Page 57 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

A Quick Guide to Welding and Weld Inspection

corrosion cracking) are achieved when there is at least 25%
ferrite, with the balance being austenite.
The ferrite in a duplex weld metal is typically in the range
of 25 to 60%. In some welding processes utilising a flux, the
phase balance of the filler is biased to increased austenite to
offset the loss of toughness associated with oxygen pickup
from the flux. Thermal expansion and conductivity of a
duplex stainless steel are between that of carbon steel and
austenitic stainless steel. The operating temperature is
normally kept below 300 8C to avoid a degradation mechan-
ism called ‘475 8C embrittlement’.
Typical defects
. The problems most typical of duplex stainless steels are
associated with the heat affected zone (HAZ) rather than
the weld metal. The HAZ can suffer from:
○ loss of corrosion resistance;
○ loss of toughness;
○ post-weld cracking.
. The duplex structure is very sensitive to contaminants,
particularly moisture.
. Detrimental reactions occur to the material properties if
heat input times keep temperatures within the 705 to
980 8C range for too long.
. Rapidly quenched autogenous welds (welds without filler)
such as arc strikes and repairs to arc strikes tend to have
ferrite levels greater than 60%. These welds can have low
toughness and reduced corrosion resistance.
Defect avoidance
. Allow rapid (but not extreme) cooling of the HAZ.
. Limit the temperature of the workpiece because it provides
the most effective cooling of the HAZ.
. Limit the maximum interpass temperature to 150 8C
(300 8F).
. When a large amount of welding is to be performed, plan

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Woodhead Publishing Ltd – A Quick Guide to Welding and Weld Inspection
Data Standards Ltd, Frome, Somerset – 17/9/200904QG Welding chap4.3d Page 42 of 48