Gas nitriding process parameters have significant effects on the nitriding layer of material. In the present work, a series of gas nitriding experiments on pearlitic grey cast iron specimens were carried out at different temperatures. To study the influence of nitriding process parameters on the nitriding layer, the numerical simulations of nitriding processing are performed, which take into account the threshold value of nitriding potential. The results show that the numerical simulations, incorporating the nitriding potential’s threshold value, can accurately predict the case depth and nitrogen concentration profile. The nitriding layer increases with increasing nitriding time and temperature, whereas the nitrogen concentration on the surface decreases with increasing temperature. Besides, the results also reveal that the nitriding potential nearly has no effect on the case depth, whereas it has great influence on nitrogen concentration on the surface and chemical composition of the compound layer.
Gas nitriding is a typical thermochemical surface treatment within the eutectoid temperature, in which the nitrogen is transferred from the ammonia gas into the workpiece surface, generating the gamma phase (
In recent years, a significant amount of experimental work has been conducted, aiming to investigate the nitriding process parameters’ effect on nitriding process. Nevertheless, due to high time-consuming and economic costs, the experimental method is not very effective and feasible in practical applications, especially when the parameters vary in a wide range. Under the circumstances, numerical analysis can be a good choice to study the processing parameters’ influence. Arif et al. [
In the present work, a series of gas nitriding experiments on grey cast iron specimens were conducted at 550, 570, and 590°C. Afterwards, the numerical simulations were performed, which combined the influence of nitriding potential’s threshold value on nitriding layer and Fick’s second law. The calculated thickness of diffusion zone was compared with the tested data to verify the numerical simulations. Finally, the effect of nitriding process parameters, including nitriding temperature, nitriding potential, and nitriding time, was further investigated.
The material tested in the study is pearlitic grey cast iron. The chemical composition of the material is shown in Table
Chemical composition (wt.%) of the studied grey cast iron.
C | S | Si | P | Mn | Cr | B | Cu | Ni | Ti | V | Mo | Nb | Fe |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
2.98 | 0.032 | 1.646 | 0.061 | 0.469 | 0.051 | 0.003 | 0.348 | 1.278 | 0.045 | 0.042 | 0.874 | 0.066 | Bal. |
SEM analysis of the grey cast iron used in an engine cylinder liner.
Gas nitriding experiments were carried out on grey cast iron specimens. The specimens were divided into three groups, with two specimens in each group. The tested temperatures were set as 550, 570, and 590°C for each group, with the same total time of 24 hours and nitriding potential of 1.5 atm−1/2.
After nitriding, the specimens were prepared by a standard grinding and polishing procedure, and microstructure observation was performed using JSM-5610LV SEM. Specimens were etched by 5% Nital in order to observe the nitriding layer by OLYMPUS PMG3. Hardness of the samples was also tested by applying a load of 1 N with holding time of 10 s. Besides, the hardness tests were conducted in 6 points of one sample and the average value was taken to represent the final hardness.
In order to obtain the composition of the nitriding layer, XRD analysis of the surface layer was performed using D8 ADVANCE X-ray diffractometer with CuK, at 40 kV and 40 mA. The scanned area was in the range of 15–90°.
In gas nitriding, the atomic nitrogen is dissociated from ammonia (NH3) gas and then diffuses into the workpiece surface of component. The dissociation obeys the following rule:
The above equation presumes that equilibrium has been established. The activated atomic nitrogen is dissolved in the iron crystal lattices, which not only promotes the nitriding of
The transformation of atomic nitrogen within grey cast iron is determined by the diffusion velocity, which is described by Fick’s second law:
Nitriding potential is the measurement of nitriding ability of NH3 gas. The phases and nitrogen concentration on the nitriding surface are closely related to the nitriding potential after local equilibrium is established. At the earliest, the well-known Lehrer diagram of pure iron, which was constructed by Lehrer through investigating the equilibrium conditions of NH3-H2 and Fe-N [
The critical value of nitriding potential is defined as the turning point when the nucleation of
At a certain nitriding temperature, the time
The boundary condition for stage I is
At the end of stage I, the compound layer begins to be formed and the nitrogen concentration in the interface between compound layer and diffusion zone is in equilibrium. In the diffusing process, the nitrogen concentration in the interface is set as unchanged. Therefore, the obtained nitrogen concentration on the surface of cast iron can be considered as the initial boundary condition for stage II. Since the thickness of compound layer is much less than that of the diffusion zone [
The parameters needed to be calibrated are the diffusion coefficient
For parameter
In practice, since the compound layer is a multiphase composition, the diffusion coefficient of atomic nitrogen in this area is hard to be determined, and the thickness of compound layer can only be determined by experimental data. Therefore, the study is aimed at predicting the thickness of diffusion zone and analyzing the influence of nitriding process parameters.
The cross-sectional observation through optical microscopes was made. Figure
Cross-sectional scanning electron microscopy observation of the nitrided samples at (a) 550, (b) 570, and (c) 590°C.
Experimental compound layer thickness as a function of nitriding temperature.
The tested hardness values in the depth of 100 and 130
The tested hardness of the compound layer at 550, 570, and 590°C.
Distance from the surface ( |
Hardness value (Hv) | ||
---|---|---|---|
550 | 570 | 590 | |
100 | 440 | 501 | 474 |
130 | 403 | 455 | 447 |
The case depth is determined by testing the microhardness of the nitrided samples. The depth is considered to be the distance between the workpiece surface and the location where the hardness is the same with the matrix material. For the analyzed grey cast iron, the obtained average case depths after nitriding are 79, 139, and 155
Figure
XRD diffractogram of the sample’s surface for the analyzed grey cast iron after nitriding.
The gas nitriding process in the study was simulated by using the model described in Section
The calculated and experimental case depths of the grey cast iron at 550, 570, and 590°C.
|
Tested ( |
Calculated ( |
Error (%) |
---|---|---|---|
550 | 74 | 79 | 6.8 |
570 | 129 | 125 |
|
590 | 142 | 141 |
|
The calculated nitrogen concentration distribution at 550, 570, and 590°C.
For the presence of calculation error, it can be attributed to the following reasons. Firstly, during calculation, the influence of the interface’s movement was not considered, which was a disadvantage of the present model. Secondly, the critical value of nitrogen potential, which was directly obtained from the Lehrer diagram of pure iron, may not be accurate for grey cast iron. Furthermore, for the analyzed grey cast iron, the largely distributed graphite may also have some uncertain effects on the nitriding potential value. Nevertheless, the accuracy of the numerical simulations is considered to be acceptable, which is a guarantee of the following effect analysis of nitriding process parameters.
Nitriding temperature is a crucial process parameter which greatly affects the gas nitriding process. In the study, a threshold value of nitrogen potential is adopted instead of the traditional critical value, which is a significant advantage of the present model in comparison with the existing ones [
The threshold value of nitrogen potential at different temperatures.
Nitrogen potential, which represents the nitrogen activity on the workpiece surface, is used to control the compound layer’s thickness and phases. In order to investigate the influence of nitrogen potential, numerical simulations were conducted with respect to different nitriding potential values, that is, 0.32, 0.75, 1.5, and 3.5 atm−1/2. Figure
The nitrogen concentration distributions with different nitriding potentials.
To investigate the influence of nitriding time on the case depth, numerical calculations were performed with different nitriding time of 7, 11, 24, and 40 hours. Figure
The nitrogen concentration distributions with different nitriding time.
Based on the gas nitriding experiments and numerical simulations, the following conclusions can be drawn. The numerical simulations, taking into account the influence of threshold value of nitriding potential, can precisely predict the thickness of diffusion zone and nitrogen concentration profile. With the increase in nitriding temperature and time, the thickness of case depth increases to some degree, whereas the nitrogen concentration on the surface decreases as nitriding temperature increases. For the analyzed grey cast iron, the nitriding potential has little effect on the thickness of case depth but has a relatively great influence on the nitrogen concentration of workpiece surface and composition of nitriding layer. The proposed numerical simulations for nitriding layer prediction and analysis can be a reference for practical engineering applications.