Zhuojin Pipe Fitting Co., LtdSurfacing Repair and Reuse of Medium Plate Hot Rolling Backup Rolls
Literature Overview
The paper by Zhou Min, published in Shandong Metallurgy (2004, Volume 26, Issue S1), documents the successful surfacing repair and reuse of four failed medium plate hot rolling backup rolls measuring Φ1800mm × 2740mm at Anyang Steel Company's second rolling mill. The study demonstrates that surfacing repair can reduce capital investment by 60% compared to purchasing new rolls, representing a significant cost-saving measure in rolling mill operations. This case study provides valuable engineering data for the evaluation of roll repair versus replacement decisions.
Technical Background
Medium plate hot rolling backup rolls are critical components in the rolling mill, subjected to extreme contact stresses, thermal cycling, and mechanical loading during the rolling process. The failure modes of backup rolls typically include:
- Surface cracking due to thermal fatigue and contact stress
- Roll body cracking from residual stresses and operational loads
- Bearing seat damage from misalignment or overload
- Surface wear and indentation from work roll interaction
The backup roll dimensions of Φ1800mm × 2740mm indicate a heavy-duty application in the medium plate rolling mill, where roll diameters and spans are substantially larger than those used in strip or wire rod mills.
Roll Failure Analysis
| Failure Mode | Typical Location | Root Cause | Repair Feasibility |
|---|---|---|---|
| Surface cracking | Roll barrel surface | Thermal fatigue, contact stress | High - surfacing repair possible |
| Roll body cracking | Near bearing seat | Residual stress, operational overload | Moderate - requires careful assessment |
| Bearing seat damage | Journal area | Misalignment, overload | Low - often requires replacement |
| Surface wear/indentation | Barrel surface | Work roll interaction | High - grinding and surfacing |
| Spalling | Surface or near-surface | Contact fatigue | Moderate - depends on depth |
Surfacing Repair Process
The repair process for the failed backup rolls involved the following key steps:
- Damage assessment and classification - Each roll was inspected to determine the extent and type of damage, classifying them into repairable and non-repairable categories. The assessment included visual inspection, magnetic particle testing (MT), and ultrasonic testing (UT) to detect surface and subsurface defects.
- Surface preparation - Damaged areas were ground or machined to remove all defective material, creating a clean, sound substrate for the surfacing weld. The surface was prepared to a smooth finish with appropriate roughness for weld adhesion.
- Preheating - The roll was preheated to a controlled temperature (typically 250-400°C for high-carbon bearing steel rolls) to reduce thermal stress during welding and minimize the risk of crack formation. Preheating was achieved using induction heating or gas flame heating, with temperature monitoring at multiple points around the roll circumference.
- Surfacing welding - Multiple passes of surfacing weld were deposited using appropriate consumables. The welding process was selected based on the required surface properties and the base metal composition. Submerged arc welding (SAW) or gas metal arc welding (GMAW) with appropriate flux or shielding gas was likely employed for this application.
- Post-weld heat treatment - After surfacing, the roll underwent a post-weld heat treatment cycle to relieve residual stresses and improve the weld metal properties. This typically involved heating to 550-650°C and holding for a duration proportional to the roll wall thickness.
- Machining and finishing - The surfaced surface was ground to the required dimensional accuracy and surface finish. The final geometry was verified against the original roll specifications.
- Quality verification - The repaired roll was subjected to dimensional inspection, hardness testing, and non-destructive examination to confirm that repair quality met the required standards.
Surfacing Repair Cost Analysis
| Cost Component | New Roll (Reference: 100%) | Surfaced Repair Roll |
|---|---|---|
| Material cost | 100% | ~15-25% |
| Manufacturing cost | 100% | ~10-20% |
| Surfacing welding cost | N/A | ~20-30% |
| Heat treatment cost | Included | ~5-10% |
| Machining cost | Included | ~10-15% |
| Inspection and testing | Included | ~5-10% |
| Total estimated cost | 100% | ~40% |
The reported 60% cost reduction is consistent with industry experience, where surfacing repair of large backup rolls typically costs 35-50% of the price of a new roll. The cost savings are most significant for large-diameter rolls where the material and manufacturing costs of new rolls are substantial.
Engineering Practice Considerations
The successful repair of these backup rolls demonstrates several important engineering principles:
- Economic viability assessment - Before initiating repair, a thorough economic analysis should be conducted comparing repair costs against new roll costs, factoring in the expected remaining service life of the repaired roll.
- Quality assurance - The repair process must include rigorous quality control at each stage to ensure that the repaired roll performs equivalently to a new roll in service.
- Documentation and traceability - Complete records of the damage assessment, repair process, and quality verification should be maintained for future reference and trend analysis.
- Service life prediction - Engineers should consider the impact of repair on the remaining service life of the roll, particularly regarding fatigue life and the potential for crack initiation at the weld/substrate interface.
The study also highlights the importance of developing in-house repair capabilities for large rolling mill components. Organizations that invest in repair infrastructure can significantly reduce operating costs and improve equipment availability by reducing dependence on external suppliers and minimizing roll change-out intervals.
Study Insights and Reflections
This case study provides a practical demonstration of the economic and technical benefits of surfacing repair for large rolling mill components. The 60% cost reduction achieved is compelling, but it is important to recognize that cost savings alone should not drive repair decisions. The technical feasibility of repair, the expected remaining service life, and the quality assurance capabilities of the repair facility must all be considered.
A key insight from this study is the importance of systematic damage assessment before repair. Not all failed rolls are suitable for repair, and the decision to repair must be based on a thorough evaluation of the damage type, extent, and root cause. Rolls with structural cracks in critical areas, such as near bearing seats or in the roll core, may not be suitable for repair regardless of the surface damage.
The study also underscores the value of developing expertise in roll repair technology within steel enterprises. The Anyang Steel Company's second rolling mill team demonstrated the capability to successfully repair large backup rolls, which requires knowledge of welding metallurgy, heat treatment, and quality control. Building this internal capability creates organizational resilience and reduces vulnerability to supply chain disruptions.
For engineers evaluating repair versus replacement decisions, this study provides a useful reference point. The cost savings from repair are substantial, but the quality of the repair must be maintained at a level that ensures equivalent service performance. The integration of non-destructive testing, dimensional verification, and mechanical property testing into the repair workflow is essential for maintaining repair quality and ensuring operational safety.