High-strength fasteners are widely used in critical connections such as wind turbine towers, bridge steel structures, construction machinery and automotive chassis. Their grade markings directly indicate mechanical performance limits. Based on GB/T 3098.1 (equivalent to ISO 898-1), this article systematically explains the marking conventions, core mechanical properties and material correspondences for Grades 8.8, 10.9 and 12.9, helping procurement teams and engineers make precise selections.
A fastener property class designation consists of two numbers separated by a decimal point. The first number multiplied by 100 gives the minimum nominal tensile strength (σb) in MPa. The second number represents 10 times the yield-to-tensile ratio. For Grade 8.8: the first "8" indicates tensile strength ≥800 MPa, and the second "8" means a yield ratio of 0.8, giving yield strength ≥640 MPa. Similarly, Grade 10.9 offers tensile strength ≥1000 MPa and yield strength ≥900 MPa; Grade 12.9 reaches tensile strength ≥1200 MPa and yield strength ≥1080 MPa.
Grade 8.8 fasteners are typically made from 35K, 45 steel or ML35 medium carbon steel, quenched and tempered to HRC 22-32, suitable for medium-load general machinery connections. Grade 10.9 requires alloy structural steels such as SCM435 (chromium-molybdenum), 40Cr or 35CrMo, achieving HRC 32-39 after quenching and high-temperature tempering — the mainstream choice for wind power and construction machinery. Grade 12.9 uses premium alloys like SCM440 or 42CrMo, reaching HRC 39-44 with extreme tensile strength, but is hydrogen-embrittlement sensitive and requires mandatory de-embrittlement after electroplating (200°C for 4+ hours).
When selecting, also evaluate proof stress (Sp), impact energy (Akv) and delayed fracture sensitivity. For Grade 10.9 and above, conventional electro-galvanizing is not recommended — Dacromet, mechanical galvanizing or phosphating are preferred to reduce hydrogen embrittlement risk. During inspection, verify head markings (e.g., "10.9" stamp), material test certificates (MTC) and third-party test reports covering wedge load, proof load and hardness tests. For low-temperature environments below -40°C, additional V-notch impact testing is required to ensure sufficient low-temperature toughness.