Research On The Structure And Properties Of The Composite Layer On The Surface Of M50NiL Steel

With the rapid development of aerospace technology,China presents an increasing demand for improving surface performance of transmission parts.M50 NiL steel is widely used in the aerospace engines as a surface hardened steel with excellent performance.The carburizing and nitriding duplex treatment is complex,the surface microstructure and performance would be changed by changing parameters in every process.To date,the relationships between microstructure and performance of M50NiL steel with carburizing and nitriding duplex treated under different carburizing,heat treatment and nitriding parameters have never been reported.Thus,in order to understand the duplex hardening behaviors with different carburizing,heat treatment and nitriding parameters,this study was carried out.The diffusion mechanism and hardening mechanism in M50 NiL steel are revealed which are combined with the changes in precipitations and solid solution by alloying elements.This study provides a theoretical basis for the subsequent application and research of M50 NiL steel.In the carburizing process,the effects carburizing temperatures and pressures on the microstructures and performances of duplex treated M50 NiL steel are studied.The results show that the contents ofγ’in the composite layers vary with the carburizing temperature and pressure.An appropriate amount of carbon content leads to an increase in the number of nanometric precipitates M_x（C,N）,an increase of the solid solution by using nitrogen and the consequential increase in the hardness of the composite layer.However,a further increase in the carbon content leads to the growth of submicron MC precipitates,changes in the shapes of the nanometric precipitates,a reduction in the solid solution by nitrogen and an increase in the number of intergranular precipitates and a reduction in hardness.Improving carburizing pressure resulted in an increase in the number of small-sized precipitations and a decrease in the thickness of composite layer.The differences in size,shape,number of precipitations,surface carbon content and solid solution amount of alloying elements affect the diffusion rate of nitrogen atoms,and also affect the redistribution of carbon atoms.The enrichment of carbon and the carbides precipitated through supersaturated solid solutions by carbon is responsible for the hardening behavior.In the heat treatment process,the effects quenching and tempering temperatures on the microstructures and performances of duplex treated M50 NiL steel are studied.The results show that the increase in quenching and tempering temperature reduce the content ofγ’to a lower value.There are large M₂₃C₆ phases in the composite layers after quenched at 1000°C.With increasing quenching temperature,the M₂₃C₆ phase would be dissolved in matrix,the size and number of other precipitates gradually decrease.At the quenching temperatures of 1000°C和1050°C,the diffusion rate of nitrogen is controlled by the size and number of precipitates,which decrease the depth of the composite layer.At the quenching temperature of 1150°C,the diffusion rate of nitrogen is controlled by the solid solution number of alloying elements,which decrease the depth of the composite layer.Tempering at a low temperature（500°C）or a high temperature（580°C）will increase the number of precipitates,resulting in a thinner thickness.The high tempering temperature greatly decreases the hardness of the surface layer.After tempered at 580°C,the surface hardness reduced to 1027HV_0.1.The amount of solid solution by alloying elements and the size,shape of precipitates control the level of the transformation from carbides into nitrides.The larger the size of the precipitate,the harder it is to change;the granular shape has the best transformation level;the higher the solid solution amount of the alloying elements,the slower the transformation rate.In the nitriding process,the effects nitriding temperatures on the microstructures and wear performances of duplex treated M50 NiL steel are studied.The results show that theγˊphase increases with the increase of nitriding temperature.The nitriding temperature of 460°C makes the compound layer and the composite layer thinner due to the lower thermodynamic effect.The increase in temperature promotes the decomposition of carbides.After the temperature raised to 540°C,the diffusion of nitrogen atoms is decreased and the thickness of the composite layer is thinner.Under the load of 20 N,the 460N specimen has the lowest wear rate,2.73×10^-55 mm³/m;with the load increasing to 40 N,the 500N specimen has the lowest wear rate,6.55×10^-5mm³/m.The wear rate under 20 N is mainly related to oxidative wear,the wear rate under 40 N is mainly related to abrasive wear.The 500N sample has the strongest resistance to abrasive wear under high load due to its thicker compound layer and optimum surface hardness.