TIG Welding of Pure Nickel and Nickel-Steel Dissimilar Joints

Literature Overview

This 1993 paper by Yu Lirong, published in the Chinese journal "Welding" (Issue 8, pp. 17-20), addresses a practically important yet technically challenging subject: the TIG (GTAW) welding of pure nickel and dissimilar joints between pure nickel and various steels, including carbon steel and stainless steel. The study is particularly relevant to the chemical process equipment industry, where nickel-based alloys are frequently employed for corrosion resistance, and dissimilar metal joints are unavoidable in equipment fabrication. The author, affiliated with Jiangsu Chemical Equipment Manufacturing and Installation Company, brings practical field experience to the analysis, making the findings directly applicable to engineering practice.

Core Technical Points on Pure Nickel Weldability

Pure nickel (UNS N07500 / Ni 99.9%) possesses unique metallurgical characteristics that significantly influence its weldability. The key issues identified in this study include:

• High thermal conductivity and diffusivity: Nickel conducts heat approximately 1.5 times more efficiently than carbon steel, which means that the molten pool cools rapidly, promoting solidification cracking and incomplete fusion defects.
• Low sulfur and phosphorus sensitivity: Unlike austenitic stainless steels, nickel is less susceptible to hot cracking from trace impurities, but it remains sensitive to copper and iron contamination from base metal dilution.
• Oxidation tendency: Nickel forms a tenacious oxide layer (NiO) at elevated temperatures, which must be removed prior to welding to avoid inclusion formation in the weld metal.
• Hydrogen embrittlement: Nickel is susceptible to hydrogen-induced cracking, particularly when arc stability is poor or when the shielding gas contains moisture.

Recommended TIG Welding Parameters for Pure Nickel

Dissimilar Nickel-Carbon Steel Joints

The dissimilar welding of pure nickel to carbon steel presents a unique set of challenges rooted in metallurgical incompatibility. The primary concerns include:

1. Carbon diffusion and carbide formation: Carbon from the steel base metal migrates into the nickel weld metal during solidification and post-weld cooling, forming brittle Ni₃C and Ni₇C₃ carbides at the fusion line. This results in a thin, embrittled zone that severely degrades ductility and can initiate cracking under thermal cycling.
2. Dilution control: The carbon steel side must be heavily diluted with nickel to reduce the carbon activity at the fusion boundary. The study recommends multiple passes with a nickel-rich filler metal (such as ERNiCrMo-3 / ENi-Clad) to build up a transition layer.
3. Residual stress: The coefficient of thermal expansion mismatch between nickel (13.2 × 10⁻⁶/°C) and carbon steel (12.0 × 10⁻⁶/°C) is relatively small, but the large difference in thermal conductivity still produces significant thermal gradients and residual stresses.

The author proposes a multi-pass strategy where the first pass is deposited on the steel side using a nickel-copper or nickel-iron alloy filler, followed by subsequent passes of pure nickel filler. This gradient approach minimizes carbon diffusion and reduces the severity of the brittle carbide zone. Post-weld stress relief at 620°C for 1 hour per 25 mm of thickness is recommended to relieve residual stresses without promoting excessive grain growth.

Dissimilar Nickel-Stainless Steel Joints

The welding of pure nickel to austenitic stainless steel (such as 304 or 316) is more metallurgically compatible than nickel-carbon steel joints, but it still requires careful process design. The key considerations include:

• Chromium carbide precipitation: At the fusion line, chromium from the stainless steel can combine with carbon to form Cr₂₃C₆ and Cr₇C₃, depleting the adjacent HAZ of chromium and reducing local corrosion resistance. This sensitization effect is most severe at temperatures between 450°C and 850°C.
• Filler metal selection: A nickel-chromium alloy filler such as ERNiCr-3 (ENi-CI) or ERNiCrMo-3 (ENi-CI) is recommended to provide adequate dilution control and to maintain high chromium and nickel content in the weld metal.
• Heat input management: Lower heat inputs (8-15 kJ/cm) are preferred to minimize the width of the sensitized zone in the stainless steel HAZ.

The study reports that weld joints produced with appropriate filler metals and controlled heat inputs achieved tensile strengths comparable to the base metals, with acceptable ductility. However, intergranular corrosion resistance testing (ASTM A262 Practice E) revealed that the HAZ adjacent to the stainless steel side showed reduced resistance, necessitating post-weld pickling or stabilization treatments in corrosive service environments.

Quality Control and Mechanical Performance

The author emphasizes several quality control measures specific to nickel welding:

The tensile test results showed that pure nickel weld joints achieved UTS values of 480-520 MPa, close to the base metal strength of 510 MPa. Dissimilar nickel-carbon steel joints showed lower UTS (approximately 350-400 MPa), with failure occurring in the transition zone rather than in the base metal or weld metal.

Engineering Practice Implications

In chemical equipment fabrication, dissimilar nickel-steel joints are commonly encountered in heat exchangers, reactors, and piping systems where nickel-lined components must be joined to carbon steel supports or piping. The findings of this study have direct practical significance:

1. Pre-weld cleaning: All surfaces must be cleaned to a bare metal finish using mechanical methods (grinding, wire brushing) or chemical pickling. No grinding dust or oxide residue should remain within 25 mm of the joint.
2. Shielding gas purity: Argon with 99.99% purity is mandatory. Gas flow rates should be 15-20 L/min for root passes and 20-25 L/min for fill and cap passes. Backfill gas flow should be 5-10 L/min.
3. Post-weld heat treatment: Stress relief at 620°C for nickel joints and 815°C for stainless steel joints should be performed separately or using a gradient approach to avoid sensitization of the stainless steel side.

Key Reflections and Study Insights

This paper, though published in 1993, remains highly relevant to contemporary practice. The fundamental metallurgical challenges of dissimilar nickel-steel welding have not changed, and the recommended approaches—gradient filler metals, controlled heat input, and stress relief—remain the industry standard. One insight that stands out is the emphasis on carbon diffusion as the primary embrittlement mechanism in nickel-carbon steel joints. This underscores the importance of understanding elemental diffusion in dissimilar welds, a concept that is equally applicable to modern weld-overlay and cladding applications.

The paper also highlights a practical gap that persists in modern engineering: the lack of standardized procedures for dissimilar nickel-steel welding. Unlike dissimilar stainless steel welds, which are governed by ASME Section IX and numerous industry standards, nickel-steel dissimilar welds often require qualification through destructive testing and service performance data. Engineers should be encouraged to document and share successful welding procedures for these joints to build a collective knowledge base.

In conclusion, this study provides a solid foundation for understanding the weldability of pure nickel and its dissimilar joints with steels. The combination of metallurgical analysis, process parameter recommendations, and mechanical performance data makes it a valuable reference for engineers working in the chemical equipment and process industry. The key takeaway is that successful dissimilar nickel-steel welding requires careful attention to filler metal selection, dilution control, and post-weld treatment to mitigate the formation of brittle carbide phases at the fusion boundary.