Discs with round hole DIN 440 Form R-steel 100 HV plain 17,5

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.

The term plain indicates a completely untreated, merely oiled, smooth surface for all DIN and ISO screws such as wood screws, hexagon head screws, threaded rods, nuts and many types of washers.