Microstructure And Performance Of Tantalum And Chromium Alloying Layer On M50 Steel Induced By High Current Pulsed Electron Beam

Magnetron sputtering and high current pulsed electron beam?HCPEB?are introduced in this research for surface alloying treatment on M50 steel in order to increase the service life of bearings in extreme environment.The research about the microstructure and formation of defects have been studied.The main purposes of this research are to reveal the non-equilibrium solidification and strengthening mechanism of the nano-sized precipitates in the surface alloying layer and to provide fundamental results for widely application and detailed researches.In the meantime,alloying layer with high hardness,low friction coefficient and high performance in corrosion resistance have been obtained.After surface alloying treatment by HCPEB,the surface morphology is sensitive to the surface alloying parameters and types of the pre-deposited films.And surface lamination,micro-cracks and crater-like defects have been found after surface alloying treatment.Gap of thermal properties,accumulation of energy at sub-layer and precipitation of intermetallics are the main factors which is related to the formation of defects.And some defects are caused by synthetical factors.The premium parameters for surface alloying treatment has been obtained by selected parameters about energy density,pulse number and thickness of pre-deposited film to eliminated the defects.After surface alloying treatment,the pre-deposited film is mixed with M50 steel substrate by metallurgical bonding.There is a “white layer” at the top of treated samples and the thickness is about 4-6 ?m.The heat-affected layer is composed of plate martensite and carbide particles with thickness of 10-20 ?m.The parameters of surface alloying have a significant influence on the microstructure.The results reveal that alloying layer can be divided into several layers.The Ta alloying layer are composed of ?-Fe phase with spinodal and small hollow spherical structure due to the segregation of alloying elements especially Ta and martensie.The Cr alloying layer can be divided into Fe-Cr solid solution,austenite and martensite.In the Fe-Cr solid solution layer,the spinodal microstructure can also be found with a small amount of amorphous phase in partial zone.The solidification interface is moved as plane state,cellular state and then back to plane state after surface alloying treatment by HCPEB,which is influenced by the factors of solidification rate,temperature gradient,degree of supercooling and concentration of alloying elements.After solidification,the nano-sized spinodal networks are caused by the short-ranged segregation through diffusion of alloying elements.After tempering treatment,the nano-sized spinodal networks in surface alloying layer provide ideal environment for the precipitation and growth of nano-sized carbide particles.During tempering treatment,a great number of defects at surrounding areas of spinodal networks provide idea place for nucleation of carbides.In the meanwhile,the segregation of alloying elements at the spinodal networks are importance supply for nucleation and growth of carbides.Ta C and M23C6 have been observed in the alloying layer after tempering treatment and the mean size are about 5-8 nm.The results indicate the parallel relationship between the following two axes: [011]?-Fe//[001]Ta C and [113]Fe-Cr//M₂3C₆[122]After surface alloying treatment and corresponding tempering treatment,there are significantly improve in the field of surface hardness,wear and corrosion resistance comparing with the original M50 steel.The surface hardness has a slightly drop only after surface alloying treatment;while the precipitations of carbides after tempering treatment help to improve the hardness of Ta and Cr alloying layer to 17.3GPa and 18.2GPa respectively.The tempering equation related to the temperature and tempered duration has also been obtained.After surface alloying treatment,the tensile stress can reach to 700 MPa at the surface layer.However,the tensile stress significantly drops after tempering treatment,or even to compressive stress.During frictional tests,the oxides with low friction coefficient form and serve as solid lubricant,which help to reduce the friction coefficient.The corrosion resistance of the specimen after surface alloying treatment is significantly improved and the solid solution is believed to contribute to the formation of passive film to increase the corrosion resistance.It can be concluded that surface alloying treatment induced by HCPEB has improved the performance of M50 steel effectively.This method can be applied for the surface modification to the bearing in extreme service environment.And it also proved to be a reliable technical mean for increasing the service life for bearing.