Zhuojin Pipe Fitting Co., LtdManual TIG Filler Wire Welding of Duplex Stainless Steel Tube Sheets
Literature Overview
The paper by Wen Mingke and Ji Qiang, published in Hot Working Technology (2009, Vol. 38, No. 15, pp. 136-137), investigates the manual TIG welding with filler wire of duplex stainless steel tube sheets in heat exchanger assemblies. The authors are affiliated with CNOOC Petrochemical Refining Co., Ltd. Huizhou Refining Branch and Fushun Chemical Machinery Equipment Manufacturing Co., Ltd., respectively. This study is practically significant because duplex stainless steel tube sheets are increasingly used in the petrochemical industry for their superior corrosion resistance and mechanical properties, yet welding these components requires careful control of the ferrite-austenite phase balance to maintain the desired properties.
Duplex Stainless Steel Welding Challenges
Duplex stainless steels derive their unique properties from the approximately equal balance of ferrite and austenite phases. This phase balance is critical because it provides a combination of high strength, excellent resistance to chloride stress corrosion cracking (SCC), and good resistance to pitting and crevice corrosion. The following table summarizes the key challenges and solutions for duplex stainless steel tube sheet welding:
| Challenge | Risk | Solution |
|---|---|---|
| Phase balance disruption | Loss of SCC resistance, reduced strength | Controlled heat input, appropriate filler metal |
| Ferrite coarsening | Reduced toughness | Limit interpass temperature, minimize HAZ dwell time |
| Sigma phase formation | Brittleness, reduced corrosion resistance | Rapid cooling, avoid prolonged exposure to 600-900°C |
| Element burn-off | Altered composition, phase imbalance | Shielding gas control, filler metal selection |
| Cracking susceptibility | Weld defects, rework | Preheating control, low hydrogen procedure |
The tube sheet welding application adds complexity because the weld joint connects the tube sheet to the tubes, creating a dissimilar joint in terms of geometry and potentially in terms of material. The tube sheet is typically a thick section with clad or solid duplex stainless steel, while the tubes may be made of the same or different duplex grade. The weld must accommodate differences in thermal expansion and must resist the combined mechanical and corrosion loading of the heat exchanger service.
Welding Procedure and Results
The manual TIG welding with filler wire procedure was designed to maintain the ferrite-austenite phase balance in the weld metal and HAZ. The key results reported in the paper are summarized as follows:
- Penetrant testing (PT): No macroscopic cracks were detected in the welds, indicating sound weld formation without cracking susceptibility.
- Microstructural examination: The ferrite-to-austenite ratio in the weld metal and HAZ was within the acceptable range specified by technical specifications, confirming that the phase balance was maintained.
- SEM line analysis: Elemental distribution across the weld was within reasonable ranges, with minimal element burn-off during welding. This is particularly important for the alloying elements (Cr, Ni, Mo) that govern the phase balance and corrosion resistance.
- Hardness testing: Hardness values were within the specified limits, with no evidence of martensitic transformation or excessive hardening in the HAZ.
The absence of quench-hardened martensite in the HAZ is a significant finding because it indicates that the heat input was sufficient to allow austenite formation but not so high as to cause excessive grain growth or sigma phase precipitation. The controlled cooling rate achieved through manual TIG welding—with its relatively low heat input and the ability to adjust parameters based on visual feedback—proved to be an effective approach for maintaining the duplex microstructure.
Engineering Practice Implications
The successful application of manual TIG welding with filler wire to duplex stainless steel tube sheets has several important implications for engineering practice. First, it demonstrates that manual welding can achieve the quality required for duplex stainless steel applications, provided that the welder is properly trained and the procedure is carefully controlled. This is particularly relevant for field welding applications where automatic welding equipment may not be practical. Second, the use of filler wire with appropriate composition is essential for maintaining the phase balance; the filler metal should typically be slightly higher in nickel than the base metal to compensate for the preferential evaporation of nickel during welding. Third, the interpass temperature must be carefully controlled—typically limited to below 150°C for duplex stainless steels—to prevent sigma phase formation and ferrite coarsening.
For heat exchanger manufacturers, the ability to reliably weld duplex stainless steel tube sheets opens up opportunities for extending equipment service life in aggressive chemical environments. Duplex stainless steels such as UNS S31803 (2205) and UNS S32750 (2507) offer significantly better chloride SCC resistance than conventional 304L or 316L austenitic stainless steels, making them attractive for applications in seawater cooling, chemical processing, and oil and gas production. The weld quality directly determines whether the corrosion resistance advantage of the base material is maintained at the joint.
Key Questions and Reflections
Several important considerations emerge from this study. First, the paper does not extensively discuss the long-term corrosion performance of the welds, which is critical for duplex stainless steel applications where the entire value proposition rests on superior corrosion resistance. Accelerated corrosion testing—such as ASTM G48 pitting and crevice corrosion tests, ASTM G55 stress corrosion cracking tests, and ASTM G150 HIC testing—should be conducted to validate the long-term performance of the welds. Second, the effect of weld geometry on stress concentration and fatigue performance should be evaluated, particularly for applications subject to cyclic pressure loading. Third, the consistency of manual welding results across different operators and shifts should be assessed, as this is a common concern in production environments.
A critical reflection is that while manual TIG welding offers flexibility and adaptability, it also introduces variability that must be managed through rigorous quality control. The welder's skill, the condition of the equipment, and the environmental conditions all influence the weld quality. For critical applications, it may be necessary to supplement manual welding with post-weld inspections including UT, PT, and possibly hydrostatic testing to ensure that the welds meet the required quality standards.
Study Insights and Outlook
The research by Wen Mingke and Ji Qiang provides practical validation that manual TIG welding with filler wire can produce acceptable welds in duplex stainless steel tube sheet applications. The systematic approach to evaluating weld quality—combining NDT, metallographic examination, elemental analysis, and mechanical testing—serves as a model for quality assurance procedures in similar applications. As the petrochemical industry continues to adopt duplex stainless steels for their superior performance in aggressive environments, the development of reliable welding procedures for tube sheet assemblies will be increasingly important. Future work should focus on extending the service life assessment of duplex stainless steel welds through long-term corrosion testing and accelerated aging studies, and on developing automated welding procedures that can replicate the quality achieved through skilled manual welding with greater consistency and productivity.