Svetsaren_nr2 definitief (Page 1) - Esab

The susceptibility to localised corrosion also increases
with rising temperatures and with increasing oxidising
capacity of the environment. Strong oxidisers such as
chlorine dioxide (ClO 2 ) and ozone (O 3 ) used in pulp
bleaching, therefore play an important role from the
point of view of localised and general corrosion
susceptibility.
The weld metal microstructure plays a major role
with respect to local corrosion resistance. The
corrosion behaviour of weld metal differs from that of
the more homogeneous base metal that has been
manufactured by rolling and subsequent heat treatment.
A solidified weld metal, instead, remains in the
”as-cast” condition, showing far more pronounced inhomogeneity
than the base metal. The main reasons for
this inhomogeneity are the microsegregation which
takes place during weld solidification and the uneven
partitioning of alloying elements between different
phases during phase transformations. In high-alloyed
austenitic stainless steel welds, the microsegregation of
Mo and Cr in particular leads to a remarkably inhomogeneous
weld composition. After weld solidification,
the dendrite cores are, therefore, depleted in
Cr and Mo, whereas the interdendritic regions are
enriched in these elements. Consequently, the dendrite
cores are more easily attacked in a chloride-containing
environment, which results in inadequate corrosion
resistance unless the weld composition is overalloyed.
The corrosion resistance of a weld may also be created
by the precipitation of Cr- carbides and intermetallic
phases such as sigma-phase in the weld metal
microstructure [1-7].
To avoid the above mentioned problems, the weld
composition is normally overalloyed to eliminate local
underalloyed regions from the weld microstructure. To
achieve this, it has been generally recommended to use
Ni-based consumables when welding high-alloyed
stainless steels. For example, the welding of 254 SMO -
steel (6 %Mo) is usually carried out with a high Mocontaining
(9 %Mo), Ni-base consumable. For 654
SMO (7.5 %Mo), the Mo- content of the consumable is
increased to 15 %.
The use of overalloyed Ni-base consumables has
some limitations, however. The weldability of these
consumables is technically quite demanding, as a result
of the more viscous weld pool behaviour, and a skilful
welder is therefore needed to avoid weld imperfections.
The weld fusion line may also show inadequate
corrosion resistance due to the formation of an
unmixed zone in the dissimilar joint, see Fig. 2. This is
due to the fact that the unmixed zone solidifies in a
similar manner as the base metal, leaving a pronounced
microsegregation in this narrow weld region.
General corrosion in highly oxidising
conditions
In highly oxidising conditions, such as in ClO 2 - bleaching,
the dominant corrosion mechanism may change
from localised corrosion to the so called transpassive
Fig. 2. Schematic presentation of different weld zones [8].
dissolution which causes general corrosion in the base
and weld metals. The resistance to general corrosion in
highly oxidising conditions depends not only on
composition but also on pH, temperature, and on the
oxidising potential (ClO 2 - content) of the environment.
Research studies as well as practical experience
have shown that Ni-base alloys and stainless steels
display a different behaviour in ClO 2 -bleaching
environments. With stainless steels, the transpassive
corrosion rate increases with the oxidising potential of
the environment as well as with increasing Cr-, Ni- and
Mo- contents of the steel. At low pH- values (< 2),
there is no striking difference between stainless steels
and Ni-based alloys. As soon as the pH- value exceeds
3, the transpassive corrosion rate of stainless steels
decreases, while that of the Ni-based alloys increases.
Consequently, the dissolution rate of Ni-based alloys is
more pronounced in neutral solutions than in acid
environments [9-12].
Both Ni-based alloys and stainless steels display an
increased transpassive corrosion in oxidising conditions
when the Cr-, Ni- and Mo- contents of the alloys are
increased. This is due to the fact that all these elements
display more pronounced dissolution rates in oxidising
conditions.
General corrosion of welds in highly oxidising
conditions
To take account of the possibility of general corrosion,
the composition of a weld metal must be optimised to
minimise transpassive dissolution. This is important,
especially when the localised corrosion resistance of a
weld has been maximised with Ni-based consumables,
without noticing any enhanced risk of general
dissolution. There is clear evidence that, in oxidising
and acidic conditions (pH = 3-6), Ni-based weld metals
can display a relatively rapid general dissolution via
transpassive mechanisms. Important alloying elements
(Cr and Mo), which are added to increase localised
corrosion resistance, as well as Ni added to maintain an
austenitic microstructure, have shown to be harmful
with respect to general corrosion resistance. This has
led to development work on new welding consumables
and welding techniques that take both corrosion
mechanisms into account.
It has been demonstrated experimentally that the
40 • Svetsaren no. 2 • 2002