Grade 8.8 high-strength hex head bolts are quenched and tempered fasteners manufactured from medium carbon steel or alloy steel, designed for structural and mechanical joints requiring a balance of high tensile strength, fatigue resistance, and cost efficiency. With a minimum tensile strength of 800 MPa and a minimum yield strength of 640 MPa, these bolts deliver approximately twice the load-bearing capacity of standard Grade 4.8 fasteners, making them a standard choice for machinery, automotive, and general engineering applications. The black oxide surface finish provides a clean, oil-retaining appearance suitable for both domestic and export markets. Available in M8 and M10 as ready stock, with a full range from M5 to M39 on request. Key Specifications Parameter Specification Strength Grade 8.8 Material Medium carbon steel / alloy steel (quenched & tempered) Standard GB/T 5783 / ISO 4017 / DIN 933 Thread Metric coarse (M), full thread Surface Finish Black oxide Thread Diameter M5 – M39 (M8, M10 in stock) Length Range 10 mm – 200 mm (custom lengths available) Head Type Hexagon head Mechanical Properties (ISO 898-1 / GB/T 3098.1) Property M5 – M16 M18 – M39 Minimum Tensile Strength (Rm) 800 MPa 830 MPa Minimum Yield Strength (Rp0.2) 640 MPa 660 MPa Minimum Proof Load Stress (Sp) 580 MPa 600 MPa Core Hardness 22–32 HRC 23–34 HRC Elongation After Fracture (A min) 12% 12% Material Condition Quenched and tempered Quenched and tempered These values correspond to the ISO 898-1 requirements for property class 8.8 fasteners. Grade 8.8 denotes a nominal tensile strength of 800 MPa and a yield strength ratio of 0.8 (hence 8.8). The "8" before the decimal indicates one-tenth of the minimum tensile strength in MPa, while the "8" after the decimal indicates 80% of the tensile strength as the minimum yield strength. Material & Heat Treatment Grade 8.8 bolts are manufactured from medium carbon steel (such as 35# or 45#) or alloy steel (such as 40Cr or SCM435). The material undergoes quenching and tempering — heating to austenitizing temperature, rapid quenching to form martensite, and controlled tempering to achieve the required combination of strength, ductility, and toughness. This heat treatment distinguishes Grade 8.8 from lower grades and is essential for meeting the mechanical property requirements of ISO 898-1. 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 including 8.8), and are dimensionally equivalent to ISO 4017 and DIN 933. The 8.8 grade marking is stamped on the bolt head for identification and traceability. Surface Treatment The standard black oxide finish offers mild corrosion protection and a clean, oil-retaining surface appearance. Black oxide is commonly applied to quenched and tempered fasteners for a uniform, professional finish. For enhanced corrosion resistance, alternative finishes such as zinc plating, zinc-nickel coating, or Dacromet are available upon request. Note: For electroplated high-strength bolts, dehydrogenation baking within 24 hours of plating is recommended to prevent hydrogen embrittlement. Applications Grade 8.8 hex bolts are a standard choice for applications requiring a balance of high strength and cost efficiency: Machinery and equipment assembly Automotive chassis and engine mounts Structural steel connections (non-preloaded) Heavy-duty truck and trailer fastening Agricultural and construction equipment Industrial plant maintenance and repair Available Sizes Thread (M) Common Length Range Hex Across-Flats (s) M8 16 – 80 mm 13 mm M10 20 – 100 mm 16 mm M12 25 – 120 mm 18 mm M16 30 – 150 mm 24 mm M20 40 – 200 mm 30 mm Full range available from M5 through M39. M8 and M10 are held in ready stock for immediate shipment. Custom lengths and thread pitches available on request. Ordering Information Thread sizes: M5–M39 standard; M8, M10 in stock Lengths: 10–200 mm standard; custom lengths available Finishes: Black oxide (standard); zinc plated, zinc-nickel, Dacromet on request Packaging: Bulk carton, small box, or customized export packing Documentation: Mill test certificates and material traceability available MOQ: Flexible for wholesale and distributor orders Please inquire for specific size combinations, volume pricing, and lead times.