Study On Controllable Hardening Of The Contact Surface Of Metal Based On Laminar Plasma Jet And The Effect On Their Service Performance

For some key mechanical parts such as rails,molds,bearings,and so on,surface damages are easy to occur under various complicated contact conditions,while the difference of the contact conditions will affect the surface damages.In typical rolling contact conditions,such as wheel and rail,the types of surface damage are usually wear and rolling contact fatigue failure,while in typical pure sliding and corrosion conditions,such as injection mold steel,corrosion wear,thermal fatigue,and mechanical fatigue are dominant.Surface damages under different contact conditions will reduce the service accuracy and life of the corresponding parts,leading to high maintenance and replacement costs.To improve the service life of these key parts,it is necessary to strengthen the contact surface of the metal under different contact conditions.Laminar plasma jet(LPJ)surface hardening is an effective method for surface strengthening.To improve the service performance of the contact surface of the metal under different contact conditions by using LPJ surface hardening.It is necessary to have a deep insight into the effect of LPJ surface hardening on the service performance of the metal contact surface.Besides,to prepare a hardened layer with ideal geometrical dimension,microstructure,and hardness distribution controllably,it is necessary to clarify the hardening mechanism of LPJ surface hardening.Therefore,in this study,U75V rail steel whose working condition is typical rolling contact condition and P20 mold steel whose working condition is typical pure sliding and corrosion condition were treated by LPJ.Further,the influence mechanism of working parameters on the geometrical dimension,microstructure,and hardness distribution of the hardened layer was clarified by numerical simulation and materials characterization.Finally,the effects of LPJ surface hardening on the service performance of the metal contact surface under typical rolling contact conditions and pure sliding and corrosion conditions were clarified by a series of experiments.The main research contents and conclusions of this thesis are as follows:(1)The effect of working parameters of the LPJ surface hardening on the hardened layer was investigated.The results show that:By increasing the arc current or decreasing the scanning velocity,a hardened layer with a larger size can be obtained.Besides,by setting a small anode nozzle,a deeper and narrower hardened layer can also be achieved.Therefore,the size and shape of the hardened layer can be adjusted by changing several working parameters of the LPJ surface hardening.In addition,based on the LPJ surface hardening,the hardened layer with a hardness gradient can be effectively prepared.The hardened layer was composed of the full hardened layer and the transition layer.The full hardened layer consisting of the complete martensite has a hardness of approximately 800 HV.While the hardness of the transition layer,whose microstructure is a mixture of martensite,pearlite,ferrite and carbides,gradually decreases from 780 HV to 380 HV.To obtain the hardened layer with a hardness gradient,a larger anode nozzle should firstly be selected.And then a suitable arc current should be set.Finally,a relatively small scanning velocity should be used.(2)The temperature field of the hardened layer was simulated by ANSYS software.The hardening mechanism of the LPJ surface hardening was clarified.The results show that:Obtaining a desired temperature field within the heat affected zone over the critical austenitizing temperature is the key to adjust the geometrical dimension of the hardened layer.By changing the arc current or anode nozzle to adjust the FWHM of the heat flux of LPJ applied on the workpiece surface or changing the scanning velocity to adjust the heating time,the temperature field within the heat affected zone(maximum temperature)of the hardened layer can be changed.A relatively slow cooling rate below the critical austenitizing temperature is the key to produce the transition layer with a hardness gradient.The microstructure with the mixture of martensite,pearlite,ferrite and carbides in the hardened layer was attributed to the relatively slow cooling rate.(3)The service performance of the rail sample after LPJ surface hardening whose working condition is typical rolling contact condition was studied by a series of rolling contact wear tests.The results of the study on the wheel-rail system showed that the LPJ surface hardening can significantly improve the service performance of the contact surface of the metal under the rolling contact condition.In this study,the wear resistance of the rail pair with double-row circular hardened spots was the best.The wear rate was 0.48 times that of the untreated rail pair.Besides,the total wear rate of the wheel/rail pair whose rail pair had single-row circular hardened spots single and the 24°distance between the hardened spots is the lowest,and the total wear rate was 0.87 times that of untreated wheel/rail pair.The microstructure of the hardened zone obtained by the LPJ discrete surface hardening was martensite with high hardness.The martensite structure has high resistance to plastic deformation,which greatly increases the wear resistance of the hardened area.Furthermore,the existence of a hardening zone can alleviate the plastic deformation of the untreated area in the contact surface of the rail pair.The depth of the surface plastic deformation of the substrate around the hardened zone after LPJ discrete surface hardening was at most 0.116 times than that of the untreated rail surface.The LPJ discrete surface hardening can improve the plastic deformation resistance of the rail specimen surface under rolling conditions,thus the wear resistance of the rail pair can be greatly improved.(4)The ball-on-disk tribological tests and the electrochemical corrosion tests were conducted to investigate the wear behavior and the corrosion behavior of the LPJ treated P20 mold steel whose working condition is typical pure sliding and corrosion conditions.Attributed to the ultra-fast cooling rate in the LPJ treatment,the P20 mold steel's microstructure transforms from tempered martensite to compact lath-martensite,resulting in the increase of hardness from 300 HV to about 600 HV.Besides,during the austenitizing process,a mass of plate-like carbides(Fe₃C)and spherical carbides(M₃C,M=Cr,Mn,Mo)were dissolved,resulting in the homogenization of the Cr element.LPJ surface hardening treatment can significantly improve the wear resistance of the pre-hardened P20 mold steel.The tribological test results showed that the average COF of the sample treated by LPJ decreased by about 1.7 times,while the wear rate decreased by about 2.3 times compared with the untreated sample.The wear mechanism of the treated sample changed from abrasive wear combined with adhesive wear to mild oxidative wear.The sample treated by LPJ presents significantly higher corrosion resistance compared with the untreated sample.The corrosion rate of the untreated sample is0.2427 mm/year,whereas that of the treated sample is only 0.0680 mm/year.Such high corrosion resistance is mainly attributed to the compact lath-martensite structure and the homogeneously distributed Cr element.