Response of a DIN 18MnCrSiMo6-4 Continuous Cooling Bainitic Steel to Different Plasma Nitriding Gas Mixtures

Continuous cooling bainitic steels has been widely used in industrial processes owing to its excellent mechanical properties and toughness. Although the surface properties of them are acceptable for many purposes, for their use in mechanical components like gears, it is necessary to improve their surface properties. Plasma nitriding treatments was carried out of a DIN 18MnCrSiMo6-4 steel at 500 °C, with three different nitrogen gas composition: 76, 24 and 5 vol.% nitrogen in hydrogen, for 3, 6 and 9 hours. The surfaces were characterized concerning the microstructure, microhardness, fracture toughness, nitrogen concentration and carbon composition, phase composition and residual stress states. Based on the results presented, layer growth constants (k) for different nitrogen gas composition was determined. The carbon profiles of samples indicate that there was decarburization during the plasma nitriding. The nitrided samples with thicker compound layers presented a fracture behavior dominated by the formation of Palmqvist cracks. X-ray phase analysis indicated the formation of biphasic compound layer on the surface of all nitrided samples with 76 and 24 vol.% nitrogen, while the nitrided samples with 5 vol.% nitrogen indicated the formation of monophasic compound layer. The diffusion zone presented compressive residual stresses with highest values near the surface.

连续冷却贝氏体钢（Continuous cooling bainitic steels）凭借优异的力学性能与韧性，已在工业生产中得到广泛应用。尽管该类钢材的表面性能可满足多数应用场景的需求，但当其应用于齿轮等机械构件时，仍需进一步优化其表面性能。本研究以DIN 18MnCrSiMo6-4钢为对象，在500℃条件下开展等离子渗氮处理，采用三种不同的氮气-氢气混合气体组分（氮气体积分数分别为76%、24%、5%），处理时长分别为3、6、9小时。通过显微组织、显微硬度、断裂韧性、氮元素与碳元素浓度、物相组成以及残余应力状态等维度对试样表面进行表征。基于所得实验结果，本研究确定了不同氮气组分下的渗层生长常数（k）。试样的碳元素分布曲线表明，等离子渗氮过程中发生了脱碳现象。复合层厚度更大的渗氮试样，其断裂行为以帕姆奎斯特裂纹（Palmqvist cracks）的形成为主导特征。X射线物相分析结果显示，氮气体积分数为76%与24%的所有渗氮试样表面均形成了双相复合层；而氮气体积分数为5%的渗氮试样表面仅形成单相复合层。扩散层内存在压应力，且表面附近的压应力数值最高。