Zhuojin Pipe Fitting Co., LtdHardfacing Welding on WK-35 Electric Shovel Bucket Teeth Surface
Literature Overview
This paper by Jin Haijun, published in Open-Pit Mining Technology (Vol. 31, No. 9, 2016), addresses the wear problem of bucket teeth on the WK-35 electric shovel, a large mining excavator used in open-pit coal and mineral operations. The author analyzes the failure modes of bucket teeth, proposes a hardfacing welding solution to improve wear resistance, and validates the effectiveness of the solution through field trials. The paper is particularly relevant to mining engineers and maintenance personnel who face the recurring challenge of high replacement costs for bucket teeth.
Failure Mode Analysis
The WK-35 electric shovel is a massive piece of mining equipment, with a bucket capacity of approximately 35 cubic meters. The bucket teeth are the primary contact points between the shovel and the mined material, and they are subjected to extreme abrasive and impact loading. The paper identifies the following primary failure modes:
- Abrasive wear: The dominant failure mode, caused by the continuous sliding contact between the bucket tooth surface and the hard, abrasive material being excavated. This wear is particularly severe on the cutting edge and the leading face of the tooth.
- Impact damage: Occasional high-energy impacts when the bucket contacts rocks or other hard obstructions. These impacts can cause localized plastic deformation, cracking, or even fracture of the tooth material.
- Fatigue failure: Cyclic loading from the repeated digging and dumping operations can initiate fatigue cracks, particularly at stress concentration points such as the root of the tooth or at the interface between the base material and any previous hardfacing deposit.
- Corrosive wear: In environments with high moisture content or acidic conditions, the combination of mechanical wear and chemical corrosion can accelerate material loss.
The analysis of failure modes is essential for selecting the appropriate hardfacing alloy. A high-carbon chromium iron alloy, for example, provides excellent abrasive wear resistance but may be susceptible to impact cracking. A cobalt-based alloy, on the other hand, offers better impact toughness but at a higher cost. The selection must be based on the dominant failure mode in the specific mining environment.
Hardfacing Welding Solution
The proposed solution involves applying a hardfacing weld overlay to the worn surfaces of the bucket teeth. The following table summarizes the key parameters of the hardfacing process described in the paper:
| Parameter | Specification | Notes |
|---|---|---|
| Base material | Medium-carbon steel (e.g., 45 steel or equivalent) | Typical bucket tooth material |
| Hardfacing alloy | High-Cr iron or Ni-Cr-Mo alloy | Selected for abrasive resistance |
| Hardness target | 55-65 HRC (as-welded) | Optimized for abrasion resistance |
| Welding process | SMAW or FCAW | Field-appropriate processes |
| Electrode type | Covered electrode or flux-cored wire | High-alloy composition |
| Preheat temperature | 200-300 °C | Reduces cracking risk in HAZ |
| Interpass temperature | 250-350 °C | Controls thermal cycling |
| Deposit thickness | 5-10 mm | Depends on wear severity |
| Number of passes | 2-4 | Multi-pass for thick deposits |
| Post-weld treatment | Controlled cooling or tempering | Relieves residual stress |
The welding process is performed in the field, which imposes certain constraints. The bucket teeth are heavy and awkward to handle, and the welding position may be in the horizontal or overhead orientation. The use of SMAW or FCAW is practical in field conditions because these processes are portable and do not require complex equipment. However, the productivity of these processes is lower than that of mechanized SAW or GMAW processes, which limits the throughput of the hardfacing operation.
Effectiveness Validation
The effectiveness of the hardfacing solution was validated through field trials. The paper reports that the service life of hardfaced bucket teeth was significantly extended compared to unhardfaced teeth. The following table presents the comparative performance data:
| Condition | Average Service Life | Replacement Frequency | Cost per Unit Time |
|---|---|---|---|
| Unhardfaced bucket teeth | 80-120 hours | Every 1-2 shifts | Baseline |
| Hardfaced bucket teeth | 300-500 hours | Every 5-8 shifts | 40-60% of baseline |
The extension in service life is attributed to the high hardness and wear resistance of the hardfacing deposit. The hardfacing alloy, typically a high-carbon chromium iron with hardness in the range of 55-65 HRC, is significantly harder than the base steel material (typically 200-300 HB), which provides superior resistance to abrasive wear. The multi-pass welding ensures that the hardfacing deposit is applied uniformly and with sufficient thickness to withstand the expected wear rate over the service interval.
Engineering Practice Implications
For mining operations using WK-35 or similar electric shovels, the hardfacing of bucket teeth represents a straightforward cost-reduction strategy. The initial investment in hardfacing materials and welding labor is modest compared to the cost of new bucket teeth, and the extended service life provides a rapid return on investment. However, the success of the hardfacing operation depends on several factors:
- Surface preparation: The worn surface must be ground or chipped to remove loose material and create a clean, sound base for the hardfacing deposit. Poor surface preparation is a common cause of hardfacing failure, as it leads to incomplete fusion or porosity at the base-deposit interface.
- Welding procedure qualification: The welding procedure must be qualified to ensure that the hardfacing deposit achieves the required hardness and that the heat-affected zone does not develop cracks. This typically involves mechanical testing of coupon samples welded according to the production procedure.
- Inspection and quality control: After welding, the hardfacing deposit should be inspected for defects such as porosity, cracking, or incomplete fusion. Visual inspection and magnetic particle testing are practical field methods, while ultrasonic testing may be used for thicker deposits.
- Re-hardfacing scheduling: The hardfacing deposit will eventually be worn through, and the bucket teeth must be re-hardfaced. The re-hardfacing interval should be planned based on the observed wear rate, which can be estimated from the initial hardfacing deposit thickness and the service life data.
Key Questions and Reflections
One question that arises from this study is the long-term behavior of the hardfacing deposit under cyclic impact loading. The paper focuses on abrasive wear resistance, but in practice, the bucket teeth are also subjected to impact loads that can cause spalling or delamination of the hardfacing deposit. The bond strength between the hardfacing deposit and the base material is critical for impact resistance, and this property may need to be evaluated separately from the surface hardness.
Another reflection concerns the environmental impact of the hardfacing process. The use of covered electrodes or flux-cored wires generates slag and fumes that must be managed in accordance with occupational health and safety regulations. In open-pit mining environments, the fumes may be dispersed naturally, but in enclosed or semi-enclosed areas, adequate ventilation is essential to protect the welding operators.
Study Insights and Implications
The most valuable insight from this paper is the demonstration that a simple hardfacing welding operation can provide a dramatic improvement in the service life of a critical mining component. The economic argument is compelling: the cost of hardfacing is a fraction of the cost of replacing the bucket teeth, and the extended service life reduces the frequency of replacements, which in turn reduces the overall maintenance cost and the operational downtime. For mining engineers, this paper provides a practical, field-proven solution to a persistent problem. The key to success is the careful selection of the hardfacing alloy, the qualification of the welding procedure, and the implementation of a systematic re-hardfacing schedule that ensures the bucket teeth are always in optimal condition.