What Do 4.8 and 8.8 on Bolts Mean?
The markings on bolts, such as 4.8 and 8.8, denote the performance grades of the bolts used in steel structure connections. These grades are categorized into over ten levels including 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, and 12.9. Among these, bolts graded 8.8 and above are made from low-carbon alloy steel or medium-carbon steel that has undergone heat treatment (quenching and tempering), commonly referred to as high-strength bolts. The rest are generally known as regular bolts.
The performance grade marking consists of two sets of numbers representing the nominal tensile strength value and the yield-to-tensile ratio of the bolt material. For example:
- A bolt with a performance grade of 4.8 signifies:
- Nominal tensile strength of the bolt material reaches 400 MPa;
- Yield-to-tensile ratio of the bolt material is 0.8;
- Therefore, the nominal yield strength of the bolt material is 400 × 0.8 = 320 MPa.
- A high-strength bolt with a performance grade of 10.9, after heat treatment, can achieve:
- Nominal tensile strength of the bolt material reaches 1000 MPa;
- Yield-to-tensile ratio of the bolt material is 0.9;
- Thus, the nominal yield strength of the bolt material is 1000 × 0.9 = 900 MPa.
The performance grade of bolts is an internationally recognized standard, ensuring that bolts of the same grade have identical performance regardless of their material or origin, allowing for design selection based solely on performance grade.
Regarding the strength grade, it should be noted that the so-called 8.8 and 10.9 grades do not refer to shear stress levels of 8.8 GPa and 10.9 GPa, but rather:
- For an 8.8 grade, the nominal tensile strength is 800 N/mm², and the nominal yield strength is 640 N/mm².
Bolt strength is typically indicated by "X.Y", where X100 equals the tensile strength of the bolt, and X100*(Y/10) equals the yield strength of the bolt (as per the marking convention: yield strength/tensile strength = Y/10).
For instance, for a grade 4.8 bolt:
- The tensile strength would be 400 MPa;
- The yield strength would be 400 * 8 / 10 = 320 MPa.
Additionally, stainless steel bolts are often marked as A4-70 or A2-70, which have different meanings specific to the standards governing stainless steel fasteners.
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