CLADDING TECHNOLOGY SHANXI CO., LTDGas Tungsten Arc Weld Overlay Process for Cobalt Chromium Tungsten Alloy
Overview and Background
Gas tungsten arc welding (GTAW), commonly known as TIG welding, is the most widely used process for depositing cobalt-chromium-tungsten (Co-Cr-W) alloy overlays on critical wear components. This process offers exceptional control over heat input, minimal dilution, and the ability to achieve dense, crack-free deposits suitable for demanding applications such as petroleum wellheads, chemical processing equipment, and aerospace components. The Co-Cr-W system, exemplified by alloys like Stellite 6 (Co-27Cr-6W-1.5Mo) and Stellite 21 (Co-25Cr-12W-1.5Mo-0.5C), provides outstanding resistance to abrasive wear, erosion-corrosion, and hot hardness retention.
Process Parameters and Optimization
GTAW overlay of Co-Cr-W alloys requires careful parameter selection to balance deposit quality, productivity, and metallurgical integrity. The process is typically performed in the AC or DCEN mode, with DCEN preferred for its deeper penetration and higher deposition rate.
| Parameter | Typical Range | Optimization Target |
|---|---|---|
| Current | 120-250 A | Deposition rate vs. dilution |
| Arc voltage | 12-18 V | Arc stability |
| Travel speed | 30-80 mm/min | Penetration profile |
| Wire feed rate | 150-400 mm/min (PUSH/PULL) | Consistent deposit width |
| Shielding gas | Ar 99.99% (±10-20 L/min) | Oxide prevention |
| Torch angle | 70-80° from surface | Uniform bead profile |
| Stick-out | 8-12 mm | Arc stability |
| Preheat | 100-200°C | Reduce cracking |
| Interpass temp | <250°C | Control cooling rate |
Heat Input Control
The heat input (Q) is calculated as: Q = (V × I × η) / v, where V is arc voltage, I is current, η is efficiency (0.85 for GTAW), and v is travel speed. For Co-Cr-W overlays, the target heat input is typically 0.5-2.0 kJ/mm. Excessive heat input leads to:
- Grain coarsening in the overlay
- Increased dilution from base metal
- Reduced hardness (HRC drops 3-5 points per 1.0 kJ/mm increase)
- Cracking susceptibility due to prolonged time at critical temperatures
Multi-Pass Strategy
For overlay thicknesses exceeding 1.0 mm, a multi-pass approach is essential:
- Transition pass: Low current (100-120 A), high travel speed (60-80 mm/min), targeting dilution <20%
- Build-up passes: Medium current (160-200 A), moderate travel speed (40-60 mm/min), targeting 95% alloy composition
- Surface finish pass: Low current (100-130 A), high travel speed (50-70 mm/min), achieving Ra <3.2 μm
Metallurgical Considerations
Microstructure Evolution
The solidification microstructure of Co-Cr-W GTAW deposits is characterized by:
- Dendritic solid solution matrix (γ-phase, FCC)
- M₇C₃ carbides at dendrite boundaries
- M₆C carbides in later solidification regions
- Possible M₂₃C₆ formation at high chromium concentrations
The carbide morphology and distribution directly influence wear resistance. Spheroidized carbides provide superior impact toughness, while network carbides offer better abrasive resistance. Post-weld heat treatment (PWHT) at 1050-1150°C for 1-2 hours followed by air cooling promotes carbide spheroidization and relieves residual stresses.
Dilution Control and Its Effects
| Dilution (%) | Hardness (HRC) | Wear Rate (mm³/N·m) | Application Suitability |
|---|---|---|---|
| <10 | 50-55 | 0.08-0.12 | Wear-critical |
| 10-20 | 45-50 | 0.12-0.18 | General wear |
| 20-30 | 40-45 | 0.18-0.25 | Moderate service |
| >30 | <40 | >0.25 | Not recommended |
Defect Analysis and Prevention
Common Defects in GTAW Co-Cr-W Overlay
| Defect | Root Cause | Prevention Strategy |
|---|---|---|
| Hot cracking | Low melting point eutectics, high S/P | Use low-sulfur wire, control preheat |
| Porosity | Gas entrapment, wire contamination | Clean wire, proper shielding |
| Incomplete fusion | Low heat input, excessive travel speed | Increase current, reduce speed |
| Excessive dilution | High heat input, poor technique | Reduce heat input, use transition pass |
| Cracking in HAZ | Thermal stress, base metal susceptibility | Preheat, control cooling rate |
| Tungsten inclusion | Contaminated electrode, excessive stick-out | Dress electrode, maintain stick-out |
FMEA for Critical Applications
For a petroleum wellhead application requiring 3.0 mm Co-Cr-W overlay on API 5CT P110 tubing:
- Failure mode: overlay cracking leading to fluid leakage
- Severity: 10 (catastrophic — safety hazard)
- Occurrence: 3 (with proper procedure, rare)
- Detection: 4 (detectable by MT before service)
- RPN: 120 (actionable)
- Mitigation: Implement 100% MT inspection, reduce interpass temperature to <200°C, use 0.5 mm transition pass with Ni-based alloy
Engineering Practice and Study Insights
The GTAW process for Co-Cr-W overlay is remarkably forgiving when proper parameters are established, yet unforgiving when shortcuts are taken. In a field repair of a chemical reactor agitator shaft, the original overlay had failed after 18 months of service due to erosion-corrosion. The repair using GTAW with Stellite 6 wire (ERCoCr-A, 1.6 mm diameter) achieved HRC 52 hardness with less than 15% dilution after three passes. The key success factors were: (1) thorough surface preparation removing all prior overlay and oxide; (2) consistent torch angle and travel speed maintained by an experienced welder; (3) post-weld stress relief at 850°C for 2 hours in a vacuum furnace. The repaired shaft has now exceeded 30 months of continuous service, demonstrating that proper process control can restore components to beyond-original performance levels.
The study of GTAW Co-Cr-W overlay reinforces that the process is fundamentally a balance between metallurgical quality and productivity. Higher deposition rates inevitably increase heat input and dilution, reducing the functional performance of the overlay. Engineers must resist the temptation to maximize productivity at the expense of deposit quality, particularly for safety-critical applications where overlay failure can lead to catastrophic consequences.