Grade 8.8 Hexagon Head Bolts' Material and Quality Requirements

Grade 8.8 is one of the most common strength classes for hexagon head bolts, specifying a defined set of mechanical properties for the fastener, regardless of its specific steel composition . The requirements are standardized internationally, primarily by ISO 898-1.

Here are the key material and quality requirements for Grade 8.8 hexagon head bolts:

1. Material and Manufacturing

· Common Materials: The bolts are typically made from medium carbon steel (e.g., 10B21, 45# steel) or alloy steel, which are heat-treated (quenched and tempered) to achieve the required strength .
· Mandatory Heat Treatment: Heat treatment is essential for all Grade 8.8 bolts. This process is what allows standard carbon or alloy steels to meet the high mechanical property benchmarks .

2. Core Mechanical Properties (ISO 898-1)

The grade designation "8.8" itself defines the key mechanical properties:

· First Digit (8): Indicates the minimum tensile strength (Rₘ) is 800 MPa (8 x 100 MPa).
· Second Digit (0.8): Indicates the yield strength (RₑL or Rₚ0.₂) is 80% of the tensile strength, which is 640 MPa (800 MPa x 0.8) .

The following table summarizes all critical mechanical property requirements per ISO 898-1 for Grade 8.8 bolts:

Mechanical Properties of Property Class 8.8 

Property Minimum Requirement Unit/Note
Tensile Strength, Rₘ 800 MPa
Yield Strength, RₑL / Rₚ0.₂ 640 MPa
Proof Load Stress, Sₚ 580 MPa (Test load stress)
Elongation after Fracture, A 12 %
Hardness (Vickers), HV 250 - 320 -
Hardness (Rockwell), HRC 22 - 32 -
Impact Strength, Kᵥ (for d≥16 mm) 27 (at -20°C) Joules

3. Quality & Performance Requirements

· Dimensional Standards: The bolts must conform to dimensional standards such as ISO4014 (full thread) / ISO 4017 (partial thread) or their national equivalents (e.g., GB/T 5783 in China, DIN 933/931 in Germany) .
· Surface Integrity: Must be free from defects detrimental to performance according to standards like ISO 6157-1 .
· Decarburization: There are strict limits on thread decarburization to prevent weakening .
· Surface Treatment: Common corrosion protection treatments include zinc plating, black oxide (blackened), or zinc-nickel plating for higher corrosion resistance . For example, a zinc-nickel coating can offer up to 720 hours of protection against base metal corrosion in a salt spray test .
· Matching Nuts: It is strongly recommended to use nuts of property class 8 or higher to ensure that the bolt fails before the thread strips .

4. Design Resistance Values (for structural calculations)

For structural design per European standards (e.g., EN 1993-1-8), the following characteristic strengths are used for Grade 8.8 bolts:

· Ultimate Tensile Strength (fᵤ₆): 800 MPa
· Shear Resistance Factor (αᵥ): 0.6 (This factor is used in the formula to calculate the bolt's shear resistance) .

Key Standards Summary

Category Standard Title / Scope
Mechanical Properties ISO 898-1 Mechanical properties of fasteners made of carbon steel and alloy steel. The core standard defining Grade 8.8.
Geometry & Dimensions ISO 4014 Hexagon head bolts with full thread.
 ISO 4017 Hexagon head bolts with partial thread.
 GB/T 5783 Chinese national standard for hexagon head bolts (full thread).
Testing ISO 898-7 Torsional test for bolts (determines fracture torque, M_B).

In summary: A Grade 8.8 hexagon head bolt must be made from heat-treated carbon or alloy steel, possess a minimum tensile strength of 800 MPa and a minimum yield strength of 640 MPa, and conform to strict dimensional, hardness, and quality standards as defined by ISO 898-1 and related product geometry standards.