Page 58 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

Materials and Their Weldability

the welding to provide enough time for cooling between
passes.
. Avoid PWHT stress relief because duplex steels are
sensitive to even relatively short exposures to temperatures
in the 300 to 1000 8C range.
. PWHT stress relief in the 300 to 700 8C range may cause
precipitation of the alpha prime phase (‘475 8C embrittle-
ment’), causing a loss of toughness and corrosion
resistance.
. Stress relief in the range of 700 to 1000 8C leads to rapid
precipitation of intermetallic phases, resulting in loss of
toughness and corrosion resistance.
. Heat treatment of duplex steel, for whatever reason,
should be a full solution anneal followed by water
quenching.
In summary, the best way to avoid problems with duplex
steels is to avoid excessive ferrite levels and limit the total
time at temperature in the HAZ.
Material properties
Materials are chosen for service use based on the properties
that they possess.
Ductility is the ability of a material to be drawn or
plastically deformed without fracture. It is therefore an
indication of how ‘soft’ or malleable the material is. The
ductility of steels varies depending on the types and levels of
alloying elements present. An increase in carbon, for
example, will increase the strength but decrease the ductility.
Hardness is the ability of a material to resist abrasion or
penetration on its surface. The harder the material, the
smaller the indentation left by an object such as a ball or
diamond being impressed upon it. As a general rule there will
be a higher risk of cracking as hardness increases.
Toughness is the ability of a material to resist impact (i.e.
absorb the energy of an impact). The general rule is that a
higher toughness will lessen the risk of cracking.

43

Woodhead Publishing Ltd – A Quick Guide to Welding and Weld Inspection
Data Standards Ltd, Frome, Somerset – 17/9/200904QG Welding chap4.3d Page 43 of 48