Study Note on Double Tungsten Electrode Automatic TIG Overlay Welding of Nickel-Based Alloy on Tube Sheets

Literature Overview

This paper, authored by Zheng Mingtao, Wang Li, Zhou Xiaolong, Sui Na, and Liu Kai from Harbin Electric Group (Qinhuangdao) Heavy Equipment Co., Ltd. and Guoneng United Power Technology (Chifeng) Co., Ltd., published in 2024, addresses a critical challenge in pressure vessel fabrication: the overlay welding of nickel-based alloys onto tube sheets using a novel double-tungsten-electrode automatic TIG (GTAW) process. The work targets hydrogenation reactors and high-pressure equipment where the tube sheet must withstand both high mechanical loads and severe corrosion environments. The dual-electrode configuration represents a significant advancement over conventional single-electrode GTAW overlay methods, offering improved deposition efficiency, reduced dilution, and enhanced metallurgical compatibility between the base carbon steel and the nickel-based overlay layer.

Core Technical Content and Process Analysis

The fundamental challenge in tube sheet overlay welding is achieving a thick, corrosion-resistant nickel-based alloy layer while maintaining structural integrity at the bond line. Conventional single-electrode TIG overlay welding suffers from low deposition rates, excessive dilution of the overlay material by the base metal, and difficulty in achieving uniform multi-layer coverage over large tube sheet surfaces. The double-tungsten-electrode approach resolves these issues by simultaneously feeding two filler wires and energizing two tungsten electrodes, effectively doubling the heat input and deposition rate while maintaining the precision and clean weld quality characteristic of GTAW.

The nickel-based alloy overlay typically employs materials such as Inconel 625 (UNS N06625), Inconel 600 (UNS N06600), or Hastelloy C276 (UNS N10276), depending on the service environment. For hydrogenation reactor tube sheets exposed to high-pressure hydrogen at elevated temperatures, Inconel 625 is frequently selected due to its excellent resistance to hydrogen embrittlement, intergranular corrosion, and stress corrosion cracking. The double-electrode TIG process enables deposition of overlay layers with thicknesses exceeding 3 mm in multiple passes, which is essential for meeting the minimum overlay thickness requirements specified in standards such as NB/T 47014 and ASME IX.

Metallurgical Considerations

The bond line between the carbon steel base and the nickel-based overlay is the most critical region from a metallurgical standpoint. During the first pass of overlay welding, the dilution ratio between the base metal and the filler material must be carefully controlled. Excessive dilution introduces carbon equivalents that promote brittle intermetallic compound formation at the bond line, while insufficient dilution leads to poor mechanical bonding. The double-electrode configuration allows independent adjustment of heat input distribution, enabling the operator to optimize the first-pass dilution ratio to typically 20–35%, which falls within the acceptable range for achieving good metallurgical bonding without excessive brittle phase formation.

The heat-affected zone (HAZ) in the base carbon steel is another concern. The reduced interpass temperature requirement of the double-electrode process helps limit the HAZ width and minimizes the formation of hard, brittle martensite in high-carbon equivalents steels. For low-alloy steels such as 16MnR or SA-516 Gr.70 commonly used in pressure vessel tube sheets, the controlled heat input of the dual-electrode GTAW process helps maintain the toughness and ductility of the base material.

Standards Compliance and Quality Assurance

The overlay welding process described in this paper must comply with multiple standards governing both the welding procedure and the final product. The welding procedure qualification follows NB/T 47014 (for Chinese pressure vessels) or ASME Section IX (for ASME-coded vessels), which require demonstration of mechanical properties, macrographic and micrographic examination, and hardness testing of the overlay layer and bond line. The final tube sheet assembly must meet the requirements of GB/T 150 or ASME VIII Division 1, including non-destructive examination of the overlay welds.

The paper's emphasis on automation is noteworthy from a quality assurance perspective. Automatic TIG overlay welding provides superior repeatability compared to manual welding, reducing operator variability and ensuring consistent bead geometry, dilution ratios, and interpass temperature control. This is particularly important for large tube sheets where hundreds of welding passes may be required to achieve the specified overlay thickness.

Engineering Practice Implications

The application of this technology in hydrogenation reactor fabrication carries significant economic and safety implications. Hydrogenation reactors operate at pressures exceeding 20 MPa and temperatures above 350 °C, with the hydrogen environment posing severe risks of hydrogen blistering, hydrogen cracking, and high-temperature hydrogen attack. The nickel-based overlay layer serves as the primary barrier against these degradation mechanisms. The double-electrode TIG process offers a practical solution for fabricating tube sheets that combine the economic advantages of carbon steel construction with the corrosion resistance of nickel-based alloys.

From a fabrication planning perspective, the process requires careful consideration of the tube sheet geometry, including tube hole spacing, edge distance, and the interaction between the overlay welds and the tube-to-tubesheet welded joints. The overlay welding sequence must be planned to minimize distortion and residual stress, particularly for large-diameter tube sheets where dimensional control is critical for subsequent tube insertion and welding operations.

Study Insights and Reflections

This paper represents a meaningful contribution to the advancement of overlay welding technology for pressure vessel applications. The transition from single-electrode to double-electrode automatic TIG welding addresses a genuine industrial bottleneck: the deposition rate limitation of conventional GTAW overlay processes. For large tube sheets requiring 3–5 mm of nickel-based alloy overlay, the time savings and improved quality consistency of the dual-electrode approach are substantial. The research also highlights an important trend in pressure vessel fabrication: the increasing use of automated and semi-automated welding processes to improve productivity, consistency, and quality assurance.

The work underscores the importance of process optimization in achieving the desired metallurgical outcome. The interplay between heat input, dilution ratio, interpass temperature, and layer thickness determines whether the overlay weld will perform reliably in service. Engineers involved in specifying and qualifying such processes must pay close attention to the qualification parameters and ensure that the production welding parameters remain within the qualified envelope. The double-electrode TIG overlay welding process described here offers a promising path toward more efficient and reliable fabrication of critical pressure vessel components in the petrochemical and hydrogen energy industries.