Microstructure and Mechanical Properties of Inconel 600 PAW Plus TIG Weld Joints Technical Study Note

Literature Overview

This 2024 publication from the Journal of Welding, authored by researchers from Lanzhou Lanshi Heavy Equipment Co., Ltd., Lanzhou University of Technology, and the Chinese Academy of Sciences, investigates a hybrid welding process combining plasma arc welding (PAW) with tungsten inert gas (TIG) welding for Inconel 600 nickel-based superalloy. Funded by Gansu Provincial Key R&D Programs, this research addresses a critical need in the fabrication of high-performance pressure vessels and heat exchangers used in hydrogenation, refining, and chemical processing industries.

Core Technical Content

Hybrid PAW+TIG Process Rationale

The combination of PAW and TIG welding in a single process leverages the complementary advantages of both techniques. PAW provides deep, narrow penetration suitable for root pass welding, while TIG offers excellent arc stability and bead appearance for cap passes. For thick-section Inconel 600 weldments, this hybrid approach can significantly reduce welding time and heat input compared to using either process alone.

Inconel 600 Metallurgical Challenges

Inconel 600 (UNS N06600) is a nickel-chromium-iron alloy with excellent corrosion resistance in oxidizing and reducing environments. However, it presents significant welding challenges:

• High hot cracking susceptibility due to dendritic solidification and segregation of chromium and molybdenum
• Sensitivity to intergranular corrosion from chromium carbide precipitation at grain boundaries
• Low thermal conductivity leading to high residual stresses and distortion
• Limited solid solubility of carbon, requiring careful filler metal selection

The PAW+TIG hybrid process must be carefully optimized to minimize these risks while achieving the required weld quality for pressure vessel applications.

Microstructural Evolution

The weld metal microstructure of Inconel 600 is typically columnar dendritic with secondary phases including Ni3Nb (gamma prime), NiCr (mu phase), and carbides. The PAW process, with its higher energy density and faster cooling rates, tends to produce finer dendrites and reduced secondary phase formation compared to conventional TIG welding. This is beneficial for mechanical properties but may require post-weld heat treatment (PWHT) to ensure adequate toughness.

Process Parameters and Optimization

Recommended Welding Parameters

Based on the study and practical experience with Inconel 600 welding, the following parameter ranges are recommended:

• PAW root pass: Current 100-150 A, voltage 25-30 V, travel speed 5-10 cm/min, arc length 2-4 mm
• TIG cap pass: Current 80-120 A, voltage 18-22 V, travel speed 8-15 cm/min, arc length 2-3 mm
• Shielding gas: Pure argon or 95% Ar / 5% H2 (reducing atmosphere)
• Backing gas: Pure argon or helium
• Interpass temperature: Below 150°C to minimize grain boundary sensitization

Defect Prevention Strategies

Quality Assurance and Standards Compliance

Applicable Standards

The fabrication of Inconel 600 pressure vessels must comply with relevant codes and standards:

• ASME VIII Div. 1 or Div. 2 for pressure vessel design and fabrication
• ASME IX for welding procedure qualification
• ASTM B166 for Inconel 600 plate specifications
• ASTM A263 for Inconel 600 weld overlay specifications
• NB/T 47014 for welding procedure qualification (Chinese standard)
• JB/T 4730 for non-destructive testing

Inspection Requirements

Engineering Practice Integration

In hydrogenation reactor fabrication, Inconel 600 is commonly used as the cladding material on carbon steel or low-alloy steel shells to provide hydrogen attack resistance. The PAW+TIG hybrid process can be used for the initial bonding of the cladding layer, providing a strong metallurgical bond with minimal dilution. Subsequent overlay passes can be applied using GTAW or FCAW to build up the required cladding thickness.

For heat exchanger applications, Inconel 600 tubes are frequently brazed or welded to carbon steel or stainless steel headers. The PAW+TIG hybrid process offers an alternative to brazing for thicker sections, providing a stronger joint with better thermal fatigue resistance.

Key Questions and Reflections

The primary technical question is whether the hybrid PAW+TIG process can achieve weld quality equivalent to all-TIG welding while offering significant productivity improvements. The answer depends on the ability to maintain consistent arc characteristics during the transition between PAW and TIG modes, and to avoid defects at the interface between the two weld zones.

Another important consideration is the effect of the hybrid process on the final mechanical properties of the weld joint. The different cooling rates and solidification conditions in the PAW and TIG zones may lead to property variations that affect the overall joint performance, particularly under cyclic loading or thermal fatigue conditions.

Study Insights and Implications

The PAW+TIG hybrid welding process represents a significant advancement in nickel-based superalloy fabrication technology. The key insight is that combining two established processes can yield synergistic benefits that neither process can achieve alone. For pressure vessel manufacturers, this technology offers the potential to reduce fabrication costs while maintaining or improving weld quality.

Engineers should carefully evaluate the applicability of this hybrid process for their specific applications, considering factors such as section thickness, joint geometry, and service conditions. The development of qualified welding procedures and the training of welders in this technique are essential steps before production implementation. The study provides valuable guidance for process development and quality assurance in the fabrication of high-performance nickel-based alloy components.