Stainless Steel Brazing

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The definition of stainless steel is the main addition of chromium to make the steel passivated, that is, the steel with the characteristics of stainless steel. Stainless steel is mainly Cr-Fe series and Cr-Fe-Ni series ternary alloys. Iron is the matrix and chromium is the main alloying element. In order for the steel to have rust-free properties, ω (Cr) must be higher than 12%. At this time, the surface of the steel can quickly form a dense Cr2O3 oxide film, which makes the electrode potential of the steel and the corrosion resistance in the oxidizing medium abruptly improved. In the non-oxidizing medium (HCl, H2SO4), the role of chromium is not obvious. In addition to chromium, elements such as Ni and Mo that can passivate the steel must be added to the stainless steel.

1. Adding Alloying Elements is Basically Divided into Two Categories

One is the element that forms or stabilizes austenite: such as carbon. Nickel, manganese, nitrogen, etc., of which carbon and nitrogen are used the most; the other is the element that shrinks or seals the γ phase to form ferrite: such as chromium, silicon, molybdenum, titanium, niobium, aluminum and so on. Due to the different alloying elements, stainless steel presents a different organization under the greenhouse. According to different structures, stainless steel can be divided into ferritic stainless steel, martensitic stainless steel, austenitic stainless steel, austenitic-ferritic stainless steel and precipitation hardening stainless steel.

Among various types of stainless steel, austenitic stainless steel is the most widely used and has the most varieties. Due to the high content of Cr and Ni in austenitic stainless steel, it has good corrosion resistance in oxidizing, neutral and weakly reducing media. Austenitic stainless steel has excellent plastic toughness and excellent cold and hot processing properties, so it is widely used in industrial fields such as architectural decoration, food industry, medical equipment, textile printing and dyeing equipment, petroleum, chemical industry, atomic energy, aviation and aerospace.

In order to highlight the contrast, the corresponding physical properties of steel carbon are listed. The density of carbon steel is not much different from that of stainless steel. The resistivity increases in the order of carbon steel, ferritic stainless steel, and austenitic stainless steel. The resistivity of austenitic stainless steel can reach about 5 times that of carbon steel. The coefficient of linear expansion of austenitic stainless steel is about 50% larger than that of carbon steel, while the coefficient of linear expansion of martensitic stainless steel and ferritic stainless steel is roughly equal to that of carbon steel. The thermal conductivity of austenitic stainless steel is lower than that of carbon steel, only about 1/3 of it. The thermal conductivity of the other two types of stainless steel is about 1/2 of that of carbon steel.

2. Brazing of Stainless Steel

①. Surface oxide film. As mentioned above, in addition to the main alloying element chromium, stainless steel often contains elements such as nickel, manganese, titanium, molybdenum, niobium, and aluminum. The main oxides formed on the surface are Me2O3 (Me=Fe, Ni, Cr, Mn, Ti) and MeO·Me”2O3 (Me’= Fe, Ni, Mn; Me”= Cr, Fe, Ni, Mn, Ti) Two categories. Among them, Cr2O3 and TiO2 are quite stable and difficult to remove. When brazing in the air, a strong active flux must be used to remove these oxides; when brazing in a protective atmosphere, the oxygen film can be reduced only in a high-purity atmosphere with a low dew point and a sufficiently high temperature; vacuum brazing At this time, a good vacuum degree (above 10-2Pa) and a sufficiently high temperature are required to achieve good results.

②. Brazing heating temperature. For ferritic stainless steel, as long as the brazing heating temperature does not cause its crystal grains to grow violently, it can be considered appropriate.

For martensitic stainless steel, the brazing heating temperature has a great influence on the performance, because the martensitic stainless steel is used in the quenched and tempered state. There are two options for the brazing heating temperature of martensitic stainless steel: one is to match the brazing heating temperature with the quenching temperature. For example, for 1Crl3 and 2Crl3 stainless steel, the brazing heating temperature is selected from 1000 to 1050 ℃, and the brazing temperature is quickly cooled after brazing to achieve the purpose of quenching the base material, and then tempering, so that the best comprehensive mechanical properties can be obtained.

The other is to select the brazing temperature to be lower than the tempering temperature of steel, for example, lower than 700 ℃ for lCrl3 stainless steel. In this way, the quenched and tempered base material will not soften during the brazing process, and the base material will still maintain the original comprehensive performance.

For austenitic stainless steel, the brazing heating temperature should not be too high. When the brazing temperature is higher than 1150°C, the crystal grains begin to grow violently. Once the grains of austenitic stainless steel grow up, they can no longer be refined by heat treatment. Therefore, when selecting the brazing filler metal and brazing process parameters, it is necessary to avoid prolonged heating above 1150°C. Austenitic stainless steels that do not contain stabilizing elements titanium or niobium but have a high carbon content, such as lCrl8Ni9, 2Crl8Ni9, etc., when staying in the range of 500~750℃, chromium carbide will precipitate along the grain boundary, resulting in chromium depletion at the grain boundary , It is easy to produce intergranular corrosion when used in corrosive media. Therefore, brazing of this type of steel should be avoided in this temperature range.

The brazing heating temperature of austenitic-ferritic steel should not be too high to avoid grain growth.

The selection of the brazing heating temperature of precipitation hardening stainless steel is in principle the same as that of martensitic stainless steel, that is, the brazing heating temperature must match the heat treatment system of the steel to obtain the best mechanical properties.

③. Austenitic stainless steel has a tendency of stress cracking, so brazing should be carried out under the state of removing internal stress.

3. Surface Preparation of Stainless Steel

The cleaning methods of stainless steel surface include gas phase degreasing; solvent sodium hydroxide degreasing; spraying or shot blasting; wiping with steel wire brush or stainless steel cotton or polishing with emery cloth and pickling, etc. For mass-produced workpieces, the following pickling solution can be used to clean (mass score):

①. 10% H2SO4, 15% HCl, 5% HNO3, the balance water. The pickling temperature is 100℃, and the pickling time is 30s. Then use 15% HNO3 aqueous solution as gloss treatment, the solution temperature is 100℃, and the time is about 10s.

②. 10% HNO3, 6% H2SO4, 50g/LHF aqueous solution, pickling temperature 20℃, pickling time 10min. After pickling, wash carefully with hot water at 60~70℃ for 10min, and then dry in hot air at 60~70℃.

③. 15% HNO3, 50g/L NaF, 85% H2O solution, etch 5~10min at room temperature, then wash with hot water, and then dry at 100~120℃.

The first solution is suitable for thick oxide scale on the surface of thick parts. The latter two solutions are used for the thin oxide film on the surface of thin parts. Pickling should be carried out in strict accordance with the process regulations to avoid over-corrosion.

The soldering of stainless steel: The soldering temperature of soldering is low, and the influence on the performance of stainless steel itself is minimal. Tin-lead solder is mainly used for soldering, and tin-lead solder with high tin content is suitable, such as HLSn63Pb, HLSn60Pb, HLSn50Pb and HLSn40Pb, because these solders have good wettability. Tin-silver solder can also be used. The choice of flux is the key. A strong active flux must be used to remove the oxide film on the surface.

There are two commonly used fluxes: one is orthophosphoric acid aqueous solution (H3PO4960g, H2O445g); the other is zinc chloride hydrochloric acid aqueous solution (ZnCl2 1360g NH4C1 140g, HCl 85g, H2O 4L). The active time of the phosphoric acid aqueous solution is short, and the brazing method with rapid heating must be adopted. The flux residue is highly corrosive and must be cleaned after brazing.

Shanghai Geheng Vacuum Technology Co., Ltd. is a vacuum heat treatment furnace manufacturer with many years of experience in design, manufacturing and production. Mainly produce vacuum heat treatment furnace, vacuum sintering furnace, vacuum brazing furnace, New Energy And Environmental Protection Equipment and other vacuum equipments.Provide customized services, equipment work area can be based on customer output.