Duplex & Super Duplex
Fasteners
Manufacturer India
Why Duplex — Microstructure Creates Both Strength and Corrosion Resistance
Duplex stainless steels have a mixed microstructure of approximately equal proportions of austenite and ferrite — two distinct crystallographic phases that coexist within the same alloy. This is not a transition state or a defect: it is deliberately designed through precise control of chromium, nickel, molybdenum, and nitrogen content to stabilise both phases simultaneously at room temperature and in service.
The austenitic phase provides corrosion resistance — the familiar Cr₂O₃ passive layer protection and resistance to stress corrosion cracking (SCC) is governed by the austenite phase. The ferritic phase provides strength — ferrite has a body-centred cubic (BCC) crystal structure that resists dislocation movement more effectively than austenite’s FCC structure, giving duplex steels minimum yield strengths approximately twice that of standard 316L. Duplex 2205 achieves ≥448 MPa YS versus 316L’s ≥170 MPa — allowing thinner section fasteners with the same bolt preload, which matters in weight-sensitive offshore and subsea applications.
The pitting resistance of duplex comes from high nitrogen (N) content — up to 0.22% in 2205 — in addition to elevated Cr (22%) and Mo (3%). Nitrogen is 16× as effective as molybdenum in the PRE formula: PRE = %Cr + 3.3×%Mo + 16×%N. This nitrogen addition raises PRE from 316L’s ≈24 to 2205’s ≈35 and 2507’s ≈43. For chloride pitting (the mechanism that destroys 316L in seawater, coastal chemical plants, and offshore topside connections), the duplex grades provide a step-change improvement.
Provides: corrosion resist.
SCC resistance
Toughness / ductility
Provides: high strength
Resistance to SCC
Work hardening
Grade Profiles — 2205 (S31803) and 2507 (S32750)
Duplex 2205 is the standard workhorse duplex grade — the correct specification for the majority of offshore, chemical, and seawater applications. Super Duplex 2507 is specified when 2205 is not sufficient: higher pressure seawater injection, aggressive chloride concentrations, and high-temperature seawater service.
Duplex 2205
| Element / Property | Value |
|---|---|
| Chromium (Cr) | 21.0–23.0% |
| Nickel (Ni) | 4.5–6.5% |
| Molybdenum (Mo) | 2.5–3.5% |
| Nitrogen (N) | 0.08–0.20% |
| Min Yield Strength | ≥448 MPa |
| Min UTS | ≥621 MPa |
| PRE (nominal) | ≈ 35 |
| NACE MR0175 | Compliant ≤36 HRC |
| Hardness limit | ≤ 36 HRC |
| Max service temp. | ~315°C (practical) |
Super Duplex 2507
| Element / Property | Value |
|---|---|
| Chromium (Cr) | 24.0–26.0% |
| Nickel (Ni) | 6.0–8.0% |
| Molybdenum (Mo) | 3.0–5.0% |
| Nitrogen (N) | 0.24–0.32% |
| Min Yield Strength | ≥550 MPa |
| Min UTS | ≥795 MPa |
| PRE (nominal) | ≈ 43 |
| NACE MR0175 | Compliant ≤36 HRC |
| Hardness limit | ≤ 36 HRC |
| Density | 7.80 g/cm³ |
Duplex Fastener Types — Both Grades
Duplex 2205 (S31803/S32205) is the primary stocking grade. Super Duplex 2507 (S32750) available on order with full ASTM A182 certification. M4–M100.
Stud Bolts
Fully threaded stud bolts in Duplex 2205 (F51) and Super Duplex 2507 (F53) for ASME B16.5 pressure flanges in offshore, chemical, and seawater service. M6–M100, lengths to 3,000mm. ASTM A182 certified material. NABL tensile and hardness on every lot. Where the application requires stainless but 316L PRE≈24 is insufficient, duplex stud bolts are the cost-effective upgrade before Hastelloy.
Hex Bolts
Full thread and partial thread hex bolts in Duplex 2205 and Super Duplex 2507. M4–M100. Offshore platform structural connections, chemical plant process equipment bolting, desalination plant flanges, seawater cooling system connections, and all stainless steel hex bolt applications where 316L shows chloride pitting failure and the higher strength of duplex reduces bolt count or allows weight reduction.
Hex Nuts
Standard hex and heavy hex nuts in Duplex 2205 and Super Duplex 2507. M4–M100. Matched from same ASTM A182 certified heat as stud bolts. NACE MR0175 hardness confirmed on every lot (≤36 HRC). Single MTC covering stud bolt and nut for ASME B16.5 duplex stud bolt assembly documentation in offshore and chemical service.
Washers
Plain flat washers in Duplex 2205 and Super Duplex 2507. M4–M100. For complete duplex stud bolt assembly sets — stud bolt, nut, and washer from the same ASTM A182 certified heat for single-document traceability. DIN 9021 large OD washers for soft-face gasket flange connections and FRP equipment in offshore service.
Socket Cap Screws
Hex socket cap screws in Duplex 2205. M4–M24. Confined-access offshore connections, chemical plant process equipment internal screws, desalination plant internal baffle screws, seawater system instrument connections, and all marine bolting where flush head access and duplex’s chloride resistance are simultaneously required.
Threaded Rods
Full thread threaded rods in Duplex 2205. M4–M100, lengths to 3,000mm. Offshore structural hanger rods, chemical plant elevated equipment supports, desalination plant structural tie-rods, and all applications requiring non-magnetic corrosion-resistant threaded rod with duplex’s higher strength and chloride resistance compared to 316L.
NACE Sour Service Sets
Duplex 2205 and Super Duplex 2507 stud bolt + nut assembly sets for NACE MR0175 / ISO 15156 sour service (H₂S environments). Hardness confirmed ≤36 HRC per ASTM A370. Both duplex grades are NACE MR0175 compliant — their higher chloride resistance compared with B7M allows duplex NACE sets to handle both chloride and H₂S simultaneously, where B7M handles only H₂S in low-chloride service.
Custom Duplex
Non-standard dimensions, unequal-end stud bolts, Class 2500 duplex stud bolt sets, and drawing-specific configurations in Duplex 2205 or Super Duplex 2507. Contact sales@akbarfasteners.in with UNS designation, ASTM standard, dimensions, NPS and Class (if B16.5), and documentation requirement.
Duplex 2205 vs 316L vs 904L vs Hastelloy C276
Duplex 2205 fills the critical gap between 316L (insufficient for chloride service) and Hastelloy C276 (expensive, over-specified for most offshore and marine applications). Know when duplex is the cost-optimal and technically correct choice.
| Criterion | SS 316L | Duplex 2205 This Page | S.Duplex 2507 This Page | Hastelloy C276 |
|---|---|---|---|---|
| CORROSION RESISTANCE | ||||
| PRE (pitting) | ≈24 | ≈35 | ≈43 | ≈66 |
| Seawater immersion (topside) | Pitting risk | ✔ Good | ✔ Excellent | ✔ Excellent |
| Chloride SCC resistance | Susceptible | ✔ Good (duplex struct.) | ✔ Excellent | ✔ Excellent |
| HCl (hydrochloric acid) | Fails | Limited | Limited | ✔ Excellent |
| NACE MR0175 sour service | ✔ Compliant | ✔ Compliant ≤36 HRC | ✔ Compliant ≤36 HRC | ✔ Compliant |
| MECHANICAL PROPERTIES | ||||
| Min yield strength | ≥170 MPa | ≥448 MPa (~2.6×) | ≥550 MPa (~3.2×) | ≥310 MPa |
| Slightly magnetic | No (austenitic) | Slightly (ferritic phase) | Slightly (ferritic phase) | No (Ni alloy) |
| COST POSITION | ||||
| Relative cost vs 316L | Baseline 1× | 1.8–2.5× | 2.5–4× | 8–12× |
| Optimal for | Non-chloride | Offshore / seawater | High-pressure seawater | Acid / conc. chloride |
HCl service: Hastelloy C276 · Seawater piping system match: Cu-Ni 90/10
Duplex Fasteners Application Areas
Offshore platforms, seawater injection, chemical chloride process, desalination, oil and gas sour service, pulp and paper — wherever 316L fails by pitting and Hastelloy is over-specified.
Offshore Platforms
structural bolt sets
Oil & Gas
seawater injection bolts
Desalination Plants
RO membrane connections
Chemical Processing
bleach plant bolts
Subsea / FPSO
FPSO produced water
Pulp & Paper
digester structural
Marine Structural
pier & jetty fasteners
Power & Utilities
seawater condenser bolts
Duplex / Super Duplex Standards Coverage
Same ISO 9001:2015 quality system as our complete stainless range. PMI XRF confirms Mo 2.5–5% and N presence — the critical check distinguishing duplex grades from standard 316L which has no nitrogen addition.
Complete Documentation Package
A182 F51/F53 + UNS
Tensile · Hardness
Cr% Confirmed
≤36 HRC Confirmed
TÜV · BV · Lloyds
Related Corrosion-Resistant Fastener Grades
316L below PRE≈24. Duplex 2205 at PRE≈35. Super Duplex 2507 at PRE≈43. Hastelloy C276 at PRE≈66 for acids and concentrated chlorides.
SS 316L Fasteners
The grade below duplex — when the application has mild chloride risk and 316L’s PRE≈24 is sufficient, 316L is the lower-cost correct choice. Specify duplex only when service conditions demonstrate that 316L is failing or CPT assessment shows pitting risk. Cost difference is significant: duplex is 1.8–2.5× the cost of 316L.
View SS 316L →SS 904L Fasteners
904L (PRE≈36) is an austenitic alternative to Duplex 2205 at similar PRE — but with the H₂SO₄ resistance benefit from its copper addition. For sulphuric acid service where duplex’s ferritic phase has limited H₂SO₄ resistance, 904L is the austenitic alternative.
View 904L →Hastelloy C276 Fasteners
When super duplex 2507 is not sufficient — concentrated HCl, mixed acids, or chloride concentrations exceeding 2507’s capability. Hastelloy C276 (PRE≈66) is the grade for environments that destroy all stainless steels including super duplex.
View C276 →Monel K500 — High-Strength Seawater
When super duplex 2507’s ≥550 MPa yield is insufficient for high-load seawater connections — Monel K500 (age-hardened, ≥1000 MPa) provides the highest-strength seawater fastener in commercial engineering, at significantly higher cost than duplex.
View K500 →Need Duplex or
Super Duplex Fasteners?
Grade (2205/2507) · form · size · NACE requirement · ASME B16.5 Class/NPS. Akbar Fasteners responds within 24 hours.
Export: +91 8696 32 4646 · Support: +91 800 9315 333 · export@akbarfasteners.in