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Duplex & Super Duplex
SS Fasteners
A comprehensive engineering reference for corrosion engineers, offshore piping engineers, EPC procurement teams and materials specialists — covering duplex 2205 and super duplex 2507/2760 fastener metallurgy, dual-phase microstructure requirements, PREN calculation, NACE compliance, ferrite content control, galling prevention, torque considerations, and full project documentation for critical offshore, sour service and seawater fastener applications.
Duplex Stainless Steel Metallurgy,
Grade Classification & Selection
Dual-Phase · Ferrite-Austenite · Cr-Mo-N · PREN
What is Duplex Stainless Steel
Duplex stainless steels are a family of corrosion-resistant alloys with a two-phase microstructure consisting of approximately equal proportions of austenite (face-centred cubic, FCC) and ferrite (body-centred cubic, BCC). This dual-phase structure gives duplex steels their characteristic combination of properties: the high strength of the ferritic phase and the excellent corrosion resistance and toughness of the austenitic phase — resulting in a material that is typically 2× stronger than standard austenitic stainless steels (A193 B8M / SS 316L) at the same corrosion resistance level.
The key alloying elements in duplex fastener grades are: Chromium (Cr) — forms the passive oxide film that provides fundamental corrosion resistance; Molybdenum (Mo) — enhances resistance to pitting and crevice corrosion in chloride environments; and Nitrogen (N) — stabilises the austenite phase, increases strength, and dramatically improves pitting corrosion resistance. The PREN (Pitting Resistance Equivalent Number) formula — PREN = %Cr + 3.3×%Mo + 16×%N — is the primary single-number measure of pitting corrosion resistance used to compare duplex grades.
Standard austenitic stainless steels (SS 316L, A193 B8M) are susceptible to chloride stress corrosion cracking (SCC) — a catastrophic failure mode where the combination of tensile stress (from bolt preload), chloride ions (from seawater, brine, or atmospheric chloride) and temperature above ~60°C causes sudden brittle fracture. Duplex 2205 is substantially more resistant to chloride SCC than 316L because the ferritic phase acts as a barrier to SCC crack propagation — cracks initiated in the austenite are arrested at the ferrite-austenite phase boundary. This makes duplex 2205 the industry-standard choice for offshore platform flange fasteners in seawater and produced water service above 60°C, where 316L fasteners would be unacceptable.
Duplex Grade Families — Engineering Overview
Mechanical Properties, PREN
& Grade Comparison
2205 vs 2507 vs 316L vs B7 · Strength Advantage
PREN Calculation
// Duplex 2205 (UNS S32205): 22%Cr + 3.3×3%Mo + 16×0.17%N
PREN_2205 = 22 + 9.9 + 2.72 = 34.6 // Typical; range 34–38 depending on actual composition
// Super Duplex 2507 (UNS S32750): 25%Cr + 3.3×4%Mo + 16×0.27%N
PREN_2507 = 25 + 13.2 + 4.32 = 42.5 // Range 40–45; PREN ≥40 = seawater pitting resistance threshold
// SS 316L (for comparison): 17%Cr + 3.3×2.2%Mo + 16×0.03%N
PREN_316L = 17 + 7.26 + 0.48 = 24.7 // Insufficient for offshore seawater service (needs PREN ≥40 for seawater)
| Grade | UNS / ASTM | PREN | Yield (MPa) | UTS (MPa) | Max HRC (NACE) | Elongation (%) | Chloride SCC |
|---|---|---|---|---|---|---|---|
| SS 316L (B8M) | S31603 / A193 B8M | 24–28 | ≥207 (Ann.) | ≥515 | N/A | ≥40 | Susceptible >60°C |
| Lean Duplex 2101 | S32101 / A276 | 24–28 | ≥450 | ≥650 | 28 HRC | ≥25 | Moderate resistance |
| Duplex 2205 | S31803/S32205 / A182 F51 | 34–38 | ≥448 | ≥620 | 28 HRC | ≥25 | Good to ~150°C |
| Super Duplex 2507 | S32750 / A182 F53 | 40–43 | ≥550 | ≥750 | 30 HRC | ≥15 | Excellent |
| Super Duplex F55 | S32760 / A276 | 40–45 | ≥550 | ≥750 | 30 HRC | ≥15 | Excellent |
| CS A193 B7 (ref.) | — / A193 B7 | 0 | ≥724 | ≥862 | 35 HRC | ≥16 | None (will corrode) |
| Inconel 625 (ref.) | N06625 / B446 | >50 | ≥275 | ≥690 | 35 HRC | ≥30 | Immune |
Mechanical properties for annealed/solution-annealed condition per ASTM standards. Duplex 2205 and super duplex 2507 provide approximately 2× the yield strength of austenitic 316L (annealed), enabling smaller fastener diameters for the same clamping force in some applications. NACE max HRC limits per NACE MR0175 / ISO 15156 Part 3 for sour service — these limits apply to the finished fastener cross-section, not just the input bar material.
NACE Compliance, Ferrite Control
& Thread Galling Prevention
Charpy · Sigma Phase · Anti-Seize · Xylan 1070
NACE MR0175 Compliance for Duplex Fasteners
Duplex stainless steel fasteners in sour H≶S service must comply with NACE MR0175 / ISO 15156 Part 3. The primary requirements for duplex 2205 (S31803/S32205) fasteners are:
- Maximum hardness ≤28 HRC (equivalent to ≤271 HV or ≤255 HB) measured on the cross-section of the finished fastener, at minimum 3 measurement locations including the thread root zone. Hardness must be reported per heat in the MTC. The thread root is the critical location — work hardening during threading can raise local hardness above 28 HRC if threading is done without proper tooling and lubrication.
- Ferrite content FN 35–65% measured per ASTM E562 (point count) or magnetic method. Outside this range, the microstructure is no longer dual-phase: below FN 35% the material behaves as austenitic SS (susceptible to SCC), above FN 65% it behaves as ferritic SS (susceptible to hydrogen embrittlement in sour service). Ferrite content must be measured and reported separately for each component (stud bolt and nut).
- Solution annealing treatment at 1020–1100°C followed by water quench — this dissolves any sigma phase or other intermetallic precipitates that form during slow cooling through the 700–950°C range. Sigma phase is brittle, non-magnetic and raises hardness above NACE limits. Post-threading solution anneal is mandatory if threading raises the hardness above the limit.
Sigma phase (σ-phase) is an intermetallic compound (FeCr) that precipitates in duplex stainless steels when held in the temperature range 600–950°C for extended periods during welding, hot forming or slow cooling after annealing. Sigma phase is: (1) extremely brittle — even small amounts (1–2 vol%) dramatically reduce toughness and elongation; (2) harder than the matrix — raises overall hardness above NACE limits; (3) chromium-rich — its formation depletes Cr from the surrounding matrix, reducing corrosion resistance. In duplex fastener supply, sigma phase is the most common non-conformance found at incoming inspection — it is caused by inadequate solution annealing or insufficient water quench rate after annealing. Always specify and verify the heat treatment cycle (time, temperature, quench method) in the MTC.
Thread Galling in Duplex Fasteners
Duplex 2205 and super duplex 2507 fasteners are highly susceptible to thread galling (cold welding) during assembly — even more so than standard austenitic stainless (316L) due to their higher surface hardness. Galling produces catastrophic seizure of the nut on the stud bolt thread, making the fastener impossible to remove without cutting. Prevention measures:
- Xylan 1070 coating (PTFE dry-film, IFI-125) applied to both stud bolt threads and nut threads before assembly — the most effective anti-galling treatment for offshore duplex fasteners. K factor 0.10–0.13. Also provides 1000+ hr salt spray corrosion protection. The preferred solution for offshore duplex flange bolt kits.
- Molybdenum disulphide anti-seize compound (Molykote 1000 or equivalent) applied to clean threads immediately before assembly. K factor approximately 0.13–0.15. Not a permanent treatment — must be re-applied at every assembly.
- Dissimilar alloy pairing — pairing duplex 2205 stud bolts with Inconel 625 nuts (or vice versa) reduces galling propensity because the different crystal structures and hardness values of the two alloys reduce the adhesive wear that causes galling. Used on critical high-pressure connections.
- Low-speed hand tightening before applying torque wrench — confirm smooth thread engagement throughout the full length of engagement before applying any torque. Impact wrenches are prohibited for duplex fasteners.
| Requirement | Duplex 2205 (S31803/S32205) | Super Duplex 2507 (S32750) | Test Method | Report Required |
|---|---|---|---|---|
| Max Hardness | ≤28 HRC (271 HV / 255 HB) | ≤30 HRC (285 HV) | ASTM E18 (Rockwell) or E92 (Vickers) | Per heat; min 3 locations per fastener cross-section |
| Ferrite Content | FN 35–65% | FN 35–65% | ASTM E562 (point count) or magnetic ferritescope | Per heat; stud bolt and nut separately |
| Solution Anneal | 1020–1080°C + water quench | 1050–1120°C + water quench | MTC heat treatment record | Anneal temp, time and quench method in MTC |
| Charpy Impact | ≥27 J at 0°C (project dep.) | ≥27 J at −46°C (project dep.) | ASTM A370 / EN ISO 148 | Required when MDMT is below 0°C or project spec. mandates |
| PMI | 100% XRF | 100% XRF | XRF or OES | Individual readings for every fastener; UNS confirmed |
| Passivation | ASTM A380 | ASTM A380 | Copper sulphate test | Required; confirms no iron contamination on surface |
Applications, Torque Considerations
& Quality Control Documentation
Torque Table · EN 10204 3.2 · PMI · Ferrite · TPI
Applications by Industry
Duplex 2205 stud bolts and nuts (Xylan 1070-coated) are the standard fastener specification for all offshore topsides duplex stainless steel piping flanges per NORSOK M-630 and most major operator DEPs. The combination of duplex 2205 flange material and duplex 2205 (or Inconel 625) fasteners provides: galvanic compatibility (no galvanic couple between flange and fastener of the same alloy), adequate chloride SCC resistance for the atmospheric seawater exposure on offshore platforms, NACE compliance for sour produced fluid contact zones, and high strength enabling smaller fastener diameters than 316L for the same bolt load. All offshore duplex fasteners require EN 10204 3.2 with TPI countersignature, 100% PMI, ferrite content measurement and NACE hardness survey as minimum documentation.
Super duplex 2507 fasteners are used for seawater cooling water, firewater and ballast water piping on platforms and FPSOs where water temperatures and chloride concentrations require PREN ≥40 for adequate pitting corrosion resistance. Duplex 2205 is adequate below approximately 35°C in clean seawater; super duplex 2507 is required above this temperature or where chlorinated seawater (typically dosed to 1–2 ppm chlorine for biofouling control) is handled, because chlorination raises the effective corrosivity of the water significantly.
For production piping handling sour (H≶S-containing) produced fluids — where the H≶S partial pressure exceeds the NACE MR0175 threshold — duplex 2205 fasteners provide a cost-effective alternative to Inconel 625 for NACE-qualified flange bolting on duplex or carbon steel flanges. The NACE requirements (≤28 HRC, FN 35–65%, solution anneal verification) must be fully documented. Duplex 2205 in NACE sour service has a upper temperature limit — above approximately 121°C in H≶S service, NACE ISO 15156-3 restrictions apply and the alloy selection must be individually qualified. Consult NACE ISO 15156-3 Annex A for the approved envelope.
Super duplex 2507 fasteners are used throughout subsea production tree, manifold and jumper connector bolted connections where both PREN ≥40 seawater resistance and NACE sour compliance must be met simultaneously. Subsea duplex fasteners require the most rigorous QC of any application — inaccessible for inspection or replacement after installation, so all quality checks must be completed and verified before dispatch. Critical additional requirements for subsea: SSC testing per NACE TM0177 Method A on representative specimens; corrosion inhibitor compatibility verification; cathodic protection compatibility (duplex is compatible with zinc and aluminium sacrificial anodes); and dimensional verification of thread engagement length against the subsea connector design.
Torque Reference for Duplex 2205 Stud Bolts
| Bolt Size | Stress Area (mm²) | 65% Proof Preload (kN) | Torque K=0.12 Xylan (N·m) | Torque K=0.18 Dry (N·m) |
|---|---|---|---|---|
| M16 | 157 | 59.1 | 113 | 170 |
| M20 | 245 | 92.2 | 221 | 332 |
| M24 | 353 | 132.8 | 382 | 573 |
| M27 | 459 | 172.7 | 560 | 840 |
| M30 | 561 | 211.1 | 760 | 1140 |
| M36 | 817 | 307.4 | 1330 | 1995 |
| M42 | 1121 | 421.8 | 2132 | 3198 |
| M48 | 1473 | 554.3 | 3200 | 4800 |
Based on Duplex 2205 annealed yield 448 MPa; preload at 65% proof load. K=0.12 with Xylan 1070 coating (preferred for offshore); K=0.18 dry (not recommended for duplex — galling risk). Always verify against the flange manufacturer's bolting specification and gasket seating stress requirements. For Class 600 and above, a formal bolted joint analysis per EN 1591-1 or ASME PCC-1 Appendix O is recommended. Super duplex 2507 (yield 550 MPa) carries approximately 23% higher preload for the same torque — verify with the relevant 2507 torque table.
Quality Control
QC for duplex and super duplex fasteners covers: (1) 100% PMI by XRF on every fastener; (2) Hardness survey per heat on cross-section — minimum 3 locations including thread root zone, all ≤28 HRC (2205) or ≤30 HRC (2507); (3) Ferrite content per ASTM E562 — FN 35–65% confirmed for each component; (4) Solution anneal certificate — temperature, time, quench method confirmed; (5) Charpy impact — when MDMT <0°C or project requires; (6) Thread gauge — Go/No-Go per ASME B1.1 or ISO 965; (7) Passivation test — copper sulphate test per ASTM A380; (8) Dimensional inspection — shank diameter, length, hex across-flats.
| # | Document | Standard | Minimum Requirement |
|---|---|---|---|
| 01 | Material Test Certificate (MTC) | EN 10204 3.2 | 3.2 with TPI (BV/DNV/Lloyds/SGS/TÜV) countersignature — mandatory for all duplex fasteners |
| 02 | PMI Report (XRF) | Project spec. | 100% of every fastener; UNS number and grade confirmed individually |
| 03 | Hardness Test Report | ASTM E18 / E92 | Per heat; min 3 cross-section locations; thread root zone included; ≤28 HRC (2205) / ≤30 HRC (2507) |
| 04 | Ferrite Content Report | ASTM E562 | FN 35–65% confirmed; stud bolt and nut reported separately per heat |
| 05 | Heat Treatment Certificate | ASTM A276 / A182 | Solution anneal temp (1020–1100°C), time and water quench method confirmed; sigma phase absence verified |
| 06 | Charpy Impact Report | ASTM A370 / EN ISO 148 | Required if MDMT <0° C or project specification mandates; ≥27 J at test temperature |
| 07 | Tensile Test Report | ASTM F606 | Per heat; yield, UTS, elongation, reduction of area confirmed against ASTM A182 minimums |
| 08 | Passivation Test Report | ASTM A380 | Copper sulphate test; no iron contamination confirmed on all surfaces |
| 09 | Thread Gauge Certificate | ASME B1.1 / ISO 965 | Go/No-Go per lot; thread class 2A/2B (UNC) or 6g/6H (metric) confirmed |
| 10 | Coating QC Report (if Xylan) | IFI-125 / ISO 9227 | DFT (ISO 2808), salt spray (1000+ hr ISO 9227), K-factor (0.10–0.13) confirmed |
RR Hydraulics manufactures and exports duplex and super duplex stainless steel fasteners — stud bolts, hex bolts, heavy hex nuts and washers — in Duplex 2205 (UNS S31803/S32205, ASTM A182 F51), Super Duplex 2507 (UNS S32750, A182 F53) and Super Duplex F55 (UNS S32760). Sizes M8–M72 metric and 5/16"–3" UNC. Xylan 1070/1424 coating available. EN 10204 3.2 MTC with TPI (BV/DNV/Lloyds/SGS/TÜV) countersignature, 100% PMI, hardness survey (thread root included), ferrite content measurement (ASTM E562), solution anneal certificate, Charpy impact testing, ASTM F606 tensile test, passivation test, thread gauge certificate and Xylan coating QC report. NACE MR0175 / ISO 15156-3 full compliance. 48-hour express dispatch on standard in-stock sizes.