Bolt performance grade is a universal national standard for identifying the mechanical properties of bolt steel. Common grades of metric high-strength bolts include Grade 4.8, Grade 8.8, Grade 10.9 and Grade 12.9. A performance grade consists of two sets of figures (such as 8.8, 10.9 and 12.9). In professional terms, the first figure represents the percentage value of the bolt's nominal tensile strength, while the second figure represents the yield ratio (the ratio of yield strength to tensile strength), rather than directly indicating the yield strength.
The following shows the standard mechanical parameters and application scenarios of common bolt performance grades:
1. Grade 4.8 Bolt: It has a minimum tensile strength of 400N/mm² and a minimum yield strength of 320N/mm². Made of ordinary low-carbon steel, this bolt features good toughness and low cost. It is suitable for conventional connection scenarios with low stress, no vibration and no heavy load, and is widely used in common hardware, civil equipment and light assembly structures.
2. Grade 8.8 Bolt: It has a minimum tensile strength of 800N/mm² and a minimum yield strength of 640N/mm². As a medium and high-strength bolt treated by quenching and tempering heat treatment, it delivers excellent comprehensive mechanical properties. It is the most widely used bolt grade in the industry, applicable to the connection of various mechanical equipment, building structures and conventional engineering machinery.
3. Grade 10.9 Bolt: It has a minimum tensile strength of 1000N/mm² and a minimum yield strength of 900N/mm². This is a high-strength bolt processed with strict quenching and tempering treatment, featuring high bearing capacity and good fatigue resistance. It is mainly used for key load-bearing connections of large machinery, bridges, steel structure projects and important equipment.
4. Grade 12.9 Bolt: It has a minimum tensile strength of 1200N/mm² and a minimum yield strength of 1080N/mm². It is the highest conventional strength grade of bolts on the market. With premium material and high precision, it boasts strong resistance to high pressure and impact. It is suitable for harsh working conditions such as high pressure, heavy load, high-strength vibration, precision machinery and high-end industrial equipment.
Selection Notes: When selecting bolt performance grades, it is not sufficient to only refer to tensile strength and yield strength. It is necessary to comprehensively consider the tightening torque, fatigue resistance, wear resistance, corrosion resistance, operating temperature and vibration conditions. Different service environments, load conditions and structural importance require matching bolt strength grades. Selection shall be strictly based on actual working conditions to ensure the safety and stability of structural connections.
