The effective hardening depth of three parts of lzqt600-3 ductile iron profile was measured. According to the measured data, the effective depth of the hardened layer at the edge, 1 / 2R and the center is different after nitrocarburizing, and the effective hardening depth of the core nitrocarburizing sample is the largest. This is due to the rapid cooling rate of molten iron at the edge of horizontal continuous casting process, and the carbon content in ferrite gradually decreases from the edge, 1 / 2R to the core. When the solidification time of molten iron is different, the solidification time of the molten iron forming the core is the longest. The carbon in the matrix is easier to diffuse into graphite, and the ferrite content in the core is lower. After nitrocarburizing, the carbon at the edge is difficult to penetrate, but 1 / 2R Compared with the carbon in the center, the edge is easier to penetrate. The carbon infiltration makes the micro carbides have a positive effect on the diffusion of nitrogen and accelerate the formation of high nitrogen compounds. Therefore, the effective hardening depth of the core is the largest after nitrogen carbon CO treatment.
The range analysis was carried out on the effective hardened layer depth data of the edge, 1 / 2R and the center, and the R value was calculated, and the results as shown in the figure were obtained. It can be seen from Figure 1 that the primary and secondary factors affecting the effective hardened layer depth in the center of the orthogonal experiment in this paper are as follows: nitriding temperature T > gas proportion s > nitriding time t; the primary and secondary factors influencing effective hardened layer depth at edge and 1 / 2R are gas proportion s > nitriding temperature T > nitriding time t. Under the actual working conditions, with the continuous wear of the workpiece, the thickness of the carburized layer decreases, the greater the effective hardening depth is, the better, and the hardness of compound layer should be higher. Therefore, the main and secondary order of influencing factors on the effective hardening layer depth of lzqt600-3 ductile iron profile is selected as the standard, so the primary and secondary order of the influence of the nitriding parameters on the effective hardened layer depth of lzqt600-3 ductile iron profile is as follows: nitriding temperature T > gas proportion s > nitriding time t. The results show that the influence of temperature T on the effective hardening depth of the samples plays a controlling role, that is, the change of temperature can greatly adjust the effective depth of hardened layer of Nitrocarburized samples, followed by the gas ratio s.
The evaluation index for the data is the intuitive analysis of the influence of various factors on the effective hardening depth of the sample, which can more clearly show the influence degree of each factor. The results are shown in Fig. 2. It can be seen from Fig. 2 that the increase of gas ratio has a tendency to increase the effective depth of hardened layer, that is to say, the increase of NH3 content has a positive effect on the effective hardening depth. This is due to the increase of ammonia content, the number of active nitrogen atoms in the nitrocarburizing process also increases, the number of nitrides, nitrocarbons and other hardening phases in the formed layer structure increases, the hardness of the structure increases obviously, and the corresponding effective hardening depth also increases. However, with the increase of the nitriding temperature, the effective depth of hardened layer is negatively correlated, and the influence of the factors is not obvious.
ANOVA was used to test the significance of the effective depth of hardened layer at the edge, 1 / 2R and the center, and the results were summarized. The results show that the factors t and s are significant, while the factor t is not significant, which is consistent with the range analysis results. All of these show that the effective hardening depth index of different parts of lzqt600-3 ductile iron profile is significantly controlled by the infiltration temperature T.