DUPLEX UNS S31803

1. Introduction to Duplex Stainless Steel Fasteners

Duplex Stainless Steel UNS S31803 / UNS S32205 (EN 1.4462) fasteners represent a high-performance category of corrosion-resistant engineering fasteners designed for severe industrial environments where conventional austenitic stainless steels may not provide adequate strength or corrosion resistance.

The duplex microstructure consists of approximately:

• 50% Austenite (γ phase)
• 50% Ferrite (α phase)

This balanced structure combines:

The resulting material provides approximately twice the yield strength of standard 304 and 316 stainless steels while maintaining excellent corrosion resistance.

2. International Material Designations

Equivalent Standards

3. Chemical Composition

UNS S31803 / S32205 Composition

4. Why Duplex Fasteners Are Used

Industrial fastening systems are expected to perform under simultaneous exposure to:

• Static loads
• Dynamic loads
• Thermal cycling
• Corrosive chemicals
• Offshore atmospheres
• Pressure fluctuations
• Vibration

Duplex S31803/S32205 fasteners are selected when engineers require:

High Strength

Approximately:

• Yield Strength: 450–550 MPa
• Tensile Strength: 620–880 MPa

compared with:

• 316 Stainless Yield ≈ 205 MPa

This enables:

• Smaller diameter fasteners
• Reduced weight
• Higher load carrying capacity

Superior Corrosion Resistance

Excellent resistance to:

• Chloride pitting
• Crevice corrosion
• Stress corrosion cracking
• Seawater attack
• Sour gas environments

Improved Lifecycle Economics

Benefits include:

• Reduced maintenance
• Longer service intervals
• Lower replacement frequency
• Reduced downtime

These advantages are particularly valuable for EPC projects and long-term infrastructure assets.

5. Functional Role of Duplex Fasteners

Fasteners create mechanical joints that generate clamp force.

The purpose of a fastener is not merely holding components together.

Its primary function is:

Creating sufficient preload to keep joint members compressed throughout service life.

A properly designed duplex fastener joint:

• Prevents separation
• Controls vibration
• Minimizes fatigue
• Transfers load safely
• Maintains sealing pressure

6. Fastener Load Mechanics

Fundamental Force Relationship

When tightened:

Bolt experiences:

Tensile Force

$$F_t$$

Joint experiences:

Compressive Force

$$F_c$$

Under equilibrium:$$F_t = F_c$$

This preload is the basis of all bolted joint performance.

7. Understanding Preload

Preload is the intentional tensile force generated during tightening.

Benefits

Engineering practice typically targets:$$70\%-80\%$$

of proof load.

8. Load Transfer Mechanism

External loads are ideally carried through friction between clamped components.

Friction Load Capacity

$$F_{friction}=\mu \times F_{clamp}$$

Where:

If external load exceeds friction capacity:

• Joint slip occurs
• Bolt shear increases
• Fatigue risk rises

9. Torque-Tension Relationship

Applied torque creates preload.

The practical engineering equation:$$T=K \times F \times D$$

Where:

Typical Nut Factors

Duplex fasteners often employ lubrication to achieve consistent preload.

10. Duplex Fastener Mechanical Properties

Typical Mechanical Properties

11. Comparison with Other Fastener Materials

Material Selection Matrix

12. Joint Design Principles

A fastener is only one element of a successful joint.

Joint performance depends upon:

Fastener

• Diameter
• Material
• Strength

Joint Members

• Thickness
• Modulus
• Surface finish

Interface Conditions

• Lubrication
• Coating
• Temperature

Service Conditions

• Vibration
• Pressure
• Corrosion
• Thermal cycling

13. Thread Engagement Design

Insufficient engagement causes thread stripping before bolt failure.

Recommended engagement:

For Duplex 2205 assemblies:$$L_e \ge 1D$$

is generally recommended.

14. Stress Area Calculation

Tensile stress area:$$A_s=\frac{\pi}{4} \left( \frac{d_2+d_3}{2} \right)^2$$

Where:

• d₂ = pitch diameter
• d₃ = minor diameter

This area determines:

• Proof load
• Yield load
• Tensile capacity

15. Preload Calculation Example

Fastener

M16 × 2.0 Duplex S32205

Stress Area:$$A_s=157mm^2$$

Assume:

Proof Strength:$$600MPa$$

Proof Load:$$F_p=A_s \times S_p$$

$$F_p=157 \times 600$$

$$F_p=94,200N$$

Target preload:

75%$$F=70,650N$$

Required torque:$$T=KFD$$

Assume:

K=0.18

$$T=0.18\times70650\times0.016$$

$$T=203Nm$$

Approximate installation torque:

200–205 Nm

16. Joint Stiffness Considerations

The ratio between:

• Bolt stiffness
• Joint stiffness

controls load distribution.

A stiffer joint:

• Retains preload longer
• Improves fatigue life
• Reduces bolt stress fluctuations

Engineers often design joints where:$$C=\frac{k_b}{k_b+k_j}$$

remains minimized.

17. Fatigue Behavior of Duplex Fasteners

Most bolted joints fail due to fatigue rather than overload.

Fatigue life improves when:

• Proper preload maintained
• Joint separation prevented
• Stress concentration minimized
• Rolled threads used

SM Fasteners utilizes precision thread manufacturing processes that support improved fatigue performance through controlled thread geometry and dimensional consistency.

18. Common Failure Mechanisms

Fatigue Failure

Caused by:

• Cyclic loading
• Insufficient preload
• Vibration

Indicators:

• Beach marks
• Progressive crack growth

Shear Failure

Occurs when:$$\tau=\frac{F}{A}$$

exceeds material capability.

Common in:

• Flange joints
• Structural assemblies

Thread Stripping

Occurs due to:

• Insufficient engagement
• Soft mating material
• Over-tightening

Galling

A concern in stainless and duplex fasteners.

Causes:

• High friction
• Lack of lubrication
• Excessive installation speed

Prevention:

• PTFE coatings
• Moly lubricants
• Controlled torque installation

Stress Corrosion Cracking

Duplex alloys provide substantially better SCC resistance than 304 or 316 stainless steels.

Common environments:

• Chloride-rich systems
• Offshore platforms
• Desalination plants

Hydrogen Embrittlement

Risk is lower than high-strength alloy steel fasteners but process control remains essential during:

• Pickling
• Electroplating
• Acid cleaning

19. Corrosion Resistance by Environment

20. Duplex Fasteners in Critical Engineering Systems

Major sectors utilizing UNS S31803/S32205 fasteners include:

Oil & Gas

• Wellheads
• Pipelines
• Pressure vessels
• Offshore platforms

Petrochemical

• Heat exchangers
• Reactors
• Process piping

Power Generation

• FGD systems
• Cooling water circuits
• Turbine auxiliaries

Marine & Shipbuilding

• Deck equipment
• Seawater systems
• Structural assemblies

Infrastructure

• Bridges
• Coastal structures
• Water treatment facilities

OEM Equipment

• Pumps
• Compressors
• Valves
• Filtration systems

21. Duplex S31803/S32205 Fastener Product Portfolio

Duplex Stainless Steel UNS S31803/S32205 fasteners are manufactured in a wide range of configurations to satisfy the requirements of structural, pressure-containing, rotating equipment, offshore, marine, petrochemical, and EPC applications.

At SM Fasteners, Duplex 2205 fasteners are produced under controlled manufacturing and inspection systems aligned with ISO 9001 quality management requirements, supporting both standard and custom-engineered fastening solutions.

22. Primary Fastener Categories

Bolts

Bolts are externally threaded fasteners intended for assembly with nuts.

Typical configurations:

Applications:

• Pressure vessels
• Flanged piping
• Structural steel
• Heat exchangers
• Offshore equipment

Nuts

Internally threaded components that generate clamping force.

Common types:

Stud Bolts

Stud bolts are preferred for critical flange connections because preload is more consistent than conventional bolt arrangements.

Common standards:

Industries:

• Refining
• LNG
• Offshore
• Power generation

Screws

Washers

Washers improve load distribution and reduce bearing stress.

Threaded Rods

Used where adjustable assembly length is required.

Applications:

• Pipe supports
• Structural bracing
• Anchor systems
• HVAC supports

Standards:

• DIN 975
• DIN 976
• ASTM A193

Rings and Specialty Components

SM Fasteners also manufactures:

• Eye bolts
• Lifting rings
• Retaining rings
• Locking rings
• Custom CNC fasteners

for project-specific engineering requirements.

23. PEEK Fasteners in Duplex Assemblies

Where galvanic isolation or electrical insulation is required, PEEK fasteners may complement duplex systems.

Typical applications:

Advantages:

• Chemical resistance
• Electrical insulation
• Low weight
• Non-magnetic behavior

24. Fastener Geometry Fundamentals

Fastener performance depends heavily upon geometry.

Critical dimensions include:

Diameter

Determines:

• Tensile capacity
• Shear capacity
• Clamp load

Thread Pitch

Determines:

• Mechanical advantage
• Vibration resistance
• Adjustment precision

Grip Length

Influences:

• Joint stiffness
• Load distribution

Head Geometry

Determines:

• Wrench engagement
• Bearing pressure
• Installation access

25. Metric Thread System

Most industrial duplex fasteners use ISO Metric Threads.

Designation format:

Example:

M20 × 2.5 × 120

Where:

26. Coarse vs Fine Threads

27. ISO Metric Thread Dimensions

Standard Metric Coarse Pitch Series

28. Hex Bolt Dimensional Logic

Hex bolt dimensions are governed by:

• ISO 4014
• ISO 4017
• DIN 931
• DIN 933

Key dimensions:

29. Standard Hex Bolt Dimensions

ISO Metric Hex Bolts

30. Duplex Bolt Length Range

Typical manufacturing range:

SM Fasteners can manufacture non-standard lengths according to EPC drawings and customer specifications.

31. Thread Standards and Tolerances

Metric Thread Standards

Unified Thread Standards

British Thread Standards

32. Thread Tolerance Classes

Metric External Threads

Metric Internal Threads

Most Duplex fasteners are supplied as:

6g / 6H

33. UNC and UNF Thread Chart

34. BSW and BSF Thread Chart

35. Proof Load Fundamentals

Proof load is the maximum load a fastener can withstand without permanent deformation.$$F_p = A_s \times S_p$$

Where:

• Fp = Proof Load
• As = Tensile Stress Area
• Sp = Proof Stress

36. Duplex Fastener Proof Load Table

Typical Values

Based on approximately 600 MPa proof stress.

37. Tensile Capacity Table

Duplex UNS S32205

Assuming:$$UTS = 720 MPa$$

38. Load Distribution in Multi-Bolt Joints

Load sharing depends upon:

• Bolt spacing
• Flange rigidity
• Gasket stiffness
• Thermal expansion

Design standards commonly referenced:

• ASME PCC-1
• ASME Section VIII
• EN 1591

39. Bolt Spacing Principles

Recommended spacing:$$3D \le Pitch \le 6D$$

Minimum edge distance:$$1.5D$$

Where D is bolt diameter.

40. Heavy Hex vs Standard Hex Geometry

Heavy hex fasteners are commonly used in:

• Refineries
• LNG terminals
• Offshore platforms
• Pressure vessel systems

41. ASTM Standards Relevant to Duplex Fasteners

42. ISO Standards Relevant to Fasteners

43. DIN Standards Commonly Applied

44. British Standards References

45. Interchangeability Considerations

Before substituting fasteners, engineers must verify:

• Thread form
• Pitch
• Strength level
• Corrosion resistance
• Temperature capability
• Certification requirements

Substituting 316 stainless with Duplex 2205 generally improves strength, but design verification remains mandatory.

46. Engineering Selection Matrix

47. Procurement Considerations

EPC buyers typically specify:

• Heat number traceability
• ASTM-compliant raw material
• EN 10204 3.1 certification
• PMI verification
• Mechanical test reports
• Dimensional inspection records

SM Fasteners supports project requirements through documented quality systems, material traceability, and custom-engineered fastener manufacturing across standard and non-standard dimensions.

48. Materials Engineering of Duplex Fasteners

Material selection is one of the most critical decisions in fastener engineering because fasteners are often the most highly stressed components within an assembly.

Selection must simultaneously satisfy:

• Mechanical strength requirements
• Corrosion resistance requirements
• Temperature limitations
• Fatigue performance
• Environmental compatibility
• Regulatory compliance
• Project lifecycle expectations

For offshore, petrochemical, LNG, power generation, and EPC projects, Duplex UNS S31803/S32205 (EN 1.4462) provides an optimized balance between strength, corrosion resistance, and cost effectiveness.

49. Metallurgical Structure of Duplex 2205

The defining characteristic of Duplex 2205 is its dual-phase microstructure.

Approximate phase balance:

Ideal target:

This balanced structure provides:

Ferrite Contribution

• High yield strength
• Improved SCC resistance
• Reduced thermal expansion

Austenite Contribution

• Toughness
• Ductility
• Improved fabrication characteristics

The resulting alloy offers superior performance compared with conventional 304 and 316 stainless steel fasteners.

50. PREN (Pitting Resistance Equivalent Number)

Resistance to localized chloride attack is commonly evaluated using PREN.

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

Typical Duplex 2205:$$PREN \approx 35$$

Comparison:

Higher PREN values generally indicate better resistance to pitting and crevice corrosion.

51. Material Selection Matrix

Comparative Engineering Properties

52. Duplex vs Super Duplex Selection

Engineering Comparison

Selection guideline:

Duplex 2205

Preferred for:

• Oil & Gas
• Petrochemical
• Power plants
• Desalination systems

Super Duplex 2507

Preferred for:

• Deepwater offshore
• High chloride exposure
• Severe marine immersion
• Critical subsea equipment

53. Corrosion Resistance by Environment

Service Environment Selection Table

54. NACE MR0175 / ISO 15156 Compliance

For oil and gas projects involving hydrogen sulfide (H₂S), fasteners frequently require compliance with:

NACE MR0175

and

ISO 15156

These standards govern:

• Material selection
• Hardness limitations
• Environmental suitability
• Sulfide stress cracking resistance

Duplex S31803/S32205 is widely accepted for sour service applications when manufactured and heat-treated within specified hardness limits.

55. Hardness Requirements for Sour Service

Typical limits:

Excessive hardness may increase susceptibility to:

• Sulfide stress cracking
• Hydrogen-assisted cracking

Therefore process control and testing are essential.

56. Service Temperature Limits

Recommended Operating Range

Duplex materials are generally not recommended for prolonged exposure above approximately 300°C because microstructural changes can reduce toughness and corrosion resistance.

57. Material Verification Prior to Manufacturing

SM Fasteners follows material verification procedures before production begins.

Verification typically includes:

Review of

• Material Test Certificate (MTC)
• Heat number
• Chemical composition
• Mechanical properties

Incoming Inspection

• Visual examination
• Dimensional verification
• Positive Material Identification (PMI)

Only approved material proceeds to manufacturing.

58. Raw Material Forms Used

Fasteners may be manufactured from:

Selection depends on:

• Fastener size
• Mechanical requirements
• Production volume

59. Manufacturing Route Overview

A controlled manufacturing workflow is essential for producing high-integrity duplex fasteners.

Typical Process Flow

Raw Material Receipt

↓

Material Verification

↓

Cutting

↓

Forging or Machining

↓

Heat Treatment

↓

Thread Manufacturing

↓

Surface Treatment

↓

Inspection

↓

Marking

↓

Packaging

↓

Dispatch

60. Hot Forging Process

Large duplex fasteners are generally produced by hot forging.

Benefits:

• Improved grain flow
• Better fatigue resistance
• Reduced material waste
• Higher structural integrity

Common products:

• Heavy hex bolts
• Stud bolts
• Large diameter fasteners

61. Cold Forming Process

Used for smaller fasteners.

Advantages:

Typically used up to medium diameters.

62. CNC Machining Operations

Precision machining may be required for:

• Custom bolts
• Special heads
• Aerospace parts
• OEM components

Processes include:

• Turning
• Milling
• Drilling
• Threading
• Grooving

63. Thread Manufacturing Methods

Thread quality directly influences:

• Preload accuracy
• Fatigue life
• Assembly reliability

Two primary methods are used.

64. Thread Rolling

Threads are formed by plastic deformation.

Advantages:

Preferred for critical fasteners.

65. Thread Cutting

Threads are machined by material removal.

Advantages:

Limitations:

• Lower fatigue performance
• Interrupted grain structure

Typically used for:

• Large studs
• Custom fasteners
• Low-volume production

66. Solution Annealing

Solution annealing is the most important heat treatment for Duplex 2205.

Typical temperature:

Followed by:

Rapid Water Quenching

Purpose:

• Restore phase balance
• Dissolve detrimental phases
• Improve corrosion resistance
• Optimize mechanical properties

67. Harmful Intermetallic Phases

Improper heat treatment may form:

Controlled processing prevents their formation.

68. Ferrite Content Control

Ferrite content significantly influences performance.

Typical acceptable range:

Measured according to:

• ASTM E562
• ASTM A923

69. Mechanical Property Development

After proper solution annealing:

70. Surface Finishing Objectives

Surface finishing provides:

• Improved corrosion resistance
• Removal of contaminants
• Better appearance
• Reduced galling risk

71. Surface Finish Types

As Forged

Characteristics:

• Economical
• Rough surface
• Industrial applications

Machined Finish

Characteristics:

• Precise dimensions
• Smooth finish
• OEM equipment

Pickled Finish

Characteristics:

• Oxide removal
• Improved corrosion resistance

Passivated Finish

Characteristics:

• Enhanced chromium oxide layer
• Improved corrosion resistance

Electropolished Finish

Characteristics:

• Ultra-clean surface
• Reduced roughness
• Enhanced hygiene

72. Surface Finish Comparison

73. Coating Considerations for Duplex Fasteners

Unlike carbon steel fasteners, Duplex 2205 generally does not require protective coatings for corrosion resistance.

However coatings may be used for:

• Anti-galling
• Friction control
• Assembly efficiency

74. Common Coating Systems

75. Surface Finish & Coating Performance Comparison

76. Galvanic Compatibility Considerations

When Duplex fasteners are coupled with dissimilar materials:

Potential concerns:

• Galvanic corrosion
• Accelerated attack of less noble metals

Common mating materials:

Insulating washers or PEEK components may be utilized where galvanic isolation is required.

77. Traceability During Manufacturing

Every production lot should maintain traceability through:

• Heat number tracking
• Batch control
• Manufacturing records
• Inspection records
• Material certificates

SM Fasteners integrates traceability practices within its ISO 9001 quality management framework to support EPC, oil & gas, power generation, and export project requirements.

78. Manufacturing Documentation Generated

Throughout production, documentation may include:

79. Engineering Selection Summary

Duplex UNS S31803/S32205 should be selected when projects require:

✓ High strength

✓ Excellent chloride resistance

✓ Sour service suitability

✓ Long service life

✓ Reduced maintenance

✓ Improved weight efficiency

✓ Compliance with international oil & gas standards