Fabricating Carpenter Stainless Steels
June 2006
Soldering and Brazing Carpenter Stainless Steels
Soldering and brazing differ
only in the temperatures used to melt the alloy being used to join the material
being soldered or brazed. The material being joined is not melted by these
joining processes.
Soft Soldering:
Soft soldering of
stainless steel is not much of a problem when the requirements of the job are
understood. The biggest problem is breaking through the passive film with a flux
so that the solder will wet the stainless.
Soft solders are weak
compared with stainless steel. Consequently, if strength is required, the edges
should first be riveted or spot-welded, then soldered for a tight seal.
Stainless steel must be
perfectly clean before soldering is attempted. Cleaning can be accomplished by
pickling with acid or with mechanical polishing. Do not expect the flux to do
the cleaning.
Stainless steel is resistant
to the corrosive attack of most soldering fluxes, and unless the flux etches the
surface, it will not function. On smooth surfaced parts, such as cold rolled
strip, it will be difficult to get the flux to spread and completely cover the
surface. Therefore, the soldering area should first be roughened by acid etching
(50:50 muriatic acid and water) or mechanical polishing. This rough surface will
take the flux quickly and the solder will flow evenly.
Use fluxes prepared
especially for soldering stainless steel. Apply the flux with a brush to the
area to be soldered and rub until the surface is wet. All flux must be properly
and completely removed after soldering to avoid continued corrosion. Be sure to
remove all splattered flux with soap and water.
Stainless steels are slower
to absorb heat, and it is, therefore, necessary to use a larger and heavier
iron. The iron need not be hotter, but it should be bigger and possess more heat
capacity. That way, the iron will heat a sufficient area to allow the
solder to flow freely. "Tinning" the joint will also assist in making the solder
flow more evenly. Keep moving as fast as the solder fills the joint.
Ordinary half-and-half solder
applied from the top of a well-tinned copper is satisfactory but for brighter,
stronger joints, use 67 percent tin and 33 percent lead dairy solder. In
general, the higher the lead content, the more quickly the joint will darken on
exposure to air.
Hard Soldering or Brazing: This process is also
called silver soldering and is applicable to all types of stainless steels. The
temperature range in which this process is applied is typically from 1150 to
1500°F (621 to 816°C), although brazing may be performed at temperatures up to
above 2000° F (1093°C), depending on the composition of the brazing material.
The straight chromium martensitic steels will air harden if heated above 1450°F
(788°C). Exercise care to limit the heating of ferritic steels to the minimum
required for flow of the solder in order to avoid grain growth and embrittlement
in these grades. The chrome-nickel austenitic steels are necessarily heated in
the carbide precipitation range, which may affect their corrosion resistance.
Lap-type joints are used in
silver brazing. Joint clearances should be between 0.002" and 0.005" (0.051 to
0.127 mm) for best distribution of filler metal in the joint by capillary
attraction. Silver brazing alloys for stainless steel contain from about 50
percent to 75 percent silver. The best color match is obtained with the alloys
containing higher percentages of silver. A flux is generally required to make a
satisfactory joint. However, for certain processes, particularly the straight
silver-copper filler metals, if brazing is done in a vacuum or in inert
atmosphere, flux may not be needed.
The bi-metallic nature of the
joint makes it very difficult to predict the corrosion resistance of
silver-brazed joints in stainless steel. Give consideration to crevice corrosion
whenever the fluid contains small amounts of chlorine compounds. Cleaning of the
flux after brazing is essential to prevent corrosion failure. The most common
method of cleaning flux is with a hot water rinse of long enough duration to
dissolve all the flux.
NOTE: American Welding Society
Specification AWS A5.8 for Brazing Filler Metal prescribes requirements for
filler metals which are added when making a braze.
