The evaluation index of the data is the visual analysis chart of the influence of various factors on the surface hardness of the sample, which can more clearly show the influence degree of each factor, and the results are shown in the figure. It can be seen from the figure that the surface hardness of the sample can be greatly increased by properly increasing the temperature and prolonging the time of the nitriding, while the change of the gas proportion, that is, the increase of NH3, has a trend of first increasing and then decreasing on the surface hardness of the sample The increase of NH3 makes the active nitrogen atoms combine again to form nitrogen molecules and separate from the surface of the substrate. Holes and cracks will appear on the surface of the sample. At the same time, there is residual oxygen in the tube furnace. With the increase of the nitriding time, the more the sample is oxidized, the microstructure of the layer will become loose, so the surface hardness increases first and then decreases.
The confidence level of α = 0.1 was selected in the analysis of variance of orthogonal test. The critical value of significance f0.90 (2,2) could be obtained by looking up the F distribution table to judge the significance level of each factor.
The surface hardness data of edge, 1 / 2R and center were tested by ANOVA, and the results were summarized. The results show that the factors t and T are more significant, but the factor s is not significant, which is consistent with the range analysis results. All of these show that the surface hardness index of different parts of lzqt600-3 ductile iron profile is controlled by the temperature and time of the process.
According to the results of range analysis and variance analysis of orthogonal test, the appropriate level is selected because of the need to consider cost saving and simple operation. Therefore, the best scheme of this test, which takes the surface hardness of different parts of lzqt600-3 ductile iron profile as index, is t2t2t2s3, that is, the nitriding temperature is 560 ℃, the time is 4 h, and the gas ratio is 4:1.