The use of a vacuum sintering method to form an alloy coating is a novel surface metallurgy technique in which a self-fluxing alloy powder layer of Ni, Co, Fe, etc. applied onto the surface of a common material is heated under vacuum conditions. Partially melted and infiltrated the surface of the workpiece, firmly bonded to the surface of the substrate, cooled and solidified to obtain a surface functional coating with high hardness, wear resistance, high temperature resistance and corrosion resistance. Vacuum fusion technology is not affected by the shape of the components, and the thickness, composition and performance of the coating are adjustable. It has greater process flexibility and is widely used in machine tool guides, cutting tools, hammers, pressure rollers, tools and molds, valves. Functional coating and repair of cylinder liners and other products have received widespread attention in recent years.

The vacuum fusion NiCrBSi alloy coating produced at a higher temperature will accelerate the diffusion of Fe element from the carbon steel base alloy to the NiCrBSi alloy coating, resulting in the defects such as the decrease of the Fe element and the increase of the crack formation probability due to the increase of Fe element. Caused early failure and destruction of the workpiece. Rare earths have excellent modification effects in surface treatment techniques such as chemical heat treatment and hot dip plating. However, there have been no reports on the modification of vacuum fusion-bonded NiCrBSi alloy coatings by rare earths. This experiment analyzed the influence of rare earth Y on the chemical composition and microstructure of NiCrBSi alloy coating, and explored the mechanism of action of rare earth.

2 Experimental method The sample was a normalized 45 steel with a size of 30mmX7mmX7mm. The coating material was a mixed powder of NiCrBSi self-fluxing alloy powder and rare earth Y. See Table 1 for details. Table 1 Chemistry of NiCrBSi self-fluxing alloy and rare earth Y Composition Date of receipt of the first draft: 2003-07-21; Date of receipt of the revised version: 2004-09-20 Fund project: "15th Five-Year Plan" scientific and technological research project in Anhui Province (01012031) Vacuum fusion welding equipment for the VF-79J type vacuum fusion Furnace, mix NiCrBSi alloy powder with 0.2% Y (mass fraction) powder and apply it on the surface of 45 steel sample with a coating thickness of 1mm. The sample is dried in a vacuum oven at a low temperature and the vacuum degree is 10Pa, the sintering temperature is 1020*C; the eutectic temperature of the non-rare earth NiCrBSi alloy is 1 for 5min, cooled to 180C with the furnace, and then cooled in the furnace, and the sample surface is ground and then analyzed. The chemical composition of the coating was analyzed using the KEVEX Sigma spectrum microanalysis system; the phase structure of the coating was analyzed with LEO.

3 Experimental results 3.1 The microstructure of the coating is the secondary electron image of the vacuum sintered Ni-based alloy. On the Ni-based alloy substrate (A region), the massive second phase (B region) is distributed in a ring, and the ring A spherical second phase (C zone) and acicular second phase (D zone) are distributed. a is the microstructure of the rare earth-free NiCrBSi alloy coating. The size of the spherical phase is different. On the substrate (especially on the left side of the tissue diagram) there are a lot of fine needle-like secondary phases. This is the second phase in the Fe-based alloy coating. Characteristics. b is the microstructure of the NiCrBSi alloy coating containing rare earth Y. The number of spherical second phases is increased and refined, and the needle-like second phase is almost disappeared. This causes the front end of the needle-like phase in the coating to separate the coating matrix. Stress concentration is relieved, reducing the possibility of cracks in the coating.

3.2 Chemical composition of the coating The main elements of the coating are Ni, Cr, Fe, Si, and non-metallic elements 0. Due to the low content of Y and the lightness of C and B, the corresponding energy spectrum does not appear in the figure. Peak; the energy spectrum of the NICrBSl alloy coating is similar to that of the NICrBSl alloy coating except that the peak height of the energy spectrum is different.

The X-ray energy spectrum of the vacuum-sintered NiCrBSiY alloy coating Fig. 2 shows the energy spectrum analysis results of the various compositional phases in the two coatings. Table 2 shows the composition of the various phases. The matrix is ​​mainly composed of higher Ni, containing Cr, Si, Fe, etc.; Ni in the needle phase is slightly lower than the matrix, but the Fe content is higher than the matrix; the bulk phase is dominated by Ni and Cr, and the Fe content is The matrix and needle phase are low; the spheroidal phase is dominated by Cr, with higher Fe and lower Ni and Si. Compared with the NiCrBSi alloy coating without the rare earth, Ni content in the NiCrBSiY coating matrix and needle phase is higher, and Cr content in the bulk and spheroidal phase is higher. It is worth noting that the content of Fe in each phase of the coating containing rare earth Y is lower than that of undoped rare earth, reflecting the fact that when preparing a vacuum-sintered Ni-base alloy coating, rare earth Y hinders the carbon steel. The diffusion of Fe in the material into the coating reduces the "dilution" effect of Fe on the Ni-base alloy coating and ensures good microstructure and performance of the coating.

Table 2 The chemical composition of the constituent phases of the vacuum-sintered Ni-based alloy coating. The phase structure of the Table 2 Chemical composition of composing 3.3 coating is the X-ray diffraction pattern of the vacuum-sintered Ni-based alloy coating. A rare earth-free NiCrBSi coating consists of a Ni-based solid solution containing various solutes and carbides Cr23C6, boride Ni3B, NiB and CrB and silicide Cr3Ni5Si2. b is the X-ray diffraction pattern of the rare earth vacuum fusion-bonded NiCrBSiY alloy coating. In addition to the above-mentioned composition phase, carbide 7C3 and boride Ni2B have newly emerged. It can be seen that the involvement of the rare earth into the Ni-based alloy coating affects the formation of the second phase. And precipitation.

X-Ray Diffraction Patterns of Vacuum Melting Ni Base Alloy Coatings 4 Analysis and Discussion In vacuum fusion NiCrBSi alloy coatings, due to the presence of various alloying elements, solute containing C, B, Si, etc. is formed during the cooling of the melted joints. Ni-based solid solution, Ni-Ni3B eutectic and carbides, borides, and silicides containing Cr, Ni provide coatings with high hardness and wear resistance.

B and Si are strong reducing agents. Besides participating in the alloying of the coating, they can also generate small amounts of relatively stable oxides B2O3 and SiO2, which can form borosilicates with low viscosity and low density with other metal oxides. ,Frosting on the surface during vacuum welding protects the coating from oxidation. However, due to the fact that the coating is in a partially molten state during vacuum fusion, the diffusion of alloying elements is difficult to fully proceed, resulting in poor uniformity in the composition and composition phase distribution and performance of the coating. NiCrBSi alloys are generally vacuum sintered above 1050*C. At this time, the mutual solubility zone of the coating and the carbon steel base metal is wider. A large amount of Fe atoms diffuses from the carbon steel matrix into the coating alloy, and the alloy coating occurs. The "diluted" phenomenon of the layers and the more fine needle-like phases exacerbated the stress concentration effect and reduced the plasticity, toughness, hardness and wear resistance of the coating.

The rare earth elements interact strongly with the transition elements Ni, Cr, and the nonmetallic elements C and B, and can form Ni-based solid solutions containing multiple solutes, and (Cr, RE)7C3, (Ni, RE)1 -3B, (Cr, RE) B and other compounds containing B, Si, C, and RE. Rare earth is a surface active element and has strong adsorption ability to the electrons of the surrounding atoms, so that the unsaturated bond of the surface atoms is compensated, and the atom distribution and interaction of the coating are affected. Therefore, compared with the vacuum fusion NiCrBSi alloy coating, the chemical compositions of the constituent phases of the NiCrBSi alloy coating containing rare earth Y have changed to varying degrees.

Rare-earth Y and Ni, Al and other alloying elements can reduce the activity of each other, improve the mutual solubility, with which it can form a variety of stable compounds. Therefore, when the alloy coating is formed, Y impedes the diffusion of Ni into the matrix of the carbon steel base material, and the constituent phases of the coating have a high Ni content.

The rare-earth Y has a very low electronegativity, which often adsorbs the surrounding atoms as a negative center and forms a large “polarization sphere”. Polarization increases the binding force between the atoms and forms a large stress in the coating. The field affects the vibration frequency, transition probability, lattice constant, etc. of the atom and reduces the diffusion coefficient of the atom, so that Fe in the carbon steel base material diffuses into the alloy coating, and alloy elements such as Ni and Cr in the coating are transferred to the mother. The resistance of the material diffusion is obviously increased, which effectively slows down the "diluted" effect of Fe on the NiCrBSi alloy coating, ensuring that the coating has a good chemical composition, organization and performance. It is well known that rare earths can form a low-melting phase with a variety of alloys. When vacuum sintering, melting of a low-melting phase facilitates the diffusion of alloying elements. NiCrBSi alloy coatings containing rare earth Y can be vacuum sintered at 1020C. With the reduction of the operating temperature, the mutual dissolution zone between the coating and the base material of carbon steel also becomes narrower, and the effect of Fe on the “dilute” of the alloy coating is hindered, so that the Fe content in the coating is reduced, and the needle is greatly reduced. The precipitation of the phases improves the composition and the microstructure of the NiCrBSi alloy coating.

The atomic radius of rare earth Y is about 40*% larger than the atomic radius of Ni. Trace rare earths are often segregated in dislocations and crystal defects of Ni-based solid solution. A large number of distortion regions are generated near these defects, leading to the system. The energy rises. When the coating melts, diffuses, and solidifies, under the action of the system's spontaneously stable state, atoms such as C, B, Si are preferentially segregated in the distorted region to form atomic groups, which in turn form carbides, borides, silicides, and other compounds. The nucleation center promotes the nucleation and precipitation of the second phase and precipitates a new carbide CC and boride Ni2B. The second phase located on the grain boundary lowers the driving force for grain growth and refines the grain. The particles are conducive to uniform diffusion of the coated alloy elements. In addition, the adsorption of the rare earth Y on the grain boundary also reduces the grain boundary energy and surface tension, hinders the precipitation of the needle-like second phase, promotes the spheroidization and refinement of the second phase, and reduces the stress concentration of the alloy coating. , inhibited the initiation and expansion of cracks. At the same time, rare earths also improve the infiltration of CrB, 7C3, Ni3B and 23C6 hard phases and Ni-based solid solutions, resulting in a uniform, dense vacuum fusion NiCrBSiY alloy coating. Therefore, the addition of rare earth Y not only changes the chemical composition of the matrix and the second phase, but also changes the morphology and relative amount of the second phase.

The rare earths also have good O, H, S removal capabilities. Y, O, H, S and other impurities can also reduce the mutual activity, improve the solubility, form rare earth compounds, reduce the non-metallic impurities in the coating, inhibit them The detrimental effects of loose tissue, make the coating more dense, and can purify, strengthen the grain boundary and improve the performance of the coating.

5 Conclusions The rare earth Y changes the chemical composition of the constituent phases of the vacuum-fused NiCrBSi alloy coating and the morphology of the second phase, basically eliminating the acicular phases, increasing the number of precipitated spherical phases, and refining them.

The rare earth Y reduces the operating temperature of the vacuum sintering and changes the chemical composition of the constituent phases of the NiCrBSi alloy coating, reduces the content of Fe in each constituent phase, and impedes the Fe in the carbon steel base material into the coating. Diffusion reduces the "dilution" of Fe to Ni-based alloy coatings

The adverse effects.

After the addition of rare earth Y, the NiCrBSi alloy coating promoted the nucleation and precipitation of the second phase, and precipitated a new carbide Cr7C and boride Ni2B.

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