Carbonitriding heat treatment process of nodular cast iron plunger pump cylinder

The diffusion coefficient of nitrogen atom in α – Fe is larger than that of carbon atom. The main mechanism of nitriding is nitriding when the temperature is less than 600 ℃. The surface defects of carbon steel or pure iron are easy to produce after the surface treatment. The hole is the most serious defect. A lot of literatures have analyzed the causes of void defects, and come to the conclusion of “rebinding theory”. Specifically, ammonia gas in the gas medium used in the process of nitriding will decompose the viable nitrogen atoms, and then combine again to change into nitrogen, so the compound layer on the surface of the sample will appear. These defects will affect the hardness, friction and wear properties of the sample surface.

Pan Jiansheng et al. Developed a “short-time high nitrogen potential nitriding technology” which has been able to obtain a surface dense strengthening layer. The main constituent phases of the compound layer are ε – fe2-3n phase and γ′ – Fe4N phase. These two phases are ceramic phases with high hardness and excellent wear resistance, so the mechanical properties of the compound layer formed on the surface of the sample are relatively excellent. However, under the condition of high load stress, due to the poor phase toughness, cracks may appear on the surface of the sample or in the infiltration layer, and the cracks may extend to the interior of the substrate, so the workpiece may fail in advance [36]. The addition of carbon atoms can promote the infiltration of nitrogen atoms, which has a positive effect on the increase of the thickness of the layer. However, if the amount of carbon atoms is too much, the thickness of the layer will decrease.

After quenching and nitrocarburizing, the microstructure and wear resistance of 45 steel were characterized. The results show that the wear resistance of the sample treated by this process is greatly improved. C. – M. karamboiki et al. Studied the influence of dry sliding friction on the compound layer formed by liquid nitrocarburizing treatment of H13 steel. The research found that after nitrocarburizing at 580 ℃ for 4 h, a white and diffusion layer was formed on the surface of H13 steel, with a thickness of about 200 Compared with the untreated samples, the wear resistance of the material increased and the friction coefficient decreased by 55%. Adhesive wear was the main wear mechanism. G. Nicoletto et al. Studied the behavior change of nodular cast iron during dry sliding friction after nitriding and nitrocarburizing, and explored the influence of process on the wear resistance of the sample. The research results show that when the dry sliding friction speed is lower (specific is ≤ 0.3 M / s), the sample treated by nitrocarburizing is more wear-resistant than that treated only by nitriding.

In nitrocarburizing, the determination of temperature, time and nitriding potential is particularly important. With the increase of temperature, time and nitrogen content, the thickness of white layer will increase. With the appearance of holes, the density, hardness, strength and toughness of the surface layer will decrease. In terms of the thickness of the carburized layer, there is a critical value of carbon concentration, which is not fixed, and its size is related to the nitrogen content in the medium. At present, there are many studies on the influence factors of nitrocarburizing process on the thickness, hardness and phase transformation of the nitrided layer on the surface of the sample, but the compactness, phase composition, friction and wear properties of the surface compounds are more deeply studied and analyzed.

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