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

Literature Overview

This paper by Zhang Jiawei and colleagues from Liaoning Petrochemical University investigates the detrimental effects of sigma phase precipitation on the microstructure and corrosion resistance of duplex stainless steel hardfacing layers. Published in Hot Working Technology in 2007, the study addresses a critical issue in the application of duplex stainless steel overlays: the formation of intermetallic phases during heat treatment that can severely degrade corrosion performance. The research employed controlled heat treatment at 900°C with varying holding times to deliberately precipitate different amounts of sigma phase, followed by comprehensive microstructural and corrosion testing.

Core Technical Findings

The study systematically examined the effects of sigma phase formation on the microstructure, hardness, and corrosion resistance of duplex stainless steel hardfacing layers. The key finding is that sigma phase precipitation, combined with changes in ferrite content, severely degrades the corrosion performance of the overlay while increasing its hardness.

The ferrite content in the duplex stainless steel hardfacing layer decreased below 40% after heat treatment, which is below the typical target range of 40-60% ferrite for optimal duplex stainless steel properties. The sigma phase, an intermetallic compound with a tetragonal crystal structure rich in chromium and molybdenum, formed preferentially in the ferrite phase, depleting the surrounding matrix of these alloying elements and creating localized areas of reduced corrosion resistance.

Metallurgical Mechanism Analysis

Sigma phase (σ-phase) is a body-centered tetragonal intermetallic compound with the general formula Cr15Mo7, which forms in duplex stainless steels during prolonged exposure to temperatures in the range of 600-900°C. The formation of sigma phase is thermodynamically favorable in alloys with high chromium and molybdenum content, particularly in the ferrite phase. The precipitation of sigma phase is accompanied by the depletion of chromium and molybdenum from the surrounding matrix, creating chromium-depleted zones that are highly susceptible to intergranular and pitting corrosion.

The decrease in ferrite content below 40% is also significant. In duplex stainless steels, the balance between austenite and ferrite phases is critical for achieving optimal mechanical and corrosion properties. A ferrite content below 40% indicates a shift toward a more austenitic microstructure, which may affect the mechanical properties and the resistance to stress corrosion cracking. The combined effect of sigma phase precipitation and ferrite content reduction creates a complex microstructure that is significantly different from the intended duplex structure.

The increase in hardness observed with sigma phase formation is expected, as sigma phase is a very hard and brittle intermetallic compound. However, this increase in hardness comes at the cost of corrosion resistance, which is the primary reason for selecting duplex stainless steels in many applications. Engineers must recognize that hardness and corrosion resistance are not always positively correlated in duplex stainless steel systems, and optimizing for one property may sacrifice the other.

Engineering Practice Implications

For engineers working with duplex stainless steel hardfacing layers, this study provides critical guidance on heat treatment limitations. The following practical recommendations can be derived:

1. Avoid heat treatments that involve prolonged exposure to temperatures above 600°C. If post-weld heat treatment is necessary, it should be performed at temperatures below 600°C to minimize sigma phase formation.
2. The total time spent in the sigma phase formation temperature range (600-900°C) should be minimized. This includes not only the intentional heat treatment but also the time spent in this range during welding, cooling, and any subsequent thermal processing.
3. The ferrite content of the hardfacing layer should be monitored and maintained within the target range of 40-60% ferrite. Ferrite content can be measured using ferrite gauges or through metallographic analysis after sectioning.
4. For applications requiring both high hardness and good corrosion resistance, alternative approaches such as martensitic stainless steel hardfacing layers or multi-layer overlays with a corrosion-resistant cap layer should be considered.
5. In welding procedures involving duplex stainless steel hardfacing, the interpass temperature should be controlled to minimize the time spent in the sigma phase formation range. Interpass temperatures below 150°C are generally recommended for duplex stainless steels.

The corrosion testing results showing severe degradation of both pitting and intergranular corrosion resistance are particularly concerning for applications in aggressive environments, such as chemical processing, marine environments, and nuclear applications. The chromium-depleted zones surrounding sigma phase particles create preferential sites for corrosion initiation, and the intergranular nature of sigma phase precipitation means that corrosion can propagate along grain boundaries, leading to rapid material loss.

Key Questions and Reflections

A significant question raised by this study is the relationship between sigma phase formation during welding and sigma phase formation during post-weld heat treatment. In practice, the welding process itself can introduce some sigma phase if the interpass temperature is too high or if the cooling rate is too slow. The study's deliberate heat treatment at 900°C may overestimate the sigma phase content that would form during normal welding practice, but it does highlight the sensitivity of the microstructure to thermal exposure.

Another important consideration is the role of welding procedure on sigma phase formation. Different welding processes, such as GTAW, GMAW, and SAW, produce different thermal cycles that can influence the amount of sigma phase formed. Engineers should select welding processes and parameters that minimize the time spent in the sigma phase formation temperature range, such as using high heat input processes with rapid cooling rates or using processes with low dilution rates.

The study also raises questions about the repairability of duplex stainless steel hardfacing layers that have already formed sigma phase. Once sigma phase has formed, it cannot be easily removed by re-welding or additional heat treatment, as the process of forming sigma phase is thermodynamically favorable and can be reversed only by solution treatment at temperatures above 1100°C, which is not practical for most applications. This means that sigma phase formation is essentially irreversible in service, and prevention is the only viable strategy.

Study Insights and Outlook

This study underscores the importance of understanding the metallurgical stability of duplex stainless steel hardfacing layers under thermal exposure. The findings have direct implications for the design and qualification of welding procedures for duplex stainless steel overlays, as well as for the inspection and maintenance of in-service components. For engineers, the key takeaway is that duplex stainless steel hardfacing layers must be protected from thermal exposure that could lead to sigma phase formation, and that the corrosion resistance of these layers is highly sensitive to microstructural changes. Future research should focus on developing duplex stainless steel hardfacing compositions that are more resistant to sigma phase formation, such as alloys with reduced molybdenum content or with the addition of elements that inhibit sigma phase nucleation. Additionally, the development of advanced inspection techniques for detecting sigma phase in service components would be valuable for condition monitoring and life assessment purposes.