Welding

Definition:

The method welding joints metals by fusion. The metals at the joint is melted and fues with additional metal from a welding rod using heat from either an electric arc or an oxyacetylene torch. The weld material and the base metal form a continuous and almost homogeneous joint after after cooling.

Shielded Arc Process:

A heavy coated welding rod is used that releases an inert gas which envelopes the arc stream to protect the weld from excessive oxidation. This techniques is called the shielded arc process.

1) Butt welds

The strength of a butt weld is equal to the allowable stress multiplied by the product of the length of the weld time the thickness of the thinner plate of the joint. The allowable stress is taken to be the same as that of the base metal.

2) Fillet welds

Normally side and transverse fillet welds are used in practical cases. The strength of side or transverse fillet welds is assumed to be determined by the shearing resistance of the throat of the weld regardless of the direction of the applied load. In the 450 fillet weld, with the leg equal to t, the shearing area through the throat depth

i.e. A = L( t sin 450)

= 0.707 L t.

Allowable stress for fillet welds

The allowable stresses for fillet welds specified by the AISC (based on recommendation of the American Welding Society) depending on the electrode used in the welding process and on the grade of steel being welded. For example, if E-70 electrode are used to weld A 36 steel ( one of the more grades of structural steel used today), the allowable shearing is 21 Ksi (145 Mpa). For this case, the strength and allowable force per unit length for the 450 fillet weld are as follows:

AISC specification for maximum size of fillet weld:

It is required to adopt special precautions to ensure that length of the leg of a fillet weld along an edge is actually equal to the thickness of the edge. This is because

1) The edge of rolled shapes is rounded and the length of the leg would be less than the nominal thickness of the shape.

2) During welding, the corner of the edge may melt into the weld; which would reduce the length of the leg. 

AISC specification

For material thickness  ≥  ¼ in.

The maximum size of a fillet weld should be 1/16 in. (2 mm) less than the material thickness along edges of ¼ in.( 6 mm) or more thick.

For material thickness  <  ¼ in.

For edges less than ¼ in. thickness, the maximum size of the weld may equal to the egge thickness.

However, weld size may exceed these specifications if the designer stipulates that the weld is to be built out to obtain full throat thickness .

Advantages of Welded Connection:

Welded connections are used extensively to supplement or replace riveted or bolted connections in structural and machine design. It is more economical to fabricate a member by welding simple component parts together than to use a completed casting and the dependance on welded connections is increased.