Zhuojin Pipe Fitting Co., LtdTIG Welding Process and Microstructure Properties of Pure Nickel and 304 Austenitic Stainless Steel
Literature Overview
The research by Zhao Hulin, Wang Xijing, Shen Liang, and Wang Jiang from the Lanzhou University of Technology (Key Laboratory of Nonferrous Metal New Materials, Gansu Province) and Jinchuan Group Nickel-Cobalt Research Institute, published in Welder in 2014 (Vol. 44, Issue 8, pp. 55–58), investigates the TIG welding of dissimilar joints between pure nickel (N6) and 304 austenitic stainless steel. The study addresses a practical challenge in the nickel industry, where equipment and components made of different materials must be joined to create functional assemblies. The authors examine the welding process parameters, mechanical properties, and microstructural evolution of the weld joints, providing valuable insights for engineers working on dissimilar metal welding in the nickel and stainless steel industries.
Dissimilar Metal Welding Challenges
Welding pure nickel to 304 austenitic stainless steel presents several unique challenges. First, the two materials have different thermal expansion coefficients, thermal conductivities, and melting points, which can lead to thermal stresses, distortion, and cracking during welding. Second, the chemical composition of the weld metal is a mixture of nickel and iron, which can lead to the formation of intermetallic compounds and brittle phases. Third, the different microstructures of the two materials—pure nickel is FCC with high ductility, while 304 stainless steel is austenitic with a complex phase structure—can lead to non-uniform microstructural evolution in the weld joint.
The study uses 3 mm thick pure nickel (N6) plates and 1.5 mm thick 304 austenitic stainless steel plates, creating a significant thickness mismatch that further complicates the welding process. The thinner stainless steel plate is more susceptible to burn-through, while the thicker nickel plate requires sufficient heat input for penetration.
Welding Process and Parameters
The authors employed automatic TIG welding to join the dissimilar materials, using a nickel-based filler wire to match the pure nickel side and to provide adequate dilution control. The optimal welding parameters were determined through a series of trial welds, and the resulting weld joints were evaluated using tensile testing, metallographic examination, scanning electron microscopy (SEM), and microhardness testing.
The key finding was that with appropriate welding parameters, the weld joints achieved a tensile strength of 636 MPa, which is comparable to the tensile strength of the 304 stainless steel base metal (approximately 520–620 MPa) and significantly higher than the pure nickel base metal (approximately 200–300 MPa). This result indicates that the weld metal is dominated by the stainless steel side, with sufficient dilution from the nickel side to provide adequate strength.
Microstructural Analysis
The metallographic examination revealed several important features of the weld joint:
| Region | Microstructure | Observations |
|---|---|---|
| Weld metal | Iron-nickel alloy | Uniform microstructure; no intermetallic compounds observed |
| Weld metal / N6 fusion line | Blurred boundary | Many grains extend across the fusion line; high dilution from N6 |
| Weld metal / 304 fusion line | Distinct boundary | Fewer grains extend across the fusion line; lower dilution from 304 |
| Base metal (N6) | FCC nickel grains | Unchanged from base metal |
| Base metal (304) | Austenitic grains | Unchanged from base metal |
The blurred fusion line between the weld metal and the N6 side indicates high dilution from the pure nickel, which is expected due to the higher thermal conductivity and lower melting point of nickel. The distinct fusion line between the weld metal and the 304 side indicates lower dilution from the stainless steel, which is also expected due to the higher melting point and lower thermal conductivity of stainless steel.
The SEM analysis of the tensile fracture surface revealed a ductile fracture mode, characterized by dimples and voids. This is a favorable result, as it indicates that the weld joint has good toughness and ductility, and that the failure occurred in the base metal rather than in the weld metal or fusion zone.
Engineering Practice and Application
The study is directly relevant to the nickel industry, where equipment and components made of pure nickel and stainless steel must be joined for applications such as heat exchangers, condensers, and chemical processing equipment. The nickel-based filler wire and the optimized welding parameters provide a practical solution for producing sound, defect-free weld joints with acceptable mechanical properties.
The thickness mismatch between the two plates (3 mm N6 and 1.5 mm 304) is a common configuration in industrial applications, where the nickel side may be thicker to provide additional corrosion resistance or structural strength. The study demonstrates that this configuration can be successfully welded using automatic TIG welding, provided that the welding parameters are carefully controlled.
The microstructural analysis provides valuable insights into the dilution behavior and phase evolution in the weld joint. The absence of intermetallic compounds and the ductile fracture mode indicate that the weld joint is metallurgically compatible and mechanically sound. However, the study does not include corrosion testing, which would be essential for evaluating the long-term performance of the weld joint in aggressive environments.
Reflections and Study Insights
This paper provides a practical and valuable contribution to the field of dissimilar metal welding, particularly for the nickel and stainless steel industries. The use of a nickel-based filler wire and the optimized welding parameters offer a reliable solution for joining pure nickel to 304 stainless steel, and the microstructural analysis provides insights into the dilution behavior and phase evolution in the weld joint.
The key finding that the weld metal is dominated by the stainless steel side, with sufficient dilution from the nickel side to provide adequate strength, is important for engineers designing dissimilar metal joints. The high dilution from the nickel side is expected due to the higher thermal conductivity and lower melting point of nickel, and it can be controlled by adjusting the welding parameters and the filler wire composition.
One limitation of the study is the absence of corrosion testing, which is essential for evaluating the long-term performance of the weld joint in aggressive environments. The nickel-based weld metal may be susceptible to selective leaching or galvanic corrosion in the presence of chloride ions, and the microstructural heterogeneity at the fusion lines may act as preferential sites for corrosion attack. Future work should include corrosion testing, such as salt spray testing, potentiodynamic polarization, and immersion testing, to provide a complete evaluation of the weld joint's performance.
Additionally, the study does not address the effects of post-weld heat treatment (PWHT) on the weld joint's properties. PWHT can relieve residual stresses, refine the microstructure, and improve the mechanical properties of the weld joint. The authors should investigate the effects of PWHT on the weld joint's microstructure and mechanical properties, and recommend an appropriate PWHT cycle for industrial applications.
In summary, this paper demonstrates that pure nickel and 304 austenitic stainless steel can be successfully joined using automatic TIG welding with a nickel-based filler wire, producing weld joints with acceptable mechanical properties and a ductile fracture mode. The microstructural analysis provides valuable insights into the dilution behavior and phase evolution in the weld joint, and the findings support the use of this welding approach for industrial applications. The study highlights the importance of controlling the welding parameters and the filler wire composition to achieve a metallurgically compatible and mechanically sound weld joint, and it serves as a useful reference for engineers working on dissimilar metal welding in the nickel and stainless steel industries.