Steel Structure Bolt Grades: 4.6, 8.8, 10.9, 12.9 Explained
Bolt grade numbers follow ISO 898-1 standard. The format X.Y means: X × 100 = ultimate tensile strength (MPa), and Y/10 × X × 100 = yield strength (MPa). For example, Grade 8.8: ultimate tensile = 800 MPa, yield strength = 0.8 × 800 = 640 MPa.
Understanding the Bolt Grade Numbering System
Bolt grade numbers follow ISO 898-1 standard. The format X.Y means: X × 100 = ultimate tensile strength (MPa), and Y/10 × X × 100 = yield strength (MPa). For example, Grade 8.8: ultimate tensile = 800 MPa, yield strength = 0.8 × 800 = 640 MPa.
Grade 4.6: Low Strength General Purpose
Grade 4.6 bolts have ultimate tensile strength of 400 MPa and yield strength of 240 MPa. Made from low/medium carbon steel, they are used in non-critical applications: light fixtures, signage mounting, wooden structure connections, and temporary fixings. Not suitable for structural steel connections.
Grade 8.8: The Workhorse Structural Bolt
Grade 8.8 is the most widely used structural bolt. With 800 MPa tensile and 640 MPa yield strength, they handle most steel structure connections. Made from medium carbon steel, quenched and tempered. Standard in building frames, bridges, industrial platforms, and pre-engineered buildings. Hot-dip galvanized 8.8 bolts are common for outdoor applications.
Grade 10.9: High Strength for Critical Connections
Grade 10.9 bolts deliver 1040 MPa tensile and 940 MPa yield strength. Made from alloy steel (typically boron steel or chromium-molybdenum steel), quenched and tempered. Used in: heavily loaded structural connections, pre-stressed concrete anchoring, crane rails, and heavy machinery mounting.
Grade 12.9: The Highest Standard Grade
Grade 12.9 is the highest standard ISO bolt grade, offering 1220 MPa tensile and 1100 MPa yield strength. Made from alloy steel with careful heat treatment. Used in: high-stress engine components, die casting equipment, hydraulic systems, and aerospace ground support equipment. Not typically used in building structures.
Selection Guide & CoreMetal Supply
When selecting bolt grades, consider: applied loads, corrosion environment, temperature range, and whether the connection is bearing-type or slip-critical. CoreMetal supplies high-strength bolts in grades 8.8, 10.9, and 12.9 in sizes M12-M36, with hot-dip galvanizing and zinc flake coating options. Contact Tracy for structural bolt specifications and pricing.
About Xi’an Coremetal Steel Co., Ltd.
CoreMetal is a leading Chinese steel supplier specializing in carbon steel, stainless steel, galvanized products, aluminum, and specialty alloys. With ISO 9001 certification and export experience to 60+ countries, we provide competitive pricing and reliable delivery for projects of all sizes. Contact Tracy at tracy@coremetalsteel.com or +86 18291910632 for inquiries.