Introduction to Basic Manufacturing Processes and ... - always yours

342 Introduction to Basic Manufacturing Processes and Workshop Technology
temperature of the base metal. The filler metal is drawn into the joint by means of capillary
action (entering of fluid into tightly fitted surfaces). Brazing is a much widely used joining
process in various industries because of its many advantages. Due to the higher melting point
of the filler material, the joint strength is more than in soldering. Almost all metals can be
joined by brazing except aluminum and magnesium which cannot easily be joined by brazing.
Dissimilar metals, such as stainless steel to cast iron can be joined by brazing. Because of
the lower temperatures used there is less distortion in brazed joints. Also, in many cases the
original heat treatment of the plates being joined is not affected by the brazing heat. The joint
can be quickly finished without much skill. Because of the simplicity of the process it is often
an economical joining method with reasonable joint strength. The brazed joints are reasonably
stronger, depending on the strength of the filler metal used. But the brazed joint is generally
not useful for high temperature service because of the low melting temperature of the filler
metal. The color of the filler metal in the brazed joint also, may not match with that of the
base metal. Because the filler metal reaches the joint by capillary action, it is essential that
the joint is designed properly. The clearance between the two parts to be joined should be
critically controlled. Another important factor to be considered is the temperature at which
the filler metal is entering the joint.
During brazing, the base metal of the two pieces to be joined is not melted. An important
requirement is that the filler metal must wet the base metal surfaces to which it is applied.
The diffusion or alloying of the filler metal with the base metal place even though the base
metal does not reach its solidus temperature. The surfaces to be joined must be chemically
clean before brazing. However, fluxes are applied to remove oxides from the surfaces. Borax
is the most widely used flux during the process of brazing. It will dissolve the oxides of most
of the common metals.
17.13.1 Methods of Brazing
Torch Brazing
It is the most widely used brazing method. Heat is produced, generally, by burning a
mixture of oxy-acetylene gas, as in the gas welding. A carbonizing flame is suitable for this
purpose as it produces sufficiently high temperature needed for brazing.
Furnace Brazing
It is suitable for brazing large number of small or medium parts. Usually brazing filler
metal in the granular or powder form or as strips is placed at the joint, and then the assembly
is placed in the furnace and heated. Large number of small parts can be accommodated in
a furnace and simultaneously brazed.
17.13.2 Braze Welding
In welding processes where the joint of the base metal is melted and a joint is prepared
having higher joint strength, it is likely to cause metallurgical damage by way of phase
transformations and oxide formation. In this process, the base metal is not melted, but the
joint is obtained by means of a filler metal.
17.14 SOLDERING
Soldering is a method of joining similar or dissimilar metals by heating them to a suitable
temperature and by means of a filler metal, called solder, having liquidus temperuatre not