CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Repair of Ball Mill Trunnion Bearings
Literature Overview and Problem Statement
Ball mills are fundamental equipment in mineral processing, cement production, and power generation industries, operating continuously under heavy radial and axial loads. The trunnion bearings, which support the rotating mill shell, are subject to severe sliding wear, corrosion, and occasional impact loading. When these bearings experience excessive wear beyond specified limits, traditional replacement procedures result in significant production downtime and material costs. The literature reviewed in this study presents a systematic approach to restoring worn ball mill trunnion bearings through weld overlay repair, offering a technically sound and economically advantageous alternative to complete component replacement.
The study addresses the specific challenge of repairing large-diameter trunnion surfaces (typically 400-1200 mm in diameter) that require restoration of dimensional accuracy, surface hardness, and load-bearing capacity while maintaining the structural integrity of the mill shell. The research methodology encompasses wear assessment, repair design, welding process selection, heat treatment optimization, and post-repair verification through comprehensive testing protocols.
Wear Mechanism Analysis and Repair Strategy
Failure Mode Identification
Before initiating repair, a thorough failure analysis is conducted to understand the wear mechanism and determine the appropriate repair strategy. Ball mill trunnion bearings typically experience a combination of wear mechanisms:
| Wear Mechanism | Contributing Factors | Typical Location | Recommended Repair Material |
|---|---|---|---|
| Abrasive wear | Mill liner fragments, ore particles | Contact zone, load-bearing area | High-carbon martensitic steel (40-50 HRC) |
| Adhesive wear | High contact pressure, insufficient lubrication | Sliding interface | Medium alloy steel with Mo addition |
| Corrosive wear | Moisture, chemical attack from process media | Non-contact areas, crevices | Stainless steel overlay (316 or duplex) |
| Impact fatigue | Mill start-up/stop cycles, liner breakage | Bearing raceway | Tough overlay with high fracture toughness |
The study emphasizes that the repair strategy must address not only the current wear condition but also account for the expected future operating environment. A comprehensive condition assessment includes dimensional measurement using laser scanning, hardness profiling across the worn area, metallographic examination of the subsurface microstructure, and evaluation of any pre-existing defects such as cracks or inclusions.
Repair Design Considerations
The repair design follows a systematic approach that considers the operational requirements, material compatibility, and manufacturability constraints. Key design parameters include:
- Overlay thickness: Typically 8-15 mm for trunnion bearings, providing sufficient material for machining to final dimensions while maintaining structural integrity
- Overlay width: Extending 20-30 mm beyond the functional bearing surface to ensure complete coverage of the wear zone and prevent undercutting at the edges
- Welding sequence: Designed to minimize residual stress through a center-out or symmetric sequence pattern
- Post-weld machining allowance: Minimum 3-5 mm reserved for final dimensional accuracy and surface finish
Welding Process Selection and Process Parameters
Process Comparison
The study evaluates multiple welding processes for trunnion bearing repair, with the following comparison:
| Process | Deposition Rate (kg/h) | Dilution (%) | Hardness Achievable (HRC) | Cost Factor | Suitability for Large Surfaces |
|---|---|---|---|---|---|
| Submerged Arc Welding (SAW) | 25-45 | 15-25 | 35-45 | Low | Excellent |
| Flux-Cored Arc Welding (FCAW) | 8-15 | 20-30 | 38-48 | Medium | Good |
| Gas Metal Arc Welding (GMAW) | 4-8 | 25-35 | 35-45 | Medium | Fair |
| TIG Welding (GTAW) | 0.5-2 | 10-20 | 40-50 | High | Limited |
| Electroslag Welding (ESW) | 50-100 | 10-15 | 30-40 | Medium | Excellent for thick deposits |
For ball mill trunnion bearings, the study recommends submerged arc welding (SAW) as the primary process due to its high deposition rate, excellent slag protection, low dilution, and suitability for large flat or slightly curved surfaces. The flux composition is carefully selected to provide adequate deoxidation, controlled cooling rate, and stress-relieving properties.
Recommended Process Parameters
| Parameter | Value | Rationale |
|---|---|---|
| Electrode wire | φ3.2 mm or φ4.0 mm low-alloy steel | Balances penetration and deposition rate |
| Flux | SJCA-101 or equivalent rutile-basic type | Good arc stability, low spatter |
| Current | 400-550 A (φ3.2 mm wire) | Adequate penetration without excessive heat input |
| Voltage | 28-34 V | Controls arc length and penetration profile |
| Travel speed | 100-150 mm/min | Optimizes deposit quality and productivity |
| Polarity | DCEP (direct current electrode positive) | Deeper penetration, cleaner weld |
| Preheat | 150-250°C | Prevents cold cracking in low-alloy base metal |
| Interpass temperature | 200-300°C | Maintains weldability without excessive grain growth |
Post-Weld Heat Treatment and Stress Relief
The residual stress distribution in the weld overlay deposit is a critical concern for trunnion bearing repair, as excessive residual stresses can lead to delayed cracking, dimensional instability during machining, or premature failure in service. The study recommends a post-weld heat treatment (PWHT) procedure that includes:
- Stress relief annealing: Heating to 580-650°C and holding for 2-4 hours per 25 mm of section thickness, followed by furnace cooling
- Tempering: For high-carbon overlay materials, tempering at 500-550°C for 2 hours to reduce hardness from the as-welded condition while maintaining adequate wear resistance
- Cooling rate control: Furnace cooling at a rate not exceeding 100°C/hour to prevent thermal shock cracking
The effectiveness of the heat treatment is verified through residual stress measurement using the hole-drilling method or X-ray diffraction, with target residual stresses below 100 MPa in the overlay layer.
Quality Assurance and Verification
The repair quality is verified through a multi-stage inspection protocol:
- Visual inspection: Examination of all weld surfaces for undercut, porosity, cracks, and incomplete fusion
- Magnetic particle testing (MT): Applied to all weld surfaces and heat-affected zones to detect surface and near-surface cracks
- Ultrasonic testing (UT): Performed on the weld overlay to detect internal defects including lack of fusion, slag inclusions, and internal cracks
- Hardness testing: Vickers hardness measurement across the weld cross-section to verify the hardness gradient and confirm adequate hardness in the functional zone
- Dimensional verification: Post-machining dimensional inspection to confirm bearing surface geometry, roundness, and surface finish within specified tolerances
- Load testing: Where applicable, application of a proof load to verify the structural integrity of the repaired bearing
Engineering Practice and Lessons Learned
The literature documents several practical lessons from field implementations. One critical finding is the importance of surface preparation prior to welding. The removal of all lubricants, oils, and corrosion products from the base metal surface is essential to prevent hydrogen-induced cracking and ensure proper metallurgical bonding. Shot blasting to SA 2.5 grade is recommended as the minimum surface preparation standard.
Another important practical consideration is the management of thermal distortion during welding of large trunnion assemblies. The study recommends the use of welding fixtures and backing plates to constrain movement, combined with a symmetric welding sequence that balances heat input on both sides of the trunnion. In cases where the trunnion is already mounted on the mill shell, additional support structures may be required to prevent gravitational sagging during the welding process.
The economic analysis presented in the study demonstrates that weld overlay repair of trunnion bearings typically costs 30-50% less than replacement with new bearings, while achieving comparable service life when properly executed. The reduced downtime associated with on-site repair, compared to the lead time for new bearing fabrication and installation, further enhances the economic advantage.
Study Insights and Reflections
The most significant insight from this research is the demonstration that systematic engineering approach—combining failure analysis, process optimization, and rigorous quality verification—can successfully restore worn critical components to full service capability. The approach validates the concept of extending component life through surface engineering rather than complete replacement, aligning with current sustainability and circular economy principles.
The study also highlights the importance of understanding the interaction between welding process parameters and the final microstructure of the overlay deposit. Small variations in heat input can significantly affect the hardness and toughness balance of the repair overlay, emphasizing the need for process qualification and parameter control in production welding.
Future work should focus on developing self-monitoring repair procedures that incorporate real-time process monitoring and adaptive parameter adjustment, as well as exploring advanced materials such as high-entropy alloys and nanocomposite overlays for next-generation bearing repair applications.