SMO 254

1 . Industry Context

Overview of SMO 254 Fasteners

SMO 254 (UNS S31254 / W.Nr. 1.4547) is a high-alloy austenitic stainless steel specifically developed for severe chloride-bearing and highly corrosive industrial environments where conventional stainless steels such as 304, 316, and even many duplex grades may not provide sufficient corrosion resistance.

The alloy was originally engineered for seawater handling systems and has since become a preferred material for:

• Offshore platforms
• Desalination plants
• Marine engineering systems
• Chemical processing facilities
• Flue gas desulfurization systems
• Pulp and paper bleaching plants
• Petrochemical installations
• LNG terminals
• Power generation facilities

In bolting applications, SMO 254 fasteners are selected when:

• Chloride concentration exceeds acceptable limits for 316L
• Pitting corrosion is a primary concern
• Crevice corrosion risk exists
• Long service life is required
• Maintenance shutdown costs are significant
• Corrosion-induced joint failures cannot be tolerated

For critical EPC projects, SMO 254 fasteners provide a balance between corrosion performance and mechanical reliability while avoiding the substantially higher cost associated with nickel-based alloys such as Hastelloy C276 or Inconel 625.

Position of SMO 254 Among Corrosion Resistant Alloys

Material Hierarchy

Material Grade	UNS Number	Corrosion Resistance	Relative Cost	Typical Use
SS304	S30400	Moderate	Low	General industrial
SS316L	S31603	Good	Low-Medium	Chemical & marine
Duplex 2205	S32205	Very Good	Medium	Offshore
Super Duplex 2507	S32750	Excellent	High	Seawater systems
SMO 254	S31254	Excellent	High	Chloride-rich environments
Alloy 625	N06625	Outstanding	Very High	Extreme service
Hastelloy C276	N10276	Outstanding	Very High	Aggressive chemical media

SMO 254 occupies a strategic position between Super Duplex and Nickel Alloys, offering exceptional chloride resistance while maintaining austenitic toughness and fabrication characteristics.

2. Technical Definition

What is SMO 254?

SMO 254 is a high-molybdenum, high-nitrogen austenitic stainless steel standardized as:

Designation System	Grade
UNS	S31254
EN	1.4547
ASTM	A182 F44
ASTM Bolting	A193 B8M Class 2 (special chemistry)
Common Trade Name	SMO 254

The alloy chemistry is optimized to maximize resistance to:

• Pitting corrosion
• Crevice corrosion
• Chloride stress corrosion cracking
• Seawater attack
• Acidic process streams

Chemical Composition

SMO 254 Typical Composition

Element	Content (%)
Chromium (Cr)	19.5 – 20.5
Nickel (Ni)	17.5 – 18.5
Molybdenum (Mo)	6.0 – 6.5
Nitrogen (N)	0.18 – 0.22
Copper (Cu)	0.5 – 1.0
Carbon (C)	Max 0.02
Manganese (Mn)	Max 1.0
Silicon (Si)	Max 0.8
Iron (Fe)	Balance

The combination of:

• High Molybdenum
• Elevated Nickel
• Controlled Nitrogen

produces superior corrosion resistance compared with conventional stainless steels.

3. Functional Role of Fasteners in Industrial Assemblies

Why Fasteners Matter

A bolted joint is fundamentally a force transmission system.

The fastener’s primary purpose is not simply holding components together but creating sufficient preload to:

• Maintain gasket compression
• Resist external loads
• Prevent separation
• Minimize vibration loosening
• Ensure structural integrity

In severe environments, corrosion-resistant fasteners become as important as the equipment materials themselves.

A corrosion-resistant pipe flange can fail prematurely if lower-grade fasteners experience:

• Crevice attack
• SCC cracking
• Thread degradation
• Galvanic corrosion

Typical SMO 254 Fastener Products

SM Fasteners manufactures and supplies:

Bolts

• Hex Head Bolts
• Heavy Hex Bolts
• Structural Bolts
• Flange Bolts
• Stud Bolts
• Anchor Bolts

Nuts

• Hex Nuts
• Heavy Hex Nuts
• Jam Nuts
• Lock Nuts
• Slotted Nuts

Washers

• Plain Washers
• Heavy Duty Washers
• Structural Washers
• Spring Washers

Threaded Components

• Fully Threaded Rods
• Double-End Studs
• Tap-End Studs
• Special Machined Fasteners

Custom Engineered Components

• Offshore bolting systems
• High-temperature assemblies
• Pressure vessel fasteners
• Corrosion-critical custom hardware

4. Load Mechanics of SMO 254 Fasteners

Fundamental Load Types

Industrial fasteners are subjected to:

Tensile Load

Force acts parallel to bolt axis.

Examples:

• Pipe flanges
• Pressure vessels
• Structural joints

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

Where:

• σ = Stress (MPa)
• F = Applied Force (N)
• A = Stress Area (mm²)

Shear Load

Force acts perpendicular to bolt axis.

Formula:$$\tau = \frac{F}{A}$$

Applications:

• Structural steel connections
• Machinery mounting
• Offshore support systems

Combined Loading

Real-world joints often experience:

• Tension
• Shear
• Bending
• Dynamic fatigue

simultaneously.

SMO 254 fasteners maintain integrity because of:

• High toughness
• Austenitic ductility
• Excellent crack resistance

5. Preload Principles

What is Preload?

Preload is the tensile force intentionally induced during tightening.

The clamping force generated by preload keeps joint members compressed.

Relationship

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

A correctly preloaded fastener:

• Prevents joint separation
• Increases fatigue resistance
• Improves sealing
• Minimizes vibration loosening

Why Preload Matters

A bolt carrying external load without sufficient preload experiences:

• Cyclic stress
• Fatigue cracking
• Thread loosening
• Leakage

Proper preload shifts most external loading into the joint members rather than the fastener itself.

6. Torque–Tension Relationship

Fundamental Equation

$$T = K \times F \times D$$

Where:

Variable	Meaning
T	Torque (N·m)
K	Nut Factor
F	Desired Preload (N)
D	Nominal Diameter (m)

Nut Factor Values

Condition	K Factor
Dry	0.20–0.25
Zinc Coated	0.18–0.22
PTFE Coated	0.10–0.15
Moly Lubricated	0.12–0.18

Because friction consumes 85–90% of tightening torque, lubrication has a major influence on achieved preload.

7. Clamping Force and Joint Integrity

Friction-Based Load Transfer

The joint relies on friction generated between mating surfaces.$$F_{friction}=\mu \times F_{clamp}$$

Where:

• μ = Coefficient of friction
• Fclamp = Clamping force

Increasing preload increases frictional resistance.

Joint Separation Prevention

For a joint to remain closed:$$F_{external} < F_{preload}$$

When external force exceeds preload:

• Gasket unloading occurs
• Joint separation begins
• Leakage becomes possible

This is particularly critical in:

• Offshore flanges
• LNG piping
• Chemical reactors
• Pressure vessels

8. Thread Engagement Principles

Importance of Thread Engagement

Insufficient thread engagement causes:

• Thread stripping
• Reduced tensile capacity
• Uneven stress distribution

Recommended minimum engagement:

Material Combination	Engagement
Steel-to-Steel	1 × Diameter
Stainless-to-Stainless	1.25 × Diameter
Soft Materials	1.5–2 × Diameter

Load Distribution Across Threads

Thread loading is not uniform.

Approximate load distribution:

Thread Position	Load Share
First Thread	34%
Second Thread	23%
Third Thread	16%
Fourth Thread	11%
Remaining Threads	16%

This explains why thread quality is critical for high-integrity bolting systems.

9. Joint Design Principles

Key Engineering Objectives

A bolted joint must:

Provide Sufficient Clamp Force

To resist:

• Separation
• Leakage
• Slip

Maintain Load Under Service Conditions

Including:

• Temperature cycling
• Vibration
• Pressure fluctuations

Prevent Fastener Failure

By controlling:

• Stress concentration
• Fatigue loading
• Corrosion attack

Joint Stiffness Concept

The external load shared by a fastener depends on:

• Bolt stiffness
• Joint stiffness

A stiffer joint transfers less load to the fastener.

This improves fatigue performance significantly.

10. Failure Mechanisms in SMO 254 Fasteners

Although highly corrosion resistant, improper design can still result in failure.

Fatigue Failure

Caused by:

• Cyclic loading
• Inadequate preload
• Stress concentration

Indicators:

• Beach marks
• Crack propagation zones

Mitigation:

• Correct preload
• Rolled threads
• Proper washer selection

Thread Stripping

Causes:

• Insufficient engagement
• Over-tightening
• Material mismatch

Mitigation:

• Correct nut height
• Controlled torque
• Thread inspection

Galling

Austenitic stainless steels are susceptible to galling.

Causes:

• High friction
• Lack of lubrication
• Rapid tightening

Prevention:

• Moly lubricant
• PTFE coating
• Controlled installation speed

Crevice Corrosion

Occurs in:

• Thread roots
• Washer interfaces
• Gasket contact regions

SMO 254 offers exceptional resistance but proper joint design remains necessary.

Stress Corrosion Cracking (SCC)

SMO 254 significantly outperforms 304 and 316 in chloride SCC environments.

Common SCC sources:

• Hot chloride solutions
• Residual stresses
• Tensile loading

Applications include:

• Desalination plants
• Offshore topsides
• Seawater cooling systems

11. Corrosion Resistance Performance

PREN Value Comparison

Pitting Resistance Equivalent Number:

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

Typical PREN

Material	PREN
304	18
316L	25
Duplex 2205	35
Super Duplex 2507	42–45
SMO 254	43–45

This places SMO 254 among the highest-performing stainless steel fastener materials for chloride resistance.

12. Corrosion Resistance vs Environment

Environment	316L	Duplex 2205	SMO 254
Fresh Water	Excellent	Excellent	Excellent
Seawater Splash Zone	Moderate	Very Good	Excellent
Full Seawater Immersion	Poor	Good	Excellent
Chloride Process Streams	Moderate	Very Good	Excellent
Sulfuric Acid Dilute	Good	Good	Excellent
Phosphoric Acid	Moderate	Good	Excellent
Caustic Solutions	Good	Good	Excellent
Offshore Atmosphere	Good	Very Good	Excellent
Desalination Systems	Moderate	Good	Excellent

13. Product Types and Variants

SMO 254 fasteners are produced in a wide range of configurations to meet the requirements of pressure-retaining equipment, offshore structures, rotating machinery, marine systems, and corrosion-critical process plants.

Selection of fastener geometry directly influences:

• Load distribution
• Joint stiffness
• Fatigue performance
• Assembly accessibility
• Maintenance requirements
• Corrosion resistance
• Inspection capability

SM Fasteners manufactures standard and custom-engineered SMO 254 fastening systems according to international specifications and project requirements.

14. SMO 254 Bolt Types

Hex Head Bolts

The most widely used industrial fastening system.

Features

• External wrench drive
• High load carrying capability
• Easy field installation
• Suitable for structural and pressure applications

Applications

• Steel structures
• Process equipment
• Pipe supports
• Offshore modules

Standards

Standard	Description
ISO 4014	Partially threaded hex bolt
ISO 4017	Fully threaded hex bolt
DIN 931	Partially threaded
DIN 933	Fully threaded
ASTM A193	Pressure service bolting

Heavy Hex Bolt

Heavy hex bolts feature larger bearing surfaces and increased wrenching dimensions.

Advantages

• Improved load distribution
• Enhanced fatigue resistance
• Better suitability for critical joints

Typical Uses

• Pressure vessels
• Heat exchangers
• Structural steel
• Flange assemblies

Flange Bolts

Feature an integrated washer face.

Benefits

• Larger bearing area
• Reduced loosening risk
• Improved vibration resistance

Used extensively in:

• Pumps
• Compressors
• Mechanical equipment

Structural Bolts

Designed specifically for steel construction.

Characteristics include:

• High tensile strength
• Controlled mechanical properties
• Reliable preload performance

Applications:

• Bridges
• Buildings
• Transmission towers
• Offshore structures

Eye Bolts

Used for:

• Lifting
• Rigging
• Equipment handling

Important design considerations:

• Load angle
• Thread engagement
• Safety factor

Anchor Bolts

Installed into concrete foundations.

Types include:

• L-Type
• J-Type
• Straight Anchor
• Sleeve Anchor
• Chemical Anchor Assemblies

Applications:

• Equipment foundations
• Pipe racks
• Structural supports

15. Stud Bolt Variants

Stud bolts are the preferred fastening system for flange joints in process industries.

Fully Threaded Stud Bolts

Threaded along entire length.

Advantages:

• Maximum adjustment flexibility
• Suitable for heavy hex nuts

Standards:

• ASTM A193
• DIN 975
• DIN 976

Applications:

• Pipe flanges
• Valves
• Heat exchangers

Tap-End Studs

Threaded on both ends.

One end screws directly into equipment.

Applications:

• Pumps
• Turbines
• Compressors

Double-End Studs

Used where repeated assembly and disassembly occur.

Benefits:

• Reduced thread wear
• Improved maintenance access

16. Nut Types

Nut selection must always correspond with:

• Bolt strength
• Thread class
• Corrosion resistance
• Operating temperature

Hex Nuts

Most common fastening nut.

Standards:

Standard	Description
ISO 4032	Standard hex nut
DIN 934	Hex nut
ASTM A194	Pressure service nut

Heavy Hex Nuts

Feature:

• Increased thickness
• Greater bearing surface
• Enhanced stripping resistance

Used in:

• Pressure vessels
• Offshore systems
• Structural assemblies

---

Jam Nuts

Reduced thickness.

Used for:

• Locking
• Positioning
• Secondary retention

Lock Nuts

Designed to resist loosening from vibration.

Variants:

• Nylon insert
• All-metal lock
• Prevailing torque

Slotted Nuts

Used with cotter pins.

Applications:

• Rotating assemblies
• Safety-critical equipment

17. Washer Types

Washers play a significant role in load distribution and joint reliability.

Plain Washers

Functions:

• Increase bearing area
• Reduce surface damage
• Improve preload distribution

Standards:

• ISO 7089
• DIN 125

Heavy Duty Washers

Used with:

• Structural bolts
• High preload joints

Benefits:

• Reduced bearing stress
• Improved fatigue performance

Spring Washers

Provide limited anti-loosening functionality.

Standards:

• DIN 127

Belleville Washers

Disc spring geometry provides:

• Load compensation
• Thermal expansion control

Applications:

• High-temperature systems
• Dynamic assemblies

18. Threaded Rods

Threaded rods are commonly used in:

• Pipe supports
• Structural bracing
• Equipment anchoring

Standards:

Standard	Description
DIN 975	Continuous thread rod
DIN 976	Stud bolt rod
ASTM A193	Pressure service rod

19. Dimensional Logic of Fasteners

Fastener dimensions are engineered to ensure:

• Adequate strength
• Assembly compatibility
• Tool accessibility
• Standard interchangeability

Critical dimensions include:

• Diameter
• Pitch
• Head size
• Head height
• Thread length
• Overall length

20. Metric Thread System

The ISO Metric thread is the most common thread form globally.

Designation:$$M \; Diameter \times Pitch$$

Example:$$M20 \times 2.5$$

Where:

• 20 mm = nominal diameter
• 2.5 mm = thread pitch

21. Metric Thread Dimensions

Standard Coarse Pitch Series

Thread Size	Pitch (mm)
M6	1.0
M8	1.25
M10	1.5
M12	1.75
M16	2.0
M20	2.5
M24	3.0
M30	3.5
M36	4.0
M42	4.5
M48	5.0
M56	5.5
M64	6.0

22. Standard Hex Bolt Dimensions

ISO 4014 / DIN 931 Reference Dimensions

Size	Across Flats (mm)	Head Height (mm)
M6	10	4
M8	13	5.3
M10	17	6.4
M12	19	7.5
M16	24	10
M20	30	12.5
M24	36	15
M30	46	18.7
M36	55	22.5
M42	65	26
M48	75	30

23. Standard Length Availability

SMO 254 fasteners are commonly supplied in:

Diameter	Standard Length Range
M6	10–100 mm
M8	12–150 mm
M10	16–200 mm
M12	20–300 mm
M16	25–400 mm
M20	30–500 mm
M24	40–600 mm
M30	50–800 mm
M36	60–1000 mm

Custom lengths can be manufactured according to EPC specifications.

24. Thread Standards and Tolerances

Fastener interchangeability depends on thread standard compliance.

Metric Threads

Standard	Description
ISO 68	Basic profile
ISO 261	Preferred diameters
ISO 262	Metric combinations
ISO 965	Tolerance system

Tolerance Class:

Component	Class
External Thread	6g
Internal Thread	6H

Unified Threads (UNC / UNF)

Widely used in:

• North America
• Offshore projects
• Oil & gas facilities

Standards:

Standard	Description
ASME B1.1	Unified Threads
ASTM Standards	Bolting Applications

UNC Series

Size	TPI
1/4″	20
5/16″	18
3/8″	16
1/2″	13
5/8″	11
3/4″	10
1″	8

UNF Series

Size	TPI
1/4″	28
5/16″	24
3/8″	24
1/2″	20
5/8″	18
3/4″	16
1″	12

---

25. British Thread Standards

Many legacy installations still utilize British thread forms.

BSW

British Standard Whitworth

Thread angle:$$55^\circ$$

Applications:

• Railways
• Heritage equipment
• Legacy infrastructure

BSF

British Standard Fine

Provides:

• Improved vibration resistance
• Higher tensile stress area

Thread Standards Comparison Table

Thread System	Angle	Typical Application
Metric ISO	60°	Global industrial
UNC	60°	Heavy engineering
UNF	60°	High strength joints
BSW	55°	Legacy systems
BSF	55°	Precision assemblies

26. Applicable International Standards

SMO 254 fasteners may be supplied according to numerous international specifications.

ISO Standards

Standard	Scope
ISO 4014	Hex bolts
ISO 4017	Hex bolts fully threaded
ISO 4032	Hex nuts
ISO 7089	Plain washers
ISO 898	Mechanical properties
ISO 965	Thread tolerances
ISO 3506	Stainless fasteners

ASTM Standards

Standard	Scope
ASTM A193	Alloy bolting materials
ASTM A194	Nuts
ASTM F593	Stainless bolts
ASTM F594	Stainless nuts
ASTM A276	Stainless bar stock
ASTM A479	Pressure service bar

DIN Standards

Standard	Description
DIN 931	Hex bolts
DIN 933	Hex bolts
DIN 934	Hex nuts
DIN 125	Plain washers
DIN 127	Spring washers
DIN 975	Threaded rod
DIN 976	Stud bolts

British Standards

Standard	Description
BS 3692	Metric fasteners
BS 4190	Hex bolts and nuts
BS 4320	Washers
BSW	Whitworth threads
BSF	Fine threads

27. Dimensional Specification Table

Standard Metric Bolt Dimensions

Size	Pitch	Head AF	Head Height	Tensile Stress Area (mm²)
M6	1.0	10	4.0	20.1
M8	1.25	13	5.3	36.6
M10	1.5	17	6.4	58
M12	1.75	19	7.5	84.3
M16	2.0	24	10	157
M20	2.5	30	12.5	245
M24	3.0	36	15	353
M30	3.5	46	18.7	561
M36	4.0	55	22.5	817

28. Fastener Selection Criteria

SMO 254 should be selected when one or more of the following conditions exist:

Corrosion Drivers

• High chloride exposure
• Seawater service
• Crevice corrosion risk
• Acid contamination

Mechanical Drivers

• High preload requirements
• Dynamic loading
• Long-term reliability demands

Operational Drivers

• Difficult maintenance access
• Long design life requirements
• Offshore exposure
• Critical safety systems

Regulatory Drivers

• Owner specifications
• EPC standards
• Corrosion management programs
• Asset integrity requirements

29. Interchangeability Considerations

Before replacing fasteners, engineers must verify:

• Diameter
• Pitch
• Length
• Head geometry
• Material grade
• Corrosion resistance
• Strength level
• Applicable project specification

Substituting standard stainless fasteners for SMO 254 can significantly reduce service life in aggressive chloride environments.

30. SM Fasteners Manufacturing Capability

SM Fasteners supplies SMO 254 fasteners in metric, UNC, UNF, BSW, and BSF thread forms, including standard and custom-engineered dimensions for EPC, offshore, petrochemical, desalination, LNG, power generation, and marine applications. Manufacturing is supported by ISO 9001 quality management systems, MSME registration, UKAF-certified quality processes, material traceability, and project-specific inspection requirements.

31. Material Grades and Selection Criteria

Understanding SMO 254 as a Fastener Material

SMO 254 (UNS S31254 / EN 1.4547) belongs to the family of high-alloy austenitic stainless steels specifically engineered for aggressive chloride-bearing environments.

The alloy combines:

• High Chromium content
• High Nickel content
• High Molybdenum content
• Nitrogen strengthening

to achieve exceptional resistance to:

• Pitting corrosion
• Crevice corrosion
• Chloride stress corrosion cracking
• Seawater attack
• Acidic process media

Unlike duplex and super duplex alloys, SMO 254 maintains a fully austenitic microstructure, providing:

• Excellent toughness
• High ductility
• Superior fabrication characteristics
• Excellent low-temperature performance

32. Material Selection Methodology

Material selection should never be based solely on mechanical strength.

Industrial fastener selection requires evaluation of:

Environmental Factors

• Chloride concentration
• Moisture exposure
• Immersion conditions
• Acid contamination
• H₂S exposure
• Process temperature

Mechanical Factors

• Static load
• Dynamic load
• Fatigue loading
• Impact loading
• Thermal expansion effects

Operational Factors

• Maintenance accessibility
• Inspection frequency
• Service life expectations
• Asset criticality

Selection Decision Matrix

Service Condition	Recommended Material
Fresh Water	SS316L
Marine Atmosphere	Duplex 2205
Seawater Immersion	SMO 254
Desalination Plants	SMO 254
Offshore Splash Zone	SMO 254
Chloride Process Streams	SMO 254
Sour Gas Service	Super Duplex / SMO 254 (Project Specific)
Aggressive Acid Service	Hastelloy C276
High Temperature Corrosion	Inconel 625

33. Mechanical Properties of SMO 254

Mechanical properties are governed by:

• Chemical composition
• Solution annealing
• Cold working
• Manufacturing process

Typical Mechanical Properties

Property	Typical Value
Tensile Strength (UTS)	≥ 650 MPa
Yield Strength (0.2%)	≥ 300 MPa
Elongation	≥ 35%
Reduction of Area	≥ 50%
Hardness	≤ 220 HB
Elastic Modulus	195 GPa
Density	8.0 g/cm³

Mechanical Property Comparison

Material	Yield (MPa)	UTS (MPa)	Elongation (%)
SS304	215	515	40
SS316L	220	515	40
Duplex 2205	450	620	25
Super Duplex 2507	550	800	20
SMO 254	300	650	35
Inconel 625	460	830	30

34. Mechanical Behavior Under Service Loads

Elastic Behavior

Below yield strength:$$\sigma = E \times \varepsilon$$

Where:

• σ = Stress
• E = Modulus of Elasticity
• ε = Strain

SMO 254 exhibits predictable elastic behavior suitable for critical bolted joints.

Plastic Deformation

When yield strength is exceeded:

• Permanent deformation occurs
• Clamp force decreases
• Joint reliability deteriorates

Proper design ensures service stresses remain below allowable limits.

35. Proof Load Fundamentals

Proof load represents the maximum load a fastener can sustain without permanent deformation.

Approximate Proof Loads for SMO 254 Fasteners

Size	Stress Area (mm²)	Proof Load (kN)
M8	36.6	9.5
M10	58	15
M12	84.3	22
M16	157	41
M20	245	64
M24	353	92
M30	561	146
M36	817	212

Actual values depend on manufacturing specification and project requirements.

36. Tensile Strength Table

Estimated Tensile Capacity

Size	Stress Area (mm²)	Ultimate Tensile Load (kN)
M8	36.6	23.8
M10	58	37.7
M12	84.3	54.8
M16	157	102
M20	245	159
M24	353	229
M30	561	365
M36	817	531

Based on minimum UTS of approximately 650 MPa.

37. Temperature Performance

Recommended Service Temperature Range

Condition	Temperature
Cryogenic Service	Down to -196°C
General Service	Ambient
Elevated Service	Up to 550°C
Intermittent Service	Up to 600°C

Thermal Expansion

SMO 254 exhibits thermal expansion similar to other austenitic stainless steels.

Property	Value
Coefficient of Thermal Expansion	16 × 10⁻⁶ /°C

This must be considered in:

• Flange design
• Heat exchangers
• Offshore piping systems

38. NACE and Sour Service Considerations

For oil and gas applications:

• NACE MR0175
• ISO 15156

requirements may apply.

Evaluation typically includes:

• Hardness limits
• Sulfide stress cracking resistance
• Process chemistry

Project specifications should always govern final acceptance.

39. Heat Treatment of SMO 254

Unlike carbon steel fasteners, SMO 254 derives corrosion resistance from its chemistry rather than quench-and-temper processing.

Solution Annealing

Primary heat treatment process.

Typical Temperature

$$1150^\circ C – 1200^\circ C$$

Followed by:

• Rapid water quenching
• Controlled cooling

Purpose of Solution Annealing

Removes:

• Harmful precipitates
• Sigma phase
• Intermetallic compounds

Restores:

• Corrosion resistance
• Ductility
• Toughness

40. Effects of Improper Heat Treatment

Improper thermal processing may cause:

Sigma Phase Formation

Results in:

• Reduced toughness
• Reduced corrosion resistance

Chromium Depletion

Leads to:

• Intergranular attack
• Premature corrosion

Reduced Mechanical Reliability

Potential consequences:

• Cracking
• Loss of ductility
• Reduced fatigue performance

41. Manufacturing Workflow

SM Fasteners follows a controlled manufacturing process to ensure compliance with customer and project specifications.

42. Raw Material Procurement

Manufacturing begins with certified raw material.

Typical forms include:

• Round bars
• Forging stock
• Wire rods

Material verification includes:

• Heat number confirmation
• Chemical composition review
• Mill Test Certificate verification

Material Documentation

Typical standards:

• ASTM A276
• ASTM A479
• EN 10088

Material is linked to heat numbers for complete traceability.

43. Incoming Material Inspection

Verification includes:

Chemical Analysis

Methods:

• Optical Emission Spectroscopy
• PMI Testing

Dimensional Verification

Checks:

• Diameter
• Straightness
• Surface condition

Documentation Review

Validation of:

• MTC
• Heat number
• Specification compliance

44. Forging Process

Hot Forging

Used for:

• Hex bolts
• Heavy hex bolts
• Special fasteners

Advantages:

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

Forging Sequence

1. Raw material cutting
2. Heating
3. Forging
4. Trimming
5. Cooling
6. Inspection

45. Machining Operations

Machining is used for:

• Custom fasteners
• Precision studs
• Special geometries

Processes include:

• CNC turning
• Milling
• Drilling
• Slotting

46. Thread Manufacturing

Thread quality directly influences fastener performance.

Thread Rolling

Preferred process.

Benefits:

• Compressive residual stresses
• Improved fatigue life
• Superior surface finish

Thread Cutting

Used when:

• Large diameters exist
• Custom thread forms are required
• Small production runs are involved

Comparison

Characteristic	Rolled Thread	Cut Thread
Fatigue Strength	Excellent	Good
Surface Finish	Superior	Moderate
Production Rate	High	Medium
Material Waste	Low	Higher
Cost Efficiency	Better	Lower

47. Surface Preparation

Before finishing:

• Deburring
• Cleaning
• Degreasing
• Inspection

are performed.

48. Passivation Process

Passivation is strongly recommended for stainless fasteners.

Purpose

Removes:

• Free iron contamination
• Surface impurities

Promotes:

• Stable chromium oxide layer
• Enhanced corrosion resistance

Standards

Standard	Description
ASTM A967	Passivation
ASTM A380	Cleaning & Passivation

49. Electropolishing

Advanced finishing option.

Benefits include:

• Reduced surface roughness
• Enhanced cleanliness
• Improved corrosion resistance
• Lower bacterial adhesion

Common applications:

• Pharmaceutical
• Food processing
• Semiconductor industries

50. Surface Finish Comparison

Finish Type	Corrosion Resistance	Surface Smoothness	Appearance
As Machined	Moderate	Moderate	Industrial
Pickled	Good	Moderate	Matte
Passivated	Excellent	Good	Clean Metallic
Electropolished	Outstanding	Excellent	Bright Metallic

51. Coating Considerations for SMO 254

Because SMO 254 already possesses exceptional corrosion resistance, coatings are generally not required for corrosion protection.

Optional Functional Coatings

PTFE (Xylan)

Provides:

• Reduced friction
• Galling resistance
• Easier assembly

Molybdenum Disulfide

Provides:

• Lower nut factor
• Consistent preload

Typical Coating Selection

Coating	Main Purpose
None	Maximum alloy exposure
Passivation	Corrosion enhancement
PTFE	Anti-galling
Xylan	Low friction
MoS₂	Torque control

52. Corrosion Resistance Comparison

Chloride Environment Performance

Material	Pitting Resistance	Crevice Resistance
SS304	Fair	Poor
SS316L	Good	Moderate
Duplex 2205	Very Good	Good
Super Duplex 2507	Excellent	Excellent
SMO 254	Excellent	Excellent
Hastelloy C276	Outstanding	Outstanding

53. Corrosion Resistance vs Industrial Environment

Environment	SMO 254 Performance
Marine Atmosphere	Excellent
Offshore Splash Zone	Excellent
Seawater Immersion	Excellent
Desalination Plants	Excellent
Brine Systems	Excellent
Chlorinated Water	Excellent
Phosphoric Acid	Very Good
Sulfuric Acid (Dilute)	Very Good
Caustic Service	Excellent
LNG Facilities	Excellent

54. Galvanic Corrosion Considerations

When joining dissimilar metals:

Potential concerns include:

• Accelerated attack
• Localized corrosion
• Reduced service life

Recommended pairing materials:

• SMO 254
• Duplex 2205
• Super Duplex 2507
• High-alloy stainless steels

Insulating washers may be required where dissimilar metals cannot be avoided.

55. PEEK Fasteners and Hybrid Assemblies

SM Fasteners also manufactures advanced PEEK fastening solutions for specialized applications.

PEEK fasteners may be used where:

• Electrical insulation is required
• Weight reduction is critical
• Chemical resistance is required
• Metal-free assemblies are necessary

Hybrid systems may combine:

• SMO 254 structural fasteners
• PEEK insulating components

for highly specialized engineering applications.

56. Material Comparison Table

Material	UTS (MPa)	Yield (MPa)	Corrosion Resistance	Relative Cost	Typical Application
SS304	515	215	Moderate	Low	General Industry
SS316L	515	220	Good	Low-Medium	Marine & Chemical
Duplex 2205	620	450	Very Good	Medium	Offshore
Super Duplex 2507	800	550	Excellent	High	Seawater Systems
SMO 254	650	300	Excellent	High	Desalination & Offshore
Inconel 625	830	460	Outstanding	Very High	High Temperature
Hastelloy C276	690	280	Outstanding	Very High	Aggressive Chemicals

SM Fasteners Manufacturing Capability

SM Fasteners manufactures SMO 254 fasteners from fully traceable raw materials under ISO 9001 quality systems with controlled forging, machining, thread rolling, passivation, inspection, and documentation processes. Production capabilities include bolts, nuts, washers, threaded rods, custom-engineered fastening systems, and corrosion-resistant assemblies for EPC, offshore, marine, petrochemical, desalination, LNG, and power generation projects requiring high-performance stainless steel fastening solutions

57. Inspection and Quality Control Philosophy

For critical fastening systems used in offshore, petrochemical, LNG, power generation, and structural applications, quality assurance extends far beyond dimensional compliance.

The objective of quality control is to ensure that every fastener satisfies:

• Material specification requirements
• Mechanical property requirements
• Dimensional tolerances
• Traceability requirements
• Corrosion resistance expectations
• Customer and EPC specifications

SM Fasteners integrates inspection controls throughout the manufacturing cycle under ISO 9001 quality management systems, ensuring consistent compliance with project specifications and international standards.

58. Quality Control Stages

Stage 1 – Raw Material Verification

Verification includes:

Material Test Certificate Review

Checks include:

• Heat number
• Chemical composition
• Mechanical properties
• Applicable standards

Positive Material Identification (PMI)

Verification of alloy chemistry before manufacturing.

Typical PMI methods:

• XRF Analysis
• Optical Emission Spectroscopy (OES)

Stage 2 – In-Process Inspection

Performed during:

• Cutting
• Forging
• Machining
• Thread rolling
• Heat treatment
• Surface finishing

Controls include:

• Dimensional verification
• Thread inspection
• Surface quality assessment

Stage 3 – Final Inspection

Verification before shipment:

• Dimensions
• Thread accuracy
• Mechanical properties
• Surface condition
• Marking compliance
• Documentation review

59. Dimensional Inspection Requirements

Critical Dimensions

Inspection typically includes:

Feature	Inspection Requirement
Diameter	Calibrated measurement
Length	Vernier / Digital measurement
Head Dimensions	GO/NO-GO verification
Thread Pitch	Thread gauge verification
Across Flats	Dimensional check
Concentricity	Project-specific requirement
Straightness	Visual and dimensional

Thread Inspection

Thread integrity directly affects joint reliability.

Inspection methods:

• Thread ring gauges
• Plug gauges
• Optical profile measurement
• Coordinate Measuring Machines (CMM)

60. Mechanical Testing

Mechanical testing validates performance under load.

Tensile Testing

Conducted according to:

• ASTM standards
• ISO standards
• Customer specifications

Determines:

• Yield strength
• Ultimate tensile strength
• Elongation

Hardness Testing

Methods include:

Method	Standard
Brinell (HB)	ASTM E10
Rockwell (HRB/HRC)	ASTM E18
Vickers (HV)	ASTM E92

Proof Load Testing

Used to verify:

• Elastic behavior
• Permanent deformation resistance
• Compliance with specification

61. Positive Material Identification (PMI)

PMI is frequently mandatory in:

• Oil & Gas
• Offshore
• LNG
• Refinery projects

Purpose

Ensures:

• Correct alloy supplied
• Material substitution prevention
• Full traceability

Typical Elements Verified

Element	Importance
Cr	Corrosion resistance
Ni	Austenitic structure
Mo	Pitting resistance
N	Strength and PREN
Cu	Acid resistance

62. Non-Destructive Testing (NDT)

NDT verifies integrity without damaging the fastener.

Liquid Penetrant Testing (PT)

Used to detect:

• Surface cracks
• Forging defects
• Machining defects

Applicable Standards:

• ASTM E165
• ASME Section V

Magnetic Particle Inspection (MPI)

Generally not applicable to fully austenitic SMO 254 due to non-magnetic structure.

Ultrasonic Testing (UT)

Used for:

• Large diameter studs
• Critical bolting
• Custom forged components

Detects:

• Internal discontinuities
• Voids
• Inclusions

63. Corrosion Testing

Project specifications may require:

ASTM G48 Testing

Evaluates:

• Pitting resistance
• Crevice corrosion resistance

Salt Spray Testing

Used for coated components where specified.

64. Traceability System

Traceability is critical for EPC projects.

Typical traceability includes:

• Heat number
• Manufacturing batch
• Inspection lot
• Production records

Each batch can be linked back to:

• Raw material source
• Manufacturing records
• Inspection reports

65. Fastener Marking Requirements

Marking typically includes:

• Manufacturer identification
• Material grade
• Heat number (where required)
• Project identification (special orders)

Marking must remain legible throughout service life.

66. Mechanical Properties Table

SMO 254 Fasteners

Property	Typical Value
Yield Strength	≥ 300 MPa
Tensile Strength	≥ 650 MPa
Elongation	≥ 35%
Hardness	≤ 220 HB
Density	8.0 g/cm³
Elastic Modulus	195 GPa

67. Proof Load and Tensile Strength Table

Size	Stress Area (mm²)	Proof Load (kN)	Ultimate Load (kN)
M8	36.6	9.5	23.8
M10	58	15	37.7
M12	84.3	22	54.8
M16	157	41	102
M20	245	64	159
M24	353	92	229
M30	561	146	365
M36	817	212	531

68. Tightening Torque Fundamentals

Torque is applied to create preload.

Relationship:$$T = K \times F \times D$$

Where:

Symbol	Meaning
T	Torque
K	Nut Factor
F	Preload
D	Diameter

69. Recommended Tightening Torque Chart

Lubricated SMO 254 Fasteners

Size	Preload (kN)	Torque (N·m)
M8	14	22
M10	22	44
M12	32	77
M16	60	190
M20	95	380
M24	137	660
M30	218	1370
M36	318	2290

Actual tightening values should always be verified against project specifications, lubrication condition, and gasket requirements.

70. Torque Comparison by Lubrication Condition

Condition	Nut Factor
Dry	0.22–0.25
Light Oil	0.18–0.22
Moly Lubricant	0.12–0.18
PTFE Coated	0.10–0.15
Xylan Coated	0.10–0.15

71. Preload Calculation Example

Formula

$$F = \frac{T}{K \times D}$$

Example

M20 SMO 254 Bolt

Given:

• Torque = 380 N·m
• K = 0.20
• Diameter = 0.020 m

Calculation:$$F = \frac{380}{0.004}$$

$$F = \frac{380}{0.20 \times 0.020}$$

$$F = 95,000 N$$

$$F = 95 kN$$

Result:

Preload generated ≈ 95 kN

72. Joint Design Example

Flange Connection

Given:

• Internal pressure = 20 bar
• Gasket seating force = 400 kN
• Number of bolts = 8

Required preload per bolt:$$\frac{400}{8}$$

$$= 50 kN$$

Applying design factor:$$50 \times 1.5$$

Recommended preload per bolt:$$75 kN$$

This methodology is commonly used for flange joint calculations.

73. Weight Chart for SMO 254 Hex Bolts

Approximate Unit Weight

Size × 100 mm	Weight/Piece (kg)	Weight/100 pcs (kg)
M8	0.04	4
M10	0.07	7
M12	0.11	11
M16	0.22	22
M20	0.39	39
M24	0.62	62
M30	1.10	110
M36	1.82	182

Actual values vary based on thread length, head style, and customer drawings.

SM Fasteners can provide project-specific weight charts aligned with manufacturing dimensions and procurement schedules.

74. Surface Finish Performance Comparison

Finish	Corrosion Resistance	Galling Resistance	Appearance
As Machined	Moderate	Moderate	Industrial
Pickled	Good	Moderate	Matte
Passivated	Excellent	Good	Clean Metallic
Electropolished	Outstanding	Good	Bright
PTFE Coated	Excellent	Excellent	Colored Finish
Xylan Coated	Excellent	Excellent	Colored Finish

75. Industry Applications

Construction & Structural Steel

Applications include:

• Coastal structures
• Marine bridges
• Corrosion-resistant steelwork
• Infrastructure exposed to salt environments

Benefits:

• Long service life
• Reduced maintenance
• Improved reliability

Oil & Gas Industry

Upstream

Used in:

• Offshore platforms
• Wellhead equipment
• Seawater injection systems

Midstream

Applications:

• Pipeline systems
• Pump stations
• Compressor facilities

Downstream

Applications:

• Refineries
• Chemical plants
• Process piping

Petrochemical Industry

Common equipment:

• Reactors
• Heat exchangers
• Pressure vessels
• Flanged piping

SMO 254 provides excellent resistance to chloride contamination commonly encountered in process plants.

LNG Facilities

Used in:

• Cryogenic systems
• Storage terminals
• Regasification plants

Advantages:

• Excellent toughness
• Corrosion resistance
• Long-term reliability

Power Generation

Applications include:

• Cooling water systems
• Desalination units
• Emission control systems
• Coastal power plants

Marine and Shipbuilding

Applications:

• Deck equipment
• Seawater systems
• Hull structures
• Offshore vessels

Railways and Infrastructure

Used in:

• Coastal railway systems
• Bridges
• Corrosion-sensitive structures

Desalination Plants

One of the most important application sectors.

SMO 254 fasteners are widely used in:

• Reverse osmosis systems
• Brine handling systems
• Seawater intake facilities

76. PEEK Fastener Applications

SM Fasteners manufactures advanced PEEK fastening solutions for specialized industries.

PEEK fasteners are selected where:

• Electrical insulation is required
• Metal contamination must be avoided
• Weight reduction is important
• Chemical resistance is critical

Applications include:

• Semiconductor manufacturing
• Medical equipment
• Electronics
• Chemical processing systems

Hybrid assemblies often combine:

• SMO 254 structural fasteners
• PEEK insulating components

to achieve both mechanical strength and electrical isolation.

77. Packaging and Preservation

Proper packaging prevents:

• Corrosion
• Mechanical damage
• Thread damage
• Contamination

Standard Packaging

Includes:

• Polybag packing
• Box packing
• Carton packaging
• Palletized shipments

VCI Protection

VCI (Volatile Corrosion Inhibitor) packaging provides:

• Corrosion prevention
• Moisture protection
• Long transit durability

Thread Protection

Methods include:

• Plastic caps
• Thread sleeves
• Protective separators

Especially important for:

• Stud bolts
• Threaded rods
• Precision machined fasteners

78. Export Crating

For international shipments:

ISPM-15 Compliant Wooden Crates

Features:

• Fumigated timber
• Export markings
• Heavy-duty construction

Suitable for:

• Offshore projects
• EPC shipments
• Long-distance transportation

79. Export Documentation

SM Fasteners supports project documentation requirements for global industrial procurement.

Material Test Certificate (MTC)

Contains:

• Heat number
• Chemical composition
• Mechanical properties

EN 10204 3.1 Certification

Includes:

• Manufacturer verification
• Material traceability
• Test results

EN 10204 3.2 Certification

May include:

• Third-party witness inspection
• Independent verification

Inspection Reports

Can include:

• Dimensional reports
• PMI reports
• Mechanical test reports
• NDT reports

Heat Treatment Reports

Where applicable:

• Furnace records
• Temperature records
• Batch traceability

Certificate of Conformity (CoC)

Confirms:

• Compliance with purchase order
• Applicable specifications
• Inspection completion

80. Global Supply Capability

SM Fasteners supports industrial projects requiring precision-manufactured SMO 254 fasteners for critical service environments.

Manufacturing capabilities include:

• Hex bolts
• Heavy hex bolts
• Stud bolts
• Threaded rods
• Hex nuts
• Heavy hex nuts
• Washers
• Custom-engineered fasteners

Supported thread standards:

• Metric ISO
• UNC
• UNF
• BSW
• BSF

Available documentation:

• MTC
• EN 10204 3.1
• EN 10204 3.2
• PMI Reports
• Mechanical Test Reports
• Dimensional Reports
• CoC

Quality systems are supported by:

• ISO 9001 Certification
• MSME Registration
• UKAF-Certified Quality Frameworks

The combination of advanced material expertise, controlled manufacturing processes, inspection traceability, corrosion-resistant alloy capability, and custom engineering support enables SM Fasteners to supply SMO 254 fastening solutions suitable for offshore, desalination, petrochemical, LNG, marine, power generation, infrastructure, and EPC projects worldwide.

Engineering Summary

SMO 254 (UNS S31254 / EN 1.4547) fasteners represent one of the highest-performing stainless steel fastening solutions available for chloride-rich and seawater environments. The alloy’s high chromium, nickel, molybdenum, and nitrogen content delivers exceptional resistance to pitting, crevice corrosion, and chloride stress corrosion cracking while maintaining excellent toughness and fabrication characteristics. When manufactured under controlled ISO 9001 quality systems with full traceability, PMI verification, dimensional inspection, and project-specific documentation, SMO 254 fasteners provide a reliable solution for critical bolted joints in offshore, marine, desalination, petrochemical, LNG, power generation, and infrastructure applications where long-term corrosion resistance and asset integrity are essential.