CLADDING TECHNOLOGY SHANXI CO., LTDKey Considerations for Online Cladding Repair of Vertical Mill Roller Sleeves
Literature Overview
This 2018 publication by Chen Changmin from Chengdu Building Materials Industry Design and Research Institute Co., Ltd., published in the New Century Cement Herald, addresses a specific and technically demanding scenario: the online cladding repair of roller sleeves in vertical mills. The term "online" indicates that the repair is performed with the mill partially or fully assembled, rather than removing the roller sleeve for off-line repair. This approach is driven by the economic reality that downtime costs in cement production can be substantial, and minimizing the duration of maintenance shutdowns is a priority. However, online repair introduces unique challenges that must be carefully managed.
Technical Challenges of Online Repair
Online cladding repair of vertical mill roller sleeves differs from off-line repair in several critical respects. The most significant constraint is the limited access to the work area. In an off-line scenario, the roller sleeve is removed, placed on a welding fixture, and can be rotated freely to ensure uniform weld coverage. In an online scenario, the roller remains installed in the mill housing, and access is restricted by the surrounding structure. This limitation affects every aspect of the welding process, from surface preparation to weld positioning to post-weld inspection.
Surface Preparation Under Constraints
The preparation of the roller sleeve surface for cladding is the most time-consuming and quality-critical step. The existing overlay layer must be removed to expose sound base metal, and the surface must be cleaned to bare metal. In an online configuration, the use of grinding equipment is constrained by the available space. The author notes that inadequate surface preparation is the leading cause of failure in online repairs, and that this step should never be rushed.
The recommended preparation sequence includes:
- Mechanical removal of the failed overlay using a heavy-duty angle grinder or a milling tool
- Verification of the substrate condition — checking for cracks, corrosion, or material degradation
- Cleaning of the substrate to a minimum of Sa 2.5 surface finish using appropriate abrasive grit
- Application of a temporary protective coating on areas that will not be welded to prevent contamination
Welding Process Selection for Online Repair
The choice of welding process for online repair is constrained by the geometry and accessibility of the work area. The following table compares the suitability of common overlay processes for online vertical mill roller sleeve repair:
| Process | Suitability for Online Repair | Key Advantage | Key Limitation |
|---|---|---|---|
| Submerged Arc Welding (SAW) | Moderate | High deposition rate, good penetration | Requires flux handling space, limited to flat/vertical positions |
| Flux-Cored Arc Welding (FCAW) | High | Good deposition rate, flexible positioning | Flux handling in confined space, slag removal |
| Gas Metal Arc Welding (GMAW) | High | Flexible, good visibility, portable | Lower deposition rate, spatter management |
| Gas Tungsten Arc Welding (GTAW) | Low-Moderate | Excellent bead control, low dilution | Very low deposition rate, difficult for thick overlays |
| Electroslag Welding (ESW) | Low | Very high deposition rate | Requires specific geometry, not practical online |
For vertical mill roller sleeve repairs, FCAW and GMAW are generally the most practical choices for online applications. The author recommends a multi-pass approach with a transition layer between the base metal and the final hardfacing layer to manage dilution and ensure metallurgical compatibility.
Thermal Management and Distortion Control
A critical concern in online repair is the thermal effect on the surrounding mill structure. The roller sleeve is typically press-fit or shrink-fit into the roller body, and excessive heating during welding can cause the fit to loosen, leading to misalignment and vibration. The author emphasizes the importance of thermal management, including:
- Limiting the total heat input per pass to prevent excessive temperature rise in the substrate
- Using a thermocouple to monitor the temperature at the press-fit interface and ensuring it does not exceed the specified limit (typically below 200 °C for steel-on-steel press fits)
- Applying controlled cooling to the areas adjacent to the weld to reduce the thermal gradient
- Performing welds in a sequence that balances the thermal input around the circumference of the roller to minimize distortion
Post-Weld Inspection in a Confined Environment
NDT in a confined online environment presents its own challenges. The author recommends the following inspection approach:
- Visual inspection of all weld beads immediately after welding, using a magnifying glass and adequate lighting
- Magnetic particle testing (MT) of the entire overlay surface using a portable wet or dry MT system
- Hardness testing at multiple locations around the roller circumference and at different depths to verify the hardness profile
- Dimensional inspection using a feeler gauge or a laser displacement sensor to verify the roller profile and runout
Engineering Practice Reflections
Having worked with vertical mill repairs in multiple cement plants, I can attest that the online repair approach described in this paper is both practical and necessary. The economic pressure to minimize downtime is real, and the ability to perform a reliable repair without removing the roller can save days of shutdown time. However, the trade-off is that the quality margin is narrower, and the consequences of a failed repair are more severe because the repair cannot be easily redone.
The author's emphasis on surface preparation and thermal management is well-founded. In my experience, the majority of online repair failures can be traced back to either inadequate surface preparation or uncontrolled thermal input. The paper's recommendations for thermocouple monitoring at the press-fit interface are particularly valuable, as this is a parameter that is often overlooked by maintenance welders who focus on the weld itself rather than the broader thermal context.
Summary and Implications
This paper provides a focused and practical guide to one of the most challenging cladding repair scenarios in cement plant maintenance. The key message is that online repair is feasible and reliable when the specific constraints of the online environment are recognized and addressed through disciplined process control. Engineers planning online cladding repairs should adopt the thermal management and inspection protocols described here as minimum requirements, and should never compromise on surface preparation to save time. The experience documented in this paper is a valuable contribution to the body of knowledge on industrial repair welding.