Experimental Research On Vacuum Brazing Of Steel Oil Cooler Using Ironchromium Based Filler Metal

In this paper, researches were carried out on the application of the iron-chromium based filler metal in the manufacture of stainless steel oil cooler. Through the trail manufacture of oil coolers of different types and structures, several problems were disclosed in the products, including faulty brazing defect, low joint strength, fracture of mounting plate brazing seam in the process of flattening, and the excessive dissolution of the base metal.To investigate the cause of the faulty brazing defect, comparative experiments on the wetting and gap filling abilities of the iron-chromium brazing filler metal and pure copper were conducted. The results show that the iron-chromium brazing filler metal has similarly excellent gap filling ability as copper, but the wetting property of the former is not so good as the latter. Besides, due to the poor dimensional precision of the fins, non- uniformity defects were observed in the process of brazing paste coating. These are possible reasons of the faulty brazing defect.T-shaped and wedge specimens were used to study the dissolution of fillet and parallel brazing seam. With the increase of temperature and the amount of filler metal applied, the dissolution area and depth are both increased. In the parallel brazing seam, the dissolution depth increases with the increasing gap. The dissolution process may be initiated by the grain bounda ry dissolution caused by the diffusion of phosphorus contained in the filler metal along the grain boundary. Based on the results of these experiments, the proper amount of brazing paste for each bump were determined for the 14.4 type oil cooler.A comparative study was conducted on the microstructure and shear strength of 304-304 joints and 304-Q235 joints brazed by iron-chromium based filler metal and pure copper, respectively. When the gap width is between 0 and 0.02 mm, the brazing seam contains only a small amount of phosphide. With the increase of gap clearance, continuous eutectic region is formed in the center of the brazing seam, and the width percentage of this region is also increasing. As a result, the shear strength of the joint first decreases and then becomes stable with the increasing gap width, and the fracture behavior turns from being ductile to being brittle. Although the phosphide content decreases with the increasing temperature, the joint strength is only improved at small gap width.By redesigning the plate, the bumps are arrayed closer, thus improving the uniformity of press applied between the plate and fin, and decreasing the assembling clearance. Although there still exist a small amount of faulty brazing defects, all the reliability standards for the products are reached, due to the improvement of the joint strength. The strength of the products can also be increased by improving the way of filler metal pre-position and the flatness of the brazing die. The problem of the fracture of mounting plate brazing seam in the process of flattening can be solved by using copper filler metal at the mounting plate and spacer joints, while still using ironchromium brazing filler metal in the channel, by which the brazing process can be finished in one step. Heat exchange property test and hot oil corrosion test were conducted for the oil cooler brazed using iron-chromium brazing filler metal. The results show that the requirements for both the heat exchange property and low-copper are satisfied. At present, the iron-chromium based filler metal has been successfully applied to 14.4-type oil cooler, and small volume production has been realized.