CLADDING TECHNOLOGY SHANXI CO., LTDOverlay Repair of ZGM95G Coal Mill Roller and Raceway Lining Plates
Overview and Background
The ZGM95G mid-speed coal mill is a widely deployed grinding unit in pulverized coal systems of coal-fired power stations. The roller and raceway lining plates are subjected to severe sliding abrasion, impact loading, and thermal cycling during operation. Over time, the original hardfacing layer degrades, leading to excessive wear, loss of grinding efficiency, and eventual equipment downtime. Overlay repair of these components—typically involving removal of the worn layer followed by reapplication of a wear-resistant hardfacing alloy—is a cost-effective and technically mature approach that restores the component to serviceable condition. This study note draws on the technical content of the referenced literature and connects it with practical field experience in coal mill maintenance.
Material and Process Selection
The original lining plates on ZGM95G rollers are commonly clad with a high-chromium cast iron or cobalt-based hardfacing alloy, designed to resist the abrasive action of lignite and bituminous coal particles. When overlay repair is performed, the choice of filler metal must match or exceed the hardness and wear resistance of the original layer while maintaining adequate toughness to resist cracking under thermal and mechanical shock.
| Parameter | Typical Specification |
|---|---|
| Base material | 42CrMo or similar low-alloy steel |
| Original lining hardness | 58–65 HRC |
| Repair overlay hardness | ≥ 58 HRC |
| Recommended filler metal | D107, D207, or Co-based Stellite-type |
| Preheat temperature | 200–300 °C |
| Interpass temperature | ≤ 300 °C |
| Post-weld heat treatment | Stress-relief at 550–600 °C |
| Maximum single-layer thickness | 3–5 mm per pass |
The welding process is typically manual arc welding (SMAW) using electrode type D107 or D207, or submerged arc welding (SAW) for thicker builds. The preheat temperature is critical to minimize hydrogen-induced cracking in the high-carbon martensitic base material. Interpass temperature control is equally important; exceeding 300 °C can cause softening of the previously deposited hardfacing layer and reduce overall hardness.
Defect Analysis and Countermeasures
Common defects encountered during overlay repair of ZGM95G lining plates include:
- Cracking at the fusion line: This arises from the high carbon and alloy content of the base material combined with inadequate preheat. Countermeasure: increase preheat to 300 °C and use a low-hydrogen electrode.
- Porosity in the overlay layer: Often caused by insufficient flux coverage in SAW or contamination of the substrate. Countermeasure: ensure thorough mechanical cleaning of the substrate to a Sa 2.5 standard and verify flux drying at 300 °C for 2 hours.
- Insufficient bond strength: Resulting from poor surface preparation or excessive dilution. Countermeasure: perform a groove preparation with a 60° included angle and limit dilution to less than 30 percent by controlling current and travel speed.
- Hardness non-uniformity: Caused by inconsistent heat input. Countermeasure: maintain a consistent welding current of 180–220 A and travel speed of 150–200 mm/min.
Engineering Practice Insights
In actual field repairs, the roller and raceway lining plates are often disassembled and sent to a workshop for overlay. The repair procedure follows a PDCA cycle: Plan the groove preparation and welding sequence; Do the welding with controlled parameters; Check the hardness, bond strength, and surface integrity; Act by adjusting parameters for the next layer. Metallographic examination of cross-sections reveals that a well-executed overlay produces a uniform martensitic microstructure with carbide particles distributed throughout the matrix, providing the desired wear resistance.
The bond strength between the overlay and base material is typically verified by a tensile test or a peel test, with a minimum acceptable value of 250 MPa for coal mill applications. A hydrostatic pressure test at 1.5 times the design pressure is not applicable to this component, but dimensional accuracy and surface roughness checks are mandatory to ensure proper contact with the coal during grinding operation.
Study Reflections
The key takeaway from this literature is that overlay repair of heavily loaded grinding components demands a systematic approach to both material selection and process control. The interplay between preheat temperature, interpass temperature, and cooling rate governs the final microstructure and, consequently, the service life of the repair. Engineers must resist the temptation to increase welding current for productivity gains, as this invariably leads to excessive dilution and hardness degradation. A disciplined approach to parameter control, combined with rigorous post-weld inspection, ensures that the repaired component performs reliably in the demanding environment of a coal mill.