Welding

Welding is a process of joining similar metals by application of heat with or without the application of pressure and addition of filler material

Modern methods of welding may be classified under two broad categories.

a)    Pressure welding

b)   Fusion or non-fusion welding

Pressure welding

In the ‘pressure welding process’ the pieces of metal to be joined are heated to a plastic state and then forced together by external pressure exerted by hammering, rolling or pressing to effect the weld. Forge welding, resistance welding and thermit welding are of this type.

Fusion welding

In the ‘fusion welding process’ the material at the joint is heated to fusion temperature and a filler metal is added externally. The weld is then allowed to cool down. Electric arc welding, electric resistance welding, gas welding and induction welding are included in this category.

General welding conditions

The following general conditions are to be observed for getting strong and efficient welded joints.

a) Keep the surfaces to be welded clean and free from foreign material. This is done by wire brushing, machining or sand blasting. If the surfaces are not properly cleaned, gas and the slag inclusions in the weld material will remain, causing weak and brittle joint.

b) Carry out the edge preparation properly to deposit and fuse weld material into the parent material. This will cause better penetration of weld material upto the opposite side.

c) Create a non-oxidising atmosphere at the point where the welding is done.The tendency to oxidise increases with temperature. At high temperatures, oxidation of weld metal is likely to have serious weakening effects in the weld. This influence is counteracted by the use of a flux which removes oxides and permits a better bond between metals. In electric arc welding flux coated electrodes are used. In inert gas welding, argon, carbon-dioxide shield is used round the electrode. In submerged arc welding, the arc is struck under the flux to avoid contact with the atmosphere and so on.

d) Maintain the required speed of the welding process, so as to avoid oxidation at high temperature. Some processes are naturally quicker than others. Select such processes as far as possible.

e) Use the correct quality and size of electrode, according to its applications.

f)  Have a perfect control over the selected process.e.g. control over the voltage, current and polarity in case of electric arc welding; control over oxygen and acetylene pressures and nozzle size, etc.

Application of welding processes

A few of them are given below

1) Fabrication of steel structures – frame work of industrial buildings, bridges, machine frames, towers etc.

2)Welding processes are employed extensively in the manufacture of automobile bodies, aircrafts, railway coaches etc.

3)   Machine tools, building industry use welding.

4)   In the oil industry, welding is extensively used at refineries and in pipe line fabrication.

5)  The ship building industry is dependent on welding.

6) Pressure vessels manufacturing industry–boilers and reservoirs and tanks for process equipment in chemical, sugar, paper and cement industries.

7)  General machine repair and maintenance work.

8) The economic and other advantages of welding has also been felt in the casting industries, since many machine parts that were formerly cast are now made of steel members, welded together. Such a construction has the advantage of being lighter and stronger than cast iron.

Types of welds

1.      Bead type

2.    Fillet type

3.    Groove type

4.    Plug type

Fig.

Bead weld

These are used for making edge joints in thin materials and for building up surfaces. The latter, known as hard surfacing or hard facing consists of the overlaying by welding of a more wear-resisting material upon a basic metal blank. This may consist of a high alloy steel on a mild steel blank or a non-ferrous alloy on a tool steel blank.

Filler weld

These are used for tee joints, lap joints and corner joints. The size of the fillet weld is measured by the ‘leg length’ of the largest 45⁰ right angle triangle that can be inscribed within the contour of the cross section of the weld.

Groove welds

The size of such welds is the depth of the groove, which may be usually taken as the thickness of the plates to be joined (say ‘t’). Welding from both sides with a double-groove preparation is to be preferred where accessibility and shop facilities permit, because less weld metal is required and a saving in time and materials is effected. The choice of the groove type depends on the type of joint to be welded and the thickness of the parts.

Fig.

Plug weld

These are used to connect two parts of welding through a round hole or a slot in either one of the parts. Plug welds are used for welding stay bolts to thick plates and for strengthening the support of a fixture, bolted or welded plates.

Types of welded joints

Bead, groove and fillets welds are used in six basic types of joints, namely

1.      Butt joint

2.    Lap joint

3.    Edge joint

4.    Corner joint

5.     Tee joint

6.    Plug joint

Lap and butt joints are most commonly used in resistance welding, gas welding and electric arc welding processes.

In selecting the type of joint and its weld preparation, the main factors to be considered are

a.     The nature of the load

b.    The cost of the edge preparation and welding

c.     The accessibility for welding