Zhuojin Pipe Fitting Co., LtdOverlay Welding Repair of Electric Shovel Drive Wheels and Bucket Teeth
Literature Overview and Industrial Context
The paper by Xu Guoling (Ningbo Port Authority Automobile Repair Factory) and Shi Meiyu (Ningbo City Equipment Installation Company), published in "Construction Mechanization" (1992, Vol. 13, No. 5, pp. 35–37), describes the overlay welding repair process for drive wheels and bucket teeth of WK-4 type electric shovels. The WK-4 electric shovel is a large mining machine with a bucket capacity of 4.6 m³, used extensively in open-pit mining operations. After prolonged service, the drive wheels, bucket teeth, bucket front walls, and support wheels of the shovels suffer from severe wear, with some components reaching the point of scrap. The authors describe the overlay welding repair process used to restore these components, emphasizing the selection of overlay materials based on the base metal properties, working conditions, and desired overlay metal properties.
Base Metal Characterization and Overlay Material Selection
Drive Wheel Base Metal
The drive wheels of the WK-4 electric shovel are made from ZG35CrMo, a cast steel with 0.35% carbon and chromium-molybdenum alloying. The base metal has a hardness of approximately 200–250 HB and provides adequate strength and toughness for the mechanical loading during operation. However, the working surface of the drive wheel is subject to severe abrasive wear from contact with the track links, and the hardness of the base metal is insufficient to resist this wear. The overlay material must provide a hard, wear-resistant surface while maintaining good bonding with the base metal.
Overlay Material Selection for Drive Wheels
The authors selected a high-carbon, high-chromium overlay material for the drive wheel working surface. The overlay material contains 1.2–1.5% carbon, 5–8% chromium, and 1–2% molybdenum, producing a high-carbon martensitic microstructure with dispersed carbides. The resulting overlay layer achieves a hardness of 55–60 HRC, which provides excellent resistance to abrasive wear. The chromium and molybdenum content also provides good resistance to oxidation and thermal fatigue at the elevated temperatures encountered during operation.
Bucket Teeth Base Metal and Overlay
The bucket teeth of the WK-4 electric shovel are subjected to severe impact and abrasive wear during the digging operation. The base metal of the bucket teeth is typically a high-strength steel with a hardness of 250–300 HB. The overlay material for the bucket teeth must provide both wear resistance and impact toughness, as the teeth are subjected to both abrasive contact with rock and soil and impact loading during digging. The authors selected a medium-carbon, medium-chromium overlay material with 0.8–1.0% carbon and 3–5% chromium, producing a bainitic or martensitic microstructure with a hardness of 45–50 HRC. This composition provides a good balance of hardness and toughness for the demanding bucket tooth application.
Welding Process and Quality Control
The overlay welding was performed using shielded metal arc welding (SMAW) with specialized overlay electrodes. The welding parameters were optimized to ensure good fusion with the base metal, adequate dilution control, and minimal residual stress. The following table summarizes the key welding parameters and quality control measures:
| Component | Base Metal | Overlay Material | Overlay Hardness | Welding Current | Preheat Temperature |
|---|---|---|---|---|---|
| Drive wheel | ZG35CrMo | 1.2–1.5% C, 5–8% Cr, 1–2% Mo | 55–60 HRC | 120–180 A | 150–200°C |
| Bucket teeth | High-strength steel | 0.8–1.0% C, 3–5% Cr | 45–50 HRC | 100–150 A | 100–150°C |
Quality control measures included visual inspection of the overlay layer for surface defects, hardness testing to verify the target hardness, and metallographic examination to confirm the microstructure and bonding quality. The overlay layer was ground to the required dimensions after welding to ensure proper fit and function.
Engineering Practice and Economic Benefits
The overlay welding repair process was applied to 12 WK-4 electric shovels during a major overhaul. The repair restored the functionality of components that would otherwise have been scrapped, saving significant costs in terms of replacement parts and downtime. The authors report that the overlay-welded components achieved a service life comparable to new components, with the added benefit of reduced environmental impact from material conservation.
The process also offered the advantage of being applicable to components that were damaged beyond the point of conventional repair. By applying a new overlay layer to the worn or damaged surface, the component could be restored to serviceable condition without the need for complete replacement.
Reflections and Practical Implications
The paper by Xu and Shi provides a practical example of overlay welding repair in heavy equipment maintenance. The approach is particularly valuable for large mining equipment where component replacement is expensive and downtime is costly. The overlay welding process offers a cost-effective means of restoring worn components to serviceable condition, extending equipment life and reducing maintenance costs.
One of the key insights from the paper is the importance of matching the overlay material to the specific working conditions of the component. The drive wheel and bucket teeth have different wear mechanisms and loading conditions, and the overlay materials were selected accordingly. The drive wheel overlay required high hardness for abrasive wear resistance, while the bucket teeth overlay required a balance of hardness and toughness for impact and abrasive wear. This material selection approach is broadly applicable to other overlay welding repair applications.
The paper also highlights the importance of quality control in overlay welding repair. The overlay layer must be free of defects to ensure long-term service life, and the hardness and microstructure must be verified to confirm the effectiveness of the repair. The use of metallographic examination and hardness testing as quality control measures is a good practice that should be adopted in all overlay welding repair applications.
Summary
The overlay welding repair process for WK-4 electric shovel drive wheels and bucket teeth is a practical and cost-effective solution to the problem of component wear in heavy mining equipment. The paper by Xu and Shi demonstrates that careful selection of overlay materials, optimization of welding parameters, and rigorous quality control can restore worn components to serviceable condition, extending equipment life and reducing maintenance costs. The principles of material selection, process optimization, and quality control discussed in the paper are broadly relevant to overlay welding repair applications in other heavy equipment industries.