Page 41 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

A Quick Guide to Welding and Weld Inspection

procedure, problems can arise. A common example is when
tack welds are done without applying the specified pre-heat.
This leads to the tacks and HAZ around them being harder
than they should be because they cool down too quickly
(remember, the main reason for pre-heat is to retard the
cooling rate and reduce temperature variations across the
weldment). These hardened areas within a highly stressed
region can then suffer cracking, with possible catastrophic
consequences if they are not found.
Weld joint: preparation methods
The type of joint preparation and how it is made can have an
effect on the final weldment properties. For example:

. Weld preps made using thermal cutting processes can be
affected by the heat of the cutting process. This can lead to
a possible loss of toughness or cracking in the HAZ, so
approximately 3 mm of material is normally removed
from the prep by mechanical means after thermal cutting.
Typical thermal cutting processes (used to form straight-
sided preps such as square edged or bevelled) include
plasma, oxy-fuel gas or oxy-arc processes.
. If cutting is by an oxygen fuel gas process such as
oxyacetylene then the weld preps can gain excess carbon
from the process, leading to cracking in the weld (the
carbon increases the weld metal hardness and therefore the
risk of cracking). This is another reason to remove 3 mm
from the prep by mechanical means after cutting.
Remember that oxy-fuel gas cutting does not actually
melt and blow away the molten material like plasma or
oxy arc cutting does. What it actually does is to heat up
the material to its ignition temperature and then
introduces a stream of oxygen, turning the metal into
instant rust, which is then blown away. This is why
stainless steel (a rust-resistant material) cannot be cut by
oxy-fuel gas unless special powders are added to the
process.

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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 26 of 35