Austenitic Grades 304 316L

1. Introduction to Austenitic Stainless Steel Fasteners

Austenitic stainless steels represent the most widely specified corrosion-resistant fastener materials in global industrial engineering applications. Their unique metallurgical structure provides an optimized combination of:

• Corrosion resistance
• Toughness
• Ductility
• Cryogenic performance
• Elevated temperature stability
• Fabrication versatility

For critical bolting systems used in:

• Oil & Gas
• Petrochemical Processing
• LNG Facilities
• Offshore Structures
• Chemical Plants
• Power Generation
• Marine Installations
• Infrastructure Projects

Austenitic grades are frequently selected where carbon steel or alloy steel fasteners would be susceptible to corrosion-induced degradation.

SM Fasteners manufactures precision-engineered stainless steel fasteners in a comprehensive range of Austenitic grades including:

Grade	UNS Number	EN Number
304	S30400	1.4301
316	S31600	1.4401
316L	S31603	1.4404
317L	S31703	1.4438
310S	S31008	1.4845
321	S32100	1.4541

These materials are supplied in accordance with international standards and quality systems maintained under ISO 9001-certified manufacturing procedures.

2. Industrial Importance of Austenitic Stainless Fasteners

Fasteners often represent less than 1% of total project value but can determine the reliability and service life of entire systems.

Failures associated with improper fastener selection commonly result from:

• Galvanic corrosion
• Pitting attack
• Crevice corrosion
• Chloride stress corrosion cracking
• Thread galling
• Fatigue fracture
• Improper preload

Because of these risks, EPC contractors and engineering organizations frequently specify Austenitic stainless fasteners for:

Corrosion Control

Protection against:

• Atmospheric exposure
• Salt-laden environments
• Chemical processing atmospheres
• Offshore splash zones

Asset Reliability

Reduced maintenance intervals and improved lifecycle cost.

Regulatory Compliance

Used where:

• NACE MR0175
• ISO 15156
• ASME
• API
• ASTM

requirements influence material selection.

3. Metallurgical Definition of Austenitic Stainless Steel

Austenitic stainless steels derive their characteristics from a face-centered cubic (FCC) crystal structure stabilized by nickel additions.

The microstructure remains predominantly austenitic at room temperature.

Typical composition ranges include:

Element	Function
Chromium	Corrosion resistance
Nickel	Austenite stabilization
Molybdenum	Pitting resistance
Carbon	Strength
Nitrogen	Strength & corrosion resistance
Titanium	Carbide stabilization
Manganese	Deoxidation and toughness

4. Fundamental Characteristics of Austenitic Fasteners

Non-Magnetic Behavior

Most austenitic grades are non-magnetic in annealed condition.

Advantages:

• Instrumentation systems
• Medical equipment
• Electronic assemblies

Excellent Toughness

Retains impact strength at:

• Cryogenic temperatures
• Sub-zero service conditions

Common in LNG facilities operating below −160°C.

Superior Corrosion Resistance

Passive chromium oxide film provides self-healing protection.

Corrosion resistance increases with:

• Chromium content
• Nickel content
• Molybdenum additions

High Ductility

Allows:

• Cold heading
• Thread rolling
• Deep forming

without cracking.

Elevated Temperature Capability

Certain grades such as:

• 310S
• 321

are engineered for high-temperature service.

5. Overview of Individual Austenitic Grades

Grade 304

The most widely used stainless fastener material.

Composition:

• ~18% Chromium
• ~8% Nickel

Common applications:

• Construction
• Architectural systems
• Water treatment
• Food processing

Advantages:

• Economical
• Readily available
• Excellent fabrication

Limitations:

• Moderate chloride resistance

Grade 316

Contains molybdenum.

Typical composition:

• 16–18% Chromium
• 10–14% Nickel
• 2–3% Molybdenum

Benefits:

• Improved pitting resistance
• Better seawater performance
• Enhanced chemical resistance

Applications:

• Offshore facilities
• Marine equipment
• Chemical plants

Grade 316L

Low carbon version of 316.

Carbon:

≤0.03%

Advantages:

• Reduced sensitization
• Improved weldability
• Superior resistance to intergranular corrosion

Widely specified for:

• Pressure equipment
• LNG systems
• Pharmaceutical facilities

Grade 317L

Higher molybdenum content than 316L.

Advantages:

• Improved chloride resistance
• Better crevice corrosion resistance

Applications:

• Chemical processing
• Desalination
• Pollution control systems

Grade 310S

High chromium and nickel grade.

Characteristics:

• Outstanding oxidation resistance
• Excellent heat resistance

Used in:

• Furnaces
• Heat exchangers
• Thermal processing equipment

Grade 321

Titanium stabilized stainless steel.

Benefits:

• Prevents chromium carbide precipitation
• Improved high-temperature reliability

Applications:

• Refineries
• Exhaust systems
• Thermal equipment

6. Functional Role of Fasteners in Industrial Assemblies

Fasteners do not primarily resist external load through shear.

Instead, properly engineered bolted joints function by:

Clamping Components Together

The bolt acts as a spring.

Tension generated during tightening creates:

• Preload
• Compression between joined parts

The resulting friction transfers operational loads.

Typical Fastener Functions

Structural Connections

Examples:

• Steel buildings
• Pipe racks
• Bridges

Pressure Containment

Examples:

• Flanges
• Heat exchangers
• Pressure vessels

Rotating Equipment

Examples:

• Pumps
• Compressors
• Turbines

Dynamic Systems

Examples:

• Rail equipment
• Marine machinery
• Heavy industrial equipment

7. Load Mechanics of Austenitic Fasteners

Understanding fastener mechanics is essential for safe design.

Tensile Loading

The most common loading mode.

External force acts parallel to bolt axis.

Example:

Flange bolting.

Tensile Stress Formula

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

Where:

• σ = tensile stress
• F = applied load
• A = stress area

Shear Loading

Force acts perpendicular to bolt axis.

Common in:

• Structural joints
• Machinery bases

Shear capacity depends upon:

• Material strength
• Bolt diameter
• Number of shear planes

Combined Loading

Many industrial joints experience:

• Tension
• Shear
• Bending

simultaneously.

Examples:

• Offshore equipment
• Crane systems
• Rotating machinery

Fatigue Loading

Critical for:

• Vibrating systems
• Pumps
• Compressors
• Engines

Repeated cyclic stress can initiate microscopic cracks.

Fatigue failures often occur below yield strength.

Thermal Loading

Thermal expansion differences create additional stresses.

Common in:

• Refineries
• Boilers
• Heat exchangers

Grades 310S and 321 are frequently selected where thermal cycling is significant.

8. Fastener Preload Fundamentals

Preload is the most important parameter in bolted joint performance.

When tightened, the bolt elongates elastically.

This elongation generates clamping force.

Simplified Preload Relationship

$F_p = k\Delta L$

Where:

• Fp = preload
• k = bolt stiffness
• ΔL = elongation

Why Preload Matters

Adequate preload:

• Prevents loosening
• Prevents gasket leakage
• Improves fatigue resistance
• Reduces joint movement

Insufficient preload causes:

• Vibration loosening
• Leakage
• Fretting damage

Excessive preload causes:

• Yielding
• Thread stripping
• Bolt fracture

9. Torque-Tension Relationship

Applied torque is converted into preload.

However:

Only about 10–15% of tightening torque actually creates bolt tension.

The remainder is consumed by friction.

Approximate distribution:

Energy Consumption	Percentage
Thread Friction	40%
Bearing Friction	45%
Useful Bolt Tension	15%

Therefore lubrication condition significantly affects preload accuracy.

Basic Torque Equation

$T = KFD$T=KFD

Where:

• T = tightening torque
• K = nut factor
• F = preload
• D = nominal diameter

This equation forms the basis for torque chart development.

10. Joint Stiffness Principles

A bolted joint behaves as two springs:

Bolt Spring

Elastic elongation.

Joint Spring

Compression of connected materials.

Performance depends on stiffness ratio.

A stiffer joint generally:

• Maintains preload better
• Improves fatigue life

11. Thread Engagement Principles

Adequate thread engagement prevents stripping.

General engineering guidelines:

Material	Minimum Engagement
Stainless Steel	1.0 × Diameter
Aluminum	1.5 × Diameter
Cast Iron	1.5 × Diameter
Soft Alloys	2.0 × Diameter

Example:

M20 bolt:

Minimum stainless engagement ≈ 20 mm.

12. Friction Effects in Stainless Fasteners

Austenitic stainless steels exhibit higher galling tendency than carbon steels.

Galling occurs when:

• Thread surfaces weld microscopically
• Metal transfer occurs
• Seizure develops during tightening

Risk increases with:

• High tightening speed
• Dry assembly
• Fine threads
• Elevated temperature

Mitigation methods:

• Molybdenum disulfide lubricants
• PTFE coatings
• Silver plating
• Controlled installation procedures

13. Load Transfer Mechanisms

Industrial joints transfer load through:

Friction Grip

Preferred method.

Load transferred through clamping force.

Advantages:

• Reduced bolt stress
• Improved fatigue life

Bearing Type Connection

Load transferred through:

• Bolt shank
• Hole contact

More common in structural applications.

14. Failure Mechanisms in Austenitic Fasteners

Understanding failure modes is essential during material selection.

Tensile Failure

Occurs when applied load exceeds ultimate tensile strength.

Indicators:

• Necking
• Ductile fracture

Thread Stripping

Occurs when:

• Engagement insufficient
• Material too soft

Common in improperly designed tapped joints.

Fatigue Failure

Initiated by cyclic loading.

Typical crack origins:

• Thread roots
• Surface defects
• Stress concentrations

Stress Corrosion Cracking (SCC)

Austenitic stainless steels may experience SCC in:

• Chloride environments
• Elevated temperatures

Grade selection becomes critical.

Pitting Corrosion

Localized attack caused by chlorides.

Resistance ranking:

317L > 316L > 316 > 304

Crevice Corrosion

Occurs in stagnant fluid zones:

• Under washers
• Flange interfaces
• Gasket regions

Galling

Particularly important for stainless steel fasteners.

Prevention must be addressed during design and installation.

15. Joint Design Principles for EPC Projects

For critical industrial installations, bolted joints should be designed around:

Load Requirement

Determine:

• Static loads
• Dynamic loads
• Shock loads

Environmental Conditions

Evaluate:

• Chlorides
• Acids
• H₂S
• Moisture
• Temperature

Corrosion Risk

Select grade according to exposure severity.

Thermal Conditions

Consider:

• Expansion mismatch
• Thermal cycling
• Creep resistance

Inspection Accessibility

Allow:

• Torque verification
• Visual inspection
• Maintenance access

16. Engineering Selection Overview

Requirement	Preferred Grade
General Industrial Service	304
Marine Atmosphere	316
Offshore Structures	316L
Chloride Chemical Service	317L
High Temperature Service	310S
Thermal Cycling Equipment	321

SM Fasteners Engineering Capability

SM Fasteners manufactures precision stainless steel fasteners in Austenitic grades 304, 316, 316L, 317L, 310S, and 321 for EPC, OEM, infrastructure, petrochemical, offshore, and heavy engineering applications. Production is controlled through ISO 9001-certified quality systems with complete material traceability, inspection documentation, and global project supply capability. The company also supports custom-engineered fastening solutions, advanced alloy materials, and high-performance PEEK fasteners for specialized industrial environments.

17. Product Types and Variants

Industrial fasteners manufactured from austenitic stainless steels are available in numerous configurations designed to satisfy specific load paths, installation constraints, maintenance requirements, and environmental conditions.

For EPC, petrochemical, offshore, power generation, and infrastructure projects, fastener selection extends beyond material grade and must consider:

• Joint geometry
• Load direction
• Accessibility
• Vibration conditions
• Corrosion exposure
• Assembly sequence
• Inspection requirements

SM Fasteners manufactures standard and custom-engineered fastening systems in accordance with international specifications and project-specific requirements.

18. Bolts

Bolts are externally threaded fasteners intended for use with mating nuts.

They are the most common fastening element used in:

• Structural steelwork
• Pipe supports
• Pressure equipment
• Heavy machinery
• Marine assemblies

Hex Head Bolts

Most widely specified industrial bolt configuration.

Characteristics:

• Six-sided head
• High torque transmission
• Easy field installation
• Compatible with standard tooling

Applicable Standards:

Standard	Description
ISO 4014	Partially threaded hex bolt
ISO 4017	Fully threaded hex bolt
DIN 931	Partial thread
DIN 933	Full thread
BS EN ISO 4014	Structural bolting
ASTM F593	Stainless steel bolts

Applications:

• Flanges
• Structural steel
• Pipe supports
• Equipment mounting

Heavy Hex Bolts

Larger bearing surface than standard hex bolts.

Advantages:

• Improved load distribution
• Better performance under high preload
• Reduced bearing stress

Standards:

Standard	Description
ASTM A193	High-temperature bolting
ASTM F593	Stainless heavy hex
ASME B18.2.1	Heavy hex dimensions

Applications:

• Pressure vessels
• Refineries
• Heat exchangers

Flange Bolts

Incorporate integrated washer face.

Advantages:

• Larger bearing area
• Reduced assembly components
• Improved load distribution

Applications:

• Automotive systems
• Mechanical equipment
• Pumps

Socket Head Cap Screws

Internal hex drive design.

Standards:

Standard	Description
ISO 4762
DIN 912
BS EN ISO 4762

Applications:

• Precision equipment
• Machinery
• OEM assemblies

Advantages:

• Compact design
• High preload capability
• Improved appearance

19. Stud Bolt

Stud bolts are threaded at both ends or continuously threaded along the entire length.

They are extensively used in pressure-retaining equipment.

Standards:

Standard	Description
ASTM A193
ASTM A320
ASTM F593
DIN 975
DIN 976

Applications:

• ASME flanges
• Heat exchangers
• Pressure vessels
• Offshore equipment

Advantages:

• Uniform preload distribution
• Easier flange alignment
• Better maintenance access

Fully Threaded Studs

Used where:

• Maximum adjustability required
• Variable grip lengths occur

Double-End Studs

Typically used in:

• Equipment housings
• Pumps
• Turbines

Tap-End Studs

One end installed permanently.

Applications:

• Cast housings
• Equipment frames

20. Nuts

Nuts provide the mating internal thread for externally threaded fasteners.

Hex Nut

Most common fastening nut.

Standards:

Standard	Description
ISO 4032
DIN 934
BS EN ISO 4032

Applications:

• General industrial assemblies

Heavy Hex Nuts

Designed for:

• Structural joints
• Pressure vessel bolting

Standards:

Standard	Description
ASTM A194
ASTM F594

Lock Nuts

Designed to resist loosening under vibration.

Types:

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

Applications:

• Railways
• Rotating equipment
• Heavy machinery

Slotted Nuts

Used with cotter pins.

Applications:

• Critical safety assemblies

21. Washers

Washers distribute preload and protect mating surfaces.

Plain Washers

Standards:

Standard	Description
ISO 7089
DIN 125
BS 4320

Functions:

• Load distribution
• Surface protection

Heavy Duty Washers

Used in:

• Structural steel
• High-preload joints

Spring Washers

Designed to provide limited resistance against loosening.

Standards:

DIN 127

Belleville Washers

Disc spring design.

Applications:

• Thermal cycling
• Dynamic loading

Advantages:

• Maintains preload

22. Threaded Rods

Threaded rods provide continuous external threads.

Standards:

Standard	Description
DIN 975
DIN 976
ASTM F593

Applications:

• Pipe supports
• Anchor systems
• HVAC supports

23. Screws

Unlike bolts, screws may engage directly with tapped holes.

Common variants:

• Machine screws
• Socket screws
• Set screws
• Self-tapping screws

Standards:

Standard	Description
ISO 7045
ISO 14579
DIN 7985
DIN 963

24. Anchor Fasteners

Used for concrete and structural attachments.

Common configurations:

• Chemical anchors
• Expansion anchors
• Stud anchors

Applications:

• Structural supports
• Equipment foundations

25. Rings and Special Components

SM Fasteners also manufactures:

• Retaining rings
• Lock rings
• Custom-machined components
• Special aerospace configurations
• High-temperature alloy fasteners
• PEEK fastening systems

26. Dimensional Logic of Fastener Design

Fastener dimensions directly influence:

• Tensile capacity
• Shear capacity
• Fatigue life
• Assembly clearance
• Tool access

Critical dimensions include:

• Diameter
• Pitch
• Thread length
• Head size
• Bearing surface

27. Nominal Diameter

Nominal diameter represents thread major diameter.

Examples:

Designation	Diameter
M6	6 mm
M8	8 mm
M10	10 mm
M12	12 mm
M16	16 mm
M20	20 mm
M24	24 mm
M30	30 mm

Increasing diameter significantly increases tensile stress area.

28. Thread Pitch

Pitch equals distance between adjacent thread crests.

Metric designation:

M20 × 2.5

Where:

• 20 = diameter
• 2.5 = pitch

Coarse Thread Advantages

• Faster installation
• Better contamination resistance
• Improved field assembly

Fine Thread Advantages

• Higher preload precision
• Better vibration resistance
• Larger tensile stress area

29. Thread Length Logic

Thread length influences:

• Load transfer
• Nut engagement
• Fatigue performance

Excessive thread exposure can increase corrosion risk.

Insufficient engagement may result in thread stripping.

30. Head Geometry Considerations

Head dimensions affect:

• Wrench access
• Bearing stress
• Torque transfer

Heavy hex heads are preferred for:

• Pressure equipment
• Structural applications

31. Standard Metric Hex Bolt Dimensions

ISO 4014 / DIN 931

Size	Pitch (mm)	Across Flats (mm)	Head Height (mm)
M6	1.0	10	4
M8	1.25	13	5.3
M10	1.5	17	6.4
M12	1.75	19	7.5
M16	2.0	24	10
M20	2.5	30	12.5
M24	3.0	36	15
M30	3.5	46	18.7

32. Standard Length Range

Industrial production capability generally covers:

Diameter	Standard Length Range
M6	10–150 mm
M8	15–200 mm
M10	20–300 mm
M12	25–400 mm
M16	30–500 mm
M20	40–600 mm
M24	50–700 mm
M30	60–1000 mm

Custom dimensions are frequently manufactured for EPC projects.

33. Thread Standards Used Globally

Several thread systems remain active worldwide.

ISO Metric Threads

Most common internationally.

Standard:

ISO 261

Designation:

M20 × 2.5

Flank angle:

60°

Unified Threads (UNC/UNF)

Used primarily in:

• North America
• Oil & Gas equipment

Standards:

ASME B1.1

Flank angle:

60°

ननमंुBSW Threads

British Standard Whitworth.

Flank angle:

55°

BSF Threads

British Standard Fine.

Common in legacy equipment.

34. Thread Standard Comparison

Thread Type	Angle	Region
Metric	60°	Global
UNC	60°	North America
UNF	60°	North America
BSW	55°	Legacy UK
BSF	55°	Legacy UK

35. Thread Tolerance Classes

Thread tolerances influence:

• Assembly fit
• Load distribution
• Interchangeability

Metric External Threads

Common classes:

Class	Application
6g	Standard
4g6g	Precision
8g	Loose fit

Metric Internal Threads

Class	Application
6H	Standard
5H	Precision
7H	General industrial

36. Thread Tolerance Table

External	Internal	Fit Type
6g	6H	Standard
4g6g	5H	Precision
8g	7H	Loose

37. Applicable ASTM Standards

The ASTM system dominates Oil & Gas and EPC projects.

Material Standards

Standard	Scope
ASTM F593	Stainless bolts
ASTM F594	Stainless nuts
ASTM A193	High-temperature bolting
ASTM A320	Low-temperature bolting
ASTM A194	Nuts for pressure service

38. Applicable ISO Standards

Standard	Description
ISO 3506	Stainless fasteners
ISO 4014	Hex bolts
ISO 4017	Fully threaded bolts
ISO 4032	Hex nuts
ISO 7089	Washers
ISO 4762	Socket screws

39. Applicable DIN Standards

Standard	Description
DIN 931	Hex bolts partial thread
DIN 933	Hex bolts full thread
DIN 934	Hex nuts
DIN 125	Washers
DIN 912	Socket head screws
DIN 975	Threaded rods

40. Applicable British Standards

Standard	Description
BS 3692	Fastener dimensions
BS 4320	Washers
BS 4190	Hex bolts and nuts
BS EN ISO Series	Harmonized ISO standards

41. ISO 3506 Stainless Fastener Property Classes

Property classes designate mechanical performance.

Common classes:

Class	Material
A2-50	304
A2-70	304
A2-80	Cold worked 304
A4-70	316
A4-80	316

Where:

• A2 = 304 family
• A4 = 316 family

42. Interchangeability Considerations

Fasteners from different standards may appear similar but possess dimensional differences.

Critical checks:

• Head dimensions
• Thread pitch
• Tolerance class
• Bearing surface
• Nut height

Improper substitution can result in:

• Reduced preload
• Fitment issues
• Inspection non-conformance

43. Engineering Selection Matrix

Requirement	Preferred Product
Structural Steel	Hex Bolt
Pressure Vessel	Stud Bolt
Rotating Equipment	Socket Head Screw
Offshore Flanges	Stud Bolt + Heavy Hex Nut
Pipe Supports	Threaded Rod
Vibration Service	Lock Nut Assembly
Thermal Cycling	Belleville Washer System

44. SM Fasteners Manufacturing Scope

SM Fasteners manufactures:

• Hex Bolts
• Heavy Hex Bolts
• Stud Bolts
• Hex Nuts
• Heavy Hex Nuts
• Threaded Rods
• Socket Head Cap Screws
• Washers
• Retaining Rings
• Special Fasteners
• Custom Engineered Components
• PEEK Fasteners

Production is aligned with ISO, ASTM, DIN, BS, and project-specific EPC requirements, supported by ISO 9001-certified quality systems, full traceability, and inspection-controlled manufacturing processes.

45. Materials Engineering of Austenitic Stainless Steel Fasteners

Material selection is one of the most critical decisions in bolted joint design. The chosen fastener material must withstand:

• Mechanical loading
• Environmental exposure
• Operating temperature
• Corrosion mechanisms
• Inspection requirements
• Project life-cycle expectations

For EPC, offshore, petrochemical, LNG, refinery, and power generation projects, material selection is frequently governed by:

• ASTM specifications
• ISO 3506
• ASME design codes
• NACE MR0175 / ISO 15156
• Client-approved vendor lists
• Project-specific engineering standards

SM Fasteners manufactures precision fasteners in multiple Austenitic stainless steel grades to satisfy these requirements while maintaining traceability through Mill Test Certificates (MTCs) and ISO 9001 quality systems.

46. Chemical Composition Comparison

Typical Chemical Composition (%)

Grade	Cr	Ni	Mo	C Max	Ti	Key Characteristic
304	18–20	8–10.5	—	0.08	—	General corrosion resistance
316	16–18	10–14	2–3	0.08	—	Marine & chloride resistance
316L	16–18	10–14	2–3	0.03	—	Welded equipment
317L	18–20	11–15	3–4	0.03	—	Superior pitting resistance
310S	24–26	19–22	—	0.08	—	High-temperature service
321	17–19	9–12	—	0.08	Stabilized	Thermal cycling service

47. Mechanical Properties Comparison

Typical Mechanical Properties

Grade	UTS (MPa)	Yield Strength (MPa)	Elongation %	Hardness HB
304	515–620	205	40	201
316	515–620	205	40	217
316L	485–620	170–205	40	217
317L	515–620	205	40	220
310S	520–650	205	40	220
321	515–620	205	40	217

48. ISO 3506 Property Class Comparison

Mechanical Classification

Property Class	Minimum Tensile Strength (MPa)
A2-50	500
A2-70	700
A2-80	800
A4-50	500
A4-70	700
A4-80	800

Where:

• A2 = 304 family
• A4 = 316 family

Most EPC specifications commonly require:

• A2-70
• A4-70
• A4-80

49. Corrosion Resistance Mechanism

Corrosion resistance is derived from a passive chromium oxide film that forms naturally on the stainless steel surface.

This passive layer:

• Self-repairs in oxygen-rich environments
• Prevents general corrosion
• Reduces oxidation

Resistance increases with:

• Chromium content
• Nickel content
• Molybdenum content
• Nitrogen content

50. Corrosion Resistance Comparison

Relative Corrosion Performance

Environment	304	316	316L	317L	310S	321
Urban Atmosphere	Excellent	Excellent	Excellent	Excellent	Excellent	Excellent
Industrial Atmosphere	Good	Excellent	Excellent	Excellent	Good	Good
Seawater Splash Zone	Fair	Good	Good	Very Good	Fair	Fair
Chloride Exposure	Fair	Good	Good	Excellent	Poor	Fair
Organic Acids	Good	Very Good	Very Good	Excellent	Good	Good
Sulfur Compounds	Fair	Good	Good	Good	Good	Good
High Temperature Oxidation	Good	Good	Good	Good	Excellent	Very Good

51. PREN (Pitting Resistance Equivalent Number)

PREN is widely used to compare resistance to chloride-induced pitting.

Formula

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

Higher PREN values indicate improved chloride resistance.

Typical PREN Values

Grade	Approximate PREN
304	18–20
316	24–26
316L	24–26
317L	30–35
310S	24
321	18–20

52. Material Selection Guide

Recommended Grade by Service Environment

Service Environment	Recommended Grade
General Industrial	304
Food Processing	304 / 316
Water Treatment	316
Offshore Platforms	316L
Marine Equipment	316L
Chloride Chemical Plants	317L
Refinery Heater Systems	321
Furnace Equipment	310S
LNG Equipment	316L
Heat Exchangers	321 / 310S

53. Temperature Capability

Continuous Service Temperature

Grade	Maximum Service Temperature
304	870°C
316	870°C
316L	870°C
317L	870°C
321	900°C
310S	1100°C

310S is the preferred selection where oxidation resistance becomes the dominant requirement.

54. NACE MR0175 / ISO 15156 Considerations

For sour service applications involving:

• H₂S
• Chlorides
• High pressure environments

material selection must satisfy NACE requirements.

Key considerations include:

• Hardness limits
• Stress corrosion resistance
• Sulfide stress cracking resistance

Austenitic stainless steels generally perform well due to their inherent toughness and ductility, but project-specific approval remains essential.

55. Heat Treatment of Austenitic Stainless Fasteners

Unlike carbon steel fasteners, austenitic stainless steels cannot be hardened through conventional quench-and-temper methods.

Their properties arise primarily from:

• Alloy chemistry
• Cold working
• Controlled processing

56. Solution Annealing

Most important heat treatment process.

Typical Temperature Range

Grade	Temperature
304	1010–1120°C
316	1040–1150°C
316L	1040–1150°C
317L	1040–1150°C
321	950–1120°C
310S	1040–1150°C

Process:

1. Heat uniformly
2. Dissolve carbides
3. Rapid quench

Benefits:

• Restores corrosion resistance
• Improves ductility
• Removes residual stresses

57. Cold Working

Austenitic fasteners frequently obtain strength through cold deformation.

Processes include:

• Cold heading
• Thread rolling
• Cold forging

Advantages:

• Increased tensile strength
• Improved fatigue performance
• Better surface finish

58. Stress Relief

Occasionally performed for:

• Machined components
• Large custom fasteners

Purpose:

• Reduce residual stresses
• Improve dimensional stability

59. Manufacturing Workflow at SM Fasteners

Industrial fastener manufacturing requires rigorous process control to ensure compliance with international specifications.

Stage 1 – Raw Material Procurement

Approved mills supply:

• Stainless steel wire rod
• Stainless steel bar stock

Materials arrive with:

• Heat numbers
• MTC documentation
• Chemical analysis reports

Stage 2 – Incoming Inspection

Verification includes:

• Heat number confirmation
• Visual inspection
• Dimensional checks
• PMI verification (when required)

Stage 3 – Material Identification

Positive Material Identification (PMI) may be conducted using:

• XRF analyzers
• OES spectrometers

Particularly important for:

• 316 vs 316L
• 317L verification
• Mixed inventory prevention

Stage 4 – Cutting Operations

Raw stock is cut into slugs or blanks according to manufacturing drawings.

Stage 5 – Cold Heading / Hot Forging

Cold Heading

Used for:

• High-volume production
• Small to medium diameters

Advantages:

• Superior grain flow
• Better mechanical properties
• High productivity

Hot Forging

Used for:

• Large diameters
• Heavy hex bolts
• Custom geometries

Advantages:

• Reduced forming forces
• Larger size capability

60. Forging vs Machining Comparison

Parameter	Forging	Machining
Grain Flow	Excellent	Interrupted
Strength	Higher	Lower
Material Utilization	Better	Less efficient
Cost for Volume	Lower	Higher
Complex Shapes	Good	Excellent

SM Fasteners Material & Manufacturing Capability

SM Fasteners supplies precision-manufactured Austenitic stainless steel fasteners in grades 304, 316, 316L, 317L, 310S, and 321, produced under ISO 9001-certified quality systems. Manufacturing capabilities include cold heading, hot forging, thread rolling, CNC machining, passivation, specialized coating systems, custom-engineered fasteners, and advanced materials such as PEEK, Duplex, Super Duplex, Inconel, Hastelloy, Monel, Incoloy, Nickel Alloys, and SMO 254. Complete traceability is maintained from raw material receipt through final inspection and shipment documentation.