Welding Terminology

TERMINOLOGY

Use of the correct terminology is important. British Standart uses BS 499 standard. Frequently the terms ‘weld’ and ‘joint’ are used incorrectly. Exact definitions are given in BS 499 PT 1 1983 - ‘Welding terminology’ and BS 499 Pt 2: 1980 ‘ Weld symbols’.

TYPES OF WELD

Butt Weld

Fillet Weld:

Edge Weld:

Spot Weld:

TYPES OF JOINT

The following are some typical joints:

PLATE EDGE PREPARATION FOR BUTT WELDS

The illustrations show standard terminology for the various features of plate edge preparations.

FEATURES OF A COMPLETED WELD

A butt weld in plate, made by welding from both sides, has two weld faces and four toes. In a full penetration weld made from one side, the protruding weld on the underside is called the penetration bead, which also has two toes. The root is defined (BS 499) as the zone on the side of the first run farthest from the welder.

If a weld is sectioned, polished and etched, the fusion boundary can be established. Metal lying between the two fusion boundaries is weld metal – a mixture of deposited metal and plate metal that has been melted. The fusion zone is the area of highest dilution between filler metal and parent plate. Adjacent to the fusion boundary is the heat affected zone (HAZ), in which the plate material has had its metallurgical structure modified by the heat of welding.

Excess weld metal is the correct term, not ‘ weld reinforcement’. Excess weld metal lying outside the plane joining the toes of the weld.

Fillet welds have similar features:.

The shape of a fillet in cross-section is described in three terms

A convex fillet has a poor toe blend – greater notch effect and sharper angle at toe, not used in fatigue situations. A concave fillet has a better toe blend for fatigue situations, however a reduced throat. The concave weld may be made by welding alone or by subsequent grinding.

SIZE OF WELDS

Full Penetration Butt Welds.

The general rules are: design throat thickness = thickness of the thinner part joined.

cap width = prep width + 10% either side (for open prep, not specified in BS 499)

Partial Penetration Butt Welds.

The term partial penetration strictly implies butt welds that are designed to have less than full penetration. Failure to achieve full penetration when it is wanted should be listed as the defect incomplete penetration.

The design throat thickness of a partial penetration weld is t1 and the actual throat thickness is t2. With a partial penetration weld made from both sides, the design throat thickness is t1 + t1 and the actual throat thickness is t2 + t2. Note that the degree of penetration must be known.

Fillet Welds.

Fillet weld sizes are calculated by reference to allowable shear stress on the throat area, i.e.

throat area = design throat thickness x length of weld.

The size required is specified on drawings in terms of leg length (l).

For fillet welds with equal leg lengths l = 1.4 t1. This does not apply to concave fillet welds.

If an asymmetrical weld is required, both leg lengths are specified and t1 is taken as the minimum throat dimension.

Deep penetration fillet weld.

With high current density processes, e.g. submerged arc and MIG (spray), penetration along the joint line can be produced. This gives an increase in throat thickness with no change in leg length.