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Why are residual stresses incurred in steel after welding?

Considerable residual stresses are induced in connecting steel members after the welding operation. The local temperature of steel where welding takes place is higher than the remaining parts of the connecting steel members.


What is the function of gap between adjoining root faces in butt weld?

In the design of butt weld strength, it is generally assumed that its strength is at least equivalent to the parent metal. To enhance proper welding operation, the gap between two metals to be welded should not be too small, otherwise the root would be inadequately fused during welding and the butt weld strength would be reduced. On the other hand, the gap should be not set too large because the weld metal would simply pass through it. The function of the gap between adjoining root faces is to increase the depth of penetration down to the root of the weld.


In welding design what are the different applications of square-groove, V-groove, U-groove and J-groove?

When the base metal is thin (i.e. 0.125” to 0.25”), square-groove can be adopted. Where welding is carried out from one side of the joint, a temporary backup strip can be used to ensure proper joint penetration and to avoid excessive leakage of metal though the joint.


Convex fillet welds are sometimes used in welding. Why?

There are three types of fillet weld cross section profile, namely, flat, convex and concave. The convexity in convex fillet welds serve as reinforcement, which is believed to provide additional strength. However, care should be taken in not introducing excessive convexity to fillet welds.


There is a general rule in fillet weld that “the leg should be equal to the thickness of metals.” Why?

Let’s take an example of 6mm thick plates to illustrate the rule. In case 12mm leg is adopted in the fillet weld, the weld volume would be 3-4 times more than required. It would result in waste of weld metal and welder’s time. Worse still, over-welding may weaken the structure and result in distortion owing to the formation of residue stress. As such, the resulting weld could support less stress than fillet weld with “the leg equal to the thickness of metals.”