Wear-Resistant Cladding Repair Process for Cold Rolling Rolls

Literature Overview

This 2006 study published in the journal of Welding Technology in China, authored by Yu Chengkui from Wuhan University of Technology and Yang Zhenlin from the Tianjin Municipal Quality and Technical Supervision Bureau, focuses on the wear-resistant cladding repair process for cold rolling rolls. Cold rolling rolls are subjected to extreme conditions in the cold rolling mill, including high contact stresses, severe sliding friction, and abrasive contact with the workpiece. The wear of roll surfaces is a major source of downtime and cost in cold rolling operations, and the development of effective cladding repair processes is essential for maintaining production efficiency and product quality. This research contributes practical process knowledge for the repair and restoration of worn cold rolling rolls using cladding technology.

Core Technical Points

The wear mechanisms in cold rolling rolls are primarily abrasive and adhesive, with the severity of wear depending on the material being rolled, the rolling conditions, and the roll surface properties. The cladding repair process must address these wear mechanisms by depositing a wear-resistant layer on the worn roll surface that can withstand the operating conditions. Common cladding materials for cold rolling rolls include high-carbon steel, high-speed steel, and tool steel alloys, each offering different combinations of hardness, toughness, and wear resistance.

The selection of the cladding process is critical to achieving a repair that is both effective and economical. Submerged arc welding (SAW) overlay is a popular choice for cold rolling roll repair due to its high deposition rate, low cost, and ability to achieve thick cladding layers. However, the high heat input of SAW can cause excessive dilution and may affect the properties of the underlying roll material. Gas tungsten arc welding (GTAW) overlay offers better control over dilution and bead geometry, making it suitable for precision repair of localized wear areas.

Engineering Practice Implications

In cold rolling mill operations, roll wear is typically assessed by measuring the reduction in roll diameter or by monitoring the surface roughness of the roll. When the roll diameter reduction exceeds the allowable limit or when the surface roughness degrades beyond acceptable levels, the roll is removed for repair. The cladding repair process involves the removal of the worn surface, preparation of the substrate, and deposition of the cladding layer using the selected welding process.

The surface preparation of the cold rolling roll is a critical step in the repair process. The worn surface must be ground or machined to remove any damaged material and to create a clean, uniform surface for cladding. The grinding process must be carefully controlled to avoid introducing residual stresses or microcracks in the roll surface. The substrate temperature during grinding should be monitored to prevent overheating, which can alter the microstructure of the roll material and reduce its fatigue strength.

The cladding process parameters must be optimized to achieve a strong bond between the cladding layer and the roll substrate while minimizing the heat-affected zone. For SAW overlay, a typical parameter set includes a current of 400 to 600 amperes, a voltage of 25 to 35 volts, a travel speed of 300 to 600 mm per minute, and a wire feed rate of 10 to 15 meters per minute. The flux composition is also important, as it influences the arc stability, slag properties, and final weld composition. For GTAW overlay, the parameters are typically lower, with a current of 100 to 200 amperes, a voltage of 15 to 25 volts, and a travel speed of 200 to 400 mm per minute.

Quality control for cladded cold rolling rolls includes hardness testing to verify that the cladding layer meets the required hardness specification, macrographic examination to check for cracks and porosity, and dimensional inspection to ensure that the roll diameter and profile are within tolerance. The repaired roll must also undergo a trial run in the mill to confirm that the cladding layer performs satisfactorily under actual operating conditions.

Study Insights and Reflections

The research provides practical insights into the challenges and solutions associated with cladding repair of cold rolling rolls. One of the key challenges is the management of thermal distortion during the repair process. The localized heat input of the welding arc can cause differential expansion and contraction in the roll, leading to barrel distortion or out-of-roundness that must be corrected by subsequent grinding. The use of multi-pass cladding with controlled interpass temperature can help to manage the thermal cycle and reduce distortion, but this approach increases the repair time and cost.

Another important consideration is the compatibility of the cladding material with the roll substrate. The thermal expansion coefficient mismatch between the cladding layer and the roll material can lead to residual stresses at the interface, which may promote crack initiation and propagation during service. The selection of a cladding material with a thermal expansion coefficient close to that of the roll substrate can help to minimize this issue. Additionally, the use of a graded interlayer between the cladding layer and the substrate can further reduce the thermal mismatch stress.

The economic aspects of cladding repair versus roll replacement are also significant. The cost of cladding repair is typically 40 to 60 percent of the cost of a new roll, and the repair time is significantly shorter than the lead time for a new roll. However, the service life of a repaired roll is typically shorter than that of a new roll, and the frequency of repairs increases with each successive repair cycle. A life-cycle cost analysis should be performed to determine the optimal repair strategy for a given roll, taking into account the cost of repair, the expected service life after repair, and the cost of downtime.

This research contributes valuable practical knowledge to the field of cold rolling roll repair, and the process parameters and quality control procedures described in the study can serve as a reference for similar applications in other rolling mills. The continued development of cladding repair technology for cold rolling rolls is important for improving the productivity and cost-effectiveness of cold rolling operations, and this study provides a solid foundation for further advancements in this area.