CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Tungsten Carbide Cladding Layer Prepared by TIG Arc Welding
Technical Background and Industrial Demand
Wear-resistant coatings for pressure vessel internals, pump components, and mining equipment face constant demands for improved hardness and wear resistance. Tungsten carbide (WC) is one of the hardest engineering ceramics (HV 2400–2800) and is widely used as a reinforcing phase in hard-facing alloys. This study examines the preparation of WC-containing cladding layers using TIG arc welding, analyzing how welding parameters affect WC integrity, microstructure, and resulting mechanical properties. The research is directly relevant to engineers specifying overlay cladding for hydrogenation reactor internals, valve seats, and pump impellers.
Process Parameters and WC Particle Behavior
During TIG cladding with WC-containing filler materials, the WC particles undergo complex thermal transformations depending on the local temperature and cooling rate experienced during solidification:
| Thermal Condition | WC Behavior | Resulting Phases |
|---|---|---|
| T < 1000 °C (matrix solidification) | WC remains intact | WC + solid solution matrix |
| 1000–1350 °C (moderate heating) | Partial decomposition | WC + W2C + η-carbide (M6C) |
| T > 1350 °C (full melting) | Complete dissolution | W dissolves → η-carbide on cooling |
The decomposition reaction is: WC → W + C (at temperatures above approximately 1350 °C). In practice, the weld pool temperature (1500–2500 °C) exceeds this threshold, meaning most WC particles decompose during welding. The key challenge is to maximize the fraction of intact WC while ensuring adequate metallurgical bonding.
| Parameter | Range | Effect on WC Integrity |
|---|---|---|
| Arc current | 100–180 A | Lower current preserves more WC |
| Travel speed | 200–400 mm/min | Higher speed reduces thermal exposure |
| Powder particle size | 25–75 μm | Larger particles survive better |
| Shielding gas | Ar | Prevents oxidation of W |
| Preheat | 0–150 °C | Lower preheat maintains WC |
| Fill ratio | 30–60% WC | Higher ratio increases WC content |
Microstructural Characterization
The resulting microstructure of TIG-cladded WC coatings exhibits a heterogeneous distribution:
- Intact WC particles: Angular to equiaxed morphology, 10–50 μm in size, retaining their original tetragonal structure (a = 0.2906 nm, c = 0.2839 nm). These provide primary wear resistance.
- Decomposed WC regions: Where WC has dissolved and re-solidified, forming η-carbide (W6C, M6C type) with hexagonal crystal structure. These regions appear as dark contrast in SEM.
- Matrix phases: Depending on the filler composition, the matrix may be austenite (for Ni-based fillers), martensite (for Fe-based fillers), or a mixture.
- Bond line: Transition zone with dissolved tungsten in solid solution, showing solid solution strengthening.
Typical volume fractions achieved:
- Intact WC: 15–35 vol%
- Decomposed WC (η-carbide): 10–25 vol%
- Matrix: 40–60 vol%
Mechanical Properties and Wear Performance
| Property | WC-30% (Fe matrix) | WC-50% (Ni matrix) | Bare Substrate |
|---|---|---|---|
| Microhardness (HV30) | 800–950 | 900–1100 | 200–250 |
| Wear rate (mm³/N·m) | 3–6 × 10⁻⁴ | 2–4 × 10⁻⁴ | 20–35 × 10⁻⁴ |
| Bond strength (MPa) | 220–300 | 250–340 | N/A |
| Impact toughness (J/cm²) | 3–6 | 4–8 | 45–55 |
| Elastic modulus (GPa) | 180–220 | 200–240 | 200 |
The Ni-based matrix with 50% WC content provides the best combination of hardness and toughness, making it suitable for applications requiring both wear resistance and some impact tolerance.
Common Defects and Countermeasures
| Defect Type | Cause | Countermeasure |
|---|---|---|
| WC decomposition | Excessive heat input | Reduce current, increase speed |
| Cracking | High residual stress from hard coating | Preheat substrate, use ductile transition layer |
| Porosity | Gas entrapment, powder oxidation | Improve shielding, preheat powder |
| Poor bonding | Inadequate wetting | Increase current slightly, add Ni or Fe to filler |
| Spalling | Thermal mismatch, brittle matrix | Multi-layer approach with graded composition |
Engineering Implementation Guidelines
For practical implementation in pressure vessel and rotating equipment applications, the following guidelines are recommended:
- Substrate preparation: Grind the base surface to Ra < 6.3 μm. Preheat to 150–200 °C for carbon steel substrates to reduce cracking risk.
- Multi-pass strategy: Apply a first pass with 20% WC (ductile transition layer), followed by subsequent passes with 40–50% WC for wear resistance.
- Post-weld treatment: Stress relief at 550–600 °C for 2 hours to reduce residual stresses without causing phase transformation.
- Inspection: Perform magnetic particle testing (MT) for surface cracks and ultrasonic testing (UT) for subsurface defects per JB/T 4730 or ASME V requirements.
- Acceptance criteria: Hardness ≥800 HV at coating surface, bond strength ≥200 MPa, no surface cracks or spalling visible under 10× magnification.
Summary and Practical Recommendations
The TIG arc welding approach for WC-containing cladding layers offers a practical balance between wear resistance and process accessibility for industrial applications. The critical insight from this study is that while complete preservation of WC particles is impossible under welding conditions, a combination of intact WC (15–35 vol%) and decomposition products (η-carbide) provides adequate wear resistance for most industrial applications. Engineers should select Ni-based filler compositions for applications requiring impact tolerance and Fe-based compositions where maximum hardness is the priority. The multi-layer approach with a ductile transition layer is strongly recommended for pressure-retaining applications to prevent coating spallation under thermal cycling conditions.