UNS S32750 Stud Bolt | Super Duplex 2507 Fasteners
UNS S32750 stud bolts are high-strength super duplex stainless steel fasteners designed for seawater, offshore, chemical-processing and chloride-containing environments. With approximately 25% chromium, 7% nickel, 4% molybdenum and 0.27% nitrogen, S32750 combines very high mechanical strength with excellent resistance to pitting, crevice corrosion and chloride stress-corrosion cracking.
We manufacture and supply UNS S32750 fully threaded studs, flange stud bolts, hex bolts, heavy hex bolts, hex nuts and custom fasteners in metric and inch sizes, with optional PTFE or fluoropolymer coating, PMI, thread inspection and EN 10204 3.1 certification.
UNS S32750 Fastener Specification
| Item | Specification |
|---|---|
| Material | UNS S32750 |
| Common Designation | Super Duplex 2507 |
| EN Material No. | 1.4410 |
| EN Name | X2CrNiMoN25-7-4 |
| Material Family | Super Duplex Austenitic-Ferritic Stainless Steel |
| Material Standard | ASTM A479 / ASME SA-479 for bar material where applicable |
| Stud Standard | ASME B18.31.2 when specified |
| Hex Bolt Standard | ASME B18.2.1 / ISO 4014 / ISO 4017 |
| Hex Nut Standard | ASME B18.2.2 / applicable metric standard |
| Thread Standard | ASME B1.1 / metric thread standard |
| Products | Stud Bolt / Hex Bolt / Heavy Hex Bolt / Hex Nut |
| Coating | Plain / PTFE / Fluoropolymer when specified |
| Certification | EN 10204 3.1 / 3.2 when specified |
What Is UNS S32750?
UNS S32750 is a high-alloy super duplex stainless steel with a mixed austenitic-ferritic microstructure.
Its combination of chromium, molybdenum and nitrogen provides high resistance to localized corrosion, while the duplex structure gives the material significantly higher strength than conventional austenitic stainless steels.
Important characteristics include:
- Very high yield strength
- High tensile strength
- Excellent pitting resistance
- Excellent crevice-corrosion resistance
- Strong resistance to chloride stress-corrosion cracking
- Good erosion-corrosion resistance
- Good resistance in many seawater environments
S32750 should not be described as a fully austenitic or non-magnetic stainless steel.
UNS S32750 Chemical Composition
| Element | UNS S32750 |
|---|---|
| Chromium, Cr | 24.0–26.0% |
| Nickel, Ni | 6.0–8.0% |
| Molybdenum, Mo | 3.0–5.0% |
| Nitrogen, N | 0.24–0.32% |
| Carbon, C | 0.030% max |
| Manganese, Mn | 1.20% max |
| Silicon, Si | 0.80% max |
| Phosphorus, P | 0.035% max |
| Sulfur, S | 0.020% max |
| Copper, Cu | 0.50% max |
| Iron, Fe | Balance |
The high Cr-Mo-N content is the main reason for the strong localized corrosion resistance of S32750.
Actual heat chemistry is reported on the Material Test Certificate.
Mechanical Properties
UNS S32750 has substantially higher strength than conventional austenitic stainless steels.
Typical solution-annealed material properties include:
| Property | Typical Minimum Level |
|---|---|
| Tensile Strength | 800 MPa / 116 ksi |
| 0.2% Yield Strength | 550 MPa / 80 ksi |
| Elongation | 15–25% depending on product specification |
Mechanical acceptance requirements can vary with bar diameter, product form and ordered ASTM A479 / ASME SA-479 requirements.
Actual mechanical-test results are reported on the supplied MTC.
Finished fastener performance should be evaluated according to:
Material Condition + Fastener Dimensions + Thread Geometry + Joint Design + Project Specification
ASTM A479 / SA-479 and Fastener Standards
The material specification and fastener dimensional standard have different functions.
ASTM A479 / ASME SA-479
ASTM A479 / ASME SA-479 covers stainless steel bars and shapes, including austenitic-ferritic grades such as UNS S32750.
It can provide the raw-material basis for S32750 fasteners where specified.
ASME B18.31.2 Stud Bolts
ASME B18.31.2 covers inch-series:
- Continuous-thread studs
- Flange bolting studs
- Double-end studs
ASME B18.2.1 Hex Bolts
Inch-series hex bolts can be manufactured to ASME B18.2.1 when specified.
Metric hex bolts can be supplied to standards such as:
- ISO 4014
- ISO 4017
ASME B18.2.2 Hex Nuts
Inch-series hex and heavy hex nuts can be supplied to ASME B18.2.2.
Unified inch threads can be specified to ASME B1.1, including UNC, UNF and 8UN.
PREN and Chloride Corrosion Resistance
A key advantage of S32750 is its high Pitting Resistance Equivalent Number (PREN).
A commonly used formula is:
PREN = %Cr + 3.3 × %Mo + 16 × %N
With approximately:
- 25% Cr
- 4% Mo
- 0.27–0.30% N
S32750 typically has a PREN above 40.
This provides strong resistance to:
- Pitting corrosion
- Crevice corrosion
- Chloride attack
- Chloride stress-corrosion cracking
PREN is an important comparison tool but should not be used as the sole material-selection criterion.
Seawater & Offshore Service
UNS S32750 fasteners are widely used in seawater, marine and offshore equipment.
Typical applications include:
- Offshore piping
- Seawater systems
- Desalination equipment
- Subsea equipment
- Marine pumps
- Valves
- Heat exchangers
- Scrubber systems
- High-chloride process piping
Final suitability depends on temperature, chlorination, crevice geometry, flow conditions and process chemistry.
S32750 Fasteners for Sour Service
UNS S32750 can be used in certain H₂S-containing oil and gas environments when the material condition and environmental limits comply with the applicable NACE MR0175 / ISO 15156 requirements.
Sour-service suitability depends on factors including:
- Material condition
- Hardness
- Temperature
- H₂S partial pressure
- Chloride concentration
- pH
- Applied stress
- Project requirements
The UNS designation alone should not be treated as automatic approval for every sour-service environment.
Available S32750 Fastener Types
| Product | Description |
|---|---|
| Fully Threaded Stud | Threaded along the complete length |
| Flange Stud Bolt | Stud used with nuts for flange joints |
| Double-End Stud | Threads provided at both ends |
| Hex Bolt | Hexagonal-head bolt |
| Heavy Hex Bolt | Heavy-pattern hex head when specified |
| Hex Nut | Matching corrosion-resistant nut |
| Heavy Hex Nut | Heavy-pattern nut for bolting assemblies |
| Custom Fastener | Manufactured according to drawing |
Typical Size & Thread Range
| Item | Typical Range |
|---|---|
| Metric Diameter | M6–M64 or project-specific |
| Inch Diameter | 1/4″–2″ or project-specific |
| Length | Custom |
| Metric Threads | Coarse / Fine |
| Inch Threads | UNC / UNF / 8UN |
| Thread Class | As specified |
Actual availability depends on material diameter, thread form, length, quantity and inspection requirements.
PTFE / Fluoropolymer-Coated S32750 Stud Bolts
S32750 stud bolts can be supplied with PTFE or fluoropolymer coating when required by the project specification.
Potential benefits include:
- Reduced thread friction
- More consistent assembly
- Reduced galling risk
- Easier disassembly
- Additional barrier protection
Coating has an important effect on the nut factor and torque-tension relationship.
Installation torque should therefore be based on the specified coating system and validated friction characteristics rather than using an uncoated fastener torque value.
Thread Galling & Assembly
Super duplex stud-and-nut combinations can be susceptible to thread galling if assembled without appropriate control.
Important factors include:
- Thread finish
- Surface condition
- Lubricant
- Coating
- Installation speed
- Nut factor
- Torque control
Suitable lubrication or an approved low-friction coating can reduce the risk of seizure during assembly.
The final tightening procedure should follow the flange-joint and project requirements.
S32750 vs S32205 Fasteners
| Item | S32750 / 2507 | S32205 / 2205 |
|---|---|---|
| Stainless Type | Super Duplex | Duplex |
| Cr | 24–26% | 22–23% |
| Mo | 3–5% | ~3% |
| N | 0.24–0.32% | Lower |
| PREN | Above 40 | Typically mid-30s |
| Strength | Very High | High |
| Chloride Resistance | Higher | Good |
| Typical Positioning | Severe chloride / seawater | General duplex service |
S32750 is generally selected where the chloride environment or localized corrosion requirements exceed the capability of standard 2205 duplex stainless steel.
Temperature Considerations
S32750 is not primarily a high-temperature bolting alloy.
Prolonged exposure to elevated temperatures can cause detrimental microstructural changes that reduce toughness and corrosion performance.
Operating temperature should therefore be evaluated according to:
- Duration of exposure
- Applicable design code
- Material condition
- Joint requirements
- Process environment
High oxidation temperature should not be used as a substitute for an allowable bolting design temperature.
Inspection & Certification
UNS S32750 fasteners can be supplied with:
- EN 10204 Type 3.1 MTC
- EN 10204 Type 3.2 when specified
- Heat chemical analysis
- Mechanical test results
- Hardness testing
- PMI
- Dimensional inspection
- Thread inspection
- Visual examination
- Surface inspection
- Raw-material UT when specified
- Coating inspection when applicable
- Third-party inspection
- Full heat-number traceability
Frequently Asked Questions
What is a UNS S32750 stud bolt?
A UNS S32750 stud bolt is a high-strength super duplex stainless steel fastener designed for severe chloride, seawater, offshore and corrosive process environments.
Is S32750 the same as Super Duplex 2507?
Yes. UNS S32750 is the standardized UNS designation commonly associated with Super Duplex 2507 and EN 1.4410.
What is the PREN of UNS S32750?
Based on its high chromium, molybdenum and nitrogen contents, UNS S32750 typically has a PREN above 40, providing very high resistance to pitting and crevice corrosion.
Can S32750 stud bolts be used in sour service?
S32750 can be used in certain H₂S-containing environments when material condition, hardness and environmental limits comply with NACE MR0175 / ISO 15156 and project requirements.
Why use PTFE coating on S32750 stud bolts?
PTFE or fluoropolymer coating can reduce thread friction and galling and improve assembly consistency.
Because coating changes the nut factor, installation torque should be selected specifically for the coated bolting system.
UNS S32750 Stud Bolt-Super Duplex 2507 Fasteners
UNS S32750 Stud Bolt Manufacturer
We manufacture and supply UNS S32750 Super Duplex stud bolts, fully threaded studs, hex bolts, heavy hex bolts, hex nuts and custom corrosion-resistant fasteners for offshore, seawater, chemical and high-chloride applications.
Available options include:
UNC / UNF / 8UN / Metric Threads, Custom Lengths, Matching Nuts, PTFE / Fluoropolymer Coating, PMI, Hardness Testing, Thread Inspection and EN 10204 Certification
For quotation, please specify:
Material + Product Type + Diameter + Thread + Length + Nut Requirement + Coating + Quantity + Standard + Inspection Requirements
Example:
UNS S32750 Stud Bolt, 1″-8UN × 250 mm, Fully Threaded, Matching S32750 Heavy Hex Nuts, PTFE Coated, EN 10204 3.1
