Steel construction screws with nuts DIN 7990-4.6 hot-dip galvanized M20x140 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.

In hot-dip galvanising, a zinc coating is applied to the component by immersing it in molten zinc and then spinning it off. DIN EN ISO 10684 restricts its application to steel fasteners with standard threads ranging from M8 to M64, as well as strength classes up to 10.9 for bolts and 12 for nuts. The average coating thickness for fasteners is 50 micrometres, which ensures corrosion protection for many years.

The wrench sizes for hexagon bolts and nuts are specified in ISO 272: 1982-01. The nominal wrench size on the workpiece (e.g. bolt, nut) is the same as that on the tool. However, there is some play between the two to allow the tool to be fitted without difficulty. The relationship between the nominal diameter d of the thread and the wrench size s is standardised in EN ISO 4014 and EN ISO 4017 for hexagon bolts, and in EN ISO 4032 for hexagon nuts (dimensions in mm).

The number before the point multiplied by one hundred gives the nominal value of the tensile strength in N/mm2, the number before the point multiplied by the number after the point and multiplied by 10 gives the nominal value of the lower yield strength in N/mm2. The number after the decimal point provides information about the yield strength, as a percentage of the tensile strength.