Are you confused by the different bolt grades? Don’t worry, I’m Monica Song, have 16 Years experience in fasteners and today I’m going to break down the different bolt grades for you. If you have questions about SAE, metric, or ASTM standards, I’m going to answer them right here.
Bolts are classified by their strength and material properties. Common SAE grades are 2 (low strength), 5 (medium strength), and 8 (high strength). Metric grades include 5.8, 8.8, 10.9, and 12.9, with increasing strength. ASTM grades like A307, A325, and A490 also exist, suited for various applications.
Intrigued? Let’s dive deeper into the world of bolt grades and discover their unique characteristics and applications.
What are Bolt Grades, Tensile Strength, and Yield Strength?
Bolt grades describe the material and mechanical properties of bolts, particularly their tensile and yield strength. These grades help in choosing the right bolt for the required strength, durability, and environmental conditions. Tensile Strength: The maximum load a bolt can handle without breaking. For example, a Grade 8 bolt has a higher tensile strength than a Grade 2 bolt, making it suitable for more demanding applications. Yield Strength: The load at which a bolt begins to deform plastically. After this point, the bolt will not return to its original shape. HYou want high yield strength if you need a bolt to maintain its form under heavy loads. Understanding these properties ensures that you select the correct bolt grade for your application, ensuring safety and performance.What are the Different Bolt Grade Standards?
In the realm of bolts, three primary classification systems exist: SAE (Society of Automotive Engineers), Metric, and ASTM (American Society for Testing and Materials). Each system has its distinct marking and usage criteria.SAE Grades
The SAE grading system primarily applies to imperial bolts and uses radial lines on the bolt head to denote different strength levels.-
- SAE Grade 2: Made of low carbon steel, these bolts are suitable for general purposes with a tensile strength of approximately 60,000 psi (about 414 MPa). They have no head markings and are perfect for light-duty applications like household fixtures, furniture assembly, and non-critical automotive and agricultural equipment parts.
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- SAE Grade 5: Composed of medium carbon steel, quenched, and tempered, these bolts boast a tensile strength of about 105,000 psi (around 724 MPa). Marked with three radial lines, they are ideal for medium-strength requirements such as automotive chassis, agricultural equipment, and machinery.
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- SAE Grade 8: These high-strength alloy steel bolts, also quenched and tempered, feature a tensile strength of around 150,000 psi (approximately 1034 MPa). Marked with six radial lines, they are designed for high-stress environments like engine parts, suspension systems, and heavy-duty structural connections.
Metric Grades
Metric bolts are graded by their material strength and are often marked directly on the bolt head with numbers such as “8.8”, “10.9”, and “12.9”.-
- Class 8.8: Made from medium carbon steel and heat-treated to achieve a tensile strength of 830 MPa, these bolts are suitable for general strength applications. They are marked with “8.8” and commonly used in general machinery, automotive parts, and light steel structures.
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- Class 10.9: High-strength alloy steel bolts with a tensile strength of 1040 MPa. Marked with “10.9”, they are used in high-strength applications such as structural connections, heavy machinery, and automotive components requiring high strength.
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- Class 12.9: The highest strength bolts made from alloy steel, boasting a tensile strength of 1220 MPa. Marked with “12.9”, they are suitable for extremely high-strength requirements like aerospace, high-performance racing, and heavy engineering machinery.
ASTM Grades
ASTM standards provide comprehensive specifications and performance requirements for bolts, using a combination of letters and numbers to denote different grades.-
- A307: These low carbon steel bolts, with a tensile strength of around 60,000 psi (about 414 MPa), are suitable for general-purpose low-stress applications. Marked with “A307”, they are commonly used in general structural connections, light steel structures, and furniture manufacturing.
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- A325: High-strength bolts with a tensile strength of 120,000 psi (approximately 827 MPa), marked with “A325”. They are ideal for high-stress structural applications like bridges, high-rise buildings, and heavy machinery.
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- A490: These alloy steel bolts have a tensile strength of 150,000 psi (around 1034 MPa) and are marked with “A490”. They are used in ultra-high-stress applications such as skyscrapers, large-span bridges, and critical aerospace structures.