Appendix B - Brewing Metallurgy
B.3 Soldering, Brazing, and Welding Tips
Soldering with a propane torch is the easiest way to join copper and brass. You can even use solder to join copper or brass to stainless steel, you just need the proper flux. But there are a couple tips to keep in mind to make it work right the first time:
- Use a liquid flux instead of a paste flux. The paste flux tends to leave tacky residue that is difficult to clean off. If you must use a paste flux, use it sparingly.
- Use plumbing (silver) solder only. Do not use electrical or jeweler's solder because these often contain lead or cadmium. These are toxic metals.
- Apply solder separately to each of your parts before joining them. This practice is known as "tinning" and makes joining the parts easier.
- Heat the parts, not the solder. Play the flame all around the joint to get it good and hot before you apply the solder. This allows the solder to flow evenly over the joint.
Brazing is like soldering but it is done at higher temperatures and is applicable to more metals. It can readily join stainless steel to itself, and is an alternative to welding. The recommended filler rod for brewing service is AWS type BAg-5, and its temperature range 1370-1550°F (743-843°C). While brazing can provide a stronger joint, the high brazing temperatures can be bad for stainless steel. At those temperatures, carbon in the stainless steel can form chromium carbides which takes the chromium out of solution, making the steel non-stainless near the joint. This area is prone to rust and cracking after it is in service. The problem cannot be fixed by re-passivation so it is best to avoid excessively heating the parts during the braze and keep the total time at temperature to four minutes or less. Propane torches are usually not adequate for brazing. You will need to use MAPP gas or acetylene.
Welding is the best methods for joining stainless steel, but it takes skill to make a good joint. There are two welding processes that will work- MIG (auto-wire feed type) and TIG (tungsten electrode type). TIG welding allows the best control for these small joints. Your best bet is to look in the Yellow Pages of your phonebook for a stainless steel welder to do the job for you. The cost should be minimal, $20-50 depending on the amount of welding needed. I had pipe nipples welded onto 3 converted kegs for 20 dollars. If you want to do it yourself, or you have a friend that welds but has not done stainless before, here is what you need to know:
Table 20 - Suggested Welding Parameters
Ideally, the backside of the weld should be purged with the argon gas to prevent heavy oxidation. But, most welders don't bother, so the backside of the weld joint should be ground/sanded down afterwards to expose clean metal. Do not use steel wool! To clean off the black/blue-ish oxides that could initiate corrosion in the heat affected zone around the weld or braze joints on stainless steel, use the oxalic acid based cleansers and procedures mentioned above in the passivating section.