What are Bolt Grades
What are Bolt Grades
Bolt grades are a standardized way of categorizing bolts based on their mechanical properties, such as tensile strength, yield strength, and hardness. These grades ensure that bolts meet the specific demands of different applications. Bolt grades are typically determined by two main standards:
Metric System (ISO): Common worldwide, used for metric bolts.
Imperial System (SAE): Common in the United States and for imperial bolts.
ey Features
Strength: Bolt grades specify the tensile strength (the maximum force a bolt can endure before breaking) and yield strength (the force at which a bolt deforms permanently).
Material: Different grades correspond to specific materials, such as low-carbon steel, medium-carbon steel, or alloy steel.
Markings: Bolt heads are marked with grade identifiers, such as numbers or radial lines, which allow for quick identification.
Standards: Bolt grades are standardized by organizations like ISO (International Standards Organization) for metric bolts and SAE (Society of Automotive Engineers) for imperial bolts.
Common Applications
Low-Grade Bolts: For light-duty applications such as furniture assembly or household repairs (e.g., Metric 4.6, SAE Grade 2).
Medium-Grade Bolts: For automotive components and moderate-strength machinery (e.g., Metric 8.8, SAE Grade 5).
High-Grade Bolts: For heavy machinery, structural connections, and critical load-bearing applications (e.g., Metric 10.9, SAE Grade 8).
Understanding Metric Bolt Grades
Metric bolt grades provide a clear classification system for identifying the mechanical properties and strength of bolts. This system, standardized by the International Organization for Standardization (ISO), is widely used across industries globally. They are indicated by a two-part number system stamped on the bolt head, such as 4.6, 8.8, 10.9, or 12.9. These numbers denote the tensile strength and yield strength ratio of the bolt, which define its load-bearing capacity and elasticity.
This chart helps quickly match the appropriate bolt grade to your application based on strength and use case.
| Grade | Tensile Strength (MPa) | Yield Strength (MPa) | Yield Ratio (%) | Applications |
| 4.6 | 400 | 240 | 60% | Light-duty applications: furniture, light fixtures, general-purpose fastening. |
| 8.8 | 800 | 640 | 80% | Medium-strength applications: automotive, machinery, construction. |
| 10.9 | 1,000 | 900 | 90% | High-load environments: heavy machinery, structural connections. |
| 12.9 | 1,200 | 1,080 | 90% | Critical, high-stress tasks: aerospace, industrial equipment, engines. |
Understanding SAE Bolt Grades
SAE (Society of Automotive Engineers) bolt grades are widely used in the United States to classify bolts based on their material composition and mechanical properties. They use a marking system on the bolt head to indicate the strength and mechanical properties of the bolt. These markings are made up of radial lines, which correspond to specific grades and provide critical information about the bolt’s performance characteristics.
This chart provides a quick overview to help choose the right SAE bolt grade based on strength requirements and application needs.
| Grade | Material | Tensile Strength (PSI) | Yield Strength (PSI) | Identification Marking | Applications |
| Grade 2 | Low-carbon steel | ~74,000 | ~57,000 | No radial lines | Light-duty tasks: furniture, household repairs, non-critical applications. |
| Grade 5 | Medium-carbon steel, heat-treated | ~120,000 | ~92,000 | Three radial lines | Automotive components, moderate-strength machinery, and structural connections. |
| Grade 8 | Medium-carbon alloy steel, heat-treated | ~150,000 | ~130,000 | Six radial lines | Heavy machinery, industrial equipment, high-load, and high-stress environments. |