Optimising the Life Cycle Costs of Continuous Caster Rolls

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.