Page 106 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

A Quick Guide to Welding and Weld Inspection

Reduce the hydrogen level by:
. use of correctly heat treated low H 2 electrodes;
. removal of moisture, oil, grease, paint, etc., from
materials;
. use of a low H 2 welding process;
. carrying out a hydrogen soak.
Reduce the stress level by:
. removal of restraints;
. ensuring a good joint fit-up;
. carrying out PWHT stress relief.
Reduce the hardness by:
. preheating the joint;
. carrying out PWHT.
Carry out delayed NDT:

. The temperature will eventually fall below the critical level
but a crack may not occur until days afterwards, so carry
out delayed NDT.

Solidification cracking
Solidification cracking (also known as centreline cracking or
hot cracking) is a fracture that occurs in the weld metal of
ferritic steels with a high sulphur or phosphorus content or in
joints with a large depth/width ratio.
It occurs in ferritic steels with a high sulphur content
because during welding the sulphur joins with iron to form
iron sulphide (FeS). This FeS has a lower melting point than
steel and therefore remains as a liquid as the steel solidifies.
The centre of the weld is the last place to cool so this liquid
FeS is pushed to the centre of the weld and forms a liquid
film on the grain boundaries, causing a lack of adhesion
between the grains. The weld metal contraction due to
cooling leaves a high tensile stress, which can pull the weld

90

Woodhead Publishing Ltd – A Quick Guide to Welding and Weld Inspection
Data Standards Ltd, Frome, Somerset – 17/9/200907QG Welding chap7.3d Page 90 of 107