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Leakage Causes of Vacuum Brazed Stainless Steel Plate Heat Exchanger
Water seepage and corrosion occurred in a heat exchanger after long-term use, and the leaking parts were inspected macroscopically, scanned electron microscope and energy spectrum analysis. The results show that: the copper solder in some areas is not completely fused with the stainless steel substrate during welding; during the use of the heat exchanger, multiple heat cycles and periodic impacts of water flow separate the copper solder in the unfused area from the stainless steel substrate, eventually leading to local Seepage.
Heat exchangers are widely used in chemical, food, pharmaceutical, machinery and other fields, and occupy an important position in production. The macroscopic appearance of the vacuum brazed stainless steel plate heat exchanger is shown in Figure 1. It is composed of stainless steel sheets, gaskets, clamping devices, etc. During manufacture, the stainless steel sheets and copper brazing foils are alternately combined and sent to the brazing furnace. After entering the furnace, the copper foil melts into a liquid at a certain temperature, and these liquids are adsorbed on the support points and boundaries of the stainless steel sheet through capillary action, thereby playing the role of sealing and strengthening.
The failure forms of the vacuum brazed stainless steel plate heat exchanger are: pitting corrosion and perforation of the stainless steel sheet leading to leakage; crevice corrosion of the stainless steel sheet of the heat exchanger and the sealing gasket leading to failure; copper solder penetration into the stainless steel grain boundary, grain boundary It becomes a cracking point and fails; the temperature of the heat exchanger brazing process is too high to cause overburning, resulting in damage and failure. The multi-layer stainless steel heat-conducting sheet (see Figure 2) in a plate heat exchanger presents an “S” shape. This curved surface structure can improve the heat transfer efficiency of the heat-exchanging sheet. The surface structure of the stainless steel sheet inside the plate heat exchanger is complex, and the diameter of the liquid circulation channel is small. During the flow of the liquid, its flow direction and flow rate are constantly changing, and the fluid is greatly disturbed. Liquids can still flow turbulently at low flow rates as they pass through the heat exchanger channels. During the working process of the heat exchanger, the flow of liquid will exert an intermittent pulse impact on the heat exchanger. The shell girth weld joint where the stainless steel sheet is connected to the shell plate of the heat exchanger is the most severely impacted area. It can be seen from Figure 2 that the inner surface and side of the heat exchanger have serious corrosion defects. The author conducts macro inspection, scanning electron microscope and energy spectrum analysis on the failure site.
1. Physical and chemical inspection of stainless steel plate heat exchanger
1.1 Macro analysis of stainless steel plate heat exchanger
Observation with a stereo microscope revealed that there was corrosion on both sides of the heat exchanger. The sample was taken along the severely corroded area of the side, and the polished sample was observed under a stereo microscope. As shown in Figure 3, it can be seen that the copper solder in some parts is separated from the stainless steel substrate on the upper and lower sides.
The part where the copper solder was separated from the stainless steel substrate was cut, and it was found that some stainless steel flakes had fallen off. The macroscopic appearance of the weld surface at the water leakage is shown in Figure 4. It can be seen from Figure 4 that there is no obvious stainless steel residue on the copper solder side, and the fallen off The surface of the stainless steel side is flat and there is no obvious copper solder residue on the surface. Combining the two sides of the peeling surface, it can be seen that the separation between the stainless steel and the copper solder occurs.
1.2 Scanning electron microscope and energy spectrum analysis
The microscopic morphology of the stainless steel side surface and the copper solder surface was observed under the scanning electron microscope. The stainless steel side surface was flat, the crystal grains were not completely melted, and only slight melting occurred at the grain boundary; there were reticular protrusions on the copper solder surface. These protruding copper solders penetrate into the stainless steel grain boundaries, thus forming a nested structure of welding. The energy spectrum (EDS) analysis of the peeled surface and the energy spectrum analysis results of the stainless steel side surface can be seen that the surface is mainly composed of iron, chromium, carbon and other elements, as well as a small amount of copper. It is the result of energy spectrum analysis of the surface of the copper solder side, the surface is mainly copper and carbon elements, and hardly contains iron and chromium elements. From the microscopic morphology of normal brazed joints, it can be seen that the grain boundaries of the stainless steel surface grains in contact with the copper solder appear to melt slightly, and the copper solder penetrates into the grain boundaries. The results of line-scan energy spectrum analysis at the stainless steel grain boundaries of normal brazed joints show that copper solder infiltrates into the grain boundaries of stainless steel surface grains at normal brazed joints.
2. Comprehensive analysis of vacuum brazed stainless steel plate heat exchanger
From the analysis results, the failure mode of the heat exchanger is the separation of the stainless steel substrate and the copper solder. From the macroscopic analysis, it can be found that although the solder in the local area of the failure sample completely fills the weld, it is poorly combined with the stainless steel base material. From the scanning electron microscope results, it can be seen that the copper solder and stainless steel base material at the failure joint and the normal joint have formed a welding nesting structure; the combination of the copper solder at the failure joint and the stainless steel base material is not tight. The oil on the surface of the stainless steel sheet and the copper solder decomposes at high temperature to form carbon. Before vacuum brazing, if the surface treatment or treatment of the stainless steel sheet and the copper solder is not up to standard, the Cr2O3 oxide film on the local stainless steel surface cannot be removed during vacuum brazing, and the stainless steel matrix in the local area is not tightly combined with the copper solder, thus There is no fusion phenomenon.
3. Conclusion and suggestion of vacuum brazing of stainless steel plate heat exchanger
After the heat exchanger is put into use, the water flow continuously exerts periodic impact on the heat exchanger, and the stainless steel matrix and the copper solder in the part with small local bonding force and incomplete fusion gradually separate, resulting in gaps, which make the heat exchanger partially Seepage. It is recommended to strengthen the surface pretreatment of welded parts before vacuum brazing, clean up the oil and residue on the surface of each stainless steel sheet and copper solder, promote the mutual integration of copper solder and stainless steel matrix, and enhance the bonding force between solder and stainless steel.
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