CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Repair of Chisel Bodies in Mining Machinery
Literature Overview
This technical report, authored by Liu Cheng from Tangshan Science and Technology Vocational College and published in 2008, addresses the weld overlay repair of chisel bodies used in coal mining machinery. Chisels are critical cutting tools used in roadheaders and continuous miners to excavate coal and rock. They are subjected to extreme impact loading, abrasive wear, and vibration during operation, leading to rapid deterioration of the cutting edge and body surface. Traditional replacement of worn chisels is costly and time-consuming, making weld overlay repair an attractive alternative for extending the service life of these components. This work presents a systematic approach to chisel body repair using weld overlay techniques, including material selection, process optimization, and quality assessment.
Core Technical Points
Service Conditions and Failure Modes
Chisels in mining applications operate under the following conditions:
- Impact loading: Repeated impact forces from the cutting action, with peak loads reaching several hundred kilonewtons.
- Abrasive wear: Contact with hard rock and coal particles, causing progressive material removal from the cutting edge and body surface.
- Vibration: High-frequency vibration from the cutting action, leading to fatigue damage and loosening of the chisel in its holder.
- Thermal cycling: Frictional heating during cutting, followed by cooling during non-cutting periods, leading to thermal stresses.
The primary failure modes of chisel bodies include:
- Cutting edge wear: Progressive blunting of the cutting edge due to abrasion.
- Body surface wear: Material removal from the body surface due to contact with rock.
- Impact fatigue cracking: Initiation and propagation of cracks from stress concentration points.
- Spalling: Delamination of the surface layer due to cyclic loading.
Overlay Material Selection
The selection of overlay material for chisel body repair must balance wear resistance, impact toughness, and weldability. The following materials are commonly used:
| Overlay Material | Hardness (HRC) | Impact Toughness (J/cm²) | Application |
|---|---|---|---|
| High-carbon steel (e.g., 65Mn) | 50–58 | 10–20 | General wear repair |
| Cr-Mo alloy steel (e.g., 42CrMo) | 40–50 | 20–35 | Combined wear and impact |
| Ductile iron (high-silicon) | 55–65 | 5–15 | Severe abrasion |
| Hardfacing alloy (Ni-Cr-B-Si) | 50–60 | 15–25 | High-temperature abrasion |
| Tungsten carbide composite | 65–75 | 5–10 | Extreme abrasion, low impact |
For chisel body repair, a layered approach is often employed, with a transition layer of lower-carbon material between the base metal and the hard overlay layer to improve weldability and reduce cracking susceptibility.
Welding Process Selection
Several welding processes are suitable for chisel body repair, each with distinct advantages and limitations:
- Submerged arc welding (SAW): High deposition rate, good penetration, suitable for building up thick overlay layers. However, the flux coverage may obscure the weld bead geometry, requiring post-weld grinding.
- Flux-cored arc welding (FCAW): High deposition rate, good slag protection, suitable for field repair. The flux-cored wire provides adequate dilution control and good mechanical properties.
- Gas metal arc welding (GMAW): Good visibility, precise bead control, suitable for thin overlay layers. However, lower deposition rate compared to SAW and FCAW.
- Oxy-acetylene welding: Simple equipment, suitable for small repairs. However, low deposition rate, high dilution, and limited process control.
For chisel body repair, FCAW is often preferred due to its balance of deposition rate, process control, and equipment portability for field applications.
Process Parameters
| Parameter | Typical Value | Notes |
|---|---|---|
| Welding current | 200–350 A | Depends on wire diameter |
| Arc voltage | 25–35 V | Adjusted for bead geometry |
| Travel speed | 150–300 mm/min | Optimized for deposition rate |
| Wire diameter | 1.2–1.6 mm | FCAW wire |
| Number of passes | 2–4 | Building to required thickness |
| Interpass temperature | <200°C | Prevents cracking |
| Preheat temperature | 100–200°C | Reduces residual stress |
Quality Control and Defect Prevention
The weld overlay repair of chisel bodies is susceptible to several common defects:
- Cracking: Cold cracking in the HAZ due to high carbon equivalent of the base metal and inadequate preheat. Countermeasures include strict control of preheat temperature, use of low-hydrogen filler materials, and post-weld heat treatment.
- Porosity: Caused by inadequate shielding gas coverage, contaminated base metal surface, or excessive travel speed. Countermeasures include proper surface preparation, adequate gas flow rates, and optimization of travel speed.
- Lack of fusion: Resulting from insufficient heat input, poor base metal cleaning, or excessive travel speed. Countermeasures include increased current, thorough surface preparation, and reduced travel speed.
- Undercut: Common in vertical and overhead positions, leading to stress concentration and potential crack initiation. Countermeasures include proper torch angle control and post-weld grinding.
Quality control measures include:
- Visual inspection: For surface defects such as undercut, porosity, and incomplete fusion.
- Magnetic particle inspection (MT): For surface and near-surface cracks.
- Ultrasonic testing (UT): For subsurface defects such as lack of fusion and internal cracks.
- Hardness testing: To verify the hardness profile across the overlay thickness and detect HAZ softening.
- Impact testing: To verify the impact toughness of the overlay layer and HAZ.
Engineering Practice and Reflections
The weld overlay repair of chisel bodies offers significant economic benefits compared to replacement, particularly for large chisels used in roadheaders and continuous miners. A typical chisel may cost several thousand dollars, and replacement is required every few hundred hours of operation. Weld overlay repair can extend the service life by 2–3 times, providing a substantial return on investment.
However, the success of weld overlay repair depends on several factors:
- Assessment of the base metal condition: The base metal must be free of cracks, excessive wear, and other defects that would compromise the repair. Ultrasonic testing is recommended to detect internal cracks before repair.
- Surface preparation: Thorough cleaning of the repair area is critical to ensure proper fusion and prevent porosity. Grinding to a clean, bright metal surface is recommended.
- Process qualification: Welding procedures must be qualified per applicable standards (such as NB/T 47014 or ASME IX) to ensure consistent and reliable results.
- Post-repair inspection: Comprehensive NDT and mechanical testing after repair ensures that the repair meets quality requirements and is safe for service.
The FMEA approach is particularly useful in identifying potential failure modes at each step of the repair process and implementing preventive measures. For example, the FMEA for the surface preparation step would identify inadequate cleaning as a potential failure mode, with porosity as the effect, and thorough grinding and cleaning as the preventive measure.
The integration of metallurgical knowledge, welding process expertise, and systematic quality management is essential for successful chisel body repair. Engineers working in mining equipment maintenance should develop in-house capabilities for weld overlay repair, supported by proper training, equipment, and quality control systems.
Summary
The weld overlay repair of chisel bodies in mining machinery is a practical and economically attractive alternative to component replacement. The key to success lies in the proper selection of overlay materials, careful control of welding processes, and rigorous quality assurance. Engineers in the mining industry should recognize that weld overlay repair is not merely a cost-saving measure but a technical solution that requires metallurgical understanding and systematic quality management. The lessons from this work extend to other heavy-duty components subject to severe wear and impact, such as bucket teeth, conveyor rollers, and crusher hammers. A lifecycle approach to component maintenance, combining in-service monitoring, timely repair, and rigorous quality control, is essential for maximizing equipment availability and minimizing total cost of ownership.