| By the reason of unique corrosion resistance characteristics and cheap raw materials, stainless steels are very widely used in automobile, medical, petrochemical, ship navigation and other industries. Therefore, to get a good stainless steel welding joint become more and more important for promoting the applications of the stainless steel. Vacuum has played a special role in removing the oxide film on the surface of base metal in vacuum brazing technology, which making it has its unique advantage in stainless steel brazing, nickel-based high-temperature fillers are mainly used for brazing stainless steel by vacuum brazing method in the industrial production, but, the P, Si and other elements in the nickel-based could produce some high hardness and brittleness compound in the center of the brazing seam during the brazing process in actual production. At the same time, some other small atoms such as B could diffuse into the center of base metal, then help to produce cavities in the joint interface, which defects make the application of vacuum brazing stainless steel restricted.For deeply understanding the formation mechanism of brittle compound in the interface of stainless steel vacuum brazing joints, while using the vacuum brazing method. In this article pure Fe, Ni, Mo have been brazed, which are the main component elements of stainless steel, observed the microstructures of pure metal vacuum brazing joint and analyze of the brazing seam phase composition under different brazing process. Pure metals Fe, Ni, Mo were used for base metal, BNi-5 and add 9%Cu element and 6%P element was as mixed filler, brazing temperature were set for 1030 ℃ and 1010 ℃, brazing gap was set for 30 μm and 100 μm, holding time fixed at 30 min. According to process parameter, 15 groups of trials were set up, by mean of metallographic microscope, Scanning Electron Microscope(SEM) and its accompanying Energy Disperse Spectroscopy(EDS) and the micro-hardness test equipment, with the help of aforementioned devices, the microstructure of the brazing joint, elements distribution of brazing seam and micro-hardness brazing joint were entirely analyzed under different process parameters. Possible phases were calculated by Thermo-Calc, a kind of thermodynamic phase diagram calculation software, in the brazing joint on definite condition. The experimental results and simulation results were processed by comparative analysis method, joint performance, law of phase formation and exist phase in brazing joint on condition of brazing temperature and different brazing gaps and filler components were researched.Research results have shown that, firstly, brazing joints of three different base metals can be divided into three regions: diffusion-affected zone(DAZ), the interface reaction isothermal solidification zone(ISZ), athermally solidified zone in middle of brazing seam(ASZ). Thermo-Calc software calculations showed that, the systems might contain the following phases, LIQUID, solid solution phase FCC_A1 and BCC_A2, compound phase such as M3 P, Cr3 Si, M5Si3, MSi and so on. In pure Fe brazing, using BNi-5+Cu as filler compared to using BNi-5 as filler, the brittle compounds content decreased obviously, and micro-hardness of joint also decreased, brazing joint with adding Cu, have generated a lot of voids in the diffusion-affected zone(DAZ), when the brazing temperature dropped to 1010 ℃, the number of voids were significantly reduced. For pure Ni brazing, filler contain Cu may have a good diffusion ability, Si and P could have diffused into the base material in large number, in the meantime, the content of brittle compounds containing P element also have a noteworthy reduce, so adding Cu could effectively improve the brazing seam defects such as cracks. For pure Mo brazing filler containing Cu in a small gap brazing, a large number of honeycomb holes were formed in the brazing seam, otherwise,large gap brazing, cannot form a complete joint, so for pure Mo metal brazing, the Cu element played a detrimental role. |
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