Stainless steel is hard to braze primarily due to the presence of stable oxide films on its surface, which hinder the wettability and spreading of the filler metal. These oxides, particularly those of chromium (Cr2O3) and titanium (TiO2), are very stable and difficult to remove. This stability is due to the high content of chromium and other alloying elements like nickel, titanium, manganese, molybdenum, and niobium in various stainless steel compositions, which can form various oxides and complex oxides.
The removal of these oxides is crucial for successful brazing. In atmospheric brazing, this is typically achieved using active fluxes that chemically reduce the oxides. However, in vacuum or protective atmosphere brazing, the conditions must be carefully controlled. Vacuum brazing requires high vacuum levels (typically 2 x 10-4 torr or better) and high temperatures to decompose the oxide films, allowing the filler metal to wet the stainless steel surface effectively. This method avoids the use of fluxes and minimizes the risk of oxidation, leading to better joint quality.
Additionally, the choice of filler metal and the brazing temperature are critical. Filler metals with self-fluxing characteristics or those that are more aggressive can improve the brazeability of stainless steel. The brazing temperature must also be carefully selected to match the heat treatment regime of the specific type of stainless steel to ensure optimal mechanical properties. For instance, austenitic stainless steel should not be heated above 1150°C during brazing to prevent excessive grain growth, which could degrade the material's mechanical properties.
Furthermore, austenitic stainless steel is susceptible to stress cracking during brazing, especially when using copper-zinc filler metals. To mitigate this, stress relief annealing is recommended before brazing, and uniform heating during the brazing process is crucial.
In summary, the challenges in brazing stainless steel stem from the stable oxide layers on its surface, the need for precise temperature control, and the susceptibility to stress cracking. These factors necessitate careful selection of brazing methods, filler metals, and process parameters to achieve successful and high-quality joints.
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