CLADDING TECHNOLOGY SHANXI CO., LTDStudy Notes on Weld Overlay Repair of Large Backup Rolls
Overview and Background
Large backup rolls in hot rolling mills are subjected to extreme thermal, mechanical, and chemical loads during continuous production. These rolls typically exhibit a base material of forged alloy steel (such as 40CrNiMo or equivalent grades) with a surface layer that undergoes progressive degradation through thermal fatigue, rolling contact fatigue, and abrasive wear. When the accumulated damage exceeds permissible limits, the roll surface must be restored through weld overlay repair rather than full replacement, given the enormous cost and lead time associated with manufacturing a new roll of the required diameter (commonly 700 mm to 1000 mm or larger).
The literature reviewed focuses on a systematic approach to weld overlay repair of large backup rolls, emphasizing the selection of overlay materials, welding process parameters, heat input control, and post-weld treatment strategies. The repair typically involves the removal of the damaged surface layer followed by multi-pass weld overlay to restore both geometry and surface properties.
Core Technical Points
The primary challenge in repairing large backup rolls lies in managing residual stresses and preventing cracking in the overlay layer. The following key technical points were identified:
- Overlay material selection: Hardfacing alloys such as Fe-Cr-C type (e.g., Cr38) or Ni-based alloys are commonly selected depending on the service environment. For hot rolling backup rolls exposed to scale and iron oxide abrasion, Fe-Cr-C alloys with high carbon and chromium content offer superior wear resistance.
- Heat input control: The linear heat input must be carefully controlled to minimize thermal distortion and reduce residual stresses. Typical values range from 12 to 20 kJ/mm for multi-pass overlay using submerged arc welding (SAW) or gas metal arc welding (GMAW) processes.
- Preheating and interpass temperature: A preheat temperature of 200 to 350 degrees Celsius is recommended to reduce thermal gradients and hydrogen-induced cracking risk. The interpass temperature should be maintained between 200 and 300 degrees Celsius to avoid excessive grain coarsening.
- Post-weld stress relief: Stress relief annealing at 550 to 650 degrees Celsius for a duration proportional to the roll diameter is essential to relieve residual stresses that could otherwise lead to delayed cracking.
Process Parameters and Defect Analysis
| Parameter | Typical Value | Purpose |
|---|---|---|
| Preheat temperature | 200-350 degC | Reduce thermal gradient, prevent HIC |
| Interpass temperature | 200-300 degC | Control microstructure, prevent cracking |
| Linear heat input | 12-20 kJ/mm | Balance penetration and distortion |
| Stress relief temperature | 550-650 degC | Relieve residual stresses |
| Overlay thickness | 3-8 mm total | Restore geometry and surface properties |
Common defects observed during backup roll repair include:
- Undercut and lack of fusion at the base metal-overlay interface, typically caused by excessive travel speed or insufficient heat input.
- Porosity in the overlay layer, often attributed to inadequate shielding gas coverage or contaminated base metal surfaces.
- Cracking in the weld overlay, which can be either hot cracking (solidification cracking) or cold cracking (hydrogen-induced). Hot cracking is mitigated by adjusting the alloy composition to promote a more ductile solidification structure, while cold cracking is addressed through preheating and post-weld stress relief.
- Thermal distortion of the roll barrel, which can compromise dimensional accuracy. This is managed through symmetric welding sequences and controlled heat input.
Engineering Practice Insights
In practice, the repair of large backup rolls requires careful planning of the welding sequence to minimize cumulative thermal distortion. A common approach is to divide the roll circumference into segments and weld alternating segments in a balanced pattern. The welder or robotic welding system must maintain consistent parameters throughout the repair, which is challenging given the large scale and curvature of the roll surface.
The use of multi-layer multi-pass welding is standard practice, with the first pass serving as a transition layer between the base metal and the final hardfacing alloy. This transition layer typically uses a filler metal with intermediate composition to reduce dilution effects and improve metallurgical compatibility. The final overlay passes use the selected hardfacing alloy to achieve the desired surface hardness (typically 50 to 60 HRC for Fe-Cr-C alloys).
A critical observation from the literature is that the bond strength between the overlay layer and the base metal is often the weakest link in the repair. Non-destructive testing using magnetic particle inspection (MT) and ultrasonic testing (UT) should be performed after each major pass to detect interfacial defects early. Additionally, microhardness profiling across the overlay-thickness is recommended to verify that the hardness distribution meets specifications and that no soft zones exist near the fusion line.
The study also highlights the importance of surface preparation prior to welding. The damaged surface must be thoroughly ground to remove all affected material, including any decarburized or scale-contaminated zones. The grinding should be performed to a minimum depth of 2 mm below the lowest visible damage to ensure a sound base for the overlay.
Summary
The repair of large backup rolls through weld overlay is a technically demanding operation that requires a deep understanding of metallurgy, welding processes, and residual stress management. The key to successful repair lies in the careful selection of overlay materials, rigorous control of welding parameters, and comprehensive post-weld treatment. Engineers should adopt a systematic approach that integrates process planning, in-process monitoring, and post-weld inspection to ensure that the repaired roll meets the stringent performance requirements of hot rolling service. The lessons learned from this study emphasize that no single parameter can be optimized in isolation; rather, a holistic approach that considers the interaction between heat input, cooling rate, material composition, and mechanical properties is essential for achieving a durable and reliable repair.