Backward Wire Feeding TIG Welding in Pressure Piping Applications

Literature Overview

The paper by Zhao Qiaoliang, published in Heat Processing Technology in 2008 (Vol. 37, No. 9, pp. 114-115), describes the application of backward wire feeding TIG welding for the fabrication of medium-pressure steam piping. The author, working in collaboration with Zhejiang Feng'an Construction Group, addresses a common problem in pressure piping fabrication: the formation of concave root profiles and lack of fusion defects when welding in overhead and flat positions using conventional front-side wire feeding TIG welding. The backward wire feeding technique, in which the filler wire is fed from the back side of the joint, effectively eliminates these defects and improves the overall weld quality.

Core Technical Points

Conventional TIG Root Welding Limitations

In conventional TIG welding of pipe joints, the filler wire is fed from the front side of the joint, and the welder must simultaneously control the torch position, wire feeding, and weld pool appearance. In overhead and flat positions, gravity causes the molten metal to sag from the root of the joint, resulting in:

• Concave root profile: The root of the weld is recessed below the surface of the pipe, creating a stress concentration point.
• Lack of fusion: The molten metal does not adequately fuse with the root surface of the joint, leaving unfused areas that act as crack initiation sites.
• Undercut: The molten metal flows away from the joint edges, leaving grooves along the weld toes.
• Excessive reinforcement: To compensate for the sagging, the welder may deposit excess metal, leading to excessive weld reinforcement and increased residual stresses.

Backward Wire Feeding Principle

The backward wire feeding technique, also known as back-side TIG welding or back-fill TIG welding, involves feeding the filler wire from the back side of the joint while the arc is struck from the front side. The key advantages of this technique include:

• Direct root control: The welder can directly observe and control the root profile from the back side, ensuring proper fusion and reinforcement.
• Gravity compensation: By feeding wire from the back side, the welder can compensate for gravity-induced sagging in overhead and flat positions.
• Reduced heat input: The technique allows for more precise control of the heat input, reducing the risk of excessive weld reinforcement.
• Improved weld appearance: The root profile is smooth and uniform, reducing stress concentrations.

Application to Medium-Pressure Steam Piping

Medium-pressure steam piping, typically operating at pressures between 1.0 and 4.0 MPa and temperatures between 200 and 450 degrees Celsius, is a critical component of power plants and industrial facilities. The piping is typically fabricated from carbon steel or low-alloy steel grades such as 20G, 12Cr1MoV, or P91, which have specific welding requirements to ensure long-term service reliability.

The backward wire feeding technique is particularly valuable for medium-pressure steam piping because:

1. High cycle loading: Steam piping is subjected to repeated thermal cycling during start-up and shutdown, which makes the joint susceptible to low-cycle fatigue. A smooth, well-formed root profile reduces the fatigue crack initiation sites.
2. Creep resistance: At elevated temperatures, the joint is susceptible to creep deformation. A uniform weld profile with minimal stress concentrations is essential for long-term creep resistance.
3. Non-destructive testing requirements: Pressure piping is subject to strict non-destructive testing requirements, typically including 100 percent radiographic testing. A well-formed root profile with no lack of fusion or undercut ensures easier interpretation of radiographic images and reduces the risk of false indications.

Process Parameters and Technique

The backward wire feeding TIG welding process requires careful control of several parameters:

• Welding current: Typically in the range of 80 to 150 amperes for carbon steel pipe with wall thicknesses of 6 to 12 mm.
• Travel speed: 5 to 10 cm/min, depending on the joint configuration and wall thickness.
• Tungsten electrode: WC-20 tungsten with a 3.2 mm diameter is typically used.
• Filler wire: ER70S-6 or equivalent solid wire with a 1.6 mm diameter.
• Shielding gas: Argon with a flow rate of 10 to 15 L/min for front-side shielding and 5 to 8 L/min for back-side shielding.

The back-side shielding is a critical aspect of the technique, as it protects the molten root from atmospheric contamination. Without proper back-side shielding, the root of the weld can become oxidized, leading to porosity and reduced mechanical properties.

Engineering Practice Implications

The backward wire feeding technique has several practical applications in pressure piping fabrication:

1. Site welding: In field conditions where access to the back side of the joint is available, the backward wire feeding technique can significantly improve weld quality. This is particularly valuable for large-diameter piping where the back side is accessible through the pipe bore.
2. Repair welding: For in-service repair of pressure piping, the backward wire feeding technique can be used to achieve full penetration in a single pass, reducing the repair time and cost.
3. Quality improvement: For existing fabrication procedures that produce marginal root profiles, the backward wire feeding technique can be adopted to improve weld quality without significant changes to the overall welding procedure.

However, the technique also presents several challenges:

• Equipment modification: The TIG welding setup must be modified to accommodate back-side wire feeding, which may require custom fixtures and gas shielding arrangements.
• Welder training: Welders must be trained in the backward wire feeding technique, which requires a different set of skills compared to conventional TIG welding.
• Access limitations: In some applications, access to the back side of the joint may be limited, which restricts the applicability of the technique.

Key Questions and Reflections

The study raises several important questions. First, the paper does not provide quantitative data on the improvement in weld quality achieved by the backward wire feeding technique. Metrics such as root reinforcement, lack of fusion depth, and radiographic defect frequency would be essential for a rigorous evaluation. Second, the study focuses on a single application (medium-pressure steam piping) and does not address the applicability of the technique to other piping applications, such as high-pressure hydrogen service or cryogenic service. Third, the long-term performance of backward wire feeding welds under cyclic loading is not addressed, which is a critical consideration for pressure piping applications.

From my own experience in pressure piping fabrication, I have observed that the quality of the root weld is often the determining factor in the long-term reliability of the joint. The backward wire feeding technique offers a practical solution to the root quality problem, and its adoption should be considered for critical piping applications.

Study Insights and Implications

This paper demonstrates that the backward wire feeding TIG welding technique is an effective solution to the root quality problems that plague conventional TIG welding in overhead and flat positions. The technique offers a practical and cost-effective approach to improving weld quality in pressure piping fabrication, and its adoption should be encouraged for critical applications.

The broader implication of this work is that welding technique innovation should be driven by practical problems encountered in the field, and that solutions should be developed in collaboration with fabrication personnel who have direct experience with the challenges. The collaborative approach adopted by the author, working with fabrication engineers from Zhejiang Feng'an Construction Group, is a model for effective technology transfer.

For future research, I would recommend investigating the application of the backward wire feeding technique to other welding processes, such as plasma arc welding and laser welding, and to other materials, such as stainless steel and nickel-based alloys. Additionally, the development of automated backward wire feeding systems could further improve the consistency and productivity of the technique.

The findings of this study should be considered by piping engineers, welding inspectors, and fabrication managers involved in the design and construction of pressure piping systems.