 28MnB5 steel after the NQT process,which not only contributes to the improvement of toughness by increasing the resistance to crack expansion and changing the crack expansion path,but also helps to enhance the resistance to dislocation movement to achieve the strength increase.In addition,since the NQT process does not significantly change the solid solution C content in the martensite,the hardness of NQT and QT 28MnB5 steel is approximately equivalent.In purpose to further improve the surface hardness and wear resistance of28MnB5 steel,NCQT 28MnB5 steel with a carburizing layer thickness of about 2.2mm and a surface hardness（723 HV₁）significantly higher than that of the core（490HV₁）was obtained by the Normalizing-Carburizing-Quenching-Tempering process.As the distance from the surface of the carburizing layer increases,the carbon concentration of NCQT 28MnB5 steel gradually decreases from the surface（0.94wt.%）to the core（0.30 wt.%）,which leads to the transition from the surface to the core to acicular（twin）martensite+residual austenite,acicular martensite+lath martensite+residual austenite and lath martensite in that order.In the meantime,the M_s point increases as the carbon concentration in the carburized layer decreases,leading to a reduction in the driving force required for martensitic phase transformation,further causing the martensitic phase content and size to gradually increase with the distance from the surface of the carburized layer,and the residual austenite phase content and size to gradually decrease.The comparative study of the wear resistance of NCQT and NQT 28MnB5 steel showed that the average friction coefficient of NCQT 28MnB5 steel did not change remarkably as the friction load（5 N,10 N and 15 N）increased,while the average friction coefficient of NQT 28MnB5 gradually increased,and the overall friction coefficient of the former was smaller than that of the latter.Furthermore,the abrasive wear mechanisms of both NCQT and NQT 28MnB5 steel contain micro-cutting,micro-ploughing and oxidation wear mechanisms.With the extension of the wear time,the wear depth on the wear surface of NQT 28MnB5 steel accelerates to increase and the number of spalling pits also gradually grows,resulting in an increased rate of material wear weight loss.Meanwhile,the initiation and propagation of microcracks on the wear surface lead to material delamination,which further reduces the wear resistance of NQT 28MnB5 steel.Unlike the NQT 28MnB5 steel,the wear surface morphology of the NCQT 28MnB5 steel is flatter and the rate of increase of the wear depth is slower,which is mainly related to the high hardness of the carburized layer and the stress/strain-induced martensite phase transformation strengthening mechanism of the residual austenite.Based on this,further field experiments showed that the wear loss of the NCQT plow tips was reduced by about 35.6%compared to the NQT plow tips made from 28MnB5 steel,and its wear performance was significantly improved.