In the previous posts for this topic, corrosion-resistant alloys were defined and techniques for obtaining the best brazing joints were discussed. The balance of our discussion addresses issues critical to these steels and helps readers to select the correct brazing filler metal (BFM).
Brazing is a highly versatile and cost-effective high-temperature method to permanently join a wide range of heat- and corrosion-resistant alloys for service in such diverse industries as automotive, aerospace, medical, electrical, hand-tools, cutlery and food-handling.
When brazing stainless steel assemblies, such as those made from 304 or 316 stainless, it is very important for the brazing shop to only specify the “L” (low-carbon) versions of these alloys for brazing (or welding). A common mistake made by many shops is to only specify, and braze, the standard versions of these alloys. That is an incorrect choice and has resulted in premature corrosive failures of stainless components in the field. As shown in Table 1, the “L” version of 304, for example, typically contains only about one-third (or less) of the amount of carbon as does the standard grade.
During brazing processes, the stainless steel can easily dwell long enough between 800-1500°F (425-815°C) – known as the “sensitization” range – for carbon in the stainless to quickly react with the chromium in the metal to form chromium carbides. This depletes those regions of the chromium-oxide protective layer, resulting in oxidation/corrosion of that depleted region. This often exhibits itself as a rust-band on each side of a weldment or as a generalized oxidation/corrosion on furnace-brazed components. Be sure your purchasing personnel are made aware of this, and understand the need to only buy the “L” version of these alloys.
Some designers specify 321 stainless for brazing, and I strongly urge caution in so doing. Refer once again to Fig. 4 (metal/metal-oxide curves) for the titanium-oxide curve on the far right side. 321 stainless uses excess titanium additions (a minimum of 5x the carbon in the alloy) so that the titanium will react with the carbon to prevent the carbon from depleting the chromium oxides.
Once the carbon has been “tied-up” so to speak, what happens with the rest of the excess Ti present in the stainless matrix? A number of brazing shops report that the excess titanium then reacts directly with any oxygen present in the furnace atmosphere, forming a dark gray/brown surface discoloration that is then difficult to braze. For this reason, I do NOT recommend specifying 321 stainless for any brazing applications. Instead specify 304L, 316L or stabilized 347 stainless, which uses niobium instead of titanium in its matrix and is therefore preferable to 321.