For applications with the highest demands:
1.6587 (18CrNiMo7-6)
Case-hardening steels are used whenever components require external strength with good internal toughness at the same time. This difference in hardness is achieved by case hardening, which is also used, for example, with 16MnCr5.
The advantage of 1.6587 is that it can be both core and edge hardened. This means that it can be used in a wide variety of components, e.g. in gears or as toothed racks.