Page 47 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

A Quick Guide to Welding and Weld Inspection

cannot be uniformly heated to the melting temperature
required to weld. The net effect of this is that some distortion
will always occur because the weld effectively acts as a form
of localised heating.
The coefficient of thermal expansion of a material plays a
large part in how much welding stress is introduced into a
material and how much distortion can occur. The higher the
coefficient of expansion, the higher the distortion level, which
is why stainless steels suffer higher distortion levels than plain
carbon steels.
Simplistically, the main distortions to consider are caused
by the weld metal shrinkage in the longitudinal, transverse
and short transverse directions. In reality, the shrinkage and
distortion will follow a very complex pattern but a simplified
exaggerated view of the effects is shown in Fig. 3.5.
The shape of a weld preparation will affect how much
distortion is produced, because bevel angles will help to
direct the distortion direction. The material thickness,
amount of weld metal required and size of individual weld
runs also affect stress and distortion levels. A single V butt
will suffer considerable distortion, especially during the
initial root runs when there is no restraint, because of the
high level of weld metal shrinkage. Although further runs
add to the distortion, the overall effect is not cumulative
because the previous runs will help to restrain the joint from
moving. A square-edged closed butt will actually suffer very
little distortion. Figure 3.6 compares distortion levels based
on the bevel angle of two single V butt welds and a square-
edged closed butt weld.
Minimising stresses and distortion
There are various ways to counteract the effects of residual
stress levels and distortion depending upon the type of
welding process and joint type that is being used.
Offsetting (Fig. 3.7) is where plates are offset to a preset
angle and welded without restraint. As the weld solidifies and
shrinks it distorts the plates and pulls them into the correct

32

Woodhead Publishing Ltd – A Quick Guide to Welding and Weld Inspection
Data Standards Ltd, Frome, Somerset – 17/9/200903QG Welding chap3.3d Page 32 of 35