Page 124 - CITS - Welder - Trade Theory

WELDER - CITS

The process is similar to other brazing processes. The points to be remembered while silver soldering are:
- The joint must be thoroughly cleaned both mechanically and chemically.
- Fit the joint closely/tightly without any gap and support the joint. (The maximum permissible gap between the
parts to be silver brazing is 0.08mm).
Apply proper flux at the joint and on the filler rod.
Heat the joint to the brazing temperature depending on the composition of the silver brazing filler rod. The brazing
temperature may vary from 600°C to 800°. Use an oxy-acetylene blow pipe for heating.
Apply the silver brazing filler rod coated with the pasty flux at the joint using leftward technique. Heat the filler
rod to the “flow temperature” which is usually 10 to 15° more than its melting temperature. i.e, for the filler metal
to flow easily into the joint and for getting the wetting and capillary action, it is necessary to heat the molten filler
metal to 10 or 15° more than its melting temperature.
Allow the joint to cool without removing the support given to the joint.
Clean the joint thoroughly to remove all residual flux.
Fluxes used for silver brazing may be chlorides or borax made into a paste with water.
Brazing and braze welding: Both brazing and braze welding are metal joining processes which are performed
at temperatures above 840°F (450°C) as compared to soldering which is performed temperatures below 840°F
(450°C).
The American Welding society defines these processes as follows:
Brazing - A group of welding processes which produces coalescence of materials by heating them o a suitable
temperature and by using a filler metal having a liquids above 840°F (450°C) and below the solidus of the base
metal . The filler metal is distributed between the closely fitted surfaces of the joint by capillary action” coalescence
is a joining or uniting of materials.
Braze welding - A welding process variation in which a filler metal, having a liquids above 840°F (450°C) and
below the solidus of the base metal, is used. Unlike brazing, in braze welding the filler is not distributed in the joint
by capillary action”
Brazing has been used for centuries. Blacksmiths, jewelers, armourers and other crafters used the process
on large and small articles before recorded history. This joining method has grown steadily both in volume and
popularity. It is an important industrial process, as well as jewelry making and repair process. The art of brazing
has become more of a science as the knowledge of chemistry, physics and metallurgy has increased.
The usual terms Brazing and Braze welding imply the use of a nonferrous alloy. These nonferrous alloys consist
of alloys of copper. tin, zinc, aluminum, beryllium, magnesium, silver, gold and others.
Brass is an alloy consisting chief of copper and zinc. Bronze is an alloy consisting chiefly of copper and tin. Most
rods used in both brazing and braze welding on ferrous metals are brass alloys rather than bronze. The brands
which are called bronze usually contain a small percent (about one percent) of tin.
Brazing and braze welding principles: Brazing is an adhesion process in which the metals being joined are
heated but not melted: the brazing filler metal melts and flows at temperatures above 840°F (450°C). Adhesion is
the molecular attraction exerted between surfaces.
A brazed joint is stronger than a soldered joint because of the strength of the alloys used. In some instances it is
as strong as a welded joint. It is used where mechanical strength and leap roof joints are desired. Brazing and
braze welding are superior to welding in some applications, since they do not affect the heat treatment of the
original metals as much as welding.
Brazing and braze welding wrap the original metals less and it is possible to joint dissimilar metals. For example,
steel tubing may be brazed to cast iron, copper tubing brazed to steel and tool steel brazed to low carbon steel.
Brazing is done on metals which fit together tightly. The metal is drawn into the joint by capillary action. (A liquid
will be drawn between two tightly fitted surfaces. This drawing action is known as capillary action). Very thin layers
of filler metal are used when brazing. The joints and the material being brazed must be specially designed for the
purpose. When brazing, poor fit and alignment result in poor joints and in inefficient use of brazing filler metal.
In braze welding, joint designs used for oxy fuel gas or arc welding are satisfactory. When braze welding, thick
layers of the brazing filler metal is used.

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CITS : CG & M - Welder - Lesson 31 & 32 CITS : CG & M - Welder - Lesson 31 & 32