Zhuojin Pipe Fitting Co., LtdSurfacing Repair and Strengthening of 1550mm Medium Plate Mill Backup Roller
Literature Overview
This study, published in Shandong Metallurgy (2003, Vol. 25, No. 3), addresses a critical industrial maintenance challenge: the repair and life extension of backup rollers in a 1550mm medium plate rolling mill at Jinan Iron and Steel Group. The authors from Shandong Academy of Metallurgical Sciences and Jinang Steel Group analyzed the failure conditions of the backup roller, selected appropriate low-crack-tendency filler wire, and developed a submerged arc surfacing combined with special heat treatment process. The reported result is a 2.54-fold improvement in service life over a new roller, which represents a significant economic and operational achievement.
Operating Conditions and Failure Analysis
The backup roller in a medium plate rolling mill operates under extreme combined loading: high radial contact stress from the work roll, alternating thermal cycling from hot rolled slabs, and abrasive wear from scale and oxide particles. The failure modes typically include surface spalling, bearing zone cracking, and progressive diameter reduction. The authors systematically evaluated the service conditions before proposing the repair strategy, which is a sound engineering approach consistent with root-cause analysis methodologies.
Key operating parameters of the 1550mm medium plate mill backup roller include:
| Parameter | Typical Value |
|---|---|
| Roller diameter | φ1550 mm |
| Material (original) | High-carbon-chromium bearing steel (e.g., GCr15 or similar) |
| Contact stress | 2000–3000 MPa |
| Rolling temperature | 800–1200 °C |
| Typical failure mode | Surface spalling, Hertzian fatigue |
Surfacing Process Design
The selection of low-crack-tendency filler wire is a critical decision. Backup rollers are typically made of high-carbon bearing steel, which is inherently susceptible to hydrogen cracking and cold cracking during welding repair. The authors selected a filler wire with controlled carbon content and appropriate alloying to minimize the carbon equivalent (CE) of the weld metal and reduce the risk of martensitic transformation in the heat-affected zone.
The submerged arc welding (SAW) process was chosen for its advantages in producing deep penetration, high deposition efficiency, and consistent weld quality suitable for large-diameter cylindrical surfaces. The surfacing layer serves multiple functions: restoring the roller to dimensional specification, providing a harder surface layer resistant to abrasive wear, and introducing compressive residual stress that counteracts the tensile stresses generated during rolling service.
Special Heat Treatment Process
The special heat treatment process is the distinguishing feature of this repair approach. After surfacing, a tailored heat treatment cycle was applied to the roller. The objectives of this heat treatment include:
- Tempering the hardened martensite in the weld metal and HAZ to reduce residual stress and improve toughness.
- Refining the grain structure in the heat-affected zone to restore mechanical properties.
- Ensuring uniform hardness distribution between the base metal, weld metal, and HAZ to avoid soft spots or hard spots that could initiate surface fatigue.
The heat treatment parameters likely included:
| Heat Treatment Step | Temperature | Time | Cooling Method |
|---|---|---|---|
| Stress relief annealing | 550–650 °C | 2–4 h | Furnace cool |
| Final tempering | 450–550 °C | 2–3 h | Air cool or furnace cool |
The combination of surfacing and heat treatment achieved a synergistic effect: the surfacing restored geometry and provided a wear-resistant surface, while the heat treatment optimized the metallurgical state of the entire cross-section.
Engineering Practice and Study Insights
From a practical standpoint, this case study demonstrates that the repair of large rolling mill components can be economically viable when the engineering design is properly executed. The 2.54-fold life improvement translates directly into reduced downtime, lower spare parts inventory, and significant cost savings.
Reflecting on this work, several lessons emerge for engineering practice. First, the selection of filler metal must be guided by the base metal chemistry and the intended service conditions, not merely by cost considerations. Second, the heat treatment step is not optional but essential; without proper post-weld treatment, the residual stress from welding could accelerate the very fatigue failure mode that caused the original roller failure. Third, the process design should be validated by field trials before full-scale implementation, as the authors apparently did by monitoring service life in actual mill operation.
This approach is directly transferable to other large rotating components in the steel industry, including work rolls, housing rolls, and backup rollers in hot strip mills. The methodology of surfacing repair combined with tailored heat treatment remains a cornerstone of rolling mill maintenance engineering.