Customers of metal treatment companies often ask what would happen if they needed stainless steel assemblies that were too large to fit inside a furnace. In most cases, the answer is that the assemblies would be brazed in open air using a torch. While this type of brazing is not ideal for metals that contain chromium, which oxidizes when it is heated in the presence of oxygen, stainless steel can be successfully brazed in open air when the right measures are taken.
Apply Specially Formulated Flux
When stainless steel assemblies are treated in an atmosphere-controlled furnace that sustains a pure nitrogen/hydrogen atmosphere, they do not need flux applied to the brazing joints and adjacent areas. The furnace’s oxygen-free atmosphere prevents the chromium in the metal from oxidizing. However, when the metal is brazed in open air using a torch, flux that is specially formulated for stainless steel should be applied to brazing joints and adjacent areas.
There are several types of stainless steel fluxes available, but paste fluxes generally work best at preventing a stable layer of oxide from forming. In addition to having a thick consistency that forms a substantive barrier between the flame and the base metal, paste flux is superior to liquid flux for its ability to wet and remain on vertical surfaces throughout the brazing process. A provider of brazing services applies flux to assemblies before heat is applied to the metal.
Use a Low-Temperature Filler Metal
To ensure that the filler metal distributes between the work pieces as evenly, as it would in a furnace, low-temperature silver brazing alloys that have a melting point between 600ºC and 718ºC are typically used for the filler. These alloys are particularly recommended when the failure of a brazing joint due to crevice corrosion can be predicted. Fillers that contain cadmium or zinc are known to cause corrosion due to phases that lead to preferential corrosion.
Using low temperature filler metal is also beneficial for keeping the temperature and duration of the brazing process as low and brief as possible. The higher the temperature and the longer the metal is exposed to heat, the greater the chance that oxidation will occur.
Even Temperature Distribution
One of the main advantages of performing brazing in a furnace is that the equipment evenly heats the assembly, which prevents the metal from developing a heterogeneous microstructure that contains stresses that could lead to compromised dimensional stability. Ensuring even heat distribution when using a torch is a highly specialized procedure that should be performed by metal treatment companies that have extensive experience with the process.
Contact Franklin Brazing
With over 40 years of experience in providing brazing services, Franklin Brazing has the expertise you need in a metal treatment company. Whether you need assemblies that can be brazed in a furnace or must be processed using a high-powered torch, we will provide you with high-quality results in a short period of time. For more information about our brazing services, call us today at (800) 450-7782, or click here to use our contact form.