Objective GCr15 steel is a high-carbon steel with high hardness and good wear resistance. It has been widely used in many fields, such as the automotive industry, aviation equipment, transport ships. However, the corrosion resistance of GCr15 steel is poor, and its components suffer early fatigue failure due to corrosion. It can be characterized by the phenomenon that when it is used in marine equipment, its service life is short due to the erosion effect of Cl ^-. The corrosion resistance of GCr15 steel can be improved by adjusting its microstructure. However, simultaneously, the corrosion resistance is closely related to the composition. There are some limitations in improving the corrosion resistance by simply adjusting the microstructure. Laser surface alloying (LSA) is a typical surface strengthening technology, which is often used to adjust the distribution of elements and microstructure nearing the metal surface, so it has a broad application prospect in improving the mechanical properties and corrosion resistance of metal materials. Therefore, in this study, LSA is used to prepare Cr alloyed layer on the surface, and the effect of B₄C on the phase, hardness, and corrosion resistance of the alloyed layer is studied.

Methods Using laser alloying, a corrosion-resistant high Cr alloyed layer is prepared on the surface of GCr15 steel. Before alloying, the substrate is preheated to avoid cracks. Then, the microstructure and phase of the alloyed layer are analyzed by optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), and X-ray diffractometer (XRD). The electrochemical performance is tested by a conventional three-electrode system. The saturated calomel electrode is used as the reference electrode, the sample as the working electrode, and the platinum electrode as the auxiliary electrode. In this work, 3.5% NaCl solution is used as the corrosive medium, the scanning speed is 1 mV/s, and the test time is 1800 s. The corrosion resistance is analyzed by polarization curve and impedance spectrum.

Results and Discussions As shown in Fig. 2, the alloyed layers obtained using B₄C/Cr powders with different mass ratios have no defects such as cracks and pores, the interface between the alloy layer and the substrate is metallurgically bonded. The distribution of Cr in the alloy layer is analyzed by EDS. It is found that the Cr content in the alloy layer is higher than that of the matrix, and the thickness of the alloy layer is about 400 μm. The remelting occurred in the overlapped region. Due to the convection in the molten pool, elements in the alloy layer are redistributed, which will promote homogenization of the composition (Fig. 3 (b)).

The microstructure of the alloyed layer is dendrite. In the process of laser alloying, due to the heat conduction of the substrate, there is a large temperature gradient in the direction perpendicular to the substrate, the direction of dendrite growth is approximately perpendicular to the substrate. Compared with the alloyed layer obtained using Cr powder, the microstructure of the alloyed layer obtained using B₄C/Cr mixed powder is finer (Fig. 4), and there are two new strengthening phases of Fe₂B and CrB in the alloyed layer (Fig. 6). Furthermore, the addition of B₄C can improve the hardness of the alloyed layer (Fig. 7). Moreover, the newly formed borides and carbides can be used as the core of heterogeneous nucleation, which can increase the nucleation rate and thus refine the microstructure of the alloyed layer. Alternatively, there are more carbides CrB and Fe₂B in the alloyed layer, which serves as a dispersion strengthening.

By analyzing the Nyquist curves of impedance spectra of different samples, it is found that they have similar capacitive arc characteristics (Fig. 8 (a)). The corrosion potential (E_corr) and corrosion current density (I_corr) are obtained from Tafel curve extrapolation. The results are listed in Table 3. It is found that corrosion resistance of the alloy is improved because Cr is a passivation element, and an increase in Cr content on the surface is beneficial to delay the corrosion rate. Compared with the alloyed layer obtained using pure Cr, the alloyed layer obtained using B₄C/Cr mixed powder has a higher corrosion potential and lower corrosion current density, which indicates that it has better corrosion resistance. This is because the microstructure is refined by adding B₄C, and the alloyed layer obtained using the B₄C/Cr mixed powder with a mass ratio of 1∶16 has a higher content of CrB, which is beneficial to increase the corrosion factor. The hard phase enriched with Cr and a solid solution of (Fe, Cr) is firmly combined with other phases, which reduces the degree of grain boundary corrosion (Fig. 10).

Conclusions In this study, a high Cr corrosion-resistant alloyed layer is prepared on the surface of GCr15 steel by laser alloying. The alloyed layer has good metallurgical bonding with the substrate material, and the microstructure is a typical dendritic structure. Compared with the alloyed layer obtained using Cr powder, the microstructure of the alloyed layer obtained using B₄C/Cr mixed powder is more refined, and there are two new strengthening phases of Fe₂B and CrB in the alloyed layer. The addition of B₄C improves the hardness and corrosion resistance to a certain extent. When the mass ratio of B₄C and Cr powder is 1∶16, the microhardness of the alloyed layer is about 621 HV, which is 2 to 3 times the hardness of the substrate, and its corrosion resistance is better.