The Researchment Of Iron-based Brazing Filler Metals And The Joint Performance For Brazing Stainless Steel EGR Oil Cooler In Vacuum

With the development of modern economy, the production and sales of the automobile increases year by year. The environment pollution problems which cased by vehicle emissions attract the attentions of researchers worldwide. Exhaust Gas Recirculation（EGR） system can effectively meet the requests of automobile off-gas purification treatment. Traditional nickel-based and pure copper brazing filler metals are mostly used for EGR coolers. But brazing temperature of nickel-based filler metal is high while the price is expensive as well. In addition, pure copper filler metal is easy to cause the corrosion failure of the engine. New-type iron-based filler metal is developed for brazing EGR coolers. But the base metals is easy to dissolve exceedingly as well as the grain growth with the increased of temperature, which eventually led to the deterioration of properties of parent metal as well. The waste gas exhausted by automotive engine produce sulfuric acid when it is cooled, which will corrode the engine. Therefore, the filler metal can not only produce a brazed joint with good corrosion resistance, but also satisfy the requirement of large gap brazing. As "use ironbased instead of nickel-based" has become the main development trend of filler metals for stainless steel brazing, it is urgent to develop a new type of filler metal. It exhibits lower melting temperature, higher joint strength with minimal erosion, better corrosion resistance, in addition to has good process window that can satisfy the requirement of large gap brazing. Compared with foreign iron-based brazing filler metal, it is more economical as well.In this paper, a new type of iron-based brazing filler metals was designed. Firstly, the chemical composition of the filler was optimized. The effects of B and Mo content on lap-joint shear strength, interface microstructure, corrosion resistance and microhardness of brazed seam were investigated. After the optimization of filler metal composition, the effects of brazing temperature and holding time on lap-joint shear strength, interface microstructure, interface reaction products were also investigated. At the same time, interface microstructure and erosion phenomena of T-shape joints were analysed. The main results are as follows:（1） B elements can effectively reduce the melting temperature of the iron-based filler metal and improve the wettability on base metals. Along with the increasing of B content from 0 to 1 wt%, the spreading performance of the filler metal becomes better. But diffusion of brittle phase into the base metal resulted in the decrease of joint shear strength. The maximum shear strength 71.2 MPa with uniform microstructure and good corrosion resistance of brazed seam were gained when B elements is 0.25 wt%. The uniformity of the microstructure of brazed joints can be improved by adding Mo element, as well as joint strength and corrosion resistance. With the increase of Mo element content, the microstructure of brazed seam become more uniform, and shear strength of the brazed joints showed an increasing trend. The best corrosion resistance in 10% H2_SO₄ solution and the maximum shear strength 71.2 MPa were obtained when Mo content is 3 wt%. The microstructure of brazed seam is herringbone areas of light and shade interweave, mainly comprised of tough and ductile Fe Cr Ni-rich phase and hard Fe Cr Ni P-rich phase respectively, hard Fe Cr Ni P-rich phase surrounded by a ductile Fe Cr Ni-rich phase, which can prevents the extension of hard and brittle phase to base metal.（2） With the increase of brazing temperature, the shear strength of brazed joints showed an increasing trend rapidly and then tends to bestable as the heating temperature and holding time increased. Maximum shear strength 77.0 MPa was gained under optimum process parameters, i.e. brazing temperature of 1050℃and holding time of 20 min. Lap-joints have no lack of filler penetration area and gain good weld forming. Brazing temperature and holding time influence the volume and quantity of the interface reaction products. In the test brazing temperature and holding time, the interface products are mainly gray ductile Fe Cr Ni-rich phase, black hard Fe Cr Ni P-rich phase, Mo PSi phase, and the punctate Mo2 C compounds existing in adjacent domains of parent metal and brazing seam. With the increase of brazing temperature and prolonging of holding time, the quantitly of Mo2 C compound phase increases, as well as the black hard Fe Cr Ni P-rich phase, which penetrate into the parent metal, resulting in enlarging the diffusion reaction zone.（3） T-shape joints gained a smooth fillet and good weld forming at brazing temperature1060℃, 1070℃ and holding time 20 min. When the holding time is 50 min, relatively high levels of T-shape joint base metal erosion were witnessed.（4） Compared properties of current iron-based brazing filler metals with H?gan?s F300 and commercial nickel-based filler BNi2, the microstructure of BJUT-Fe brazing filler metals is more uniform, as well as better wettability on base metals. It exhibits much higher in shear strength and corrosion resistance of brazed joint than F300, which is lower than BNi2.