Sigma Phase: An Intermetallic That Can Harm Stainless Steel
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Quick Answer: Sigma phase is a hard, brittle intermetallic compound, roughly FeCr in composition, that precipitates in stainless steels held in the approximate range of 565 to 925°C, with the fastest formation typically around 700 to 900°C. It forms mainly from delta ferrite and drains chromium and molybdenum from the surrounding matrix, causing a sharp drop in impact toughness and a loss of corrosion resistance. Duplex and super duplex grades, and austenitic welds with elevated delta ferrite, are the most susceptible.
Stainless steel earns its name from a passive chromium oxide layer, but the same chromium and molybdenum that give it corrosion resistance can also work against it under the wrong thermal history. Sigma phase is the best-known example: an intermetallic compound that quietly precipitates during welding, post-weld heat treatment, or prolonged high-temperature service, and leaves behind a material that looks like stainless steel but no longer behaves like it in an impact test or a corrosion test.
This guide explains what sigma phase actually is, why duplex and austenitic stainless steels are both susceptible in different ways, the temperature range where it forms fastest, how it is detected, and the practical steps fabricators and welding engineers use to keep it out of a finished weldment.
- Sigma phase is a hard, non-magnetic, brittle intermetallic compound with an approximate FeCr composition.
- It forms mainly by transformation of delta ferrite, and forms faster with higher chromium, molybdenum, titanium, and silicon content.
- The typical formation range is approximately 565 to 925°C, with the fastest kinetics usually cited around 700 to 900°C.
- Sigma phase depletes chromium and molybdenum from the surrounding matrix, causing both a loss of impact toughness and a loss of corrosion resistance at the same time.
- Duplex and super duplex stainless steels are especially susceptible due to their higher chromium and molybdenum content and multi-phase microstructure; ASTM A923 provides standard test methods to detect harmful intermetallic phases in duplex grades.
What Is Sigma Phase?
Sigma phase is a hard, brittle intermetallic compound, with an approximate chemical formula of FeCr, that precipitates within the microstructure of iron-chromium alloys, including stainless steels, when they are exposed to a specific elevated temperature range for a sufficient time. Unlike a carbide or nitride, sigma phase is a true intermetallic: a compound with its own distinct crystal structure formed directly between metal atoms, rather than between a metal and carbon or nitrogen.
Sigma phase is not magnetic, is significantly harder than the surrounding austenite or ferrite matrix, and is essentially non-ductile. A microstructure that develops a meaningful fraction of sigma phase behaves, mechanically, less like a tough stainless steel and more like a brittle ceramic distributed through a metal matrix, particularly once the sigma fraction becomes continuous along grain or phase boundaries.
Where it comes from: In both austenitic and duplex stainless steels, sigma phase forms predominantly from delta ferrite. Austenitic stainless steel weld metal is deliberately formulated to retain a small amount of delta ferrite, commonly a few percent, specifically to resist solidification (hot) cracking. That same delta ferrite becomes the primary source of sigma phase if the weld metal is later exposed to the sigma formation temperature range for long enough.
How and When Does Sigma Phase Form?
Sigma phase formation is a time-and-temperature dependent solid-state transformation, meaning it requires both the right temperature range and enough time at that temperature to develop a significant fraction. The commonly cited formation range is approximately 565 to 925°C (1050 to 1700°F), with the fastest formation kinetics typically reported in the 700 to 900°C (1290 to 1650°F) band.
| Situation | Why It Matters |
|---|---|
| Multi-pass welding, slow interpass cooling | Each subsequent pass can reheat earlier passes and the HAZ back into the sigma formation range |
| Post-weld heat treatment (PWHT) | A PWHT temperature or heating/cooling ramp that dwells in the sigma range can precipitate sigma even if the peak temperature is chosen to avoid it |
| Elevated-temperature service | Components operating continuously within the sigma range accumulate sigma phase over months or years, even without any single high-temperature excursion |
| Improper solution annealing or quenching | Slow cooling through the sigma range after solution annealing can reintroduce sigma phase that the anneal was meant to dissolve |
Alloying content strongly influences how readily sigma phase forms. Higher chromium and molybdenum content accelerate sigma formation, which is exactly why duplex and super duplex grades, with their higher Cr and Mo content compared to standard austenitic grades, are more prone to sigma phase and form it faster. Titanium and silicon also promote sigma phase formation, while nitrogen and carbon tend to reduce the tendency to form it, one of several reasons nitrogen is a deliberate alloying addition in modern duplex stainless steel grades.
What Effects Does Sigma Phase Have on Stainless Steel?
Sigma phase causes two distinct and equally serious problems at the same time, because both stem from the same underlying mechanism: chromium and molybdenum are drawn out of the surrounding matrix to form the sigma particles, leaving the adjacent matrix locally depleted.
- Loss of impact toughness. Charpy impact toughness has been shown to drop sharply, in some studies exponentially, as sigma phase content increases, since the hard, brittle sigma particles provide easy crack initiation and propagation paths, particularly once they form a continuous network along phase or grain boundaries.
- Loss of corrosion resistance. The chromium and molybdenum depletion in the matrix immediately adjacent to sigma particles reduces the local corrosion resistance, even though the bulk chemical analysis of the material may still meet specification. This can produce localized corrosion or intergranular attack that a simple mill certificate would not predict.
- Increased hardness. Sigma-affected material typically shows elevated hardness relative to unaffected material, which is one of the simpler field indicators that something has changed in the microstructure, though hardness testing alone cannot confirm sigma phase is the cause.
Caution: Sigma phase embrittlement is especially dangerous because it can be present without any visible surface indication. A component can pass visual and dimensional inspection, and even pass a standard hardness check, while still carrying a significantly reduced fracture toughness and corrosion margin due to sigma phase in the microstructure.
Duplex vs Austenitic Stainless Steel Susceptibility
Both duplex and austenitic stainless steels can develop sigma phase, but the practical risk profile differs between the two families:
| Aspect | Duplex / Super Duplex | Austenitic |
|---|---|---|
| Typical source phase | Ferrite phase (roughly half the microstructure) | Residual delta ferrite in weld metal (commonly a few percent) |
| Relative susceptibility | Higher, due to elevated Cr and Mo content Higher risk | Lower, but still significant in high-restraint, multi-pass welds |
| Grades of particular concern | Super duplex grades such as UNS S32750, S32760 | Fully austenitic or low-ferrite grades welded with high heat input |
| Standard detection method | ASTM A923 (Practices A, B, and C) | Metallographic examination, hardness survey, impact testing |
Because delta ferrite is the primary feedstock for sigma phase in austenitic welds, the same delta ferrite content that protects against solidification cracking during welding becomes a long-term embrittlement liability if the weld is later exposed to elevated temperature. Filler metal and welding parameter selection for austenitic stainless steel therefore has to balance these two competing requirements, commonly by keeping delta ferrite content low enough to limit long-term sigma risk while still high enough to resist hot cracking during solidification.
Sigma Phase vs 475°C Embrittlement
Sigma phase is sometimes confused with a related but distinct phenomenon called 475°C embrittlement (also written as 885°F embrittlement). Both cause a loss of toughness in ferrite-containing stainless steels, but the mechanisms and temperature ranges are different:
- Sigma phase embrittlement is caused by the formation of a distinct intermetallic phase, typically in the 565 to 925°C range, and is associated with chromium and molybdenum depletion of the surrounding matrix.
- 475°C embrittlement occurs in the lower temperature range of roughly 400 to 550°C (with the effect most pronounced near 475°C), and results from spinodal decomposition of the ferrite phase into chromium-rich and iron-rich domains within the ferrite itself, without forming a separate sigma phase.
Both mechanisms reduce toughness through chromium-related microstructural changes in ferrite, and both become more pronounced with higher chromium content, but they occur in different temperature windows and respond differently to remediation. A material susceptible to one is not necessarily equally susceptible to the other, and both should be considered separately when reviewing a component’s thermal history.
How Is Sigma Phase Detected?
Because sigma phase can be present without visible surface indication, detecting it reliably requires methods that go beyond routine visual inspection:
- Metallographic examination. A polished and etched cross-section, examined under a microscope, can reveal sigma phase particles directly, particularly with an etchant selected to highlight the phase.
- Impact (Charpy) testing. A drop in impact toughness compared to the expected value for the material and condition is a strong practical indicator, since toughness loss is one of the most sensitive consequences of sigma phase formation.
- ASTM A923 test methods. For duplex stainless steel, ASTM A923 provides standardized test methods, commonly referred to as Practices A, B, and C, covering etch testing, impact testing, and a ferric chloride corrosion test, specifically to detect harmful intermetallic phases including sigma phase.
- Hardness surveys. An unexpectedly elevated hardness reading can prompt further investigation, though hardness alone cannot confirm sigma phase without corroborating metallography or impact data.
Field tip: Where a component’s thermal history is uncertain, for example after an unplanned furnace excursion or an extended shutdown at elevated temperature, prioritize impact testing and, where practical, ASTM A923-style corrosion testing over hardness testing alone. Hardness can look normal even when corrosion resistance has already been meaningfully degraded by localized chromium depletion.
How Is Sigma Phase Prevented?
Sigma phase prevention focuses on limiting time spent in the formation temperature range and on controlling the alloy content and microstructure that make sigma formation more likely:
- Control welding heat input and interpass temperature. Lower heat input and controlled interpass temperature reduce the time each weld pass and the surrounding HAZ spend in the sigma formation range.
- Select filler metal and delta ferrite content carefully. Balance the delta ferrite level needed to resist solidification cracking against the long-term sigma risk that delta ferrite represents, per the applicable welding procedure and material specification.
- Control PWHT parameters precisely. Where PWHT is required, control both the peak temperature and the heating and cooling rates through the sigma formation range to minimize dwell time in that band.
- Solution anneal and quench correctly. Where solution annealing is used to dissolve existing sigma phase, ensure the subsequent quench is fast enough to prevent sigma from reprecipitating while cooling back down through the formation range.
- Avoid unplanned high-temperature exposure. Fire events, process upsets, or extended operation above the design temperature can inadvertently expose equipment to the sigma range; components with a known or suspected such exposure should be evaluated before returning to service.
Quick Reference: Sigma Phase at a Glance
| Property | Value / Behavior |
|---|---|
| Approximate composition | FeCr intermetallic compound |
| Formation temperature range | Approximately 565-925°C (1050-1700°F) |
| Fastest formation kinetics | Approximately 700-900°C (1290-1650°F) |
| Primary source in the microstructure | Delta ferrite |
| Promoted by | Chromium, molybdenum, titanium, silicon |
| Suppressed by | Nitrogen, carbon |
| Main effects | Loss of impact toughness, loss of corrosion resistance, increased hardness |
| Most susceptible grades | Duplex and super duplex stainless steels (e.g. S32750, S32760) |
Common Mistakes and Limitations
- Relying on visual inspection alone. Sigma phase leaves no reliable visual signature; it requires metallography, impact testing, or standardized methods such as ASTM A923 to detect with confidence.
- Confusing sigma phase with 475°C embrittlement. The two mechanisms occur in different temperature ranges and involve different microstructural changes; treating them as interchangeable can lead to the wrong root-cause conclusion after a failure.
- Assuming a passed hardness test rules out sigma phase. Hardness can remain within an acceptable range even when localized corrosion resistance has already been compromised by chromium depletion around sigma particles.
- Ignoring cumulative service exposure. Sigma phase can accumulate gradually during long-term elevated-temperature service, not just during a single welding or heat treatment event; equipment thermal history over its full service life matters, not just the fabrication record.
- Over-restricting delta ferrite without considering hot cracking risk. Reducing delta ferrite content to suppress sigma phase risk can increase susceptibility to solidification cracking in austenitic welds; both risks need to be balanced against each other in filler metal selection.
Susceptibility and exact formation kinetics vary by specific alloy composition and thermal history; confirm the applicable material specification and welding procedure requirements for the actual grade and service conditions on your project.
Key Terms
- Sigma Phase
- A hard, brittle intermetallic compound, approximately FeCr in composition, that precipitates in iron-chromium alloys within a specific elevated temperature range.
- Delta Ferrite
- A ferrite phase retained in austenitic stainless steel weld metal, typically a few percent, to resist solidification cracking; also the primary source phase for sigma formation.
- Intermetallic Compound
- A compound formed between two or more metal elements with its own distinct crystal structure, generally hard and brittle compared to the surrounding metal matrix.
- 475°C Embrittlement
- A separate toughness-loss mechanism in ferrite-containing stainless steels, caused by spinodal decomposition of ferrite in the 400-550°C range, most pronounced near 475°C.
- Chromium Depletion
- A local reduction in chromium content in the matrix immediately surrounding a precipitate, reducing corrosion resistance in that zone even when bulk composition meets specification.
- Super Duplex Stainless Steel
- A high-alloy duplex stainless steel family, such as UNS S32750 and S32760, with elevated chromium and molybdenum content and correspondingly higher sigma phase susceptibility.
Frequently Asked Questions
What temperature range causes sigma phase to form in stainless steel?
Sigma phase typically forms in the approximate range of 565 to 925 degrees Celsius (1050 to 1700 degrees Fahrenheit), with the fastest formation kinetics generally reported in the 700 to 900 degree Celsius band. Actual formation time and extent depend on the specific alloy composition and how long the material dwells in this range.
Why are duplex stainless steels more prone to sigma phase than austenitic grades?
Duplex and super duplex stainless steels contain roughly equal parts ferrite and austenite, with the ferrite phase acting as a larger, more readily available source for sigma phase transformation. They also generally have higher chromium and molybdenum content than standard austenitic grades, both of which accelerate sigma phase formation, making super duplex grades such as S32750 and S32760 particularly susceptible.
Does sigma phase affect corrosion resistance as well as toughness?
Yes. Sigma phase forms by drawing chromium and molybdenum out of the surrounding matrix, which locally depletes those elements right where corrosion resistance depends on them most. This means sigma phase reduces both impact toughness and localized corrosion resistance at the same time, even when the bulk chemical composition of the material still meets specification.
How is sigma phase different from 475°C embrittlement?
Sigma phase embrittlement is caused by the formation of a distinct intermetallic phase in the 565 to 925 degree Celsius range. 475 degree Celsius embrittlement occurs at a lower temperature range, roughly 400 to 550 degrees Celsius, and results from spinodal decomposition within the ferrite phase itself rather than the formation of a separate sigma phase. Both reduce toughness but through different mechanisms and in different temperature windows.
Can sigma phase be reversed once it has formed?
Sigma phase can generally be dissolved back into the matrix through a solution anneal at a sufficiently high temperature, followed by a fast enough quench to prevent it from reprecipitating while cooling back down through the sigma formation range. Whether this is practical depends on the component’s size, the achievable cooling rate, and whether re-solutionizing is compatible with the equipment’s design and service history.
What test method is used to detect sigma phase in duplex stainless steel?
ASTM A923 provides standardized test methods for detecting detrimental intermetallic phases, including sigma phase, in duplex stainless steels, commonly referred to as Practice A (etch test), Practice B (impact test), and Practice C (ferric chloride corrosion test).
Does delta ferrite content in austenitic welds increase sigma phase risk?
Yes. Delta ferrite, deliberately retained in austenitic stainless steel weld metal to resist solidification cracking, is the primary source phase for sigma formation. Higher delta ferrite content generally increases long-term sigma phase susceptibility if the weld is later exposed to the sigma formation temperature range, which is why filler metal selection has to balance hot cracking resistance against long-term embrittlement risk.
Standards and References
- ASTM A923, Standard Test Methods for Detecting Detrimental Intermetallic Phase in Duplex Austenitic/Ferritic Stainless Steels, ASTM International.
- ASTM A790/A790M, Standard Specification for Seamless and Welded Ferritic/Austenitic Stainless Steel Pipe, ASTM International – referenced for duplex stainless steel pipe covered by A923 testing.
- ASTM A262, Standard Practices for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels, ASTM International – related intergranular corrosion test methods.
Conclusion
Sigma phase is a reminder that a stainless steel’s corrosion resistance and its long-term mechanical integrity are governed by the same chromium and molybdenum content, and that content can work against the material if it is held in the wrong temperature range for too long. Whether the source is an uncontrolled multi-pass weld, an imprecise PWHT cycle, or years of elevated-temperature service, the outcome is the same: a component that still looks like sound stainless steel but has lost a meaningful share of its toughness and corrosion margin. Controlling heat input, filler metal delta ferrite content, and PWHT parameters, combined with the right detection method when sigma phase is suspected, is what keeps this intermetallic from becoming a field failure. For related metallurgy topics, see the duplex stainless steel welding guide and the high-temperature material properties guide on WeldFabWorld, both of which intersect directly with sigma phase risk.
About This Guide: This article was prepared by the WeldFabWorld technical team from the standards listed in the References section above. Susceptibility and formation kinetics vary by specific alloy and thermal history; verify current test methods and acceptance criteria against the specific standard edition referenced in your project’s material specification before use.