Class 8.8 bolts are high-strength fasteners defined by ISO 898-1, with a minimum tensile strength of 800 MPa and yield strength of 640 MPa. Made from quenched and tempered medium carbon steel, they are the workhorse of structural and mechanical connections.
What are 8.8 Bolts?
8.8 bolts refer to bolts with a property class of 8.8 conforming to ISO 898-1 or similar standards (e.g., China’s GB/T 3098.1, Germany’s DIN 933/931).
“8.8” is a property class marking, not a specific material grade. It specifies the minimum mechanical properties the bolt must achieve but allows some flexibility in chemical composition.
Meaning of "8.8"
This is a code:
The first digit “8”: Indicates the bolt’s nominal tensile strength (σb) is 8 x 100 = 800 MPa.
The second digit “.8”: Indicates the ratio (yield ratio) of the bolt’s nominal yield strength (σs) to its tensile strength is 0.8.
Therefore, nominal yield strength = 800 MPa x 0.8 = 640 MPa.
Core Requirement: The bolt’s actual measured tensile strength must be ≥ 800 MPa, and its yield strength must be ≥ 640 MPa.
8.8 Bolts Typical Material
While the standard doesn’t prescribe a single grade, to achieve the Class 8.8 requirements cost-effectively, manufacturers typically use the following materials, subject to a crucial heat treatment:
Common Steels: Medium carbon steel (e.g., Chinese grades 35, 45; European grades C35, C45).
Core Process: Quenching and Tempering.
Trempe (water or oil quenching) creates a high-hardness martensitic structure.
Followed by medium/high-temperature tempering to achieve a well-balanced, strong, and tough tempered sorbite structure.
Appearance: Typically black oxidized (blued). This is a high-temperature oxidation surface treatment primarily for rust prevention and aesthetics, with minimal dimensional change.
8.8 Bolts Mechanical Properties (per ISO 898-1)
| Propriété | Exigence |
| Nominal Tensile Strength (σb) | 800 MPa |
| Minimum Tensile Strength | ≥ 800 MPa |
| Nominal Yield Strength (σs) | 640 MPa |
| Minimum Yield Strength | ≥ 640 MPa |
| Dureté Vickers (HV) | 250 – 320 HV |
| Dureté Rockwell (HRC) | Approx. 22 – 32 HRC |
| Elongation after Fracture (A) | ≥ 12% |
| Reduction of Area (Z) | ≥ 48% |
8.8 Bolts Application Scenarios
Class 8.8 bolts are one of the most widely used high-strength bolts due to their excellent strength, toughness, and cost-effectiveness.
- Steel Structure Connections: Main structural bolted connections in buildings, bridges, stadiums, etc.
- Machinery & Equipment: Critical connection points in heavy machine tools, mining machinery, construction machinery (e.g., excavators, cranes).
- Industrie automobile : Key stressed areas like chassis, suspension, and engine mounts.
- General Industry & Maintenance: Virtually any application requiring reliable, removable fastening.
Characteristics Summary
| Avantages | Disadvantages / Considerations |
| ✅ High Strength: Far greater load-bearing capacity than low-carbon steel bolts (e.g., Class 4.8). | ❌ Poor High-Temperature Resistance: Continuous service temperature should not exceed 250°C. Strength degrades rapidly above 300°C. |
| ✅ Good Strength-Toughness Balance: The quenching and tempering process provides both strength and a degree of toughness. | ❌ Moderate Corrosion Resistance: Black oxide offers limited rust protection. Use galvanized or other coatings for humid/corrosive environments. |
| ✅ Cost-Effective: Mature material and process technology offer good value. | ❌ Susceptible to Hydrogen Embrittlement: If electroplated (e.g., zinc-plated), proper dehydrogenation baking is essential to prevent brittle fracture. |
| ✅ Highly Standardized: Easy to source and replace globally. | ❌ Not Weldable: Welding destroys the heat-treated microstructure, drastically reducing performance. |
Common FAQs of 8.8 Bolts
Yes. In general terminology, Class 8.8 and above (10.9, 12.9) are typically called "high-strength bolts." Classes like 4.8, 5.8 are considered medium or low strength.
- Primary difference is strength: Class 10.9 (1000/900 MPa), Class 12.9 (1200/1080 MPa) are stronger.
- Material differs: Class 10.9 often uses low/medium-carbon alloy steel (e.g., 20MnTiB), Class 12.9 uses higher-grade alloy steel (e.g., 35CrMo) for greater hardenability.
- Generally decreasing toughness: Higher strength often comes with increased notch sensitivity and relatively lower toughness.
- Selection Principle: Stronger isn't always better. Choose the class matching the design load, safety factor, and connected material thickness.
Black oxide 8.8 bolts have weak rust protection and are unsuitable for long-term outdoor/damp use. Choose hot-dip galvanized ou Dacromet-coated Class 8.8 bolts for significantly improved corrosion resistance.
- Look for the head marking: The bolt head is usually raised or indented with "8.8".
- Observe the appearance: The surface is typically a uniform black or blue-black color (black oxide).
Strongly discouraged! Must use a nut of matching property class. The nut for an 8.8 bolt should be Class 8 (marked "8"). A weaker nut can strip its threads, causing the high-strength bolt to fail. The proper pairing is Class 8.8 bolt with Class 8 nut.
Very difficult and not recommended. Its hardness will rapidly wear down standard drill bits. If absolutely necessary, use a carbide drill bit at low speed with ample cooling.
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