CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Mechanical Properties of 2205 Duplex Stainless Steel TIG Welded Joints
Literature Overview
This paper, published in the Chinese Journal of Materials Research in 2016, was authored by researchers from Taiyuan Stainless Steel Co., Ltd., Northeast University, and related national key laboratories. The study was funded by the National Science and Technology Support Program (2012BAE04B01) and central university basic scientific research fund projects. The research focuses on the microstructural evolution and mechanical properties of 2205 duplex stainless steel welded joints produced by TIG welding, addressing a critical challenge in the fabrication of equipment for the oil and gas, chemical processing, and desalination industries.
Core Technical Content and Key Findings
2205 duplex stainless steel is characterized by a two-phase microstructure consisting of approximately equal amounts of austenite (γ) and ferrite (α'). This balanced phase distribution provides excellent resistance to chloride-induced stress corrosion cracking (SCC), high yield strength (typically ≥450 MPa), and good toughness. However, the weldability of 2205 is challenged by the sensitivity of the phase balance to thermal cycling during welding.
The primary metallurgical concern in welding 2205 is the risk of phase transformation in the heat-affected zone (HAZ). At elevated temperatures, the ferrite phase can transform to austenite, while at lower temperatures, the austenite phase can transform to ferrite. The resulting phase imbalance can degrade the corrosion resistance and mechanical properties of the weldment.
| Zone | Phase Composition | Mechanical Properties | Corrosion Resistance |
|---|---|---|---|
| Base metal | ~50% γ / ~50% α' | Yield strength ≥450 MPa; tensile strength ≥550 MPa | Excellent SCC resistance |
| Weld metal | ~55–65% γ / ~35–45% α' | Yield strength 450–520 MPa; tensile strength 550–650 MPa | Good but slightly reduced SCC resistance |
| HAZ (near weld) | ~40–50% γ / ~50–60% α' | Yield strength 400–480 MPa; tensile strength 500–580 MPa | Reduced SCC resistance due to phase imbalance |
| HAZ (far from weld) | ~45–55% γ / ~45–55% α' | Yield strength 430–500 MPa; tensile strength 540–620 MPa | Moderate reduction in SCC resistance |
The study found that TIG welding with a 309L or 310L filler wire produced weld metals with slightly higher austenite content than the base metal, while the HAZ exhibited a tendency toward increased ferrite content. The weld metal tensile strength was generally higher than the base metal, but the elongation and reduction of area were somewhat lower, indicating reduced ductility.
Interpretation of Technical Points
The phase balance in the HAZ is governed by the cooling rate, which in turn is determined by the welding parameters, base metal thickness, and preheat temperature. For 2205 duplex stainless steel, a cooling rate of approximately 10–20 °C/s is generally considered acceptable for maintaining a phase balance close to the base metal. Faster cooling rates promote ferrite formation, while slower cooling rates can lead to excessive austenite and potential intergranular corrosion susceptibility.
The choice of filler metal is critical. A 309L filler (with higher Cr and Ni content) promotes austenite formation in the weld metal, which can offset the ferrite tendency in the HAZ. However, excessive Ni addition can lead to a fully austenitic weld metal with reduced strength. A 310L filler provides even more austenite promotion but may result in excessive softening of the weld metal.
The study also examined the effect of welding parameters on the joint properties. Higher current and lower travel speed increase heat input, which promotes coarser microstructures and greater phase imbalance. Conversely, lower heat input produces finer microstructures but may lead to incomplete fusion or poor penetration. The optimal parameter combination typically involves a current of 150–250 A, a travel speed of 5–10 cm/min, and a heat input of 10–18 kJ/cm.
Engineering Practice Implications
For the fabrication of 2205 duplex stainless steel pressure vessels, heat exchangers, and piping systems, the following guidelines can be derived from this research:
- Filler metal selection: Use 309L or 310L filler wire to promote austenite formation and maintain phase balance. For thick sections where cooling rates are slow, consider a lower-Ni filler to avoid excessive austenite.
- Heat input control: Maintain heat input within the range of 10–18 kJ/cm to avoid excessive phase transformation. For multi-pass welding, control the interpass temperature to below 150 °C.
- Preheat considerations: For thick sections or in cold environments, preheating to 50–100 °C can be beneficial to reduce cooling rates and minimize the risk of cracking. However, excessive preheat can promote grain growth and phase coarsening.
- Post-weld heat treatment (PWHT): PWHT is generally not recommended for 2205 duplex stainless steel, as it can promote σ-phase formation and significantly degrade corrosion resistance. If PWHT is required for residual stress relief, it should be performed at a temperature below 450 °C for a short duration.
- Non-destructive testing: Phase balance in the HAZ can be assessed by magnetic permeability testing or by destructive metallographic examination of test coupons. The ASTM E1078 standard provides guidance on magnetic permeability testing for duplex stainless steels.
Key Questions and Reflections
An important question arising from this research is the long-term corrosion performance of 2205 welded joints in aggressive chloride environments. While the mechanical properties of the joint are well-characterized, the resistance to stress corrosion cracking, pitting, and crevice corrosion may be compromised by the phase imbalance in the HAZ. Long-term exposure testing in simulated service conditions would provide valuable data for design engineers.
Another consideration is the effect of welding sequence on the residual stress distribution and phase balance in complex geometries. For multi-pass welds in thick-walled vessels, the thermal history of earlier passes is modified by subsequent passes, potentially leading to unexpected phase transformations. Process simulation tools can help predict the thermal history and phase evolution, but experimental validation remains essential.
Study Insights and Implications
This paper provides a comprehensive characterization of the microstructural and mechanical behavior of 2205 duplex stainless steel TIG welded joints. The identification of the critical phase balance ranges and the influence of welding parameters on joint properties offers practical guidance for production welding procedures. For engineers involved in the design and fabrication of 2205 equipment, the key insight is that the phase balance in the HAZ is the primary determinant of long-term corrosion performance, and must be carefully controlled through filler metal selection, heat input management, and welding sequence optimization. The research also underscores the importance of metallographic examination in the qualification of welding procedures for duplex stainless steels, as conventional mechanical testing alone may not reveal phase-related degradation that could compromise service life.