Technical knowledge
Welding duplex stainless flanges: keeping the phase balance
How to weld duplex and super duplex flanges without losing corrosion resistance or toughness: filler selection, heat input and interpass limits, shielding gas with nitrogen, and verification testing.
Why the phase balance matters
The weld thermal cycle melts and rapidly resolidifies the material, which produces an almost fully ferritic structure. Austenite then reforms during cooling, but only if the cooling rate and the filler chemistry allow it. Too little heat input gives excessive ferrite; too much allows intermetallic precipitation. Both outcomes reduce toughness and corrosion resistance.
Filler selection
| Parent material | Typical filler | Why |
|---|---|---|
| Duplex 2205 | ER2209 | Over alloyed in nickel to promote austenite formation |
| Super duplex 2507 | ER2594 | Matches the higher pitting resistance |
| Duplex to carbon steel | ER309LMo or nickel base per procedure | Manages dilution and expansion mismatch |
| Duplex to austenitic | ER2209 or per procedure | Agree on the governing service |
Controls that determine the outcome
- Heat input window. Stay inside the qualified range; both too low and too high are harmful.
- Interpass temperature. Typically limited to around 150 C, with lower limits for super duplex.
- Shielding and backing gas. Use argon, often with a small nitrogen addition, and purge the root.
- No stress relieving. Post weld heat treatment in the precipitation range risks sigma phase; full solution annealing is the only thermal option and is rarely practical.
- Cleanliness. Remove all contamination, and use dedicated tools for stainless work.
Verification that proves the procedure worked
- Ferrite measurement on the weld metal and the heat affected zone within the specified range.
- Impact testing at the design minimum temperature.
- Corrosion testing to ASTM G48 where the specification requires it, at the stated temperature and acceptance mass loss.
- Dye penetrant and visual examination of all sealing surfaces and weld ends.
- Positive material identification of weld metal where filler verification is required.
Questions about this topic
Why is the filler over alloyed in nickel?
Nickel promotes austenite formation during weld cooling. Without the extra nickel, the rapid thermal cycle leaves too much ferrite, which reduces toughness and corrosion resistance.
Can duplex be stress relieved after welding?
Generally no. Stress relieving temperatures fall in the range where sigma phase precipitates, which damages both toughness and corrosion resistance. Design the fabrication to avoid the need for relief.
How do I know if the weld is acceptable?
Through ferrite measurement, impact testing and corrosion testing on a qualified procedure and, where required, on production test plates. Visual appearance alone tells you nothing about the phase balance.
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 demi@nickel-alloy.com 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.