Elimination of surface spottiness defects and characterisation of the nitrocarburised surface for 40Cr steel

Nitrocarburising was an effective technique to enhance the surface strength of iron-based alloys and has been widely used in engineering. However, an unfavourable issue was the potential development of spottiness defects on specimen surfaces due to nitrocarburising treatment. This paper focused on the nitrocarburised 40Cr steel shafts with surface spottiness defects in production conditions. Optimisation of the production process was conducted by pre-cleaning the furnace with organic solvents before nitrocarburising treatment to address spottiness defects on the workpiece surfaces. The microhardness values of the specimens were measured, and surface characteristics were analysed using scanning electron microscope, energy-dispersive spectroscopy, and X-ray diffractometer. The surface microhardness results indicated that the specimens with spottiness defects displayed lower levels, which implied that spottiness defects were detrimental to surface microhardness. The main elements distributed on the surface of the specimens were iron, oxygen, nitrogen, and carbon. The phase identification results proved that the nitrocarburised surface layers were mainly composed of Fe₃O₄, Fe₃C, and Fe₄N. The spottiness defects were most likely caused by the accumulation of iron oxides, which hindered the diffusion of nitrogen and carbon atoms in the nitrocarburising process. As a result, the Fe₃O₄ levels on the surface of the optimised-treated specimens decreased, while the Fe₃C and Fe₄N content increased. This research served as valuable insights for enhancing the visual appearance and properties of alloy steels post nitrocarburising treatment.