Figure 1. Brazed Stainless Steel Components
During the brazing process, a brazed joint is formed by the filler metal melting and flowing via a capillary effect into the pores of the closely fitted surfaces of the joint to form an alloy of the metals upon solidification.
Figure 2. Brazing Schematic
The key to successfully achieving a good brazed connection is surface preparation. The presence of contaminants or oxides prevents the filler metal from coming into contact with one of the surfaces to be brazed. In the case of minor oxidation, the pores of the surfaces to be brazed will be sealed by the oxide. This prevents the capillary action and, ultimately, the brazing from occurring. Hence, the initial cleanliness of the surfaces to be brazed is extremely important, but it is equally important that the cleanliness of these surfaces be maintained during the brazing process.
Figure 3. Effects of Oxidation on Brazing
Achieving and maintaining the necessary level of cleanliness is much more difficult for brazing stainless steel components than brazing steel components. The Chromium in the stainless steel forms a much more stable oxide at a much lower Oxygen level than iron.
Figure 4. Oxide Stability
The oxides present on the surface must be reduced prior to the part reaching the melting temperature of the filler metal. The reduction is typically achieved through a reaction of Hydrogen with the Oxygen present in the oxide to form water vapor.
Figure 5. Oxide Reduction Mechanism
The presence of too much water vapor or Oxygen in the system will prevent the reaction from proceeding. The dew point is used to determine the amount of water vapor in the system at given conditions. The dew point is the temperature at which an amount of water vapor in the system will saturate the atmosphere. The typical dew point required for brazing stainless steel joints in 100% Hydrogen is –50º F.
Figure 6. Dew Point Requirements