What Grade Bolt is a Socket Head Cap Screw

Introduction to Socket Head Cap Screws and Bolt Grades

Socket head cap screws, commonly referred to as Allen bolts or hex socket screws, are among the most versatile and widely used fasteners in industrial applications. Their distinctive cylindrical head with an internal hexagonal drive provides excellent torque transmission and a low-profile design, making them ideal for applications where space is limited.

One of the most critical aspects of selecting the right socket head cap screw is understanding its bolt grade, which indicates its mechanical properties, including tensile strength, yield strength, and hardness. This article provides an in-depth analysis of socket head cap screw grades, their material compositions, performance characteristics, and suitable applications.

Understanding Bolt Grades for Socket Head Cap Screws

Bolt grades are standardized classifications that define a fastener's strength and material properties. The most common grading systems include:

• Metric grades (e.g., 8.8, 10.9, 12.9)

• SAE grades (e.g., Grade 2, Grade 5, Grade 8)

• ASTM standards (e.g., ASTM A574 for socket head cap screws)

1. Metric Bolt Grades for Socket Head Cap Screws

Metric socket head cap screws are typically graded according to ISO 898-1 or DIN 912 standards. The most common grades include:

Grade 8.8

• Tensile Strength: 800 MPa (~116,000 psi)

• Yield Strength: 640 MPa (~93,000 psi)

• Material: Medium carbon steel, quenched & tempered

• Applications: General machinery, automotive components, construction

Grade 10.9

• Tensile Strength: 1000 MPa (~145,000 psi)

• Yield Strength: 900 MPa (~130,000 psi)

• Material: Alloy steel, heat-treated for high strength

• Applications: Heavy-duty machinery, structural engineering, high-stress environments

Grade 12.9

• Tensile Strength: 1200 MPa (~174,000 psi)

• Yield Strength: 1080 MPa (~156,000 psi)

• Material: Alloy steel with additional heat treatment

• Applications: Aerospace, high-performance automotive, precision equipment

2. SAE Bolt Grades for Socket Head Cap Screws

In the U.S., socket head cap screws often follow SAE (Society of Automotive Engineers) grading:

Grade 2

• Tensile Strength: ~74,000 psi

• Material: Low or medium carbon steel

• Applications: Light-duty fastening, non-critical assemblies

Grade 5

• Tensile Strength: 120,000 psi

• Material: Medium carbon steel, heat-treated

• Applications: Automotive, machinery, general engineering

Grade 8

• Tensile Strength: 150,000 psi

• Material: Medium carbon alloy steel, heat-treated

• Applications: High-stress applications, heavy machinery, military equipment

3. ASTM Standards for Socket Head Cap Screws

ASTM A574 is the most common standard governing socket head cap screws, specifying high-strength alloy steel fasteners with tensile strengths up to 180,000 psi.

Material Composition and Heat Treatment

The bolt grade of a socket head cap screw is heavily influenced by its material and heat treatment process:

• Carbon Steel (Grades 8.8, Grade 5): Affordable and widely used, but prone to corrosion without coating.

• Alloy Steel (Grades 10.9, 12.9, Grade 8): Enhanced strength due to chromium, molybdenum, or vanadium additives.

• Stainless Steel (A2/A4): Corrosion-resistant but lower strength (~80,000 psi for A2-70).

• Titanium: Lightweight, high strength-to-weight ratio, used in aerospace and medical applications.

Heat Treatment Processes:

• Quenching & Tempering: Increases hardness and strength (used for Grades 10.9, 12.9, and Grade 8).

• Case Hardening: Surface hardening for wear resistance (used in some specialty screws).

How to Identify the Grade of a Socket Head Cap Screw?

Most socket head cap screws have markings on the head indicating their grade:

• Metric Grades: Number markings (e.g., "8.8," "10.9").

• SAE Grades: Radial lines (Grade 5 = 3 lines, Grade 8 = 6 lines).

• Stainless Steel: "A2" or "A4" markings.

If unmarked, the screw is likely a low-grade fastener (e.g., Grade 2 or equivalent).

Applications Based on Bolt Grade

Factors to Consider When Choosing a Socket Head Cap Screw Grade

1. Load Requirements: High-stress applications require Grade 10.9 or 12.9.

2. Environmental Conditions: Stainless steel or coated screws for corrosive environments.

3. Weight Considerations: Titanium for lightweight applications.

4. Cost vs. Performance: Grade 8.8 offers a good balance for general use.

Conclusion

The bolt grade of a socket head cap screw determines its strength, durability, and suitability for different applications. Whether you need a standard Grade 8.8 for general machinery or a high-performance Grade 12.9 for aerospace, selecting the right grade ensures optimal performance and safety.

For specialized applications, consult a fastener expert to choose the best socket head cap screw grade for your needs.