Study On Active Screen Nitriding Process For M50 Steel

Bearings are important transmission parts in mechanical equipment.It is of great significance to improve bearings' performance for the development of all kinds of equipment.M50 bearing steel has good comprehensive performance,and it is widely applied for main engine bearings and bearings of medium and high temperature equipment.In order to further improve the performance and fatigue life of M50 bearing steel,the surface can be strengthened by nitriding technology to increase the hardness of the bearing surface and form a surface compressive stress layer.By rationally designing the nitriding process parameters,the bearing can be improved wear resistance and fatigue life of steel.The novel active screen plasma nitriding(ASPN)technique,has the advantages of good nitriding uniformity,avoiding edge effects and hollow cathode effects,and is suitable for bearings nitriding.In this paper,the temperature uniformity in the active screen is first tested.On the premise of ensuring uniform temperature,the active screen nitriding test of M50 steel discs,balls,and bearing ring cut samples is performed by different processes.The cross-sectional morphology of the nitrided sample was observed by the metallographic microscope.The X-ray diffractometer was used to analyze the phases of the sample before and after nitriding.The hardness distribution of the nitrided layer of the sample was tested using a Vickers microhardness tester.The distribution of elements in the nitrided layer was detected by scanning electron microscope energy spectrum test.The structures and properties of the samples at the floating potential and grounded potential during nitriding were compared.The nitriding uniformity of balls and bearing rings has been studied.The effects of these three nitriding processes—atmosphere,temperature,and time parameters—on the sample nitriding layer were studied.It was found that the samples treated under the two different potential states obtained the nitriding hardened layer with equal thickness.The phases of the nitriding layer were both ?N phase and nitride phase.Because the edge effect was avoided,the nitrided layer of the discs was uniform.However,because of the complex shapes of the balls and bearing rings,their nitriding layers were not uniform after being nitrided by ASPN.The nitriding layer thickness,nitrogen content and hardness of the samples subjected to nitriding treatment in a nitrogen-hydrogen mixed atmosphere are higher than those of the samples subjected to nitriding treatment in a nitrogen-argon mixed atmosphere.And as the content of N2 in the atmosphere increases,the thickness,nitrogen content and hardness of the nitrided layer of the sample increase.As the temperature increases,the infiltrated layer thickness,nitrogen content,and hardness of the sample increase.With the extension of the nitriding time,the thickness and hardness of the sample increase.Finally,in this paper,the relationship between the composition and performance of the sample nitrided by ASPN is discussed.The kinetic model of nitriding is established,and the nitrogen concentration distribution of the nitriding layer is calculated and fitted.