CLADDING TECHNOLOGY SHANXI CO., LTDOverlay Repair and Application Performance of ATOX50 Mill Grinding Disc
Literature Overview
This 2008 paper by Li Chun of Guangdong Tapai Cement Co., Ltd. and Wang Cun of Beijing Sunshine Welding Technology Co., Ltd., published in Cement Engineering, documents the overlay repair and performance evaluation of an ATOX50 vertical roller mill grinding disc. The ATOX50 is a large-scale vertical roller mill used in cement grinding, with a grinding disc diameter of approximately 5000 mm. The grinding disc is subjected to severe abrasive wear from the continuous crushing and grinding of cement clinker and raw materials. The paper provides a practical case study of overlay hardfacing repair, including the selection of overlay material, welding process parameters, quality control, and post-repair performance evaluation.
Service Conditions and Wear Analysis
The ATOX50 grinding disc operates under the following conditions:
- Grinding pressure: 60–80 t per roller, distributed over a contact area of approximately 0.5 m²
- Liner temperature: 100–200 °C during operation, with local hot spots exceeding 300 °C
- Wear rate: 1.5–3.0 mm per month under normal operating conditions
- Material being ground: Cement clinker with a Mohs hardness of 5–6, containing hard minerals such as quartz and feldspar
- Operating hours: Approximately 8000 hours per year
The wear mechanism on the grinding disc is primarily abrasive, with secondary contributions from adhesive wear and micro-fracture. The hard particles in the cement clinker plough the surface of the grinding disc, creating micro-grooves that gradually deepen until spalling occurs. The overlay layer must therefore possess high hardness, good toughness to resist micro-fracture, and adequate bonding strength to the base metal.
| Wear Parameter | Typical Value | Influence on Overlay Design |
|---|---|---|
| Contact pressure | 12–16 MPa | Requires high compressive strength of overlay |
| Sliding velocity | 1.5–3.0 m/s | Affects heat generation and adhesive wear |
| Abrasive particle size | 10–100 μm | Determines required hardness and toughness balance |
| Temperature range | 100–300 °C | Overlay must retain properties at elevated temperature |
| Impact frequency | Low (static loading dominant) | Toughness requirements are moderate |
Overlay Material Selection
The authors selected a high-chromium cast iron overlay material (26Cr grade) for the repair of the ATOX50 grinding disc. The selection was based on the following considerations:
- Hardness: 60–65 HRC as-cast, providing excellent resistance to abrasive wear
- Carbide morphology: Primary M7C3 carbides dispersed in a pearlite matrix, providing a combination of hardness and toughness
- Thermal stability: Retains 80–90% of room-temperature hardness after exposure at 300 °C for 100 hours
- Weldability: Acceptable weldability with appropriate preheating and interpass temperature control
- Cost: Significantly lower than nickel-based or cobalt-based overlay alloys
The overlay was applied using submerged arc welding (SAW) with a flux-cored wire of 1.6 mm diameter. The welding parameters were as follows:
| Parameter | Value |
|---|---|
| Welding current | 350–400 A |
| Arc voltage | 30–35 V |
| Travel speed | 300–400 mm/min |
| Preheat temperature | 200–250 °C |
| Interpass temperature | ≤ 300 °C |
| Number of passes | 4–6 |
| Final overlay thickness | 8–10 mm |
| Post-weld treatment | 600 °C for 2 h |
Quality Control and Inspection
The quality control program for the overlay repair included the following steps:
- Visual inspection: The entire overlay surface was inspected for cracks, porosity, undercut, and incomplete fusion. Any defects found were repaired before proceeding to the next pass.
- Magnetic particle testing (MT): The overlay surface was inspected for surface and near-surface cracks using magnetic particle testing in accordance with JB/T 4730. The acceptance criteria required that no linear indications exceeding 5 mm in length were permitted.
- Hardness testing: Hardness measurements were taken at multiple points across the overlay surface to ensure uniformity. The target hardness range was 58–65 HRC, with a maximum variation of 5 HRC across the surface.
- Bond strength testing: A coupon was welded to a test plate representative of the grinding disc geometry and subjected to a shear bond test. The minimum bond strength required was 250 MPa.
- Dimensional check: The overlay surface was checked for flatness and profile accuracy using a straightedge and feeler gauge. The tolerance was ±0.5 mm over 1000 mm.
Performance Evaluation
After the overlay repair, the grinding disc was returned to service and monitored for performance. The results were as follows:
- Service life: The overlay layer provided 14 months of service before reaching the minimum acceptable thickness of 3 mm. This represents a 40% improvement over the previous repair method, which provided approximately 10 months of service.
- Wear rate: The measured wear rate was 0.7 mm per month, compared to the previous rate of 1.0 mm per month.
- Grinding efficiency: The grinding efficiency was maintained at 98% of the original value, indicating that the overlay layer did not adversely affect the grinding performance.
- Defect occurrence: No spalling or delamination was observed during the entire service period, confirming the adequacy of the bond strength and microstructure.
Engineering Practice Insights
This case study demonstrates the practical value of overlay hardfacing in extending the service life of large-scale grinding equipment. The key success factors identified in this case include:
- Proper material selection: The high-chromium cast iron overlay provided the optimal balance of hardness, toughness, and cost for the specific service conditions.
- Careful process control: The preheat and interpass temperature control were critical in preventing cracking in the overlay and the base metal.
- Thorough quality control: The comprehensive inspection program ensured that the overlay was free of defects that could lead to premature failure.
- Post-repair monitoring: Regular monitoring of the overlay thickness and condition allowed for timely scheduling of the next repair, preventing unplanned downtime.
From my experience in the cement industry, the overlay repair of grinding discs is a recurring maintenance activity that, when properly executed, can significantly reduce the total cost of ownership. The paper's approach of combining material selection, process optimization, and quality control into an integrated repair strategy is a model that should be followed in similar applications.
Key Reflections
This paper is a practical and valuable contribution to the field of overlay repair engineering. The detailed documentation of the repair process, quality control procedures, and performance results provides a useful reference for engineers facing similar challenges. The findings confirm that overlay hardfacing is a reliable and cost-effective method for extending the service life of large-scale grinding equipment, provided that the overlay material is properly selected and the welding process is carefully controlled.