2.2 - ICR DEC 05 - Fractured Kiln Welding
2.2 - ICR DEC 05 - Fractured Kiln Welding
2.2 - ICR DEC 05 - Fractured Kiln Welding
KILN MAINTENANCE
Repair option
If the repair route is selected, a number
of decisions must be made very quickly.
These include whether to do the repair inhouse with existing staff and/or contract
welders, or whether to subcontract the
work to a company with experience and
equipment to do the entire job. In recent
years the latter has increasingly been
the most prevalent choice, especially in
light of the limited welding personnel
available in most plants today. It should
also be considered that once the repair
has begun, it is imperative and more
efficient to continue welding around the
clock until it is finished. This is much more
cost effective than frequently stopping
and starting, and minimises the difficulties
involved in maintaining correct base metal
preheating temperatures throughout the
entire repair cycle.
Process considerations
There are generally two alternative
options for selecting the welding alloys
and procedures to be used:
Option 1: The use of an autogenous,
AWS specification low hydrogen electrode
that meets the general standards for
welding this type of steel.
Option 2: The use of a dissimilar,
non-ferrous nickel-based alloy electrode
with a basic coating specially formulated
by Castolin Eutectic for repairing air
hardening steels.
While the ferritic alloy (Option1) closely
matches the chemistry and alloy content
of the base metal, the air hardenability
of the large volumes of weld metal to
be deposited would require rather high
preheat requirements, creating both
practical and procedural problems. In
addition, Post Weld Heat Treatment
(PWHT) would be mandatory. The
difficulties associated with these levels of
heating, considering the mass involved
Consumable
recommendations
When making the final selection of the
electrode to be used on a kiln tyre repair,
it is important to remember the severe
penalties associated with having the repair
fail again. The cost of the right electrode,
then becomes much less important than
knowing it will not only do the job, but
will continue to perform safely and reliably
for many years to come.
Primary factors that affect an electrodes
suitability include:
linear coefficient of thermal expansion
tolerance to the effects of dilution
microstructural stability
resistance to Hydrogen Induced
Cracking (HIC).
Unless the electrode is specifically
designed to accommodate all four of
the above factors, residual welding
stresses and the presence of hydrogen
in hardenable zones will increase the
risk of provoking unexpected cold
cracking phenomena 24 48 hours after
completing the repair!
Linear coefficient of
thermal expansion
Various metals expand and contract at
different rates during the welding process.
Stainless steel alloys, for example, expand
DECEMBER 2005 ICR 91
KILN MAINTENANCE
Conclusion
Tolerance to the
effects of dilution
Dilution refers to the
percentage of base metal
melted into the weld deposit.
When conventional stainless steel alloys
are used to join or repair carbon steels,
the molten weld will invariably be diluted
by at least 20-30 per cent of the base
metal, which creates a weld deposit prone
to embrittlement due to the formation of
a hard martensitic structure.
Xuper NucleoTec 2222, on the other
hand, will tolerate up to 50 per cent
dilution with carbon steels and still
retain its tough austenitic structure again
promoting maximum safety margin welds.
Microstructural stability
When welding thick carbon steel
sections with conventional stainless steel
electrodes, hard, brittle Sigma phases may
be formed in the weld. This is especially
true when welding with the traditional
austenitic ferritic alloys often selected
for this type repair that contain chromium
and iron in high quantities. Sigma phase
precipitation is associated with time and
temperature and forms within minutes at
900C and after a few days at 500C. It
should be noted that these temperatures
are cycled several times during each weld,
regardless of what service conditions are
encountered later.
As seen from the Ternary Phase
Diagram (Figure 6), Xuper NucleoTec
2222 is situated far away from the critical
Sigma Phase zones. Even with dilution,
Xuper NucleoTec 2222 remains immune
to the formation of harmful, embrittling
Sigma Phases provoked by thermal
92 ICR DECEMBER 2005
Resistance to HIC
The risk of delayed cold-cracking in
steels is multiplied whenever a hardened
welding zone accumulates critical tensile
stresses and excessive residual hydrogen
which is unable to escape by diffusion
phenomena (HIC).
Xuper NucleoTec 2222 minimises HIC
risks in two complementary ways:
1. The basic extruded flux coating ensures
tough, ductile welds with lowest diffusible
hydrogen contents.
2. The austenitic weld deposit structure is
Figure 7: repaired kiln tyre with Xuper
NucleoTec 2222