Hot Rolling Backup Roll Surfacing Welding Technology Research

Literature Overview

This paper, published by Lin Jiandong from the Panzhihua Steel Research Institute in the journal Electrician (2009, Vol. 39, No. 7, pp. 85–89), addresses a critical operational challenge in hot strip mills: the short service life of backup rolls after surfacing repair. The backup rolls under study are made of 70Cr3NiMo surface-hardened forged steel, a material widely used in the finishing mills of hot rolling lines due to its excellent combination of strength, toughness, and surface hardness. The author identifies two interrelated problems: the low post-repair service life of these rolls and the absence of mature surfacing welding materials in China at the time. To resolve these issues, the author independently developed a new flux-cored wire designated PRJ225, matched with flux 107, and established an optimized surfacing welding procedure.

Core Technical Findings

The central achievement of this research is the development of PRJ225 flux-cored wire, which was benchmarked against the then-conventionally used H25Cr3Mo2MnVA wire paired with SJ301 flux. The comparison revealed that PRJ225 outperformed the conventional wire in three critical performance indicators: hardness, wear resistance, and crack propagation resistance. These results are particularly significant because backup rolls operate under extreme contact stress and abrasion conditions, where even minor degradation in surface integrity can lead to premature failure and costly production downtime.

Performance Comparison

Process Analysis and Engineering Practice

The selection of flux-cored wire (FCAW) over solid wire or covered electrode processes reflects the practical requirements of large-diameter roll surfacing. Flux-cored wire offers higher deposition rates, better arc stability, and the ability to incorporate alloying elements directly into the wire core, which is essential for achieving the desired microstructure in the surfacing layer. The matching of PRJ225 with flux 107 suggests a carefully designed slag system that promotes proper wetting, adequate penetration, and controlled cooling rates in the surfacing deposit.

The achievement of over 650,000 tonnes of average steel throughput per repaired roll is a remarkable industrial benchmark. For context, a typical hot strip mill finishing stand may process 200,000 to 400,000 tonnes per roll change cycle in standard operation. The fact that the repaired roll exceeded this range by a significant margin demonstrates the effectiveness of the new surfacing material and procedure. The maximum throughput of 1,126,000 tonnes is especially noteworthy, as it suggests that certain rolls benefited from particularly favorable operating conditions, possibly related to the specific steel grades being rolled or the压下量 (reduction) parameters.

Key Technical Insights

From a metallurgical perspective, the superior crack propagation resistance of PRJ225 is likely attributable to a more favorable microstructure in the surfacing layer. Backup rolls experience cyclic contact stresses that can initiate surface cracks, and the rate at which these cracks propagate determines the remaining service life of the roll. A surfacing layer with finer grain structure, controlled carbide morphology, and lower residual stress would naturally exhibit better crack resistance. The development of a dedicated welding consumable rather than adapting an existing one represents a more fundamental approach to solving the problem, addressing the root cause rather than merely mitigating symptoms.

The research also highlights an important aspect of Chinese metallurgical engineering: the transition from relying on imported or generic welding consumables to developing application-specific materials. This approach is particularly valuable for heavy equipment repair operations where the operating conditions are unique and demanding.

Reflections and Implications

This study carries forward the philosophy that material development and process optimization must proceed hand in hand. Simply selecting a harder surfacing material would not have addressed the crack propagation issue, which is equally critical for service life. The holistic approach of developing a new wire, matching it with an appropriate flux, and optimizing the welding procedure demonstrates mature engineering thinking. For practitioners in steel pipe manufacturing and heavy equipment repair, this paper serves as a reminder that consumable development tailored to specific service conditions can yield dramatic improvements in component life and cost efficiency.