A strength grade consists of two numerical components, taking Grade 10.9 as an example:
1. The number before the decimal point: 10 → Tensile strength ≥ 1000 MPa
2. The number after the decimal point: 9 → Yield-to-tensile ratio of 0.9, yield strength ≥ 900 MPa
• Grade 4.8: Tensile strength ≥ 400 MPa, yield strength ≥ 320 MPa; for general fastening applications with limited load-bearing capacity
• Grade 8.8: Tensile strength ≥ 800 MPa, yield strength ≥ 640 MPa
• Grade 12.9: Tensile strength ≥ 1200 MPa, yield strength ≥ 1080 MPa; far superior to ordinary bolts in tensile resistance, fatigue resistance and fracture resistance
III. Material & Heat Treatment (Essential Differences)
• Ordinary Bolts (mainly Grade 4.8)
Mostly made of Q235 low-carbon steel, formed only by cold heading without quenching and tempering. They have average toughness and are prone to deformation or fracture under heavy loads and vibration.
• High-Strength Bolts (Grade 8.8 and above)
Fabricated from medium-carbon or alloy steels such as 45# steel, 40Cr and 35CrMo. They must undergo quenching and tempering to refine grain structure, achieving a balance of high strength and toughness. Grades 10.9 and 12.9 demand stricter material purity and heat treatment precision.
IV. Auxiliary Identification via Appearance, Details & Surface Treatment
1. Head Finish: High-strength bolt heads feature neat edges, uniform chamfers and minimal burrs; ordinary bolts have relatively rough workmanship.
2. Thread Precision: High-strength bolts adopt tighter thread tolerances for smooth engagement and reduced risk of thread stripping.
3. Surface Treatment: High-strength bolts often use high-end anti-corrosion finishes including Dacromet, hot-dip galvanizing and Teflon coating; ordinary bolts mostly apply basic electro-galvanizing or black oxide finishing.
4. Dimensional Specifications: For the same nominal diameter, high-strength bolts are designed with rational minimum cross-sections and effective cross-sectional areas, suitable for high preload conditions.
V. Application Scenarios & Installation Requirements
• Ordinary Bolts: Used for furniture, civilian equipment, non-load-bearing supports and temporary fastening; loaded via thread clamping with no strict preload requirements.
• High-Strength Bolts: Applied to steel structure bridges, wind power facilities, automotive chassis, engineering machinery, pressure vessels and flange connections. They must be tightened per specified torque/axial force and transmit load via friction. Substitution with ordinary bolts is strictly prohibited, as it may lead to severe safety accidents.
VI. Professional Verification (For Uncertain Cases)
1. Material Spectroscopy: Rapidly identifies alloy steel composition.
2. Hardness Testing:
◦ Grade 4.8: approx. 120–220 HB
◦ Grade 8.8: 250–320 HB
◦ Grade 10.9: 320–380 HB
◦ Grade 12.9: 390–440 HB
3. Tensile Testing: Measures tensile and yield strength to issue mechanical performance reports.
4. Document Verification: Reputable manufacturers provide tempering reports and material certificates with shipments.