Stainless steels may be brazed by all processes, but with tighter process controls than required to braze carbon steels. The most rigorous requirements are imposed by inherent chemical characteristics of the stainless steels and the generally more arduous service environments. Success in brazing stainless steel components depends on a knowledge of the properties of stainless steels and rigid adherence to the appropriate process controls.

Base Metals- Stainless steels may be grouped into five categories: (1) austenitic (nonhardenable) steels, (2) ferritic (nonhardenable) steels, (3) martensitic (hardenable) steels, (4) precipitation hardening steels, and (5) duplex stainless steels. All these alloys are iron based and contain chromium, the basic element that imparts corrosion resistance. The corrosion resistance of the stainless steels varies from one alloy tio another, and for any given alloy, varies from one corrosive medium to another. If doubt exists about the correct stainless steel to use in a given environment, standard reference works or manufacturer’s representatives should be consulted.

Filler Metals- Factors to be considered in selecting a filler metal for a particular application include the following:

(1) Service conditions, including operating temperature, stresses, and environment

(2) Heat treatment requirements for martensitic or precipitation hardening steels

(3) Brazing process

(4) Cost

(5) Special precautions, such as sensitization of unstabilized austenitic steels at certain temperatures.

Commercially available brazing filler metals used for joining stainless steels are commonly the copper, silver, nickel, cobalt, platinum, palladium, and gold based alloys.

Process and Equipment- Stainless steels can be brazed with any brazing process. Much controlled atmosphere brazing is performed on stainless steels, and the acceptability of this technique is attributed to the ready availability of reliable atmospheres and vacuum furnaces. The primary requirements are that the furnaces have good temperature control, plus or minus 8°C (15°F), at brazing temperature and be capable of fast heating and cooling. All gases used in atmosphere furnaces must be of high purity (>99.995%pure). Commercial vacuum brazing equipment operates at pressures varying from 0.0015 to 13.5 Pa (10-5  to 10-1 torr). The necessary vacuum level depends on the particular grade of stainless steel, with those containing titanium or aluminum requiring better vacuums.

Precleaning and Surface Preparation- Stainless steels require more stringent precleaning than carbon steels. During the heating cycle, residual contaminants often form tenacious films which are difficult to remove by fluxes or reducing atmospheres. These films form as a direct reaction between the contaminant and stainless steel surface.

Precleaning for brazing should include a degreasing operation. The joint surfaces should also be cleaned mechanically or with an acid pickling solution. Wire brushing should be avoided, but if necessary, stainless steel brushes should be used. Cleaned surfaces should be protected to prevent soiling by dirt, oil or fingerprints. The parts should be brazed immediately after cleaning. When this is not practical, the cleaned parts should be enclosed in containers such as sealed polyethylene bags or desiccator jars to exclude moisture and other contaminants until the part can be brazed.

Fluxes and Atmospheres- Stainless steel assemblies are routinely furnace brazed in atmospheres of dry hydrogen, argon, helium, dissociated ammonia, or vacuum,, without the aid of flux. When fluxes are required, there are a number of special compositions available for use with stainless steels. There are many special requirements for brazing stainless steels; appropriate references should be consulted.

Postbraze Operations- The major stainless steel postbraze operations that may be necessary are removal of flux or stopoff residues, and any required postbraze heat treatment.

Depending on the materials used, flux or stopoff residues can be removed by water rinsing, chemical cleaning, or mechanical means. With abrasive cleaning, the grit should be sand or another nonmetallic material. Metallic shot, other than stainless steel, should be avoided because particles may become embedded in the stainless steel surface and cause rusting or pitting corrosion in service.

Unless the brazing cycle is compatible with the heat treating requirements of the base metal, heat treatment after brazing will be required for assemblies which are made with martensitic or precipitation hardening stainless steels. Since treatments vary so widely, no general rules can be made except that supplier recommendations should be followed.

Repair Methods- When furnace brazed assemblies contain many joints, minor defects may occur that are beyond acceptance limits, but which are not economically or technically feasible to repair by rebrazing the entire assembly. In some cases, repairs can be made by localized rebrazing using oxyacetylene or gas tungsten arc torches. The manual gas tungsten arc method is useful for braze fillet repairs on applications like turbine engine stators. Filler metal is added, as required.

Prototype work on a mock-up with proper evaluation prior to actual repair work is recommended.

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