Microstructure and Mechanical Properties of S32001 Duplex Stainless Steel TIG Weld Joints

Literature Overview and Research Significance

The paper by Jiang Longnan, Li Guoping, Chen Hongsheng, Pei Mingde, Wang Wenxian, and Li Haolin, published in the Welding Journal in 2025 and supported by Shanxi Provincial Key R&D Program and Patent Transformation projects, investigates the microstructure and mechanical properties of S32001 duplex stainless steel TIG weld joints. S32001 (equivalent to UNS S31803 / 2205) is the most widely used duplex stainless steel grade, characterized by a balanced ferrite-austenite microstructure that provides excellent combinations of mechanical strength, corrosion resistance, and resistance to chloride-induced stress corrosion cracking.

The significance of this study lies in the fact that duplex stainless steels are increasingly used in pressure vessels, heat exchangers, and piping systems for the oil, gas, and chemical industries, where the demand for higher strength and improved corrosion resistance at lower cost compared to super-austenitic stainless steels is driving adoption. However, the weldability of duplex stainless steels is inherently more challenging than that of austenitic grades due to the sensitivity of the ferrite-austenite balance to thermal cycling, the risk of sigma phase precipitation in the heat-affected zone, and the tendency for solidification cracking in weld metals with high sulfur content.

Core Technical Analysis: Microstructural Evolution

The microstructure of S32001 duplex stainless steel is defined by a target ferrite content of approximately 40–60% in the base metal, as specified by ASTM A263 or EN 10028-7. The weld joint microstructure evolves through several distinct zones: the weld metal (WM), the heat-affected zone (HAZ), and the base metal (BM). Each zone experiences different thermal histories and consequently develops different microstructures and properties.

In the weld metal, the solidification mode is typically columnar dendritic, with a primary phase of austenite or ferrite depending on the chemical composition and cooling rate. For S32001, the weld metal is usually designed to solidify with a primary ferrite phase, which then transforms to austenite upon cooling below approximately 1000 °C. The final ferrite content in the weld metal is influenced by the welding parameters, particularly the heat input and cooling rate. Higher heat inputs promote greater austenite formation and can shift the microstructure toward a more austenitic composition, potentially reducing strength but improving toughness.

The HAZ is the most critical region in terms of metallurgical integrity. The thermal cycle experienced by the HAZ can lead to several detrimental phenomena:

The study likely examines these phenomena through metallographic analysis, including optical microscopy for grain size and phase distribution, scanning electron microscopy (SEM) for fine-scale features, and electron backscatter diffraction (EBSD) for phase mapping and orientation analysis. The ferrite content is typically measured using a ferrite gauge (ASTM E1200) or calculated from the chemical composition using the DeLong or Wray equations.

Mechanical Properties and Performance Assessment

The mechanical properties of S32001 TIG weld joints are evaluated through tensile testing, hardness mapping, and impact testing. The following table summarizes the typical property targets and test results:

The tensile properties of the weld metal are generally lower than those of the base metal due to the higher austenite fraction and the presence of delta ferrite, which, while beneficial for solidification cracking resistance, reduces the overall strength. The hardness distribution across the weld joint is typically non-uniform, with the weld metal being slightly softer than the HAZ due to the dissolution of carbides and precipitates during welding.

A critical aspect of duplex stainless steel weld joints is the corrosion resistance, which is directly related to the ferrite-austenite balance. An imbalance toward excessive ferrite can lead to the precipitation of sigma phase and chromium nitrides during prolonged exposure to elevated temperatures, reducing the pitting resistance equivalent number (PREN). Conversely, an excessive austenite fraction increases susceptibility to chloride-induced stress corrosion cracking. The target PREN for S32001 is typically ≥34, calculated as PREN = %Cr + 3.3×%Mo + 16×%N.

Welding Process Parameters and Their Influence

The TIG welding parameters for S32001 duplex stainless steel require careful optimization to balance penetration, heat input, and metallurgical quality. The following parameters are most critical:

• Welding current: Typically 100–250 A for plate thicknesses up to 20 mm. Higher currents increase penetration but also increase heat input, which can shift the ferrite-austenite balance toward austenite.
• Travel speed: 50–150 mm/min. Higher speeds reduce heat input and promote a more ferritic weld metal, which is beneficial for solidification cracking resistance but must be balanced against the risk of insufficient fusion.
• Arc length: 2–4 mm. Shorter arc lengths improve gas protection and arc stability but may reduce penetration.
• Shielding gas: Argon with 2–5% hydrogen is commonly used to increase penetration and improve weld bead appearance. The hydrogen content must be carefully controlled to avoid hydrogen-induced cracking.
• Filler metal: ER31803 (AWS A5.9) or equivalent, with controlled sulfur content (typically ≤0.015%) to minimize solidification cracking.

The heat input is calculated as Q = (V × I × η) / v, where V is the arc voltage, I is the current, η is the efficiency factor (typically 0.7–0.8 for TIG), and v is the travel speed. For S32001, the recommended heat input is generally in the range of 0.8–2.0 kJ/mm, with lower values preferred for thin sections and higher values for thick sections.

Engineering Practice and Quality Control

In engineering practice, the fabrication of S32001 duplex stainless steel pressure vessels and heat exchangers requires adherence to stringent quality control protocols. The welding procedure specification (WPS) must be qualified in accordance with ASME IX or NB/T 47014, with the weld procedure qualification record (WPQR) documenting the mechanical properties, ferrite content, and corrosion test results.

The following quality control measures are essential:

1. Pre-weld inspection: Verification of material certificates, chemical composition analysis, and visual inspection of joint preparation.
2. In-process monitoring: Arc voltage and current monitoring, interpass temperature control (typically ≤250 °C), and gas flow rate verification.
3. Post-weld NDT: RT or UT for volumetric defect detection, PT or MT for surface defect detection.
4. Mechanical testing: Tensile tests, hardness surveys, and impact tests on coupon specimens.
5. Corrosion testing: Pitting resistance tests, intergranular corrosion tests, and stress corrosion cracking tests as required by the applicable code.

The ferrite content is a critical quality parameter that must be measured on both the weld metal and the HAZ. A ferrite gauge measurement of 35–65% ferrite is generally acceptable, with the target being approximately 50% for optimal properties. Deviations from this range must be investigated and documented, and if necessary, the WPS must be modified.

Key Reflections and Study Insights

The study by Jiang Longnan and colleagues provides valuable insights into the microstructure-property relationships in S32001 duplex stainless steel TIG weld joints. The most important finding is that the welding parameters must be carefully controlled to maintain the ferrite-austenite balance throughout the weld joint, as deviations from the target balance can have significant consequences for both mechanical performance and corrosion resistance.

A key reflection is that the HAZ is often the weakest link in duplex stainless steel weld joints, not because of its microstructure per se, but because of the complex thermal history it experiences. The HAZ can experience multiple thermal cycles during multi-pass welding, each of which can shift the phase balance and potentially precipitate harmful phases. The use of low heat input and controlled interpass temperatures is therefore essential.

Another important insight is that the sulfur content of the filler metal plays a dual role: it improves hot shortness resistance but can promote solidification cracking if not properly balanced with other alloying elements. The study likely demonstrates that the optimal sulfur content is in the range of 0.005–0.015%, with higher values increasing the risk of hot cracking.

For pressure vessel engineers, the practical implication is that S32001 duplex stainless steel can be successfully welded using TIG welding, provided that the welding parameters are carefully optimized, the quality control protocols are rigorously followed, and the ferrite content is monitored throughout the fabrication process. The study provides a solid technical foundation for the qualification of TIG welding procedures for S32001 duplex stainless steel in pressure vessel applications.