Oval-head drill screws inner square DIN 7504N-steel zinc plated blue 3,5x16mm

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.

Blue galvanised finishes are produced either by electroplating in accordance with DIN EN ISO 4042 or by mechanical plating in accordance with DIN EN ISO 12683. For fasteners with a hardness exceeding 390 HV, there is a risk of hydrogen embrittlement during galvanising (electrolytic process). This necessitates specific measures (such as annealing), although these do not provide a 100 per cent guarantee against embrittlement.

The internal square enables a good frictional connection and the screwdriver slips out of the screw head less easily than is the case with screws with a slot. The system was therefore more suitable for industrial use. A common application in Europe is self-drilling sheet metal screws for installing ventilation ducts.

Case hardening is used to impart significantly greater hardness to the surface layer of steel workpieces and to improve their mechanical properties. Case-hardened workpieces are characterised by increased wear resistance, a tough core and enhanced fatigue strength.