Shank screws with slot and conical head DIN 427 / ISO 2342-14H tempered black M8x25 mm

Slotted screw drives are the oldest form of rotary drives for fastening screws. The Schltz is usually manufactured in a pressed or milled version. Because of its uneconomical processing method (slow, unwieldy), the slot is increasingly disappearing in favor of other drives (hexagon socket, cross slot, hexagon socket).

Hardness classes are specified for set screws and similar fasteners that are not subject to tensile stress, which are regulated for stainless austenitic steels in ISO 3506-3 and for carbon steel and alloy steel in ISO 898-5. ISO 3506-3 defines the two hardness classes 12H and 21H, ISO 898-5 defines the four hardness classes 14H, 22H, 33H and 45H. The hardness class designation 14H means 140 HV (Vickers hardness).

Steel is an iron-carbon alloy with a maximum carbon content of 2% by mass. The steels achieve their intended physical and mechanical properties through various hardening, annealing and tempering processes. It is also possible to harden the edge zone by surface hardening, case hardening or nitriding. Well-known steel grades are structural steels, free-machining steels, case-hardening steels, nitriding steels, tempering steels, tool steels and stainless steels.

The tempering process of screws and nuts (heating - holding - quenching - tempering) results in a thin baked oxide layer, which is referred to as tempered black. This surface condition is not a surface coating and therefore offers no corrosion protection.