Effect of Sigma Phase on Duplex Stainless Steel Hardfacing Layer Microstructure and Corrosion Performance

Literature Overview

The paper by Zhang Jiawei, Huang Meiping, Hu Chuanshun, Ai Zhongyang, and Yan Lijun, published in Hot Working Technology (Vol. 36, No. 19, 2007, pp. 7-9), investigates the formation and detrimental effects of sigma (σ) phase in duplex stainless steel hardfacing layers subjected to heat treatment at 900°C for various holding times. This study is critically important for engineers working on corrosion-resistant overlay applications in the petrochemical, offshore, and marine industries, where duplex stainless steel hardfacing is commonly used to protect carbon steel components from aggressive environments.

Core Technical Findings

The research demonstrates that heat treatment at 900°C, which is within the sigma phase formation temperature range for duplex stainless steels, leads to progressive sigma phase precipitation with increasing holding time. The sigma phase formation is accompanied by a decrease in ferrite volume fraction to below 40%, increased hardness, and severe degradation of both pitting and intergranular corrosion resistance.

Sigma Phase Formation Mechanism

The sigma phase (FeCr) is a brittle intermetallic compound with a tetragonal crystal structure that forms in duplex stainless steels during prolonged exposure to temperatures between 600-1000°C. The formation mechanism involves:

• Nucleation: Sigma phase nucleates preferentially at ferrite-austenite phase boundaries and within the ferrite phase, where chromium diffusion is most active.
• Growth: The phase grows by chromium diffusion from the surrounding ferrite, leading to chromium depletion in the adjacent matrix.
• Ferrite consumption: As sigma phase forms, the ferrite phase is consumed, reducing the ferrite volume fraction below the optimal 40-60% range for duplex stainless steels.
• Chromium depletion: The chromium-depleted regions adjacent to sigma phase particles become susceptible to intergranular corrosion, as the local chromium content falls below the critical threshold of approximately 12% for pitting resistance.

Engineering Implications for Hardfacing Applications

The findings of this study have profound implications for the design and application of duplex stainless steel hardfacing:

• Service temperature limitation: Duplex stainless steel hardfacing should not be used in applications where the component is exposed to temperatures exceeding 600°C for extended periods, as sigma phase formation will compromise corrosion resistance.
• Heat input control: During hardfacing welding, the heat input should be minimized to reduce the time spent in the sigma phase formation temperature range. Low heat input processes such as plasma arc welding or laser cladding are preferred over conventional submerged arc or GMAW processes.
• Post-weld heat treatment: If post-weld heat treatment is required for stress relief, the temperature should be limited to below 600°C, and holding times should be minimized. Alternatively, solution treatment at 1050-1100°C followed by rapid quenching can dissolve sigma phase, but this may cause grain growth and other microstructural issues.
• Welding procedure optimization: Multi-pass welding with controlled interpass temperatures below 150°C can limit the cumulative time at elevated temperatures and reduce sigma phase formation.

Corrosion Performance Assessment

The severe degradation of both pitting and intergranular corrosion resistance observed in this study is consistent with the well-documented detrimental effects of sigma phase in duplex stainless steels. The pitting resistance is compromised by chromium depletion in the ferrite matrix, while the intergranular corrosion resistance is reduced by the formation of chromium-depleted zones along grain boundaries. For engineers specifying duplex stainless steel hardfacing for chloride-containing environments, the following guidelines should be followed:

• Material selection: For applications involving elevated temperature service, consider alternative hardfacing materials such as austenitic stainless steels (309L, 316L) or nickel-based alloys (Alloy 625, Alloy 825) that are more resistant to sigma phase formation.
• In-service monitoring: For existing duplex stainless steel hardfacing in service, periodic non-destructive examination should include assessment of corrosion resistance, particularly after any thermal events that may have exposed the overlay to elevated temperatures.
• Repair procedures: When repairing duplex stainless steel hardfacing, the repair welding procedure should be designed to minimize heat input and avoid re-exposure to sigma phase formation temperatures.

Study Insights and Reflections

This research provides a clear and quantitative demonstration of the sigma phase problem in duplex stainless steel hardfacing, reinforcing the importance of thermal management in both the welding process and the service environment. The observation that ferrite volume fraction drops below 40% is particularly significant, as this falls outside the optimal duplex range and compromises both mechanical and corrosion properties. For engineers involved in the selection and application of duplex stainless steel hardfacing, this study underscores the need for a holistic approach that considers not only the initial weld properties but also the long-term microstructural stability under service conditions. The findings also highlight the value of heat treatment studies in understanding the microstructural evolution of hardfacing alloys, as such knowledge is essential for predicting and preventing in-service degradation. In practice, the sigma phase problem represents one of the most significant limitations of duplex stainless steel hardfacing, and engineers must carefully evaluate the thermal history of the component throughout its service life to ensure continued corrosion protection.