Dacromet coating

1 Industrial Context

1.1 Industrial Context of Dacromet Coated Fasteners

Dacromet coating technology is extensively used where conventional zinc electroplating or hot-dip galvanizing may be inadequate due to:

• Severe atmospheric exposure
• Marine chloride environments
• High humidity conditions
• Temperature cycling
• Corrosive industrial emissions
• Offshore installations
• Automotive underbody exposure
• Petrochemical processing environments

Dacromet is classified as a water-based inorganic zinc-aluminum flake coating system designed to provide sacrificial and barrier corrosion protection without hydrogen embrittlement risk associated with electroplating processes.

The coating system is widely adopted across:

• Offshore structures
• Wind turbines
• Bridge infrastructure
• Automotive chassis assemblies
• LNG terminals
• Pipe support systems
• Transmission towers
• Rail infrastructure
• Structural steel assemblies
• EPC modular fabrication

Compared with conventional electro-zinc coatings, Dacromet systems provide:

• Superior salt spray resistance
• Lower friction variability
• Better temperature resistance
• Improved galvanic compatibility
• Enhanced coating uniformity on threaded assemblies

1.2 Technical Definition of Dacromet Coating

Dacromet coating consists primarily of:

• Zinc flakes
• Aluminum flakes
• Chromium-based or chromium-free inorganic binder
• Water-based carrier medium

The coating is applied through:

• Dip-spin process
• Spray application
• Dip-drain method

The coated fastener is then cured at elevated temperatures typically between:

• 280°C to 320°C

The resulting coating forms:

• Layered metallic flake structures
• Barrier protection against moisture ingress
• Sacrificial zinc protection
• Controlled galvanic shielding

Typical coating thickness:

• 5 µm to 20 µm

Unlike electroplating:

• No hydrogen generation occurs during application
• Risk of hydrogen embrittlement is significantly reduced

This makes Dacromet highly suitable for:

• High tensile bolts
• Property class 10.9 and 12.9 fasteners
• ASTM A490 structural bolts
• High-strength automotive fasteners

1.3 Functional Role in Mechanical Assemblies

Dacromet-coated fasteners perform several simultaneous engineering functions:

Structural Functions

• Joint clamping
• Tensile load transfer
• Shear resistance
• Dynamic load stabilization

Corrosion Functions

• Barrier protection
• Sacrificial corrosion resistance
• Thread protection
• Surface isolation

Maintenance Functions

• Reduced seizure risk
• Improved dismantling capability
• Lower maintenance frequency
• Better preload retention

1.4 Load Mechanics and Force Behavior

Fastener joints function by generating clamp load through controlled elastic deformation.

Primary load conditions include:

• Tensile loading
• Shear loading
• Combined loading
• Cyclic fatigue loading
• Vibrational loading
• Thermal expansion loading

The effectiveness of Dacromet-coated fasteners depends on maintaining:

• Stable preload
• Controlled friction coefficient
• Thread integrity
• Corrosion resistance

1.5 Torque–Tension Relationship

Fastener preload is generated through tightening torque.

Fundamental relationship:

$T = K \times F \times d$

Where:

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

For Dacromet-coated fasteners:

• Typical nut factor range: 0.12–0.18
• More consistent than hot-dip galvanized systems
• Reduced torque scatter compared to zinc-plated fasteners

1.6 Preload Principles

Proper preload ensures:

• Joint integrity
• Fatigue resistance
• Vibration resistance
• Load distribution
• Leak prevention

Recommended preload:

• 70%–85% of proof load

Insufficient preload may cause:

• Joint separation
• Fatigue failure
• Loosening

Excessive preload may cause:

• Thread stripping
• Bolt yielding
• Stress concentration

1.7 Worked Preload Calculation Example

Example:

M16 × 2.0 — Property Class 10.9 — Dacromet Coated

Assumptions:

• Proof load stress area = 157 mm²
• Proof stress = 830 MPa
• Target preload = 75%
• Nut factor K = 0.14

Step 1 — Proof Load

$F_p = A_s \times S_p$​$$F_p = 157 \times 830$$

​$$F_p = 130310 \, N$$

Step 2 — Target Preload

$$F = 0.75 \times 130310$$

$$F = 97732 \, N$$

Step 3 — Torque Requirement

$$T = 0.14 \times 97732 \times 0.016$$

$$T \approx 219 \, Nm$$

1.8 Friction Behavior in Dacromet Coatings

Friction coefficient stability is critical for:

• Torque-controlled tightening
• Clamp load accuracy
• Automated assembly systems

Typical coefficient of friction:

• 0.10–0.18

Topcoat lubricants may further reduce friction variability.

1.9 Thread Engagement Principles

Minimum thread engagement depends on:

• Material strength
• Loading direction
• Thread form
• Base material hardness

Recommended engagement:

• Steel-to-steel: 1 × diameter
• Aluminum base materials: 1.5 × diameter
• Cast iron: 1.5–2 × diameter

1.10 Joint Design Principles

Important Design Considerations

Clamp Load Distribution

Higher clamp force improves fatigue performance.

Grip Length

Short grip lengths increase bending stress.

Washer Usage

Washers improve:

• Bearing distribution
• Coating protection
• Surface isolation

Differential Thermal Expansion

Dacromet-coated joints perform well under thermal cycling due to stable coating integrity.

1.11 Fatigue Behavior

Fatigue failures initiate from:

• Thread roots
• Surface defects
• Corrosion pits
• Stress concentrations

Dacromet coatings improve fatigue life by:

• Preventing corrosion pitting
• Reducing surface degradation
• Stabilizing friction behavior

1.12 Failure Mechanisms

Fatigue Failure

Occurs under cyclic loading.

Shear Failure

Occurs when transverse loads exceed material shear capacity.

Hydrogen Embrittlement

Dacromet significantly reduces risk due to non-electrolytic application.

Stress Corrosion Cracking

Improved resistance compared with electro-zinc systems.

Galling

Reduced with lubricated topcoats.

1.13 Environmental Performance

Dacromet coatings are highly effective in:

• Marine atmospheres
• Chloride exposure
• Industrial pollution
• Offshore installations
• Humid climates

Limitations:

• Strong acids
• High-abrasion applications
• Continuous immersion in aggressive chemicals

1.14 Corrosion Resistance vs Environment Table

Environment	Dacromet Performance	Typical Life Expectancy	Remarks
Marine Atmosphere	Excellent	15–25 Years	Offshore suitable
Coastal Infrastructure	Excellent	15–20 Years	Chloride resistant
Industrial Pollution	Very Good	10–15 Years	SO₂ resistant
High Humidity	Excellent	15+ Years	Stable coating
H₂S Environment	Good	Depends on substrate	NACE compliance required
Mild Acids	Moderate	Limited	Additional topcoat preferred
Strong Acids	Poor	Not recommended	Use alloy materials
LNG Facilities	Excellent	Long-term	Low-temperature suitable
Automotive Undercarriage	Excellent	OEM standard	High salt resistance
Rail Infrastructure	Excellent	Long service life	Weather resistant

1.15 Mechanical Properties Table — Metric Property Classes

Property Class	Tensile Strength MPa	Yield Strength MPa	Typical Use
4.6	400	240	Light assemblies
5.8	500	400	General engineering
8.8	800	640	Structural joints
10.9	1040	940	Heavy-duty machinery
12.9	1220	1100	High-load precision assemblies

1.16 Typical Salt Spray Resistance

Coating System	Salt Spray Resistance (Hours)
Electro Zinc	72–240
Zinc Nickel	500–1000
Hot Dip Galvanized	300–1000
Dacromet	1000–2000+
Geomet	1500–3000

1.17 Dacromet vs Hot Dip Galvanized

Parameter	Dacromet	Hot Dip Galvanized
Coating Thickness	Thin	Thick
Thread Fit	Excellent	Requires oversize tapping
Hydrogen Embrittlement	Minimal risk	None
Torque Consistency	Excellent	Moderate
Surface Finish	Smooth	Rough
High Strength Bolts	Highly suitable	Limited
Salt Spray Resistance	Excellent	Very good
Temperature Resistance	Higher	Moderate

1.18 Relevance in EPC and Industrial Procurement

EPC buyers select Dacromet-coated fasteners for:

• Long project life
• Reduced maintenance cost
• Corrosion-critical installations
• Standardized torque performance
• Global standards compliance
• Offshore durability
• Improved lifecycle reliability

SM Fasteners supports EPC procurement with:

• Full material traceability
• Batch identification
• EN 10204 certification
• Controlled coating systems
• Global export packaging
• Project-specific documentation
• Custom engineered fastener capability

2. Product Types of Dacromet Coated

2.1 Product Types of Dacromet Coated Fasteners

Dacromet coating systems are applicable across a broad range of industrial fastening products where corrosion resistance, controlled friction behavior, and long-term mechanical reliability are required.

SM Fasteners manufactures Dacromet-coated fastening systems for:

• Structural steel assemblies
• Petrochemical modules
• Offshore platforms
• LNG terminals
• Transmission infrastructure
• Rail systems
• OEM equipment
• Heavy engineering systems
• Automotive structural applications
• Wind and renewable energy systems

2.2 Dacromet Coated Bolts

Hex Head Bolts

Most widely used industrial fastener type for:

• Structural joints
• Pipe supports
• Flange systems
• Mechanical assemblies
• Equipment mounting

Typical standards:

• ISO 4014
• ISO 4017
• DIN 931
• DIN 933
• ASTM A325
• ASTM A490

Advantages with Dacromet:

• Stable tightening torque
• Corrosion resistance
• Reduced thread seizure
• High preload reliability

Heavy Hex Bolt

Used in:

• Structural steel connections
• Offshore modules
• Wind towers
• Bridge construction

Characteristics:

• Larger bearing surface
• Higher preload capability
• Improved structural load distribution

Applicable standards:

• ASTM A325
• ASTM A490
• EN 14399

Socket Head Cap Screws

Used in:

• Precision machinery
• Compact assemblies
• OEM systems
• Automation equipment

Standards:

• ISO 4762
• DIN 912

Dacromet advantages:

• Reduced corrosion inside recesses
• Improved installation repeatability

Flange Bolt

Integrated flange improves:

• Load distribution
• Surface protection
• Anti-loosening performance

Common in:

• Automotive assemblies
• Heavy equipment
• Vibrational systems

2.3 Dacromet Coated Nuts

Hex Nuts

Used with:

• Structural bolts
• Stud systems
• Threaded rods

Standards:

• ISO 4032
• DIN 934
• ASTM A563

Heavy Hex Nuts

Used for:

• Structural assemblies
• High preload joints
• Offshore steelwork

Characteristics:

• Increased wrenching area
• Higher proof load capability

Lock Nuts

Used in:

• Dynamic loading environments
• Vibrating machinery
• Rotating systems

Variants:

• Nylon insert
• All-metal prevailing torque
• Serrated flange

2.4 Dacromet Coated Screws

Self-Drilling Screws

Applications:

• Roofing systems
• Cladding
• HVAC systems
• Light structural assemblies

Advantages:

• Fast installation
• Corrosion resistance
• Outdoor durability

Self-Tapping Screws

Used in:

• Sheet metal assemblies
• Automotive interiors
• Electrical enclosures

Machine Screws

Applications:

• Precision assemblies
• Electronics housings
• Equipment covers

2.5 Dacromet Coated Washers

Washers protect coated surfaces and improve preload consistency.

Flat Washers

Functions:

• Load distribution
• Surface protection
• Prevention of embedding

Standards:

• ISO 7089
• DIN 125

Spring Washers

Used for:

• Anti-vibration applications
• Moderate locking requirements

Structural Washers

Used in:

• Structural steel assemblies
• High-strength bolting systems

Standards:

• ASTM F436

2.6 Dacromet Coated Threaded Rods

Used in:

• Pipe support systems
• Structural anchoring
• Equipment support
• HVAC suspension

Thread forms:

• Metric coarse
• Metric fine
• UNC
• UNF

2.7 Specialty and Custom Fasteners

SM Fasteners manufactures:

• Stud bolts
• Anchor bolts
• U-bolts
• Eye bolts
• Foundation bolts
• Custom cold-forged fasteners
• CNC-machined components
• Large-diameter structural fasteners

Custom engineering capability includes:

• Non-standard dimensions
• Project-specific coatings
• Hybrid material assemblies
• High-temperature applications
• Offshore-grade fasteners

2.8 PEEK Fasteners with Dacromet-Compatible Assemblies

PEEK fasteners are used where:

• Electrical insulation is required
• Chemical exposure is severe
• Weight reduction is critical
• Non-metallic fastening is necessary

Applications:

• Semiconductor systems
• Chemical processing
• Aerospace interiors
• Electrical insulation assemblies

Hybrid assemblies may combine:

• Dacromet-coated steel bolts
• PEEK spacers
• PEEK washers
• Composite insulating systems

2.9 Dimensional Logic of Fasteners

Fastener geometry directly influences:

• Clamp load
• Fatigue resistance
• Stress concentration
• Joint stiffness
• Installation torque

Critical dimensions include:

• Nominal diameter
• Thread pitch
• Head height
• Across flats dimension
• Grip length
• Thread length
• Fillet radius

2.10 Metric Thread Geometry

Metric threads follow ISO profile standards.

Fundamental thread angle:

• 60°

Key parameters:

• Major diameter
• Minor diameter
• Pitch diameter
• Pitch spacing

2.11 Coarse vs Fine Threads

Parameter	Coarse Thread	Fine Thread
Installation Speed	Faster	Slower
Stripping Resistance	Better	Moderate
Vibration Resistance	Moderate	Better
Tensile Stress Area	Lower	Higher
Adjustment Precision	Lower	Higher
Dirty Environment Suitability	Better	Moderate

2.12 Thread Standards and Tolerances Table

Thread Standard	Region	Thread Angle	Typical Applications
ISO Metric	Global	60°	General engineering
UNC	USA	60°	Structural & industrial
UNF	USA	60°	Precision assemblies
BSW	UK	55°	Legacy equipment
BSF	UK	55°	Fine-thread assemblies
NPT	USA	60°	Pipe threads
BSP	UK/Global	55°	Piping systems

2.13 Metric Thread Pitch Table

Diameter	Coarse Pitch	Fine Pitch
M6	1.0	0.75
M8	1.25	1.0
M10	1.5	1.25
M12	1.75	1.5
M16	2.0	1.5
M20	2.5	1.5
M24	3.0	2.0
M30	3.5	2.0

2.14 Dimensional Specification Table — Hex Bolts

Size	Pitch	Head Across Flats (mm)	Head Height (mm)	Standard Thread Length
M6	1.0	10	4	18
M8	1.25	13	5.3	22
M10	1.5	17	6.4	26
M12	1.75	19	7.5	30
M16	2.0	24	10	38
M20	2.5	30	12.5	46
M24	3.0	36	15	54
M30	3.5	46	18.7	66

2.15 Length Selection Logic

Proper bolt length ensures:

• Full nut engagement
• Adequate clamp length
• Thread protrusion

Recommended protrusion:

• 2–3 threads beyond nut face

Insufficient length may cause:

• Incomplete engagement
• Reduced proof load

Excessive length may create:

• Installation interference
• Corrosion exposure

2.16 Grip Length Principles

Grip length is the total thickness of clamped materials.

Important effects:

• Longer grip improves elasticity
• Reduced fatigue sensitivity
• Better preload retention

Short grip lengths may cause:

• Joint relaxation
• Higher bending stress

2.17 Head Geometry and Load Distribution

Hex Head

Best for:

• General industrial use
• High torque applications

Socket Head

Best for:

• Space-constrained assemblies
• Precision equipment

Countersunk Head

Best for:

• Flush surface requirements

Flange Head

Best for:

• Improved bearing area

2.18 Applicable ISO Standards

Standard	Description
ISO 4014	Hex bolts partially threaded
ISO 4017	Hex bolts fully threaded
ISO 4032	Hex nuts
ISO 4762	Socket head cap screws
ISO 7089	Plain washers
ISO 898-1	Mechanical properties of bolts
ISO 898-2	Mechanical properties of nuts
ISO 965	Thread tolerances
ISO 10683	Zinc flake coatings
ISO 9227	Salt spray testing

2.19 Applicable ASTM Standards

ASTM Standard	Description
ASTM A325	Structural bolts
ASTM A490	High-strength structural bolts
ASTM A563	Carbon steel nuts
ASTM F436	Hardened washers
ASTM B117	Salt spray testing
ASTM F3125	Structural bolting assemblies
ASTM F1136	Zinc/aluminum coatings
ASTM A193	Alloy steel bolting
ASTM A194	High-pressure nuts

2.20 Applicable DIN Standards

DIN Standard	Description
DIN 931	Hex bolts partially threaded
DIN 933	Fully threaded hex bolts
DIN 912	Socket head cap screws
DIN 934	Hex nuts
DIN 125	Flat washers
DIN 127	Spring washers
DIN 6921	Flange bolts

2.21 Applicable British Standards (BS)

BS Standard	Description
BS 3692	Metric precision fasteners
BS 4190	ISO metric fasteners
BS 1083	Hexagon bolts
BS 1768	Hexagon nuts
BS 4320	Washers

2.22 Interchangeability Considerations

Critical interchangeability factors:

• Thread pitch compatibility
• Head dimensions
• Property class equivalence
• Coating thickness
• Tolerance class

Improper interchangeability may cause:

• Galling
• Preload inconsistency
• Thread interference
• Reduced fatigue performance

2.23 Structural Fastener Geometry Requirements

Structural assemblies require:

• Hardened washers
• Controlled thread engagement
• Specified grip lengths
• Tight dimensional tolerances

Critical in:

• Bridges
• Offshore structures
• Steel buildings
• Heavy industrial modules

2.24 Dacromet Coating Thickness Logic

Typical thickness ranges:

Coating Thickness	Application
5–8 µm	Automotive
8–12 µm	General industrial
12–15 µm	Structural outdoor
15–20 µm	Offshore & marine

Excessive thickness may affect:

• Thread fit
• Torque consistency
• Assembly compatibility

2.25 Surface Finish and Friction Classes

Surface Finish	Friction Coefficient	Typical Use
Plain Dacromet	0.14–0.18	Structural joints
Lubricated Dacromet	0.10–0.14	Controlled preload
Topcoat-Sealed	0.12–0.16	Offshore systems
Waxed Finish	0.09–0.13	Automated assembly

2.26 Proof Load and Tensile Strength Table

Size	Property Class	Proof Load (kN)	Ultimate Tensile Load (kN)
M8	8.8	23	26
M10	8.8	36	42
M12	8.8	52	60
M16	8.8	96	116
M20	8.8	150	181
M12	10.9	75	84
M16	10.9	136	157
M20	10.9	212	245
M24	10.9	305	353

2.27 Tightening Torque Chart — Dacromet Coated Fasteners

Size	Grade	Dry Torque (Nm)	Lubricated Torque (Nm)
M8	8.8	25	20
M10	8.8	49	39
M12	8.8	86	68
M16	8.8	210	168
M20	8.8	410	328
M12	10.9	121	97
M16	10.9	295	236
M20	10.9	580	464
M24	10.9	1000	800

2.28 Weight Chart — Hex Bolts

Size × Length	Approx. Weight Per Piece	Approx. Weight Per 100 pcs
M8 × 30	0.017 kg	1.7 kg
M10 × 50	0.039 kg	3.9 kg
M12 × 60	0.068 kg	6.8 kg
M16 × 80	0.158 kg	15.8 kg
M20 × 100	0.312 kg	31.2 kg
M24 × 120	0.590 kg	59.0 kg
M30 × 150	1.180 kg	118 kg

SM Fasteners weight charts are aligned with:

• ISO dimensional standards
• Manufacturing tolerances
• Coating allowance
• Industrial packing calculations
• Export shipment planning

2.29 Selection Criteria for Engineers and Procurement Teams

Selection of Dacromet-coated fasteners should consider:

• Corrosion severity
• Mechanical loading
• Torque requirements
• Service temperature
• Inspection requirements
• Regulatory compliance
• Dissimilar metal interaction
• Lifecycle maintenance expectations

Critical procurement considerations:

• Traceability
• Coating certification
• Salt spray validation
• Mechanical property compliance
• Batch-level inspection records
• EN 10204 documentation
• Packaging integrity
• Export compliance

SM Fasteners supports project procurement through:

• Custom dimensional manufacturing
• Controlled coating systems
• Batch traceability
• EPC-grade documentation
• International standards compliance
• Large-volume export capability

3. MATERIAL GRADES, HEAT TREATMENT, MANUFACTURING

3.1 Material Selection Principles for Dacromet Coated Fasteners

Material selection for Dacromet-coated fasteners depends on:

• Mechanical load requirements
• Environmental exposure
• Temperature conditions
• Corrosion severity
• Fatigue loading
• Sour service requirements
• Weight considerations
• Galvanic compatibility
• Lifecycle expectations

The Dacromet coating itself provides corrosion protection; however, substrate material selection remains critical for:

• Tensile performance
• Yield strength
• Fracture resistance
• Thermal stability
• Long-term reliability

SM Fasteners manufactures Dacromet-coated fasteners in:

• Carbon steel
• Alloy steel
• Stainless steel
• Duplex stainless steel
• Super duplex stainless steel
• Nickel alloys
• Hastelloy
• Inconel
• Incoloy
• Monel
• SMO 254
• Engineering polymers including PEEK

3.2 Carbon Steel Grades

Carbon steel is widely used for:

• Structural assemblies
• Construction
• General industrial equipment
• Infrastructure projects

Typical grades:

• SAE 1018
• SAE 1022
• SAE 1045
• ASTM A307
• ASTM A325

Advantages:

• High strength-to-cost ratio
• Good machinability
• Suitable for coating systems

Limitations:

• Requires corrosion protection
• Limited high-temperature resistance

3.3 Alloy Steel Grades

Alloy steels provide:

• Higher tensile strength
• Improved fatigue resistance
• Better heat resistance
• Enhanced hardenability

Typical grades:

• ASTM A193 B7
• ASTM A320 L7
• 4140
• 4142
• 4340

Applications:

• Pressure vessels
• Flange bolting
• Oil & gas
• Petrochemical systems
• Offshore modules

3.4 Stainless Steel Grades

Austenitic Stainless Steel

SS 304 / A2

Applications:

• General corrosion environments
• Food processing
• Water systems

SS 316 / A4

Applications:

• Marine environments
• Chloride exposure
• Offshore installations

Advantages:

• Excellent corrosion resistance
• Non-magnetic properties
• Good formability

Limitations:

• Lower strength compared with alloy steel

3.5 Duplex & Super Duplex Stainless Steel

Duplex Stainless Steel

Typical grades:

• UNS S31803
• UNS S32205

Super Duplex Stainless Steel

Typical grades:

• UNS S32750
• UNS S32760

Advantages:

• High chloride resistance
• Excellent strength
• Superior pitting resistance
• Offshore suitability

Applications:

• Seawater systems
• Offshore oil platforms
• Desalination systems
• Chemical processing

3.6 Nickel Alloy Fasteners

Inconel

Applications:

• High-temperature service
• Gas turbines
• Exhaust systems

Incoloy

Applications:

• Chemical processing
• Heat exchangers

Monel

Applications:

• Marine systems
• Hydrofluoric acid exposure

Hastelloy

Applications:

• Aggressive chemical environments
• Acidic process systems

3.7 SMO 254 Fasteners

SMO 254 provides:

• Exceptional chloride resistance
• Superior pitting resistance
• Excellent crevice corrosion resistance

Used in:

• Seawater systems
• Offshore platforms
• Chemical plants
• LNG systems

3.8 PEEK Fasteners

PEEK (Polyether Ether Ketone) fasteners are used where:

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

Applications:

• Semiconductor systems
• Aerospace interiors
• Electronics
• Chemical handling systems

Temperature capability:

• Continuous service up to approximately 250°C

3.9 Material Comparison Table

Material	UTS (MPa)	Yield Strength (MPa)	Corrosion Resistance	Relative Cost	Typical Applications
Carbon Steel	400–800	240–640	Moderate	Low	Structural
Alloy Steel	800–1200	640–1100	Moderate	Medium	High-strength joints
SS 304	515–700	205–310	Good	Medium	Water systems
SS 316	515–700	205–310	Very Good	Medium-High	Marine
Duplex SS	620–880	450–650	Excellent	High	Offshore
Super Duplex	750–950	550–750	Exceptional	Very High	Seawater
Inconel	900–1400	500–1100	Exceptional	Premium	High-temperature
Hastelloy	700–1200	300–700	Exceptional	Premium	Acidic systems
Monel	550–900	240–550	Excellent	Premium	Marine
SMO 254	680–850	300–500	Exceptional	Premium	Chloride service
PEEK	90–110	80–100	Exceptional	High	Non-metallic systems

3.10 Mechanical Property Considerations

Key mechanical selection parameters:

• Tensile strength
• Yield strength
• Hardness
• Elongation
• Impact toughness
• Fatigue resistance

Critical in:

• Dynamic loading
• Offshore systems
• Rotating equipment
• Pressure systems

3.11 NACE MR0175 / ISO 15156 Compliance

Sour gas environments containing H₂S require strict hardness control.

Critical considerations:

• Sulfide stress cracking resistance
• Hardness limitation
• Controlled heat treatment
• Traceability

Common hardness limits:

• Maximum 22 HRC for many sour service alloy steels

Applicable industries:

• Upstream oil & gas
• Refining
• LNG
• Offshore production

SM Fasteners supports:

• NACE-compliant manufacturing
• Controlled heat treatment
• Batch-level hardness verification

3.12 Temperature Resistance of Materials

Material	Maximum Recommended Temperature
Carbon Steel	400°C
Alloy Steel	500°C
SS 304	870°C
SS 316	925°C
Duplex SS	300°C
Super Duplex	300°C
Inconel	1000°C+
Hastelloy	1000°C+
PEEK	250°C

3.13 Heat Treatment Objectives

Heat treatment is performed to:

• Increase strength
• Improve toughness
• Reduce brittleness
• Optimize hardness
• Improve fatigue resistance

3.14 Common Heat Treatment Processes

Annealing

Purpose:

• Reduce hardness
• Improve machinability
• Relieve stresses

Normalizing

Purpose:

• Refine grain structure
• Improve mechanical consistency

Quenching

Purpose:

• Increase hardness and strength

Media:

• Oil
• Water
• Polymer

Tempering

Purpose:

• Reduce brittleness after quenching
• Improve toughness

3.15 Heat Treatment of High-Strength Fasteners

Property classes:

• 8.8
• 10.9
• 12.9

Typically undergo:

1. Austenitizing
2. Quenching
3. Tempering

Mechanical performance depends on:

• Time-temperature cycle
• Cooling rate
• Alloy chemistry

3.16 Heat Treatment Impact on Mechanical Properties

Process	Strength	Hardness	Toughness
Annealing	Decreases	Decreases	Increases
Quenching	Increases	Increases	Decreases
Tempering	Controlled	Controlled	Improves
Normalizing	Moderate increase	Moderate	Good

3.17 Hydrogen Embrittlement Considerations

High-strength fasteners above:

• 1000 MPa tensile strength

are vulnerable to hydrogen embrittlement.

Dacromet coating minimizes risk because:

• No acid pickling dependency
• No electrolytic hydrogen charging
• Non-electroplated process

This is a major advantage over:

• Electro zinc plating
• Cadmium plating

3.18 End-to-End Manufacturing Workflow

SM Fasteners follows controlled industrial manufacturing systems aligned with ISO 9001 quality management requirements.

Manufacturing stages include:

1. Raw material verification
2. Cutting and preparation
3. Forging or machining
4. Heat treatment
5. Thread manufacturing
6. Surface preparation
7. Dacromet coating
8. Curing
9. Inspection
10. Packaging
11. Traceability marking
12. Dispatch documentation

3.19 Raw Material Verification

Incoming raw materials are verified through:

• Mill Test Certificates (MTC)
• Chemical composition analysis
• Mechanical property review
• PMI testing where applicable
• Heat number traceability

Applicable standards:

• ASTM
• EN
• ISO
• ASME

3.20 Forging vs Machining

Cold Forging

Advantages:

• High production efficiency
• Improved grain flow
• Better fatigue performance
• Reduced material waste

Used for:

• Bolts
• Screws
• Nuts

Hot Forging

Used for:

• Large diameters
• High-strength components
• Structural bolts

Advantages:

• Better formability
• Lower forming stress

CNC Machining

Used for:

• Precision fasteners
• Custom geometries
• Special tolerances
• Low-volume production

3.21 Thread Rolling vs Thread Cutting

Thread Rolling

Advantages:

• Improved fatigue strength
• Compressive surface stress
• Better dimensional consistency

Preferred for:

• High-strength fasteners
• Dynamic loading applications

Thread Cutting

Advantages:

• Flexible manufacturing
• Large-diameter threading
• Custom thread forms

Limitations:

• Lower fatigue resistance

3.22 Surface Preparation Before Coating

Proper surface preparation is essential for:

• Coating adhesion
• Corrosion resistance
• Uniform thickness

Typical preparation methods:

• Degreasing
• Shot blasting
• Alkaline cleaning
• Mechanical cleaning

3.23 Dacromet Coating Application Process

Step 1 — Cleaning

Removal of:

• Oil
• Scale
• Oxides
• Contaminants

Step 2 — Dip-Spin Coating

Fasteners are immersed in coating slurry.

Excess coating removed through spinning.

Step 3 — Drying

Initial moisture removal.

Step 4 — Thermal Curing

Typical curing temperature:

• 280–320°C

Result:

• Inorganic metallic bonded layer formation

3.24 Coating Thickness Control

Controlled through:

• Viscosity management
• Spin speed
• Immersion time
• Number of coating layers

Typical thickness:

• 5–20 µm

3.25 Surface Finish Comparison Table

Surface Finish	Corrosion Resistance	Friction Stability	Temperature Resistance	Hydrogen Embrittlement Risk
Electro Zinc	Moderate	Moderate	Moderate	High
Hot Dip Galvanized	Very Good	Moderate	Moderate	Low
Phosphate	Low	Good	Moderate	Low
Dacromet	Excellent	Excellent	High	Very Low
Geomet	Exceptional	Excellent	High	Very Low
PTFE Coated	Excellent	Exceptional	Moderate	Low

3.26 Dacromet vs Geomet Coating

Parameter	Dacromet	Geomet
Chromium Content	May contain Cr	Chromium-free
Environmental Compliance	Good	Superior
Salt Spray Resistance	Excellent	Exceptional
Friction Control	Excellent	Excellent
OEM Acceptance	Extensive	Increasing
Offshore Suitability	Excellent	Excellent

3.27 Topcoat Systems

Topcoats improve:

• Friction control
• Chemical resistance
• UV resistance
• Color identification

Variants:

• Organic sealers
• PTFE lubricants
• Wax topcoats
• Color-coded systems

3.28 Galvanic Corrosion Considerations

When dissimilar metals are connected:

• Galvanic corrosion may occur

Critical combinations:

• Carbon steel with aluminum
• Stainless steel with carbon steel
• Copper alloys with steel

Dacromet coatings help reduce:

• Direct galvanic contact
• Surface oxidation

3.29 Coating Performance in Marine Environments

Dacromet performs well under:

• Salt spray
• Humidity
• Atmospheric chlorides
• Splash zones

Used extensively in:

• Offshore platforms
• Coastal bridges
• Marine terminals
• Shipbuilding structures

3.30 Manufacturing Traceability

SM Fasteners maintains traceability through:

• Heat number identification
• Batch coding
• Production routing records
• Inspection records
• Coating batch control
• Packaging traceability

This supports:

• EPC audit requirements
• Third-party inspection
• Global export compliance
• Long-term maintenance documentation

4. INSPECTION, QUALITY CONTROL, INDUSTRY APPLICATIONS

4.1 Inspection Philosophy for Dacromet Coated Fasteners

Industrial fastening systems used in EPC, offshore, structural, petrochemical, and power generation environments require strict inspection systems to ensure:

• Mechanical integrity
• Dimensional conformity
• Coating performance
• Corrosion resistance
• Traceability
• Long-term reliability

SM Fasteners operates under ISO 9001 quality management systems with documented inspection procedures aligned with:

• ISO standards
• ASTM standards
• DIN standards
• BS standards
• Project-specific EPC specifications

Inspection activities are performed across:

1. Incoming raw materials
2. In-process manufacturing
3. Heat treatment
4. Thread verification
5. Coating application
6. Final inspection
7. Dispatch verification

4.2 Dimensional Inspection

Dimensional inspection ensures compliance with:

• ISO tolerances
• Thread class requirements
• Assembly interchangeability
• Structural fit-up requirements

Measured parameters include:

• Major diameter
• Minor diameter
• Pitch diameter
• Thread pitch
• Head dimensions
• Overall length
• Grip length
• Washer thickness
• Nut height
• Coating thickness

Inspection tools:

• Vernier calipers
• Micrometers
• Thread plug gauges
• Ring gauges
• Optical comparators
• Coordinate measuring machines (CMM)

4.3 Thread Tolerance Verification

Thread tolerances directly influence:

• Clamp load generation
• Assembly fit
• Galling resistance
• Torque consistency

Typical tolerance classes:

Thread System	External Thread	Internal Thread
ISO Metric	6g	6H
UNC/UNF	2A	2B
Precision Assemblies	4g6g	5H

Critical inspections:

• Go/No-Go gauge verification
• Pitch consistency
• Crest and root profile inspection

4.4 Mechanical Testing

Mechanical testing validates:

• Tensile performance
• Yield strength
• Hardness
• Ductility
• Proof load capacity

Applicable standards:

• ISO 898-1
• ASTM F606
• ASTM A370

4.5 Tensile Testing

Tensile testing determines:

• Ultimate tensile strength (UTS)
• Yield strength
• Elongation
• Reduction of area

Performed using:

• Universal Testing Machines (UTM)

4.6 Proof Load Testing

Proof load testing validates:

• Elastic performance
• Thread integrity
• Load-bearing capability

The fastener must withstand specified proof load without:

• Permanent deformation
• Thread failure

4.7 Hardness Testing

Hardness testing verifies:

• Heat treatment effectiveness
• NACE compliance
• Surface condition

Common methods:

• Rockwell
• Brinell
• Vickers

4.8 Hardness Limits for Sour Service

NACE MR0175 / ISO 15156 environments require controlled hardness.

Material Category	Typical Maximum Hardness
Carbon Steel	22 HRC
Alloy Steel	22 HRC
Stainless Steel	Project-specific
Duplex Stainless	Controlled by procedure

Importance:

• Prevention of sulfide stress cracking
• H₂S resistance
• Long-term reliability

4.9 Coating Thickness Inspection

Coating thickness directly affects:

• Corrosion resistance
• Thread fit
• Torque behavior

Measurement methods:

• Magnetic thickness gauges
• X-ray fluorescence (XRF)
• Microscopic cross-section analysis

Typical Dacromet thickness:

• 5–20 µm

4.10 Salt Spray Testing

Salt spray testing evaluates:

• Corrosion resistance
• Coating integrity
• Red rust formation resistance

Applicable standards:

• ASTM B117
• ISO 9227

Typical Dacromet performance:

• 1000–2000+ hours

4.11 Adhesion Testing

Adhesion testing ensures:

• Coating bonding integrity
• Resistance to peeling
• Handling durability

Methods:

• Cross-hatch testing
• Bend testing
• Impact testing

4.12 Non-Destructive Testing (NDT)

NDT methods identify defects without damaging components.

Magnetic Particle Inspection (MPI)

Detects:

• Surface cracks
• Near-surface discontinuities

Applicable for:

• Alloy steel fasteners
• Structural bolts

Dye Penetrant Testing (PT)

Used for:

• Surface crack detection
• Stainless steel inspection

Ultrasonic Testing (UT)

Used for:

• Large diameter fasteners
• Internal flaw detection

4.13 Positive Material Identification (PMI)

PMI confirms alloy composition using:

• XRF analyzers
• Optical emission spectroscopy

Critical for:

• Duplex stainless steel
• Nickel alloys
• Offshore projects
• Refinery systems

4.14 Chemical Composition Verification

Material chemistry is verified against:

• ASTM specifications
• EN standards
• Customer requirements

Elements monitored:

• Carbon
• Chromium
• Nickel
• Molybdenum
• Sulfur
• Phosphorus

4.15 Coating Friction Testing

Controlled friction is critical for:

• Torque-preload relationship
• Automated assembly systems
• Structural tightening procedures

Measured parameters:

• Coefficient of friction
• Torque scatter
• Clamp load consistency

4.16 Failure Analysis Procedures

Failure investigations may include:

• Fractography
• Metallography
• Hardness mapping
• SEM analysis
• Corrosion analysis
• Load reconstruction

Common failure causes:

• Improper preload
• Fatigue
• Hydrogen embrittlement
• Stress corrosion cracking
• Over-torquing
• Coating damage

4.17 EN 10204 Certification Requirements

SM Fasteners supports:

• EN 10204 3.1 certification
• EN 10204 3.2 certification

Documentation may include:

• Material Test Certificates
• Mechanical test reports
• Coating reports
• Heat treatment records
• PMI reports
• Dimensional inspection reports

4.18 ISO 9001 Quality Integration

ISO 9001 systems support:

• Process control
• Calibration management
• Traceability
• Corrective actions
• Supplier qualification
• Inspection documentation
• Continuous improvement

SM Fasteners integrates ISO 9001 quality systems throughout:

• Procurement
• Manufacturing
• Inspection
• Packaging
• Export logistics

4.19 Industry Applications — Construction & Structural Steel

Dacromet-coated fasteners are widely used in:

• Structural steel connections
• Bridges
• Stadiums
• Industrial buildings
• Transmission towers

Advantages:

• Weather resistance
• Long-term durability
• Reduced maintenance

Typical products:

• ASTM A325 bolts
• Structural washers
• Heavy hex nuts

4.20 Oil & Gas Applications

Upstream

Used in:

• Drilling structures
• Offshore platforms
• Wellhead equipment

Midstream

Used in:

• Pipeline supports
• Compressor stations

Downstream

Used in:

• Refineries
• Process plants
• Pressure systems

Critical requirements:

• NACE compliance
• Corrosion resistance
• Traceability
• High preload integrity

4.21 Power Generation Applications

Applications include:

• Turbine assemblies
• Boiler systems
• Structural support systems
• Renewable energy infrastructure

Power sectors:

• Thermal
• Nuclear
• Hydro
• Wind
• Solar

4.22 Petrochemical & Chemical Processing

Requirements:

• Chemical resistance
• Corrosion resistance
• Temperature resistance
• Reliability under cyclic conditions

Common materials:

• Alloy steel
• Duplex stainless steel
• Hastelloy
• Inconel

4.23 LNG & Offshore Applications

Offshore environments expose fasteners to:

• Chlorides
• Humidity
• Salt spray
• Cyclic loading

Dacromet systems provide:

• Long-term atmospheric protection
• Controlled friction behavior
• Reduced maintenance requirements

Used in:

• FPSOs
• LNG terminals
• Offshore modules
• Subsea support structures

4.24 Automotive & Heavy Equipment

Applications:

• Chassis systems
• Suspension systems
• Undercarriage assemblies
• Engine compartment structures

Advantages:

• OEM-approved corrosion resistance
• Controlled torque behavior
• High-volume production compatibility

4.25 Railway & Infrastructure Applications

Used in:

• Rail fastening systems
• Signal infrastructure
• Bridge systems
• Electrification assemblies

Requirements:

• Weather resistance
• Fatigue durability
• Vibration resistance