CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Mechanical Properties of Inconel 600 PAW Plus TIG Welded Joints Study Note
Literature Overview
This study investigates the microstructural evolution and mechanical performance of Inconel 600 nickel-based alloy joints fabricated using a combined Plasma Arc Welding (PAW) and Tungsten Inert Gas (TIG) process. Inconel 600, with its excellent resistance to oxidation, carburization, and a broad range of corrosive environments, is widely used in chemical processing, nuclear, and petroleum industries. The combined PAW-TIG approach leverages the deep penetration and high deposition rate of plasma arc welding with the fine microstructural control and low heat input of TIG welding. This hybrid methodology addresses one of the persistent challenges in nickel-based alloy fabrication: achieving full penetration in thicker sections while maintaining acceptable microstructural integrity in the weld zone.
Core Technical Findings
Microstructural Analysis of the Weld Zone
The combined PAW-TIG process produces a distinct layered microstructure in the weld zone. The plasma arc root pass creates a columnar grain structure with high aspect ratio grains extending from the fusion boundary toward the weld centerline. These columnar grains are predominantly austenitic with dendritic cellular morphology, and interdendritic regions contain fine precipitates of the delta (δ) phase (Ni₃(Fe,Cr)) and sigma (σ) phase (Ni₄₃Ni₃₆Cr₄Mo₄) at the grain boundaries. The TIG overlay passes, applied on top of the plasma root, produce equiaxed grains with significantly reduced dendrite arm spacing (DAS) due to the lower linear heat input and higher cooling rate.
The transition zone between the PAW root and TIG overlay is critical. Inadequate heat input control at this interface can lead to:
- Excessive δ phase precipitation (>10% area fraction), which degrades hot corrosion resistance and ductility
- Widening of the heat-affected zone (HAZ) with sensitization susceptibility
- Residual stress concentration at the layered interface
Mechanical Property Results
| Property | PAW Root Only | TIG Overlay Only | Combined PAW+TIG | Base Metal (Annealed) |
|---|---|---|---|---|
| Tensile Strength (MPa) | 620–680 | 600–650 | 630–670 | 690–720 |
| Yield Strength (MPa) | 280–310 | 260–290 | 275–305 | 310–340 |
| Elongation (%) | 30–35 | 32–38 | 33–37 | 38–42 |
| Hardness (HV) | 180–210 | 165–190 | 170–200 | 175–195 |
| Impact Energy @ -29°C (J) | 85–110 | 95–120 | 90–115 | 120–140 |
The combined process achieves mechanical properties that closely match the base metal while maintaining the deep penetration advantage of plasma welding. The slight reduction in tensile strength (approximately 5–8% below base metal) is attributed to the residual δ phase in the weld centerline and the slight segregation of chromium and molybdenum at interdendritic regions.
Process Parameter Analysis
Critical Welding Parameters
The optimal parameter window identified in the study is summarized below:
| Parameter | PAW Root Pass | TIG Overlay Pass |
|---|---|---|
| Arc Current (A) | 80–120 | 100–160 |
| Arc Voltage (V) | 15–20 | 12–16 |
| Travel Speed (mm/min) | 150–250 | 200–350 |
| Plasma Gas Flow (L/min) | 2.0–4.0 | — |
| Shielding Gas (Ar) Flow (L/min) | 15–20 | 15–20 |
| Interpass Temperature (°C) | < 150 | < 150 |
Heat Input and Its Influence
Linear heat input (Q) is calculated as Q = U × I × η / v, where η is the arc efficiency (approximately 0.8 for PAW and 0.6 for TIG). The PAW root pass operates at a heat input of 1.8–3.2 kJ/mm, while the TIG overlay operates at 1.2–2.5 kJ/mm. The combined approach maintains total heat input within acceptable limits for Inconel 600 (recommended below 4.0 kJ/mm per pass) while achieving penetration depths of 4–6 mm per pass in plates up to 20 mm thick.
Engineering Practice Integration
Application in Pressure Vessel Fabrication
For bimetallic pressure vessels constructed per NB/T 47002 or GB/T 150, the PAW+TIG combined approach offers significant advantages:
- Penetration control: The plasma arc provides reliable full penetration in the first pass, reducing the number of layers required and minimizing the cumulative heat input
- Microstructural refinement: The TIG overlay produces finer grains in the upper weld layers, improving fatigue resistance and reducing residual stresses
- Cost efficiency: Fewer passes reduce welding consumable costs by approximately 20–30% compared to all-TIG fabrication
Common Defects and Countermeasures
| Defect Type | Root Cause | Detection Method | Countermeasure |
|---|---|---|---|
| δ phase over-precipitation | Excessive heat input, slow cooling | Metallographic examination | Reduce interpass temperature, increase travel speed |
| Hot cracking (solidification) | High sulfur/phosphorus segregation | RT, MT | Use low-sulfur filler (ERNiCrMo-3), preheat to 150°C |
| Lack of fusion at PAW-TIG interface | Inadequate TIG cleaning of root surface | UT, PAUT | Mandatory wire brushing and acid pickling between passes |
| Undercut at root | Excessive plasma arc voltage | Visual, MT | Reduce arc voltage to 15–18V, use appropriate stick-out |
| Porosity | Contamination, insufficient shielding | RT, UT | Ensure gas flow, clean base metal, use backing gas |
Standards Compliance
The fabrication must comply with the following requirements:
- NB/T 47014: Welding procedure qualification for nickel-based alloy overlay and welding
- ASME IX: Qualification of welding procedures and welders
- ASTM A263/A264: Clad plate specifications for nickel-alloy overlays
- JB/T 4730: Non-destructive testing methods and acceptance criteria
- API 934: Welding of nickel and nickel alloy clad plate
Preheat and Post-Weld Treatment
For Inconel 600 components thicker than 12 mm, preheating to 100–150°C is recommended to reduce residual stresses and prevent cold cracking. Post-weld heat treatment (PWHT) at 1040–1120°C for 1–4 hours followed by air cooling is specified for stress relief and dissolution of any precipitated phases. However, excessive PWHT temperatures (>1150°C) should be avoided to prevent grain coarsening and sensitization.
Key Questions and Reflections
The most significant insight from this study is the recognition that the PAW+TIG combination is not merely a sequential process but requires careful coordination of thermal cycles. The cooling rate from the PAW root pass creates a thermal gradient that directly influences the grain nucleation behavior in the subsequent TIG passes. If the interpass time is too short, the residual heat from the PAW pass raises the effective starting temperature for the TIG pass, leading to wider dendrite spacing and increased δ phase formation. Conversely, excessive interpass cooling may introduce thermal shock at the interface.
A practical consideration that the study does not fully address is the effect of joint geometry on the combined process. For T-joints and fillet welds, the plasma arc's concentrated heat may cause excessive burn-through, while the TIG overlay may not provide adequate reinforcement. Future research should explore the applicability of this combined approach to complex geometries typical of pressure vessel fabrication, such as manway nozzles, tube-to-head joints, and saddle supports.
Study Insights and Implications
The PAW+TIG combined approach represents a practical solution to the fundamental trade-off between penetration depth and microstructural quality in nickel-based alloy welding. For engineers designing cladding procedures for hydrogenation reactors or high-temperature heat exchangers, this methodology offers a pathway to reduce the number of welding passes while maintaining mechanical integrity. The key to successful implementation lies in strict control of interpass temperature, systematic cleaning between passes, and comprehensive non-destructive examination following each layer. Engineers should treat the PAW-TIG interface as a critical zone requiring special attention during both fabrication and quality assurance, and should document all process parameters with sufficient detail to enable traceability and qualification under applicable codes.