Zhuojin Pipe Fitting Co., LtdDevelopment and Application of Overlay Composite Grinding Roller for Vertical Mill in Cement Production
Literature Overview
The paper by Wu Hong (Xi'an University of Architecture and Technology) and Li Wenjie (Tongchuan Shengwei Building Materials Co., Ltd.) (2013), published in Cement (No. 1, pp. 67–68), addresses the practical challenge of grinding roller sleeve degradation in vertical roller mills (VRM) used in cement production lines. The case study focuses on two 5000 t/d production lines at Tongchuan Shengwei, where MPF2116-type medium-speed mills were equipped with monolithic cast KMTBCr20 high-chromium alloy roller sleeves that suffered from cracking, fracture, and uneven wear during extended service.
Core Technical Content
Problem Statement and Failure Analysis
The MPF2116 vertical mill is a widely used medium-speed mill in Chinese cement plants. The original roller sleeve design—a monolithic KMTBCr20 (high-chromium white iron, typically 13–18% Cr, 2.5–3.5% C) casting—suffers from several inherent limitations:
| Failure Mode | Cause | Consequence |
|---|---|---|
| Cracking | Poor impact toughness of cast high-Cr alloy | Unplanned shutdown, safety hazard |
| Fracture | Stress concentration at casting defects | Complete roller sleeve failure |
| Uneven wear | Non-uniform material properties from casting | Reduced grinding efficiency |
| Thermal damage | Hot gas exposure (200–350 °C) | Thermal fatigue cracking |
| Poor repairability | Low weldability of high-Cr alloy | High replacement cost |
The KMTBCr20 alloy, while offering excellent abrasion resistance (due to hard carbides in a pearlitic/martensitic matrix), has inherent problems with impact toughness (typically < 5 J at room temperature) and weldability due to its high carbon equivalent and tendency to form brittle martensite in the heat-affected zone.
Composite Overlay Solution
The developed solution replaces the monolithic cast sleeve with a composite structure consisting of a base material (typically a medium-carbon steel or low-alloy steel with good toughness and weldability) overlaid with a wear-resistant alloy layer through welding. This approach offers several advantages:
- Improved impact resistance: The base material provides ductility and toughness, absorbing impact loads from large feed particles.
- Controlled wear layer: The overlay alloy can be optimized specifically for abrasion resistance without compromising toughness.
- Repairability: When the overlay layer wears through, it can be rebuilt by re-welding rather than replacing the entire sleeve.
- Reduced casting defects: Welded overlay avoids the porosity, shrinkage, and segregation problems inherent in large castings.
Overlay Welding Process and Material Selection
The typical overlay composite roller sleeve construction involves:
| Layer | Material | Thickness | Function |
|---|---|---|---|
| Base | Q345R or 42CrMo | 60–80 mm | Structural support, impact resistance |
| Transition | 309L or custom buffer | 3–5 mm | Reduce dilution, prevent cracking |
| Build-up | Medium-Cr alloy (e.g., Cr26, Cr30) | 15–25 mm | Primary wear resistance |
| Surface (optional) | High-Cr (KMTBCr20 equivalent) | 5–10 mm | Maximum abrasion resistance |
The welding process typically employs either:
- SAW (Submerged Arc Welding): For large production runs, providing high deposition rates and consistent quality.
- FCAW (Flux-Cored Arc Welding): For field repair and smaller production volumes.
- GTAW + SAW combination: GTAW for the root pass to ensure fusion quality, followed by SAW for fill and cap passes.
Performance Comparison
| Property | Monolithic KMTBCr20 | Composite Overlay Sleeve |
|---|---|---|
| Impact toughness (J) | 2–5 | 20–40 (base) / 10–15 (overlay) |
| Surface hardness (HV) | 700–850 | 600–750 |
| Service life | 6–12 months | 18–36 months |
| Repairability | Poor | Good (rebuildable) |
| Cost per ton cement | Higher | Lower |
| Downtime frequency | High | Low |
Engineering Practice Integration
The implementation of this technology at Tongchuan Shengwei's 5000 t/d lines demonstrated significant operational improvements. The key engineering considerations included:
- Surface preparation: The base sleeve surface must be machined to remove any casting imperfections before overlay welding.
- Preheating: Base material preheat to 150–200 °C to reduce residual stress and prevent cold cracking.
- Interpass temperature control: Maintain below 250 °C to avoid excessive grain growth in the overlay.
- Post-weld machining: The overlay surface must be machined to precise geometric tolerances (typically Ra 3.2–6.3 μm) for proper grinding function.
- Quality assurance: UT inspection of overlay welds to detect lack of fusion, porosity, and internal cracks.
Key Technical Insights and Reflections
This case study exemplifies the practical application of overlay welding technology in the cement industry, where cost-effectiveness and reliability are paramount. The transition from monolithic cast components to composite welded structures represents a paradigm shift in maintenance engineering philosophy—from "replace when failed" to "design for repairability."
The economic analysis strongly favors the composite approach: while the initial fabrication cost may be slightly higher than a cast sleeve, the extended service life, reduced downtime, and rebuildability result in significantly lower total cost of ownership. For a 5000 t/d line operating 24/7, each day of unplanned downtime represents approximately 5000 tons of lost production, making reliability improvements highly valuable.
From a metallurgical perspective, the composite approach solves the fundamental materials contradiction between toughness and wear resistance. By separating these functions into different layers of the component, engineers can optimize each layer for its specific function without compromise.
Study Insights and Implications
This practical engineering paper, though brief in scope, addresses a critical pain point in cement production operations. The technology transfer from research to industrial application is clearly demonstrated through the case study at Tongchuan Shengwei. For engineers working in bulk material handling, grinding, and crushing applications, this work provides a template for evaluating composite overlay solutions against monolithic designs. The approach is particularly relevant for applications involving combined mechanical and thermal loading where single-material solutions cannot simultaneously satisfy all performance requirements.