With only a few exceptions, most types of stainless steel can be brazed. There are three common methods when it comes to stainless steel brazing, which are vacuum brazing, brazing under reducing atmosphere and brazing in air with flux.
Vacuum Brazing
In most stainless steel brazing applications where the vacuum method is utilized, high-temperature filler metals are used. High temperatures in excess of 1832°F are the standard for vacuum brazing. This allows for the application of heat treating during the brazing operational cycle.
As a part of the vacuum brazing process, the furnace being used may be back-filled with an inactive gas to aid in flushing out any remaining air from capillary paths of brazed parts. Then, the gas is eliminated prior to the beginning of the brazing operation. Inactive gas may also be used in order to accelerate cooling following the solidification of the filler.
Brazing Under Reducing Atmosphere
Stainless steel brazing with a reducing atmosphere furnace has been gaining popularity for the past two decades. This comes as a result of demands for systems and fuel rails fabricated in stainless steel by the automotive industry.
With this particular application type, chemicals are reduced in surface oxides. A surface that is free of oxides is necessary in order to allow the flow and wetting by molten filler material. For this reason, the brazing is typically done in a continuous conveyor furnace that has been lined with heat resistant alloys. Thus, the atmosphere is properly contained.
Carefully controlled levels of water vapor, oxygen and hydrogen is crucial in the furnaces that are used for stainless steel brazing under reducing atmosphere. In general, copper or copper based alloys are the filler materials used in this method of brazing where the temperatures are usually in excess of 1985°F.
Brazing In Air with Flux
For stainless steel brazing in air with flux, silver brazing alloys at low temperatures are typically used. Fillers that contain either zinc or cadmium, which will result in the corrosion of the stainless steel because of the formation of phases resulting in some preferential corrosion.
Stainless steel brazing requires some consideration because the properties of the alloys that are used to create joints must be harmonious with the base metals. Nevertheless, a primary benefit is that many different metals can be joined to stainless steels through brazing, yielding clean, smooth and strong joints that are incredibly ductile.