Page 122 - CITS - Welder - Trade Theory

WELDER - CITS

LESSON 31 & 32 : Brazing - Principal, Types of brazing,

Application

Objectives

At the end of this lesson you shall be able to
• describe the method of brazing
• describe the types brazing
• explain the application of brazing.

Brazing: Brazing is a metal joining process which is done at a temperature of above 450°C as compared to
soldering which is done at below 450°C
So brazing is a process in which the following steps are followed.

- Clean the area of the joint thoroughly by wire brushing, emerging and by chemical solutions for removing oil,
grease, paints etc.
- Fit the joints tightly using proper clamping. (Maximum gap permitted between the two joining surfaces is only
0.08 mm)
- Apply the flux in paste form (for brazing iron and steel a mixture of 75% borax powder with 25% boric acid
(liquid form) to form a paste is used). Usually the brazing flux contains chlorides, fluorides, borax, borates,
flurodorates, boric acid, wetting agents and water. So suitable flux combination is selected based on metal
being used.
Brazing is employed where a ductile joints is required.
Brazing filler rods/ metals melt at temperature from 860°C to 950°C and are used to braze iron and its alloys.
Brazing fluxes: Fused borax is the general purpose flux for most metals.
It is applied on the joint in the form of a paste made by mixing up with water.

If brazing is to be done at a lower temperature, fluorides of alkali materials are commonly used. These fluxes will
remove refractory oxides of aluminium, chromium, silicon and beryllium.
Various methods of brazing

Torch brazing: The base metal is heated to the required temperature by the application of the oxy-acetylene
flame. (Fig 1)
Furnace brazing: The parts to be brazed are aligned with the brazing material placed in the joint. The assembly
is kept in the furnace. The temperature is controlled to provide uniform heating. (Fig 2)
Dip brazing: The parts to be brazed are submerged in a molten metal or chemical bath (Fig 3) of brazing filler
metal.
Induction brazing: The parts to be brazed are heated to the melting point of the brazing material by means of a
high frequency electric current. This is done by encircling the joint with a water cooled induction coil (Fig 4).
Silver brazing: Silver brazing is also sometimes called silver soldering. It is one of the best methods used to
connect/join parts which are to be leak proof and has to give maximum strength of the joint. It is a very useful
and easy process for joining copper brass, bronze parts as well as for joining dissimilar metal tubes like copper
to stainless steel tubes etc. The melting point of silver brazing alloy filler rods will be around 600 to 800°C which
is always less than that of the base metals joined. Fig 5 shows silver brazing of stainless steel tube to be with a
copper tube.

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CITS : CG & M - Welder - Lesson 27 - 30