Zhuojin Pipe Fitting Co., LtdSurfacing Repair of ZGM95G Coal Mill Rollers and Grinding Table Linings
Literature Overview
The paper by Gong Junfeng (Beijing Jingneng Thermal Power Co., Ltd., 2009) provides a practical case study of surfacing repair applied to ZGM95G coal mill rollers and grinding table lining tiles in a power generation facility. The work documents the operating conditions, wear mechanisms, material selection, welding procedure, and economic evaluation of surfacing repair as a maintenance strategy for these critical components. The study is valuable for its direct relevance to power plant maintenance engineering and its quantification of the economic benefits achieved through surfacing repair.
Operating Conditions and Wear Mechanisms
ZGM95G coal mills are used in coal-fired power plants to grind raw coal into fine powder for combustion. The rollers and grinding table linings are subjected to a severe combined loading regime that includes:
- Abrasive wear: Coal particles embedded in the material stream abrade the roller and table surfaces continuously during operation.
- Impact loading: Coal lumps are crushed between the roller and table, generating high localized impact stresses.
- Fatigue: Repeated cyclic loading causes rolling contact fatigue damage, leading to surface spalling and cracking.
- Thermal cycling: The mill operates at elevated temperatures (80–120 °C), and the thermal cycling can exacerbate fatigue damage.
- Corrosive wear: Moisture and sulfur compounds in the coal can contribute to corrosion-assisted wear.
The dominant wear mechanism for ZGM95G rollers and table linings is typically a combination of abrasive and fatigue wear, with the relative contribution depending on the coal characteristics (hardness, moisture content, ash content) and the mill operating parameters (roller pressure, table speed, coal feed rate).
Material Selection for Surfacing Repair
The authors selected a high-chrome cast iron type self-shielded flux-cored wire for the surfacing repair. This selection is based on the following considerations:
| Requirement | Specification | Rationale |
|---|---|---|
| Hardness | HRC 58–65 | Must exceed base metal hardness to resist abrasive wear |
| Microstructure | Martensitic with carbide particles | Provides combination of hardness and toughness |
| Crack resistance | High | Must withstand impact loading and thermal cycling |
| Weldability | Good | Must be applicable to large cast iron components without excessive preheating |
| Self-shielded | Yes | Eliminates need for external shielding gas in plant environment |
High-chrome cast iron surfacing alloys typically contain 12–20% Cr, 2–4% C, and 1–3% Mo, producing a microstructure of tempered martensite with M7C3 and M23C6 carbide particles. The carbides provide the primary wear resistance, while the martensitic matrix provides toughness and resistance to cracking.
Welding Procedure and Construction Control
Pre-Weld Preparation
The preparation of the ZGM95G rollers and table linings for surfacing repair involves several critical steps:
- Inspection and assessment: The worn area must be thoroughly inspected to determine the extent of damage, identify any cracks or spalling, and establish the required repair dimensions.
- Material removal: All cracked, spalled, or severely worn material must be removed by grinding or machining. The repair area should be ground to a smooth, slightly undercut profile that promotes good fusion.
- Cleaning: The repair area must be cleaned to remove all contaminants including oil, grease, rust, and coal residue. Solvent cleaning followed by wire brushing is typically sufficient.
- Preheating: Preheating to 200–300 °C is recommended for cast iron components to reduce the risk of cracking during welding. The preheat temperature must be maintained throughout the welding sequence.
Welding Sequence
The surfacing repair is performed in a multi-pass sequence:
- First pass (transition pass): A thin transition layer is deposited at the edge of the repair area to blend the surfacing alloy with the base metal and minimize dilution effects.
- Intermediate passes: Additional passes are deposited to build up the required repair thickness. Interpass temperature must be maintained between 200–300 °C.
- Final pass: The final pass is deposited with a slight overlap onto the adjacent base metal to ensure a smooth transition and avoid a hard shoulder that could become a stress concentrator.
Post-Weld Treatment
After surfacing repair, the following post-weld treatments are applied:
- Stress relief: The component is heated to 550–650 °C and held for a sufficient duration to relieve welding residual stresses. This is critical for cast iron components to prevent delayed cracking.
- Grinding: The surfacing deposit is ground to the required surface profile and finish. The grinding also removes any surface defects and provides a uniform contact surface.
- Inspection: Magnetic particle testing (MT) or penetrant testing (PT) is performed to detect any surface or near-surface cracks. Hardness testing is performed across the fusion zone to confirm a gradual hardness transition.
Economic Evaluation
The economic benefits of surfacing repair for ZGM95G coal mill rollers and table linings are substantial:
| Cost Item | Replacement | Surfacing Repair |
|---|---|---|
| Material Cost | High (new roller/table) | Low (consumable wire) |
| Labor Cost | High (dismantle, install, align) | Moderate (surface preparation, welding, grinding) |
| Downtime | Long (days to weeks) | Short (hours to 1–2 days) |
| Logistics | High (shipping, storage) | Minimal |
| Service Life Extension | Full new life | 60–80% of new life |
| Total Cost per Unit | Baseline (100%) | 20–35% of replacement cost |
The economic analysis demonstrates that surfacing repair can reduce the total cost of ownership for ZGM95G coal mill components by 65–80% compared to complete replacement, while extending the service life by a factor of 1.5–2.5 compared to the original component life.
Study Insights and Reflections
This case study provides a practical and quantified demonstration of the value of surfacing repair in power plant maintenance. The ZGM95G coal mill application is representative of many heavy-duty rotating equipment components in the power generation industry, and the principles established here can be applied to similar components in other industries.
One important observation from this study is the emphasis on process control and construction discipline. The success of surfacing repair on large cast iron components depends critically on maintaining consistent preheat and interpass temperatures, controlling the welding sequence to minimize distortion, and performing thorough post-weld stress relief. Any deviation from the qualified procedure can result in cracking, reduced bond strength, or premature failure of the repair.
The paper also highlights an important consideration for maintenance engineers: the decision between surfacing repair and component replacement should be based on a comprehensive economic analysis that includes not only direct material and labor costs but also the cost of downtime, the risk of repair failure, and the remaining useful life of the repaired component. In many cases, surfacing repair is the economically optimal choice, but in cases where the base component has sustained significant structural damage or has limited remaining life, replacement may be more appropriate.
The practical value of this study extends to the broader field of industrial maintenance engineering, where the systematic application of surfacing repair technology can significantly extend equipment life, reduce maintenance costs, and improve operational reliability. The key to successful implementation lies in the careful selection of surfacing materials, the rigorous control of welding procedures, and the thorough inspection and verification of repair quality.