CLADDING TECHNOLOGY SHANXI CO., LTDField Overlay Repair of Metal Matrix Composite Ceramic Grinding Discs
Literature Overview
This 2013 study by Ni Junjie and Yang Wei from the Zhengzhou Machinery Research Institute documents the practical application of overlay welding techniques for the in-situ repair of metal matrix composite (MMC) ceramic grinding discs used in the cement industry. Published in the New Century Cement Guide (Xin Shiji Shui Ni Daobao), the work addresses a specific and economically significant maintenance challenge: the restoration of worn grinding discs in ball mills and vertical roller mills used for cement grinding. The study provides a rare example of field-level cladding technology application in heavy industry.
Background and Economic Context
In cement production, grinding mills consume a substantial portion of the plant's energy budget, and the condition of the grinding media (balls, segments, or discs) directly affects grinding efficiency and product quality. Metal matrix composite ceramic grinding discs, which combine a steel matrix with embedded ceramic particles (typically alumina or silicon carbide), offer enhanced wear resistance compared to conventional forged steel grinding media. However, even these advanced composites suffer from wear, edge chipping, and surface degradation during service, necessitating periodic repair or replacement.
The economic case for overlay repair is compelling: replacing an entire grinding disc assembly is costly and time-consuming, while overlay welding can restore the disc geometry and wear resistance at a fraction of the replacement cost. The challenge lies in achieving a metallurgically sound bond between the overlay alloy and the MMC substrate, which has a different thermal expansion coefficient, thermal conductivity, and microstructure compared to homogeneous steel.
Repair Process and Technical Approach
The study describes a multi-step repair process that includes surface preparation, preheating, overlay welding, and post-weld treatment. The following table summarizes the key process parameters and materials used:
| Process Step | Details |
|---|---|
| Surface preparation | Grit blasting to remove wear debris and oxide; machining to remove severely worn zones |
| Substrate preheating | 200-300 °C using oxy-fuel torch or induction heating |
| Overlay process | Multi-pass submerged arc welding (SAW) or flux-cored arc welding (FCAW) |
| Consumable | Hardfacing alloy with Cr 20-30%, C 2-3%, Mo 5-10% (high-chromium white iron type) |
| Number of passes | 3-5 passes depending on wear depth |
| Interpass temperature | 200-300 °C |
| Post-weld treatment | Controlled cooling in furnace or under insulation blankets |
| Final machining | Grinding to restore disc geometry and surface finish |
The choice of a high-chromium white iron hardfacing alloy was based on its excellent abrasion resistance, good weldability with steel substrates, and proven performance in cement mill applications. The alloy forms a microstructure consisting of a martensitic matrix with M7C3 carbide particles, providing a hardness of 55-65 HRC.
Challenges and Defect Control
The repair of MMC grinding discs presents several unique challenges that are not encountered in conventional hardfacing applications:
| Challenge | Description | Countermeasure |
|---|---|---|
| Thermal mismatch | MMC substrate has different thermal expansion from overlay alloy | Controlled preheating and slow cooling |
| Ceramic particle effects | Embedded ceramic particles affect arc stability and heat distribution | Surface preparation to expose steel matrix |
| Cracking | Thermal stresses from mismatch can cause interfacial cracking | Low heat input, multiple thin passes |
| Dilution | MMC composition differs from homogeneous steel | Use of compatible hardfacing alloy |
| Geometry restoration | Worn discs require significant material buildup | Multi-pass welding with controlled buildup |
The study reports that careful attention to surface preparation was critical: the embedded ceramic particles in the MMC substrate could cause arc deflection and uneven melting if not properly exposed by machining or blasting. The interpass temperature was maintained at 200-300 °C to reduce thermal gradients and minimize cracking risk.
Performance Results
After repair and return to service, the overlay-repaired grinding discs demonstrated acceptable performance in cement grinding operations. The overlay layers maintained their hardness and wear resistance for the expected service interval, and no catastrophic failure (spalling or delamination) was observed during the monitoring period. The repair extended the service life of the grinding discs by an estimated 30-50% compared to unrepaired condition, providing a significant economic benefit.
However, the study also notes that the overlay layer thickness was limited to 8-12 mm due to the risk of cracking in thicker deposits on the MMC substrate. For severely worn discs requiring more than 15 mm of buildup, replacement was recommended rather than overlay repair.
Key Reflections
This study provides a valuable practical reference for field engineers dealing with the maintenance of composite grinding media in cement and mining operations. The work demonstrates that overlay welding can be successfully applied to MMC substrates with appropriate process control, but also highlights the limitations imposed by thermal mismatch and microstructural heterogeneity. Engineers should note that the success of such repairs depends heavily on the quality of surface preparation, the selection of a compatible hardfacing alloy, and the discipline of maintaining controlled thermal conditions throughout the repair process. The economic benefits of overlay repair over component replacement are significant, but the technical risks must be carefully managed to avoid premature failure and unplanned downtime.