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

Literature Overview

The study under review investigates the microstructural evolution and mechanical behavior of gas tungsten arc welding (GTAW/TIG) joints fabricated from S32001 duplex stainless steel plate. S32001, corresponding to UNS S31803 / EN 1.4462, is a 22Cr-5Ni-3Mo-0.15N duplex austenite-ferrite stainless steel widely employed in oil and gas platforms, desalination plants, and chemical processing equipment where chloride stress corrosion resistance and high yield strength are critical. The paper examines how the welding thermal cycle redistributes the austenite-ferrite phase balance in the weld metal and heat-affected zone (HAZ), and how this redistribution affects tensile strength, hardness distribution, and corrosion resistance.

Core Findings on Phase Distribution

The base metal of S32001 typically exhibits an approximately 50:50 austenite-to-ferrite ratio by volume fraction, which provides the hathe writing systemark mechanical and corrosion properties of duplex grades. During TIG welding, the rapid heating and cooling associated with the narrow weld profile causes significant phase redistribution. The study reports that in the weld metal, the ferrite content can rise to 65-75% depending on the filler wire composition and heat input parameters. This occurs because the welding process tends to produce a leaner Ni equivalent in the weld pool relative to the base metal, shifting the Schaeffler diagram toward the ferrite region.

In the HAZ, the microstructure evolves through a transformation sequence that depends on peak temperature. At temperatures exceeding 1350°C, the original ferrite partially dissolves, and upon cooling, new austenite forms preferentially at ferrite grain boundaries and within ferrite grains. The study found that the HAZ adjacent to the fusion line (the "narrow HAZ") can develop a ferrite content as high as 70-80%, which raises concerns about the precipitation of brittle intermetallic phases such as sigma (σ) and chi (χ) during subsequent aging or high-temperature service.

Mechanical Property Assessment

The tensile properties of the TIG weld joints demonstrate that the weld metal retains acceptable strength levels, with ultimate tensile strength (UTS) typically in the range of 580-650 MPa, slightly above the minimum requirement of 550 MPa specified in ASTM A928 for S32001 plate. However, elongation tends to be lower in the weld metal (15-20%) compared to the base metal (25-30%), reflecting the higher ferrite fraction and the presence of harder phases. The fracture analysis reveals mixed ductile-brittle behavior, with cleavage fracture initiating preferentially in the high-ferrite regions of the weld metal.

Hardness mapping across the joint cross-section reveals a clear gradient: the base metal shows the lowest hardness (220-260 HV30), the weld metal exhibits moderate hardness (260-310 HV30), and the narrow HAZ adjacent to the fusion line shows the highest hardness values (270-330 HV30). This hardness gradient is consistent with the ferrite redistribution pattern and indicates that the narrow HAZ is the most susceptible region for intergranular corrosion and cracking during service.

Process Parameters and Their Influence

The study systematically varied the TIG welding parameters including current (120-200 A), travel speed (4-8 mm/min), shielding gas composition (pure Ar vs. Ar-2% He), and interpass temperature. The key findings regarding process optimization are summarized below:

The most critical finding is that controlling the interpass temperature below 150°C is essential to prevent the precipitation of sigma phase in the HAZ. When the interpass temperature exceeds 200°C, the study observed measurable sigma phase formation even in the as-welded condition, leading to a significant drop in impact toughness and intergranular corrosion resistance.

Corrosion Behavior and Engineering Implications

Electrochemical testing and salt spray tests reveal that the weld metal and narrow HAZ exhibit inferior pitting corrosion resistance compared to the base metal. The pitting resistance equivalent number (PREN) of the base metal is approximately 35-38, while the weld metal PREN drops to 32-35 due to the altered Cr/Ni/Mo/N composition. The study recommends that for critical applications, a post-weld heat treatment (PWHT) at 1020-1100°C for 1-2 hours followed by rapid cooling should be considered to restore the phase balance and dissolve any incipient intermetallic phases.

Key Reflections and Practical Recommendations

From an engineering practice perspective, this study underscores several critical points for fabricators working with S32001 duplex stainless steel. First, the selection of filler metal is paramount: using a standard austenitic filler (e.g., ER308L) would produce a weld with excessive ferrite and poor corrosion resistance. A duplex filler wire matching the base metal composition (ER31803, ER2209, or equivalent) is mandatory. Second, heat input must be carefully controlled; excessive heat input not only increases ferrite content but also widens the HAZ, increasing the volume of material susceptible to sensitization. Third, the narrow HAZ is the weakest link in terms of corrosion resistance, and in applications where the joint will be exposed to chloride-containing environments, additional surface treatment or protective measures for the HAZ region should be considered.

In summary, this study provides valuable quantitative data on the microstructural and mechanical behavior of S32001 TIG weld joints, and its findings directly inform welding procedure qualification (WPQ) and welding procedure specification (WPS) development for duplex stainless steel fabrication. The emphasis on controlling interpass temperature, selecting appropriate filler metal, and limiting heat input should be incorporated into all welding procedures for S32001 components, particularly for pressure vessels and piping systems in the oil and gas industry.