CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Repair of φ1550 Backup Roller in Hot Rolling Mill
Literature Overview
This 1997 research paper published in the Journal of Shenyang University of Technology, authored by Zhao Hui (Shenyang University of Technology) and Han Zhong (Institute of Metal Research, Chinese Academy of Sciences), investigates the weld overlay repair process for a φ1550 backup roller. Backup rollers in hot strip mills are large-diameter cylindrical components that support the work rolls and transmit rolling force. They are subjected to extreme thermal and mechanical loading, and their surface condition directly affects strip surface quality and rolling force stability.
Core Technical Analysis
Operating Environment of Backup Rollers
Backup rollers operate under conditions that demand exceptional surface integrity:
- Temperature: Surface temperatures reaching 800–1000°C due to contact with hot steel strip
- Rolling force: Contact stress exceeding 2000 MPa at the roll/strip interface
- Thermal cycling: Rapid heating and cooling during each pass of the strip
- Chemical attack: Potential oxidation and scale adhesion from the hot strip
These conditions make the backup roller surface prone to cracking, spalling, and wear, necessitating periodic restoration through weld overlay.
Material Selection for Overlay
The base material of the backup roller is typically a medium-carbon steel or low-alloy steel cast iron. The overlay material must provide:
- High temperature hardness retention
- Thermal fatigue resistance
- Resistance to scale adhesion
- Compatibility with the base material to prevent cracking
| Overlay Material | Hardness (HRC) | Temperature Resistance | Application |
|---|---|---|---|
| Fe3Cr2Mo (high-carbon Cr-Mo cast iron) | 55–62 | 600°C | Moderate duty |
| Fe3Cr2W (high-carbon Cr-W cast iron) | 58–65 | 700°C | Heavy duty |
| 310 stainless steel overlay | 35–45 | 1000°C | High temperature service |
| Ni-based alloy overlay | 30–40 | 1100°C | Extreme thermal cycling |
| High-speed steel (W6Mo5Cr4V2) | 60–65 | 600°C | Wear-critical applications |
Process Development and Optimization
The research likely involved systematic process development to optimize overlay parameters for the specific roller geometry and service requirements. Key process variables include:
| Parameter | Range | Effect on Overlay Quality |
|---|---|---|
| Heat input (J/mm) | 15–40 | Higher input increases dilution but reduces cracking |
| Travel speed | 50–200 mm/min | Faster speed reduces dilution but may cause incomplete fusion |
| Electrode/wire diameter | 3.2–6.0 mm | Larger diameter increases deposition rate |
| Number of passes | 2–5 | Multiple thin passes reduce residual stress |
| Preheat temperature | 150–350°C | Higher preheat reduces cracking tendency |
| Post-weld heat treatment | 500–700°C | Relieves residual stress, improves toughness |
Defect Prevention Strategies
The φ1550 backup roller presents unique challenges due to its large diameter and the need to maintain dimensional accuracy after overlay. Key defect prevention measures include:
- Cracking prevention: Preheating to 200–300°C, using low-hydrogen electrodes, and post-weld annealing at 600°C for 2 hours per 25 mm thickness
- Dilution control: Using a "capping layer" technique where the first pass uses a transition alloy and subsequent passes use the hardfacing material
- Dimensional control: Limiting total heat input per pass to prevent thermal distortion of the roller journal geometry
- Bonding quality: Ensuring 100% fusion bond through adequate current and proper surface preparation
Quality Control and Inspection
| Test Method | Frequency | Acceptance Criteria |
|---|---|---|
| Visual inspection | Every pass | No visible defects, uniform bead profile |
| MT (magnetic particle) | Final surface | No cracks, no indications > 1 mm |
| UT (ultrasonic) | Full circumference | 100% bond, no internal voids |
| Hardness survey | 10 points per meter | Uniform within ±3 HRC |
| Dimensional check | After completion | Within ±0.5 mm of nominal diameter |
| Penetrant testing (PT) | Critical areas | No linear indications |
Engineering Practice Integration
The backup roller overlay work at Shenyang University of Technology represents a significant contribution to the hot rolling industry's maintenance practices. The φ1550 roller size indicates a medium-to-large hot strip mill, where roller replacement costs are substantial and downtime is extremely expensive. A single backup roller can weigh over 10 tons, and replacement requires complete mill shutdown.
The collaboration between Shenyang University of Technology and the Institute of Metal Research, Chinese Academy of Sciences, suggests that this research involved fundamental metallurgical investigation in addition to applied process development. This academic-industrial partnership model is a hathe writing systemark of Chinese engineering research and has produced many practical advances in heavy industry maintenance.
Study Reflections
The backup roller overlay problem is fundamentally a materials selection and process control challenge. The research demonstrates that successful overlay of large, heavily loaded components requires careful consideration of the entire process chain—from surface preparation through post-weld treatment. The work also highlights the importance of understanding the metallurgical interactions between the overlay and base materials, particularly the formation of brittle intermetallic phases at the interface that can compromise long-term service life.