Overlay Repair of Hot Rolling VSB Vertical Rolls

Literature Overview

This paper by Wang Yinjun, Li Yungang, Dong Hanjun, and Wang Kaisheng (2006) presents a practical overlay welding repair technology applied to hot rolling VSB (Vertical Strip Bulger) vertical rolls at Meishan Steel Company. The study addresses the critical challenge of roll wear in heavy plate rolling mills, where vertical rolls experience severe abrasive and adhesive wear under extreme thermal and mechanical loads. The authors describe a systematic overlay repair process that extended the service life of VSB vertical rolls by 33% to 66% over a complete rolling campaign, with the overlayed rolls passing through 200,000 to 250,000 tonnes of steel per rolling plan cycle.

Core Technical Content

The VSB vertical roll in a heavy plate mill operates under conditions that are far more demanding than conventional roll applications. The roll surface is subjected to combined loading from strip bulging forces, frictional contact with hot steel strips at temperatures exceeding 900°C, and periodic thermal cycling during each pass. The original roll surface material, typically a medium-carbon low-alloy steel, cannot withstand the cumulative wear over an extended production campaign. The overlay repair approach involves grinding the worn surface down to a specified depth, preparing the substrate, and applying a multi-layer overlay weld deposit with progressively increasing alloy content to build a wear-resistant surface layer.

Key Process Parameters

Welding Metallurgy Considerations

The transition from the base metal to the overlay deposit involves a complex dilution gradient. In the first overlay layer, dilution from the substrate can reach 40% to 60%, which reduces the effective alloy content of the deposited metal. This necessitates a multi-layer strategy where the first layer acts as a transition buffer, and subsequent layers progressively build up the full alloy composition. The residual stress distribution in the overlay is particularly critical for vertical rolls, as these components operate under high bending moments and tensile hoop stresses. Excessive residual tensile stress at the weld root can initiate cracking during service. The authors employed post-weld stress relief to reduce residual stresses below 100 MPa, which is essential for preventing delayed cracking in the heat-affected zone.

Defect Control and Quality Assurance

Common defects in vertical roll overlay repair include porosity at the weld root, lack of fusion between layers, and undercut at the toe of the weld bead. The authors implemented a rigorous inspection protocol:

The grinding and dressing of the overlay surface is a critical finishing step. The final surface profile must meet geometric tolerances of ±0.1 mm per meter to ensure proper strip contact and avoid localized overheating or galling during rolling.

Engineering Practice Insights

The achievement of 33% to 66% life extension is significant from an economic perspective. For a heavy plate mill rolling millions of tonnes annually, even a modest extension of roll life translates into substantial savings in roll procurement, downtime, and labor. However, the success of this approach depends heavily on the consistency of the repair process. Batch-to-batch variation in flux composition, wire chemistry, and welding parameters can lead to inconsistent overlay properties. I would emphasize that in engineering practice, the repair qualification should include a full-scale rolling trial before full production deployment, with periodic metallographic checks during the rolling campaign to monitor overlay degradation.

Study Insights and Reflections

This paper represents a pragmatic approach to extending the service life of critical rolling mill components. The VSB vertical roll is not a pressure-containing component, and the failure mode is gradual wear rather than catastrophic fracture, which provides some tolerance for overlay imperfections. Nevertheless, the metallurgical requirements remain stringent. The dilution management, residual stress control, and surface finish quality all demand skilled welders and disciplined process control. For engineers working in similar applications, the key takeaway is that overlay repair is not merely a matter of depositing hard material; it is a systematic engineering exercise that requires understanding of welding metallurgy, residual stress management, and tribological behavior under hot rolling conditions.