Grade 12.9 high-strength hex head bolts are quenched and tempered alloy steel fasteners engineered for critical structural and mechanical joints requiring maximum load-bearing capacity. With a minimum tensile strength of 1,220 MPa and a minimum yield strength of 1,100 MPa, these bolts deliver approximately three times the strength of standard Grade 4.8 fasteners, making them suitable for applications where joint integrity is essential. The black oxide surface finish provides a clean, oil-retaining appearance for both domestic and export markets. Key Specifications Parameter Specification Strength Grade 12.9 Material Alloy steel (42CrMo / SCM435 / 35CrMo) Standard GB/T 5783 / ISO 4017 / DIN 933 Thread Metric coarse (M), full thread Surface Finish Black oxide Thread Diameter M6 – M64 (M64–M100 available on request) Length Range 8 mm – 600 mm (extended up to 1,000 mm) Head Type Hexagon head Mechanical Properties (ISO 898-1 / GB/T 3098.1) Property Value Minimum Tensile Strength (Rm) 1,220 MPa Nominal Tensile Strength 1,200 MPa Minimum Yield Strength (Rp0.2) 1,100 MPa Minimum Proof Load Stress (Sp) 970 MPa Core Hardness 39–44 HRC Elongation After Fracture (A min) 8% Material Condition Quenched and tempered These values correspond to the ISO 898-1 requirements for property class 12.9 fasteners, which specify a minimum tensile strength of 1,220 MPa and a minimum 0.2% yield strength of 1,100 MPa for sizes up to M39. Material & Heat Treatment Grade 12.9 bolts are typically manufactured from quenched and tempered alloy steels such as 42CrMo, SCM435, or 35CrMo. For SCM435 steel, a representative optimized heat treatment process consists of quenching at 880 °C for 30 minutes followed by tempering at 500 °C for 60 minutes, which achieves the strength and plasticity requirements of Grade 12.9 bolts. Standards & Compliance These bolts are produced in accordance with GB/T 5783 (Chinese national standard for fully threaded hex head bolts, covering M1.6–M64 and property classes up to 12.9), and are dimensionally equivalent to ISO 4017 and DIN 933. The 12.9 grade marking is stamped on the bolt head for identification and traceability. Applications Grade 12.9 hex bolts are specified where high clamping force and fatigue resistance are required. Typical industries include: Structural steel and bridge construction Railway and high-speed rail equipment Mining and heavy machinery Automotive engine and chassis fastening Wind power and photovoltaic mounting systems Petroleum and chemical equipment Shipbuilding and offshore engineering Surface Treatment The standard black oxide finish offers mild corrosion protection and a clean, oil-retaining appearance. For enhanced corrosion resistance, alternative finishes such as zinc plating, nickel plating, or Dacromet are available upon request. Due to the high hardness of Grade 12.9 bolts (39–44 HRC), hydrogen embrittlement risk must be managed — if any electroplating is applied, post-bake dehydrogenation within 24 hours is strongly recommended.