Stainless steels

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Stainless steels

Stainless steels are chromium containing iron-based alloys, used in applications demanding thermal or corrosion resistance.

Stainless steels may be grouped into four types:
• austenitic (non-hardenable) (16-26%Cr)
• ferritic (non-hardenable) (10.5-30%Cr)
• martensitic (hardenable)(10.5-18%Cr)
• precipitation hardened (10.5-30%Cr)

These materials can be brazed by all the common processes available; however, they require rigid process control, i.e. combined braze cycle and heat treatment in order to maintain their characteristic properties.
Filler metals

Filler metals used for joining stainless steels include copper, silver, nickel, platinum, palladium and gold based alloys.

For applications where corrosion resistance is required, then silver-based filler metals with nickel should be used. If possible this brazing operation should be carried out in a protective atmosphere without flux.

Nickel-based filler metals offer excellent corrosion resistance, high temperature strength, and oxidation resistance up to 1090°C.

The precious metal based alloys such as, gold, platinum and palladium offer good joint ductility, high operating temperature capabilities along with minimal formation of intermetallics with the base metal.


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Joint performance

The most frequently used protective atmospheres for furnace brazing of stainless steels are dissociated ammonia and dry hydrogen. They are effective in reducing the surface oxides, protecting the base metal and promoting filler metal flow. However, they have associated handling problems due to their potentially explosive nature.

The use of carbon jigging must be avoided, due to the formation of methane with the hydrogen. Designing parts to be self-jigging will remove problems associated with differential expansion between parts and fixtures.

Care should be taken with dissociated ammonia and nitrogen atmospheres as nitriding of the steel surface may result.

The use of vacuum brazing offers excellent corrosion resistance due to the elimination of fluxes. Brazing under vacuum may also eliminate the need for nickel plating (to reduce surface oxidation) for some steels.