*A butt weld or groove weld is defined as one in which metal lies substantially within the planes of the surfaces of the joined components.
A full penetration butt weld is normally expected to develop high strength.
A partial penetration butt weld achieved a specified penetration depth only.The most commonly weld method is manual metal arc welding, automatic welding with a continuous coated electrode feeding automatically off a drum , gas shielded welding, where a bare electrode is used,and submerged arc welding which utilizes an arc entirely submerged in a granular flux that is fed progressively onto the molten weld pool as welding proceeds.
*A fillet weld is is approximately triangular in section formed within a recentrant corner of a joint.Its strength is achieved through shear capacity of the weld across the throat , the weld size is being specified as leg length.
*Fillet welds are suitable for lap joints and Tee joints and groove
welds for butt and corner joints. Butt welds can be of complete penetration or incomplete penetration depending upon whether the penetration is complete through the thickness or partial. Generally a description of welded joints requires an indication of the type of both the joint and the weld.
*Though fillet welds are weaker than butt welds, about 80% of the connections are made with fillet welds. The reason for the wider use of fillet welds is that in the case of fillet welds, when members are lapped over each other, large tolerances are allowed in erection. For butt welds, the members to be connected have to fit perfectly when they are lined up for welding. Further butt welding requires the shaping of the surfaces to be joined . To ensure full penetration and a sound weld, a backup plate is temporarily provided.
*Butt welds have high strength, high resistance to impact and cyclic stress. They are most direct joints and introduce least eccentricity in the joint. But their major disadvantages are: high residual stresses, necessity of edge preparation and proper aligning of the members in the field. Therefore, field butt joints are rarely used.
*Owing to their economy, ease of fabrication and adaptability, fillet welds are widely used. They require less precision in the fitting up because the plates being joined can be moved about more than the Butt welds. Another advantage of fillet welds is that special preparation of edges, as required by Butt welds, is not required. In a fillet weld the stress condition in the weld is quite different from that of the connected parts.