CLADDING TECHNOLOGY SHANXI CO., LTDOffline Cladding Device for Vertical Mill Roller Sleeve in Cement Industry
Literature Overview
This study, published in 2020 by researchers from Nanyang Zhonglian Cement Co., Ltd. and Sichuan University, addresses a critical maintenance challenge in the cement grinding industry: the offline repair and hardfacing of vertical mill roller sleeves. Vertical roller mills are workhorse equipment in modern cement plants, where the roller sleeves undergo severe abrasive wear from grinding clinker, raw meal, and mixed materials. The development of a dedicated offline cladding apparatus represents a significant advancement in extending roller sleeve service life while minimizing production downtime.
Core Technical Challenges
The vertical mill roller sleeve presents unique cladding challenges that distinguish it from conventional cylindrical or planar overlay applications:
- Geometry complexity: The roller sleeve is a large-diameter cylindrical component (typically 2.2–4.2 m in diameter, 1.5–3.0 m in length) with a taper or flat working surface, requiring specialized positioning and tracking mechanisms.
- Material selection: The base material is typically low-alloy steel (Q345R or similar), while the overlay layer must provide exceptional abrasion resistance, often requiring high-chromium white cast iron, cobalt-based alloys (Stellite), or tungsten carbide composite coatings.
- Thermal management: The large mass of the roller sleeve creates significant thermal inertia, leading to steep thermal gradients at the cladding interface and elevated residual stresses that can cause spalling or cracking.
- Offline versus online repair: Online repair during mill operation is impractical for full-sleeve rebuilds, necessitating a dedicated offline apparatus capable of handling components weighing 20–50 tonnes.
Process Analysis and Key Parameters
The offline cladding device integrates mechanical positioning, thermal preheating, and automated welding into a coordinated system. Based on engineering principles and industry practice, the typical process parameters are as follows:
| Parameter | Typical Value | Rationale |
|---|---|---|
| Preheat temperature | 200–350 °C | Reduces thermal gradient; prevents cold cracking in HAZ |
| Interpass temperature | 150–250 °C | Controls heat input per pass; maintains metallurgical integrity |
| Welding method | SAW (Submerged Arc Welding) or FCAW | High deposition rate; suitable for thick overlay layers |
| Overlay thickness | 8–20 mm | Balances abrasion resistance with cost and distortion control |
| Number of passes | 3–5 layers | Ensures dilution control; achieves desired overlay chemistry |
| Post-weld heat treatment (PWHT) | 550–650 °C, 2–4 h | Relieves residual stresses; stabilizes microstructure |
| Cooling rate | ≤ 50 °C/h | Prevents hydrogen-induced cracking in HAZ |
Defect Analysis and Countermeasures
The following table summarizes common defects encountered during roller sleeve cladding and their countermeasures:
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Overlay spalling | Excessive dilution; thermal shock; high residual stress | Reduce heat input; increase preheat; apply PWHT |
| Cracking in HAZ | High carbon equivalent of base metal; hydrogen embrittlement | Use low-hydrogen consumables; preheat adequately |
| Pore formation | Contaminated surface; insufficient flux coverage | Thorough surface preparation; verify flux dryness |
| Uneven overlay thickness | Tracking inaccuracy; operator inconsistency | Automated tracking system; programmed arc manipulation |
| Interlayer cold cracking | High restraint; inadequate interpass temperature | Increase interpass temperature; reduce pass thickness |
Integration with Engineering Practice
In cement plant operations, roller sleeve cladding is typically performed during scheduled maintenance shutdowns. The offline device allows the roller sleeve to be removed from the mill housing, transported to the cladding facility, and processed in a controlled environment. Key engineering considerations include:
- Logistics planning: The removal, transport, and reinstallation of a roller sleeve must be scheduled within the plant maintenance window, typically 7–14 days.
- Dimensional accuracy: Post-cladding machining to restore the sleeve profile to specified tolerance (typically ±0.5 mm per meter) is essential for proper mill operation.
- Quality verification: Visual inspection, magnetic particle testing (MT) for surface defects, and ultrasonic testing (UT) for subsurface bonding quality are mandatory per relevant standards such as NB/T 47013.
- Cost-benefit analysis: A typical cladding operation costs 15–25% of a new roller sleeve, while extending service life by 3–5 times compared to an uncladded sleeve.
Study Insights and Implications
This literature highlights the convergence of specialized equipment design with welding metallurgy in heavy industry maintenance. The development of purpose-built offline cladding apparatus reflects a broader trend toward standardizing repair processes that were previously artisanal and inconsistent. From a materials engineering perspective, the challenge lies in achieving a metallurgically sound bond between a low-alloy steel substrate and a high-alloy, high-carbon overlay without introducing brittle intermetallic phases at the interface. The collaboration between an industrial user (Nanyang Zhonglian Cement) and an academic institution (Sichuan University) exemplifies the productive synergy between practical problem-solving and fundamental research. For engineers working on similar applications, the key takeaway is that successful cladding of large, heavily restrained components demands not only proper consumable selection and process parameters but also a holistic approach to thermal management, equipment capability, and quality assurance integration.