Hot Wire TIG Surfacing of UNS S32707 Super-Duplex Stainless Steel

Literature Overview

This 2021 study published in Pressure Vessel Technology by Wang Chun and colleagues from Harbin Well Welding Co., Ltd. and Lanzhou Lancheng Heavy Equipment Co., Ltd. investigates hot wire TIG (HW-TIG) surfacing of UNS S32707 super-duplex stainless steel. UNS S32707 is a high-strength super-duplex stainless steel with a PREN (Pitting Resistance Equivalent Number) of approximately 43, designed for aggressive chloride-containing environments. The research was conducted in collaboration with the Gansu Provincial Key Laboratory of Special Material Welding for Pressure Vessels, reflecting the critical nature of this material in pressure vessel and process equipment applications.

Core Technical Content

The authors performed HW-TIG surfacing trials on UNS S32707 and evaluated the overlay layers through chemical composition analysis, mechanical property testing, metallographic examination, and corrosion resistance testing. The study demonstrates that HW-TIG is an effective surfacing method for this material when appropriate welding parameters and post-weld heat treatment are applied.

Key Material and Process Parameters

Interpretation of Technical Points

The HW-TIG process offers several distinct advantages for surfacing super-duplex stainless steels. The hot wire provides additional heat input without increasing the arc current, which allows for higher deposition rates while maintaining a controlled thermal cycle. This is particularly important for super-duplex stainless steels, where excessive heat input can promote the formation of intermetallic phases such as sigma phase (σ) and chi phase (χ), which severely degrade corrosion resistance and ductility.

The ferrite content control is the central metallurgical challenge in welding super-duplex stainless steels. UNS S32707 is designed to maintain a balanced microstructure of approximately 40% ferrite and 60% austenite. During welding, the rapid cooling of the weld metal can shift the balance toward higher ferrite content, which is generally acceptable up to a point but excessive ferrite (>70%) can lead to intermetallic phase precipitation during subsequent heat treatment or service.

Process Parameter Considerations

HW-TIG surfacing of super-duplex stainless steels requires careful parameter selection:

• Arc current: Typically 120-200 A for single-pass surfacing, depending on wire diameter
• Hot wire current: 20-50 A, providing supplemental heat input
• Travel speed: 5-15 cm/min, balancing deposition rate with cooling rate control
• Interpass temperature: Must be maintained below 250°C to prevent sensitization and intermetallic phase formation
• Shielding gas: High-purity argon (99.99%) or argon-helium mixtures for improved arc stability and penetration
• Post-weld heat treatment: Solution treatment at 1050-1100°C followed by water quenching may be required for critical applications

The thermal cycle control is paramount. Super-duplex stainless steels have a critical cooling rate window: cooling rates between 10 and 100°C/s in the temperature range of 900-400°C promote intermetallic phase precipitation. HW-TIG's ability to control heat input independently of arc current provides a mechanism to maintain cooling rates outside this critical window.

Engineering Practice Implications

For pressure vessel and process equipment manufacturers, this research validates HW-TIG as a production-capable process for super-duplex stainless steel surfacing. The implications extend to:

• Repair of super-duplex components in service, avoiding costly replacement
• Application of corrosion-resistant overlays on carbon steel substrates in chloride-containing environments
• Manufacturing of super-duplex-lined equipment for the oil and gas, chemical, and desalination industries

The collaboration between a welding equipment manufacturer (Harbin Well Welding) and a heavy equipment manufacturer (Lanzhou Lancheng) with a provincial key laboratory underscores the industry-academia-research partnership model that drives practical welding technology development in China.

Key Questions and Reflections

A significant question is the long-term corrosion resistance of HW-TIG surfaced overlays compared to homogeneously cast UNS S32707. The weld metal microstructure, even when well-controlled, may differ from the base material in terms of grain size, inclusion content, and phase distribution. Long-term immersion testing in simulated service environments (e.g., seawater at 60°C with 5% chlorides) would provide more definitive data on overlay performance.

Another consideration is the effect of multiple surfacing passes on microstructure evolution. Each subsequent pass re-heats the previous pass, potentially altering the phase balance and promoting unwanted phase transformations. The number of passes and the interpass temperature must be carefully controlled to maintain microstructural integrity throughout the overlay thickness.

Study Insights and Implications

This research contributes practical process knowledge for one of the most demanding surfacing applications in the pressure vessel industry. UNS S32707 super-duplex stainless steel is increasingly specified for high-chloride, high-temperature applications where conventional duplex stainless steels such as UNS S31803 are insufficient. The validation of HW-TIG as an effective surfacing process for this material opens new possibilities for equipment design, where selective overlaying can reduce material costs while maintaining corrosion resistance where it matters most. Engineers should note that while the study confirms process feasibility, qualification testing specific to the intended service environment remains essential before production deployment.