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

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.