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Copper-430 series stainless steel vacuum brazing process

In recent years, a beautiful and practical stainless steel pot with 430 series stainless steel (commonly known as stainless iron) and copper bottom, which can be used for induction cooker, has become popular. The processing of this product involves large-area composite brazing of 430 stainless steel-copper-304 stainless steel materials. In the production, the vacuum brazing process is used to complete the composite brazing of the above three heterogeneous materials at one time. Due to the different thermal expansion coefficients of the three materials, in the cooling process after brazing, the shrinkage of the three materials is inconsistent, resulting in deformation stress. If the brazing process is improperly selected, the strength of the brazing seam is not enough to overcome the deformation stress, which will easily break the brazing surface or the brazing seam, and the workpiece will blister and concave. The main reason is: Compared with 304 stainless steel, 430 series stainless steel is a single-phase structure. When the brazing temperature rises to about 600 C, the grains are severely coarsened and become brittle, the plasticity decreases significantly, and the brittleness increases, resulting in a significant decrease in the bonding force between itself and the solder. At the same time, the magnetic induction coefficient of 430 stainless steel is also much larger than that of 304 stainless steel. Under the same process conditions, brazing 430 stainless steel-copper workpieces melt faster than the flux and solder of 304 stainless steel-copper workpieces, and the solder is in a molten state. The longer the time is, the more brittle alloy phase is formed, so that the brazing strength of 430 stainless steel-copper is lower than that of 304 stainless steel-copper.

Vacuum Brazing Process

The process parameters of brazing composite surface include workpiece surface treatment, matching of flux and brazing material, brazing temperature, speed, heating uniformity of induction head, pressure of induction head, etc. Brazing surface strength is an important indicator to measure the quality of brazing, and the quality of brazing surface depends on the above process parameters.

2.1 Pretreatment of vacuum weldments

The workpiece to be welded must remove all kinds of dirt and excessively thick oxide film on the surface, otherwise, a good brazing seam cannot be obtained. Stainless steel workpiece cleaning and degreasing can use w (NaOH) 5% solution. For a large number of workpieces, you can use w (HNO3) 5% and 50 g/L NaF aqueous solution at room temperature for 5~10 min, then wash with hot water, and then dry at 100~120 °C. Pure copper workpiece can be in w(H2SO4)(10%~20%) cold water solution for 10~20 min.

2.2 Selection of copper solder and flux

At room temperature, the surface of copper is covered by Cu20, which is easy to remove, so copper has good brazing ability. The oxides formed on the surface of stainless steel mainly include Me2O, (Me=Fe, Ni, Cr, Mn, Ti) and Me·O·Me2O3 (Me=Fe, Ni, Mn), among which Cr2O3 and TiO2 are quite stable, It is difficult to remove, and brazing in the air must use a highly active flux to remove these oxides. Therefore, choosing a suitable flux is also the key to the success or failure of copper and 430 series stainless steel brazing. According to the material characteristics, the use of Ag-based or Ag-containing copper-based brazing filler metals of course has an ideal brazing effect, but the brazing cost is too high, which is not conducive to the development of such new products by kitchenware manufacturers, which fundamentally limits copper and 430 series. Brazing of stainless steel pots. In order to save silver, Sn can be added to the copper-phosphorus solder to reduce the melting temperature. At the same time, Sn can also improve the strength of Cu-6P. According to the influence characteristics of Sn on copper-phosphorus alloy, it can be seen that after w(Sn) exceeds 1%, the change of tensile strength is small. The Ag-free multi-component copper-phosphorus-based solder for area composite brazing: Cu-1, the brazing process performance, brazing physical properties and chemical properties of the solder are not lower than those of the Ag-containing copper-based solder, but can be greatly reduced The cost of copper-stainless steel large-area composite brazing has been successfully applied to copper-stainless steel large-area composite brazing, and its welding quality is comparable to that of copper brazing filler metal containing Ag.

Brazing copper and stainless steel with Ag-based and brass brazing filler metals, due to the low high temperature strength and creep strength of the brazing filler metals, when the temperature of the weldment exceeds 400 ℃, the strength of the weldment drops sharply. Therefore, these brazing materials cannot braze workpieces with higher working temperature. At the same time, according to the special needs of brazing one end face of copper in the intermediate layer weldment with 430 stainless steel and the other end face and brazing with 304 stainless steel, the melting point of the brazing material used should not be too high, so as to avoid excessive brazing temperature. Grains grow, resulting in poor strength and corrosion resistance of the base metal. In the test, the self-developed Cu-1 solder with a melting temperature of 630-650 C was selected, and the self-developed FB-1 copper flux was selected, and the ratio of the flux and the solder was 1:3.

2.3 Vacuum brazing temperature

Both 304 and 430 stainless steels are prone to grain coarsening. When the temperature is above 900°C and 1150°C, respectively, the grains begin to grow violently, making their strength and corrosion resistance sharply worse. Once the grain of stainless steel grows, it cannot be refined by heat treatment, so the brazing temperature should be lower than 900 ℃, generally 30~50 ℃ higher than the melting point of the brazing filler metal.

2.4 Vacuum brazing speed

During vacuum brazing, the heating and cooling speed of the workpiece is collectively referred to as the vacuum brazing speed, which has a great influence on the welding quality of the brazing surface of the workpiece.

2.4.1 Heating rate

When heating rapidly, the melting speed of the solder is fast, and the segregation phenomenon is not serious after melting, which is conducive to the generation of greater stress during the spreading of the molten solder; if the heating speed is too slow, the high temperature residence time is long, which may cause the flux to be completed after brazing. At the same time, the oxidation of the brazing surface is aggravated, which hinders the spreading of the molten brazing filler metal. Moreover, the molten brazing filler metal is in contact with the base metal for a long time, which will also cause erosion and deteriorate the performance of the weldment. For 304 stainless steel, in the temperature range of 500~750 °C, the chromium carbide in the material will precipitate along the grain boundary, causing

It is depleted in Cr, and it is easy to cause intergranular corrosion when it is in contact with the corrosive medium. Therefore, heating in this temperature area for a long time should also be avoided. In repeated tests, the 304 stainless steel pot body-sub-copper plate-430 stainless steel plate with a diameter of 20 cm was brazed as the test workpiece (hereinafter referred to as the workpiece). Only when the brazing power is 20~35 kW and the heating time is 80 When ~100 s, the flux and brazing material can be completely melted and brazing can be achieved, but not all the best brazing results can be obtained. The results of the effect of the heating power and heating time on the brazing effect are shown in the following table. It can be seen from the table that when the heating power is 20kW, the heating time is 110s, and the solder can be completely melted; when the heating power is 35kW, the heating time is 85~90s, and the solder is completely melted. Therefore, the heating method of rapid heating should be used in vacuum brazing. But it can’t be too fast, too fast wetting and diffusion will be more perfect, but it will cause oxidative burn loss to the alloy.

2.4.2 Cooling rate

When the workpiece is cooled below 538°C after welding, the thermal expansion coefficient of 304 stainless steel is 17.2~18.4 μm/(m.), which is much larger than that of 430 stainless steel (10.4~11.4 μm/(m.°C)). 304 stainless steel produces The cooling stress is larger than that of 430 stainless steel. If the cooling rate is too fast, the weldment will have a large stress difference, which will easily deform the weldment and even crack the brazing seam. If the cooling rate is too slow, the molten solder will contact the base metal. For a long time, the phenomenon of erosion will occur and the performance of the weldment will be deteriorated. Although it will reduce the welding stress, it is not good for the quenching of the welded workpiece. After repeated tests, generally 70~80 °C/s is appropriate. The melting range of the copper brazing filler metal used in this brazing is 630~650°C, which is about 60°C higher than the aluminum filler metal used for brazing stainless steel and aluminum in the same industry. , The power of the welding machine should be improved, and the power of stainless steel welding aluminum cannot be used. The copper brazing filler metal Cu-1 will be oxidized due to heating during the brazing process. Therefore, the qualified products cannot be obtained by repeated heating and brazing. It is suggested that a high-power, high-temperature, short-time, and one-time brazing process should be used for vacuum brazing.

The weldment is pretreated, the amount of flux and brazing material is 1:3, and the temperature rises rapidly. In high power, high temperature, short time, one-time brazing, the horizontal alignment error parallelism of the heating surface of the furnace head is less than 1mm, and the furnace head is brazed. When the pressure is 5.0MPa, the brazing temperature is 30~50 C higher than the solder, and the cooling rate is 70~80 C/s, the brazing seam of the obtained weldment meets the brazing quality requirements, and the shear strength can reach 19.6 MPa.

Selection of vacuum brazing equipment: The RHVB series vacuum brazing furnace produced by SIMUWU is a high-quality product for vacuum heat treatment of tooling and moulds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum brazing process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum sintering furnace, etc., which are widely sold in developed and developing countries.