CLADDING TECHNOLOGY SHANXI CO., LTDOverlay Welding of Tungsten Carbide on Hammer Crusher Hammer Heads
Literature Overview
Hammer crusher hammer heads are subjected to severe abrasive wear during the crushing of hard materials such as coal, ore, and rock. The overlay welding of tungsten carbide (WC) particles onto hammer heads is an established method to extend service life by orders of magnitude compared to the base material. The literature under review examines the metallurgical challenges, process optimization, and practical considerations involved in WC overlay welding, providing valuable insights for engineers tasked with improving the durability of crushing equipment.
Metallurgical Challenges of WC Overlay Welding
The primary challenge in overlay welding tungsten carbide is the significant difference in thermal properties between the WC particles and the iron-based matrix. Tungsten carbide has a thermal conductivity of approximately 111 W/(m·K) compared to 50–60 W/(m·K) for steel, and a coefficient of thermal expansion of 5.5×10⁻⁶/°C compared to 12×10⁻⁶/°C for steel. This mismatch creates significant residual stresses during cooling, which can lead to cracking and spallation of the overlay layer.
Key Metallurgical Issues
- Cracking of WC particles: The brittle nature of WC combined with thermal stress during cooling causes microcracking within the particles, reducing their effectiveness as hard phases.
- Decomposition of WC: At high temperatures above 1200°C, WC can decompose into W₂C and free carbon, which reduces hardness and creates porosity.
- Bonding interface weakness: The interface between WC particles and the iron-based matrix can be weak if the bonding mechanism is not properly designed.
- Crack propagation: Cracks can initiate at the WC-matrix interface and propagate through the overlay layer, causing spallation.
Process Selection and Parameters
The overlay process selected must balance deposit thickness, dilution control, and the ability to maintain WC particle integrity. The following table compares common processes for WC overlay welding:
| Process | Heat Input | Dilution Rate | WC Integrity | Deposit Thickness | Throughput |
|---|---|---|---|---|---|
| SAW (Submerged Arc) | High | 10–25% | Good (flux protection) | 3–8 mm | High |
| GMAW (MIG) | Medium | 15–30% | Moderate | 1–3 mm | Medium |
| Oxy-fuel (Flame) | High | 20–35% | Poor (oxidation) | 2–5 mm | Low |
| TIG (GTAW) | Low | 5–15% | Excellent | 0.5–2 mm | Low |
| Plasma transfer | Low-Medium | 5–10% | Excellent | 0.5–2 mm | Medium |
For hammer crusher applications, SAW is the most commonly used process due to its high deposition rate and ability to build up thick overlay layers. The flux provides excellent protection against oxidation, and the relatively high heat input promotes good wetting and bonding of WC particles to the matrix.
Recommended Process Parameters for SAW WC Overlay
| Parameter | Value | Notes |
|---|---|---|
| Current | 350–500 A | Depends on electrode diameter |
| Voltage | 30–36 V | Short arc for good penetration |
| Travel speed | 200–350 mm/min | Lower for thicker deposits |
| Flux coverage | ≥15 mm | Essential for WC particle protection |
| Preheat | 200–300°C | Reduces thermal stress, prevents cracking |
| Interpass temperature | 250–350°C | Maintains plasticity, reduces cracking |
| Post-weld cooling | Controlled (≤50°C/h below 300°C) | Prevents cracking during cooling |
The controlled cooling rate after welding is critical. Rapid cooling from the welding temperature creates high thermal gradients that can cause cracking in the brittle WC particles and at the overlay-base interface. A controlled cooling rate of 50°C/h or less below 300°C allows for stress relief through plastic deformation before the material becomes brittle.
Overlay Layer Composition and Hardness
The overlay layer composition directly influences the wear resistance and service life of the hammer head. The following table shows typical compositions and resulting properties:
| WC Content (wt.%) | Matrix Type | Hardness (HV) | Wear Life Improvement |
|---|---|---|---|
| 30–40% | Austenitic | 600–700 | 3–5× base material |
| 40–50% | Martensitic | 700–850 | 5–8× base material |
| 50–60% | Martensitic | 800–950 | 8–12× base material |
| 60–70% | Mixed | 900–1100 | 12–15× base material |
The hardness increases with WC content, but beyond 60% WC, the toughness decreases significantly, increasing the risk of spallation under impact loading. For hammer crusher applications, a WC content of 40–55% is typically optimal, providing a good balance of hardness and toughness.
Microstructure of WC Overlay Layer
The overlay microstructure consists of WC particles (typically 20–100 μm in size) dispersed in an iron-based matrix. The matrix composition determines the phase structure:
- Austenitic matrix (Ni-Cr based): Provides toughness and ductility, suitable for high-impact applications.
- Martensitic matrix (Cr-based): Provides higher hardness and compressive strength, suitable for moderate impact applications.
- Mixed matrix: Combines austenitic and martensitic phases for balanced properties.
The bonding mechanism between WC particles and the matrix is primarily mechanical interlocking, with some metallurgical bonding through iron carbide (Fe₃C) formation at the WC-matrix interface. This interfacial reaction layer is approximately 1–5 μm thick and provides the critical bonding strength.
Engineering Practice and Quality Control
Base Material Preparation
The base material preparation is critical for successful WC overlay welding. The following steps are essential:
- Surface cleaning: Remove all rust, scale, oil, and paint from the welding area using grinding or shot blasting.
- Preheating: Heat the base material to 200–300°C uniformly across the entire hammer head to prevent thermal cracking.
- Roughening: Lightly roughen the surface to improve mechanical bonding of the first overlay pass.
- Flux conditioning: Ensure the flux is dry (moisture content <1%) to prevent hydrogen-induced porosity.
Quality Inspection Methods
| Inspection Method | Purpose | Acceptance Criteria |
|---|---|---|
| Visual inspection (VT) | Surface defects, porosity | No visible cracks, porosity <5% |
| Magnetic particle testing (MT) | Surface and near-surface cracks | No indications exceeding 2 mm |
| Ultrasonic testing (UT) | Internal defects, bonding quality | No lack of fusion, no large inclusions |
| Hardness testing | Verify overlay hardness | Within specified range ±10% |
| Bond strength test | Verify overlay-base bonding | ≥150 MPa for critical applications |
Common Defects and Solutions
| Defect | Cause | Solution |
|---|---|---|
| Overlay spallation | Excessive thermal stress, poor bonding | Controlled cooling, proper preheat, adequate flux coverage |
| WC particle cracking | Rapid cooling, excessive heat input | Lower travel speed, controlled cooling, flux protection |
| Porosity | Flux moisture, base contamination | Dry flux, clean base surface |
| Excessive dilution | High heat input, thin first pass | Lower current, thicker first pass, transition layer |
| Cracking at interface | Thermal mismatch, residual stress | Post-weld stress relief, controlled cooling |
Study Insights and Reflections
The most important insight from this literature is that the success of WC overlay welding depends less on the welding parameters themselves and more on the overall thermal management strategy. The preheat, interpass temperature, and post-weld cooling rate are more critical than the arc parameters. This is because the primary failure mode is thermal stress-induced cracking, not welding defects such as porosity or lack of fusion.
The choice of matrix composition is also a critical design decision. For hammer crusher applications where impact loading is significant (such as in coal crushing), an austenitic matrix provides better impact resistance despite lower hardness. For applications where abrasion is the dominant wear mechanism (such as in ore crushing), a martensitic matrix provides better wear resistance at the expense of some toughness.
A practical consideration that is often overlooked is the geometry of the hammer head. The overlay should be applied to the wear surface only, avoiding the striking face and the mounting areas. The transition between the overlay and the base material should be gradual to avoid stress concentration. In practice, this means using a tapered deposit profile rather than a sharp edge.
The economics of WC overlay welding must also be considered. While the initial cost of WC overlay is significantly higher than the base material, the service life improvement of 5–15 times makes it economically justified for most applications. The key is to optimize the WC content and matrix composition for the specific service conditions to maximize the cost-benefit ratio.
Reference Value and Outlook
This literature provides a comprehensive understanding of the metallurgical and process aspects of WC overlay welding for hammer crusher applications. The documented process parameters, quality control methods, and defect analysis provide a practical guide for engineers implementing WC overlay solutions. Future developments in this area may include the use of advanced WC-Co composite powders with improved bonding characteristics, and the development of automated overlay systems with real-time thermal monitoring to optimize the cooling rate during welding.