254 SMO vs Super Duplex 2507 vs AL-6XN: Alloy Selection Guide for Instrumentation Fittings, Valves & Tubing
254 SMO, Super Duplex 2507 and AL-6XN are three premium corrosion-resistant alloys built for chloride, seawater and acid exposed fluid systems: 254 SMO and AL-6XN are super-austenitic "6-moly" stainless steels, while 2507 is a duplex grade, part austenite and part ferrite, and all three land in a pitting resistance equivalent number (PREN) range of roughly 42 to 47. Pick Super Duplex 2507 when you need the highest strength around, especially for thin-wall, high-pressure or weight-saving designs. When chloride stress-corrosion cracking and seawater resistance matter most, go with AL-6XN; it carries the highest nickel content and the highest PREN of the three. 254 SMO remains the proven, cost-balanced 6Mo super-austenitic choice for seawater and acid instrumentation. What follows lays these three grades side by side on composition, PREN, mechanical strength, corrosion resistance, temperature limits, weldability, cost and lead time, then points you toward the grade that's right for the job, delivered as finished instrumentation components.
> Scope: This comparison covers instrumentation fluid-system components only: tube fittings, valves and tubing. Structural plate, marine hardware and fasteners aren't part of it. "6 moly," as used here, means super-austenitic stainless, namely 254 SMO and AL-6XN, not molybdenum tool steel.
254 SMO vs Super Duplex 2507 vs AL-6XN at a glance
All three alloys beat standard 316 and 904L on pitting and chloride resistance, though each gets there by a different route. Below is the full side-by-side table; the sections after it break down each row in detail. The figures listed are standard published values for the grades. Confirm them against the material test certificate for each heat, since actual composition can be tailored to requirement.
Table A: 254 SMO, Super Duplex 2507, and AL-6XN compared
| Attribute | 254 SMO (S31254) | Super Duplex 2507 (S32750) | AL-6XN (N08367) |
|---|---|---|---|
| Microstructure | Super-austenitic (6Mo) | Duplex (~50% austenite / 50% ferrite) | Super-austenitic (6–7Mo) |
| Chromium (Cr), % | 19.5–20.5 | 24.0–26.0 | 20.0–22.0 |
| Nickel (Ni), % | 17.5–18.5 | 6.0–8.0 | 23.5–25.5 |
| Molybdenum (Mo), % | 6.0–6.5 | 3.0–5.0 | 6.0–7.0 |
| Nitrogen (N), % | 0.18–0.22 | 0.24–0.32 | 0.18–0.25 |
| PREN (typical) | ~43 | ~42–43 | ~45–47 |
| 0.2% yield, min | ~45 ksi (310 MPa) | ~80 ksi (550 MPa) | ~45 ksi (310 MPa) |
| Tensile strength, min | ~94 ksi (650 MPa) | ~116 ksi (795 MPa) | ~100 ksi (690 MPa) |
| Relative strength | Moderate | Highest (~2×) | Moderate |
| Chloride SCC resistance | Very high | High (ferrite aids CSCC) | Highest (high Ni) |
| Pitting / crevice (PREN) | Very high | High | Highest |
| Practical upper temperature | Wide (austenitic) | Limited ~250–300°C (475°C embrittlement / sigma) | Wide (austenitic) |
| Cryogenic toughness | Excellent | Good, ferrite ductile-to-brittle caution | Excellent |
| Relative raw-cost driver | High (18% Ni + 6% Mo) | Often lowest (lower Ni) | Highest (24% Ni + 6.5% Mo) |
Also known as: designations, UNS numbers & trade names
These three alloys go by more than one name on drawings and purchase orders. The same grade might turn up as a UNS number, a Werkstoff number, an ASTM forging designation, or simply a trade name, depending on who wrote the spec. 254 SMO carries the designation UNS S31254, Werkstoff 1.4547, and ASTM F44. Engineers often just call it a "6Mo" or "6 moly" super-austenitic stainless. Super Duplex 2507 shows up as UNS S32750, Werkstoff 1.4410, F53, or SAF 2507. A few closely related super-duplex grades sit nearby: Zeron 100 (UNS S32760 / 1.4501) and Ferralium / Ferrinox 255. AL-6XN, for its part, is designated UNS N08367 and falls into the 6-7Mo super-austenitic family, the 1.4529 group. The designations listed below are all standard published values.
| Designation | 254 SMO | Super Duplex 2507 | AL-6XN |
|---|---|---|---|
| UNS number | S31254 | S32750 | N08367 |
| Werkstoff / EN | 1.4547 | 1.4410 | 1.4529 family |
| ASTM forging designation | F44 | F53 | — |
| Common names & synonyms | 254SMO, 6Mo, 6-moly super-austenitic, S31254 stainless | SAF 2507, super duplex, DSS 2507; adjacent Zeron 100 (S32760), Ferralium, Ferrinox 255 | AL6XN, AL-6XN®, N08367, 6–7Mo super-austenitic |
| Phrasing variants | 254 SMO vs super duplex · super duplex vs 254 SMO · 254 SMO vs AL-6XN · AL-6XN vs 254 SMO · super duplex vs AL-6XN · 6 moly vs super duplex · super-austenitic vs super duplex | ||
Full single-grade specifications and the available instrumentation forms are laid out on Crestflo's own pages: the 254 SMO material page, the Super Duplex 2507 material page, and the AL-6XN material page.
Austenitic vs duplex: why the metallurgy drives the differences
Microstructure is the one fact that explains nearly all the differences below. 254 SMO and AL-6XN are super-austenitic: fully austenitic stainless steels carrying roughly 6–7% molybdenum, which is why they're nicknamed "6 moly." Super Duplex 2507, by contrast, is a duplex stainless steel, built from a roughly 50/50 mix of austenite and ferrite phases. It's that dual-phase makeup that gives 2507 its standout mechanical strength, about twice the yield of a comparable austenitic grade. The ferrite phase isn't free, though. It also brings the temperature and toughness constraints covered later. Being single-phase, the two super-austenitic grades trade away some strength for a wider working temperature range, easier fabrication, and excellent toughness at low temperatures. Boiled down, super austenitic versus super duplex is really a strength-for-temperature-range trade. Corrosion performance stays broadly comparable between them, set mainly by composition.
Chemical composition compared: S31254, S32750 & N08367
Composition is what drives PREN and how these alloys behave against corrosion. AL-6XN sits highest on nickel, at roughly 24%, and that's what gives it its resistance to chloride stress-corrosion cracking. Both 254 SMO and AL-6XN carry molybdenum at 6% or above. 2507, on the other hand, relies on high chromium, around 25%, plus nitrogen, with less nickel in the mix. The figures below reflect standard published chemical requirements. Always check them against the governing product-form specification and the MTC, since these can be adjusted to meet specific requirements.
Table B, Nominal composition (wt %)
| Element | 254 SMO (S31254) | Super Duplex 2507 (S32750) | AL-6XN (N08367) |
|---|---|---|---|
| Chromium (Cr) | 19.5–20.5 | 24.0–26.0 | 20.0–22.0 |
| Nickel (Ni) | 17.5–18.5 | 6.0–8.0 | 23.5–25.5 |
| Molybdenum (Mo) | 6.0–6.5 | 3.0–5.0 | 6.0–7.0 |
| Nitrogen (N) | 0.18–0.22 | 0.24–0.32 | 0.18–0.25 |
| Copper (Cu) | 0.5–1.0 | ≤0.5 | ≤0.75 |
| Iron (Fe) | Balance | Balance | Balance |
254 SMO corresponds to UNS S31254, also designated Werkstoff 1.4547 and covered under ASTM F44. It's a 6Mo super-austenitic alloy, roughly on par with AL-6XN (N08367) in service class. One common mix-up: people sometimes take it for a duplex grade. It isn't. S31254 is single-phase austenitic, plain and simple.
PREN and pitting resistance equivalent number
The pitting resistance equivalent number, or PREN, scores how well an alloy stands up to chloride pitting, based solely on its composition. Here's the formula most engineers use:
PREN = %Cr + 3.3 × %Mo + 16 × %N
Run the nominal chemistries above and all three sit in the high-performance band. 254 SMO comes in at roughly 43. Super Duplex 2507 lands around 42–43, while AL-6XN tops the group at 45–47, thanks to its combination of molybdenum and nitrogen. Keep in mind that PREN only ranks pitting resistance, nothing more. It doesn't predict chloride stress-corrosion cracking, crevice corrosion, or acid behavior on its own, so treat it as one data point, not the full basis for selection.
Table C, PREN inputs and result
| Alloy | Cr % | Mo % | N % | PREN (typical) |
|---|---|---|---|---|
| 254 SMO (S31254) | ~20 | ~6.1 | ~0.20 | ~43 |
| Super Duplex 2507 (S32750) | ~25 | ~4.0 | ~0.28 | ~42–43 |
| AL-6XN (N08367) | ~21 | ~6.3 | ~0.22 | ~45–47 |
For a broader ranking across the full alloy range, see the PREN comparison table.
Mechanical strength & temperature limits
This is where duplex and super-austenitic grades part ways most sharply. Super Duplex 2507 carries roughly twice the minimum yield strength of the austenitic grades: about 80 ksi (550 MPa) against roughly 45 ksi (310 MPa). That difference lets designers spec thinner walls or push higher pressures through the same section. A real advantage for weight-critical and high-pressure fluid systems. The ferrite phase is where the cost shows up: duplex grades are prone to 475°C embrittlement and sigma-phase formation, so 2507's practical upper service temperature caps out around 250–300°C. Ferrite also raises a ductile-to-brittle concern at cryogenic temperatures, so cold service isn't a given either. 254 SMO and AL-6XN, both super-austenitic and single-phase, don't carry that same baggage. They cover a wider temperature window and hold up well in cryogenic toughness, though they give up some strength at room temperature. These numbers are standard published minimums, not guarantees. Confirm them against the governing spec, and adjust to the actual requirement.
Table D, Mechanical & temperature summary
| Property | 254 SMO | Super Duplex 2507 | AL-6XN |
|---|---|---|---|
| 0.2% yield, min | ~45 ksi (310 MPa) | ~80 ksi (550 MPa) | ~45 ksi (310 MPa) |
| Tensile strength, min | ~94 ksi (650 MPa) | ~116 ksi (795 MPa) | ~100 ksi (690 MPa) |
| Practical upper temperature | Wide (austenitic) | ~250–300°C limit | Wide (austenitic) |
| Cryogenic toughness | Excellent | Good, ferrite caution | Excellent |
When temperature drives grade selection, two resources are worth checking: cryogenic and LNG selection and pressure-temperature derating.
Chloride SCC, sour service & acid resistance
On chloride stress-corrosion cracking, three grades line up in a clear order. AL-6XN takes the top spot, and its high nickel content (roughly 24%) is why: nickel at that level strongly suppresses SCC. Super Duplex 2507 isn't far behind; its two-phase structure gives it strong resistance in its own right. 254 SMO lands in the middle, performing well but not quite matching either extreme.
Pitting and crevice corrosion tell a different story, one that tracks PREN directly. AL-6XN sits at the top of that ranking, 254 SMO comes next, and 2507 trails both.
Acid service is where 254 SMO has built its reputation. It's a proven performer in seawater, in pulp-and-paper bleach environments, and in moderate sulfuric or phosphoric acid duty. As a rule, the high-molybdenum super-austenitic grades hold up better under reducing conditions than the leaner-nickel duplex does.
Concepts like critical pitting temperature (CPT) and critical crevice temperature (CCT) put these alloys in the same order as PREN, but the exact figure hinges on the test solution and the surface condition. Don't rely on a single headline number; check published CPT/CCT data for your specific environment instead.
Sour (H₂S) service is where all three can be supplied to meet NACE MR0175 / ISO 15156. But the PREN and hardness limits you'll actually need depend on H₂S partial pressure, chloride levels and temperature, so check the envelope against your specific service before committing. Crestflo supplies 254 SMO, Super Duplex 2507 and AL-6XN instrumentation components in NACE-compliant condition wherever it's required.
For related selection guidance, Crestflo offers a few resources worth checking. Start with offshore / chloride selection if you're weighing corrosion resistance in chloride-rich environments. For sour-service applications, see the sour-service (NACE) selection guide. Need to compare materials against specific media? The corrosion-by-media alloy matrix lays that out. And for background on the governing standard itself, there's the NACE MR0175 / ISO 15156 standard page.
Weldability & fabrication
For welded tubing and orbital-weld fittings, the super-austenitics tend to be the easier fabrication choice: 254 SMO and AL-6XN weld with matching or over-alloyed nickel-base fillers, and neither needs phase-balance control. Super Duplex 2507 welds well too, but it demands ferrite-balance and heat-input control to keep the austenite-ferrite ratio right and preserve corrosion performance. That control matters most where thin-wall instrumentation tube gets orbitally welded. All three, in the annealed condition, form readily.
How they compare to 316, 904L and Alloy 20
These three sit a clear step above the common stainless grades. Once 316/316L starts pitting, or 904L reaches its chloride limit, the usual move is up to the 6Mo super-austenitics, 254 SMO and AL-6XN, or to super duplex 2507. Alloy 20 is typically the pick for strong reducing acids among the austenitics. Nickel alloys handle what's left, the most aggressive media out there. See 904L vs 316 for that first chloride upgrade, Alloy 20 vs 316 for acid service, duplex vs super duplex for the family view, and 2205 vs 2507 for standard against super duplex.
Which alloy should you specify?
None of the three stands out as the best option on its own. Which one fits depends on what's driving the service. Check the decision matrix to confirm.
Table E, 254 SMO vs Super Duplex 2507 vs AL-6XN decision matrix
| Dominant service driver | Recommended alloy | Why |
|---|---|---|
| Highest strength / thin-wall / weight-critical / high pressure | Super Duplex 2507 | ~2× yield lets you run thinner walls at higher pressure |
| Maximum chloride SCC / seawater / offshore | AL-6XN | Highest nickel and PREN (~45–47) |
| Balanced seawater + acid, proven 6Mo choice | 254 SMO | Well-established super-austenitic, PREN ~43 |
| Sustained elevated temperature | 254 SMO or AL-6XN | Austenitic, no 475°C / sigma limit |
| Cryogenic / LNG | 254 SMO or AL-6XN | Excellent low-temperature toughness |
| Sour service (H₂S) | Any, NACE-compliant | All three available to NACE MR0175 / ISO 15156 |
Cost and lead time
Raw-material cost tracks nickel content pretty closely. AL-6XN, with roughly 24% nickel and 6.5% molybdenum, usually costs the most of the three. 254 SMO isn't far behind, sitting around 18% nickel and 6% moly. Super Duplex 2507 carries less nickel, so it's often the cheapest per unit mass. Its higher strength can shrink the amount of metal a design actually needs, though. Prices on all three shift with alloy surcharges, so there's no fixed list price; Crestflo quotes per requirement, against the current heat. On availability: Crestflo keeps ready stock for standard items, builds specials to order, and delivers in 6–8 weeks. That's a real edge over the 20-plus-week lead times common from the major mills. For a fuller comparison, see the alloy lead-time and cost benchmark.
254 SMO, Super Duplex 2507 & AL-6XN instrumentation components
Crestflo covers these grades broadly in this piece, but what it actually supplies is instrumentation forms only: fittings, valves and tubing. Bar, sheet, plate and mill products aren't part of the offering. All three alloys count as in-scope US materials, and they're available as:
- Compression / tube fittings: end connections up to 2" (50 mm) OD, single and double ferrule, offered in imperial and metric, tested per ASTM F1387. Check the matrix cells for 254 SMO tube fittings and Super Duplex 2507 tube fittings.
- Instrumentation valves: needle, ball, check, manifold and more, in sizes from 1/16" to 1", qualified to ASME B16.34 / API. Take a look at 254 SMO valves and Super Duplex 2507 valves.
- Alloy instrumentation tubing: available seamless and welded. Browse 254 SMO tubing and Super Duplex 2507 tubing.
Why Crestflo for 254 SMO, Super Duplex 2507 & AL-6XN
- Makes all three alloys as finished instrumentation components, not a reseller of a single grade.
- Drop-in interchange with Swagelok®, Parker®, Hoke®, Hy-Lok® and similar fittings, backed by ASTM F1387 performance qualification for compression tube fittings. Ask our engineering team for a part-number cross-reference. (Swagelok® is a trademark of Swagelok Company; Parker® of Parker Hannifin Corporation; Hoke® of Circor International, Inc.; Hy-Lok® of Hy-Lok Corporation; used for identification only.)
- Every part carries a heat-number mark for full traceability. Standard documentation is EN 10204 3.1 MTC (3.2 available on request), and each lot undergoes PMI verification.
- NACE MR0175 / ISO 15156-compliant material available for sour service.
- ISO 9001 / 14001 / 45001 & PED certified.
- Ready stock for standard items · made-to-order for specials · 6–8 week delivery.
- Indian origin; US / European melt available on request, with full material traceability.
- Small MOQ, private-label and custom / made-to-print capability.
Instrumentation arm of a four-decade export house recognized by the Government of India, with a track record of delivering across the Middle East. It's a new brand, but the manufacturing heritage behind it runs deep.
We Engineer Confidence.
- Request an RFQ, Name · Company · Email · Phone · Requirement description.
- Request a sample.
- Talk to an engineer, sales@crestflousa.com · +1 346 594 9005 · Houston, TX.
Standards & specifications
Every grade carries its own product-form specifications, so you'll need to name the grade and the applicable ASTM/spec on your RFQ. Crestflo holds the standard approvals for each product type: ASTM F1387 performance qualification for compression tube fittings, and ASME B16.34 / API qualification for valves. The company is also ISO 9001 / 14001 / 45001 & PED certified. Certificates are listed as held; they aren't numbered on the content pages.
Table F, Product-form standards by grade
| Grade | Tube / pipe | Bar / forgings | Reference |
|---|---|---|---|
| 254 SMO (S31254) | ASTM A312 / A213 | ASTM A276 / A479 | A312 · A213 |
| Super Duplex 2507 (S32750) | ASTM A789 / A790 | ASTM A182 (F53) | A789 · A790 |
| AL-6XN (N08367) | ASTM B675 / B676 | ASTM B691 | Standards hub |
| Material test certificate | EN 10204 3.1 / 3.2 | EN 10204 MTC | |
| Sour service | NACE MR0175 / ISO 15156 | NACE MR0175 | |
Frequently asked questions
What is the difference between 254 SMO, Super Duplex 2507 and AL-6XN?
254 SMO and AL-6XN both belong to the super-austenitic "6-moly" family of stainless steels. 2507, by contrast, is a duplex grade, meaning it combines austenite and ferrite. Yield strength tells the real story here: 2507 comes in at roughly double what the other two offer. AL-6XN carries the highest nickel content of the three, and the highest PREN too, around 45 to 47, which gives it an edge in resisting chloride stress corrosion cracking. 254 SMO doesn't chase extremes. It's the balanced, well-proven 6Mo choice, sitting at a PREN near 43.
Which alloy has the highest PREN?
AL-6XN typically runs about 45 to 47. Compare that to roughly 43 for 254 SMO and 42 to 43 for Super Duplex 2507. PREN's formula is %Cr + 3.3 × %Mo + 16 × %N, and it doesn't measure anything beyond pitting resistance.
Which resists chloride stress-corrosion cracking best?
AL-6XN carries roughly 24% nickel, and that gives it excellent resistance to chloride stress corrosion cracking. Super Duplex 2507 takes a different route: its dual-phase structure holds up strongly against SCC too. 254 SMO lands somewhere in between the two, performing very well in its own right.
Which of the three is the strongest?
Super Duplex 2507 comes in with a minimum yield of about 80 ksi (550 MPa). Compare that to roughly 45 ksi (310 MPa) for the austenitic 254 SMO and AL-6XN, and the gap is stark. That strength edge means walls can run thinner even as pressure climbs.
What material is 254 SMO equivalent to?
254 SMO carries the designation UNS S31254 (Werkstoff 1.4547, ASTM F44), a 6Mo super-austenitic stainless steel. Its service class puts it right alongside AL-6XN (N08367). It's austenitic, not duplex.
Is 2205 duplex or super duplex?
Standard duplex is 2205. Super duplex, by comparison, is 2507, also known as UNS S32750. Crestflo's guide on duplex vs super duplex lays out the distinction in detail, and the 2205 vs 2507 comparison covers the specifics.
Is super duplex better than 316 stainless?
For strength and resistance to chloride attack, 2507 wins by a wide margin over 316. Still, 316 stays the practical, lower-cost pick for everyday instrumentation work where those extreme conditions never come into play.
Can I use these alloys in sour (H₂S) service?
Yes. Crestflo supplies 254 SMO, Super Duplex 2507 and AL-6XN components in NACE MR0175 / ISO 15156-compliant condition. Just confirm the specific PREN and hardness limits for your H₂S partial pressure.
Which alloy is best for seawater or offshore chloride?
AL-6XN, which carries the highest PREN and nickel content, and 254 SMO are the go-to super-austenitic picks. Super Duplex 2507's the better call for high-strength, weight-critical parts. For more detail, see offshore chloride selection.
Do these alloys have temperature limits?
254 SMO and AL-6XN, both austenitic grades, cover a wide temperature range and hold up well in cryogenic service. 2507, a duplex grade, doesn't share that flexibility: keep it under roughly 250–300°C, or you risk 475°C embrittlement and sigma-phase formation.
Which is the most cost-effective?
Price follows nickel content. AL-6XN, at roughly 24% Ni, usually costs the most of the three, while Super Duplex 2507 tends to run cheapest. Crestflo doesn't publish live prices. It quotes per heat instead; request a quote.
Can Crestflo supply fittings, valves and tubing in all three?
Yes. All three count as in-scope US materials. The compression fittings meet ASTM F1387 and ship ASTM F1387-tested; the valves are qualified to ASME B16.34 and API; and the tubing rounds out the set. Each item carries heat-number traceability and an EN 10204 3.1 MTC, with delivery running 6–8 weeks.
Related guides & materials
- 254 SMO (UNS S31254) full datasheet
- Super Duplex 2507 (S32750) grade data
- AL-6XN (UNS N08367) properties
- 904L super-austenitic
- Full PREN comparison table
- Duplex vs super duplex explained
- 2205 vs 2507, standard vs super duplex
- 904L vs 316 step-up
- Sour-service (NACE) alloy selection
- Corrosion-by-media alloy matrix