Letter code for material quality bolts, nuts, etc.
07 May 2026
With standard fasteners, a strength class designation such as 8.8 or 10.9 primarily indicates:
- Tensile strength (Rm)
- Yield strength (Rp0.2%)
- Properties at room temperature
This is usually sufficient for applications up to 100°C.
However, when used at higher temperatures, the behaviour of steel changes rapidly. In such cases, other material properties play a greater role, such as:
- creep resistance | slow deformation under prolonged loading at high temperatures
- heat resistance | retention of strength at, for example, 500–650 °C
- oxidation resistance | resistance to scaling and burning
- relaxation behaviour | loss of preload in bolted joints due to temperature
For this reason, when used at high temperatures, the designation ‘10.9’ alone, for example, is not sufficient. A standard 10.9 bolt can, at high temperatures:
rapidly lose strength,
creep,
lose its preload,
and eventually break.
For this reason, special alloys are used at high temperatures. These are often CrMo steels, CrMoV steels or heat-resistant (stainless steel) grades.
If there is insufficient space to mark the material number, these fasteners are marked with a letter referring to the material type.
For example:
- V = Material 1.4923
The material coding below is not officially standardised but is – in most cases – used worldwide.
| Letter | Material number | Material type |
| . | ||
| C | 1.0503 | C45 |
| D | 1.4462 | Alloy 225, Superduplex, AISI 318 |
| F | 1.4571 | AISI 316Ti |
| G | 1.7258 | 24CrMo5 |
| GA | 1.7709 | 21CrMoV5-7 |
| GB | 1.7711 | 40CrMoV4-6 |
| GC | 1.7225 | 42CrMo4 |
| H | 1.7709 | 21CrMo45-7 |
| KA | 1.7219 | 26CrMo4 |
| KB | 1.5680 | X12Ni5 |
| KG | 1.7218 | 25CrMo4 |
| L | 1.4301 | AISI 304 |
| M | 1.4404 | AISI 316L |
| P | 1.4903 | X10CrMoVNb9-1 |
| S | 1.4986 | X7CrNiMoBNb16-16 |
| SB | 2.4952 | NiCr20TiAl |
| SD | 1.4980 | A-286/Alloy 660 |
| SM | 1.4547 | F44, 254SMO |
| T | 1.7711 | CrMoV6-9 |
| V | 1.4923 | X22CrMoV12 |
| VW | 1.4913 | X19 |
| W | 1.4980 | Alloy 800H/HT |
| YB | 1.5511 | 35B2 |
| Z | 1.7709 | 21CrMoV5-7 |
| . |
! There is a possibility that a manufacturer may use a different letter code to mark its products. It is therefore important not to rely solely on the letter indicated on the material. You should also consult the manufacturer’s documentation, material certificates in accordance with EN 10204, etc., to ensure that the letter marked on the material corresponds to the grade ordered.
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Despite the care taken in compiling the above article, we cannot guarantee its accuracy.
