Study on Microstructure and Properties of 20MnMoNb Steel by Gas Oxynitriding

The 20MnMoNb steels were gas-oxynitrided for 2 h at 610 °C in a controlled atmosphere by the self-making process control system and gas oxynitriding technology. The microstructure, hardness, wear and corrosion resistance of the 20MnMoNb steels by gas oxynitriding were studied by OM, XRD, hardness test, wear test and neutral salt spray test. The results show that the thickness of diffuse layer is about 0.25 mm and that of the compound layer formed on the surface is about 50 μm. The compound layer is made of bulky columnar crystals, which have different morphology in the different form positions. The compound layer surface of gas-oxynitrided specimens consists of ε-Fe₃N, magnetite—Fe₃O₄ and inner surface consists of ε-Fe₃N and Fe₄N. The different oxygen and nitrogen concentrations lead to different morphologies in the different positions of bulky column crystals. The compound layer has a good hardness distribution and high wear and corrosion resistance.