Hex socket set screws and spigot ISO 4029-45H tempered black M4x25 mm

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.

Compared with a Phillips-head screw of the same size, a hex socket screw can withstand around ten times the torque. Hex socket screws are well suited to bolted joints where the screw heads are difficult to access due to the design of the structure. Hex socket screws can be countersunk to save space if protruding screw heads are a problem.

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 45H means 450 HV (Vickers hardness).