볼트 강도 등급 (4.6–12.9)

The designation is a coded system per ISO 898-1:

• First number × 100 = nominal tensile strength R_m in MPa
• First × second × 10 = nominal yield strength R_p0.2 in MPa

For class 8.8: R_m = 8 × 100 = 800 MPa, R_p0.2 = 8 × 8 × 10 = 640 MPa.

The second digit thus represents the yield-to-tensile ratio (0.8 = 80%).

R_p0.2 (yield strength) is the stress causing 0.2% permanent elongation — determined by tensile testing.

S_p (proof stress) is a lower, guaranteed minimum stress the bolt must withstand with zero permanent set — used for proof load testing per ISO 898-1.

Typically S_p ≈ 90–95% of R_p0.2. The proof load F_p = S_p × A_s is the key parameter for bolt specification.

8.8 is the most common class for general engineering:

• Good strength (R_m = 800 MPa) with adequate ductility
• Suitable for most machine frames, flanges, brackets
• Cost-effective, readily available

10.9 is used when higher clamp force is needed:

• Automotive engine/chassis (OEM standard)
• Structural steel connections (EN 14399)
• High-vibration environments requiring higher preload
• Space-constrained joints (smaller bolt, same force)

12.9 is for the most demanding applications where maximum preload per bolt is critical, e.g. precision machinery, motorsport.

Per ISO 898-1, hex bolts must be permanently marked with:

• Property class — e.g. "8.8", "10.9", "12.9"
• Manufacturer’s ID — symbol or code

Marking location: top or side of hex head. Socket head cap screws: side of head or in the recess.

Unmarked bolts should be assumed ≤ class 4.6 and must not be used in structural applications.

Proof load F_p = S_p × A_s

• S_p = proof stress (MPa) from ISO 898-1 for the chosen property class
• A_s = tensile stress area (mm²) for the thread size per ISO 898-1
• Result is in Newtons (÷ 1000 for kN)

The proof load test per ISO 898-1 applies this force axially for 15 seconds. The bolt must not show permanent elongation.