Multi-Pulse One-Pass TIG Automatic Welding of Economizer Tubes

Literature Overview

This paper by Wang Jian, published in Welding journal (1991, Vol. 8, pp. 19-21) from Wuxi Boiler Factory, documents the application of multi-pulse one-pass TIG automatic welding for the production of 25,000 economizer tubes in boiler manufacturing. The tubes have specifications of φ32 mm × 4 mm, φ42 mm × 4 mm, and φ42 mm × 5 mm. The paper reports that the process was validated through nearly two years of production line practice, demonstrating its reliability and practicality.

Background and Technical Context

Economizer tubes are critical components in boiler systems, where they preheat feedwater by absorbing heat from the flue gases. These tubes operate under high temperatures (up to 450°C), high pressures (up to 25 MPa), and cyclic thermal loading, making the quality of the circumferential welds critical for long-term reliability. The tubes are typically made of carbon steel or low-alloy steel (such as 20G or 12Cr1MoV), and the welds must withstand the same conditions as the base material.

Traditional TIG welding of tube joints often requires multiple passes to achieve full penetration and adequate weld reinforcement, particularly for tubes with wall thicknesses of 4-5 mm. This multi-pass approach increases production time, labor costs, and the risk of defects such as interpass oxidation, porosity, and incomplete fusion. The multi-pulse one-pass TIG welding process addresses these challenges by using a carefully designed pulse waveform to achieve full penetration and adequate weld geometry in a single pass.

Multi-Pulse Process Principles

The multi-pulse TIG welding process uses a pulse waveform with multiple segments, each with different current levels and durations. The pulse waveform can be designed to achieve several objectives simultaneously:

1. High peak current: Provides the energy necessary for deep penetration into the tube joint.
2. Low base current: Allows the weld pool to solidify between pulses, preventing excessive sagging and distortion.
3. Rapid current transitions: Enable precise control of the weld pool shape and size.
4. Tailored pulse sequence: The pulse waveform can be optimized for specific material thicknesses and joint geometries.

The key advantage of the multi-pulse approach is that it can achieve the penetration of a high-current weld while maintaining the quality characteristics of a low-current weld. This is achieved by using a series of short, high-current pulses for penetration, followed by lower-current pulses for filling and finishing.

Process Parameters and Optimization

The optimization of these parameters is critical for achieving consistent, high-quality welds. The authors do not provide specific parameter values in the abstract, but the successful production of 25,000 tubes over two years suggests that a robust process window was established.

Key optimization considerations include:

• Penetration depth: Must be sufficient to achieve full fusion of the tube walls without excessive burn-through.
• Weld reinforcement: Must meet the specifications for the boiler application, typically 1-2 mm above the tube surface.
• Weld width: Must be consistent to ensure uniform mechanical properties and avoid stress concentrations.
• Surface quality: Must be free of spatter, oxidation, and other surface defects.

Quality Control and Production Experience

The two-year production line practice is a strong indicator of the process's reliability. For a production run of 25,000 tubes, the quality control measures likely included:

1. Visual inspection (VT): 100% inspection of all welds for surface quality and geometry.
2. Radiographic testing (RT): Random sampling or 100% inspection for volumetric defects.
3. Mechanical testing: Tensile tests and bend tests on coupon specimens to verify weld strength and ductility.
4. Hydrostatic testing: Pressure testing of each tube to verify weld integrity under operating conditions.

The successful production of 25,000 tubes without major quality issues suggests that the process was well-controlled and that the equipment was reliable. The multi-pulse TIG welding process, being automated, provides consistent parameter control, which is a significant advantage over manual welding in terms of quality consistency.

Engineering Practice Implications

The application of multi-pulse one-pass TIG welding to economizer tube production demonstrates several important engineering principles:

1. Process innovation: The multi-pulse approach represents a significant improvement over conventional TIG welding for tube joints, reducing production time and improving quality.
2. Automation: The use of automatic welding equipment ensures consistent parameter control, which is essential for high-volume production.
3. Scalability: The process was successfully applied to three different tube specifications, demonstrating its flexibility and adaptability.
4. Long-term reliability: The two-year production track record provides confidence in the process's long-term performance.

Study Insights and Reflections

This paper, although published in 1991, represents an early application of advanced TIG welding technology to industrial production. The multi-pulse one-pass approach was ahead of its time, and its principles remain relevant in modern welding practice. The successful production of 25,000 tubes over two years is a testament to the robustness of the process and the effectiveness of the quality control measures. In today's context, similar multi-pulse TIG welding processes are widely used in the production of heat exchanger tubes, boiler tubes, and other critical components, and the principles established in this paper continue to guide process development and optimization.

Summary and Conclusions

These five studies collectively illustrate the breadth and depth of TIG welding technology across diverse engineering applications. From the dissimilar steel joining of hydro turbine runners to the advanced hybrid welding of aluminum-steel joints, from numerical simulation of arc-laser interactions to corrosion behavior analysis of pipeline welds, and from industrial-scale tube welding to process innovation, each paper contributes valuable insights to the field of welding engineering. The common thread is the emphasis on process control, metallurgical understanding, and quality assurance, which are the cornerstones of successful welding practice. Engineers should draw upon these studies to inform their own work, recognizing that the principles of sound welding science remain timeless, even as technology evolves.