Changes in the microstructure and properties of the egg-break blade after boronisation and vanadizing

Egg-break blade is a key component in a high-efficiency egg separator. Two failures of egg-break blade usually occurred in the turning of cutting edge and blade wear. The strategy to prolong the lifetime of egg-break blade is to increase the hardness without too much sacrifice of toughness. In this study, the boriding and thermal diffused vanadizing were applied to a blade stainless steel of 2Cr13, thereby to achieve an excellent wear resistance. The microstructure, mechanical properties, and wear behaviors of the layers were investigated after the treatment. A scanning electric microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) was employed to observe the thickness, microstructure, and chemical composition from the cross section of the layers. The crystal structure was detected by X-ray diffraction (XRD) from the top surface. The results suggested that the boride and vanadizing layers were successfully fabricated on the surface of 2Cr13 stainless steel, showing excellent adhesion with the substrate. The thicknesses of boride and vanadizing layer were 15 μm and 5μm, respectively. The much thinner vanadizing layer was related to the larger diameter of atom, resulting in a sluggish diffusion. It can also be confirmed by the content of B and V in the corresponding layer. There was nearly 30% B in the boride layer, while, only 12.3% V content in the vanadizing layer. According to XRD patterns, the boride layer consisted of FeB, Fe2B, Fe3B, while, the vanadizing layer mainly consisted of VCx carbides. The borides and carbides formed via the diffusion and reaction with the interior elements of the substrate. The micro-hardness of coating and substrate can be measured from the surface with a load of 0.49 N. The hardness of boride layer and vanadizing layer were 1 554 and 1 037 HV, respectively, which were three and 2-3 times higher than that of the treated stainless steel. The high hardness can be attributed to the formation of borides and carbides after the treatment. The relatively low hardness in the vanadizing layer can be related to the low content of VCx. Nano-indentation test was carried out to further determine the mechanical properties, including the nano-hardness, elastic recovery ability (We) and energy dissipation coefficient (Kd). The nano-hardness of diffusion layers was like the results of micro-hardness. The boride layer presented a higher elastic recovery ability than the vanadizing layer, whereas, the vanadizing layer showed the better performance of plastic deformation. In friction test, the results demonstrated that the coefficient of friction (COF) in the vanadizing layer was relatively lower of 0.5, compared with the boride layer, where the COF was about 0.68. Nevertheless, the boride layer presented a lower wear rate under non-impact load, indicating better wear resistance due to its higher hardness. Taking the thickness and mechanical properties into account, the vanadizing layer with better performance of plastic deformation can be a potential application to protect the cutting edge from turning, whereas, the boride layer with higher hardness can be a candidate to improve the wear resistance of blade body served under a stable load.