What harmful phases are easily precipitated in duplex steel within certain temperature ranges

Duplex stainless steel (DSS) is widely used in the oil and gas, chemical, and offshore engineering sectors for its high strength and excellent corrosion resistance. However, the high performance of DSS relies on its precisely balanced microstructure of austenite (γ) and ferrite (δ). When DSS is exposed to or operated for extended periods within certain temperature ranges, the ferrite phase decomposes, precipitating various "deleterious phases." These precipitates severely impair the material's mechanical toughness and corrosion resistance, posing a significant threat to the reliability of engineering applications.

1. Brittleness Killer: Precipitation of σ and χ Phases

Of all deleterious phases, the σ phase is undoubtedly the most well-known and destructive.

Precipitation Temperature Range: σ phase precipitates primarily between 600°C and 950°C, with its precipitation kinetics peaking around 800°C to 880°C.

Chemical Composition: σ phase is an intermetallic compound rich in chromium (Cr) and molybdenum (Mo). It forms through the decomposition of δ ferrite or the eutectoid decomposition reaction at the interface between δ ferrite and γ austenite.

Performance Impact: The precipitation of σ phase has a two-pronged impact on the engineering properties of DSS. First, the σ phase itself is a hard and brittle phase. Its presence sharply reduces the material's impact toughness, making it susceptible to brittle fracture at low temperatures or under stress concentration conditions. Second, during precipitation, the σ phase consumes significant amounts of Cr and Mo from the surrounding δ ferrite matrix, resulting in Cr- and Mo-depleted regions surrounding the σ phase. These depleted regions significantly reduce the corrosion resistance, becoming vulnerable to pitting and intergranular corrosion.

Chi phase is also a Cr- and Mo-rich intermetallic compound that typically forms within a similar temperature range as σ phase (700°C to 900°C). However, χ phase typically precipitates preferentially as a metastable phase at the beginning of aging, only later transforming into the more stable σ phase. Its negative impact on properties is similar to that of σ phase, leading to embrittlement and decreased corrosion resistance.

2. 475°C Embrittlement: A Hidden Threat at Low Temperatures

In addition to the σ phase in high-temperature regions, duplex stainless steel also experiences a danger zone at lower temperatures, known as 475°C embrittlement.

Precipitation Temperature Range: This phenomenon occurs between 350°C and 550°C, with peak severity around 475°C.

Micromechanism: Within this temperature range, the delta ferrite phase undergoes spinodal decomposition, breaking down into two nanoscale ferrite structures: a chromium-rich α′ phase (Cr-rich α′) and a chromium-poor α phase (Cr-poor α).

Performance Impact: This nanoscale phase separation significantly increases the material's hardness and strength, but sharply decreases its impact toughness. While this low-temperature embrittlement is less severe and pervasive than σ phase precipitation on corrosion resistance, the chromium-rich α′ phase can also lead to increased corrosion susceptibility in certain media. It is worth noting that spinodal decomposition typically requires a long aging period, but precipitation kinetics may be accelerated in cold-worked material.

3. Carbonitrides and Secondary Austenite

In addition to the primary precipitates mentioned above, other deleterious phases may form under certain conditions:

Carbides and Nitrides: Between 550°C and 750°C, chromium carbides (Cr23​C6​) or nitrides may precipitate. Although the carbon (C) content of modern DSS is typically kept to extremely low levels (≤0.03%), these precipitates may still form at grain boundaries, consuming Cr and posing a risk of intergranular corrosion.

Secondary Austenite (γ2​): During the precipitation of the σ phase, the decomposition of δ ferrite simultaneously forms nickel-rich secondary austenite (γ2​). While γ2​ itself is not a directly deleterious phase, its formation mechanism is closely linked to the precipitation of the σ phase. Its presence signals the decomposition of δ ferrite, indirectly signaling deterioration in material properties.