Regenerative Surfacing Repair of Roll Press Rollers at Jiangxi Yadong Cement Plant

Literature Overview

This 2008 paper by Xu Jian and colleagues from Zhengzhou Machinery Research Institute documents the regenerative surfacing repair of five large-diameter roll press rollers (Phi 1700 x 1800 mm) at Jiangxi Yadong Cement Co., Ltd. These rollers, manufactured by German company KHD (Humboldt), were used in both clinker grinding and raw material final grinding systems on three advanced dry-process cement production lines with a combined annual capacity of 8 million tonnes. The paper addresses the challenge of restoring worn rollers to their original or improved dimensional specifications through surfacing welding, a critical maintenance activity for continuous cement production operations.

Core Technical Content

Roll press rollers in cement grinding circuits are subjected to extremely severe service conditions. The rollers press clinker or raw meal at pressures of 100-200 MPa, with continuous sliding contact, high temperatures from the grinding process (often exceeding 300°C for clinker), and abrasive wear from the feed material. The original rollers were likely made from a high-chromium white iron or a similar wear-resistant cast iron, with a hardened surface layer designed to resist the combined effects of abrasion, impact, and thermal cycling.

Roller Specifications and Service Conditions

The scale of these rollers is substantial, and the repair methodology must account for the large mass, the need for dimensional accuracy after surfacing, and the requirement for uniform hardness across the entire working surface. The paper refers to "regenerative" surfacing, which implies not merely restoring the rollers to their original condition but potentially improving the wear resistance beyond the original specification.

Surfacing Material Selection and Process

While the paper abstract does not provide extensive detail on the specific surfacing materials and welding processes used, the context of roller repair in cement plants typically involves the following considerations:

The most likely approach for these large rollers would be a multi-layer surfacing strategy, beginning with a transition layer of a ductile alloy to ensure good bonding with the base metal and minimize residual stress, followed by one or more layers of a high-hardness wear-resistant alloy. The process would likely involve submerged arc welding (SAW) or flux-cored arc welding (FCAW) for high deposition rates, given the large surface area requiring repair.

Preheating and Interpass Temperature Control

For rollers of this size, thermal management during surfacing is critical. The large mass of the roller acts as a heat sink, but the thick cross-section also means that residual stresses can be significant. Typical preheat temperatures for surfacing on cast iron or high-carbon steel rollers would be in the range of 250-400°C, with interpass temperatures maintained at similar levels. The surfacing would be applied in a systematic sequence, typically starting at the center and working outward, or using a balanced pattern to minimize distortion.

Engineering Practice Insights

The repair of large rollers through surfacing welding is a well-established practice in heavy industry, but the specific challenges of cement plant rollers deserve attention. Unlike grinding rollers in mineral processing, cement rollers operate under sustained high-pressure contact with relatively hot material, leading to a combination of abrasive, adhesive, and thermal fatigue wear. The surfacing layer must therefore possess not only high hardness but also thermal stability at elevated temperatures.

PDCA Cycle in Roller Repair

Applying the Plan-Do-Check-Act cycle to this repair activity:

Plan: Assess the extent of wear through dimensional measurement (laser scanning or coordinate measurement machine), determine the required build-up thickness, select appropriate surfacing materials, develop a welding procedure specification (WPS), and plan the preheat and post-weld heat treatment (PWHT) schedule.

Do: Execute the surfacing welding according to the WPS, maintaining strict control of preheat and interpass temperatures, applying layers in the planned sequence, and performing in-process inspection.

Check: After surfacing, measure the final dimensions and roundness, perform hardness testing across the surface, conduct magnetic particle or dye penetrant inspection for surface defects, and if necessary, perform ultrasonic testing for internal defects.

Act: Based on inspection results, perform any necessary grinding, machining, or additional surfacing to achieve the required specifications. Document the results and update the repair procedures for future applications.

Key Technical Considerations

The dimensional accuracy required after surfacing is a major challenge. Roll press rollers must maintain a diameter tolerance of typically +/- 0.1 to +/- 0.3 mm and a runout of less than 0.05 mm. This means that the surfacing must be applied with sufficient build-up to allow for subsequent grinding, but not so much as to create excessive distortion. The surfacing layer typically needs to be 3-5 mm thicker than the final dimension to allow for grinding and finishing.

Residual stress management is another critical aspect. The high thermal input from multi-pass surfacing on a thick roller can generate significant residual stresses that may lead to delayed cracking or reduced fatigue life. Post-weld heat treatment, typically involving stress relief annealing at 550-650°C for several hours, is usually required. However, this must be carefully controlled to avoid tempering softening of the high-hardness surfacing layer.

Study Insights and Implications

This paper highlights the economic importance of surfacing repair in heavy industry. For rollers of this size, the cost of replacement can be substantial, often exceeding several hundred thousand RMB per unit. The ability to restore rollers through surfacing welding, potentially improving their wear resistance beyond the original specification, represents a significant cost saving and reduces downtime. The term "regenerative" in the title is particularly apt, as it implies that the repaired rollers may have a longer service life than the originals.

From a metallurgical perspective, the success of roller surfacing depends on achieving a metallurgical bond between the surfacing layer and the base metal that can withstand the combined mechanical and thermal loads. The interface between the surfacing layer and the base metal is the critical region, and any defects such as lack of fusion, porosity, or microcracking at this interface will lead to premature failure. The use of a transition layer with good ductility and low carbon content is essential to accommodate the differences in thermal expansion and mechanical properties between the base metal and the high-hardness wear layer.

The paper also implicitly addresses the issue of production continuity. In a cement plant with three production lines, the repair of five rollers must be scheduled to minimize production disruption. This requires careful planning, potentially involving the repair of rollers in batches, with some rollers in service while others are being repaired.

Summary

The regenerative surfacing repair of large cement plant rollers documented in this paper exemplifies the application of welding technology to critical heavy equipment maintenance. The successful repair of KHD-made Phi 1700 x 1800 mm rollers through surfacing welding demonstrates that modern welding practices can restore and even enhance the performance of large, high-value components. The key technical challenges include dimensional control, residual stress management, and ensuring a robust metallurgical bond between the surfacing layer and the base metal. Engineers undertaking similar repairs should pay close attention to the welding procedure development, thermal management, and comprehensive post-weld inspection to ensure long-term reliability in severe service conditions.