Zhuojin Pipe Fitting Co., LtdOverlay Welding Repair of Hot Rolling VSB Vertical Rolls
Literature Overview
The paper by Wang Yinjun, Li Yungang, Dong Hanjun, and Wang Kaisheng, published in Welding Technology (2006, Vol. 35, No. 4, pp. 69-71), addresses the overlay welding repair technology applied to vertical rolls in a hot rolling VSB (Vertical Shape Box) mill at Meishan Steel Company. The authors, affiliated with the Meishan Steel Technology Center and the Eighth Institute of Nanjing University of Aeronautics and Astronautics, present a systematic approach to restoring worn vertical rolls through overlay welding, demonstrating a throughput of 200,000 to 250,000 tonnes per rolling campaign cycle and a service life improvement of 33% to 66%.
Core Technical Content
The VSB mill represents a critical component in the hot rolling process, where vertical rolls are subjected to extreme mechanical and thermal loading conditions. These rolls endure repeated contact with hot steel slabs at temperatures often exceeding 1000°C, combined with compressive forces and abrasive contact from scale and iron oxide layers. The wear patterns typically manifest as barrel-shaped profiles, local indentations, and surface spalling, necessitating periodic removal from service for grinding and repair.
The overlay welding repair process described in this literature involves several key stages:
- Pre-treatment: The worn roll surface undergoes thorough cleaning, removal of residual scale, and preparation of the welding area with appropriate bevel geometry to ensure adequate fusion and metallurgical bonding.
- Preheating: Controlled preheating is applied to reduce thermal gradient and minimize residual stress concentration at the interface between the roll substrate and the deposited overlay layer.
- Overlay deposition: Multiple passes of wear-resistant alloy are deposited using arc welding processes, with careful control of interpass temperature and heat input.
- Post-weld treatment: Stress relief annealing is performed to reduce welding residual stresses that could otherwise lead to cracking during subsequent service.
- Machining and finishing: The overlay surface is ground to restore dimensional accuracy and surface roughness specifications required for VSB rolling operations.
Key Technical Parameters and Process Control
| Parameter | Typical Range | Purpose |
|---|---|---|
| Preheat temperature | 200-350°C | Reduce cooling rate, prevent HAZ cracking |
| Interpass temperature | ≤250°C | Control microstructure, limit carbon accumulation |
| Heat input per pass | 1.5-3.0 kJ/mm | Balance penetration and dilution |
| Overlay thickness | 8-15 mm | Provide adequate wear reserve |
| Post-weld stress relief | 550-650°C × 2h | Eliminate residual tensile stresses |
| Grinding tolerance | ±0.1 mm | Restore dimensional accuracy |
Engineering Practice Analysis
From a practical standpoint, the 33% to 66% improvement in service life is highly significant for rolling mill economics. The VSB vertical roll operates under unique conditions compared to conventional horizontal mill rolls. The vertical orientation subjects the roll to asymmetric loading, and the box-forming action creates concentrated stress at the roll barrel surface. The overlay alloy selection must therefore consider not only wear resistance but also thermal fatigue resistance and hot hardness retention at elevated temperatures.
The authors' achievement of 200,000 to 250,000 tonnes throughput per campaign cycle indicates that the overlay material and process parameters were well-matched to the specific service conditions at Meishan Steel. This throughput level suggests that the overlay layer maintains adequate hardness and integrity throughout the rolling campaign, without premature spalling or delamination.
Defect Analysis and Countermeasures
Common defects encountered in overlay welding of heavy rolls include:
- Cracking at the interface: Caused by excessive cooling rates, high carbon content in the substrate, or inadequate preheating. Countermeasures include increasing preheat temperature, using lower-carbon transition layers, and controlling interpass temperature.
- Porosity: Resulting from inadequate flux coverage, moisture contamination, or excessive travel speed. Mitigation involves proper flux management and optimized welding parameters.
- Delamination during service: Related to poor metallurgical bonding between overlay and substrate, or inadequate stress relief. Solutions include multi-layer deposition with compatible transition alloys and thorough post-weld heat treatment.
- Hot cracking in the overlay: Associated with low melting point phases in the weld metal. Prevention requires careful alloy design and controlled solidification conditions.
Study Insights and Implications
This literature demonstrates the economic viability of overlay welding repair as an alternative to complete roll replacement. The collaborative approach between an industrial user (Meishan Steel) and a research institution (Nanjing University of Aeronautics and Astronautics) exemplifies the effective translation of academic research into industrial practice. The key insight is that successful overlay repair of hot rolling rolls requires not only appropriate alloy selection but also meticulous process control throughout the entire repair sequence, from surface preparation through post-weld treatment.
The 33% to 66% life improvement, while substantial, suggests that there remains room for optimization. Factors such as overlay alloy composition refinement, multi-pass welding strategy optimization, and advanced thermal management during deposition could potentially yield further gains. This work provides a solid foundation for engineers seeking to implement or improve overlay repair programs for heavy industrial rolls.