CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Treatment Technology for Ball Mill Rotor Disks
Overview and Engineering Background
Ball mill rotor disks are critical structural components in mineral processing and cement grinding operations, subjected to severe impact loading, abrasive wear, and corrosion from slurry environments. The conventional approach of replacing worn rotor disks involves significant downtime, high material costs, and extended maintenance schedules. The study of weld overlay technology for ball mill rotor disks addresses these challenges by restoring dimensional accuracy and introducing a wear-resistant surface layer through controlled welding processes. The core philosophy is to transform a replacement problem into a repair problem, thereby reducing lifecycle costs and improving equipment availability.
Core Technical Approach
The overlay process typically employs flux-cored arc welding (FCAW) or submerged arc welding (SAW) for multi-layer deposition of hardfacing alloys onto the base rotor disk material. The base material is usually low-carbon steel or low-alloy steel (e.g., Q345R or equivalent), while the overlay layers consist of carbide-forming alloys such as Ni-Cr-Mo cast iron, Co-Cr alloy, or high-chromium white iron.
| Parameter | Specification |
|---|---|
| Base material | Q345R / 16MnR low-alloy steel |
| Overlay alloy type | Ni-Cr-Mo cast iron or Co-Cr alloy |
| Welding process | FCAW or SAW multi-layer |
| Typical overlay thickness | 3-6 mm |
| Number of overlay layers | 3-5 layers |
| Preheating temperature | 150-250°C |
| Interpass temperature | ≤250°C |
| Post-weld heat treatment | 550-650°C, 2-4 hours |
| Hardness target | 55-65 HRC |
| Dilution rate target | ≤15% |
Process Control and Key Technical Points
The critical challenge in rotor disk overlay lies in managing residual stresses and preventing cracking in the hardfacing layer. The following process control measures are essential:
- Surface preparation: Complete removal of rust, scale, and old weld material through grinding to bare metal. A transition groove with a 30°-45° included angle is machined at the boundary between the base material and the overlay zone to minimize stress concentration.
- Preheating strategy: Uniform preheating to 150-250°C reduces thermal gradients and hydrogen-induced cracking susceptibility. The preheat must cover a width of at least 150 mm beyond the weld zone on both sides.
- Layer sequencing: The first layer (transition layer) uses a low-dilution alloy compatible with the base steel. Subsequent layers progressively increase alloy content. A common sequence is: transition layer (Ni-Cr) → intermediate layer (Ni-Cr-Mo) → surface layer (high-carbon Ni-Cr-Mo or Co-Cr).
- Heat input control: Heat input per layer should be maintained between 15-25 kJ/mm to prevent excessive grain coarsening in the base material while ensuring adequate fluidity of the overlay alloy.
- Post-weld stress relief: Stress relief treatment at 550-650°C for 2-4 hours is mandatory to prevent delayed cracking. The cooling rate should be controlled to avoid secondary stress generation.
Defect Analysis and Countermeasures
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Cracking in overlay layer | High carbon content, rapid cooling | Increase preheat, reduce heat input, use lower carbon filler |
| Poor bond strength | Insufficient base preparation, high dilution | Improve surface cleaning, add transition layer, reduce dilution |
| Hardness variation | Uneven alloy distribution | Optimize wire feeding rate, ensure consistent travel speed |
| Porosity | Hydrogen absorption from base | Thorough drying of flux, increase preheat temperature |
| Dimensional distortion | Thermal imbalance | Use symmetric welding sequence, back-step welding |
Integration with Engineering Practice
In a practical application at a cement grinding plant, a 3.2-meter diameter rotor disk with 25 mm of material loss was successfully restored using a three-layer FCAW overlay. The process parameters were: preheat 200°C, interpass ≤250°C, wire diameter 1.6 mm, current 380-420 A, voltage 32-36 V, travel speed 250-300 mm/min. After overlay and stress relief, the hardness was measured at 58-62 HRC across the entire surface, with bond strength exceeding 200 MPa in shear testing. The restored disk returned to service with a projected service life of 18 months compared to the original 6 months before repair, demonstrating a significant improvement in the cost-effectiveness ratio.
Study Insights and Reflections
The most important insight from this study is that weld overlay for rotor disks is not merely a surface treatment but a structural repair that requires careful consideration of the entire weldment's mechanical behavior. The transition layer design is the single most critical factor determining overlay durability. A poorly designed transition can lead to interfacial cracking within weeks of service, regardless of the surface layer quality. Engineers must adopt a systematic approach—using FMEA to identify potential failure modes at each process step—and implement PDCA cycles to continuously refine process parameters based on field performance data. Metallographic examination of failed overlays should always be conducted to feed back into process improvement.