TIG Welding Process Trials for Thin-Plate Dissimilar Steel Spherical Vessels

Literature Overview

This 1989 study by Jiang Youqing from Huazhong University of Science and Technology addresses one of the most challenging problems in pressure vessel fabrication of that era: the TIG welding of thin-plate dissimilar steel spherical vessels. The research emerged from the practical need to join incompatible material systems—typically carbon steel or low-alloy steel on one side and stainless steel or nickel-based alloys on the other—while maintaining structural integrity and corrosion resistance. Spherical vessels, used extensively in chemical storage and cryogenic service, demand exceptional weld quality due to their geometric complexity and the stress concentrations inherent in their doubly curved surfaces. The study represents an early systematic investigation into the metallurgical and process challenges of dissimilar steel joining in thin sections using gas tungsten arc welding.

Core Technical Challenges and Metallurgical Considerations

The fundamental difficulty in dissimilar steel TIG welding lies in the vast difference in thermal expansion coefficients, thermal conductivity, and melting behavior between the base metals. When joining carbon steel to austenitic stainless steel in thin plate (typically 3–8 mm thickness), the heat input must be carefully controlled to prevent excessive dilution of the stainless side, which would compromise corrosion resistance, or to avoid cracking on the carbon steel side due to high carbon equivalents and restricted cooling rates.

The following table summarizes the key metallurgical concerns and corresponding process responses identified in the study:

The study demonstrated that the weld sequence for spherical vessels must account for the unique stress state created by the spherical geometry. Unlike cylindrical vessels, spherical shells experience biaxial membrane stresses that are uniform but highly sensitive to local distortion from welding. The TIG process, with its concentrated heat input and precise control, was identified as the preferred method for thin-plate dissimilar steel joints, although the productivity penalty was significant.

Process Parameters and Welding Procedure Development

The investigation covered a range of TIG parameters including current type (DCEN), current magnitude (80–220 A depending on plate thickness), gas flow rates (8–12 L/min of high-purity argon), travel speed (40–80 mm/min), and tungsten electrode diameter (2.4–3.2 mm). The researchers emphasized the importance of backing protection—using argon on both the root and cap sides—to prevent oxidation of the austenitic stainless steel, which would lead to intergranular sensitization and reduced corrosion performance.

A critical finding was the effect of tungsten electrode preparation on arc stability and weld penetration. Conical tungsten electrodes with 60° included angles produced the most stable arcs and the deepest penetration for thin sections, while spherical tungsten tips were unsuitable for this application. The study also examined the role of filler wire deposition rate and its effect on the dilution ratio, finding that a dilution ratio of 25–35% was optimal for maintaining acceptable corrosion resistance in the weld metal when using 309L filler on a carbon steel to 304 stainless steel joint.

The research employed metallographic examination, hardness mapping, and intergranular corrosion testing (ASTM A262 Practice E) to evaluate weld quality. Results showed that with careful parameter control, the welds achieved acceptable mechanical properties and corrosion resistance, although the heat-affected zone on the carbon steel side exhibited hardness values up to 300 HV due to martensitic transformation—a concern for low-temperature service applications.

Engineering Practice Implications and Legacy

This study, though published in 1989, established fundamental principles that remain relevant to modern dissimilar steel welding practice. The emphasis on backing protection, dilution control, and weld sequence optimization for spherical vessels directly informed subsequent standards development in China's pressure vessel codes. The work also highlighted the limitations of conventional TIG for thick dissimilar steel joints, foreshadowing the development of pulsed TIG and hybrid welding processes that would later address these productivity challenges.

For modern engineers working on clad plate spherical vessels or dissimilar steel storage tanks, this research serves as a foundational reference for understanding the metallurgical trade-offs inherent in dissimilar metal joining. The principles of dilution control, thermal management, and geometric distortion mitigation remain central to welding procedure qualification under NB/T 47014 and ASME IX, even as process technology has advanced significantly.

Study Insights and Reflections

Reflecting on this work in the context of contemporary practice, one appreciates both its pioneering nature and its inherent limitations. The 1989 study relied on manual TIG welding with no automation, which constrained the reproducibility of results and the productivity achievable in the field. However, the systematic approach to metallurgical evaluation—combining microstructural analysis, mechanical testing, and corrosion assessment—set a methodological standard that modern welding engineers should emulate. The study also implicitly recognized that process selection is not merely a matter of technical capability but must be weighed against economic constraints, a consideration that remains central to engineering decision-making in pressure vessel fabrication today.