Page 42 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

Analysis of a Fusion Weld

. If mechanical cutting such as machining (normally
required for weld joint preparations containing a radius
such as a U or J prep) or shearing is used then
consideration must be given to the possibility of having
cutting fluids trapped in the cut edges. These can cause
porosity or other weld defects unless they are properly
cleaned out (the edge may look smooth but under a
microscope it will be full of peaks and troughs).
Weld joint: shape
The shape of the joint (joint type) can affect not only the final
mechanical properties of the weldment but also the level of
distortion produced in it. The mechanical properties can be
affected because a change of joint type may affect the overall
heat input. This is due to the possible requirement for more
(or less) weld metal to be welded into the joint. The actual
level of final distortion is determined by factors such as the
weld metal shrinkage direction (based on the angle of the
groove) and whether the weld is formed from one or both
sides.
Figure 3.2 shows the components of a typical single V joint
and U joint, and some advantages of the single U over a
single V in thicker materials.
Residual stress and distortion
Residual stresses are those stresses that remain in a
component after a procedure such as bending or welding
has been carried out. If a plate is bent then bending stresses
will be introduced into the bend and the maximum tensile
stress will be at the outer convex surface. This stress remains
in the plate and can cause failure, especially if a defect or
stress raiser is present in the stressed region (this of course is
how bend tests are carried out).
Residual welding stress in a welded joint comes from the
uneven expansion and contraction of the joint coupled with a
restraint. Consider the following example.
An unrestrained strip of metal is heated up and expands. If

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Woodhead Publishing Ltd – A Quick Guide to Welding and Weld Inspection
Data Standards Ltd, Frome, Somerset – 17/9/200903QG Welding chap3.3d Page 27 of 35