Page 123 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

Fracture Modes and Welding Defects

Spatter
Spatter is molten globules of consumable electrode that are
ejected from the weld and quench quickly wherever they land
on the weldment. They can therefore cause cracking on
susceptible materials so they should be removed and then the
area tested with PT or MT. Other problems caused by spatter
include prevention of UT (because UT needs a smooth
surface for the probes), unwanted retention of penetrant
during PT and problems with paint retention. Causes of
spatter include:
. excessive current;
. damp electrodes;
. surface contamination from oil, paint, moisture or grease;
. incorrect wire feed speed during MAG welding.
Magnetic arc blow
Magnetic arc blow is an uncontrolled deflection of the
welding arc due to magnetism. This causes defects such as
lack of root fusion or lack of sidewall fusion. Causes of
magnetic arc blow include:
. deflection of the arc by the Earth’s magnetic field (can
occur in pipelines);
. poor position of the current return cable (the magnetic
field surrounding the welding arc interacts with the current
flow in the material to the current return cable and is
sufficient to deflect the arc);
. residual magnetism in the material causing distortion of
the magnetic field produced by the arc current.
Some methods of avoiding arc blow are:
. welding towards or away from the clamp;
. using a.c. instead of d.c.;
. demagnetising the steel before welding.

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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 107 of 107