Page 108 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

A Quick Guide to Welding and Weld Inspection

Keep stress levels low:
. Reduce restraints to decrease restraint stress and residual
stress levels.
. Keep heat inputs low.

Maintain low carbon levels. An increase to the carbon content
can rapidly increase the manganese to sulphur ratio required
to minimise cracking. Therefore use low carbon fillers and
low carbon materials.
Solidification cracking occurs in austenitic stainless steels
because the austenitic grain structure is intolerant to
contaminants such as sulphur or phosphorus between the
grains in the weld metal. A high level of these contaminants
can cause the weld to crack.
This cracking can be avoided by making the weld metal
about 5% ferrite because the ferrite structure is more
accommodating to contaminants. Weld metal can be
partially ferrite by the addition of a filler metal chosen
using a Schaeffler diagram. It is worth noting that although
austenitic stainless steel is non-magnetic the weld metal may
be found to be slightly magnetic due to the slight ferrite
content.
In welds with a large depth/width ratio (usually in excess of
around 2:3) the centre of the weld is the last place to cool and
therefore contains large columnar-shaped grains at this
point. These grains impinge on each other as they form
from both directions within the weld, leading to voids
forming in the centre of the weld that cannot be filled with
the molten filler being added from above. These voids
weaken the weld metal, which is then subjected to severe
contraction stresses caused by the shrinkage of the large weld
metal volume, producing a centreline crack. Processes with
deep penetration or large deposition rates such as SAW are
particularly susceptible to this type of failure.

92

Woodhead Publishing Ltd – A Quick Guide to Welding and Weld Inspection
Data Standards Ltd, Frome, Somerset – 17/9/200907QG Welding chap7.3d Page 92 of 107