The performance level of a bolt is typically represented by two numbers, such as 8.8 or 10.9. The first number indicates 1/100 of the tensile strength (Rb), while the second number represents 10 times the yield ratio. For example, in an 8.8 bolt, the tensile strength is 800 MPa, and the yield strength is 640 MPa (80% of the tensile strength). Common performance levels for bolts and screws include 3.6, 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9, and 12.9. These grades are usually made from medium to low carbon alloy steels.
When marking bolts, the performance level is often placed after the thread size. For instance, GB5785 specifies that for bolts with a diameter less than 39 mm, only 8.8 and 10.9 grades are used, and the 8.8 grade can be omitted in the marking. Similarly, the h9 tolerance can sometimes be omitted, such as in GB27 M12@80.
Hexagon flange bolts (GB/T 16674-1996) come in two types: F-type, which has a flat bearing surface, and U-type, which includes a slotting groove. If a manufacturer offers both options, the marking may include "F" or "U" to specify the type. For example, a bolt marked as GB/T 16674 M12@80-F would indicate an F-type bolt.
In special cases, such as when a thin rod is required, the marking might include "R" to denote a reduced shank. For example, a bolt could be labeled as GB/T 16674 M12@80-R (F-type) or GB/T 16674 M12@80-RF (both F and U types).
Cross recessed countersunk screws (GB/T 81912-1997) are divided into two series: Series 1 (deep) with a step under the head, and Series 2 (shallow) without a step. The mechanical grade is typically 8.8. If a specific series is required, it will be noted in the marking, such as GB/T 81912 M5@20-H1 for Series 1.
Slotted taper end set screws (GB 71-85) have performance grades based on Vickers hardness, such as 14H, 22H, 33H, and 45H. The lowest hardness value is shown as 1/10 of the actual value. These screws are commonly graded as 14H or 22H, with 22H being the most common. Surface treatments like oxidation or galvanization may be included, such as GB 71 M5@12-22H.
Slotted cylindrical end set screws (GB 829-88) also use Vickers hardness grading, including 14H and 33H. Their markings might include surface treatment details, such as GB 829 M5@10-ZnD, where "ZnD" denotes zinc passivation.
Studs are classified into Type A (with a chamfered end) and Type B (with a twisted end). They have performance levels such as 4.8, 5.8, 6.8, 8.8, 10.9, and 12.9. The 4.8 grade can be omitted in some cases. When the screw-in end requires an interference fit, the marking might include "YM," and the length is calculated accordingly.
Nuts are selected based on the performance grade of the bolt or stud they are paired with. Coarse nuts (GB 6170-86) have three grades: 6, 8, and 10, with 10 sometimes omitted. Thin nuts (GB 9459-88) have grades 0.4 and 0.5, representing 1/10 of the guaranteed stress. Markings like GB 6170 M12-8 or GB 9459 M10@1-05 show these specifications.
Flat washers (GB 9711-85) are marked with their performance level in Vickers hardness, such as 140HV. Open washers (GB 851-88) come in two types, A and B, with B having a shallow groove. They are marked according to size and material, such as GB 851 16-50 for a 16 mm washer with a 50 mm outer diameter.
Threaded fasteners are often drawn in simplified form, omitting small features like chamfers and fillets. This helps in making technical drawings clearer and more efficient.
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