TIG Welding of High-Temperature Alloy GH3030 - Study Note

Literature Overview

The 1999 paper by Wei Xinhua, published in Welding (焊接), addresses the TIG welding of GH3030, a nickel-chromium-iron-based superalloy developed for high-temperature service in aggressive environments. This alloy, equivalent to Alloy 600 in the Western designation system, is widely used in chemical processing equipment, heat exchangers, and pressure vessel components where resistance to oxidation, corrosion, and thermal cycling at elevated temperatures is required.

The paper is particularly relevant to engineers involved in the fabrication of hydrogenation reactors, high-temperature heat exchangers, and other critical equipment where nickel-based alloy overlays or full nickel alloy construction is specified. Understanding the welding behavior of GH3030/Alloy 600 is essential for ensuring the integrity of these components throughout their service life.

Material Characteristics and Weldability

GH3030 (Alloy 600) is a solid-solution strengthened nickel-chromium-iron alloy with the following typical composition:

The alloy exhibits excellent resistance to oxidation and corrosion in a wide range of environments, including organic acids, molten salts, and reducing atmospheres at temperatures up to 1000°C. However, its weldability presents several challenges:

1. Sensitivity to hot cracking: The alloy is susceptible to solidification cracking (hot cracking) in the weld metal, particularly when sulfur and phosphorus impurities are present. The crack susceptibility is related to the wide solidification range and the tendency to form low-melting eutectics at grain boundaries.
2. Creep rupture susceptibility: Welded joints in GH3030 may exhibit reduced creep rupture strength compared to the base metal, particularly at temperatures above 650°C. This is related to grain growth in the heat-affected zone and the absence of precipitation hardening in the weld metal.
3. Intergranular corrosion: While GH3030 has good general corrosion resistance, sensitization (chromium carbide precipitation at grain boundaries) can occur during welding if the heat input is excessive or the cooling rate is too slow.
4. Thermal cracking: The high thermal expansion coefficient (13.1 × 10⁻⁶ /°C) and low thermal conductivity (19 W/m·K) create high thermal stresses during welding, increasing the risk of thermal cracking.

Welding Procedure Development

Pre-Weld Preparation

Proper preparation is critical for successful TIG welding of GH3030:

Welding Parameters

The paper describes typical TIG welding parameters for GH3030:

Critical Process Variables

The following variables have the most significant impact on weld quality:

1. Back purge: GH3030 is highly susceptible to oxidation at elevated temperatures. A continuous back purge of high-purity argon (99.999%) is essential to prevent oxidation of the weld root and the back side of the joint. Inadequate purge results in a dark, oxidized weld root that is prone to intergranular corrosion and cracking.
2. Heat input control: Excessive heat input promotes grain growth, sensitization, and hot cracking. The heat input should be kept to the minimum required for full penetration. For multi-pass welding, interpass temperature must be strictly controlled.
3. Filler wire selection: Using Alloy 600 equivalent filler wire (ERNiCrFe) is standard for GH3030 welding. However, for applications requiring higher strength or resistance to specific corrosion environments, Alloy 617 (ERNiCrFe-18-12) or Alloy 625 (ERNiCrMo-3) may be specified.
4. Travel speed consistency: Variations in travel speed lead to variations in heat input, which can cause localized sensitization or hot cracking. Manual TIG welding requires the welder to maintain a consistent travel speed, which is challenging over long weld lengths.

Defect Analysis and Countermeasures

Engineering Practice and Standards Compliance

For pressure vessel applications, the welding of GH3030/Alloy 600 must comply with the following standards:

The welding procedure qualification (WPS/PQR) must include:

• Essential variables: Electrode type, filler metal type, preheat temperature, interpass temperature, current range, travel speed range, gas flow rate, gas type.
• Performance requirements: Tensile strength, elongation, hardness, bend tests (face, side, root), and corrosion resistance tests if required.
• Acceptance criteria: No hot cracks, no porosity exceeding ASME Section V acceptance criteria, no lack of fusion, acceptable intergranular corrosion resistance.

Case Study: Hydrogenation Reactor Shell Lining

In a recent project involving the fabrication of a hydrogenation reactor shell with GH3030 overlay, the following approach was adopted:

1. Base metal: SA-516 Gr.70 carbon steel shell, 32 mm thick.
2. Overlay material: GH3030 (Alloy 600), 6 mm thick, applied by multi-pass TIG welding.
3. Welding procedure: Qualified per NB/T 47014, with parameters optimized for minimum dilution and maximum overlay thickness.
4. Dilution control: The first pass (tie-in pass) was welded with a low-current, high-speed technique to minimize dilution. Subsequent passes were welded with progressively higher current to build up the overlay thickness.
5. Post-weld treatment: Solution heat treatment at 1100°C for 1 hour, followed by air cooling, to dissolve any precipitated carbides and restore corrosion resistance.

The overlay thickness was verified by magnetic thickness measurement, with a minimum of 4.5 mm achieved (specification: ≥4.0 mm). Intergranular corrosion testing per ASTM A263 confirmed acceptable corrosion resistance.

Key Technical Challenges and Solutions

1. Dilution control: The dilution of the overlay by the base metal is a critical concern. For GH3030 overlay on carbon steel, dilution of 25–35% is typical and acceptable. Excessive dilution (>50%) significantly reduces corrosion resistance. The dilution can be controlled by:

• Using a tie-in pass with minimum penetration
• Building up the overlay in multiple passes
• Using a dilution-resistant filler (Alloy 625 instead of Alloy 600 for high-dilution applications)

1. Residual stress management: The high thermal expansion coefficient of GH3030 creates significant residual stresses during welding. These stresses can lead to distortion and stress-corrosion cracking in service. Mitigation strategies include:

• Symmetric welding sequence to balance thermal input
• Stress relief heat treatment (650°C for 2 hours, or as specified)
• Use of low-restraint welding sequences

1. Hydrogen-induced cracking: Although GH3030 itself is not susceptible to hydrogen-induced cracking, the carbon steel base metal may be susceptible if the overlay weld introduces hydrogen. Preheating and post-weld baking are recommended for thick sections.

Study Insights and Implications

The paper by Wei Xinhua provides a practical guide to the TIG welding of GH3030, addressing the key challenges of hot cracking, oxidation, and dilution control. For engineers involved in the fabrication of nickel-based alloy lined pressure vessels, the paper's recommendations on back purge, heat input control, and filler wire selection are directly applicable.

The paper also highlights the importance of post-weld heat treatment in restoring corrosion resistance. In many fabrication shops, post-weld treatment is neglected or performed inadequately, leading to corrosion failures in service. Engineers must ensure that the fabrication specification includes appropriate post-weld treatment requirements.

In conclusion, the TIG welding of GH3030/Alloy 600 requires careful attention to material preparation, welding parameters, back purge, and post-weld treatment, and the principles described in this paper remain essential for ensuring the long-term integrity of nickel-based alloy lined pressure vessels and heat exchangers operating in aggressive high-temperature environments.