Page 50 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

Analysis of a Fusion Weld

correctly. By allowing the plates to move and not restraining
them the level of residual stress in the welded joint is reduced.
Sequential welding (Fig. 3.8) is a technique utilised to
control the level of distortion by welding the joint in a
specific way. There are various sequential welding techniques
including balanced welding (welding about a neutral axis of
the weld), back skip welding or back step welding (a short
length is welded and the next weld starts a short distance
behind the previous weld length).
Mechanical restraints including clamps and jigs allow
accurate positioning of the component parts. They are
normally left in position until the components are tack-
welded together and then removed to allow the welder full
access. Restraints will reduce the level of distortion by
physically holding the components in position but will
increase the stress levels introduced into the weld.
Flame straightening uses an oxyacetylene torch (but not a
cutting torch) to give a localised intense heat. The distortion
caused by this localised heating can be used in some cases to
straighten or modify the shape of a component. Typical
examples of this method would be straightening flanges or
removing bulges from insert plates.
Heat treatment methods such as post-weld heat treatment
stress relief can remove a high percentage of residual stresses.
Mechanical stress relieving methods include:
. using ultrasound to stress relieve fabrications;
. peening weld faces using pneumatic needle guns, to
redistribute residual stresses by placing the weld face in
compression.

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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 35 of 35