Surfacing Repair Process for Electric Shovel Drive Wheels and Bucket Teeth

Literature Overview

The paper by Xu Guoling and Shi Meiyu, published in Construction Mechanization (建筑机械化) in 1992, documents the surfacing repair of drive wheels and bucket teeth on WK-4 type electric shovels at the Ningbo Harbor Bureau. The WK-4 is a large mining shovel with a bucket capacity of 4.6 m³, used extensively in port and mining operations. After prolonged service, the authors found that critical components including drive wheels, bucket teeth, bucket front walls, and ground wheels had suffered severe wear, with some parts approaching scrap condition. Rather than replacing these components with new parts—a costly and time-consuming option—the authors adopted surfacing welding as a repair strategy.

Component Analysis and Material Selection

The drive wheel material is identified as ZG35CrMo, a chromium-molybdenum cast steel with adequate strength and toughness for the high-load conditions in shovel drive applications. The selection of surfacing material for each component is guided by the specific wear mechanism:

Process Development and Implementation

The authors adopted SMAW (shielded metal arc welding) with surfacing electrodes as the primary repair process. This choice was driven by practical considerations:

• Equipment availability: SMAW equipment is widely available at repair shops and does not require specialized power sources or shielding gas infrastructure.
• Operator skill requirements: SMAW is a well-established process with a large pool of trained welders.
• Field applicability: SMAW can be performed in various positions and on components of different sizes, making it suitable for on-site repair of large shovel components.
• Cost effectiveness: SMAW consumables are relatively inexpensive compared to FCAW or SAW consumables.

The repair process for the drive wheel involved the following steps:

1. Surface preparation: Grinding of the worn surface to remove oxide scale, rust, and damaged material, exposing clean base metal.
2. Preheating: Moderate preheating (typically 150–250°C) to reduce the risk of cold cracking in the ZG35CrMo base metal.
3. Surfacing welding: Multi-pass surfacing with appropriate overlap and heat input control to achieve uniform hardness and minimize residual stress.
4. Post-weld stress relief: Localized or full heat treatment to relieve welding residual stresses.
5. Machining: Final machining to restore dimensional accuracy and surface finish.
6. Inspection: Visual inspection, hardness testing, and magnetic particle testing to verify repair quality.

Metallurgical Considerations

The ZG35CrMo base material has a carbon equivalent of approximately 0.45%, placing it in the category of materials with moderate cold cracking susceptibility. The authors' choice of preheating temperature and surfacing electrode type reflects an understanding of this metallurgical constraint. The surfacing layer must be compatible with the base metal in terms of thermal expansion and mechanical properties to avoid interfacial cracking during thermal cycling.

For the bucket teeth, which experience the most severe abrasive wear, the surfacing material must provide high hardness while maintaining adequate toughness to resist impact loading during digging and bucket rotation. The authors likely selected a surfacing electrode with a martensitic microstructure containing hard carbides (such as Cr₇C₃ or Mo₂C) dispersed in a tempered martensite matrix, a classic design for abrasion-resistant surfacing.

Economic and Operational Benefits

The authors explicitly stated that the surfacing repair approach was adopted to save funds and steel, reduce dependence on spare parts supply, and extend component service life. These economic considerations are particularly relevant in the context of large mining equipment, where replacement parts can be expensive, have long lead times, and may not always be readily available. The surfacing repair approach offers several advantages:

• Cost reduction: Surfacing repair costs are typically 30–50% of the cost of new component replacement.
• Reduced downtime: Repair can be performed on-site or at a nearby workshop, avoiding the long lead times associated with ordering and shipping new parts.
• Material conservation: Only the worn layer is replaced, conserving expensive alloy materials.
• Flexibility: The surfacing composition can be tailored to specific service conditions, potentially improving performance over the original design.

Quality Control and FMEA Considerations

Applying a Failure Mode and Effects Analysis (FMEA) framework to the surfacing repair process reveals several critical failure modes:

The authors' emphasis on the practical success of the repair in production service suggests that these failure modes were adequately controlled through process parameter optimization and quality inspection.

Study Insights and Implications

This 1992 paper is a valuable record of practical welding repair technology applied to large mining equipment. The authors' approach—selecting appropriate surfacing materials based on wear mechanism analysis, optimizing welding parameters for the specific base metal, and validating the repair through production service—represents a systematic and rigorous methodology that remains relevant today. The economic argument for surfacing repair over component replacement is compelling, particularly in an era of increasing resource scarcity and environmental consciousness.

From a metallurgical perspective, the work demonstrates that sound understanding of wear mechanisms and material selection principles can yield significant improvements in component life and economic performance. The authors' choice of ZG35CrMo as the base material for the drive wheel, combined with appropriate surfacing alloys, illustrates the principle of material zoning that was also discussed in the hot shear blade paper. This convergence of approaches across different applications underscores the universality of the material zoning concept in engineering design.

For contemporary engineers, this paper serves as a reminder that practical welding repair technology, grounded in metallurgical understanding and validated through production experience, remains a valuable tool for extending the service life of critical equipment components. The fundamental principles of process selection, material compatibility, and quality control have not changed, even as new welding processes and advanced materials have become available.