How to Determine the "Strength Grade" of a High-Strength Bolt?
The strength grade of a high-strength bolt is typically indicated on the bolt head with a numerical designation (e.g., Grade 8.8, Grade 10.9). It serves as a core indicator of its mechanical properties. This number, separated by a decimal point, represents the bolt's "tensile strength" and the ratio of its yield strength to tensile strength (yield strength ratio), respectively. These numbers directly reflect the bolt's load-bearing capacity and material properties.
The specific meanings of 8.8 and 10.9
Using the common 8.8 and 10.9 as examples, their numerical meanings are broken down as follows:
The number before the decimal point represents the bolt's minimum tensile strength (σb), expressed in megapascals (MPa).
Grade 8.8: An "8" before the decimal point indicates a minimum tensile strength of 800 MPa (8 x 100) or greater.
Grade 10.9: A "10" before the decimal point indicates a minimum tensile strength of 1000 MPa (10 x 100) or greater.
Note: Tensile strength is the maximum stress a bolt can withstand before breaking. The higher the value, the greater the ultimate tensile force the bolt can withstand.
The number after the decimal point represents the bolt's yield strength ratio (the ratio of yield strength to tensile strength), which is calculated as "yield strength = tensile strength × this number ÷ 10."
Grade 8.8: The decimal point has an "8" and a yield strength ratio of 0.8, so its minimum yield strength is ≥800 MPa × 0.8 = 640 MPa.
Grade 10.9: The decimal point has a "9" and a yield strength ratio of 0.9, so its minimum yield strength is ≥1000 MPa × 0.9 = 900 MPa.
Note: The yield strength is the stress at which the bolt begins to deform plastically (permanently). The higher the value, the less likely the bolt is to deform under load, and the better its safety.
Additional Notes
High-strength bolt grades typically start at 8.8 (e.g., 8.8, 9.8, 10.9, 12.9, etc.), with grade 12.9 being the highest strength among the common grades (tensile strength ≥ 1200 MPa, yield strength ≥ 1080 MPa).
Grade numbers are not arbitrary; they are achieved through material selection (e.g., alloy steel) and heat treatment processes (quenching and tempering), and must be verified through rigorous mechanical property testing (tensile testing).
When selecting a grade, it's important to match it to the actual load requirements. For example, grade 10.9 is commonly used for heavy-duty applications like bridges and wind turbines, while grade 8.8 can be used for general industrial machinery to avoid either "overstrength" resulting in cost waste or "understrength" resulting in safety hazards.
The grade marking on the bolt head allows for quick identification of its load-bearing capacity, which is a crucial basis for project selection and quality inspection.
How to Determine the "Strength Grade" of a High-Strength Bolt?
The strength grade of a high-strength bolt is typically indicated on the bolt head with a numerical designation (e.g., Grade 8.8, Grade 10.9). It serves as a core indicator of its mechanical properties. This number, separated by a decimal point, represents the bolt's "tensile strength" and the ratio of its yield strength to tensile strength (yield strength ratio), respectively. These numbers directly reflect the bolt's load-bearing capacity and material properties.
The specific meanings of 8.8 and 10.9
Using the common 8.8 and 10.9 as examples, their numerical meanings are broken down as follows:
The number before the decimal point represents the bolt's minimum tensile strength (σb), expressed in megapascals (MPa).
Grade 8.8: An "8" before the decimal point indicates a minimum tensile strength of 800 MPa (8 x 100) or greater.
Grade 10.9: A "10" before the decimal point indicates a minimum tensile strength of 1000 MPa (10 x 100) or greater.
Note: Tensile strength is the maximum stress a bolt can withstand before breaking. The higher the value, the greater the ultimate tensile force the bolt can withstand.
The number after the decimal point represents the bolt's yield strength ratio (the ratio of yield strength to tensile strength), which is calculated as "yield strength = tensile strength × this number ÷ 10."
Grade 8.8: The decimal point has an "8" and a yield strength ratio of 0.8, so its minimum yield strength is ≥800 MPa × 0.8 = 640 MPa.
Grade 10.9: The decimal point has a "9" and a yield strength ratio of 0.9, so its minimum yield strength is ≥1000 MPa × 0.9 = 900 MPa.
Note: The yield strength is the stress at which the bolt begins to deform plastically (permanently). The higher the value, the less likely the bolt is to deform under load, and the better its safety.
Additional Notes
High-strength bolt grades typically start at 8.8 (e.g., 8.8, 9.8, 10.9, 12.9, etc.), with grade 12.9 being the highest strength among the common grades (tensile strength ≥ 1200 MPa, yield strength ≥ 1080 MPa).
Grade numbers are not arbitrary; they are achieved through material selection (e.g., alloy steel) and heat treatment processes (quenching and tempering), and must be verified through rigorous mechanical property testing (tensile testing).
When selecting a grade, it's important to match it to the actual load requirements. For example, grade 10.9 is commonly used for heavy-duty applications like bridges and wind turbines, while grade 8.8 can be used for general industrial machinery to avoid either "overstrength" resulting in cost waste or "understrength" resulting in safety hazards.
The grade marking on the bolt head allows for quick identification of its load-bearing capacity, which is a crucial basis for project selection and quality inspection.
