Countersunk screws hexagon socket ISO 10642-10.9 tempered black M12x140 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.

Due to their head geometry, screws with reduced load-bearing capacity may not achieve the minimum breaking force for the strength classes as specified in ISO 898-1 and ISO 3506-1. For standardised screws with reduced load-bearing capacity, the minimum breaking forces are 80 per cent of those for screws with full load-bearing capacity. They are now marked with the additional number 0 next to the strength class.