CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Mechanical Properties of S32001 Duplex Stainless Steel TIG Weld Joints
Literature Overview
This study, published in the Welding Journal in 2025 by researchers from Taiyuan University of Technology and Taiyuan Stainless Steel Co., Ltd., investigates the microstructural evolution and mechanical behavior of S32001 duplex stainless steel welded joints produced by gas tungsten arc welding (GTAW/TIG). The work is supported by Shanxi Provincial Key R&D Program (202202050201015) and the Shanxi Provincial Patent Transformation Program (202402003), reflecting a strong industry-academia collaboration model that is increasingly vital in advanced materials research. S32001 is a lean duplex stainless steel with a nominal composition of approximately 22% Cr, 3% Ni, 1.5% Mo, and 0.15% N, offering a balanced ferrite-austenite microstructure with excellent yield strength (typically 450-550 MPa) and superior resistance to chloride stress corrosion cracking compared to austenitic grades such as 304 or 316L.
Core Technical Points on Duplex Stainless Steel Welding
The fundamental challenge in welding S32001 duplex stainless steel lies in maintaining the target ferrite-to-austenite phase balance during solidification and post-weld thermal cycles. The base metal typically contains 45-55% delta ferrite, which provides resistance to chloride pitting and stress corrosion cracking. However, the welding thermal cycle can cause several detrimental transformations:
- Phase instability in the heat-affected zone (HAZ): At temperatures above 1100°C, the equilibrium phase is austenite. Upon cooling through the 900-1100°C range, sigma phase (Cr23C6 and related intermetallics) may precipitate, leading to severe embrittlement and loss of corrosion resistance.
- Freeze ratio control: The solidification mode (ferrite-first vs. austenite-first) depends on the Cr/Ni equivalent ratio and the cooling rate. For S32001, ferrite-first solidification is typical, but excessive cooling rates can trap excessive retained austenite, while too slow cooling promotes sigma phase formation.
- Nitrogen loss: During GTAW, nitrogen content in the weld pool can decrease due to spattering and gas evolution, shifting the weld metal toward a more ferritic microstructure and reducing pitting resistance.
Welding Process Parameters and Their Influence
| Parameter | Typical Range | Effect on Microstructure |
|---|---|---|
| Welding current | 80-150 A | Higher current increases weld pool volume, promotes grain growth |
| Travel speed | 50-100 mm/min | Affects cooling rate; faster speed increases cooling rate |
| Shielding gas | Ar (99.99%) or Ar-2%N2 | Nitrogen addition can compensate for N loss, adjust freeze ratio |
| Wire feed rate | 2.0-4.0 m/min | Controls dilution and weld metal composition |
| Interpass temperature | <200°C | Prevents sigma phase precipitation in HAZ |
| Pulse frequency | 50-200 Hz (pulsed GTAW) | Controls heat input, reduces porosity |
Microstructural Analysis and Phase Transformation
The weld metal microstructure of S32001 duplex stainless steel typically exhibits a dendritic cellular structure with alternating ferrite and austenite lamellae. The key observations from this study include:
- Weld centerline: The solidification microstructure shows a Widmanstätten-type morphology with primary ferrite dendrites and secondary austenite precipitating at grain boundaries during solidification. The ferrite content in the weld metal is typically 50-65%, slightly higher than the base metal due to preferential partitioning of Ni and N into austenite during solidification.
- HAZ transformation zone: The region heated above 1100°C and cooled rapidly shows a fully austenitic structure that reverts to ferrite-austenite upon cooling. The cooling rate in this zone can exceed 100°C/s, leading to a fine-grained duplex microstructure. However, if the cooling rate is too slow (below 10°C/s), sigma phase precipitation begins to occur.
- Intergranular corrosion susceptibility: The sigma phase and Cr23C6 carbides formed in the HAZ create chromium-depleted regions adjacent to these precipitates, making them highly susceptible to intergranular corrosion. The standard ASTM A263/A264 intergranular corrosion test (acidic solution, 65°C, 24h) is the primary evaluation method.
Mechanical Properties and Engineering Implications
The tensile properties of S32001 duplex stainless steel weld joints are characterized by a yield strength of 500-600 MPa and an ultimate tensile strength of 600-750 MPa, with elongation typically in the range of 25-35%. The weld metal often exhibits slightly lower strength than the base metal due to grain coarsening and phase composition changes, but the overall joint strength remains adequate for pressure vessel applications per GB/T 150 and ASME VIII Div.1 requirements.
The impact toughness behavior is particularly important for duplex stainless steels because of their inherently higher yield strength. The Charpy V-notch (CVN) impact energy at room temperature typically exceeds 100 J, but drops significantly at sub-zero temperatures due to the ferrite phase's lower ductility. For cryogenic applications, careful control of the ferrite content and grain size is essential.
Common Defects and Countermeasures
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Excessive ferrite in weld metal | Nitrogen loss during welding | Use Ar-2%N2 shielding gas; pre-heat wire with nitrogen |
| Sigma phase in HAZ | Slow cooling through 900-1100°C | Control interpass temperature <200°C; increase travel speed |
| Porosity | Contamination of base metal or wire | Thorough cleaning; use dry shielding gas; avoid spatter |
| Hot cracking | High sulfur/phosphorus content | Use low-sulfur filler metal; control thermal input |
Study Insights and Engineering Practice
The practical significance of this research extends to the fabrication of duplex stainless steel pressure vessels used in the petrochemical, offshore oil and gas, and desalination industries. These vessels often operate under conditions where chloride stress corrosion cracking is a primary concern, making the weld integrity critical for long-term service life. The GTAW process is particularly well-suited for the first pass and root weld in pipe and vessel fabrication because of its high energy density and precise heat input control.
For engineering practice, I recommend the following quality control measures:
- Perform visual inspection and dye penetrant testing (PT) on all welds per JB/T 4730.
- Conduct ultrasonic testing (UT) or phased array UT (PAUT) for internal defect detection, especially for welds thicker than 6 mm.
- Perform intergranular corrosion testing on production welds at a frequency of at least one sample per heat of material, per the requirements of ASTM A263/A264.
- Monitor the ferrite content using a ferritecope during production to ensure the weld metal ferrite number (FN) remains within the target range of 35-65 FN.
The industry-academia collaboration model demonstrated in this research is commendable, as it directly addresses real manufacturing challenges faced by stainless steel producers. The findings provide valuable guidance for optimizing GTAW parameters to achieve welds with balanced microstructures, adequate mechanical properties, and superior corrosion resistance, which are all essential for the reliable performance of duplex stainless steel components in aggressive environments.