Zhuojin Pipe Fitting Co., LtdNovel Online Overlay Welding Method for MLS3726 Vertical Mill Grinding Rollers
Literature Overview
Zhao Yunfeng's paper, published in Cement (2013, No. 8, p. 34), addresses a specific and practical challenge in vertical mill maintenance: the difficulty of removing and reinstalling grinding rollers for overlay welding repair. The paper focuses on the MLS3726 raw meal vertical mill at Feixian Yizhou Cement Co., Ltd., where two production lines each equipped with two MLS3726 mills operate without automatic roller-lifting capability. Each grinding roller weighs 36 tonnes, and the maintenance cycle requires roller removal every six months. This literature presents an innovative online welding method that eliminates the need for roller removal.
The Problem Statement
The MLS3726 vertical mill design presents unique maintenance challenges:
| Challenge | Description | Impact |
|---|---|---|
| No automatic roller lifting | Hydraulic station lifts pressure frame only, not individual rollers | Manual removal required |
| Roller weight | 36 tonnes per roller | Requires heavy crane, extended downtime |
| Ball bearing connection | Roller connects to pressure frame via spherical steel balls | Complex disassembly |
| Maintenance frequency | Every 6 months | High cumulative downtime cost |
| Traditional method | Remove roller, replace liner plates, overlay weld table liner | 2-3 days downtime per roller |
The traditional maintenance procedure involves: lifting the roller out of the mill housing using a crane, removing worn roller liner plates, performing overlay welding on the table liner plates, reassembling the roller, and reinstalling it. For a 36-tonne roller, this process is labor-intensive, time-consuming, and costly in terms of production downtime.
The Novel Online Method
The key innovation described by Zhao Yunfeng is performing overlay welding on the grinding roller surface while the roller remains installed in the mill. This "online" approach eliminates the need for roller removal, crane operation, and reassembly.
Method Description
- The mill is stopped and the roller is locked in position within the housing.
- The grinding table liner plates are removed or access is provided to the roller surface.
- Overlay welding is performed directly on the roller surface in situ, with the roller still mounted on its trunnion.
- Post-weld machining is performed using a mobile grinding head or by temporarily installing a machining fixture on the roller axis.
- The table liner plates are reinstalled, and the mill is returned to service.
Advantages of the Online Method
| Aspect | Traditional Method | Online Method |
|---|---|---|
| Downtime | 2-3 days per roller | 0.5-1 day per roller |
| Crane requirement | Heavy crane (50+ tonne capacity) | None or minimal |
| Labor intensity | High (disassembly/reassembly) | Moderate (welding only) |
| Cost per maintenance cycle | High | Reduced by 40-60% |
| Risk of misalignment | Reassembly may introduce eccentricity | Roller remains in original position |
Technical Considerations for Online Welding
The online welding method introduces several technical challenges that must be addressed:
- Access and positioning: The roller surface is curved and partially enclosed by the mill housing. Welding access may be limited to specific angular sectors, requiring the roller to be rotated between passes to achieve uniform coverage.
- Heat input control: With the roller in situ, heat dissipation through the trunnion and bearing assembly may be different from a free-standing roller. This can lead to higher localized temperatures and increased risk of distortion.
- Post-weld machining: Achieving the required surface finish and geometric tolerance on a roller that is still mounted requires specialized tooling. A mobile grinding head mounted on a temporary trunnion extension is one practical solution.
- Safety: Working inside or near the mill housing with a heavy roller in place requires strict safety protocols, including lockout-tagout procedures and fall protection.
Welding Process Parameters
For the MLS3726 roller surface overlay, the following parameters are typical:
| Parameter | Value | Rationale |
|---|---|---|
| Welding process | SMAW or GMAW | Flexibility for field conditions |
| Electrode/wire | E506打底 + E707 or E801 overlay | Compatibility and wear resistance |
| Preheat | 200-250 °C | Reduce cracking risk in high-carbon base |
| Interpass temperature | ≤250 °C | Control microstructure |
| Overlay thickness | 4-6 mm total | Balance wear life with distortion |
| Post-weld treatment | Stress relief at 550-600 °C | Eliminate residual stress |
Engineering Practice Insights
The innovation in this paper is not in the welding metallurgy itself but in the maintenance methodology. The online welding approach represents a shift from "remove and repair" to "repair in place," which is a common theme in modern maintenance engineering aimed at minimizing downtime. This approach is particularly valuable for large, heavy components where removal and reinstallation are costly and risky.
From a 5W2H perspective:
- What: Overlay welding of vertical mill grinding roller surface
- Why: Reduce maintenance downtime and cost
- Who: Maintenance welding team with specialized training
- Where: In situ, with roller mounted in mill housing
- When: During scheduled maintenance windows (6-month cycle)
- How: SMAW/GMAW with打底 + overlay sequence, followed by mobile machining
- How much: Reduced cost by 40-60% per maintenance cycle
The method also has implications for the design of future vertical mills. If online welding is viable, then mill designs could incorporate features that facilitate in-situ maintenance, such as access hatches, roller rotation capability during welding, and integrated machining fixtures.
Key Questions and Reflections
The primary question is whether the online welding method can achieve the same weld quality and roller surface integrity as the traditional remove-and-repair method. The answer likely depends on the discipline of process control: preheat, interpass temperature, and post-weld treatment must be as rigorously controlled in the online method as in the traditional method. Without these controls, the online method may produce welds with higher residual stress and poorer surface finish, leading to premature roller failure.
Another consideration is the long-term effect of repeated welding on the roller base metal. Each welding cycle introduces thermal cycling, which can degrade the base metal microstructure over time. After several repair cycles, the base metal near the weld zone may become embrittled, increasing the risk of catastrophic failure. This suggests a limit on the number of overlay repair cycles before the roller must be replaced entirely.
Study Insights and Implications
Zhao Yunfeng's paper is a practical contribution to maintenance engineering that demonstrates how process innovation—rather than material or equipment innovation—can significantly improve operational efficiency. The online welding method for vertical mill grinding rollers addresses a real and costly maintenance bottleneck, and the approach is transferable to other large rotating equipment where component removal is difficult or expensive.
For engineers managing grinding equipment maintenance, this paper encourages thinking beyond the conventional "remove-repair-reinstall" paradigm. The key enablers of the online method are: (1) sufficient access to the welding zone, (2) controlled heat input to prevent distortion of the in-situ roller, and (3) a practical post-weld machining solution. These three elements must be carefully planned and executed to ensure that the time and cost savings of the online method do not come at the expense of weld quality and roller integrity.
The broader implication is that maintenance engineering should embrace methodology innovation as a legitimate and valuable path to improving reliability and reducing costs. The online welding approach for vertical mill rollers is a clear example of how a seemingly simple change in maintenance strategy—welding in place rather than removing the component—can yield substantial operational benefits. Future work should focus on developing standardized procedures and qualification protocols for online welding of large grinding equipment, ensuring that the benefits are realized consistently and safely.