In the current economic climate, efficiency improvements and cost saving
within the metals industry have become the norm. The role of weld reclamation
and cladding as a tool to both improve caster roll life and reduce the life
cycle cost of the rolls is therefore as pertinent as ever. Despite many aspects
of the technology approaching maturity, there are still many areas where
material and process development can contribute to improved roll life.
It’s not all about hardness.
Combining good corrosion and wear resistance, martensitic stainless steel cladding materials, such as Weldclad 3, have proven a cost effective workhorse
for caster roll cladding. The ~12wt%Cr ensures stainless characteristics and
the 0.08-0.15wt%C contribute towards a martensitic matrix with a hardness
between approximately 40-48HRc. Additionally, the high yield strength and low
coefficient of thermal expansion of the martensitic microstructure, compared to
typical Cr-Ni austenitic stainless steels reduces the susceptibility to thermal
fatigue, whilst the temper resistance ensures good high temperature hardness.
Furthermore, it is now generally recognised that the optimum microstructure for
a weld deposit is characterised by a martensitic microstructure with less than
10vol% delta-ferrite.
The success of the martensitic stainless steel has resulted in many
casters using a single roll welding strategy for all rolls within the
continuous caster, a one-size-fits-all approach. However, it is evident that
the service conditions experienced by the rolls within the caster are not the
same - with the level of abrasive wear, corrosion, erosion and thermal and
mechanical fatigue varying from the foot rolls below the mould, through the bow
segment to the withdrawal rolls. The application of a single roll cladding
strategy does not, therefore, fully realise the benefits of the roll welding
technology.
Corewire have
addressed this by promoting the strategy of Zoning in which cladding materials
and welding strategies have been developed and applied to address the needs of
individual rolls within the caster.
As an example of Zoning consider the foot rolls. Situated just below
the mould, the foot rolls are probably subject to the most extreme conditions
of any roll within the caster. Laboratory work demonstrated the improved
corrosion characteristics of austenitic cladding materials such as Weldclad 800
and the nickel-based Weldclad 900, compared to standard 12wt%Cr materials.
Such laboratory
observations have been confirmed by commercial trials where foot roll life has
been increased by a factor of ca. 10 by substituting the incumbent 12wt%Cr
material with WLDC 900. Although more expensive than 12%Cr materials due to the higher alloy
content this increased cost is off-set by the significant increase in roll life.
In the caster bow, where the segment rolls experience less corrosive
conditions than the foot rolls, martensitic stainless steels such as Weldclad 3
have proven a cost effective solution, with typical wear rates well below 1mm
per million tonnes cast. However, with the adoption of modern bearing
technologies and a desire to increase casting speeds, improved roll life is
being sought. Products such as Weldclad 3XCR and Weldclad 3HT with modified
molybdenum, chromium and nitrogen additions have been shown to improve the
corrosion and wear resistance of the clad layer compared to standard 410/414
type materials. These materials are now being rolled out
commercially.
The drive rolls can experience up to
three times more wear than the surrounding idle rolls. Indeed, many roll
replacement strategies are based around the wear of the drive roll. In such
cases materials with improved abrasive wear resistance, such as WLDC 8 and WLDC
2000, have been developed to help optimise roll life. More recently, the
benefits of higher chromium tool steels, such as WLDC 5HT, have been investigated
for applications where abrasion dominates roll degradation.
In addition to correct material
selection, the welding procedure adopted can also influence the cost
effectiveness of the roll reclamation process.
For example, dilution of the weld metal by the base material occurs
during welding. Multiple layers of weld
metal are therefore required to overcome the effect of dilution and achieve the
correct surface composition. However, by
using an over-alloyed buffer material such as Weldclad 19 or over-alloyed
consumables such as WLDC 3M2H/L the number of layers of welding can be reduced
so lowering the cost of the cladding process without compromising the
performance of the clad roll. In the extreme, welding consumables such as
Weldclad 3S allow a 12%Cr martensitic cladding to be achieved in one layer of
welding on a new roll.
A Step in the Right
Direction: the Cortech Approach.
Ensuring the quality of new rolls and reclaimed rolls to the same high
standard can be problematic due to the often broken nature of the clad roll
supply chain. The cladding consumables used in the original roll manufacture
are often unknown and a different welding consumable regularly used for
subsequent roll repair. It is therefore difficult for the end user to receive
consistent, high quality, new and reclaimed rolls.
Corewire address this through the Cortech range of clad rolls. Cortech rolls are manufactured by
accredited workshops around the world using Weldclad cladding consumables to a
distinct and transparent quality standard. This ensures the end-user is aware
of the quality of the base material, weld cladding consumables and welding
process employed. By adopting complimentary cladding consumables from the
Weldclad range, the end user is able to repair the roll surface to the same
high standard as the original Cortech roll.
For further information, you can visit the Corewire website.
