等离子体渗氮气压对合金钢渗氮层微观结构和性能的影响

目的 选择M50NiL钢（高合金钢）和AISI 4140钢（低合金钢）2种合金钢，研究渗氮气压对合金钢等离子体渗氮层组织结构、渗层厚度、硬度、韧性和摩擦磨损性能的影响规律。方法 根据离子渗氮GB/T30883—2017，在0～500 Pa渗氮气压范围内选择170、250、350 Pa 3个渗氮气压进行等离子体渗氮，研究渗层微观结构和性能。结果 对于M50NiL和AISI 4140两种合金钢，350 Pa时渗层厚度均最大，170 Pa次之，250 Pa厚度最小。M50NiL钢在350 Pa渗氮和AISI 4140钢在170 Pa渗氮时，表面层具有最优的强韧性。摩擦磨损性能显示，170 Pa和350 Pa气压渗氮的摩擦磨损性能明显优于250 Pa气压渗氮，其中磨损率规律与渗氮层的韧性值测试结果吻合。结论 气压影响了氮离子的能量和分布，从而影响了渗层厚度，钢中的合金元素含量和气压共同影响表面强韧化效果，并且表面强韧化效果直接影响渗氮层的摩擦磨损性能。

Effect of Gas Pressure on Microstructures and Properties of Plasma Nitrided Layer on Alloy Steels

Plasma nitriding is one of the surface hardening technology of alloy steels. Apart from nitriding temperature, nitriding duration and ratio of nitrogen to hydrogen, the gas pressure is a key parameter in the plasma nitriding process. Air pressure affects the number and energy of active particles (electrons, ions and energetic neutral atoms, etc.). The effect of alloying elements in steel and the effect of plasma nitriding pressure on the properties of the nitrided layer have not been considered in current studies. Thus, the effect mechanism is not clear. According to the national standard GB/T 30883-2017 (ion nitriding), the gas pressure of plasma nitriding is usually 0-500 Pa. However, the gas pressure has different effects on the properties of the plasma nitrided layer on the different alloy steels. Thus, M50NiL (high alloy steel) and AISI 4140 (low alloy steel) steels were selected as the objects in the present work. The effects of gas pressure on the microstructure, mechanical and tribological properties of the plasma nitrided layer on alloy steels were investigated. The plasma nitriding experiment with a gas pressure of 170 Pa, 250 Pa and 350 Pa was conducted. There were compound layers on the AISI 4140 steels, and the thickness of the compound layer of nitrided specimen decreased as the gas pressure increased. No compound layer was formed on the M50NiL steel due to the high content of alloy elements. The results demonstrated that the plasma nitrided layer of the specimen with a gas pressure of 350 Pa was the thickest. The specimen with a gas pressure of 170 Pa came the second. The specimen with a gas pressure of 250 Pa had the thinnest plasma nitrided layer. The gas pressure during the plasma nitriding treatment had an effect on the energy and distribution of nitrogen ions. Low gas pressure lead to a small number of high-energy nitrogen ions, and high gas pressure contributed to a mass of low-energy nitrogen ions. Thus, the gas pressure affected the thickness of the nitrided layer. Based on the indentation tests, the M50NiL steel with a gas pressure of 350 Pa and AISI 4140 steel with a gas pressure of 170 Pa showed excellent hardness and toughness. In other words, the high-density low-energy nitrogen ions were more helpful to the surface hardening of M50NiL steel. High-energy nitrogen ions contributed to the surface hardening of AISI 4140 steel. The alloy elements in steels and the gas pressure of plasma nitriding had a combined effect on the surface hardness and toughness of the nitrided layer. The test of tribological properties demonstrated that the wear performance of the nitrided specimen with a gas pressure of 170 Pa and 350 Pa was significantly better than the specimen of 250 Pa. The wear mechanism of the nitrided layer was abrasive wear and oxidation wear. The wear rate of the nitrided layer was consistent with the toughness values of the nitrided layer. The promising surface-hardening technology should be a method that not only increases the hardness of the surface layer but also provides a tough plastic surface. The present work provides a theoretical basis for the choice of gas pressure during the plasma nitriding.