Flat discs, normal row ISO 7091-steel 100 HV zinc plated blue 5

The combination of strength classes for flat washers with those for bolts and nuts is governed by ISO 898-3:2018, Table 1. Washers with a hardness of 100 HV are suitable for applications with unhardened bolts up to strength class 5.8 and nuts up to class 5. Applications involving bolts of strength class 6.8 and nuts of class 6 must be checked during installation. Combinations with higher strength classes must not be used.

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.