Torch Brazing Stainless Steel

Torch Brazing Stainless Steel
Stainless steels encompass a variety of iron-based alloys, having more than 10% chromium, which offer corrosion resistance through a transparent chromium oxide layer. The AWS Handbook lists five categories of stainless steels:

• Austenitic (nonhardenable) steels (i.e. 300 series)

• Ferritic (nonhardenable) steels (i.e. 400 series)

• Martensitic (hardenable) steels (i.e. 400 series - heat treatable)

Precipitation hardenable steels, due to their heat treatment, are often furnace brazed. Duplex stainless steels are not as common as the rest and will not be discussed.

Stainless steels are often specified in product designs for these reasons:

• Resistance to corrosion

• Retention of strength  

• Resistance to scaling at high temperatures

• Stability at very low temperatures

• Cleanliness and smooth finishes

There are several processes that can be used to braze stainless steels-furnace, induction, resistance, or torch. Today, our focus is on flux brazing with a torch. When mechanized, torch brazing offers production-line efficiency, can be applied to short- and long production runs, and requires a relatively low investment in equipment and fixtures.

Filler Metals
When selecting a filler metal for torch brazing stainless steel, the AWS Handbook recommends considering:

• Service conditions - operating temperatures, stresses and environment

• Base metal composition, or type of stainless steel being used

• Heat-treatment requirements for martensitic or precipitation-hardening steels

• Part thickness or geometry

• Brazing process

• Production rate or cost

• Special precautions, such as sensitization of unstabilized austenitic stainless steels at certain temperatures.

There are several good options for alloy selection, depending on your application and the properties of the stainless steel:

• Alloys like Easy-Flo 3, Silvaloy 404 and Silvaloy 505 provide corrosion resistance (all nickel bearing).

• Cadmium-free alloys containing nickel offer moisture resistance in applications involving food processing or medical/dental devices.

• Although more expensive, silver-base alloys carry the benefits of convenience and low production costs.

Be aware that some combinations of alloys and stainless steels can result in corrosion at the interface. See the Lucas-Milhaupt blog on Interfacial Corrosion for more information.

Brazing Flux
Flux protects both the base metal and filler metal from oxidation during the brazing process. It limits the effects of surface tension by removing or reducing surface oxides, which then helps the filler metal to flow freely when molten.

To select the proper flux, analyze the properties and features you require. Choose a flux that minimizes corrosive action for your particular base metal and filler metal. Also, check the temperature range of the flux, to ensure it covers the brazing temperature of your filler metal. Then, consider the brazing time for your process and the ability of the flux to withstand the process without breaking down. Finally, consider the flux removal required for your application.

AWS FB3-C paste fluxes are often used for brazing steels, nickel/alloys and carbides with high-temperature filler metals. FB3-C fluxes contain boric acid, borates and fluorine compounds. Lucas-Milhaupt offers several options, including Handy Flux® Type B-1 or Black Ultraflux®.

Brazing Steps
To achieve the highest-quality results from your brazing process, follow these fundamentals:

1. Proper fit and clearance - for BAg alloys, use a 0.002-0.005in./0.051-0.127mm gap for flux brazing.
   
   
2. Cleaning - to achieve the metal-to-metal surface contact needed for a strong joint, remove all dust, dirt, debris and oxides with an appropriate cleaning method.
   
   
3. Proper flux/atmosphere - for stainless steel, an FB3-C boron-modified flux helps remove chromium oxide (refractory oxide). 
   
   
4. Proper fixturing - keep fixturing to a minimum; self-fixturing is best, to minimize points of contact with the fixture. If needed, stainless steel is a good material for fixtures because of its low thermal conductivity.
   
   
5. Proper heating - stainless steel base metal is a poorer conductor of heat than copper/brass; if joining stainless steel to copper, the heat should be focused on the better conductor (Cu) for similar size/mass materials. Avoid forming chrome carbides in the stainless steel while brazing by limiting the time at a temperature where the filler metal is molten. Remember, the base metal-not heat from the torch-should melt the filler metal.
   
   
6. Final cleaning - flux is corrosive and must be removed after the brazing process by water rinsing, chemical cleaning or mechanical means.

Quality Joints
Your stainless steel brazing process should yield high-quality brazed joints with these benefits:

• Corrosion resistance - aided by braze alloys containing silver, gold or nickel, plus the use of lap joints

• Leak tightness and ductility - resistant to liquid and gas leakage, and able to withstand vibration and pressure changes

• Strength - aided by triaxial loading conditions and diffusion of the filler metal into the stainless steel; able to withstand both high and low temperatures-from -300 to 400°F (-184 to 204°C), or even 700°F (371°C) for BAg-13

• Appearance - smooth and clean, braze joints can be almost invisible

CONCLUSION:
Brazing stainless steel requires some forethought, as the alloys used to form joints must have properties compatible with the base metal. However, a key advantage is that many dissimilar metals can be joined to stainless steels by brazing. This process can yield strong joints that are ductile, clean and smooth.

Lucas-Milhaupt is dedicated to providing expert information for Better Brazing. Please feel free to share this blog posting with associates. See Lucas-Milhaupt's complete line of brazing filler metals for your operation, and contact us if we may be of assistance.