CLADDING TECHNOLOGY SHANXI CO., LTDRepair Welding of Mining Pick Body A Technical Study Note
Overview of the Topic
The pick body is a critical consumable component used in roadheaders and continuous miners for coal and rock excavation. During operation, the pick body experiences intense abrasion, impact loading, and thermal cycling, leading to progressive material loss on the working surfaces. When the pick body is worn beyond its service limit, complete replacement is costly and wasteful. Weld overlay repair offers an economical and efficient alternative by rebuilding the worn surface and restoring geometry and performance. This study note examines the technical principles, process parameters, material selection, and quality considerations involved in the overlay repair welding of pick bodies.
Service Conditions and Failure Modes
Mining picks operate under extremely harsh conditions. The pick body is subjected to cyclic impact forces exceeding 100 kN during rock breaking, while simultaneously enduring abrasive contact with hard rock formations containing quartz, feldspar, and other hard minerals. The contact pressure can reach several hundred MPa, and the sliding velocity may exceed 2 m/s. These combined loading conditions produce a complex failure pattern involving three-body abrasion, two-body abrasion, adhesive wear, and fatigue spalling. The base material of the pick body is typically a medium-carbon alloy steel or low-alloy steel (such as 42CrMo or 40MnB), which provides adequate toughness and strength for the structural body but lacks sufficient surface hardness and wear resistance for prolonged service.
The typical service life of a pick body before repair or replacement is on the order of 30 to 80 hours of continuous operation, depending on the rock hardness (f=4 to f=10 on the Mohs scale) and the specific operating conditions. The primary wear zones are concentrated on the cutting edges, the shoulder surfaces, and the root fillet regions where stress concentration is highest.
Material Selection for Overlay Repair
The selection of overlay material is the most critical decision in pick body repair welding. The overlay material must provide high hardness (typically HV 600–900), good abrasion resistance, adequate toughness to resist chipping, and sufficient bonding strength with the base steel. The following table summarizes the most commonly used overlay materials for pick body repair:
| Overlay Material Type | Typical Composition | Hardness (HV) | Key Advantage | Typical Application |
|---|---|---|---|---|
| High-carbon martensitic | C 1.5–2.5%, Cr 5–10% | 600–750 | Excellent abrasion resistance | Medium-hard rock |
| High-chromium cast iron | Cr 25–30%, C 2.5–4.0% | 800–950 | Superior sliding wear resistance | Hard abrasive rock |
| Nickel-cobalt alloy | Ni 30%, Co 20%, Cr 5% | 500–650 | Good toughness, thermal stability | High-impact zones |
| Tungsten carbide composite | WC 30–50% in matrix | 800–1000 | Extreme abrasion resistance | Very hard rock |
| Hardfacing alloy (Fe-Cr-C) | Cr 20%, C 3.0%, Mo 5% | 700–850 | Balanced properties | General purpose |
For most pick body repair applications, a high-carbon martensitic overlay or a high-chromium cast iron overlay is preferred. The high-carbon martensitic type offers a good balance of hardness and toughness, making it suitable for general rock conditions. The high-chromium type provides superior abrasion resistance in highly abrasive environments but is more susceptible to cracking due to its high hardenability and brittleness.
Welding Process Selection and Parameters
The most common welding processes used for pick body overlay repair are shielded metal arc welding (SMAW), submerged arc welding (SAW), and gas metal arc welding (GMAW). Each process has distinct advantages and limitations for this application.
| Process | Typical Current (A) | Travel Speed (mm/s) | Layer Thickness (mm) | Advantages | Limitations |
|---|---|---|---|---|---|
| SMAW | 180–280 | 3–6 | 3–5 | Portable, flexible | Lower deposition rate, operator dependent |
| SAW | 400–600 | 8–15 | 5–10 | High deposition rate, consistent quality | Requires positioner, limited to flat/horizontal |
| GMAW | 200–350 | 5–10 | 3–6 | Good versatility, medium speed | Spatter, requires gas shielding |
| Flux-cored (FCAW) | 200–350 | 4–8 | 4–7 | High deposition rate, all-position | Flux disposal, fumes |
For field repair conditions, SMAW is the most practical choice due to its portability and flexibility. In workshop or shop-floor conditions, SAW or GMAW can be employed for higher productivity. The welding parameters must be carefully controlled to ensure adequate penetration and fusion with the base metal while minimizing dilution of the overlay material. Excessive dilution (above 25–30%) can significantly reduce the hardness and wear resistance of the overlay layer.
Multi-Pass Strategy
For pick body repair, the worn surface typically requires rebuilding of 5 to 15 mm of material. This is achieved through multiple passes:
- Preparation pass: The worn surface is ground to remove loose material, oxide scale, and undercut. The surface is cleaned to a near-white finish to ensure good adhesion.
- Transition pass (first layer): A low-dilution alloy or the base steel itself is deposited to create a transition zone between the base metal and the overlay alloy. This pass uses a lower current (15–20% below subsequent passes) and a tighter weave to minimize dilution.
- Build-up passes (intermediate layers): Multiple layers of the selected overlay alloy are deposited to rebuild the lost material. Each pass is deposited with a current 20–30% above the transition pass. The interpass temperature is maintained between 150 and 300 °C to prevent cracking while ensuring proper heat input for transformation of the overlay material.
- Surface pass (final layer): The final pass is deposited with a slightly lower current to produce a smoother surface with better hardness uniformity. This pass defines the final geometry of the pick body.
Preheating and Interpass Temperature Control
Preheating is essential for pick body repair welding. The base steel is preheated to 200–300 °C to reduce the cooling rate and minimize the risk of hydrogen-induced cracking and hard martensite formation in the heat-affected zone. The interpass temperature is maintained between 150 and 300 °C. If the interpass temperature drops below 100 °C, reheating is required. If it exceeds 350 °C, there is a risk of grain growth and reduced toughness in the overlay.
Post-weld heat treatment (PWHT) is typically applied at 550–620 °C for 2 hours to temper the martensitic overlay and relieve residual stresses. This treatment reduces hardness by approximately 50–100 HV but significantly improves toughness and reduces the risk of delayed cracking.
Defect Analysis and Countermeasures
The following table summarizes the most common defects encountered in pick body overlay repair and their countermeasures:
| Defect Type | Root Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Cracking (longitudinal) | High carbon equivalent, low preheat | Visual, MT | Increase preheat to 300 °C, use low-hydrogen electrode |
| Cracking (transverse) | Excessive restraint, high dilution | Visual, MT | Reduce restraint, use transition layer, lower current |
| Porosity | Contaminated surface, improper gas flow | RT, UT | Thorough surface cleaning, verify gas flow rate |
| Lack of fusion | Insufficient heat input, wrong technique | UT, MT | Increase current, improve travel technique |
| Excessive dilution | Too much penetration, wrong electrode angle | Hardness test | Use lower current, adjust electrode angle, add transition layer |
| Undercut | Excessive current, too fast travel speed | Visual, PT | Reduce current, slow travel speed, improve technique |
Engineering Practice and Quality Control
In practical engineering applications, the quality of pick body repair welding is verified through the following inspection protocol:
- Visual inspection: 100% inspection of all welds for surface defects, undercut, and geometric conformity.
- Magnetic particle testing (MT): 100% inspection of all overlay welds to detect surface and near-surface cracks.
- Hardness testing: Hardness is measured at 5 points across each weld cross-section. The overlay layer hardness should be at least 600 HV for high-carbon martensitic or 800 HV for high-chromium cast iron. The hardness gradient from base metal to overlay should be gradual.
- Impact testing (optional): Charpy V-notch impact tests at room temperature may be performed on coupon specimens to verify toughness. A minimum absorbed energy of 27 J at 25 °C is generally required for the overlay layer.
- Service verification: The repaired pick body is tracked in service to verify that the repair extends service life by at least 50–70% of the original pick body life.
Study Insights and Reflections
The study of pick body repair welding highlights several important engineering principles. First, the success of overlay repair is not solely determined by the overlay material but is equally dependent on the welding process parameters, surface preparation, and post-weld treatment. A high-quality overlay on a poorly prepared surface will inevitably fail prematurely. Second, the concept of dilution management is critical. In pick body repair, the transition layer technique is not merely a recommendation but a necessity to ensure that the overlay material retains its designed properties. Third, the economics of repair welding must be considered holistically: the cost of repair (including downtime, consumables, and labor) must be compared with the cost of new pick body replacement. In most cases, repair welding reduces the total cost of ownership by 40–60%, making it a highly attractive option for mining operators.
A key reflection from this study is the importance of understanding the interaction between the overlay material and the base metal microstructure. The base steel of the pick body undergoes significant microstructural changes in the heat-affected zone during welding. If the base steel is in a tempered martensitic condition (as is typical for 42CrMo), the HAZ may develop a mixture of martensite, bainite, and retained austenite, which can lead to cracking. Understanding this interaction and selecting appropriate welding parameters to manage the HAZ microstructure is essential for achieving a durable repair.
Summary
The repair welding of mining pick bodies is a well-established practice that combines metallurgical knowledge, welding technology, and practical engineering judgment. The key to successful repair lies in proper material selection, meticulous surface preparation, controlled welding parameters with emphasis on dilution management, appropriate preheating and post-weld treatment, and thorough quality inspection. By applying these principles systematically, the service life of pick bodies can be extended significantly, reducing both material costs and environmental impact. Engineers engaged in this work should continuously monitor repair performance in service and refine their process parameters based on actual wear data, creating a feedback loop that drives continuous improvement in repair quality and reliability.