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Precautions for vacuum brazing of stainless steel

Stainless steel contains at least 11 wt% chromium and is known as a material with a minimum chromium content of 10.5~12 wt%. Compared with carbon steel and low alloy steel, chromium produces a passivated chromium oxide layer on the surface of stainless steel, which can improve its oxidation resistance and corrosion resistance.
Among the five types of stainless steels, the ones with the lowest chromium content and therefore the worst corrosion resistance are martensitic stainless steels, such as type 403 (nominal 12.25 wt-%Cr), type 410 and type 414 (nominal 12.50 wt-%Cr. This is also This means that martensitic stainless steel contains approximately 85% to 90% iron. Each stainless steel contains less chromium than type 304 austenitic stainless steel (nominal 18%Cr–8%Ni).
ASTM A709 Grade 50CR stainless steel contains 10.5-12.5 wt% chromium and has a maximum carbon content of 0.03 wt%, which is much lower than the above-mentioned martensitic stainless steels with a maximum carbon content of 0.08 wt% or higher. Grade 50CR is similar to Type 410, but has a much lower carbon content, similar to the low carbon 304L and 316L austenitic grades of stainless steel.
As we all know, in vacuum brazing of stainless steel, it is very important to ensure the cleanliness of the base metal, and the separation of stainless steel and carbon steel during the welding process is very critical. The main problems are free iron contamination, such as rust or tool contamination, and surface contaminants or carbon absorption from carbon and low alloy steels. Even with the low chromium content in martensitic stainless steels, it is crucial to prevent contamination of the base metal, and especially the welded joints, by free iron or carbon from carbon or low-alloy steels during preparation.
Contamination with free iron or carbon can reduce the overall corrosion resistance of stainless steel, can also cause intergranular corrosion, and may reduce stress corrosion cracking and crevice corrosion.
Pre-weld cleaning for vacuum brazing typically involves brushing with a stainless steel wire brush used only on stainless steel, blasting or abrasive cleaning with clean sand or grit, and using appropriate machining tools and chlorine-free cutting fluids.
Carbon steel begins to oxidize (weight gain) at about 900 °F and begins to oxidize very rapidly thereafter; 12 wt% chromium steel (Type 410 martensitic stainless steel) does not begin to oxidize until approximately 1400 °F and oxidizes rapidly thereafter ; 304 begins to oxidize at approximately 1500 °F, and the oxidation rate is significantly slower.
All stainless steel should be kept clean and free from iron and carbon contamination. But this is not as big a problem as it is with austenitic, ferritic, precipitation hardening and duplex stainless steels.
Therefore, even when using ASTM A709 Grade 50CR martensitic stainless steel (basically Type 410), it is important to ensure that the stainless steel is not contaminated by steel or other forms of iron or carbon.