TIG Welding of Copper Heat Exchanger Tubes to Stainless Steel Tube Sheets

Literature Overview

This paper by Huang Renguo, Zhang Guodong, Ren Shihong, and Zhang Jianxiao, published in Electric Welder in 2009, addresses a critical engineering challenge in heat exchanger manufacturing: the dissimilar metal welding of pure copper (T2) heat exchanger tubes to austenitic stainless steel (0Cr18Ni10Ti) tube sheets using TIG welding. The study was conducted jointly by Wuhan University, Liuzhou Wuling Automobile Joint Development Co., Ltd., and Lanzhou Lanshi Mechanical Equipment Co., Ltd., reflecting a strong industry-academia collaboration. The authors performed welding procedure qualification tests and production trials to establish a reliable welding method that avoids copper penetration cracking.

Core Technical Findings

The study identified a nickel-based filler metal as the key to successful dissimilar metal welding of copper and stainless steel. The critical findings are summarized below:

The selection of a nickel-based filler metal is based on the metallurgical principle that nickel forms unlimited solid solutions with both copper and austenitic stainless steel, thereby preventing the formation of brittle intermetallic compounds (such as Cu-Ni or Cu-Fe phases) that would lead to cracking.

Technical Interpretation and Process Analysis

Dissimilar Metal Welding Challenges

The welding of copper to austenitic stainless steel presents several unique challenges. First, the large difference in thermal conductivity between copper (approximately 390 W/m·K) and stainless steel (approximately 15 W/m·K) creates an asymmetric heat distribution, leading to uneven melting and potential lack of fusion on the copper side. Second, the melting point difference (copper: 1085°C, stainless steel: approximately 1400°C) means that the copper side tends to melt more readily, creating a risk of excessive copper dilution in the weld metal. Third, the formation of brittle copper-iron intermetallic compounds at the fusion boundary is a major cracking risk if an inappropriate filler metal is used.

Role of Nickel-Based Filler Metal

The nickel-based filler metal serves as a metallurgical bridge between copper and stainless steel. Nickel's crystal structure (FCC) is compatible with both copper and austenitic stainless steel, allowing for continuous solid solution formation without the precipitation of brittle phases. The filler metal composition should contain sufficient nickel (typically above 60%) to ensure that the weld metal solidifies in the austenitic structure, which has good ductility and resistance to cracking.

The welding process parameters must be carefully controlled to manage the asymmetric heat flow. A common practice is to preheat the stainless steel side to compensate for its lower thermal conductivity and higher melting point, while avoiding overheating of the copper side. The TIG welding parameters should include a relatively low current density to minimize dilution of the base metals into the weld pool.

Application of 5W2H Methodology in Process Development

The 5W2H framework provides a structured approach to analyzing and optimizing the welding process:

This framework ensures that all aspects of the welding process are systematically addressed, from material selection to welder qualification to production execution.

Engineering Practice Integration

In heat exchanger manufacturing, the tube-to-tubesheet joint is a critical pressure boundary that must withstand cyclic thermal loading, corrosion, and mechanical stress. The TIG welding method described in this paper is particularly suitable for small-diameter tubes (typically below 19 mm) where the joint geometry allows for accessible TIG welding from the tube sheet side. For larger tubes, automatic orbital welding or resistance welding may be more appropriate.

A key practical consideration is the post-weld heat treatment (PWHT) requirement. Dissimilar metal welds involving nickel-based filler metals may require a solution heat treatment to dissolve any residual brittle phases and to relieve welding residual stresses. The PWHT temperature must be carefully controlled to avoid sensitization of the stainless steel base metal, which could lead to intergranular corrosion.

Another practical aspect is the visual inspection and non-destructive testing (NDT) protocol. Given the asymmetric weld geometry and the potential for lack of fusion on the copper side, ultrasonic testing (UT) or radiographic testing (RT) should be employed to verify weld integrity. The acceptance criteria should be based on applicable standards such as ASME Section VIII or EN 13445.

Key Questions and Reflections

One important question is the long-term durability of the nickel-based weld metal in corrosive service environments. While nickel-based alloys offer excellent corrosion resistance, their performance in specific media such as sulfuric acid, hydrochloric acid, or chlorinated environments must be verified through corrosion testing. The study does not address corrosion resistance, which is a critical consideration for heat exchangers operating in aggressive chemical environments.

Another reflection concerns the mechanical properties of the weld metal and the heat-affected zone. The nickel-based weld metal may have different strength and creep resistance characteristics compared to the base metals. Engineers should ensure that the weld joint meets the minimum strength requirements specified in the applicable design code, particularly for high-pressure and high-temperature applications.

Study Insights and Implications

The most significant contribution of this paper is the practical demonstration that nickel-based filler metals can successfully bridge the metallurgical gap between pure copper and austenitic stainless steel in TIG welding. This finding has broad applicability to other dissimilar metal welding scenarios involving copper and nickel-based alloys. The study also highlights the importance of filler metal selection in dissimilar metal welding, where the choice of filler metal can determine the success or failure of the joint.

For welding engineers, the study reinforces the principle that dissimilar metal welding requires a thorough understanding of the metallurgical interactions between the base metals and the filler metal. The use of a metallurgically compatible filler metal, combined with careful process parameter control, is the key to achieving reliable dissimilar metal welds.

Reference Value and Outlook

This paper holds significant reference value for engineers in the heat exchanger manufacturing industry, particularly those working with copper-tube and stainless-steel-tubesheet configurations. Its primary limitation is the lack of long-term durability and corrosion resistance data. Future research should focus on the long-term performance of nickel-based dissimilar metal welds in aggressive service environments, including cyclic thermal loading tests and accelerated corrosion tests. The methodology presented, however, provides a sound foundation for the development of reliable dissimilar metal welding procedures.