The term stainless steel actually refers to a general group of metals, all of which have different heat resistant and corrosion properties. All stainless steels can be categorized as austenitic, martensitic and ferritic steels. For successful steel brazing, it is important to understand each of these material groups.
Ferritic Stainless Steel
The steels in this group include 0.5% nickel with up to 12-18% chromium. In addition, these metals are magnetic and their properties will not improve with heat treatment. Also known as stainless irons, steel brazing in this group can prove to be difficult and are better suited for welding.
Martensitic Stainless Steel
Martensitic steels are heat treated, thus temperature influenced the corrosion and mechanical properties of this material. It may be necessary to localize heat or heat treat following the steel brazing process. Like ferritic steels, these steels are also magnetic.
Austenitic Stainless Steel
These steels are based on an 8% nickel 18% chromium composition. However, the chromium addition may vary from 15% to 22% and nickel can vary from 6% to 11%. Austenitic steels cannot be hardened through heat treating and rely on mechanical working instead. Therefore, any steel brazing will lessen the mechanical properties.
Brazing with Care
Many applications that require stainless steel subject the material to stress and corrosive environments. While strong ductile joints can be made, care must be taken in alloy choice and steel grade to achieve the best results. Steel brazing requires the steel to be heated to a temperature where weld decay will occur.
Special care must be taken when deciding on the brazing alloys to use for stainless steel when the joints are going to be exposed to humidity or water. Joint failure due to corrosion, known as crevice or interfacial corrosion, can result at the alloy/steel interface.
It is essential to use flux when steel brazing in air. It is important to note that when prolonged heating is required, a flux metal reaction can occur. This reaction produces a film on the stainless steel surface and it cannot be wetted by the brazing alloy. To remove the film, additional fresh flux must be used. In this case, a flux with improved high temperature properties must be used because it will not react with the stainless steel. However, residues are not water soluble and must be removed with mechanical methods or caustic soda. Fluxes that contain boron should never be used for steel brazing when crevice corrosion is a known service risk.