How to Read the Property Class Numbers

Carbon steel and alloy steel bolts follow a two-number code separated by a dot. The number to the left of the dot is one-hundredth of the nominal tensile strength in MPa. The number to the right is the ratio of yield strength to tensile strength, expressed as a percentage. So an 8.8 bolt has a nominal tensile strength of 8 x 100 = 800 MPa, and a yield ratio of 0.8, giving a yield strength of 800 x 0.8 = 640 MPa. A 10.9 bolt has 1000 MPa tensile and 900 MPa yield. A 12.9 bolt has 1200 MPa tensile and 1080 MPa yield.

This two-number system is defined in GB/T 3098.1-2010, which is identical to ISO 898-1. It applies to bolts, screws, and studs made from carbon steel or alloy steel, with nominal thread diameters from M1.6 up to M39. Above M39, the standard references a separate larger-size bolt specification because cross-section effects and manufacturing limits change the achievable properties. For stainless steel fasteners, the code works differently: A2-70 means austenitic stainless group A2 with a tensile strength of 700 MPa minimum.

8.8 / 10.9 / 12.9 Core Parameter Comparison

The jump from 8.8 to 10.9 is not a small step. It roughly doubles yield strength and moves the material from medium-carbon quenched-and-tempered steel to alloy steel (typically Cr-Mo or Cr-Mo-B). The 12.9 class pushes hardness above 380 HV, which makes the bolt susceptible to hydrogen embrittlement if electroplated, and it is less ductile than 10.9. Choosing 12.9 when 8.8 would do is not conservative; it introduces brittleness and hydrogen risk for no real benefit.

Typical Applications for Each Class

4.6 / 4.8 / 5.6 / 6.8 class is for light-duty non-structural joints: equipment covers, decorative trim, low-stress housing fasteners where the bolt size is chosen by installation convenience rather than calculated load. 8.8 class is the workhorse of mechanical engineering: structural frames, general machinery, pump and valve flange joints, and most equipment assembly. It is ductile, readily available, and tolerant of normal assembly variation. 10.9 class is the automotive standard for highly loaded joints: engine mounting, chassis components, wheel bolts, and brake system connections. It is usually used with controlled torque and friction specifications. 12.9 class is reserved for the highest load-to-weight ratio applications: high-performance engine connecting rod bolts, aerospace hardware, and specialty tools where space is at a premium and the joint must carry maximum load in a small envelope.

Choosing Between A2-70 and A4-70 Stainless

Stainless steel fasteners use a different coding system, defined in GB/T 3098.6 (modified adoption of ISO 3506-1). The letter number prefix (A1, A2, A3, A4, A5) identifies the austenitic stainless group, and the trailing number is one-tenth of the minimum tensile strength. A2-70 means A2 (304-type) stainless with 700 MPa minimum tensile. A4-70 means A4 (316-type) stainless with 700 MPa minimum tensile. The key difference is molybdenum: A4 contains roughly 2% Mo, which dramatically improves pitting corrosion resistance in chloride environments. If the joint sees coastal atmosphere, road salt, or chemical process fluids, A4 is worth the premium. For indoor, dry, or mildly corrosive environments, A2 is usually sufficient. A4-80 (cold-worked 316, 800 MPa tensile) is used for higher-strength corrosion-resistant joints, while C1-50 (410 martensitic, 500 MPa) offers wear and heat resistance with only moderate corrosion protection.

Nuts and Washers Must Match

A bolt class is only half the joint. The nut must be strong enough to develop the bolt's full strength without stripping internally. GB/T 3098.2 specifies nut property classes that pair with bolt classes. An 8.8 bolt uses a class 8 nut; a 10.9 bolt uses a class 10 nut; a 12.9 bolt uses a class 12 nut. The nut class number indicates the maximum bolt property class it can support. Using a class 6 nut with a 10.9 bolt means the nut thread strips before the bolt reaches its design preload. For stainless, A2-70 bolts pair with A2-70 nuts, and mixing a carbon steel nut with a stainless bolt creates galvanic corrosion risk.

Selection Decisions and Common Pitfalls

A practical decision flow: first, calculate or estimate the required clamping force from the joint load and safety factor. Second, pick the smallest bolt diameter that provides that clamping force at a reasonable torque. Third, choose the property class that delivers the required yield strength without overshooting. Fourth, consider the environment: if corrosion is a factor, evaluate stainless or coated carbon steel rather than upsizing the class. Fifth, verify that the nut, washer, and surface treatment are compatible with the chosen class.

Common pitfalls include: using 12.9 bolts on electroplated hardware without bake-out for hydrogen embrittlement; mixing stainless bolts with plain carbon steel nuts; choosing 8.8 when the application is vibrational and needs 10.9 for fatigue margin; and ignoring that higher class bolts are less ductile and more notch-sensitive. When in doubt, consult the VDI 2230 calculation or ask your fastener supplier to run the joint analysis.