Zhuojin Pipe Fitting Co., LtdOverlay Welding Process and Application Scheme for Cold Continuous Rolling Mill Backup Rolls
Literature Overview
The paper by Zhang Yuntao, Geng Bo, Wang Jixiang, and Jiang Bo (2021), published in Metal World (Issue 3, pp. 77-79), presents a systematic study of overlay welding repair technology for backup rolls in a 2180 mm cold continuous rolling mill at Handan Iron and Steel Company. This research is significant because backup roll overlay welding repair has been relatively uncommon in China's cold rolling industry due to the stringent requirements for roll surface quality, dimensional accuracy, and operational reliability. The study provides a comprehensive technical framework for implementing overlay welding repair as a cost-effective alternative to full forged steel backup rolls.
Technical Requirements and Challenges
Cold continuous rolling mill backup rolls face unique challenges that distinguish them from other roll types:
| Requirement | Specification | Challenge |
|---|---|---|
| Surface hardness | ≥68 HSD (Shore D) | High hardness requires careful control of welding parameters to avoid cracking |
| Hardness uniformity | Within 2 HSD variation | Requires consistent welding procedure and adequate post-weld machining |
| Surface roughness | Ra ≤ 0.8 μm | Demands precise post-weld grinding and finishing |
| Dimensional tolerance | ±0.02 mm for barrel diameter | Requires high-precision machining and minimum machining allowance |
| Roll diameter | 800-1000 mm (typical) | Large diameter requires specialized welding equipment and access |
| Service life | 1-2 million tons of steel throughput | Overlay must maintain performance throughout the service life |
| Coating thickness | 3-5 mm (typical) | Must be thick enough for multiple regrinding cycles |
The primary challenge is achieving the required surface quality and dimensional accuracy after overlay welding, which is inherently a rough surface process. The solution involves a carefully designed welding and machining sequence that balances deposition rate, surface quality, and cost.
Overlay Welding Process Design
The process design follows a systematic approach to ensure quality and reliability:
Welding Procedure Specification
| Parameter | Value | Rationale |
|---|---|---|
| Welding process | Submerged arc welding (SAW) | High deposition rate, good penetration, minimal spatter |
| Electrode type | Low-alloy steel electrode with high-carbon content | Provides high hardness after heat treatment |
| Flux type | Low-alkali flux | Minimizes hydrogen-induced cracking risk |
| Welding current | 600-800 A | Optimized for deposition rate and penetration |
| Welding voltage | 30-38 V | Controls arc stability and heat input |
| Welding speed | 400-600 mm/min | Balances deposition rate and bead quality |
| Number of passes | 2-3 passes | Achieves required overlay thickness with adequate fusion |
| Interpass temperature | <200°C | Prevents excessive grain growth and cracking |
| Post-weld heat treatment | 500-550°C for 4-6 hours | Achieves target hardness through tempering of martensitic structure |
Process Control Key Points
The study identifies several critical process control points:
- Base roll surface preparation: The backup roll surface must be thoroughly cleaned and ground to remove scale, rust, and previous coatings. A minimum of 2 mm of surface material should be removed to expose clean base metal.
- Welding sequence: For large-diameter rolls, a symmetrical welding sequence is essential to minimize distortion. The roll should be rotated during welding to ensure uniform heat distribution.
- Heat input control: The welding heat input must be carefully controlled to achieve the desired microstructure. Excessive heat input can lead to coarse grain growth and reduced hardness, while insufficient heat input can result in poor fusion and reduced bonding strength.
- Post-weld heat treatment: The heat treatment is critical for achieving the target hardness. The tempering temperature and duration must be optimized to balance hardness and toughness.
- Post-weld machining: The overlay surface must be ground to achieve the required surface roughness and dimensional accuracy. A minimum of 1.5-2.0 mm of machining allowance should be provided.
Performance Evaluation and Cost Analysis
The study conducted comprehensive tracking of the overlay-welded backup rolls during service:
| Performance Metric | Overlay-Welded Roll | Forged Steel Roll | Comparison |
|---|---|---|---|
| Surface hardness | ≥68 HSD | ≥68 HSD | Equivalent |
| Hardness uniformity | Within 2 HSD | Within 1 HSD | Slightly less uniform but acceptable |
| Average grinding allowance per mm of steel throughput | Equivalent to forged roll | Baseline | No performance penalty |
| Service life | 1-2 million tons | 1-2 million tons | Equivalent |
| Cost per roll | ~50% of forged roll | 100% (baseline) | 50% cost reduction |
| Annual cost savings (F1-F5 stands) | Significant | Baseline | Economically attractive |
The results demonstrate that overlay-welded backup rolls can achieve performance equivalent to forged steel rolls at approximately 50% of the cost. If all five stands (F1-F5) in the cold rolling mill use overlay-welded backup rolls, the annual cost savings are substantial, making this technology economically attractive for widespread adoption.
Quality Assurance and Inspection Protocol
The study establishes a comprehensive quality assurance system:
- Incoming inspection: Verification of base roll material, surface condition, and dimensional accuracy
- In-process inspection: Monitoring of welding parameters, interpass temperature, and bead quality
- Post-weld inspection: Visual inspection, magnetic particle testing (MT) for surface cracks, and dimensional verification
- Post-machining inspection: Surface roughness measurement, dimensional accuracy verification, and hardness testing
- Service monitoring: Regular tracking of grinding allowance consumption and surface condition during operation
Study Insights and Reflections
This research represents a significant advancement in cold rolling mill roll technology, demonstrating that overlay welding repair can meet the demanding requirements of cold rolling applications. The key insight is that the combination of careful process design, rigorous quality control, and systematic service monitoring can overcome the traditional concerns about overlay welding quality. In my experience, the success of this technology depends heavily on the availability of skilled welding personnel and the implementation of a robust quality management system. The cost savings are substantial, but they must be weighed against the potential risks of roll failure during operation. I would recommend a phased implementation approach, starting with less critical stands and gradually expanding to all stands as confidence in the technology grows. The research also highlights the importance of data-driven decision-making in industrial applications, with comprehensive tracking and analysis of roll performance providing valuable feedback for process improvement.