Effect of Heat Input on Microstructure and Mechanical Properties of 2205 Duplex Stainless Steel MIG Welded Joints

Literature Overview

This paper, published in the Journal of Materials and Metallurgy (2016, Vol. 15, No. 2, pp. 137–142) by Wang Jianjun, Wen Yanhui, Wu Tianhai, Li Guoping, Jiang Zhouhua, and Liu Chunming from Northeastern University and Taiyuan Stainless Steel Co., Ltd., systematically investigates the influence of heat input on the microstructure and mechanical properties of 2205 duplex stainless steel MIG welded joints. The study was supported by the National Science and Technology Support Program (2012BAE04B01), the 2011 Plan Steel Common Technology Collaborative Innovation Project, and Central University Basic Scientific Research Fund (N140206001, L1502045).

Technical Background

2205 duplex stainless steel is a ferrite-austenite (F-A) two-phase alloy with an approximately 50:50 phase balance in the base material. This microstructure provides an excellent combination of high strength (approximately 450–550 MPa yield strength), good corrosion resistance, and resistance to chloride stress corrosion cracking. It is widely used in oil and gas production equipment, chemical processing, marine applications, and desalination systems.

The welding of 2205 presents unique challenges:

• Maintaining the ferrite-austenite balance in the weld metal and heat-affected zone (HAZ) is critical for corrosion resistance
• Excessive heat input promotes phase transformations that shift the balance toward austenite, reducing strength and increasing susceptibility to pitting corrosion
• Widespread formation of intermetallic phases (such as sigma phase) at high temperatures can severely degrade toughness and corrosion resistance
• The Widmanstätten ferrite morphology in the HAZ can affect fatigue performance and crack propagation behavior

Heat Input Levels Investigated

Microstructural Observations

Heat-Affected Zone (HAZ)

The study examined the incomplete recrystallization zone (IRZ), which is the region of the HAZ experiencing temperatures between the Ac3 transformation temperature and the solidus temperature. Key findings include:

• Band austenite morphology: The undulation (waviness) at the edges of band austenite increases progressively with increasing heat input. At low heat input, the band boundaries are relatively straight; at high heat input, they become increasingly irregular and wavy.
• Band width: The width of band austenite increases with heat input. This is attributed to enhanced austenite growth during the prolonged thermal exposure associated with higher heat input.
• Phase balance: The ferrite-austenite ratio shifts toward increased austenite fraction with increasing heat input, potentially compromising the corrosion resistance properties that depend on maintaining adequate ferrite content.

Weld Metal

The weld metal microstructure exhibits distinct morphological variations across the weld cross-section:

• Weld center: Predominantly equiaxed blocky austenite particles dispersed in a ferrite matrix. The equiaxed morphology results from nucleation at heterogeneous sites during solidification.
• Near fusion line: Primarily Widmanstätten austenite (plate-like morphology). This morphology forms due to the constrained growth conditions near the rapidly cooled fusion boundary.
• Evolution with heat input: As heat input increases, Widmanstätten austenite gradually decreases while blocky austenite increases. The higher heat input promotes more time for austenite to nucleate and grow in equiaxed form rather than forming the constrained plate-like Widmanstätten morphology.

Phase Evolution Summary

Mechanical Properties

The mechanical property evolution with heat input follows predictable trends:

1. Tensile strength: Slight decrease with increasing heat input. The reduction is attributed to the coarsening of microstructural features and the shift toward higher austenite fraction, which has lower yield strength than ferrite.
2. Yield strength: Similar slight decrease with increasing heat input. The trend is consistent with the microstructural changes—coarser grains and increased austenite fraction reduce the overall strength level.
3. Elongation after fracture: Slight increase with increasing heat input. The enhanced ductility is attributed to the increased austenite fraction, which provides greater deformation capacity, and the reduced residual stress levels associated with higher heat input.
4. Microhardness distribution: The hardness profile from base metal to weld metal shows a characteristic pattern—hardness increases from the base metal toward the HAZ (where the highest hardness values are found), then decreases toward the weld center. The HAZ hardness maximum is attributed to the fine grain structure and martensitic transformation in the thin layers near the fusion boundary.
5. Hardness and phase fraction correlation: As heat input increases, the austenite volume fraction increases in all regions, and correspondingly, the microhardness decreases. This inverse relationship between austenite fraction and hardness is consistent with the known mechanical properties of the two phases (ferrite being harder than austenite).

Engineering Practice Implications

For engineers welding 2205 duplex stainless steel in industrial applications, this study provides critical guidance on heat input control:

• Optimal heat input range: The study suggests that moderate heat input (approximately 1.0–1.5 kJ/mm for typical MIG parameters) provides the best balance between joint strength, ductility, and corrosion resistance
• Maximum heat input: Heat input should be limited to prevent excessive austenite formation that would compromise pitting corrosion resistance and chloride stress corrosion cracking resistance
• Multi-pass welding: Each pass should be designed to maintain interpass temperature below 200°C to prevent excessive phase coarsening in previously deposited layers

For specific applications, additional considerations include:

• In high-chloride environments, maintaining ferrite content above 40% in the weld metal is essential for pitting corrosion resistance
• For cryogenic service, the toughness of the weld joint must be verified at the design temperature
• Post-weld heat treatment (PWHT) may be required for thick sections to relieve residual stresses without adversely affecting the phase balance

Key Insights and Reflections

The most significant finding of this study is the systematic documentation of how heat input drives microstructural evolution across all regions of the 2205 weld joint, with clear and predictable trends in both morphology and mechanical properties. The progressive transformation from Widmanstätten to blocky austenite morphology with increasing heat input is particularly important for fatigue performance, as Widmanstätten austenite plates can act as preferential crack paths under cyclic loading.

The correlation between microhardness and austenite volume fraction provides a practical non-destructive quality indicator. Since austenite fraction can be measured using magnetic methods (ferrite gauge), engineers can use ferrite content measurements as a proxy for hardness and, by extension, for the overall microstructural quality of the weld joint.

The slight but measurable decrease in strength with increasing heat input, combined with the increase in ductility, suggests that there exists an optimal heat input that maximizes the strength-ductility balance. For most structural applications, this optimum likely lies in the lower-to-moderate heat input range, where the weld retains sufficient strength while achieving adequate toughness.

This research contributes essential knowledge to the welding metallurgy of duplex stainless steels, providing engineers with quantitative understanding of heat input effects that can be directly applied to welding procedure specification and quality control in production environments.