Page 44 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

Analysis of a Fusion Weld

Figure 3.3 Unequal expansion and contraction
it is then allowed to cool it will shrink back to its original size
and there will be no residual stress present. If, however, the
expanded metal was to be restrained by jigs (see Fig. 3.3) to
try and prevent it shrinking when it cools, then a tensile stress
will be introduced into the material and can cause the
material to deform plastically or fracture. This, in effect, is
residual stress induced through uneven expansion and
contraction coupled with restraint.
Now consider the mechanics of welding where a pool of
molten weld metal cools down and contracts (shrinks). The
base material acts as a restraint and tries to prevent the weld
pool contracting, providing the ideal conditions required to
leave residual stress in the completed weldment. It can be
deduced from this that there will always be residual stresses
present in welded joints. In some cases this may be high
enough to approach the yield point.
Mechanical restraints such as jigs and clamps increase
restraint stresses further by preventing the movement that
would normally occur as the molten weld metal cools down
and shrinks. This causes the joined components to distort.
Allowing distortion to take place helps to reduce the overall

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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 29 of 35