Welding is a method of connecting two pieces of metal by heating to a plastic fluid state, so that fusion occurs. Bolted connection and welding are the oldest and reliable methods of jointing. The different process of arc welding that are used in structural steel applications are as follow:

• Shielded metal arc welding (SMAW),
• Subgenerged are welding (SAW),
• Gas-shielded metal are welding (GMAW),
• Flux core are welding (FCAW),
• Electro slag welding (ESW),
• Stud welding (SW).

The welds may be grouped into four types as follows:

• Groove welds (fig. 1) are used to connect structural members that are aligned in the same plane and often used in butt joints. Groove welds may also be used in
  T-connection.

• Fillets welds are approximately triangular in cross section and a few examples of application of fillet weld are shown in fig. 2.

• Slot and plug welds are not used exclusively in steel connection. When it becomes impossible to fillet welds or when the length of the fillet weld is limited slot and plug welds are used to supplement the fillet welds. They are also assumed to fail in shear.

Type of joints

The five basic types of welded joints which can be made in four different welding positions such as flat, horizontal, vertical, and overhead are as follows:

• Butt joint is used to joint the ends of flat plates of nearly equal thickness. This type of joints eliminates the eccentricity developed using a lap joint (fig.3).

• Lap joint are most commonly used because they offer ease of fitting and easy of jointing. They do not require any special preparation (fig. 4.).

• T-joint ‘Tee joints’ are often used to fabricate built-up section as T-shapes, I-shapes, plate girders, hangers, brackets and stiffeners, where two plates are joined at right angles (fig. 5).

• Corner joints are used to form built-up rectangular box sections, which may be used as columns or beams to resist high torsional forces. Filled weld and a few groove weld edge shapes for corner joints are shown in (fig. 6).

• Edge joint are not used in structural engineering applications. They are used to keep two or more plates in a given plane (fig. 7).

Type of joints

The production of sound welds is governed by the type of joint, its preparation and fit-up, the root opening, etc. In additions to this the choice of electrode, the welding position, the welding current and voltage, that are length, and the rate of travel also affect the quality of weld. Accessibility of the welding operation is also important, since the quality of weld is determined to a considerable extent by the positioning of the electrode. Some of the common defects in the welds are as follow:

• Incomplete fusion,
• Incomplete penetration,
• Porosity,
• Inclusion of slag,
• Cracks,
• Undercutting,
• Lamellar tearing.

Since a small error in a weld may lead to a catastrophic collapse, checks are to be made before, during, and after welding. The non-destructive tests usually employed include the following:

• Liquid penetrant inspection,
• Magnetic particle inspection,
• Radiographic inspection,
• Ultrasonic inspection.