CLADDING TECHNOLOGY SHANXI CO., LTDOverlay Repair of ZGM95G Coal Mill Roller and Grinding Plate Lining
Literature Overview
The reference by Gong Junfeng (Beijing Jingneng Thermal Power Co., Ltd., 2009) addresses a highly practical problem in coal-fired power plant maintenance: the wear repair of the ZGM95G medium-speed coal mill rollers and grinding plate lining. The ZGM95G mill is one of the most widely deployed coal pulverizers in Chinese utility boilers, and its rollers and grinding plates are subjected to severe abrasive wear from coal particles, moisture, and iron contaminants. The study focuses on weld overlay techniques to restore worn surfaces and extend component service life, which is critical given the high cost and long lead time of new rollers and grinding plates.
Core Technical Content
The key challenge in this application is that the base metal of the rollers is typically a high-strength cast steel or forged alloy, while the grinding plate lining is a wear-resistant alloy with a hardness typically in the range of HRC 55-65. The overlay material must satisfy several competing requirements: high hardness for abrasion resistance, adequate toughness to resist spalling under impact loading, good bonding strength to the base metal, and resistance to thermal cracking during the welding and subsequent cooling cycles.
The overlay process most commonly employed for such heavy-duty repairs is submerged arc welding (SAW) or flux-cored arc welding (FCAW) for the bulk build-up, followed by a finishing pass with a hardfacing material. The typical overlay material system for coal mill applications involves a multi-layer approach:
| Layer | Function | Typical Material | Hardness (HRC) |
|---|---|---|---|
| Bond layer | Compatibility with base metal, prevent cracking | Low-carbon austenitic or nickel-based | 20-30 |
| Transition layer | Gradual hardness increase, reduce residual stress | Medium-carbon martensitic | 40-50 |
| Wear layer | Abrasion resistance | High-carbon martensitic or carbide-forming | 55-65 |
The multi-layer strategy is essential because depositing a high-carbon hardfacing directly onto a low-carbon steel base would result in severe carbon segregation at the interface, leading to brittle phases and poor bond strength. The bond layer acts as a diffusion barrier, while the transition layer provides a gradual gradient in composition and hardness.
Process Parameters and Defect Analysis
For the roller and grinding plate repair, the following process parameters are typical:
- Preheating temperature: 200-300°C to reduce thermal gradient and prevent cold cracking
- Interpass temperature: 250-350°C, maintained throughout the overlay build-up
- Heat input: 15-30 kJ/cm for the bond layer, reduced to 8-15 kJ/cm for the wear layer
- Post-weld heat treatment: 550-650°C for 2-4 hours to relieve residual stresses and temper the martensitic structure
Common defects encountered include:
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Cracking at bond interface | Excessive carbon diffusion, high residual stress | Use proper bond layer, control heat input, preheat adequately |
| Porosity in overlay | Flux contamination, inadequate shielding | Clean surface, use proper flux, ensure gas coverage |
| Spalling of wear layer | Poor metallurgical bond, thermal cycling | Multi-layer approach, controlled cooling, PWHT |
| Hardness unevenness | Inconsistent dilution, improper layer thickness | Control layer thickness to 3-5 mm, maintain interpass temperature |
Engineering Practice Insights
From a practical standpoint, the repair of ZGM95G mill components requires careful consideration of the operational environment. The rollers experience cyclic thermal loading during the coal milling process, with temperatures fluctuating between ambient and 150-200°C. The overlay must therefore have good thermal fatigue resistance in addition to abrasion resistance. A common mistake observed in field practice is the use of a single-layer hardfacing deposit without a proper bond layer, which leads to premature failure within 2-3 months of operation.
The layered overlay approach described in this literature provides a sound engineering solution. The bond layer ensures metallurgical compatibility, the transition layer manages the hardness gradient, and the wear layer delivers the required abrasion resistance. This philosophy is consistent with modern overlay design principles and aligns with the requirements specified in standards such as GB/T 150 and ASME VIII Div.1 for weld overlay applications on pressure-containing components.
Summary and Reflections
The study by Gong Junfeng represents a valuable contribution to the practical knowledge base of coal mill maintenance engineering. The multi-layer overlay strategy it advocates is not only technically sound but also economically advantageous, as it can extend the service life of expensive mill components by 3-5 times compared to single-layer repairs. The key takeaway for engineers is that overlay design must always consider the full spectrum of service conditions—abrasion, impact, thermal cycling, and chemical exposure—and that the layering philosophy is the most reliable approach to meeting these competing requirements. This work exemplifies how a well-designed overlay scheme can transform a routine maintenance task into a cost-effective asset management strategy.