CLADDING TECHNOLOGY SHANXI CO., LTDInconel 600 Nickel-Based Alloy PAW+TIG Joint Microstructure and Mechanical Properties
Literature Overview
This study investigates the microstructure evolution and mechanical properties of Inconel 600 weld overlay joints produced using a Plasma Arc Welding (PAW) combined with TIG arc hybrid process. Inconel 600 is a nickel-chromium-iron alloy widely used in chemical processing, nuclear applications, and high-temperature service. The PAW+TIG hybrid approach represents an advancement in overlay welding technology that seeks to combine the precision of plasma arc with the high deposition rate of TIG welding.
Base Material and Process Characteristics
Inconel 600 Composition
| Element | C | Cr | Ni | Fe | Mo | Si | Mn | Al | Ti |
|---|---|---|---|---|---|---|---|---|---|
| wt% | 0.10 | 14.0–17.0 | Bal. | 6.0–10.0 | 0.30–0.80 | 0.35 | 0.35 | 0.30 | 0.15 |
Hybrid Process Parameters
| Parameter | PAW Component | TIG Component | Combined Effect |
|---|---|---|---|
| Current | 30–60 A | 120–200 A | 150–260 A total |
| Voltage | 20–30 V | 12–18 V | Effective 15–22 V |
| Gas flow | 15–25 L/min | 8–15 L/min | 25–40 L/min total |
| Travel speed | 150–400 mm/min | — | 150–400 mm/min |
| Wire feed | 100–300 mm/min | — | 100–300 mm/min |
Microstructural Analysis
Weld Zone Characterization
The PAW+TIG hybrid joint exhibits a complex microstructure resulting from the interaction of two distinct heat sources:
- Fusion zone — Columnar dendrites with primary Ni-rich dendrites and inter-dendritic Ni-Cr rich phases; grain size 50–150 μm
- Heat affected zone (HAZ) — Precipitation-free zone near fusion boundary transitioning to precipitation-hardened microstructure further from the weld
- Substrate HAZ — δ-ferrite formation (0.5–2%) at grain boundaries in the base material
Precipitation Behavior
The microstructural evolution is dominated by precipitation phenomena:
| Phase | Composition | Formation Temperature | Effect on Properties |
|---|---|---|---|
| Ni₃(Nb,Ti) | γ' phase | 600–900°C | Strengthening |
| Ni₃Si | γ'' phase | 400–700°C | Strengthening |
| Cr₂₃C₆ | Carbide | 500–800°C | Embrittlement |
| δ-ferrite | Ni₃Fe | Solidification | Cracking susceptibility |
Mechanical Properties
Room Temperature Properties
| Property | Base Metal | PAW Only | TIG Only | PAW+TIG Hybrid |
|---|---|---|---|---|
| Tensile strength (MPa) | 620–700 | 580–650 | 520–600 | 600–680 |
| Yield strength (MPa) | 280–350 | 260–320 | 220–280 | 270–340 |
| Elongation (%) | 35–45 | 30–40 | 25–35 | 32–42 |
| Hardness (HV) | 180–210 | 170–200 | 150–180 | 175–205 |
High Temperature Performance
The hybrid joint demonstrates superior high-temperature retention compared to TIG-only overlay:
- At 600°C: 85–90% of room temperature strength retained
- At 700°C: 75–80% retention
- At 800°C: 60–65% retention
This performance is attributed to the refined microstructure and controlled dilution achieved by the PAW component, which limits substrate contamination while maintaining adequate bonding.
Defect Analysis Using FMEA Approach
| Failure Mode | Severity | Occurrence | Detection | RPN | Countermeasure |
|---|---|---|---|---|---|
| Hot cracking | 9 | 4 | 5 | 180 | Control δ-ferrite < 1%, preheat 150°C |
| Cold cracking | 7 | 3 | 6 | 126 | Low hydrogen consumables, post-weld heat treatment |
| Porosity | 6 | 4 | 4 | 96 | Optimize gas shielding, wire cleanliness |
| Lack of fusion | 8 | 3 | 5 | 120 | Increase heat input, optimize travel speed |
| Excessive dilution | 7 | 5 | 3 | 105 | Reduce TIG current, increase PAW contribution |
Engineering Practice Implications
For pressure vessel fabrication involving Inconel 600 overlay on carbon steel or low-alloy steel substrates, this research provides critical guidance:
- WPS development — The PAW+TIG hybrid process requires qualification under NB/T 47014 with specific attention to the dual heat source interaction
- Heat treatment — Solution treatment at 1050°C for 1 hour followed by air cooling is recommended to dissolve harmful carbides and restore ductility
- Inspection requirements — UT and MT inspection of both surfaces is mandatory, with particular attention to the dilution interface
Study Insights
The most significant finding is that the PAW component acts as a "process stabilizer" for the TIG arc, reducing arc wandering and improving bead profile consistency. This is attributed to the plasma jet's directing effect on the molten pool. The practical implication is that hybrid PAW+TIG overlay produces more repeatable results than either process alone, which is critical for production environments requiring consistent quality across multiple welders and shifts.
The δ-ferrite content control remains the primary metallurgical challenge. The study demonstrates that maintaining δ-ferrite below 1% requires careful control of welding parameters and may necessitate post-weld solution heat treatment for critical applications.