Page 122 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

A Quick Guide to Welding and Weld Inspection

reduce the weld pool size before breaking the arc and/or by
adding filler to compensate for the weld pool shrinkage.
Other processes may require run-off plates to be used at the
end of weld runs or special techniques to be utilised to reduce
the effects of breaking the arc too quickly.
Burn-through
Burn-through is a localised collapse of the weld pool during
the root run indicated by excessive root penetration or an
irregular cavity in the root bead. It is normally assessed on
radiographs by looking at the density. Causes of burn-
through include:
. excessive welding current;
. having a small or uneven root face;
. having a large or uneven root gap;
. using too slow a travel speed.

Root oxidation
Root oxidation (sometimes referred to as ‘coking’) is most
common when welding stainless steels and leaves a blackened
poorly fused root. It is caused by insufficient back purging
gas.
Arc strike
An arc strike (or stray flash) is accidental arcing on to the
parent material. This can lead to cracking on crack-sensitive
materials due to the fast quenching of the arc strike, causing
localised hardened regions. These hardened regions are
susceptible to brittle fracture. They can also cause stress
concentrations leading to in-service failures such as fatigue
fractures. Arc strikes caused by poorly insulated cables or
loose earth clamps may introduce copper or other dissimilar
materials into the weldment, causing liquation cracking or
other contamination problems. Arc strikes on susceptible
materials require removal and PT or MT to ensure no
cracking is present.

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Woodhead Publishing Ltd – A Quick Guide to Welding and Weld Inspection
Data Standards Ltd, Frome, Somerset – 17/9/200907QG Welding chap7.3d Page 106 of 107