In the super duplex family, three grades - S32750 (2507), S32760 (Zeron 100), and S32550 (Ferralium 255) - all clear the PREN 40 line and all get sold under the same "super duplex" label. They are not the same alloy. They are not interchangeable. And if your MTR shows S32760 but your spec said S32750, your pipe may not be compliant with your project standard, regardless of how high its PREN number is.
You Specified "Super Duplex" But Not the UNS Number - and Got S32760 Instead
This is the most common error. A buyer writes "super duplex pipe" in the RFQ, three suppliers quote three different grades, and the one with the lowest price may be quoting Zeron 100 (S32760) instead of 2507 (S32750). They are both super duplex. They are not the same alloy.
| Grade | UNS | Defining Chemistry Difference | Forging Spec | Common Use |
|---|---|---|---|---|
| Super Duplex 2507 | S32750 | 25Cr / 7Ni / 4Mo / 0.27N - no Cu, no W | ASTM A182 F53 | Most super duplex pipe; standard spec |
| S32760 | S32760 | Same base + 0.5–1.0% Cu + 0.5–1.0% W | ASTM A182 F55 | Better in dilute reducing acids (Cu helps) |
| S32550 | S32550 | Same base + 1.5–2.5% Cu | ASTM A182 F61 | Sulfuric acid service (Cu helps) |
Why this matters: S32760 (Zeron 100) costs 8–15% more than S32750 because of the copper and tungsten additions. A supplier quoting S32760 in place of S32750 is overcharging you - unless your spec actually calls for it. Conversely, if your spec says S32750 and the mill delivers S32760, your certificate may not match your specification, which will fail a third-party inspection on most oil and gas projects (Saudi Aramco 9COM, Shell DEP, NORSOK M-630 all name the UNS explicitly).
What to put in your RFQ: write the UNS number. "Super duplex 2507 pipe to ASTM A790, UNS S32750, solution annealed 1040–1100 °C and water quenched" leaves no room for substitution. If your spec says "super duplex" without a UNS, you have not written a spec - you have written a wish list.
2. You Got a Heat With 32 HRC - but Your Project Is Sour Service and Wants 28 HRC
ASTM A790 (the standard for super duplex pipe) allows up to 32 HRC maximum hardness. NACE MR0175 / ISO 15156-3 (the sour service standard for oil and gas) allows 28 HRC for super duplex - and Saudi Aramco, Shell, and most subsea projects ask for 28 HRC anyway, regardless of service classification, because the lower hardness buys you extra margin against sulfide stress cracking.
This is a 4-point Rockwell gap. It sounds small, but it is the difference between a heat that passes SSC testing and one that fails.
| Spec Source | Maximum Hardness | When It Applies |
|---|---|---|
| ASTM A790 (general) | 32 HRC | Non-sour service |
| NACE MR0175 / ISO 15156-3 | 28 HRC | Sour service (H2S-bearing) |
| NORSOK M-630 (Norwegian offshore) | 28 HRC (often 25 HRC for subsea) | Norwegian Continental Shelf |
| Saudi Aramco 9COM / SABIC / ADNOC | Typically 28 HRC | GCC sour service |
| Subsea umbilicals / flowlines | Often 25–28 HRC | Deepwater, high-pressure H2S |
If your PO says "ASTM A790, hardness 32 HRC max" and your service is sour, you have written the wrong number. A pipe at 30 HRC will pass the 32 HRC check and still fail the 28 HRC NACE check. You will be told at the third-party inspection that your pipe cannot be released.
What to put in your RFQ: be explicit about both. "ASTM A790 UNS S32750, hardness 28 HRC max per NACE MR0175" is the line that gets you the heat you can actually use in sour service.
3. You Specified Solution Annealing but Not the Water Quench - and Lost Toughness
Super duplex 2507 has to be solution annealed at 1040–1100 °C and then water quenched - not air cooled, not furnace cooled. The reason is the metallurgical "danger zone" between 600 °C and 1000 °C. If the pipe cools through that band slowly, intermetallic phases (sigma phase, chi phase) precipitate at the grain boundaries. Sigma phase cuts Charpy impact toughness by 80% or more, and it destroys the pitting resistance that you paid for.
Here is what that looks like in real numbers:
| Cooling Method | Time in 600–1000 °C Band | Resulting Charpy at –46 °C | Sigma Phase Present? | Compliant? |
|---|---|---|---|---|
| Water quench from 1050 °C | < 5 seconds | 150–250 J typical | No | ✅ Yes |
| Forced air cool | 60–120 seconds | 80–120 J | Trace | ⚠️ Borderline |
| Furnace cool | 30+ minutes | 20–40 J | Heavy | ❌ No |
| Air cool (shop floor, large diameter) | 5–15 minutes | 40–80 J | Significant | ❌ No for sour/cold service |
Most pipe mills do this correctly. The risk is in heavy-wall pipe (Sch 80S and above) and large-diameter pipe (NPS 16 and above) - the cross-section is thick enough that even water spray on the outside does not pull the core through the danger zone fast enough. Some mills add internal water cooling or use a rotary quench tank for large OD pipe. Ask if your supplier has done this.
What to put in your RFQ: "solution annealed 1040–1100 °C followed by rapid water quenching with the entire cross-section passing through 600–1000 °C in less than 5 minutes; Charpy impact test at –46 °C per ASTM A370, minimum 150 J average of three specimens." A supplier who cannot answer the quench question is not a supplier you want to argue about downstream.
4. You Let the Temperature Go Above 300 °C and Wondered Why the Pipe Embrittled
ASTM A790 lists the maximum service temperature for 2507 as 300 °C (572 °F) - not 315 °C like 2205, and not 600 °C like Inconel 625. The reason is the same sigma-phase risk as the water quench problem: above 300 °C, in service, the alloy can precipitate intermetallic phases over months and years, slowly embrittling the pipe. This is a hard upper limit, not a guideline.
| Temperature | Time to Embrittle (Typical) | Effect |
|---|---|---|
| < 250 °C | No embrittlement in design life | Safe operating zone |
| 250–300 °C | Slow embrittlement; check after 5 years | Monitor |
| 300–350 °C | Embrittlement in 1–3 years | Avoid for long-term service |
| > 350 °C | Rapid embrittlement; Charpy cut in months | Do not use |
The buyers who hit this problem most often are specifying 2507 for FGD (flue gas desulfurization) absorber recycle lines in coal-fired power plants. The operating temperature is sometimes above 300 °C at the inlet. The spec says "super duplex 2507" because the chloride is high. The pipe embrittles within the first overhaul window. The right call for FGD above 300 °C is Inconel 625 (continuous service to ~600 °C) - at roughly 2.5× the cost of 2507, but with no embrittlement ceiling.
What to put in your RFQ: state the design temperature explicitly. "Maximum design temperature 300 °C continuous service; for service above 300 °C, refer to nickel alloy alternatives." If you do not know your design temperature, do not order super duplex yet.
5. You Bought "Super Duplex" Without Checking the Ferrite Range - and Welded with the Wrong Filler
Super duplex 2507 has to maintain a ferrite content of roughly 35–65% in the base metal and the weld. ASTM E562 (or Feritscope measurement) verifies this. If ferrite drops below ~25% (too austenitic), you lose the strength and corrosion resistance the ferrite phase provides. If ferrite climbs above ~75% (too ferritic), the weld becomes brittle and loses toughness.
The weld filler you choose changes the ferrite balance. The two approved fillers for 2507 are:
| Filler | AWS A5.9 Class | PREN (vs Base ~42) | When to Use |
|---|---|---|---|
| ER2594 | Overmatching | ~44 (slightly higher PREN) | Default; subsea; sour service |
| ER2553 | Matching | ~40 (slightly lower PREN) | Less aggressive service; shop fabrication only |
The buyer mistake here: specifying "309LMo" or "316L" filler because the fabricator has it in stock. Neither filler has the nickel content to balance the 2507 phase ratio. You will get a weld that is 80% ferrite and a weld that fails ASTM A923 Method A (the standard corrosion test for duplex welds).
What to put in your RFQ: "weld procedure qualified per ASME Section IX using ER2594 filler (or ER2553 if qualified), ferrite content 35–65% per ASTM E562 in weld and HAZ, ASTM A923 Method A corrosion test on procedure qualification coupon." Then ask to see the WPS and PQR before the welder strikes an arc.
FAQ
What is the difference between super duplex 2507 and Zeron 100?
Both are super duplex stainless steels with PREN above 40. The difference is chemistry: S32750 (2507) contains only chromium, molybdenum, and nitrogen; S32760 (Zeron 100) adds about 0.5–1.0% copper and 0.5–1.0% tungsten. The copper gives Zeron 100 better resistance to dilute sulfuric acid; the tungsten adds pitting resistance margin. For most chloride service, S32750 is the standard spec. For sulfuric acid or for spec compliance with F55 forgings, S32760 may be specified. They are not drop-in substitutes - your RFQ must name the UNS.
Can I use ER2209 filler on 2507?
No. ER2209 is the correct filler for duplex 2205, not super duplex 2507. For 2507, use ER2594 (overmatching, default for sour and subsea) or ER2553 (matching, for less aggressive service). Both are nitrogen-enhanced fillers that maintain the ferrite balance in the weld. Using ER2209 on 2507 gives a weld with ferrite outside the 35–65% range and probable ASTM A923 failure.
What is the maximum service temperature for 2507 pipe?
300 °C (572 °F) for continuous service. Above 300 °C, sigma phase and chi phase precipitation embrittle the alloy over months to years. For service above 300 °C (FGD absorber inlets, high-temperature process lines), upgrade to Inconel 625 (continuous service to ~600 °C) or another nickel alloy.
Does 2507 require PWHT after welding?
No. Post-weld heat treatment in the 600–950 °C range precipitates sigma phase in the weld and HAZ. 2507 welds must cool rapidly through that band. If your fabricator proposes PWHT for 2507, stop the order and re-qualify the WPS. The only heat treatment applied to 2507 welds is the original solution anneal at the mill.
What documentation should I require with super duplex 2507 pipe?
For sour or subsea service, EN 10204 3.1 mill certificate showing heat chemistry (Cr, Mo, N must hit UNS S32750 limits), ferrite content per ASTM E562, hardness per ASTM E956 (28 HRC max for sour), Charpy test at –46 °C per ASTM A370 (≥ 150 J average), solution annealing record (1040–1100 °C with rapid water quench), NACE MR0175 / ISO 15156-3 compliance statement. For GCC delivery, expect dual tagging on pipe and certificate, AVL registration, and country-of-origin certification.
What is the relative cost of 2507 pipe vs 2205 and 316L?
Indicative price index: 316L = 1.0; duplex 2205 ≈ 1.8; super duplex 2507 ≈ 3.5–4.0; Inconel 625 ≈ 5.5. The premium for 2507 over 2205 is 80–100%, but 2507 extends service from ~35 °C seawater (2205 ceiling) to ~50 °C (2507 ceiling) and from ~5,000 ppm chloride (2205 limit) to above ~30,000 ppm (2507 limit). For sour service with H2S, 2507 is the lowest-cost material that qualifies at all.
Need super duplex 2507 pipe with the chemistry, hardness, ferrite range, and documentation that actually pass your third-party inspection?
HUITONG supplies UNS S32750 pipe and tube to ASTM A790, A928, A789 with full traceability - heat chemistry to UNS limits, ferrite 35–65% per ASTM E562, hardness ≤ 28 HRC per NACE MR0175, Charpy at –46 °C ≥ 150 J, solution annealing records with rapid water quench, EN 10204 3.1 / 3.2 certificates. Send your pipe schedule, quantity, and service conditions (chloride, H2S partial pressure, design temperature, third-party inspection requirement) to market@htpipe.com or WhatsApp +86-19339900201 - and if you are not sure whether 2507 is the right grade (or whether you actually need Zeron 100 / 2205 / 625), tell us the service and we will tell you the minimum-cost grade that passes.
