42CrMo Steel Plasma Nitriding Surface Strengthening And Numerical Simulation

With the development of the industrial processing,manufacturing industry is in rapid development,the service life of machine parts put forward higher requirements, and studies show the failure of parts often starts from the surface, such as bearings, gears at service status for a long time. This needs to improve the surface performance of the parts to meet the technological requirements.As a kind of commonly used surface strengthening technique, plasma nitriding can improve the surface hardness, wear resistance, corrosion resistance and fatigue performance.Plasma nitriding has the advantages of small distortion, low cost,can easily realize local nitriding, hence it was widely used in themany areas, and made great development. With the development of plasma nitriding,engineers urgently needsa effective mathematical model to predict the total nitriding process,so as to formulate reasonable process parameters.In this paper, the plasma nitriding experiment was carried out in thermal refining 42CrMo steel, three nitriding temperatures (510??540??560?)and four nitriding times (1 hour?3 hours?5hours?7hours) were chosen.The influence of nitriding temperature and nitriding time on the nitriding layer thickness and compound layer thickness werestudied in this paper.Research has shown that with the increasing of nitriding time and nitriding temperature, the depth of nitriding layer and compound layer is increase.According plasma nitriding specimen cross section optical metallograph and SEM photos,wemeasurethe thickness of compound layer, and analyze the advantage and disadvantage of the two methods.The results showed that both methods can explain the relationship between the change of the thickness of compound layer with nitriding time/temperature, and the second method is more reasonable. The results of XRD experimentsshowed thatwith the increasing of nitriding temperature the relative content of ?'phaseis increase,and the relative content of ? phase is decreae. On the contrary, with the increasing of nitriding temperature the relative content of ?phase is increase, and ?' phase is decrease.According to the actual situation, this paper puted forward some hypothesis, the diffusion - reaction model of plasma nitriding is established. The model consided the diffusion of nitrogen in ?-Fe and the precipitate of alloy nitride(such as CrN, MoN). The model calculated the total nitrogen quality fraction in diffusion layer, solid solution nitrogen quality fraction in ?-Fe, chromium quality fraction in matrix. After that, the model consided the formation of ?' phase, and calculated the nitrogen quality fraction of ?' phase-diffusion layer.This paper has also established plasma nitriding model with variational diffusion coefficient,the model consided the whole plasma nitriding process is nitrogen diffusion,and the diffusion coefficient is variational with nitrogen quality fraction in matrix. First, in this model, we should measuremicro hardness of sample, and fit the micro hardness and depth.Second, in the model we thougththe nitrogen quality fraction and the micro hardness value was a linear relationship. Third, we used cubic spline interpolation method to fit diffusion coefficient with nitrogen mass fraction.In the model, three layer difference schemeswas used to deduce the Crank-Nicolson equation, and the Crank-Nicolson equation can calculate the nitrogen quality fraction of any time, and also deduce the micro hardness value of this time.Compared the calculated value and measured value, found that they were match very well.