| Cr18-Ni8 steel was the most common manufacturing EGR stainless steel, it had good corrosion resistance, heat resistance, low temperature strength, mechanical properties,and the price was low. Cr18-Ni8 steel mainly contained the composition such as Fe,Ni, Cr.so this article used the BNi7+Cu composite solder vacuum to brazing Fe, Ni and Cr18-Ni8 steel. It was controled vacuum around 8×10-3Pa. Heat preservation respectively used 5min,30 min. Brazing temperature adopted 940℃,960℃ and 980℃. Brazing spacing was 30μm,100μm. Through optical microscope(OM), scanning electron microscope(SEM) and energy spectrum analyzer(EDS) and digital microhardness tester to study the dissolution and diffusion behavior of the new composite solder and pure metal brazed joint, and relationship to Cr18-Ni8 steel welding head. It established brazing process parameters on the microstructure and mechanical properties of brazed joint. It revealed the diffusion-solidification law of brazed joint.When we used the BNi7+9%Cu to braze pure Fe, the brazing seam organization was mainly made up of γ(Ni,Cu)solid solution of the rich(Cr, Fe) on both sides, theγ(Ni)+Ni3P eutectic organization and Cr-Ni-P black metal compounds in the middle.The parent metal formed holes with different temperature. The distribution of microhardness present on both sides of the solid solution region was low. Middle eutectic area had high hardness. And with the increase of temperature, the hardness was the lower.The BNi7+9%Cu brazing pure Ni, when clearance of brazed joint was 30μm,the temperature of 960℃, the brazing joint was made up of rich Cr-γ(Ni, Cu) solid solution on both sides, γ(Ni) + Ni3 P eutectic tissue in the middle and a small amount of eutectic organization diffusion effect area and γ(Ni) of solid solution. Brazed joint was organized by Ni3 P in the center and diffusion effect of γ(Ni) solid solution at 980℃. The higher the temperature and the greater the spacing, the greater the spread affected zonewould be. Brazed joint microhardness distribution was high in the middle and low on both sides. Under the same braze welding head clearance, the higher the temperature of brazing seam area, the lower the hardness would be, but diffusion influence area was just the opposite. At the same temperature, the greater the microhardness spacing of brazed joint,the lower the hardness would be too.The BNi7+3%Cu or BNi7+9%Cu composite braze Cr18-Ni8 stainless steel on vacuum brazing. Brazing seam area was mainly made of γ(Ni,Cu) solid solution of the rich Cr, Fe and Fe Ni3 phase on both sides near parent metal, Ni based solid solution in the middle and Ni3 P eutectic organization and black Cr Ni P compounds. The higher the temperature and the longer the time of heat preservation, The thickness of the brazing seam on both sides of the solid solution gradually becomes thicker. When we added 3%Cu, as temperature rised, microhardness of braze welding head reduce before they rised. When we Added 9%Cu, as temperature rised,the microhardness of brazed joint gradually increased. when Brazing gap was 30μm, the microhardness of joint increased along with the time of heat preservation.Clearance increased to 100μm, the hardness was less affected by the heat preservation time. When the temperature was 940℃, the brazing seam would appear a lot of holes and solder of high melting point that was not molten, which decreased the shear strength of brazed joint.When the temperature was 980℃,braze welding defects disappeared, and the strength of the joint increased. Under the same parameters,composition of BNi7+9% Cu brazing Cr18-Ni8 steel had higher joint strength than BNi7 +3%Cu. |
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