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Research on Properties of H13 After Vacuum Nitriding
1 Vacuum nitriding test
H13 hot work die steel is the most widely used in many hot work die steels, and its alloying elements are mainly V, Cr, and Mo. Before the experiment, the H13 steel sample was cut out for vacuum oil quenching at 1075 °C, and then subjected to vacuum tempering at 550 °C for 2 h to obtain a sample with an average hardness of 47HRC. Before vacuum nitriding, the samples were degreasing and repolished, and then the samples were cleaned in an ultrasonic apparatus with an acetone cleaning solution. The vacuum nitriding process is carried out with the aid of an RVN vacuum nitriding furnace. In the process of vacuum nitriding, the nitriding furnace is first evacuated to about 50Pa, then H2 is passed into the furnace for cleaning for about 10 minutes, and then N2 is passed through for ion nitriding treatment. In the plasma nitriding process, the influence of different temperature and time conditions on the properties of H13 die steel was mainly investigated.
2 Performance changes after vacuum nitriding
2.1 Effect of vacuum nitriding temperature on wear resistance
The friction and wear test results of the samples after nitriding treatment for 8 h are shown in Figure 1. It can be seen from the figure that with the prolongation of wear time, the wear quality of die steel samples treated with different nitriding temperatures shows a linear increasing trend. During the first 4 hours of wear, the wear amount increased slowly, but after 4 hours of wear, the increase of the wear amount of the sample gradually accelerated. At the same time, it can also be found that the wear amount of die steel samples subjected to nitriding treatment at lower temperatures such as 460 °C and 480 °C is significantly greater than that at higher temperatures.
(500 ℃, 520 ℃ and 540 ℃) the die steel samples after nitriding treatment. When the wear time was 14 h, the wear amount of the die steel samples after nitriding at 460 ℃, 480 ℃, 500 ℃, 520 ℃ and 540 ℃ were 345 mg, 315 mg, 206 mg, 231 mg and 245 mg, respectively. The analysis shows that the wear of die steel samples is related to the surface hardness value. The higher the surface hardness value, the better the wear resistance, so the wear resistance of the die steel samples after nitriding treatment at a higher temperature is relatively better. In addition, in the primary stage of friction and wear, the main wear material is the nitriding layer, and the samples with higher nitriding temperature have thicker nitriding layer, so the wear amount is small. In the later stage of wear, the surface nitriding layers of the samples were all worn away, and the substrates were all worn at this time, so the wear rates of the samples were basically the same.
2.2 Influence of vacuum nitriding temperature on the thickness of nitriding layer
The effect of vacuum nitriding temperature on the thickness of nitriding layer is shown in Figure 2. With the increase of vacuum nitriding temperature, the thickness of nitriding layer shows a step-like upward trend. At 480 ~ 500 ℃, the thickness of the nitrided layer increases the fastest. At 480 °C, the thickness of the nitrided layer is 175 um, and at 500 °C, the thickness of the nitrided layer is 241 um; It is platform-shaped; when the temperature continues to rise to 540 °C, the thickness of the nitrided layer is 265 um. To analyze the reasons, as mentioned above, the diffusion rate of nitrogen atoms will increase with the increase of temperature, which will increase the thickness of the nitrided layer.
2.3 Microstructure analysis of nitrided layer of H13 die steel sample
Comprehensive analysis shows that the H13 die steel sample has the best performance after nitriding at 500 ℃ for 8h. Then, the phase composition analysis of the compounds in the nitriding layer of the best sample was carried out, as shown in Figure 3. The results show that the compounds in the nitriding layer are mainly Fe2N, Fe4N and Fe3O4. It is calculated by XRD through the diffraction intensity integration. Fe2N phase It accounts for 42.5%, the Fe4N phase accounts for 55.3%, and the Fe304 phase accounts for 2.2%. At the same time, the morphology of the best sample nitrided layer was observed by scanning electron microscope, and the compound layer could be clearly seen from the morphology. Therefore, comprehensive analysis shows that the H13 die steel sample has a compound layer and nitriding layer with appropriate thickness after being vacuum nitrided at 500 ℃ for 8 hours, which has a significant effect on the improvement of the wear resistance and thermal fatigue performance of the die steel.
In this paper, friction and wear experiments on die steel samples were carried out. The results showed that the wear quality of die steel samples showed a linear increasing trend. The die steel samples with vacuum nitriding temperature of 500 °C and holding time of 8 h were worn for 14 hours and the wear quality was 206 mg. . The phase composition shows that the compounds in the nitrided layer are mainly Fe2N, Fe4N and Fe3O4. The compound layer and the nitrided layer with appropriate thickness have a significant effect on the wear resistance and thermal fatigue performance of the die steel.
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