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Characteristics of Welding Joints

Characteristics of Welding Joints - (a) Butt-Joints 1. Square butt joints (without or with gap) are recommended for smaller plate thicknesses (up to 4.8 mm) and much lesser loading as compared to single V, U, J or bevel joints. However, there is no edge preparation (except normal cleaning) and the total cost of welding is low. 2. Double joints are preferred over single joints for bigger plate thicknesses, more severe loading conditions (especially fatigue, impact and bending) or reduced warpage and where welding can be carried out on both sides, e.g., in case of longitudinal and circumferential joints of pressure vessels which can accommodate a welder inside the shell. There is not much difference between the total welding costs of single and double joints. 3. Out of various joints, V joint finds much more extensive use and applications even though the weld distortion is more in V joints as compared to J and U joints.
4. Bevel joints are employed for plates with thicknesses from 1035 mm and subjected to medium loading. The edge preparation is simpler and the electrode consumption is less as compared to V joints, but it is difficult to obtain full (root) penetration. 5. U joints are preferred over V joints when it is required to make high quality joints in certain pressure vessels (plate thickness 10-20 mm). Though the load carrying capacity is less and edge preparation is difficult and more costly (as compared to V joints), it is simpler to get full (root) penetration and the electrode consumption is also less. 6. J-joints are recommended for normal loading in some pressure vessels. Though the total cost of welding is lesser as compared to V or U joints, it is difficult to get full (root) penetration. 7. Among the double joints, double V joint is employed under most severe loading conditions.
	(b) T-Joints As the plate thickness and the severity of loading on the joint increases, the choice of the joint shifts from (a) to (f), Single fillet T joint is preferred for smaller plate thicknesses when subjected to low or almost no (bending) load conditions whereas double T joint is recommended for most severe loading conditions. The cost of edge preparation increases and the electrode consumption decreases for joints from (a) to (f) respectively. (c) Lap Joints Single fillet and double fillet joints are used on all thicknesses; double fillet joint is better as compared to single fillet when the joint is subjected to severe loading. Single fillet joints are not recommended on plates under bending, fatigue or impact loading conditions:
Plug weld may be made without or with a hole (tapered or parallel) in the upper member. This joint is used where bottom/second plate is not easily accessible for fillet welding. Plug weld can be employed to impart added strength to the structure. (d) Corner Joints Close and half open corner joints are recommended for plates of smaller thicknesses not subjected to severe loading. Full open corner joint can be used on plates of practically all thicknesses under severe load conditions. The load bearing capacity increases when the joint is welded from both sides. (e) Edge Joint Two pieces of metals are lapped, with their edges in line and the pieces are joined by welding the two edges together. Fillet welds (i.e., corner, tee, lap, etc.) are the cheapest type of welds to make, for no edge preparation is required and setting up is simple.
However, since fillet welds do not possess the continuity of butt welds, they are not capable of carrying high dynamic stresses and in practice this generally limits the use of lap joints to applications of secondary importance. If some dynamic loading is expected, the profile of the weld should be specified as concave or flat, not convex.

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