CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay of High-Speed Steel Coating on Coal Mining Machine Picks
Literature Overview
The study by Yao Shuyu and Li Huiqi from the School of Mechanical and Electronic Engineering, Shandong University of Science and Technology (published in Metal Heat Treatment, 2004) addresses a critical engineering problem in the coal mining industry: the rapid wear of cutting picks on continuous mining machines and roadheaders. These picks operate under extremely harsh conditions involving high-impact loading, severe abrasive wear from hard coal and rock strata, and elevated temperatures. The conventional approach of using solid high-speed steel picks or hardfacing with standard carbide materials often results in insufficient service life, leading to frequent replacement cycles and increased operational downtime. This research investigates the feasibility and effectiveness of applying high-speed steel overlay coatings on pick bodies to extend service life while maintaining economic viability.
Core Technical Content
Material Selection and Composition
The research focuses on the selection of high-speed steel grades suitable for overlay application on pick substrates. Typical high-speed steel compositions include:
| Element | W6Mo5Cr4V2 (M2) | W6Mo5Cr4V (M1) | W12Cr4V (T1) |
|---|---|---|---|
| C (%) | 0.75–0.85 | 0.80–0.90 | 1.45–1.65 |
| W (%) | 5.50–6.75 | 5.50–6.75 | 11.50–12.50 |
| Mo (%) | 4.50–5.50 | 4.50–5.50 | — |
| Cr (%) | 3.80–4.40 | 3.80–4.40 | 3.50–4.50 |
| V (%) | 1.10–1.40 | 1.00–1.30 | — |
| Hardness (HRC) | 63–66 | 63–65 | 62–65 |
The selection between M2 (W6Mo5Cr4V2) and T1 (W12Cr4V) depends on the balance between hardness, toughness, and resistance to thermal softening. M2-type compositions are preferred for applications requiring better toughness due to the presence of molybdenum which provides superior red hardness compared to tungsten-only compositions.
Overlay Process Parameters
The study examines submerged arc welding (SAW) and gas metal arc welding (GMAW) as the primary overlay processes. Key process parameters include:
| Parameter | SAW | GMAW (Flux-cored) |
|---|---|---|
| Current (A) | 300–450 | 200–350 |
| Voltage (V) | 28–35 | 24–30 |
| Travel speed (cm/min) | 5–10 | 8–15 |
| Wire diameter (mm) | 2.4–3.2 (flux-cored) | 1.2–1.6 |
| Preheat temperature (°C) | 150–250 | 100–200 |
| Interpass temperature (°C) | 150–250 | 100–200 |
| Overlay thickness (mm) | 3–6 | 2–5 |
Heat Treatment Considerations
A critical aspect of this research is the post-overlay heat treatment to achieve the desired microstructure in the high-speed steel overlay. The overlay deposit, when solidified from the weld, forms a non-equilibrium structure that must be tempered to develop the characteristic carbide dispersion responsible for high-speed steel properties.
The recommended heat treatment cycle includes:
- Austenitization at 1150–1200°C (for M2 type) or 1220–1280°C (for T1 type)
- Oil quenching or air cooling
- Multiple tempering cycles at 560–580°C (typically 3 passes)
The multiple tempering cycles are essential to transform retained austenite to martensite and to precipitate secondary carbides that provide the final hardness and wear resistance. Without proper tempering, the overlay retains high residual stresses and retained austenite, leading to poor dimensional stability and potential cracking during service.
Defect Analysis and Countermeasures
Common Defects Observed
Through metallographic examination, the following defects were identified:
- Bond line cracks: Caused by excessive cooling rates at the substrate-overlay interface, particularly when the steel substrate has high carbon equivalent (Ceq > 0.45%). Countermeasure: adequate preheating and controlled interpass temperature.
- Porosity: Resulting from improper flux coverage or contaminated wire surface. Countermeasure: ensure flux dryness (moisture content < 0.1%) and proper shielding gas flow.
- Cracking in overlay: Due to high carbon and alloy content promoting martensitic transformation with associated volume expansion. Countermeasure: multi-pass welding with lower heat input per pass, and strict interpass temperature control.
- Delamination: Caused by inadequate bonding at the interface, often due to substrate surface contamination or insufficient fusion. Countermeasure: thorough surface preparation and ensuring at least 0.5 mm penetration into the base metal on the first pass.
Microstructural Analysis
The overlay microstructure consists of primary M6C carbides (MC type vanadium carbides) dispersed in a tempered martensitic matrix. The carbide morphology and distribution are critical:
- Primary carbides formed during solidification appear as large, irregular particles (5–20 μm)
- Secondary carbides formed during tempering appear as fine, uniformly distributed particles (0.1–1 μm)
- The volume fraction of total carbides is typically 15–25%
The wear resistance is directly correlated with the hardness, toughness, and carbide distribution. The M6C (VC) type carbides provide excellent resistance to abrasive wear, while the tempered martensitic matrix provides the necessary toughness to resist impact loading during cutting.
Engineering Practice Integration
Application on Mining Picks
In practical application, the overlay is applied to the cutting edge of the pick, typically covering the tip and the front 30–50 mm of the cutting edge. The substrate is usually made of 42CrMo or 40MnB quenched and tempered steel, providing adequate toughness for the pick body while the overlay provides the wear-resistant cutting surface.
Key engineering considerations include:
- Geometry design: The overlay thickness should be sufficient to maintain cutting edge integrity after grinding, typically 3–5 mm of usable overlay material
- Heat-affected zone control: The HAZ in the substrate should not exceed acceptable hardness limits (typically < 350 HB for 42CrMo)
- Post-weld grinding: The overlay surface must be ground to achieve the required cutting edge geometry (typically 15–25° rake angle)
Performance Results
The study reports significant improvements in pick service life:
| Condition | Conventional pick (solid HSS) | Overlay pick (steel body + HSS overlay) | Improvement |
|---|---|---|---|
| Average life (hours) | 8–12 | 20–35 | 200–300% |
| Wear rate (mm/100h) | 1.5–2.5 | 0.4–0.8 | 60–70% reduction |
| Cost per hour of operation | Baseline | 35–50% of baseline | Significant savings |
The improvement is attributed to the combination of the tough steel substrate absorbing impact loads while the hard overlay resists abrasive wear. This bimetallic approach provides an optimal balance between toughness and wear resistance that neither material alone can achieve.
Study Insights and Reflections
The fundamental principle demonstrated in this research is the strategic combination of materials with complementary properties through overlay welding. The substrate provides structural integrity and toughness, while the overlay provides surface functionality (wear resistance). This philosophy is directly applicable to many other engineering applications including pressure vessel internals, heat exchanger tubesheets, and wear-resistant components in mining and cement industries.
A particularly important insight is the critical role of heat treatment in achieving the desired overlay properties. Unlike simpler hardfacing deposits that rely primarily on carbide volume fraction for wear resistance, high-speed steel overlays require proper tempering to develop the full complement of secondary carbides. This means that the welding process must be compatible with subsequent heat treatment, and the thermal cycling during welding must not create conditions that prevent effective austenitization and tempering.
Another practical consideration is the residual stress state. The high hardness of the overlay creates significant compressive and tensile stresses in the deposit and substrate respectively. These residual stresses can affect dimensional stability and fatigue performance. Stress relief at 520–540°C (below the tempering temperature) for 2 hours can reduce residual stresses without adversely affecting the overlay hardness.
The research also highlights the importance of process selection. Submerged arc welding provides the highest deposition rate and best gas protection, making it suitable for thick overlay deposits. However, for complex pick geometries where access is limited, flux-cored GMAW offers better positional flexibility at the expense of slightly lower deposition rates.
Reference Value and Outlook
This research establishes a solid foundation for the application of high-speed steel overlay technology in mining equipment. The principles can be extended to other wear-critical applications such as drill bits, augers, and conveyor components. Future developments should focus on:
- Advanced high-speed steel compositions incorporating cobalt (Co-HSS) for improved red hardness in applications with elevated contact temperatures
- Multi-layer overlay designs combining a bond layer (lower carbon, better weldability) with the functional high-speed steel surface layer
- Integration of laser cladding technology for more precise control of overlay thickness and geometry on complex pick shapes
- Development of in-situ monitoring systems for wear tracking and predictive maintenance of overlay-coated picks
The economic benefits of overlay technology are substantial, particularly when considering the reduced downtime for pick replacement and the extended interval between maintenance stops. This makes the technology particularly attractive for large-scale mining operations where even small improvements in component life translate to significant cost savings.