Knowledge of stainless steel brazing

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Stainless steel is defined as the main chromium element to make steel in the passivation state, is a stainless steel characteristics. Stainless steel is mainly Cr-Fe and Cr-Fe-Ni ternary alloy. Iron is the matrix and chromium is the main alloying element. In order to make steel rust – proof, the (Cr) must be higher than 12%. At this point, the surface of the steel can quickly form a dense Cr2O3 oxide film, so that the electrode potential of the steel and the corrosion resistance of the oxidation medium have mutability. In the non-oxidizing medium (HCl, H2SO4), in fact, the role of chromium is not obvious, in addition to chromium, stainless steel must be added can passivate steel Ni, Mo and other elements, adding alloy elements are basically divided into two types:

One is the element that forms or stabilizes austenite: carbon, for example. Nickel, manganese, nitrogen, etc., among which carbon and nitrogen are the most used;
The other is to shrink or block the gamma-phase elements that form ferrites: chromium, silicon, molybdenum, titanium, niobium, aluminum, etc.
Due to the different alloying elements, stainless steel presents different tissues under the greenhouse. According to its different structure, stainless steel can be divided into ferritic stainless steel, martensitic stainless steel, austenitic stainless steel, austenitic – ferrite stainless steel and precipitation hardened stainless steel.

Austenitic stainless steel in various types of the most widely used, varieties are also the most. Due to the high content of Cr and Ni in austenitic stainless steel, it has good corrosion resistance in oxidizing, neutral and weak reducing medium. Austenitic stainless steel is widely used in architectural decoration, food industry, medical equipment, textile printing and dyeing equipment, petroleum, chemical, atomic energy, aerospace and other industrial fields because of its excellent plastic toughness and excellent cold and hot processing performance.

To highlight the contrast, the corresponding physical properties of steel carbon are listed. Carbon steel has little difference in density from stainless steel. The resistivity of carbon steel, ferritic stainless steel, austenitic stainless steel sequence increase. The resistivity of austenitic stainless steel can reach 5 times that of carbon steel. The linear expansion coefficient of austenitic stainless steel is about 50% larger than that of carbon steel, while the linear expansion coefficient 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. The thermal conductivity of another type of stainless steel for carbon steel about 1/2.

Brazing property of stainless steel
1. Surface oxidation film. As mentioned above, stainless steel in addition to the main alloy element chromium, often contain nickel, manganese, titanium, molybdenum, niobium, aluminum and other elements. 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). Cr2O3 and TiO2 are quite stable and difficult to remove. Active brazing agent must be used to remove these oxides when brazing in air. When brazing in a protective atmosphere, oxygen film can be reduced only at a low dew point and a high enough temperature. In vacuum brazing, good vacuum degree (above 10-2Pa) and high enough temperature are required to achieve good results.

2. Brazing heating temperature. For ferrite stainless steels, as long as the brazing heating temperature does not cause violent grain growth, it is considered appropriate.
For martensitic stainless steel, the brazing temperature has a great effect on the performance because martensitic stainless steel is used in quenching and tempering. There are two choices for brazing heating temperature of martensite stainless steel: one is to match brazing heating temperature with quenching temperature. For example, for 1Crl3 and 2Crl3 stainless steel, select the brazing heating temperature of 1000~1050℃, after brazing, rapid cooling, to achieve the purpose of quenching base material, and then temper, so as to obtain the best comprehensive mechanical properties.

The other is to choose the brazing temperature below the tempering temperature of the steel, such as below 700℃ for lCrl3 stainless steel. In this way, the quenched and tempered base material will not soften during brazing, and the base material will still maintain its original comprehensive properties.

For austenitic stainless steel brazing heating temperature should not be too high. When the brazing temperature is higher than 1150℃, the grain begins to grow vigorously. Once the austenitic stainless steel grains grow, they cannot be refined by heat treatment. Therefore, prolonged heating above 1150℃ should be avoided when selecting solder and brazing process parameters. Austenitic stainless steel without stable element titanium or niobium and with higher carbon content, such as lCrl8Ni9, 2Crl8Ni9, etc., when it stays within the range of 500~750℃, chromium carbide will be precipitated along the grain boundary, resulting in grain boundary poor chromium, in the corrosive medium used to generate intercrystal corrosion easily. Such steels should therefore avoid brazing within this temperature range.

The brazing heating temperature of austenitic – ferrite steel should not be too high to avoid grain growth.
In principle, the brazing heating temperature of precipitation hardened stainless steel is the same as that of Martensite stainless steel, that is, the brazing heating temperature must match the heat treatment system of steel to obtain the best mechanical properties.

3, austenitic stainless steel has the tendency of stress cracking, should be in the state of the removal of internal stress brazing.
Stainless steel surface preparation
Cleaning methods for stainless steel surfaces include gas phase oil removal; Solvent sodium hydroxide for oil removal; Peening or shot peening; Wipe with steel wire brush or stainless steel cotton or abrasive cloth grinding and pickling. For mass production workpieces can be cleaned with the following acid wash solution (mass fraction) :

1. 10%H2SO4, 15%HCl, 5%HNO3, residual water. Pickling temperature 100℃, pickling time 30s. Then 15%HNO3 aqueous solution was used as the gloss treatment, the solution temperature was 100℃, and the time was about 10s.

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

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

The first solution is suitable for the thick oxide scale on the thick piece surface. The latter two solutions are used for thin oxide films on thin surfaces. Pickling should be carried out in strict accordance with the technical procedures to avoid excessive corrosion.

Soft brazing of stainless steel: the brazing temperature of soft brazing is low, which has little effect on the performance of stainless steel itself. Soft brazing is mainly made of tin-lead filler metals, preferably those with high tin content, such as HLSn63Pb, HLSn60Pb, HLSn50Pb and HLSn40Pb, because of their good wettability. Tin-silver solder is also available. The choice of brazing agent is the key. A strong flux must be used to remove the oxide film on the surface.

Learn More : Sintering Furnace