Sigma Phase Embrittlement - Importance of Phase Balance in Duplex Stainless Steel (DSS)

Introduction to Duplex Stainless Steel (DSS):

Duplex stainless steels (DSS) are the preferred material for many engineering applications in the petroleum and refining industry, combining characteristics of both ferritic and austenitic stainless steel (SS) when welded correctly. When welded incorrectly, the potential to form detrimental intermetallic phases drastically increases, which could lead to a catastrophic failure.

• Ferritic Stainless Steels (FSS) have better resistance to Chloride Stress corrosion cracking & high strength (hardness) than ASS.
• Austenitic Stainless Steels (ASS) have better resistance to pitting corrosion, good ductility, good toughness than FSS.

When comparing DSS to Stainless Steels, DSS is more resistant than Austenitic SS to Stress Corrosion Cracking (SCC) but not as resistant as Ferritic SS; also, DSS toughness is typically superior to that of Ferritic SS but not as good as Austenitic SS.

Understanding the formation of Sigma Phase:

At high temperature applications, DSSs are prone to formation of secondary phases. Intermetallic based secondary phases like sigma and chi occur between 650 °C and 950 °C temperature range.

Sigma Phases can be formed by either of the following mechanisms

• Nucleation and growth from Ferrite
• Eutectoid Transformation of Ferrite into Secondary Austenite and Sigma
• Growth from Austenite after total consumption of Ferrite.

The preferred site for nucleation of Sigma phase is the ferrite-austenite and ferrite-ferrite grain boundaries. Sigma phase is a chromium rich and thus, it grows into chromium rich ferrite following the nucleation at grain boundaries.

The factors affecting the formation of Secondary Phases are the parameters of solution treatment process such as

• the cooling condition,
• holding temperature and duration.

Microstructure of Duplex Stainless Steel:

DSS are two phase alloys based on the iron-chromium-nickel (Fe-Cr-Ni) system. These materials typically comprise approximately equal amounts of Body-Centered Cubic (BCC) Ferrite, α-phase and Face-Centered Cubic (FCC) Austenite, γ-phase, in their microstructure.

• Maximum corrosion resistance and mechanical properties are achieved when the phase balance of ferrite to austenite is 50:50. However, achieving a 50:50 phase balance of ferrite to austenite (α → γ) in a weldment has proven to be difficult due to many variables as metal chemistry, welding processes, and thermal history of the steel.
• Experience coupled with testing has shown that DSS have optimal corrosion resistance and mechanical properties when 35% to 60% ferrite content is maintained throughout the weldment.
• DSS alloys entirely solidify primarily as ferrite at approximately 1425°C (2597°F) and partially transform to austenite at lower temperatures by a solid state reaction.
• If the cooling rate is rapid, very little ferrite will transform to austenite resulting in an excessive ferrite phase at room temperature. Consequently, the cooling rate of duplex welds must be slow enough to allow the transformation of approximately 50% of the ferrite to austenite and, at the same time, fast enough to prevent the formation of intermetallic phases and deleterious microstructures.

?Factors Affecting Phase Balance:

As discussed earlier, the parameters of solution treatment process such as the cooling condition, holding temperature and duration are the major factors responsible for the phase balance.

Let us try to understand the impact of these factors based on the experimental results with several heating temperatures and cooling cycles shown below

Samples - S1, S3, S5 have been Rapid Cooled

Samples – S2, S4, S6 have been Steady Cooled

Impact on Phase Balance:

?

• It is evident from the results that the steady cooled samples were consisted of ferrite, austenite and sigma phases. Steady cooling condition revealed lower cooling rates and sigma phase occurred at 900–600 °C temperature range during the cooling of solution-treated samples.
• Highest sigma phase ratio is obtained for the lowest solution treatment temperature and time. Sigma phase forms by the eutectoid decomposition of the ferrite.
• However, Rapid cooled samples were consisted of ferrite and austenite phases. It is also to be noted that Ferrite ratio was increased with increasing solution treatment temperature and time.
• Optimal Phase Balance can be observed at Sample 6 because with the higher solution treatment temperature and time, ferrite phase became more stable and during the steady cooling, reformation of austenite occurred at higher rates, while the formation of sigma was obtained at the lowest rate.
• Higher Solution Treatment temperature and duration provides optimal phase balance.

Impact on Grain Size:

• It can be noted that the Higher treatment temperature and time decreases the Grain boundary length?
• Decreased Grain Boundary length means the nucleation sites for Sigma formation are reduced.

Examination / Tests performed to Detect Sigma Phase Embrittlement

1. Chemical Composition Analysis - To check if the chemical composition of the material is in accordance with ASTM specification requirements.
2. Tensile Test - To check if the mechanical properties (Ductility, Toughness) is in accordance with ASTM specification requirements. Sigma Phase usually reduces the ductility & toughness of the affected material.
3. Ferrites Check Measurement - To check the ferrite percentage. Sigma Phase formation will deplete the ferrite percentage in the material.
4. Metallographic Examination - To check the microstructure for detecting the formation of sigma phases.
5. Hardness Testing - To check the hardness of material. Sigma Phase formation will significantly increase the harness of the material.
6. Scanning Electron Microscopy - To detect the mode of fracture. Sigma phase embrittlement will undergo brittle fracture indicating chevron marks.

Summary:

• DSS have optimal corrosion resistance and mechanical properties when 35% to 60% ferrite content is maintained.
• With increasing solution treatment temperature and time, the ferrite content of the alloy increases, while the ferrite grain boundary length decreases.
• The ferrite grain boundary is very important for sigma phase formation. As the ferrite grain grew and the ferrite grain boundary decreased, the ratio of the sigma phase occurred in samples decreased.
• Ferrite phase is more effective on hardness than sigma phase.

Reference:

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