according to ISO 3506

ISO-designated steel groups

1) Stabilized against intergranular corrosion through addition of titanum, possibly niobium, tantalum.
2) Low carbon austenitic stainless steels with carbon content not exceeding 0,03 % may additionally be marked with an «L», e.g. A4L-80.


Descriptions using a letter / figure combination mean the following:

ISO designated steel groups

The designation of the steel grade (first block) consists of one of the letters:

  • A for austenitic steel
  • C for martensitic steel
  • F for ferritic steel


A2-70 indicates: austenitic steel, cold worked, min. 700 N/mm2 tensile strength
C4-70 indicates: martensitic steel, hardened and tempered, min. 700 N/mm2 tensile strength
The designation of the property class consists of two digits representing 1/10 of the tensile strength of the fasteners respectively 1/10 of the proof load of the nuts.

If fastener elements are classified over the hardness, the hardness class is given according to Vickers by 2 digits standing for 1/10 of the minimum hardness value. The letter H refers to the hardness.

Designation example of a minimum hardness 250 HV: A4 25 H, austenitic steel, work hardened

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