Fasteners are fundamental components across equipment manufacturing, construction, aerospace, and other critical industries. The choice of fastener material directly determines connection reliability, service life, load-bearing capacity, and application suitability.

From an industrial application perspective, this guide provides a comprehensive breakdown of commonly used fastener materials. It covers chemical composition, mechanical properties, key characteristics, typical applications, and important considerations. Both metallic and non-metallic materials are included, balancing general-purpose and specialized use cases.Fastener Selection Guide.

1. Fastener Material Classification & Core Selection Principles

1.1 Key Dimensions of Fastener Material Classification

Fastener materials can be broadly divided into two categories based on their intrinsic properties: metallic materials and non-metallic materials.

In industrial applications, metallic materials account for over 95% of fastener usage due to their superior strength and structural reliability. Non-metallic materials are primarily used as supplements in special operating conditions, such as electrical insulation, lightweight requirements, corrosion resistance, or extreme low-temperature environments.

From a performance perspective, fastener materials can be further classified into the following categories:

• High-strength materials: Designed for load-bearing and impact resistance applications
• Corrosion-resistant materials: Suitable for humid, acidic/alkaline, or marine environments
• Lightweight materials: Commonly used in aerospace, electronics, and rail transportation
• Insulating materials: Applied in electrical and electronic equipment
• High-/low-temperature resistant materials: Engineered for extreme operating conditions

1.2 Core Fastener Material Selection Principles

• Match Load Requirements
  Select fastener materials based on the tensile stress, shear stress, and impact loads the fastener will experience. Avoid over-specification, which increases cost unnecessarily, or under-specification, which may lead to premature failure.
• Adapt to Service Environment
  Consider environmental factors such as temperature, humidity, and exposure to media (acids, alkalis, salt spray, oils). Focus on mitigating risks like corrosion, oxidation, and embrittlement.
• Ensure Process Compatibility
  Fastener materials must be compatible with manufacturing processes such as cold heading, hot forging, machining, heat treatment, and surface finishing. Improper material selection can lead to defects such as cracking, deformation, or insufficient thread precision.
• Balance Cost and Reliability
  Under the premise of meeting performance requirements, prioritize materials with the best cost-performance ratio. For critical applications (e.g., aerospace, nuclear power), high-end specialty materials may be justified.
• Comply with Industry Standards
  Follow recognized standards such as GB/T 3098, ISO 898, and ASTM to ensure material properties meet required specifications and guarantee connection safety and reliability.

2. Metallic Fastener Materials (Industrial Mainstream)

Metallic fastener materials are primarily based on steel, supplemented by specialty metals such as copper alloys, aluminum alloys, and titanium alloys. Their key advantages include high strength, excellent wear resistance, and broad adaptability. Performance can be further enhanced through heat treatment and surface finishing, making them the most widely used fastener material category in industrial fasteners.

2.1 Steel Materials (Ferrous Metals)

Steel materials, also known as ferrous metals, refer to iron-carbon alloys, including both steel and cast iron. The primary distinction lies in carbon content:

• Cast iron: Carbon content > 2.0% — hard and brittle, poor ductility, mainly used for casting or steelmaking
• Steel: Carbon content < 2.0% — excellent ductility, strength, toughness, and machinability

Due to these advantages, steel is the most widely used fastener material for industrial fasteners.

Based on chemical composition, steel can be divided into:

• Carbon steel
• Alloy steel
• Stainless steel (a specialized category of alloy steel with superior corrosion resistance, often classified separately in applications)

2.1.1 Carbon Steel

Carbon steel refers to iron-carbon alloys containing only small amounts of elements such as manganese, silicon, sulfur, and phosphorus. It offers the lowest cost and the most mature processing technology, making it the preferred material for general-purpose fasteners. Its properties are primarily determined by carbon content.

It is broadly classified into:

• Low carbon steel
• Medium carbon steel
• High carbon steel

2.1.1.1 Low Carbon Steel (C ≤ 0.25%)

Typical Grades:
Q195, Q235 (Grades A/B/C/D), 10#, 15#, 20#, AISI 1018, AISI 1022

Chemical Composition (Typical):

• C ≤ 0.25%
• Mn: 0.30–0.60%
• Si ≤ 0.35%
• P ≤ 0.045%, S ≤ 0.050% (standard grade)
• Q235D: P ≤ 0.035%, S ≤ 0.035% (higher quality grade)

Mechanical Properties:

• Tensile strength: 300–420 MPa
• Yield strength: 195–235 MPa
• Elongation: ≥ 25%
• Hardness: HB ≤ 137

Key Characteristics:

• Very low cost
• Excellent ductility and toughness
• Superior cold heading and machinability
• Can be formed without complex heat treatment
• Poor corrosion resistance → requires surface treatments such as zinc plating, chrome plating, or phosphating

Typical Applications:

• Property class 4.8 and below fasteners
• Standard bolts, nuts, self-tapping screws, flat washers, cotter pins
• Used in civil construction, light equipment, furniture, simple machinery, and electrical enclosures
• AISI 1018: commonly used for class 4.8 bolts
• AISI 1022: widely used for self-tapping screws

2.1.1.2 Medium Carbon Steel (C 0.25%–0.60%)

Typical Grades:
35#, 45#, 35K, AISI 1035, AISI 1045

Chemical Composition (Typical):

• C: 0.25–0.60%
• Mn: 0.50–0.80%
• Si: 0.17–0.37%
• P ≤ 0.035%, S ≤ 0.035%

Mechanical Properties:

• As-rolled:
  • Tensile strength: 450–650 MPa
  • Yield strength: 235–355 MPa
  • Hardness: HB ≤ 197
• After quenching & tempering:
  • Tensile strength: 600–800 MPa
  • Hardness: HB 200–250

Key Characteristics:

• Balanced strength and toughness
• Improved wear resistance compared to low carbon steel
• Can be enhanced via heat treatment (quenching & tempering)
• Good cold forming and machining performance
• Still requires corrosion protection
• Cost: higher than low carbon steel, lower than alloy steel

Typical Applications:

• Property class 6.8 and 8.8 fasteners
• Machinery bolts, automotive fasteners, structural bolts and nuts
• Suitable for medium-load and moderate wear conditions
• Common in general machinery, electrical equipment, and non-critical engineering components
• AISI 1035: suitable for class 8.8
• AISI 1045: can reach 10.9 strength class after proper heat treatment

2.1.1.3 High Carbon Steel (C ≥ 0.60%)

Typical Grades:
65#, 70#, 65Mn, 50Mn, AISI 1065

Chemical Composition (Typical):

• C ≥ 0.60%
• Mn: 0.70–1.00% (65Mn: 0.90–1.20% for higher elasticity)
• Si: 0.17–0.37%
• P ≤ 0.035%, S ≤ 0.035%

Mechanical Properties:

• As-rolled:
  • Tensile strength ≥ 650 MPa
  • Yield strength ≥ 360 MPa
  • Hardness: HB ≤ 229
• After quenching & tempering:
  • Tensile strength: 800–1000 MPa
  • Hardness: HRC 35–45

Key Characteristics:

• High hardness and excellent elasticity
• Superior wear resistance
• Lower ductility → more difficult to process
• Requires full annealing before cold heading
• Risk of cracking if improperly processed
• Poor corrosion resistance → requires strict surface treatment

Typical Applications:

• Spring washers, retaining rings, spring pins, cotter pins
• Used in anti-loosening and positioning components
• Suitable for applications requiring high elasticity, hardness, and wear resistance

2.1.2 Alloy Steel

Alloy steel is developed from carbon steel by intentionally adding one or more alloying elements such as chromium (Cr), molybdenum (Mo), manganese (Mn), boron (B), nickel (Ni), and vanadium (V). By adjusting the composition, key properties such as strength, toughness, hardenability, wear resistance, and corrosion resistance can be significantly enhanced.

Alloy steel is primarily used for high-strength, high-load, and high-toughness fasteners, making it a critical material in high-end equipment and heavy machinery.

Based on total alloying content, alloy steel can be classified into:

• Low alloy steel (≤ 5%)
• Medium alloy steel (5%–10%)
• High alloy steel (> 10%)

Among these, low alloy steel is the most widely used category for high-strength fasteners in industrial applications.

2.1.2.1 Low Alloy Steel (Total Alloy Content ≤ 5%)

Typical Grades:
40Cr, 35CrMo, 20MnTiB, 30CrMnSiA, AISI 10B21 (boron steel), SCM435 (JIS standard)

Chemical Composition (Typical):

• 40Cr: C 0.37–0.44%, Cr 0.80–1.10%, Mn 0.50–0.80%
• 35CrMo: C 0.32–0.40%, Cr 0.80–1.10%, Mo 0.15–0.25%
• 20MnTiB: C 0.17–0.23%, Mn 1.30–1.60%, Ti 0.04–0.10%, B 0.001–0.005%
• All grades: P ≤ 0.035%, S ≤ 0.035%
• Must contain at least one alloying element (e.g., Cr ≥ 0.30%, Ni ≥ 0.03%, Mo ≥ 0.20%, V ≥ 0.10%)

Mechanical Properties (After Quenching & Tempering):

• 40Cr:
  • Tensile strength ≥ 980 MPa
  • Yield strength ≥ 785 MPa
  • Hardness: HB 286–321
  • Good toughness
• 35CrMo:
  • Tensile strength ≥ 1080 MPa
  • Yield strength ≥ 930 MPa
  • Hardness: HB 311–369
  • Superior fatigue and high-temperature resistance
• 20MnTiB:
  • Tensile strength ≥ 1040 MPa
  • Yield strength ≥ 835 MPa
  • Excellent hardenability, ideal for mass production
• 10B21:
  • Tensile strength ≥ 800 MPa
  • Yield strength ≥ 600 MPa
  • Excellent cold heading performance, lower cost than 40Cr

Key Characteristics:

• High strength and good toughness
• Excellent hardenability → suitable for heat treatment
• Capable of achieving ultra-high strength levels after quenching and tempering
• Good cold forming and machining performance
• Some grades (e.g., 20MnTiB, 10B21) are suitable for direct cold heading
• Better corrosion resistance than carbon steel, but still requires surface treatment
• Cost: higher than carbon steel, lower than medium/high alloy steel

Typical Applications:

• Property class 10.9 and 12.9 high-strength fasteners
• Automotive chassis bolts, engine bolts
• Heavy machinery critical fasteners
• High-strength structural bolts for bridges and wind power equipment
• Suitable for high load, fatigue resistance, and impact conditions
• Widely used in aerospace components, high-pressure equipment, and construction machinery
• 40Cr and SCM435: key materials for class 12.9 fasteners
• 20MnTiB: commonly used for mass production of class 10.9 bolts

2.1.2.2 Medium Alloy Steel (Total Alloy Content 5%–10%)

Typical Grades:
40CrNiMoA, 30CrNi3A

Chemical Composition (Typical):

• 40CrNiMoA:
  • C 0.37–0.44%
  • Cr 0.60–0.90%
  • Ni 1.20–1.60%
  • Mo 0.15–0.25%
• Total alloy content: approximately 5%–8%

Mechanical Properties (After Heat Treatment):

• Tensile strength ≥ 1180 MPa
• Yield strength ≥ 980 MPa
• Hardness: HB 341–398

Key Characteristics:

• Excellent overall performance
• Superior strength, toughness, wear resistance, and fatigue resistance compared to low alloy steel
• High hardenability → suitable for thick-section components
• More difficult to machine
• Higher cost
• Moderate corrosion resistance → requires advanced surface treatment

Typical Applications:

• Property class 12.9 and above ultra-high-strength fasteners
• Aerospace fasteners
• Nuclear power equipment bolts
• Deep-sea equipment fasteners
• Critical components in heavy-duty machine tools
• Suitable for extreme loads, high impact, and high fatigue cycles

2.1.2.3 High Alloy Steel (Total Alloy Content > 10%)

Note:
Within high alloy steels, stainless steel is widely used and will be discussed separately. This section focuses on non-stainless high alloy steels, typically used in extreme environments.

Typical Grades:
1Cr18Ni9Ti (legacy grade, gradually replaced), GH4169 (superalloy)

Key Characteristics:

• Exceptional high-temperature resistance
• Excellent resistance to high pressure and aggressive corrosion
• Extremely high strength under extreme conditions
• Difficult to machine → requires specialized processes such as forging and precision grinding
• Very high cost

Typical Applications:

• Fasteners for extreme environments
• Aerospace engine high-temperature bolts
• Nuclear reactor fasteners
• Deep-sea high-pressure fasteners
• Used in limited quantities, primarily for high-end specialized equipment

2.1.3 Stainless Steel

Stainless steel is a specialized category of alloy steel, defined by a chromium (Cr) content ≥ 10.5%. Chromium forms a dense passive oxide film on the surface, providing excellent corrosion resistance. As a result, stainless steel fasteners can perform reliably in humid, acidic, alkaline, and salt spray environments without requiring complex surface treatments.

Based on microstructure, stainless steel is classified into:

• Austenitic stainless steel (most widely used)
• Martensitic stainless steel (high strength focus)
• Ferritic stainless steel (cost-effective with moderate corrosion resistance)

For fasteners, stainless steel follows specific property class designations:

• Austenitic stainless steel: marked as A2, A4 series
• Martensitic and ferritic stainless steel: no unified class system, selected based on application requirements

2.1.3.1 Austenitic Stainless Steel (Most Widely Used)

Typical Grades:
304 (A2 series), 316 (A4 series), 304L, 316L, 201 (economical grade)

Chemical Composition (Typical):

• 304 (A2):
  • C ≤ 0.08%
  • Cr 18.00–20.00%
  • Ni 8.00–12.00%
  • Mn ≤ 2.00%
• 316 (A4):
  • C ≤ 0.08%
  • Cr 16.00–18.00%
  • Ni 10.00–14.00%
  • Mo 2.00–3.00% (Mo significantly improves salt spray and chloride corrosion resistance)
• 304L / 316L:
  • C ≤ 0.03%
  • Same composition as 304/316 → improved weldability and resistance to intergranular corrosion
• 201:
  • C ≤ 0.15%
  • Cr 16.00–18.00%
  • Ni 3.50–5.50%
  • Mn 5.50–7.50% → lower corrosion resistance than 304

Mechanical Properties (Cold Work Strengthening, No Heat Treatment):

• 304 (A2-70):
  • Tensile strength ≥ 700 MPa
  • Yield strength ≥ 450 MPa
  • Elongation ≥ 40%
  • Non-magnetic / slightly magnetic
  • Hardness: HB ≤ 201
• 304 (A2-80):
  • Tensile strength ≥ 800 MPa
  • Yield strength ≥ 600 MPa
• 316 (A4-70):
  • Tensile strength ≥ 700 MPa
  • Yield strength ≥ 450 MPa
  • Superior corrosion resistance compared to 304
• 316 (A4-80):
  • Tensile strength ≥ 800 MPa
  • Yield strength ≥ 600 MPa
• 201:
  • Tensile strength ≥ 650 MPa
  • Yield strength ≥ 350 MPa
  • Lower corrosion resistance, prone to rusting

Key Characteristics:

• Excellent corrosion resistance in moist, salt spray, and mild chemical environments
• Non-magnetic or weakly magnetic
• Good surface finish and aesthetics
• Excellent cold forming, machining, and welding performance
• 316 (Mo-containing): significantly better resistance to chloride corrosion than 304
• 304L / 316L: low carbon → no intergranular corrosion after welding
• 201: cost-effective alternative with reduced corrosion resistance

Typical Applications:

• 304 (A2-70): general corrosion-resistant environments (equipment assembly, decorative parts, machinery, food processing, medical devices)
• 304 (A2-80): higher strength requirements under moderate corrosion conditions
• 316 (A4-70): marine, chemical, coastal, food, and medical applications
• 316 (A4-80): high-strength + high-corrosion environments (marine engineering, advanced chemical equipment, nuclear auxiliary systems)
• 201: indoor, dry, low-cost applications; not recommended for outdoor or corrosive environments

2.1.3.2 Martensitic Stainless Steel

Typical Grades:
410, 420, 430 (semi-martensitic)

Chemical Composition (Typical):

• 410:
  • C 0.15–0.20%
  • Cr 11.50–13.50%
  • Ni ≤ 0.60%
  • Can be strengthened through heat treatment

Mechanical Properties:

• Annealed condition:
  • Tensile strength ≥ 540 MPa
  • Yield strength ≥ 345 MPa
• After quenching & tempering:
  • Tensile strength ≥ 700 MPa
  • Hardness: HRC 35–45

Key Characteristics:

• High hardness and good wear resistance
• Strength can be significantly improved by heat treatment
• Corrosion resistance: better than carbon steel, lower than austenitic stainless steel
• Magnetic
• متوسط machinability
• Lower cost than 304/316

Typical Applications:

• Applications requiring high hardness + basic corrosion resistance
• Valve components, tool-related fasteners
• Outdoor wear-resistant parts
• Medical instruments (e.g., surgical tool fasteners)
• High-strength fasteners in mildly corrosive environments

2.1.3.3 Ferritic Stainless Steel

Typical Grades:
430, 409L

Key Characteristics:

• Moderate corrosion resistance
• Better than carbon steel, lower than austenitic stainless steel
• Lower cost
• Magnetic
• Cannot be strengthened by heat treatment
• Relatively lower strength

Typical Applications:

• Low-cost, low-load, mildly corrosive environments
• Decorative fasteners
• Indoor applications in dry conditions
• Fasteners for household appliances and enclosures

2.2 Non-Ferrous Metal Materials

Non-ferrous metals (non-ferrous alloys) are primarily used in applications requiring lightweight performance, electrical/thermal conductivity, or specialized corrosion resistance. Although their usage volume is lower than steel, they are indispensable in industries such as aerospace, electronics, and high-end equipment manufacturing.

The core materials include copper alloys, aluminum alloys, and titanium alloys, supplemented by zinc alloys, magnesium alloys, and others.

2.2.1 Copper Alloys

Copper alloys offer excellent electrical and thermal conductivity, along with moderate corrosion resistance and strong decorative appeal. They are easy to process and are categorized into:

• Brass (most widely used)
• Pure copper (red copper)
• Bronze

Typical Grades:

• Brass: H62, H65, H70
• Pure copper: T2
• Bronze: QSn6.5-0.1

Mechanical Properties:

• H62 Brass:
  • Tensile strength ≥ 330 MPa
  • Yield strength ≥ 130 MPa
  • Elongation ≥ 30%
  • Hardness: HB ≤ 105
• T2 Copper:
  • Tensile strength ≥ 205 MPa
  • Yield strength ≥ 60 MPa
  • Elongation ≥ 35%
  • Electrical conductivity ≥ 98%
• QSn6.5-0.1 Bronze:
  • Tensile strength ≥ 450 MPa
  • Yield strength ≥ 180 MPa
  • Excellent wear resistance

Key Characteristics:

• Excellent electrical and thermal conductivity
• Brass (H62): low cost, good machinability, attractive appearance
• Pure copper (T2): conductivity, moderate corrosion resistance
• Bronze: superior wear and corrosion resistance
• Non-magnetic
• Distinct appearance: golden (brass), reddish (copper)

Typical Applications:

• Brass (H62, H65): electronics, electrical devices, instrumentation, plumbing, decorative fasteners (screws, nuts, rivets)
• T2 Copper: precision electronics, conductive fasteners (PCB screws, conductive rivets)
• Bronze (QSn6.5-0.1): wear-resistant fasteners in machinery (bearing bolts, gear rivets)

2.2.2 Aluminum Alloys

Aluminum alloys are valued for their lightweight nature (density ≈ 1/3 of steel), excellent corrosion resistance, and superior machinability. They can be strengthened through heat treatment and are widely used in weight-sensitive applications such as aerospace, rail transportation, and electronics.

Typical Grades:
6061, 6063, 7075 (high-strength), 5052

Mechanical Properties:

• 6061:
  • Tensile strength ≥ 310 MPa
  • Yield strength ≥ 276 MPa
  • Elongation ≥ 10%
  • Good corrosion resistance, heat-treatable
• 7075:
  • Tensile strength ≥ 570 MPa
  • Yield strength ≥ 503 MPa
  • Hardness: HB 150–190
  • High strength, suitable for heavy-load lightweight applications
• 5052:
  • Tensile strength ≥ 205 MPa
  • Yield strength ≥ 90 MPa
  • Excellent corrosion resistance

Key Characteristics:

• Lightweight with good corrosion resistance (natural oxide layer)
• Excellent machinability and formability
• Suitable for cold forming, machining, and welding
• 6061: best overall balance, most widely used
• 7075: very high strength (approaching medium carbon steel)
• Higher cost than carbon steel and brass, lower than titanium

Typical Applications:

• 6061 / 6063: aerospace interiors, rail systems, automotive lightweight components, electronics, renewable energy (e.g., solar mounting bolts)
• 7075: aerospace and high-end equipment (aircraft structural bolts, UAV fasteners)
• 5052: marine and humid environments (ship interior fasteners, outdoor lightweight equipment)

2.2.3 Titanium Alloys

Titanium alloys combine lightweight properties (density ~4.5 g/cm³) with high strength and exceptional corrosion resistance, outperforming stainless steel in harsh environments. They also offer good high-temperature performance, making them essential for high-end specialized applications.

Typical Grades:
TC4 (Ti-6Al-4V), TA2 (commercially pure titanium)

Mechanical Properties:

• TC4 (Ti-6Al-4V):
  • Tensile strength ≥ 895 MPa
  • Yield strength ≥ 825 MPa
  • Elongation ≥ 10%
  • High-temperature resistance (≤ 400°C)
  • Excellent corrosion resistance
• TA2:
  • Tensile strength ≥ 480 MPa
  • Yield strength ≥ 370 MPa
  • Outstanding corrosion resistance

Key Characteristics:

• High strength-to-weight ratio
• Exceptional corrosion resistance (resistant to strong acids, alkalis, seawater)
• Excellent fatigue and high-temperature performance
• Difficult to machine → requires specialized tooling and processes
• Very high cost (typically 5–10× stainless steel)

Typical Applications:

• Aerospace, nuclear power, deep-sea equipment
• Medical devices (implant-grade fasteners)
• High-end chemical processing equipment
• Examples: aircraft engine bolts, deep-sea probe fasteners, reactor fasteners, orthopedic implant screws

2.2.4 Other Non-Ferrous Metals

• Zinc Alloys
  • Typical grade: ZZnAl4
  • Low strength, moderate corrosion resistance, low cost
  • Applications: decorative fasteners, light-duty applications (furniture screws, toy fasteners)
• Magnesium Alloys
  • Even lighter than aluminum
  • Moderate strength, relatively poor corrosion resistance
  • Applications: ultra-lightweight fasteners in aerospace and high-end electronics (e.g., satellite components)

3. Non-Metallic Fastener Materials (Specialized Applications)

Non-metallic fastener materials are primarily used in special operating conditions such as electrical insulation, lightweight design, corrosion resistance, low-temperature environments, and non-magnetic requirements. They complement the limitations of metallic materials.

Core categories include engineering plastics, nylon, rubber, and composite materials. Although used in smaller volumes, they are highly application-specific.

3.1 Engineering Plastics

Engineering plastics offer high strength, excellent corrosion resistance, electrical insulation, and lightweight properties. They are typically manufactured via injection molding and widely used in electronics, electrical, and chemical industries.

Typical Materials: POM, PEEK, PC, ABS

• POM (Polyoxymethylene)
  • Tensile strength ≥ 60 MPa
  • Yield strength ≥ 40 MPa
  • Hardness: HB ≈ 120
  • Features: excellent wear resistance, corrosion resistance, and insulation
  • Applications: insulating fasteners in electronics, corrosion-resistant fasteners in chemical equipment, gear fixing screws
  • Limitation: not suitable for high temperatures (>100°C)
• PEEK (Polyether Ether Ketone)
  • Tensile strength ≥ 100 MPa
  • Yield strength ≥ 70 MPa
  • Features: outstanding high-temperature resistance (≤260°C), exceptional corrosion resistance, excellent insulation
  • Applications: aerospace, high-end electronics, and chemical environments (e.g., high-temperature insulating bolts)
  • Note: high cost
• PC (Polycarbonate)
  • Tensile strength ≥ 65 MPa
  • Yield strength ≥ 50 MPa
  • Features: high transparency, strong impact resistance, good insulation
  • Applications: decorative transparent fasteners, lighting fixtures, electronic insulation screws
• ABS
  • Tensile strength ≥ 40 MPa
  • Yield strength ≥ 25 MPa
  • Features: low cost, good processability, moderate insulation
  • Applications: consumer and light industrial fasteners (toys, appliance housings)

Internal Link Suggestion:

• “Injection Molding Process for Plastic Fasteners”
• “High-Temperature PEEK Fastener Applications”

3.2 Nylon (Polyamide)

Nylon is the most widely used non-metallic fastener material, offering a balance of insulation, corrosion resistance, low weight, and toughness.

Typical Grades: Nylon 6, Nylon 66, Nylon 1010

Mechanical Properties:

• Tensile strength ≥ 50 MPa
• Yield strength ≥ 30 MPa
• Elongation ≥ 200%

Key Characteristics:

• Excellent toughness and flexibility
• Good electrical insulation
• Resistant to weak acids, alkalis, and oils
• Cost-effective and easy to process

Typical Applications:

• Insulating fasteners in electronics
• Low-temperature applications
• Light-duty fastening (PCB screws, appliance insulation screws, plastic assemblies)
• Outdoor light corrosion environments

Note: Nylon 66 has higher strength than Nylon 6 and is suitable for slightly higher load applications.

3.3 Rubber

Rubber materials provide excellent elasticity, sealing performance, and vibration damping. They are typically used as supporting components (e.g., washers, seals) rather than standalone fasteners.

Typical Materials: NBR, Silicone Rubber (SR), Fluororubber (FKM)

• NBR (Nitrile Rubber)
  • Features: oil resistance, wear resistance, good elasticity
  • Applications: sealing washers for automotive and machinery fasteners
• Silicone Rubber (SR)
  • Temperature range: -60°C to 200°C
  • Features: excellent thermal stability and insulation
  • Applications: high-/low-temperature sealing washers in aerospace and electronics
• Fluororubber (FKM)
  • Temperature resistance up to 250°C
  • Features: exceptional chemical and corrosion resistance
  • Applications: chemical processing and high-temperature sealing components

3.4 Composite Materials

Composite materials (e.g., carbon fiber composites, glass fiber composites) offer ultra-lightweight, high strength, corrosion resistance, and non-magnetic properties.

Key Characteristics:

• Extremely high strength-to-weight ratio
• Excellent corrosion resistance
• Non-magnetic
• High cost and complex manufacturing

Typical Applications:

• Aerospace, high-end electronics, and new energy industries
• Extreme lightweight and high-performance requirements
• Examples: aircraft structural fasteners, satellite components, UAV fasteners

4. Fastener Property Classes and Material Mapping

The fastener property class is the key indicator of mechanical performance. The designation system consists of two numbers separated by a dot:

• The number before the dot represents 1/100 of the nominal tensile strength (Rm) in MPa
• The number after the dot represents 10× the ratio of yield strength (ReL) to tensile strength (Rm)

Example:

• Property class 8.8:
  • Nominal tensile strength: Rm = 800 MPa
  • Yield ratio: 0.8
  • Nominal yield strength: 800 × 0.8 = 640 MPa

This classification follows standards such as GB/T 3098.1-2010, and aligns with ISO 898 systems.

In practical applications, property classes are directly correlated with:

• Material selection (e.g., carbon steel, alloy steel, stainless steel)
• Heat treatment processes (e.g., quenching and tempering)
• Application scenarios (load level, environment, safety requirements)

Strength Grade	Material	Heat Treatment	Processing	Typical Applications
≤4.8	Low carbon steel (Q195, Q235, 1018, 1022)	Generally no heat treatment	Cold heading, tapping (nuts), rolled threads (bolts)	Low-load applications, such as civil construction, light equipment, furniture, simple mechanical connections
6.8	Low or medium carbon steel (35K, 35#)	No heat treatment / Normalizing (mild strengthening)	Cold heading, thread machining, partial light normalizing	Medium-load applications, e.g., general machinery, electrical equipment, non-critical parts of small engineering machinery
8.8	Medium carbon steel (45#, 1035), low alloy steel	Quenching + high-temperature tempering (tempering ≥425℃)	Cold heading, quench & temper, rolled threads	Higher-load applications, e.g., mechanical equipment, automotive components, steel structure connections
10.9	Low alloy steel (40Cr, 20MnTiB, 10B21, 1045)	Quenching + tempering (tempering ≥425℃)	Cold heading, quench & temper, precision thread machining	High-load, fatigue-resistant applications, e.g., automotive chassis, engines, core components of heavy machinery
12.9	Low alloy steel (35CrMo, 40Cr, SCM435)	Quenching + high-temperature tempering (≥425℃; some grades ≥380℃)	Cold heading, quench & temper, precision machining, surface anti-corrosion treatment	Ultra-high load and high-impact applications, e.g., construction machinery, aerospace components, high-pressure equipment
A2-70	304 Austenitic stainless steel	No heat treatment (strengthened by cold working)	Cold heading, cold working, thread rolling, pickling & passivation	General corrosion-resistant applications, e.g., equipment assembly, decorative, mechanical structures, ambient wet environments
A2-80	304 Austenitic stainless steel	No heat treatment (strengthened by heavy cold working)	Cold heading, heavy cold working, thread rolling, pickling & passivation	Higher-strength 304 fasteners, e.g., medium-duty corrosion-resistant equipment connections
A4-70	316 Austenitic stainless steel	No heat treatment (strengthened by cold working)	Cold heading, cold working, thread rolling, pickling & passivation	High corrosion-resistance applications, e.g., seawater, chemical, coastal equipment, food machinery, medical devices
A4-80	316 Austenitic stainless steel	No heat treatment (strengthened by heavy cold working)	Cold heading, heavy cold working, thread rolling, pickling & passivation	High-strength + high corrosion-resistance applications, e.g., marine engineering, high-end chemical equipment, harsh corrosive environments
No fixed grade (high-hardness & corrosion-resistant)	Martensitic stainless steel (410, 420)	Quenching + tempering (strengthening by heat treatment)	Cold heading, quench & temper, thread machining, surface anti-corrosion treatment	High-hardness + corrosion-resistant applications, e.g., valve bolts, tool fasteners, outdoor high-wear and corrosion-resistant parts

5. Common Issues with Fastener Materials and Mitigation Strategies

In industrial applications, fastener materials may face issues such as corrosion, insufficient strength, or processing defects. The following provides practical strategies to mitigate these problems.

5.1 Material Corrosion

Root Causes:

• Material selection incompatible with the operating environment (e.g., carbon steel in humid or saline environments).
• Inadequate surface treatment.
• Galvanic corrosion from contact between dissimilar metals.
• Damage to passive films (e.g., stainless steel).

Mitigation Strategies:

• Choose corrosion-resistant materials according to the environment (e.g., 316 stainless steel for coastal environments, PEEK for chemical exposure).
• Apply surface treatments to carbon steel or alloy steel (galvanizing, Dacromet, phosphating).
• Avoid direct contact between dissimilar metals; use insulating washers if necessary.
• Prevent surface scratches during stainless steel processing and regularly inspect the passivation layer.

5.2 Insufficient Material Strength

Root Causes:

• Improper material selection (e.g., using 4.8-grade carbon steel bolts for high loads).
• Improper heat treatment (e.g., insufficient tempering, incorrect quenching).
• Material defects (e.g., excessive impurities, internal cracks).

Mitigation Strategies:

• Select material and strength grade strictly according to load requirements (e.g., 10.9 or 12.9 alloy steel for high loads).
• Follow GB/T 3098.1 standards to regulate heat treatment, controlling quenching and tempering temperature and duration.
• Use high-quality materials; avoid fasteners produced from substandard or recycled scrap materials.

5.3 Processing Defects

Root Causes:

• Mismatch between material properties and processing method (e.g., cold heading high-carbon steel without prior annealing).
• Unreasonable processing parameters (e.g., excessive cold heading speed, large cutting depth).
• Excessive material impurities.

Mitigation Strategies:

• Select processing methods appropriate for the material (e.g., anneal high-carbon steel before cold heading).
• Optimize processing parameters, controlling cold heading speed, deformation, and cutting speed.
• Use high-purity materials to reduce impurities.

5.4 Hydrogen Embrittlement (“Silent Killer”)

Root Causes:

• High-strength fasteners (grade ≥8.8) may absorb hydrogen during acid pickling, electroplating, or other surface treatments, leading to embrittlement and potential fracture under high load conditions.

Mitigation Strategies:

• Perform hydrogen relief (baking) after surface treatment of high-strength fasteners (180–220°C for 2–4 hours).
• Prefer hydrogen-free plating processes (e.g., Dacromet).
• Avoid using untreated high-strength fasteners in high-impact or high-cycle fatigue applications.

5.5 Stainless Steel Galling (Cold Welding)

Root Causes:

• Thread friction and adhesion during tightening of austenitic stainless steel fasteners.
• Common causes: unlubricated threads, poor thread precision, excessive torque.

Mitigation Strategies:

• Apply thread lubricants (e.g., molybdenum disulfide grease).
• Improve thread precision to reduce friction.
• Control tightening torque; avoid over-tightening.

6. Summary of Fastener Material Selection

The core principle of fastener material selection is “match the operating conditions while balancing performance and cost.” The following guidelines apply for industrial applications:

1. Ordinary loads, low/no corrosion, low-cost requirements:
   • Prefer low carbon steel (Q195, Q235) or medium carbon steel (35#, 45#) with appropriate surface treatment; best cost-performance ratio.
2. High load, fatigue resistance, high-impact requirements:
   • Choose low alloy steel (40Cr, 35CrMo, 20MnTiB) with strength grades 8.8, 10.9, 12.9; follow proper quenching, tempering, and hydrogen relief procedures.
3. Corrosive environments (humid, saline, acidic/alkaline):
   • Prefer stainless steel (304, 316); for strong corrosion, use 316L or PEEK; marine environments favor 316 stainless steel.
4. Lightweight requirements (aerospace, electronics, rail transit):
   • Use aluminum alloys (6061, 7075); for extreme lightweight applications, consider titanium alloys or composite materials.
5. Insulation, low temperature, non-magnetic requirements:
   • Use nylon, engineering plastics (POM, PEEK), or rubber; electronic applications prioritize nylon or POM.
6. Extreme operating conditions (high temperature, high pressure, strong corrosion):
   • Use high-temperature alloys, titanium alloys, or PEEK; select precisely according to specific working conditions.

As manufacturing moves toward higher complexity and performance, fasteners are no longer “off-the-shelf” components. Fastener material selection now directly affects product reliability and service life. Proper selection, standardized processing, and strict material quality control are key to ensuring fastener safety and stability, supporting high-quality development in equipment manufacturing and engineering.