Catalogue data and standard-part designations in this guide follow the cBallast C-Parts range, supplied through Bufab (Värnamo, Sweden). Values are nominal reference data; confirm against the controlling standard revision before procurement.Property classes of metric fasteners with the numbers behind the marking: nominal and minimum tensile strength, yield or 0.2 % proof stress, proof stress, Vickers hardness range and typical material for classes 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9 and 12.9 (ISO 898-1); matching nut classes 4 to 12 and thin-nut classes 04/05 (ISO 898-2); stainless steel groups A1–A5 with property classes 50, 70 and 80 (ISO 3506-1); how to read the head marking; and a proof-load table for 8.8, 10.9 and 12.9 from M5 to M36.
Catalogue data and standard-part designations in this guide follow the cBallast C-Parts range, supplied through Bufab (Värnamo, Sweden). Values are nominal reference data; confirm against the controlling standard revision before procurement.
The two figures in a metric property class encode the mechanical properties: the first figure × 100 is the nominal tensile strength in MPa, and the two figures multiplied together × 10 is the nominal yield (or 0.2 % proof) stress. A class 8.8 bolt therefore has a nominal tensile strength of 800 MPa and a yield of 640 MPa; class 10.9 is 1000 / 900 MPa. ISO 898-1 sets minimum values slightly above nominal for most classes and, for 8.8, distinguishes threads up to M16 from larger sizes. Nuts are classed by the highest bolt class they can be loaded with (ISO 898-2), and stainless fasteners use a different scheme entirely (ISO 3506-1: steel group A2/A4 plus property class 50/70/80).
| Class | Rm nominal / min (MPa) | Yield ReL or Rp0.2 min (MPa) | Proof stress Sp (MPa) | Hardness HV (min–max) | Typical material / heat treatment |
|---|---|---|---|---|---|
| 4.6 | 400 / 400 | 240 | 225 | 120 – 220 | Low- or medium-carbon steel, not heat treated |
| 4.8 | 400 / 420 | 340 (Rp0.2) | 310 | 130 – 220 | Low- or medium-carbon steel, cold worked |
| 5.6 | 500 / 500 | 300 | 280 | 155 – 220 | Medium-carbon steel |
| 5.8 | 500 / 520 | 420 (Rp0.2) | 380 | 160 – 220 | Low- or medium-carbon steel, cold worked |
| 6.8 | 600 / 600 | 480 (Rp0.2) | 440 | 190 – 250 | Medium-carbon steel, cold worked |
| 8.8 (≤ M16) | 800 / 800 | 640 | 580 | 250 – 320 | Medium-carbon or boron steel, quenched and tempered |
| 8.8 (> M16) | 800 / 830 | 660 | 600 | 255 – 335 | Medium-carbon, boron or alloy steel, quenched and tempered |
| 9.8 | 900 / 900 | 720 | 650 | 290 – 360 | Medium-carbon or boron steel, quenched and tempered |
| 10.9 | 1000 / 1040 | 940 | 830 | 320 – 380 | Alloy or boron steel, quenched and tempered |
| 12.9 | 1200 / 1220 | 1100 | 970 | 385 – 435 | Alloy steel, quenched and tempered |
Proof load = proof stress Sp × tensile stress area As. The bolt must carry this load without permanent set. Values below are rounded from ISO 898-1 (Sp 580 MPa for 8.8 up to M16, 600 MPa above M16; 830 MPa for 10.9; 970 MPa for 12.9). M5 8.8: 8.2 kN · 10.9: 11.8 · 12.9: 13.8. M6: 11.7 / 16.7 / 19.5. M8: 21.2 / 30.4 / 35.5. M10: 33.6 / 48.1 / 56.3. M12: 48.9 / 70.0 / 81.8. M14: 66.7 / 95.5 / 112. M16: 91.1 / 130 / 152. M18: 115 / 159 / 186. M20: 147 / 203 / 238. M22: 182 / 251 / 294. M24: 212 / 293 / 342. M27: 275 / 381 / 445. M30: 337 / 466 / 544. M36: 490 / 678 / 792. Tensile stress areas As used: M5 14.2, M6 20.1, M8 36.6, M10 58.0, M12 84.3, M14 115, M16 157, M18 192, M20 245, M22 303, M24 353, M27 459, M30 561, M36 817 mm².
Designation = steel group + property class, e.g. A2-70 or A4-80. Groups: A1 (free-machining austenitic, lower corrosion resistance), A2 (18/8 austenitic, EN 1.4301/1.4307 type — general atmospheric and food service), A3 (stabilised A2), A4 (austenitic with 2–3 % molybdenum, EN 1.4401/1.4404 type — chloride, marine and process service), A5 (stabilised A4). Property classes: 50 = soft/annealed, Rm ≥ 500 MPa, Rp0.2 ≥ 210 MPa; 70 = cold worked, Rm ≥ 700, Rp0.2 ≥ 450 (standard for ≤ M20; above M20 and for fine threads the class 50 values apply unless agreed); 80 = high-strength cold worked, Rm ≥ 800, Rp0.2 ≥ 600 (≤ M20). Proprietary grades such as BUMAX 88 / 109 reach 8.8- and 10.9-equivalent strength in A4 material. The 2020 edition of ISO 3506-1 also introduced duplex (D grades) and martensitic/ferritic groups; specify the edition when it matters.
Class 8.8 is the general-purpose structural and machine-building bolt in Europe and pairs with class 8 nuts. Class 10.9 gives about 45 % more preload for the same size and is the default for highly loaded, fatigue-critical joints (engines, gearboxes, HV-type structural connections), at the price of stricter corrosion-protection choices because of hydrogen-embrittlement risk above 320 HV. Class 12.9 is reserved for socket head cap screws and special joints; it should not be hot-dip galvanised or electroplated without a qualified process, and its lower ductility makes it unsuitable for joints with impact loading. Below 8.8 (4.6, 4.8, 5.8) is for non-structural, shear-only or low-load applications and for parts that must yield rather than fracture.
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