A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
3. In welding design what are the different applications of
square-groove, V-groove, U-groove and J-groove? (DW3)
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.
V-groove is commonly used for medium to thick metals (i.e. 0.25” to 0.375”).
For even thicker metal plates, U-groove and J-groove can be adopted to
provide good penetration of welded metal. One demerit of U-groove and
J-groove is the preparation of the metal plates. For instance, air carbon arc
and special mechanical cutting tools are required for preparing the joints.
4. What is the function of gap between adjoining root faces in butt
weld? (DW4)
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.
However, it is not always possible to have access to both sides of the butt
weld. Hence, the use of backing plates or rings can enhance the quality of
welding from one side only. By inserting a backing plate inside the steel
member, the correct alignment could be maintained and certain amount of
tolerance on longitudinal fit can be permitted [47].
5. Why are residual stresses incurred in steel after welding? (DW5)
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. This causes thermal expansion locally during welding and the
subsequent contraction after welding. Tensile stresses associated with the
thermal contraction generated during the cooling process are balanced by
compressive stresses in the remaining parts of connecting steel members.
As a result, residual stresses are induced during the welding operation.
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