Inconel Grades 718 & X-750

1. Industry Context

The Growing Demand for Nickel Alloy Fasteners

Modern industrial facilities increasingly operate under severe environmental conditions where conventional stainless steel and alloy steel fasteners experience accelerated degradation.

Critical challenges include:

• High-temperature oxidation
• Chloride-induced corrosion
• Sulfide stress cracking
• Crevice corrosion
• Galvanic corrosion
• Thermal cycling fatigue
• Hydrogen-assisted cracking
• High-pressure service environments

Industries requiring superior fastening systems include:

Industry	Operating Challenge	Inconel Requirement
Offshore Oil & Gas	Seawater + H₂S	Corrosion resistance
LNG Plants	Cryogenic temperatures	Toughness retention
Petrochemical	Acid exposure	Chemical resistance
Power Generation	High temperature	Creep resistance
Aerospace	Cyclic loading	Fatigue resistance
Nuclear	Radiation + temperature	Structural integrity
Chemical Processing	Aggressive chemicals	Corrosion control
Marine	Saltwater exposure	Long-term durability

Nickel-based Inconel alloys have become preferred fastening materials because they maintain mechanical properties where conventional stainless steels lose strength or become susceptible to corrosion mechanisms.

2. Technical Definition

What Are Inconel Fasteners?

Inconel fasteners are threaded mechanical joining components manufactured from nickel-chromium-based superalloys engineered for:

• Elevated temperature service
• Severe corrosion resistance
• Oxidizing environments
• Reducing environments
• Cryogenic service
• High-pressure systems

Common products include:

• Hex bolts
• Heavy hex bolts
• Stud bolts
• Tap-end studs
• Threaded rods
• Hex nuts
• Heavy hex nuts
• Lock nuts
• Socket head cap screws
• Washers
• Special machined fasteners

SM Fasteners manufactures Inconel fasteners according to customer specifications and international standards for EPC and OEM projects.

3. Overview of Major Inconel Grades

Inconel 600

UNS N06600

Primary alloying elements:

Element	Typical %
Nickel	72 min
Chromium	14–17
Iron	6–10

Characteristics:

• Excellent oxidation resistance
• High-temperature strength
• Resistance to chloride ions
• Good carburization resistance
• Suitable up to approximately 1093°C

Applications:

• Furnace hardware
• Heat treatment fixtures
• Nuclear systems
• Chemical plants

Inconel 625

UNS N06625

Primary alloying elements:

Element	Typical %
Nickel	Balance
Chromium	20–23
Molybdenum	8–10
Niobium	3–4

Characteristics:

• Outstanding seawater resistance
• Exceptional pitting resistance
• High tensile strength
• Excellent weldability
• Superior fatigue performance

Applications:

• Offshore platforms
• Subsea equipment
• Marine structures
• LNG facilities

Inconel 718

UNS N07718

Primary alloying elements:

Element	Typical %
Nickel	50–55
Chromium	17–21
Niobium	4.75–5.5
Molybdenum	2.8–3.3

Characteristics:

• Precipitation hardenable
• Extremely high strength
• Excellent fatigue resistance
• Creep resistance
• Suitable to approximately 700°C

Applications:

• Gas turbines
• Aerospace engines
• High-pressure bolting
• Power generation systems

Inconel X-750

UNS N07750

Primary alloying elements:

Element	Typical %
Nickel	70 min
Chromium	14–17
Titanium	2.25–2.75
Aluminum	0.4–1.0

Characteristics:

• Age hardenable
• Excellent relaxation resistance
• Good oxidation resistance
• High fatigue strength

Applications:

• Gas turbines
• Nuclear reactors
• Springs
• High-temperature bolting

4. Functional Role in Mechanical Assemblies

Inconel fasteners primarily serve as:

Structural Load-Carrying Elements

They transfer:

• Tensile loads
• Shear loads
• Dynamic loads
• Impact loads

through clamped joints.

Sealing Integrity Components

Critical in:

• Pressure vessels
• Heat exchangers
• Flanges
• Valve assemblies

where loss of preload may result in leakage.

Thermal Expansion Compensation

Nickel alloys exhibit predictable thermal behavior that supports:

• High-temperature joints
• Thermal cycling equipment
• Reactor systems

5. Load Mechanics and Force Behavior

Understanding fastener mechanics is essential for reliable joint performance.

Tensile Loading

Tensile force acts along the fastener axis.

Typical examples:

• Pressure vessel flanges
• Structural connections
• Equipment anchoring

Stress equation:$$\sigma=\frac{F}{A}$$

Where:

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

A properly selected Inconel fastener must maintain:

• Elastic behavior
• Sufficient preload
• Adequate safety factor

throughout operating conditions.

Shear Loading

Shear occurs when connected components attempt to slide relative to each other.

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

Where:

• τ = shear stress
• F = shear force
• A = shear area

Common examples:

• Structural steel connections
• Offshore equipment supports
• Rotating machinery assemblies

Combined Loading

Industrial fasteners frequently experience:

• Tension
• Shear
• Bending
• Vibration

simultaneously.

Examples:

• Turbine casings
• Compressor housings
• Offshore riser systems
• Pump assemblies

Inconel 718 is frequently selected because of its exceptional combined-load performance.

6. Elastic Behavior of Fasteners

A properly designed bolted joint behaves like a spring.

The fastener stretches:$$\Delta L=\frac{FL}{AE}$$

Where:

• ΔL = elongation
• F = force
• L = grip length
• A = tensile area
• E = modulus of elasticity

This elastic stretch generates clamping force.

Loss of elasticity leads to:

• Joint separation
• Leakage
• Fatigue cracking
• Bolt failure

7. Preload Principles

What is Preload?

Preload is the intentional tension introduced during tightening.

Purpose:

• Prevent loosening
• Improve fatigue life
• Maintain gasket compression
• Distribute external loads

Typical industrial target:

70–80% of proof load.

Importance in High-Temperature Applications

As temperature increases:

• Materials expand
• Gaskets relax
• Thermal cycling occurs

Insufficient preload may result in:

• Flange leakage
• Bolt loosening
• Joint movement

Inconel alloys maintain preload significantly better than carbon steel at elevated temperatures.

8. Clamping Force Mechanism

When tightened:

1. Bolt stretches
2. Joint compresses
3. Elastic energy develops
4. Components remain clamped

The clamping force must exceed external separating forces.

Relationship:$$F_c > F_e$$

Where:

• Fc = clamp force
• Fe = external force

9. Torque–Tension Relationship

Applied torque creates preload.

Approximate formula:$$T=KFD$$

Where:

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

Variables affecting preload:

Factor	Influence
Surface finish	High
Lubrication	High
Thread quality	High
Coating thickness	Medium
Nut geometry	Medium
Washer condition	Medium

Only about 10–15% of applied torque produces useful preload.

The remaining energy is lost to friction.

10. Friction Effects

Friction occurs at:

Thread Interface

Male and female threads interact during tightening.

Bearing Surface

Nut or bolt head rotates against mating surface.

Typical torque distribution:

Energy Use	Percentage
Thread friction	40%
Bearing friction	50%
Useful preload	10%

Therefore lubrication control is critical for repeatable bolt loading.

11. Thread Engagement Requirements

Adequate engagement prevents thread stripping.

General engineering guideline:

Material	Minimum Engagement
Steel	1D
Stainless Steel	1.5D
Nickel Alloys	1–1.5D
Aluminum	2D

D = nominal diameter.

For high-pressure flange assemblies, greater engagement may be specified.

12. Joint Design Principles

Principle 1: Fastener Stronger Than Applied Load

The bolt must possess sufficient:

• Yield strength
• Tensile strength
• Fatigue strength

with appropriate safety margin.

Principle 2: Joint Stiffness Balance

An optimal bolted joint balances:

• Bolt flexibility
• Joint rigidity

Excessively rigid bolts may experience fatigue failure.

Principle 3: Adequate Grip Length

Longer grip lengths generally improve:

• Elastic stretch
• Fatigue life
• Preload retention

Principle 4: Proper Washer Selection

Washers provide:

• Load distribution
• Surface protection
• Improved preload consistency

Common selections:

• Inconel washers
• Hardened alloy washers
• Special high-temperature washers

13. Fatigue Performance Considerations

Fatigue is one of the most common causes of fastener failure.

Causes:

• Vibration
• Cyclic loading
• Thermal cycling
• Pressure fluctuations

Mitigation:

• Proper preload
• Rolled threads
• Controlled surface finish
• Correct material selection

Inconel 718 and X-750 exhibit superior fatigue resistance compared with conventional stainless steels.

14. Thermal Expansion Effects

Different materials expand differently.

Thermal expansion mismatch may cause:

• Preload loss
• Bolt overstressing
• Joint leakage

Nickel alloys are frequently selected to match:

• High-nickel process equipment
• Turbine components
• Heat exchangers

15. High-Temperature Load Retention

A major advantage of Inconel fasteners is retention of mechanical properties at elevated temperatures.

Alloy	Temperature Capability
Inconel 600	Up to 1093°C
Inconel 625	Up to 982°C
Inconel 718	Up to 700°C
Inconel X-750	Up to 815°C

This makes them suitable for:

• Furnace equipment
• Turbines
• Exhaust systems
• High-temperature reactors

16. Design Selection Considerations

Engineers typically evaluate:

Load Requirements

• Static
• Dynamic
• Cyclic

Environment

• Seawater
• Chlorides
• Acids
• H₂S

Temperature

• Cryogenic
• Ambient
• Elevated

Inspection Requirements

• NDT
• PMI
• Third-party witnessing

Regulatory Compliance

• ASTM
• ISO
• DIN
• BS
• NACE MR0175
• ISO 15156

17. Why EPC Projects Specify Inconel Fasteners

Large-scale EPC projects require:

• Long design life
• Reduced maintenance
• Predictable performance
• Traceability
• Global standards compliance

Inconel grades satisfy these requirements through:

• Exceptional corrosion resistance
• High-temperature capability
• Superior fatigue strength
• Excellent preload retention
• Long-term structural integrity

SM Fasteners supports these project requirements through controlled manufacturing processes, material traceability, inspection documentation, and precision production of Inconel 600, 625, 718, and X-750 fasteners for critical industrial applications.

18. Product Types and Variants

Nickel alloy fasteners are manufactured in numerous configurations to accommodate varying load paths, assembly methods, maintenance requirements, and environmental conditions.

Primary Product Categories

Product Type	Function	Typical Applications
Hex Bolts	General structural fastening	Steel structures, machinery
Heavy Hex Bolts	High-load joints	Pressure vessels, flanges
Stud Bolts	Flange connections	Oil & Gas piping
Tap End Studs	Equipment mounting	Pumps, valves
Threaded Rods	Long clamping assemblies	Structural and piping supports
Hex Nuts	Standard mating component	General assemblies
Heavy Hex Nuts	High-pressure service	ASME flanges
Lock Nuts	Vibration resistance	Rotating equipment
Socket Head Cap Screws	Limited access assemblies	Precision equipment
Set Screws	Position retention	Mechanical drives
Plain Washers	Load distribution	General bolting
Hardened Washers	High-load joints	Critical flange systems
Spring Washers	Vibration control	Equipment assemblies
Special Machined Fasteners	Custom applications	OEM equipment

19. Hex Bolt

Engineering Characteristics

Hex bolts remain the most commonly specified industrial fastener.

Features:

• Six-sided external drive
• Full or partial threading
• Suitable for high clamp loads
• Compatible with standard tools

Advantages

• High torque transmission
• Easy installation
• Global standardization
• Wide interchangeability

Typical standards:

• ASTM A193
• ISO 4014
• ISO 4017
• DIN 931
• DIN 933

Applications:

• Structural steel
• Pipe supports
• Process equipment
• Offshore modules

20. Heavy Hex Bolts

Heavy hex bolts possess a larger head bearing area compared with standard hex bolts.

Benefits

• Higher preload capability
• Improved load distribution
• Reduced bearing stress
• Preferred for pressure-containing equipment

Common standards:

• ASME B18.2.1
• ASTM A193
• ASTM A320

Industries:

• Refineries
• LNG facilities
• Pressure vessels
• Power generation

21. Stud Bolts

Stud bolts are threaded on both ends and generally supplied with two nuts.

Advantages

• Uniform gasket compression
• Simplified maintenance
• Improved preload control
• Reduced flange distortion

Widely used in:

• ASME B16.5 flanges
• ASME B16.47 flanges
• Heat exchangers
• High-pressure valves

Typical specifications:

• ASTM A193 Grade B166 (Inconel 600)
• ASTM B446 (Inconel 625)
• ASTM B637 (718 & X-750)

22. Tap-End Studs

Tap-end studs have:

• One threaded end installed permanently
• One exposed threaded end for assembly

Applications:

• Turbine casings
• Pump housings
• Compressor bodies
• Heat exchangers

Benefits:

• Reduced thread wear
• Easier maintenance
• Enhanced alignment

23. Threaded Rods

Threaded rods provide continuous thread engagement along the entire length.

Uses

• Pipe hangers
• Structural bracing
• Equipment supports
• Long clamping systems

Available lengths:

Standard Length
1000 mm
2000 mm
3000 mm
Custom lengths

SM Fasteners manufactures project-specific threaded rods according to customer drawings and specifications.

24. Hex Nuts

Hex nuts provide the mating thread necessary to generate clamp load.

Important considerations:

• Thread class
• Proof load
• Bearing surface quality
• Material compatibility

Standards:

• ASTM A194
• ISO 4032
• DIN 934
• BS 3692

25. Heavy Hex Nuts

Heavy hex nuts provide:

• Greater wrenching area
• Increased thread engagement
• Enhanced strength

Preferred in:

• Pressure vessel construction
• Pipeline systems
• Offshore structures

26. Washers

Washers are critical load-distribution components.

Functions

• Prevent embedment
• Protect surfaces
• Improve preload consistency
• Reduce bearing stress

Common Types

Washer Type	Application
Plain Washer	General service
Hardened Washer	High preload joints
Structural Washer	Steel construction
Spherical Washer	Misalignment correction
Belleville Washer	Thermal cycling applications

27. Custom Engineered Fasteners

Many EPC projects require non-standard fasteners.

Examples include:

• Extended-length studs
• Reduced shank bolts
• Double-end studs
• Special head configurations
• Hollow fasteners
• Precision machined components

SM Fasteners supports custom-engineered fasteners manufactured to project drawings and inspection requirements.

28. Fastener Geometry Fundamentals

Fastener geometry directly influences:

• Strength
• Fatigue life
• Installation efficiency
• Clamp load consistency

Critical dimensions include:

• Diameter
• Thread pitch
• Head dimensions
• Bearing area
• Thread engagement length
• Grip length

29. Diameter Selection Logic

Diameter is the primary determinant of load-carrying capacity.

As diameter increases:

• Tensile area increases
• Shear area increases
• Preload capacity increases

However:

• Weight increases
• Cost increases
• Installation torque increases

Engineering optimization is required.

30. Thread Pitch Selection

Thread pitch affects:

• Preload control
• Vibration resistance
• Thread stripping resistance

Coarse Threads

Advantages:

• Faster assembly
• Better contamination tolerance
• Improved field installation

Fine Threads

Advantages:

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

31. Metric Thread Dimensions

ISO Metric Coarse Thread Series

Dimensional Specification Table

Size	Pitch (mm)	Stress Area (mm²)
M6	1.0	20.1
M8	1.25	36.6
M10	1.5	58.0
M12	1.75	84.3
M16	2.0	157
M20	2.5	245
M24	3.0	353
M30	3.5	561
M36	4.0	817
M42	4.5	1120
M48	5.0	1473

32. Standard Hex Bolt Dimensions

ISO 4014 / ISO 4017 Reference

Size	Head Width Across Flats (mm)	Head Height (mm)
M8	13	5.3
M10	16	6.4
M12	18	7.5
M16	24	10
M20	30	12.5
M24	36	15
M30	46	18.7
M36	55	22.5

33. Length Selection Principles

Fastener length must provide:

• Full thread engagement
• Proper grip length
• Adequate protrusion

General recommendation:

Thread protrusion beyond nut:

2–3 full threads.

Excessive protrusion:

• Increases corrosion exposure
• Creates snag hazards

Insufficient protrusion:

• Reduces inspection confidence

34. Unified Thread Standards

Widely used in oil & gas equipment.

UNC Series

Unified National Coarse

Advantages:

• Faster assembly
• Robust threads
• Better field performance

Examples:

Diameter	Threads per Inch
1/4″	20
3/8″	16
1/2″	13
5/8″	11
3/4″	10
1″	8

UNF Series

Unified National Fine

Advantages:

• Higher preload
• Better fatigue resistance
• Increased tensile area

Examples:

Diameter	Threads per Inch
1/4″	28
3/8″	24
1/2″	20
5/8″	18
3/4″	16
1″	12

35. British Thread Standards

Still encountered in maintenance and legacy equipment.

BSW (British Standard Whitworth)

Characteristics:

• 55° thread angle
• Rounded roots and crests

Applications:

• Railway equipment
• Legacy machinery
• Heritage infrastructure

BSF (British Standard Fine)

Characteristics:

• Fine-pitch Whitworth profile
• Improved adjustment precision

Applications:

• Aerospace legacy equipment
• Instrumentation

36. Thread Standards & Tolerances Table

Mandatory Engineering Table

Thread System	Standard	Typical Tolerance
Metric Coarse	ISO 261	6g / 6H
Metric Fine	ISO 965	6g / 6H
UNC	ASME B1.1	Class 2A / 2B
UNF	ASME B1.1	Class 2A / 2B
BSW	BS 84	Medium Fit
BSF	BS 84	Medium Fit

37. International Standards Applicable to Inconel Fasteners

ASTM Standards

ASTM A193

High-temperature bolting materials.

Common grades:

Alloy	ASTM Designation
Inconel 600	B166
Inconel 625	B446
Inconel 718	B637
X-750	B637

ASTM A194

Covers high-temperature nuts.

Used with:

• Stud bolts
• Heavy hex bolts
• Pressure vessel fasteners

ASTM B166

Applicable to:

• Inconel 600 bars
• Rods
• Forgings

ASTM B446

Applicable to:

• Inconel 625 bars
• Rods
• Forgings

ASTM B637

Applicable to:

• Inconel 718
• Inconel X-750

For:

• High-strength precipitation-hardened fasteners

38. ISO Standards

ISO 898

Mechanical properties of carbon and alloy steel fasteners.

Frequently referenced for comparison purposes.

ISO 261

Metric thread dimensions.

ISO 965

Thread tolerances.

ISO 4014

Hex bolts with partial thread.

ISO 4017

Fully threaded hex bolts.

ISO 4032

Hex nuts.

ISO 7089

Plain washers.

39. DIN Standards

Commonly specified by European EPC projects.

Standard	Description
DIN 931	Hex bolts, partial thread
DIN 933	Hex bolts, full thread
DIN 934	Hex nuts
DIN 125	Plain washers
DIN 127	Spring washers
DIN 975	Threaded rods
DIN 976	Stud bolts

40. British Standards (BS)

Standard	Application
BS 3692	Metric fasteners
BS 4320	Washers
BS 4190	Hex bolts and nuts
BS 84	Whitworth threads

41. Interchangeability Considerations

Fastener substitution must consider:

• Diameter
• Pitch
• Strength
• Material compatibility
• Corrosion performance

Incorrect substitution may result in:

• Galling
• Leakage
• Fatigue failure
• Thread damage

42. Engineering Selection Matrix

Selecting the Correct Inconel Grade

Requirement	Preferred Grade
High Temperature	Inconel 600
Offshore Seawater	Inconel 625
Maximum Strength	Inconel 718
High Fatigue Resistance	X-750
Turbine Applications	718
Chemical Processing	625
Heat Treatment Equipment	600
Nuclear Service	X-750

43. Dimensional Specification Table (Engineering Reference)

Diameter	Standard Pitch	Typical Length Range
M6	1.0	10–100 mm
M8	1.25	10–150 mm
M10	1.5	15–200 mm
M12	1.75	20–300 mm
M16	2.0	25–400 mm
M20	2.5	30–500 mm
M24	3.0	40–600 mm
M30	3.5	50–800 mm
M36	4.0	60–1000 mm
M42	4.5	Custom
M48	5.0	Custom

44. Procurement Considerations

When specifying Inconel fasteners, purchase documentation should define:

Material

• Inconel 600
• Inconel 625
• Inconel 718
• Inconel X-750

Standard

• ASTM
• ISO
• DIN
• BS

Configuration

• Bolt
• Nut
• Stud
• Washer

Thread Form

• Metric
• UNC
• UNF
• BSW
• BSF

Inspection Requirements

• PMI
• Mechanical testing
• Third-party inspection

Documentation

• EN 10204 3.1
• MTC
• CoC

SM Fasteners supports project-specific dimensional requirements, custom geometries, special thread forms, and international standards compliance for critical EPC and industrial applications.

45. Materials Engineering of Inconel Fasteners

Inconel alloys belong to the family of nickel-chromium superalloys specifically engineered for environments involving:

• High temperatures
• Corrosive media
• Oxidizing atmospheres
• Cryogenic service
• High cyclic stresses
• Severe pressure conditions

The high nickel content provides:

• Excellent corrosion resistance
• Stable microstructure
• Resistance to stress corrosion cracking
• High-temperature strength retention

The addition of chromium, molybdenum, niobium, titanium, and aluminum further enhances mechanical and corrosion performance.

46. Chemical Composition Comparison

Material Composition Table

Element (%)	Inconel 600	Inconel 625	Inconel 718	Inconel X-750
Nickel	72 Min	Balance	50–55	70 Min
Chromium	14–17	20–23	17–21	14–17
Iron	6–10	5 Max	Balance	5–9
Molybdenum	—	8–10	2.8–3.3	—
Niobium	—	3.15–4.15	4.75–5.5	0.7–1.2
Titanium	—	—	0.65–1.15	2.25–2.75
Aluminum	—	—	0.2–0.8	0.4–1.0

47. Mechanical Property Fundamentals

Fastener performance depends on:

Yield Strength

Stress level where permanent deformation begins.

Ultimate Tensile Strength (UTS)

Maximum stress sustained before fracture.

Elongation

Material ductility indicator.

Hardness

Resistance to indentation and wear.

Fatigue Strength

Resistance to cyclic loading.

Creep Resistance

Ability to resist deformation under long-term elevated temperature exposure.

48. Mechanical Properties Table

Mandatory Engineering Table

Property	Inconel 600	Inconel 625	Inconel 718	Inconel X-750
UTS (MPa)	550–760	827–1034	1240–1380	1035–1310
Yield Strength (MPa)	240–550	414–758	1035–1200	690–1100
Elongation (%)	30–45	30–42	12–25	15–25
Hardness (HB)	150–220	180–250	330–420	250–380
Density (g/cm³)	8.47	8.44	8.19	8.28
Modulus (GPa)	207	205	200	214

49. Material Comparison Table

UTS, Yield Strength, Corrosion Resistance & Cost Comparison

Material	UTS	Yield	Corrosion Resistance	Relative Cost	Typical Applications
SS 304	Medium	Medium	Good	Low	General industry
SS 316	Medium	Medium	Very Good	Low-Medium	Marine service
Duplex 2205	High	High	Excellent	Medium	Offshore
Super Duplex 2507	Very High	Very High	Exceptional	High	Subsea
Inconel 600	High	Medium	Excellent	High	Heat treatment
Inconel 625	High	High	Outstanding	High	Offshore/LNG
Inconel 718	Extremely High	Extremely High	Excellent	Very High	Turbines
Inconel X-750	Very High	High	Excellent	Very High	Nuclear/Turbines

50. Temperature Capability Comparison

Alloy	Maximum Recommended Service Temperature
Inconel 600	1093°C
Inconel 625	982°C
Inconel 718	700°C
Inconel X-750	815°C

51. Corrosion Resistance Mechanisms

Corrosion resistance is achieved through the formation of a stable chromium oxide passive layer.

This passive film:

• Self-heals
• Protects against oxidation
• Resists chloride attack
• Reduces localized corrosion

52. Corrosion Resistance vs Environment Table

Mandatory Engineering Table

Environment	600	625	718	X-750
Seawater	Good	Excellent	Very Good	Good
Chlorides	Good	Excellent	Very Good	Good
Sulfuric Acid	Good	Excellent	Good	Good
Nitric Acid	Excellent	Excellent	Very Good	Good
Hydrochloric Acid	Moderate	Good	Moderate	Moderate
H₂S Service	Good	Excellent	Good	Good
Offshore Splash Zone	Good	Excellent	Very Good	Good
LNG Service	Good	Excellent	Excellent	Excellent
High Temperature Oxidation	Excellent	Excellent	Very Good	Excellent

53. NACE MR0175 / ISO 15156 Considerations

Oil and gas projects often specify:

• NACE MR0175
• ISO 15156

These standards address:

• Sulfide stress cracking
• Hydrogen embrittlement
• Sour service suitability

Selection criteria include:

• Hardness limits
• Yield strength restrictions
• Environmental exposure

Inconel 625 is frequently preferred for severe sour service applications because of its excellent resistance to sulfide stress cracking.

54. High Temperature Performance

At elevated temperatures conventional alloy steels lose strength rapidly.

Inconel alloys maintain:

• Yield strength
• Oxidation resistance
• Fatigue performance
• Structural stability

Applications:

• Gas turbines
• Furnace systems
• Heat exchangers
• Petrochemical reactors

55. Cryogenic Performance

Cryogenic systems demand:

• High toughness
• Crack resistance
• Dimensional stability

Inconel alloys maintain ductility at:

• LNG temperatures
• Liquid nitrogen service
• Cryogenic process systems

Typical temperature range:

Down to approximately −196°C.

56. Heat Treatment Fundamentals

Heat treatment modifies:

• Strength
• Hardness
• Ductility
• Residual stress

The process depends upon alloy chemistry.

57. Heat Treatment of Inconel 600

Primary treatment:

Solution Annealing

Typical range:

980–1150°C

Purpose:

• Stress relief
• Microstructure stabilization
• Improved ductility

Cooling:

• Rapid air cool
• Water quench where applicable

58. Heat Treatment of Inconel 625

Typically supplied:

Solution Annealed

Temperature:

1090–1200°C

Benefits:

• Corrosion resistance optimization
• Stress reduction
• Improved toughness

59. Heat Treatment of Inconel 718

Inconel 718 derives strength through precipitation hardening.

Typical Sequence

1. Solution treatment
2. Controlled cooling
3. Age hardening

Results:

• Exceptional tensile strength
• High fatigue resistance
• Improved creep performance

60. Heat Treatment of Inconel X-750

Primary treatment:

Age Hardening

Produces:

• Increased hardness
• High-temperature strength
• Improved relaxation resistance

Used extensively for:

• Springs
• Turbine hardware
• Nuclear fasteners

61. Heat Treatment Comparison Table

Alloy	Heat Treatment	Primary Objective
600	Solution Annealed	Ductility
625	Solution Annealed	Corrosion Resistance
718	Solution + Aging	Maximum Strength
X-750	Age Hardened	High Temp Strength

62. Raw Material Verification

Manufacturing begins with certified raw material procurement.

Verification includes:

• Heat number validation
• Chemical composition review
• Material Test Certificate verification
• Visual inspection
• Dimensional verification

Documentation typically follows:

EN 10204 3.1 requirements.

63. End-to-End Manufacturing Workflow

Manufacturing Sequence

Step 1: Raw Material Receipt

Inputs:

• Bars
• Rods
• Forgings

Verification:

• MTC review
• Heat traceability

Step 2: Material Identification

Methods:

• Heat number marking
• PMI verification
• Batch traceability

Step 3: Cutting

Equipment:

• CNC band saws
• Precision cutting systems

Objective:

Maintain length tolerances.

Step 4: Hot Forging

Used for:

• Bolt heads
• Special fasteners
• High-strength components

Benefits:

• Grain flow improvement
• Enhanced fatigue life
• Increased structural integrity

64. Forging vs Machining Comparison

Factor	Forged Fasteners	Machined Fasteners
Strength	Higher	Lower
Fatigue Resistance	Better	Moderate
Grain Flow	Continuous	Interrupted
Production Rate	High	Lower
Complex Shapes	Limited	Excellent

Critical bolting typically favors forged components.

65. CNC Machining

Used for:

• Custom fasteners
• Precision components
• Special thread profiles

Capabilities include:

• Tight tolerances
• Complex geometries
• High repeatability

66. Thread Rolling vs Thread Cutting

Thread Rolling

Material is displaced rather than removed.

Benefits:

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

Thread Cutting

Material is removed.

Benefits:

• Large diameter flexibility
• Custom thread forms
• Low-volume production

67. Thread Manufacturing Comparison

Characteristic	Rolled Threads	Cut Threads
Fatigue Strength	Excellent	Good
Surface Finish	Superior	Moderate
Production Speed	High	Lower
Strength Retention	Excellent	Moderate
Cost Efficiency	Better for volume	Better for custom

68. Surface Finishing Fundamentals

Surface finish affects:

• Corrosion resistance
• Friction coefficient
• Galling tendency
• Appearance

69. Common Surface Finishes

As Manufactured

Used where no additional treatment is required.

Mechanical Polishing

Provides:

• Improved appearance
• Reduced surface roughness

Electro-Polishing

Provides:

• Enhanced corrosion resistance
• Improved cleanliness
• Reduced contamination risk

Used in:

• Pharmaceutical plants
• Food processing
• Semiconductor industries

70. Surface Finish Comparison Table

Mandatory Engineering Table

Finish Type	Corrosion Resistance	Friction Control	Appearance
As Forged	Moderate	Moderate	Industrial
Machined	Good	Good	Smooth
Polished	Very Good	Good	Excellent
Electro-Polished	Excellent	Good	Premium
Passivated	Excellent	Moderate	Clean

71. Galling Prevention

Nickel alloys may experience galling under high loads.

Mitigation methods:

• Controlled lubrication
• Silver coating
• Dry film lubricants
• Anti-seize compounds
• Surface finishing optimization

72. Coating Considerations for Inconel Fasteners

Unlike carbon steel fasteners, Inconel alloys generally do not require corrosion-protection coatings.

However, specialty coatings may be applied for:

• Friction control
• Galling prevention
• High-temperature lubrication

Examples:

• Xylan
• PTFE
• MoS₂ coatings
• Silver plating

73. Weight Chart (Reference for Procurement Planning)

Approximate Hex Bolt Weight Table

Size	Approx. Weight Per Piece (kg)	Weight Per 100 Pieces (kg)
M8 × 50	0.025	2.5
M10 × 50	0.045	4.5
M12 × 60	0.085	8.5
M16 × 80	0.190	19
M20 × 100	0.380	38
M24 × 120	0.700	70
M30 × 150	1.450	145
M36 × 180	2.600	260

Note: Actual weight varies by alloy grade, head style, thread length, and project specification. SM Fasteners can provide project-specific weight schedules aligned with manufacturing drawings and procurement requirements.

74. Material Selection Guidelines

Service Requirement	Recommended Alloy
General High Temperature	Inconel 600
Offshore Seawater	Inconel 625
Sour Gas Service	Inconel 625
Highest Strength Requirement	Inconel 718
Turbine Bolting	Inconel 718
Nuclear Applications	X-750
High Fatigue Conditions	X-750
Cryogenic LNG Systems	625 / 718

SM Fasteners supports manufacturing of Inconel 600, 625, 718, and X-750 fasteners with full material traceability, controlled heat treatment, precision machining, documented inspection procedures, and compliance-oriented production systems suitable for global EPC and industrial procurement requirements.