Zhuojin Pipe Fitting Co., LtdKey Considerations in Online Surfacing Repair of Vertical Mill Roller Sleeves
Literature Overview
The paper by Chen Changmin, published in New Century Cement Guide in 2018, addresses the practical challenges encountered during online surfacing repair of vertical mill roller sleeves in cement and mineral processing industries. The author, affiliated with Chengdu Building Materials Industry Design and Research Institute, draws upon extensive field experience to outline critical factors that influence the quality, schedule, safety, and cost of such repair operations. This literature is particularly valuable because vertical mill rollers operate under extreme conditions—high temperature, severe abrasion, and continuous loading—and their failure directly impacts production continuity. The study emphasizes that online repair, while offering significant advantages in terms of reduced downtime and cost savings compared to off-site replacement, demands rigorous process control and professional discipline.
Core Technical Content and Process Analysis
Online surfacing repair of roller sleeves typically employs submerged arc welding (SAW) or flux-cored arc welding (FCAW) processes, with the selection depending on the available equipment and the geometric constraints of the working environment. The repair procedure generally follows a sequence that includes surface preparation, preheating, multi-pass surfacing, post-weld treatment, and final inspection. Each stage carries specific technical requirements that, if neglected, can lead to repair failure or premature degradation.
Surface Preparation Requirements
Before any deposition begins, the damaged area must be thoroughly prepared. This involves grinding away the failed wear layer to expose sound base material, followed by cleaning to remove oxide scale, rust, and contamination. The preparation zone should extend at least 3 to 5 mm beyond the visible defect boundary to ensure adequate mechanical interlock between the new surfacing and the base metal. In cases where the base material has undergone significant work hardening or thermal degradation from prolonged service, additional machining may be necessary to achieve a flat, uniform substrate.
Preheating and Interpass Temperature Control
Vertical mill roller sleeves are typically fabricated from medium-carbon or low-alloy steels such as 42CrMo or similar grades, which are susceptible to hydrogen-induced cracking. The preheating temperature must be carefully controlled based on the carbon equivalent (CE) of the base material. The following table summarizes typical preheating parameters for common roller sleeve materials:
| Base Material Grade | Carbon Equivalent (CE) | Recommended Preheat Temperature | Interpass Temperature Limit |
|---|---|---|---|
| 42CrMo | 0.38–0.45 | 200–250 °C | ≤300 °C |
| 35CrMo | 0.35–0.42 | 180–220 °C | ≤280 °C |
| 45 Steel | 0.32–0.38 | 150–200 °C | ≤250 °C |
| Low-carbon Steel | <0.25 | 100–150 °C | ≤200 °C |
Failure to maintain adequate preheating temperatures is one of the most common causes of repair failure, resulting in cold cracks in the weld metal or heat-affected zone (HAZ). During online repair, maintaining these temperatures is particularly challenging because the massive roller body acts as a heat sink, rapidly dissipating thermal energy.
Surfacing Layer Design
The surfacing layer design must address two distinct functional requirements: the transition layer, which ensures metallurgical compatibility between the base material and the wear layer, and the wear layer itself, which provides the necessary abrasion resistance. A typical three-layer approach is recommended:
- Transition Layer (1st Pass): A low-alloy filler matching the base material composition, ensuring ductility and crack resistance. Common fillers include E8010-D1 or E8018-D2 electrodes.
- Intermediate Layer (2nd Pass): A medium-alloy filler that gradually increases hardness and introduces carbide-forming elements. Fillers such as E5015-A1 with Cr and Mo additions are suitable.
- Wear Layer (3rd Pass): A high-carbon, high-chromium filler that provides the required hardness, typically in the range of 45–60 HRC. Common choices include Fe-based alloys with Cr, Mo, V, and W additions.
Quality Control Measures
The author emphasizes that quality control during online repair must be comprehensive and continuous. Key inspection points include:
- Pre-weld inspection: Visual examination of surface preparation quality, verification of preheat temperature using calibrated thermocouples, and confirmation of filler metal dryness.
- In-process monitoring: Tracking of welding parameters (current, voltage, travel speed, electrode diameter), interpass temperature measurement, and periodic visual checks of weld bead appearance.
- Post-weld inspection: Magnetic particle testing (MT) for surface and near-surface defects, hardness profiling across the weld cross-section, and dimensional verification using coordinate measuring machines or laser scanning.
Safety and Risk Management
Online repair operations present unique safety hazards that differ from conventional shop welding. The working environment is often confined, with limited access to the roller surface. Hot work is performed in proximity to residual dust and potentially combustible materials. The author highlights several risk categories:
- Falling object hazards: Tools and equipment must be secured against falling from elevated positions onto the roller surface or onto personnel below.
- Electrical hazards: Temporary power supply arrangements must comply with electrical safety standards, with proper grounding and isolation procedures.
- Fire hazards: Residual organic materials near the welding zone must be removed, and fire watch personnel must be stationed throughout the operation.
- Ergonomic hazards: Prolonged work in awkward positions can lead to fatigue, increasing the risk of both safety incidents and quality lapses.
A systematic risk assessment using the FMEA (Failure Mode and Effects Analysis) approach is recommended before commencing any online repair. Each potential failure mode—such as cold cracking, porosity, incomplete fusion, or spalling of the wear layer—should be assigned a severity rating, occurrence likelihood, and detectability score to prioritize mitigation measures.
Engineering Practice Insights
From my decades of experience in welding engineering, I have observed that the most successful online repair programs share several common characteristics. First, they invest in proper training of field welders, ensuring that operators are qualified not only in welding technique but also in understanding the metallurgical implications of their work. Second, they maintain disciplined documentation practices, recording all welding parameters, temperatures, and inspection results in a traceable manner. Third, they establish clear interfaces between the repair contractor and the plant maintenance team, with defined responsibilities for equipment preparation, safety measures, and post-repair verification.
The economic argument for online repair is compelling: it eliminates the need to remove and transport heavy rollers to an off-site repair facility, reducing downtime from weeks to days. However, this economic benefit is only realized if the repair quality is assured. A poorly executed online repair that fails within months of completion can be far more costly than a properly executed off-site repair, due to the cumulative effects of unplanned downtime, secondary repairs, and potential damage to downstream equipment.
Summary and Reflection
The paper by Chen Changmin serves as a practical guide for engineers and maintenance managers who must decide when and how to perform online surfacing repair of vertical mill roller sleeves. Its strength lies in the integration of technical requirements with operational realities—recognizing that quality, safety, and cost are interdependent variables that must be managed holistically rather than in isolation. The emphasis on professional personnel configuration is particularly noteworthy, as it underscores the principle that welding quality is ultimately determined by human competence rather than equipment sophistication alone. For engineers responsible for asset integrity in cement and mineral processing plants, this literature reinforces the need for systematic approach to online repair, grounded in metallurgical understanding and disciplined execution.