SUPER DUPLEX

1. Introduction to Super Duplex Fasteners

Super Duplex Stainless Steel fasteners manufactured from UNS S32750 (EN 1.4410) and UNS S32760 (EN 1.4501) are engineered fastening components developed for highly corrosive and mechanically demanding industrial environments where conventional stainless steel, carbon steel, and standard duplex grades may not provide sufficient performance.

These alloys combine:

• High tensile strength
• Excellent chloride resistance
• Superior pitting resistance
• Outstanding crevice corrosion resistance
• Enhanced fatigue performance
• Improved stress corrosion cracking resistance

As a result, Super Duplex fasteners are widely specified across:

• Offshore platforms
• FPSOs
• Subsea systems
• LNG terminals
• Desalination plants
• Chemical processing facilities
• Petrochemical complexes
• Marine structures
• Seawater cooling systems
• Power generation projects

SM Fasteners manufactures precision-engineered Super Duplex fastening systems under controlled quality procedures aligned with ISO 9001 requirements and supports global EPC, OEM, maintenance, and infrastructure projects through traceable production and inspection systems.

2. Industry Context

Why Super Duplex Fasteners Were Developed

Traditional fastening materials face limitations in aggressive industrial environments:

The development of Super Duplex metallurgy addressed the need for:

• Higher mechanical strength
• Improved corrosion resistance
• Reduced maintenance intervals
• Increased asset life
• Lower lifecycle cost

The combination of approximately equal ferrite and austenite phases creates a microstructure capable of resisting multiple degradation mechanisms simultaneously.

3. Technical Definition

What is UNS S32750?

UNS S32750 is a super duplex stainless steel containing:

• Chromium ≈ 25%
• Nickel ≈ 7%
• Molybdenum ≈ 4%
• Nitrogen ≈ 0.3%

It is standardized under:

What is UNS S32760?

UNS S32760 is a modified super duplex grade with:

• Tungsten additions
• Enhanced pitting resistance
• Improved corrosion resistance in acidic environments

Equivalent standards include:

4. Chemical Composition

UNS S32750 Chemical Composition

UNS S32760 Chemical Composition

5. Metallurgical Structure

The superior performance of Super Duplex fasteners results from a balanced microstructure.

This dual-phase structure provides:

Ferrite Contribution

• High strength
• SCC resistance
• Improved yield strength

Austenite Contribution

• Toughness
• Ductility
• Fabrication capability

The balanced structure prevents many failure mechanisms common in conventional stainless steels.

6. PREN Value and Corrosion Resistance

Pitting Resistance Equivalent Number (PREN):

$$PREN = \%Cr + 3.3(\%Mo) + 16(\%N)$$

Typical values:

A PREN above 40 is generally considered suitable for severe chloride exposure.

7. Functional Role of Super Duplex Fasteners

Fasteners perform a structural function beyond simply joining components.

They must:

• Generate preload
• Maintain clamp force
• Resist vibration
• Prevent separation
• Transfer loads
• Accommodate thermal expansion
• Maintain sealing pressure

Typical fastening assemblies include:

• Bolt
• Nut
• Washer
• Clamped members

The bolt acts as a tension spring while the joint members act as compression springs.

8. Load Mechanics in Fastener Assemblies

Understanding load transfer is critical for fastener selection.

Tensile Loading

When tightened:

• Bolt elongates
• Joint compresses

Result:$$\sigma = \frac{F}{A}$$

Where:

Tensile Stress Example

M20 bolt:

Tensile stress area:$$A_s = 245 mm^2$$

Applied preload:$$F = 140 kN$$

Stress:$$\sigma = \frac{140000}{245}$$

$$\sigma = 571 MPa$$

9. Bolt as a Spring

A fastener behaves elastically according to Hooke’s Law:$$F = kx$$

Where:

The elastic stretch stores energy that maintains joint integrity.

10. Clamping Force Fundamentals

The primary objective of tightening is generating preload.

$$F_{clamp}=F_{preload}$$

Benefits:

• Prevents joint separation
• Improves fatigue life
• Maintains gasket compression
• Controls vibration loosening

A correctly preloaded joint experiences lower fatigue stress than an under-tightened joint.

11. Torque–Tension Relationship

The relationship between torque and preload:$$T = KFD$$

Where:

Typical Nut Factors

Super Duplex fasteners often use controlled lubrication to improve preload accuracy.

12. Friction Distribution During Tightening

Applied torque is consumed as:

Therefore only a small fraction of tightening torque becomes actual clamping force.

13. Preload Calculation Example

M24 Super Duplex Bolt

Desired preload:$$F=200kN$$

Diameter:$$D=24mm$$

Nut Factor:$$K=0.18$$

Torque:$$T=0.18\times200000\times0.024$$

$$T=864Nm$$

Required tightening torque:

864 N·m

14. Joint Stiffness Principles

Load sharing depends on stiffness ratio.$$C=\frac{K_b}{K_b+K_j}$$

Where:

Higher joint stiffness reduces bolt load increase during service.

15. Thread Engagement Principles

Thread engagement directly influences load-carrying capacity.

Recommended minimum engagement:

For M20:

Minimum engagement:$$20mm$$

Preferred:$$25mm$$

16. Load Types Encountered in Service

Static Load

Characteristics:

• Constant magnitude
• Minimal fluctuation

Examples:

• Structural supports
• Pipe supports

Dynamic Load

Characteristics:

• Variable loading

Examples:

• Pumps
• Compressors
• Rotating equipment

Impact Load

Examples:

• Crane structures
• Mining equipment
• Heavy machinery

17. Shear Loading

Fasteners may experience transverse loading.

Single shear:$$\tau = \frac{F}{A}$$

Double shear:$$\tau = \frac{F}{2A}$$

Design preference:

Avoid direct shear by using friction-grip joints whenever possible.

18. Combined Loading Conditions

Real-world joints often encounter:

• Tension
• Shear
• Bending
• Thermal stress

Simultaneously.

Design engineers evaluate combined stress using interaction equations specified by project codes.

19. Fatigue Behavior

Fatigue is one of the leading causes of fastener failure.

Failure process:

1. Crack initiation
2. Crack propagation
3. Sudden fracture

Factors affecting fatigue:

Super Duplex fasteners exhibit significantly improved fatigue resistance compared with standard stainless steels.

20. Stress Corrosion Cracking Resistance

A major advantage of Super Duplex grades is resistance to chloride-induced SCC.

This makes them preferred for offshore and seawater service.

21. Galvanic Compatibility Considerations

When dissimilar metals are connected:

• Galvanic cells can form
• Corrosion rates increase

Best practice:

• Match fastener and flange materials
• Use isolation washers where necessary
• Evaluate seawater exposure conditions

22. Joint Design Principles for EPC Projects

Proper fastener selection requires evaluation of:

Mechanical Requirements

• Tensile load
• Shear load
• Fatigue load
• Impact load

Environmental Requirements

• Chlorides
• H₂S
• CO₂
• Acids
• Temperature

Maintenance Requirements

• Accessibility
• Retightening frequency
• Inspection intervals

Regulatory Requirements

• ASTM compliance
• ISO standards
• NACE MR0175
• Client specifications

23. Design Safety Factors

Typical industrial practice:

Final values depend on governing design codes.

24. Why Super Duplex Fasteners Are Selected

Engineering selection is usually based on:

25. Product Types and Variants

Super Duplex fasteners are manufactured in a broad range of configurations to satisfy structural, pressure-containing, rotating equipment, offshore, subsea, petrochemical, LNG, and heavy engineering requirements.

Selection of the correct geometry directly influences:

• Load transfer efficiency
• Assembly reliability
• Installation accessibility
• Torque transmission
• Fatigue resistance
• Corrosion performance

SM Fasteners manufactures Super Duplex fasteners in accordance with international dimensional and material specifications, including standard and custom-engineered designs.

26. Super Duplex Bolt Types

Hex Head Bolts

Most commonly used industrial fastener.

Characteristics:

• External wrenching
• High torque transmission
• Widely standardized
• Suitable for structural and pressure applications

Applications:

• Flanges
• Structural steel
• Offshore modules
• Pressure vessels
• Pipe supports

Applicable Standards:

• ISO 4014
• ISO 4017
• DIN 931
• DIN 933
• ASTM A193/A320 project requirements

HEAVY HEX BOLT

Feature:

• Larger bearing surface
• Improved load distribution
• Preferred for critical bolted joints

Applications:

• Pressure vessels
• ASME flanges
• Petrochemical plants
• Refineries

Standards:

• ASME B18.2.1
• ASTM-compatible designs

Socket Head Cap Screws

Characteristics:

• Internal hex drive
• Compact head design
• High clamping efficiency

Applications:

• Pumps
• Compressors
• Precision equipment
• Rotating machinery

Standards:

• ISO 4762
• DIN 912

Hex Socket Countersunk Screws

Designed for flush installation.

Advantages:

• Smooth external surface
• Reduced snagging
• Improved aesthetics

Standards:

• ISO 10642
• DIN 7991

Stud Bolts

Widely used in flange assemblies.

Advantages:

• Uniform load distribution
• Easier maintenance
• Improved gasket performance

Standards:

• ASTM A193
• ASME B16.5
• ASME B16.47

Applications:

• LNG
• Refinery piping
• Offshore topsides

Tap-End Studs

Designed for blind-hole assemblies.

Applications:

• Pumps
• Valves
• Heat exchangers

Double-End Studs

Provide equal thread engagement on both ends.

Used in:

• High-pressure equipment
• Rotating machinery
• Heavy industrial systems

27. Super Duplex Nut Types

Nut geometry affects:

• Clamp retention
• Bearing stress
• Joint reliability

Hex Nut

Most widely specified.

Standards:

• ISO 4032
• DIN 934

Applications:

• General industrial assemblies

Heavy Hex Nuts

Provide:

• Greater thread engagement
• Improved load capacity

Lock Nuts

Designed to resist vibration loosening.

Types:

• Prevailing torque nuts
• Nylon insert nuts
• All-metal lock nuts

Slotted Nuts

Used with cotter pins.

Applications:

• Rotating equipment
• Safety-critical assemblies

28. Washer Types

Washers play a significant role in load distribution.

Flat Washers

Functions:

• Reduce bearing stress
• Protect mating surfaces

Standards:

• ISO 7089
• DIN 125

Heavy Duty Washers

Applications:

• Structural connections
• Offshore systems

Standards:

• ASTM F436 equivalent geometry

Spring Washers

Provide limited resistance to loosening.

Standards:

• DIN 127

Belleville Washers

Provide:

• Elastic preload retention
• Thermal compensation

Applications:

• High-temperature joints
• Dynamic systems

29. Threaded Rods

Continuous-thread fasteners used for:

• Structural anchoring
• Pipe support systems
• Equipment mounting

Standards:

• DIN 975
• DIN 976

Available lengths:

• 1 meter
• 2 meter
• 3 meter
• Custom lengths

30. Custom Super Duplex Fasteners

SM Fasteners supports custom-engineered products including:

• Special head configurations
• Non-standard threads
• Large diameter fasteners
• Offshore bolting systems
• Anchor bolts
• Foundation bolts
• Eye bolts
• U-bolts
• Ring bolts
• Precision-machined components

31. Fastener Geometry Fundamentals

Fastener geometry directly affects:

• Strength
• Fatigue resistance
• Torque characteristics
• Installation requirements

Key geometric elements include:

• Nominal diameter
• Pitch
• Thread angle
• Head dimensions
• Bearing area
• Thread length

32. Metric Thread System

The ISO Metric Thread is the most common international fastening system.

Thread profile angle:$$60^\circ$$

Example:

M20 × 2.5

Where:

33. Unified Thread System

Common in North America.

Types:

• UNC
• UNF
• UNEF

Thread angle:$$60^\circ$$

Example:

3/4″-10 UNC

34. British Thread Systems

Used in legacy equipment and infrastructure.

Types:

• BSW
• BSF

Thread angle:$$55^\circ$$

Applications:

• Railways
• Older power stations
• Heritage infrastructure

35. Thread Standards & Tolerances Table

36. Thread Engagement Requirements

Recommended minimum thread engagement:

Preferred engagement in corrosion-critical service:$$1.25D$$1.25D

37. Dimensional Logic of Fasteners

Fastener sizing is based on:

• Tensile area
• Shear area
• Required preload
• Installation clearance
• Available wrench space

Design engineers generally select the smallest diameter capable of meeting load requirements with acceptable safety factors.

38. Standard Metric Dimensions Table

ISO Hex Bolt Dimensions

Based on ISO 4014 / ISO 4017 dimensional systems.

39. Metric Tensile Stress Area Table

These values are used for preload and tensile calculations.

40. Standard Length Range

SM Fasteners can manufacture standard and custom lengths.

Custom lengths available upon project requirements.

41. Applicable International Standards

Super Duplex fasteners may be produced according to multiple international standards depending on project specifications.z

ISO Standards

*Mechanical property classes of carbon/alloy steel differ from Super Duplex material requirements.

ASTM Standards

DIN Standards

British Standards

42. NACE Compliance Requirements

For sour service applications:

Applicable standards:

• NACE MR0175
• ISO 15156

Requirements include:

• Material chemistry control
• Hardness limitations
• Heat treatment control
• Traceability

These requirements are common in:

• Upstream oil production
• Offshore platforms
• H₂S service pipelines

43. ASME Integration

Super Duplex bolting is frequently used in:

• ASME B16.5 flanges
• ASME pressure vessels
• ASME piping systems

Relevant standards include:

44. Fastener Selection Logic

Selection typically follows:

Step 1

Determine service environment:

• Seawater
• Chemical
• Offshore
• H₂S

Step 2

Determine load requirements:

• Tensile
• Shear
• Fatigue

Step 3

Select diameter

Based on:$$F=\sigma A$$

Step 4

Verify corrosion resistance

Step 5

Verify compliance

• ASTM
• ISO
• DIN
• NACE
• Client specifications

45. Interchangeability Considerations

Before substituting fasteners, verify:

Improper substitution can lead to:

• Joint failure
• Galling
• Corrosion issues
• Reduced fatigue life

46. Engineering Design Considerations

When specifying Super Duplex fasteners, engineers should evaluate:

• Design life
• Corrosion allowance
• Accessibility
• Torque requirements
• Inspection intervals
• Maintenance philosophy
• Asset criticality

Particular attention should be given to flange joints, offshore structures, pressure-retaining equipment, and subsea systems where fastener reliability directly affects plant safety and operational continuity.

47. Material Engineering Overview

The performance of a fastener is fundamentally determined by its metallurgy. For critical applications in offshore, oil & gas, LNG, chemical processing, and power generation sectors, material selection is often more important than dimensional selection.

Super Duplex UNS S32750 and UNS S32760 are selected when applications demand:

• High mechanical strength
• Exceptional chloride resistance
• Resistance to stress corrosion cracking
• Resistance to pitting and crevice corrosion
• Long-term service life
• Compliance with NACE MR0175 / ISO 15156 requirements

SM Fasteners manufactures precision-engineered fasteners from a broad range of metallic and advanced polymer materials, allowing EPC contractors and OEMs to optimize both mechanical performance and lifecycle cost.

48. Material Selection Criteria

Material selection should be based on:

49. Industrial Fastener Material Categories

SM Fasteners supports manufacturing in:

Stainless Steel Grades

• SS 304
• SS 304L
• SS 316
• SS 316L
• SS 321
• SS 347
• SS 904L

Duplex Grades

• UNS S31803
• UNS S32205

Super Duplex Grades

• UNS S32750
• UNS S32760

Nickel Alloys

• Monel 400
• Inconel 600
• Inconel 625
• Inconel 718
• Incoloy 800
• Incoloy 825
• Hastelloy C276
• Hastelloy C22

High Alloy Stainless

• SMO 254 (UNS S31254)

Carbon & Alloy Steel

• ASTM A193 B7
• ASTM A193 B16
• ASTM A320 Grades

Engineering Polymers

• PEEK Fasteners

50. Material Comparison Table

Engineering Material Selection Matrix

51. Mechanical Properties of Super Duplex Fasteners

UNS S32750 / UNS S32760

52. Mechanical Property Comparison

53. Proof Load Fundamentals

Proof load represents the maximum tensile load a fastener can withstand without permanent deformation.$$Proof\ Load = A_s \times Proof\ Stress$$

Where:

• $A_s$​ = Tensile Stress Area
• Proof Stress based on material properties

54. Proof Load & Tensile Strength Table

Typical Engineering Reference Values

Values are representative engineering calculations based on minimum yield and tensile requirements.

55. Corrosion Resistance Mechanisms

Super Duplex fasteners resist corrosion through:

Chromium

Provides passive oxide film.

Molybdenum

Improves pitting resistance.

Nitrogen

Enhances:

• Strength
• Pitting resistance
• Crevice corrosion resistance

Ferritic Structure

Improves SCC resistance.

56. Corrosion Resistance vs Environment

57. Pitting Resistance Comparison

PREN (Pitting Resistance Equivalent Number):$$PREN = Cr + 3.3(Mo) + 16(N)$$

A PREN greater than 40 generally qualifies materials for severe chloride environments.

58. Temperature Capability

Material selection should consider both temperature and environment simultaneously.

59. NACE MR0175 / ISO 15156 Compliance

Super Duplex fasteners are commonly specified for:

• Sour gas production
• H₂S environments
• Offshore drilling systems
• Wellhead equipment
• Subsea production systems

Requirements include:

• Controlled chemistry
• Controlled heat treatment
• Hardness limitations
• Full traceability

60. Heat Treatment Fundamentals

Heat treatment is critical for achieving the correct ferrite-austenite balance.

Improper heat treatment can cause:

• Sigma phase formation
• Embrittlement
• Reduced toughness
• Reduced corrosion resistance

61. Solution Annealing Process

The most important thermal process for Super Duplex materials.

Typical Cycle

1. Heat to approximately:

$$1040^\circ C – 1120^\circ C$$

2. Hold at temperature.
3. Rapid water quench.

Benefits

• Restores corrosion resistance
• Dissolves harmful phases
• Improves toughness
• Restores phase balance

62. Sigma Phase Prevention

Sigma phase is a chromium-rich intermetallic compound.

Effects:

• Severe embrittlement
• Reduced corrosion resistance
• Reduced impact toughness

Formation range:$$600^\circ C – 1000^\circ C$$

Controlled heat treatment prevents its formation.

63. Hardness Control

Hardness verification is essential.

Typical requirements:

Typical acceptance:$$\leq 35 HRC$$

subject to project requirements.

64. Manufacturing Workflow Overview

SM Fasteners follows a controlled manufacturing process integrated with ISO 9001 quality management principles.

65. Raw Material Verification

Incoming materials are verified using:

• Mill Test Certificates
• Heat Number Verification
• PMI Testing
• Visual Inspection
• Dimensional Verification

Traceability begins at this stage.

66. Manufacturing Route

Step 1: Raw Material Procurement

Materials supplied according to:

• ASTM A479
• ASTM A276
• ASTM A182

Step 2: Material Identification

Methods:

• Heat Number Control
• PMI Testing
• Material Segregation

Step 3: Cutting

Raw bars cut into blanks.

Methods:

• Band Sawing
• CNC Cutting

Step 4: Forging

Hot forging produces:

• Improved grain flow
• Better fatigue resistance
• Improved mechanical integrity

Applications:

• Hex bolts
• Heavy hex bolts
• Custom heads

Step 5: Machining

CNC machining used for:

• Precision dimensions
• Custom geometries
• Tight tolerances

Step 6: Thread Production

Two principal methods:

Thread Rolling

Advantages:

• Higher fatigue strength
• Better surface finish
• Compressive residual stresses

Preferred for critical fasteners.

Thread Cutting

Used when:

• Large diameters
• Special threads
• Low-volume production

67. Thread Rolling vs Thread Cutting

68. Dimensional Inspection

Verification includes:

• Diameter
• Pitch
• Head dimensions
• Thread form
• Length
• Concentricity

Inspection equipment includes:

• Micrometers
• Thread Gauges
• CMM Systems

69. Surface Engineering Overview

Although Super Duplex inherently possesses excellent corrosion resistance, surface treatment may be specified for performance enhancement or contamination removal.

70. Pickling

Purpose:

• Remove oxide scale
• Remove contamination
• Restore passive layer

Common after heat treatment.

71. Passivation

Chemical treatment used to enhance chromium oxide film formation.

Benefits:

• Improved corrosion resistance
• Cleaner surface condition
• Reduced contamination risk

72. Electropolishing

Removes microscopic surface irregularities.

Benefits:

• Lower surface roughness
• Enhanced cleanability
• Improved corrosion performance

Used in:

• Pharmaceutical systems
• Food processing
• High-purity applications

73. Coating Considerations

Unlike carbon steel fasteners, Super Duplex generally does not require protective coatings.

However, specialized coatings may be specified for:

• Anti-galling
• Controlled friction
• Installation efficiency

74. Surface Finish Comparison Table

75. Galling Prevention

Austenitic and Duplex stainless fasteners can experience galling during assembly.

Recommended controls:

• Lubrication
• PTFE coatings
• Controlled tightening speed
• Proper thread finish
• Clean assembly practices