Manual TIG Filler Wire Welding of Duplex Stainless Steel Tubesheet

Literature Overview

This 2009 research by Wen Mingke from CNOOC Huizhou Refining Branch and Ji Qiang from Fushun Chemical Machinery Equipment Manufacturing Co. focuses on the manual TIG (GTAW) welding process for duplex stainless steel tubesheets—a critical component in heat exchangers operating in aggressive chloride-containing environments. Duplex stainless steels (2205, 2507) offer approximately twice the yield strength of conventional austenitic grades with superior resistance to chloride stress corrosion cracking, making them indispensable in offshore platforms, desalination plants, and petrochemical refineries.

Core Technical Content

Duplex Stainless Steel Welding Fundamentals

The weldability of duplex stainless steels is governed by the austenite-to-ferrite phase balance. The parent material typically contains 40–60% ferrite (measured by magnetic penetration method per ASTM E491). During welding, the rapid cooling rates inherent to TIG welding can cause:

Ferrite fraction increase due to limited austenite formation at lower temperatures
Sigma phase precipitation in the HAZ at cooling rates between 10–100°C/s
475°C embrittlement if the weld resides in the temperature range for extended periods

Parameter	2205 Duplex	2507 Super Duplex	Notes
Typical ferrite content	45–55%	40–50%	Target range
Weld ferrite target	35–65%	35–65%	ISO 21548 requirement
Maximum cooling rate	<10°C/s preferred	<10°C/s preferred	To limit sigma phase
Interpass temperature	50–150°C	50–150°C	Lower limit for hydrogen control
Filler metal	2205 or 2507 grade	2507 grade	Matching or upgrading

Manual TIG Process Parameters

The study investigates manual TIG welding with solid filler wire for tubesheet applications where automated welding may be impractical due to complex geometries:

Parameter	Typical Value	Rationale
Current	120–250 A	DCEN for adequate penetration
Arc voltage	14–18 V	Correlated with current and travel speed
Travel speed	80–200 mm/min	Manual control, operator dependent
Filler wire	ER2209 (2205) or ER2594 (2507)	1.6–2.4 mm diameter
Shielding gas	100% Ar or 98% Ar/2% He	Helium for higher heat input
Preheat	100–200°C	Reduce cooling rate, minimize HAZ hardness
Post-weld heat treatment	Solution anneal 1050–1100°C	Restore phase balance

Tubesheet-Specific Considerations

Tubesheet welding presents unique challenges including:

Multi-pass welding in confined spaces (tube hole diameter typically 12–51 mm)
High constraint leading to residual stress accumulation
Requirement for both root and cap weld quality
Interaction between weld and tube-to-tubesheet joint

Process Analysis and Defect Control

Phase Balance Control

The critical quality criterion for duplex stainless steel welds is maintaining the austenite-ferrite balance within acceptable limits. The study emphasizes:

Ferrite measurement via magnetic penetration method after welding
Metallographic examination of weld cross-section for phase identification
Intergranular corrosion testing per ASTM A923 Practice A (65% boiling HNO3)

Sigma Phase Prevention

Sigma phase (FeCr intermetallic) formation is the primary metallurgical concern in duplex welds. The study identifies the following countermeasures:

Limit interpass temperature to below 150°C to avoid sigma formation during multi-pass welding
Minimize heat input to reduce time in the sigma-forming temperature range (600–870°C)
Apply post-weld solution heat treatment when feasible
Use filler metals with higher nitrogen content to promote austenite stability

Common Defects

Defect	Mechanism	Prevention
Sigma phase	Slow cooling in HAZ	Preheat, PWHT, low heat input
Excessive ferrite	High cooling rate	Increase heat input, use 2507 filler
Cracking	HAZ softening + constraint	Control interpass temp, reduce拘束
Porosity	Nitrogen pickup	Adequate shielding, clean surfaces
Undercut	Excessive travel speed	Reduce speed, optimize torch angle

Engineering Practice Integration

In the context of pressure vessel and heat exchanger fabrication per GB/T 150 and NB/T 47002, duplex stainless steel tubesheets require weld procedure qualification per NB/T 47014 with specific attention to:

Mechanical property testing at -20°C (impact toughness)
Corrosion testing including ASTM A923 and ASTM G48 (pitting)
Intergranular corrosion resistance verification
Ferrite content measurement and documentation

The manual TIG approach, while less repeatable than automated processes, remains essential for repair welding, field fabrication, and complex geometries where automation is impractical.

Key Reflections and Study Insights

The most significant takeaway is that duplex stainless steel welding is fundamentally a metallurgical control problem disguised as a welding process problem. The welder's skill in controlling arc characteristics, travel speed, and interpass temperature directly determines the metallurgical outcome. For engineers involved in bimetal pressure vessel design, this reinforces the need for comprehensive weld procedure specifications that address not just mechanical properties but corrosion resistance and phase balance. The distinction between 2205 and 2507 welding requirements—where 2507's higher alloy content provides greater resistance but also greater susceptibility to solidification cracking—must be carefully considered in material selection for critical applications.