Technical knowledge
Sigma phase in super duplex: how it forms and how to prevent it
What sigma phase is, the temperature range where it forms, why it removes toughness and corrosion resistance, how it is detected and the heat treatment and welding controls that prevent it.
What sigma phase actually does
Sigma phase forms from the ferrite phase and is rich in chromium and molybdenum. Because it removes those elements from the surrounding matrix, the adjacent material becomes depleted and loses pitting resistance. At the same time the hard, brittle particles provide crack initiation sites, so impact energy falls dramatically. Both effects occur at volume fractions far too small to see in a normal microstructure survey.
Where it comes from in real projects
| Cause | How it happens | Control |
|---|---|---|
| Slow cooling after annealing | Air cooling a thick section | Water quench, recorded cooling curve |
| Multi pass welding | High heat input and high interpass temperature | Qualified procedure, controlled interpass, limited bead size |
| Stress relieving | Cycle in the precipitation range | Avoid; use full solution anneal if relief is needed |
| Hot forming | Forming below the recommended temperature | Follow the alloy supplier forming limits |
| Long service at temperature | Operation above the recommended limit | Respect the 300 C guidance |
Detection and acceptance
- Impact testing. The most sensitive and most commonly specified check.
- Corrosion testing. ASTM G48 pitting or crevice testing at a defined temperature with an acceptance mass loss.
- Metallography. Microstructural examination with a trained eye, useful for failure investigation.
- Ferrite measurement. A drop in magnetic ferrite reading can indicate transformation, though it is not conclusive on its own.
Prevention on the purchase order
State the solution annealing temperature range and require water quenching, request the heat treatment chart for every heat lot, specify impact testing at the design minimum temperature, and require corrosion testing to ASTM G48 where the service is aggressive. For welded fabrication, approve the procedure, limit heat input and interpass temperature, and require production test plates where the specification calls for them.
Questions about this topic
Can sigma phase be removed once it has formed?
Yes, by a full solution anneal within the specified temperature range followed by water quenching, provided the component geometry tolerates it. The cycle must be controlled and the material re-tested afterwards.
Is sigma phase visible to the naked eye?
No. It is a microstructural feature that requires metallographic preparation and examination, which is why impact and corrosion testing are the practical acceptance methods.
Does duplex 2205 have the same problem?
Yes, but super duplex is more susceptible because its higher chromium and molybdenum content provides a greater driving force for precipitation. The same controls apply to both.
Questions about our products
What products does Hangbo Alloy supply?
Four product families, all available in nickel-based alloys, titanium, duplex and special stainless steel: flanges, butt weld pipe fittings, elbows, and seamless tube and pipe. Flanges cover weld neck, long weld neck, slip-on, blind, socket weld, threaded and lap joint patterns. Fittings and elbows cover concentric and eccentric reducers, equal and reducing tees, caps, stub ends, and 45°, 90° and 180° returns in both long and short radius, plus 3D and 5D bends for piggable lines. Seamless tube and pipe is supplied cold drawn or hot finished from 6 mm to 630 mm outside diameter.
Which alloy grades are available for these products?
More than 90 grades are published in our grade library. The most requested are Inconel 625, 718, 600 and X-750, Hastelloy C-276, C-22 and B-2, Monel 400 and K-500, Incoloy 800H, 825 and 925, duplex 2205 and super duplex 2507, 316L, 904L and 254SMO stainless steel, and titanium Grade 2 and Grade 5. Nickel-based grades can also be supplied with VIM plus ESR double melting where cleanliness or fatigue life is critical.
What sizes, wall thicknesses and pressure classes can you supply?
Flanges run from NPS ½ to NPS 24 to ASME B16.5 and from NPS 26 to NPS 60 to ASME B16.47 Series A or B, in pressure classes 150 through 2500. Butt weld fittings and elbows run from NPS ½ to NPS 48 (DN 15 to DN 1200) in wall thickness Schedule 5S through XXS. Seamless tube and pipe covers 6 mm to 630 mm outside diameter, in random lengths of about 5 to 7 metres or fixed lengths up to 12 metres. Non-standard and intermediate sizes are quoted on request.
Which material and dimensional standards do you work to?
Forged flanges to ASTM B564, B381 and A182 with dimensions to ASME B16.5 and ASME B16.47. Butt weld fittings and elbows to ASTM B366 and A403 with dimensions to ASME B16.9 and MSS SP-43. Seamless tube and pipe to ASTM B444, B423, B622, B163, B167, B861, A312, A213 and A790. Marking follows MSS SP-25 and the applicable product standard. If your project is governed by EN, DIN, JIS or a company specification, send the document and we will confirm equivalence before quoting.
Is there a minimum order quantity?
There is no minimum for standard sizes and grades held in stock, and prototypes, spares and small trial batches are accepted. Made-to-order melts and special forgings carry a minimum weight that depends on the alloy and the route, typically from about 500 kg for special grades.
What inspection and certification is issued with the goods?
Every shipment carries an EN 10204 3.1 mill test certificate quoting the actual chemical and mechanical results for the specific heat lot, together with dimensional records and heat treatment data where applicable. Testing is carried out in house by optical emission spectrometry, tensile, impact and hardness testing, plus ultrasonic and eddy current examination, with hydrostatic pressure and intergranular corrosion testing to ASTM G28 or A262 Practice E where the specification requires it. Third party inspection by SGS or another recognised agency can be arranged, and their report is issued alongside the shipment.
What are typical lead times?
Standard stock items ship within days. Common production grades such as Inconel 625 or duplex 2205 are typically a few weeks. Special chemistries, larger forgings and items that require third party inspection take longer and are quoted individually against the requirement.
How are the products packed for export?
Flanges and fittings receive a rust preventive coating, are individually wrapped and are packed in plywood cases secured with steel banding. Tube ends are capped and bundles are strapped. Packing lists quote heat numbers so every case can be cross-checked against the mill certificate on arrival. We quote EXW, FOB, CFR, CIF and DAP, and mixed-alloy orders can be consolidated into one shipment with a single documentation package.
How do I request a quotation or a sample?
Send the grade or UNS number, the dimensions including schedule or wall thickness, the governing material and dimensional standards, quantity, any heat treatment or NDT scope and the certificate type you need; a drawing is ideal for special flanges and fittings. Email [email protected] or message us on WhatsApp +86 15618279078 and we normally issue a formal quotation within 24 hours on working days. Samples and previous test reports can be arranged before the first order.
Talk to an engineer
Share your duty conditions and we will shortlist candidate alloys within one working day.