If you look closely at any bolt head, you will find marked grades such as 4.8, 8.8, 10.9 and 12.9. These numbers represent the mechanical strength grade of fasteners according to ISO 898 and GB/T 3098 standards. Different grades mean different steel materials, heat treatment processes, load-bearing capacity and application environments. Choosing the correct bolt grade is essential for equipment safety, structural stability and cost control.
What Do Bolt Grade Numbers Mean?
The marking rule is universal in the fastener industry:
• The first number represents 1/100 of the minimum tensile strength (MPa).
• The second number represents the ratio of yield strength to tensile strength.
The higher the grade, the stronger the hardness, tension resistance and fatigue resistance.
1. Grade 4.8 (Low Strength / Civil Use)
Grade 4.8 bolts are made of low-carbon steel without quenching or tempering treatment. They are the most common low-cost fasteners for light-load applications.
• Tensile strength: ≥400 MPa
• Yield strength: ≥320 MPa
• Material: Q235 / low carbon steel
• Features: good toughness, low hardness, easy processing, economical price
• Application: furniture, shelves, electrical appliances, doors and windows, simple brackets, indoor ordinary fixing
2. Grade 8.8 (Medium Strength / General Industrial Use)
Grade 8.8 is the most widely used industrial bolt grade. Produced with medium carbon steel and complete quenching and tempering heat treatment, it balances strength, stability and cost.
• Tensile strength: ≥800 MPa
• Yield strength: ≥640 MPa
• Material: 45# carbon steel
• Features: stable performance, good wear resistance, suitable for most standard mechanical structures
• Application: general machinery, steel structures, construction parts, pipeline brackets, automotive non-key components
3. Grade 10.9 (High Strength / Heavy Industry Use)
Grade 10.9 belongs to high-strength alloy steel fasteners. With strict heat treatment, it provides excellent anti-vibration, anti-fatigue and anti-deformation performance under heavy loads.
• Tensile strength: ≥1000 MPa
• Yield strength: ≥900 MPa
• Material: alloy steel (40Cr, 35CrMo)
• Features: high hardness, strong tension resistance, long service life under continuous heavy load
• Application: engineering machinery, automobile chassis, railway equipment, large steel structures, pressure vessels
4. Grade 12.9 (Ultra-High Strength / Precision Heavy Load)
Grade 12.9 is the highest strength level among standard metric bolts. Made of high-grade alloy structural steel with precise temperature-controlled heat treatment, it offers extreme mechanical stability.
• Tensile strength: ≥1220 MPa
• Yield strength: ≥1080 MPa
• Material: high-quality chromium-molybdenum alloy steel
• Features: ultra-high hardness, impact resistance, fracture resistance, maximum safety factor
• Application: molds, hydraulic equipment, heavy engineering vehicles, high-vibration machinery, precision industrial equipment
Grade Comparison Overview
• 4.8: Low strength, light load, civil and general hardware
• 8.8: Medium strength, universal industrial standard
• 10.9: High strength, heavy machinery and structural engineering
• 12.9: Ultra-high strength, precision and extreme working conditions
Why Correct Grade Selection Is Important
Using low-grade bolts on heavy-load equipment will cause loosening, sliding thread or bolt breakage, leading to serious safety risks.
Using ultra-high-grade bolts for ordinary light-load structures will cause unnecessary cost waste.
Our Factory Strength
As a professional fastener manufacturer in China, we supply full-grade bolts from 4.8, 8.8, 10.9 to 12.9. All products pass strict hardness testing, tensile testing and dimension inspection. We support standard DIN/ISO/ANSI bolts and customized non-standard fasteners for global industrial projects.