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

Literature Overview

This paper by Jiang Longnan, Li Guoping, Chen Hongsheng, Pei Mingde, Wang Wenxian, and Li Haolin, published in the Transactions of the China Welding Institute in 2025, Volume 46, Issue 10, provides a comprehensive investigation into the welding behavior of S32001 low-nickel duplex stainless steel. The study was conducted by a collaborative team from Taiyuan University of Technology and Taiyuan Stainless Steel Co., Ltd., supported by Shanxi Provincial Key R&D Program (202202050201015) and Shanxi Provincial Patent Transformation Program (20240003). The research systematically examines the effects of TIG welding parameters on joint microstructure, phase composition, and mechanical properties.

Material Background and Welding Challenges

S32001 is a low-nickel duplex stainless steel that offers a favorable combination of high strength, good corrosion resistance, and reduced material cost compared to conventional duplex grades such as 2205. The duplex structure, consisting of roughly equal amounts of austenite and ferrite, provides resistance to both pitting corrosion and stress corrosion cracking. However, the welding of duplex stainless steels presents unique challenges related to maintaining the austenite-ferrite balance during the thermal cycle, particularly in the heat-affected zone (HAZ) where temperature excursions can shift the phase balance.

Experimental Methodology and Parameter Matrix

The study employed automatic TIG welding with varying parameters to investigate the effects on joint characteristics. The key parameters studied included welding current, welding speed, and welding frequency. The following table summarizes the parameter ranges and their effects:

Microstructure Analysis

The metallographic examination revealed several important findings:

1. The weld metal microstructure consisted of austenite and ferrite phases, consistent with the duplex nature of S32001.
2. Secondary austenite phases were observed precipitating within the ferrite grains and at ferrite/austenite phase boundaries. This secondary austenite formation is a critical feature that contributes to the strengthening of the weld metal.
3. The hardness distribution exhibited a characteristic "W" pattern across the joint, with the base metal showing slightly higher microhardness and the HAZ showing slightly lower microhardness values.
4. As heat input increased, grain size in the weld region increased and hardness values decreased.

The formation of secondary austenite within the ferrite phase is particularly noteworthy. This phase transformation occurs during the cooling cycle when the local composition and temperature conditions favor austenite precipitation within the ferrite matrix. The secondary austenite acts as a strengthening phase, contributing to the overall mechanical properties of the weld metal.

Mechanical Properties and Failure Analysis

The mechanical property results demonstrated a clear relationship between welding parameters and joint performance:

The optimal parameter combination of 52 A current, 0.3 m/min speed, and 2 Hz frequency yielded the highest microhardness and tensile strength. The fracture surface analysis revealed a mixed failure mode consisting of small planar facets, microvoids, and tearing ridges. Some micron-scale inclusions were observed on the fracture surface, and the weld metal exhibited quasi-cleavage fracture characteristics.

Strengthening Mechanisms

The study identifies multiple strengthening mechanisms contributing to the mechanical properties of S32001 TIG welded joints:

• Solid solution strengthening from alloying elements (Cr, Mo, N, and residual Ni)
• Precipitation strengthening from secondary austenite formation within ferrite
• Grain refinement at lower heat input conditions
• Strain hardening during solidification

The "W" hardness distribution pattern reflects the complex thermal history experienced by different regions of the joint. The base metal, having not undergone melting, retains its original hardened condition. The HAZ, having experienced peak temperatures below the melting point, undergoes partial recrystallization and phase transformation, resulting in slightly reduced hardness. The weld metal, having undergone complete melting and solidification, develops a new microstructure with properties determined by the solidification cooling rate and subsequent phase transformations.

Engineering Practice Implications

For engineers working with S32001 duplex stainless steel in pipe and fitting applications, this research provides several practical guidelines:

• Low heat input parameters should be preferred to maintain fine grain structure and optimal mechanical properties
• The welding frequency of 2 Hz provides beneficial pulse arc characteristics that improve penetration control and reduce thermal distortion
• Post-weld inspection should pay particular attention to the HAZ region where hardness is lowest and potential for stress corrosion cracking may be highest
• The presence of secondary austenite should be verified through metallographic examination to ensure adequate phase balance

Study Insights and Outlook

This research contributes valuable knowledge to the welding of low-nickel duplex stainless steels, which are increasingly being specified for applications where cost reduction is important without sacrificing critical performance characteristics. The identification of secondary austenite as a strengthening mechanism provides new insight into the microstructural evolution during welding. For future work, the investigation should extend to include corrosion performance evaluation under specific service conditions, fatigue behavior assessment, and long-term aging effects on the phase stability of the weld metal. The findings also suggest that parameter optimization should be conducted for each specific application, considering the unique thermal constraints and mechanical requirements of the service environment.