Welding Processes
The welding processes may be broadly classified into the following two groups:
1. Welding processes that use heat alone e.g. fusion welding.
2. Welding processes that use a combination of heat and pressure
e.g. forge welding.
These processes are discussed in detail, in the following pages.
10.4 Fusion Welding
In case of fusion welding, the parts to be jointed are held in position while the molten metal is supplied to the joint. The molten metal may come from the parts themselves (i.e. parent metal) or filler metal which normally have the composition of the parent metal. The joint surface become plastic or even molten because of the heat from the molten filler metal or other source. Thus, when the molten metal solidifies or fuses, the joint
is formed.
The fusion welding, according to the method of heat generated, may be classified as:
1. Thermit welding, 2. Gas welding, and 3. Electric arc welding.
10.5 Thermit Welding
In thermit welding, a mixture of iron oxide and aluminium called thermit is ignited and the iron
oxide is reduced to molten iron. The molten iron is poured into a mould made around the joint and
fuses with the parts to be welded. A major advantage of the thermit welding is that all parts of weld
section are molten at the same time and the weld cools almost uniformly. This results in a minimum
problem with residual stresses. It is fundamentally a melting and casting process.
The thermit welding is often used in joining iron and steel parts that are too large to be manufactured
in one piece, such as rails, truck frames, locomotive frames, other large sections used on steam
and rail roads, for stern frames, rudder frames etc. In steel mills, thermit electric welding is employed
to replace broken gear teeth, to weld new necks on rolls and pinions, and to repair broken shears.
10.6 Gas Welding
A gas welding is made by applying the flame of an oxy-acetylene or hydrogen gas from a
welding torch upon the surfaces of the prepared joint. The intense heat at the white cone of the flame
heats up the local surfaces to fusion point while the operator manipulates a welding rod to supply the
metal for the weld. A flux is being used to remove the slag. Since the heating rate in gas welding is
slow, therefore it can be used on thinner materials.
10.7 Electric Arc Welding
In electric arc welding, the work is prepared in the same manner as for gas welding. In this case
the filler metal is supplied by metal welding electrode. The operator, with his eyes and face protected,
strikes an arc by touching the work of base metal with the electrode. The base metal in the path of the
arc stream is melted, forming a pool of molten metal, which seems to be forced out of the pool by the
blast from the arc, as shown in Fig. 10.1. A small
depression is formed in the base metal and the
molten metal is deposited around the edge of this
depression, which is called the arc crater. The slag
is brushed off after the joint has cooled.
The arc welding does not require the metal
to be preheated and since the temperature of the
arc is quite high, therefore the fusion of the metal
is almost instantaneous. There are two kinds of
arc weldings depending upon the type of electrode.
1. Un-shielded arc welding, and
2. Shielded arc welding.
When a large electrode or filler rod is used for welding, it is then said to be un-shielded arc welding.
In this case, the deposited weld metal while it is hot will absorb oxygen and nitrogen from the atmosphere.
This decreases the strength of weld metal and lower its ductility and resistance to corrosion.
In shielded arc welding, the welding rods coated with solid material are used, as shown in Fig.
10.1. The resulting projection of coating focuses a concentrated arc stream, which protects the globules
of metal from the air and prevents the absorption of large amounts of harmful oxygen and nitrogen.
10.8 Forge Welding
In forge welding, the parts to be jointed are first heated to a proper temperature in a furnace orforge and then hammered. This method of welding is rarely used now-a-days. An electric-resistance welding is an example of forge welding.
In this case, the parts to be joined are pressed together and an electric current is passed from one part to the other until the metal is heated to the fusion temperature of the joint. The principle of applying heat and pressure, either sequentially orsimultaneously, is widely used in the processesknown as *spot, seam, projection, upset and flash
welding.
The welding processes may be broadly classified into the following two groups:
1. Welding processes that use heat alone e.g. fusion welding.
2. Welding processes that use a combination of heat and pressure
e.g. forge welding.
These processes are discussed in detail, in the following pages.
10.4 Fusion Welding
In case of fusion welding, the parts to be jointed are held in position while the molten metal is supplied to the joint. The molten metal may come from the parts themselves (i.e. parent metal) or filler metal which normally have the composition of the parent metal. The joint surface become plastic or even molten because of the heat from the molten filler metal or other source. Thus, when the molten metal solidifies or fuses, the joint
is formed.
The fusion welding, according to the method of heat generated, may be classified as:
1. Thermit welding, 2. Gas welding, and 3. Electric arc welding.
10.5 Thermit Welding
In thermit welding, a mixture of iron oxide and aluminium called thermit is ignited and the iron
oxide is reduced to molten iron. The molten iron is poured into a mould made around the joint and
fuses with the parts to be welded. A major advantage of the thermit welding is that all parts of weld
section are molten at the same time and the weld cools almost uniformly. This results in a minimum
problem with residual stresses. It is fundamentally a melting and casting process.
The thermit welding is often used in joining iron and steel parts that are too large to be manufactured
in one piece, such as rails, truck frames, locomotive frames, other large sections used on steam
and rail roads, for stern frames, rudder frames etc. In steel mills, thermit electric welding is employed
to replace broken gear teeth, to weld new necks on rolls and pinions, and to repair broken shears.
10.6 Gas Welding
A gas welding is made by applying the flame of an oxy-acetylene or hydrogen gas from a
welding torch upon the surfaces of the prepared joint. The intense heat at the white cone of the flame
heats up the local surfaces to fusion point while the operator manipulates a welding rod to supply the
metal for the weld. A flux is being used to remove the slag. Since the heating rate in gas welding is
slow, therefore it can be used on thinner materials.
10.7 Electric Arc Welding
In electric arc welding, the work is prepared in the same manner as for gas welding. In this case
the filler metal is supplied by metal welding electrode. The operator, with his eyes and face protected,
strikes an arc by touching the work of base metal with the electrode. The base metal in the path of the
arc stream is melted, forming a pool of molten metal, which seems to be forced out of the pool by the
blast from the arc, as shown in Fig. 10.1. A small
depression is formed in the base metal and the
molten metal is deposited around the edge of this
depression, which is called the arc crater. The slag
is brushed off after the joint has cooled.
The arc welding does not require the metal
to be preheated and since the temperature of the
arc is quite high, therefore the fusion of the metal
is almost instantaneous. There are two kinds of
arc weldings depending upon the type of electrode.
1. Un-shielded arc welding, and
2. Shielded arc welding.
When a large electrode or filler rod is used for welding, it is then said to be un-shielded arc welding.
In this case, the deposited weld metal while it is hot will absorb oxygen and nitrogen from the atmosphere.
This decreases the strength of weld metal and lower its ductility and resistance to corrosion.
In shielded arc welding, the welding rods coated with solid material are used, as shown in Fig.
10.1. The resulting projection of coating focuses a concentrated arc stream, which protects the globules
of metal from the air and prevents the absorption of large amounts of harmful oxygen and nitrogen.
10.8 Forge Welding
In forge welding, the parts to be jointed are first heated to a proper temperature in a furnace orforge and then hammered. This method of welding is rarely used now-a-days. An electric-resistance welding is an example of forge welding.
In this case, the parts to be joined are pressed together and an electric current is passed from one part to the other until the metal is heated to the fusion temperature of the joint. The principle of applying heat and pressure, either sequentially orsimultaneously, is widely used in the processesknown as *spot, seam, projection, upset and flash
welding.
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