Research on φ1550 Backup Roll Overlay Repair Process

Literature Overview

This 1997 publication by Zhao Hui and Han Zhong from Shenyang University of Technology and the Institute of Metal Research, Chinese Academy of Sciences, addresses the overlay welding repair of a φ1550 backup roll. Backup rolls are critical components in hot strip mills, where they support the work rolls and transmit rolling force to the mill housing. The φ1550 designation indicates the roll barrel diameter, which is typical for medium-thickness hot rolling operations. Overlay repair of worn or damaged backup rolls is a cost-effective alternative to complete roll replacement, particularly when the damage is localized to the barrel surface.

Technical Analysis of Backup Roll Overlay Repair

Backup rolls in hot strip mills operate under extreme conditions:

• Temperature: Surface temperature reaches 800-1000°C during hot rolling
• Contact pressure: 100-300 MPa between work roll and backup roll
• Rolling speed: 1-3 m/s at the roll surface
• Thermal cycling: Repeated heating and cooling with each pass of the strip

These conditions lead to several failure modes that necessitate overlay repair:

1. Surface wear: Gradual reduction in barrel diameter from abrasive contact with work rolls
2. Roll surface cracking: Thermal fatigue cracks initiating at surface defects
3. Indentation: Localized plastic deformation from contact with hard inclusions in the strip
4. Spalling: Flaking of surface material due to subsurface crack initiation

Overlay Material Selection for Backup Rolls

The selection of overlay material for backup roll repair depends on the specific failure mode and the desired service life extension:

For the φ1550 backup roll application, a Ni-Cr-Mo alloy overlay is typically most appropriate, as it provides a balance of thermal fatigue resistance, adequate hardness, and good toughness to withstand the combined loading conditions.

Process Parameters and Technique

The overlay repair of backup rolls presents unique challenges due to the large diameter and the need to maintain precise geometric accuracy. The process typically involves:

1. Surface preparation: Grinding or machining to remove the damaged layer, creating a uniform substrate surface
2. Preheating: Induction heating to 200-300°C to reduce residual stress and prevent cracking
3. Overlay deposition: Multi-pass welding using SAW or GMAW with controlled parameters
4. Post-weld treatment: Stress relief at 550-650°C for 2-4 hours
5. Machining: Grinding to final dimensional accuracy (±0.02 mm per 100 mm)

The choice between SAW and GMAW depends on the required overlay thickness and the availability of equipment. SAW offers higher deposition rates and better penetration for thicker overlays, while GMAW provides better control for thinner layers and more complex geometries.

Defect Prevention and Quality Control

Common defects in backup roll overlay repair include:

• Cracking: Due to high carbon equivalent of the base material or excessive residual stress. Preheating and post-weld stress relief are essential.
• Delamination: Incomplete bonding between overlay passes, often caused by surface contamination or insufficient current.
• Uneven deposition: Variation in overlay thickness across the roll barrel, leading to geometric inaccuracy after machining.
• Porosity: From inadequate shielding or contamination. Particularly problematic in thick multi-pass deposits.

Quality verification for backup roll overlay repair includes:

Engineering Practice and Economic Considerations

The economic justification for overlay repair of backup rolls is compelling. A new φ1550 backup roll costs approximately 150,000-250,000 CNY, while overlay repair costs 20,000-40,000 CNY for material and labor. The repair extends the roll's service life by 6-12 months, compared to 2-3 months for an unprotected roll. The downtime savings from avoiding complete roll replacement—typically 24-48 hours of mill shutdown—further enhance the economic case for overlay repair.

However, overlay repair is not suitable for all damage conditions. If the barrel has lost more than 10% of its original diameter, or if there are deep subsurface cracks extending beyond the overlay depth, complete roll replacement is necessary. A systematic assessment using the PDCA (Plan-Do-Check-Act) cycle should guide the decision:

• Plan: Assess damage extent through UT and dimensional measurement
• Do: Perform overlay repair with selected material and process
• Check: Verify quality through NDT and mechanical testing
• Act: Monitor service performance and refine process for future repairs

Study Insights

This work exemplifies the application of metallurgical research to practical industrial problems. The collaboration between Shenyang University of Technology and the Institute of Metal Research demonstrates the value of academic-industry partnerships in advancing welding technology. The research provides systematic guidance on material selection, process parameters, and quality verification for backup roll overlay repair—knowledge that remains relevant in modern hot rolling operations.

For contemporary engineers, the key takeaway is that overlay repair is a viable and economical solution for extending the service life of large, expensive rolling mill components. However, success depends on rigorous engineering analysis of the damage mechanism, careful selection of overlay material and process, and thorough quality verification. The principles of thermal management, residual stress control, and metallurgical compatibility established in this work form the foundation for modern roll repair practices.