The effect of Na₂S₂O₃ on RE-boronizing behavior of 45 carbon steel

Highlights: • The interface changes flatter and flatter with the increase of Na₂S₂O₃. • The content of B in the surface of the coatings increases with Na₂S₂O₃ increase. • The amount of the Fe₂B phase decreases with the increase of Na₂S₂O₃ content. • The wear resistant of the B-3SL layer is the best when a high load is applied. - Abstract: In this study, the effect of Na₂S₂O₃ on RE-boronizing behavior of 45 carbon steel was investigated by varying the Na₂S₂O₃ content in the treatment agent. The content of Na₂S₂O₃ was set to 0%, 1%, 2%, 3%, 4% and 5%, respectively, to study the effect of sulfur on the property of the boride layer. The diffusion layers obtained by the above-described agents containing different levels of Na₂S₂O₃ were named B-0SL, B-1SL, B-2SL, B-3SL, B-4SL and B-5SL, respectively. Pack RE-boronizing of 45 carbon steel was carried out at 300 °C for 2 h, and then at 850 °C for 6 h. The diffusion coatings were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), Vickers microhardness measurement, profilometry, the Daimler-Benz Rockwell-C adhesion test, and friction test. Testing results have revealed a significant morphology change in the borides with different Na₂S₂O₃ contents. The thickness of the layer increases dramatically with increasing content of Na₂S₂O₃, reaches its maximum (152 μm) with an agent of 3 wt.% Na₂S₂O₃, then remains unchanged with further increasing Na₂S₂O₃ content. The concentrations of B, S and O in the diffusion coatings increase with increasing the content of Na₂S₂O₃. B-rich, O-rich and S-rich regions form successively from the surface to the inner layer in the B-5SL layer. FeS₂ and Fe₂O₃ phases with a low hardness were observed in the B-3SL and B-5SL layers, respectively. The microhardness of the B-5SL layer is the highest, while the adhesion between the coating and the substrate in the B-3SL layer is the best among all the coatings. Compared with other coatings, the B-3SL layer can still keep a lower friction coefficient as well as a lower wear rate even when a high load is applied