🔩 Bolt Strength Classes 4.6–12.9

Mechanical properties, tightening torques, head markings — per ČSN EN ISO 898-1 / EC3

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Mechanical Properties (ISO 898-1)
Grade fub (MPa)
Tensile		 fyb (MPa)
Yield / proof		 fyb/fub Tensile strength
visual		 αv
EC3 §3.6		 Typical material Typical use
4.6 4002400.60
400
 0.60 Low carbon steel Light structures, non-critical
4.8 4003200.80
400
 0.60 Low carbon steel, cold-formed General fastening
5.6 5003000.60
500
 0.60 Medium carbon steel Structural (mild)
5.8 5004000.80
500
 0.60 Low/medium carbon steel, cold-formed Machine parts
6.8 6004800.80
600
 0.60 Medium carbon steel Machine components
8.8 8006400.80
800
 0.60 Medium carbon or alloy steel, Q+T Standard structural, machinery
10.9 10009000.90
1000
 0.50 Alloy steel, Q+T High-load structural, preloaded
12.9 120010800.90
1200
 0.50 Alloy steel, Q+T (high quality) Critical joints, automotive, tooling
αv (EC3 Table 3.4): Shear coefficient for bolt shear resistance. αv = 0.60 for grades 4.6–8.8 (shear plane through threaded part). αv = 0.50 for grades 10.9 and 12.9. Formula: Fv,Rd = αv · fub · A / γM2
EC3 Design Parameters (γM2 = 1.25)
Grade fub (MPa) αv Fv,Rd/A (MPa)
Threaded, 1 shear plane		 Fv,Rd M10 (kN)
As = 58 mm²		 Fv,Rd M12 (kN)
As = 84.3 mm²		 Fv,Rd M16 (kN)
As = 157 mm²		 Fv,Rd M20 (kN)
As = 245 mm²
4.6 4000.60192 11.116.230.147.0
4.8 4000.60192 11.116.230.147.0
5.6 5000.60240 13.920.237.758.8
5.8 5000.60240 13.920.237.758.8
6.8 6000.60288 16.724.345.270.6
8.8 8000.60384 22.332.460.394.1
10.9 10000.50400 23.233.762.898.0
12.9 12000.50480 27.840.575.4117.6
Tightening Torques MA (Nm) — dry, μ ≈ 0.14
Values per VDI 2230 / ISO 16047 for hexagon head bolts, coefficient of friction μ ≈ 0.14 (dry, uncoated). Target: 90% of proof load. Reduce by ~20% for lubricated / zinc-plated bolts. Metric coarse thread.
Size d0 (mm) As (mm²) 4.6 5.6 8.8 10.9 12.9
M448.781.21.52.84.04.8
M5514.22.33.05.57.99.5
M6620.13.95.29.51416
M8836.69.513233339
M101058.01925466578
M121284.3334380110135
M14141155269125175210
M161615780105195275330
M1818192110145270380450
M2020245155205385540650
M2222303210280520730880
M24243532653506559201100
M272745939052096013501600
M3030561530705130018502200
Bolt Head Markings (ISO 898-1)

Grading system explained

The grade designation X.Y encodes two values:

  • X × 100 = nominal tensile strength fub in MPa
    (e.g. 8 × 100 = 800 MPa)
  • X × Y × 10 = nominal yield strength fyb in MPa
    (e.g. 8 × 8 × 10 = 640 MPa)

Material & surface notes

  • 4.6–6.8: plain low/medium carbon steel
  • 8.8: medium carbon or alloy steel, Q+T
  • 10.9, 12.9: alloy steel, quenched & tempered
  • 12.9 is susceptible to hydrogen embrittlement — avoid electroplating
  • Hot-dip galvanizing: suitable up to 10.9
  • Stainless A2/A4: ~70/80 equivalent (≠ ISO 898 series)

Preloaded connections (EC3 §3.9)

  • Preload force: Fp,Cd = 0.7 · fub · As
  • Only grades 8.8 and 10.9 allowed for preloaded joints per EC3
  • Requires controlled tightening (torque wrench or turn-of-nut)
  • Category B (slip-resistant at SLS) / Category C (slip-resistant at ULS)

Common standards

  • ISO 4014 / ČSN EN ISO 4014 — hex bolts, part thread
  • ISO 4017 — hex bolts, full thread
  • ISO 4032 — hex nuts
  • ISO 4762 — socket head cap screws
  • ISO 898-1 — property classes (bolts)
  • ISO 898-2 — property classes (nuts)
  • EN 1993-1-8 (EC3) — structural design of joints