What counts least for the screw gauge

Strengths (standardized in EN-ISO 898-1 EN 20898-2)

The strength of stainless fasteners is given in a combination of letters and numbers. This information is stamped in the screw head or in the nut.

Example: A2-70

The letter indicates the material group (A = austenitic steel; C = martensitic steel; F = ferritic steel)

The number after the letter is a key to chemical composition

1 = free-cutting steel with added sulfur
2 = cold heading steel alloyed with chrome and nickel
3 = cold heading steel alloyed with chrome and nickel, stabilized with Ti, Nb, Ta
4 = cold heading steel alloyed with chrome, nickel and molybdenum
5 = cold heading steel alloyed with chrome, nickel and molybdenum, stabilized with Ti, Nb, Ta

The numbers after the hyphen indicate the strength class.

Stainless screws are divided into strength classes such as 50, 70 or 80. These numbers give 1/10 of the minimum tensile strength R.m at. 50 means a minimum tensile strength of 500 N / mm2. A test load is specified for nuts of low design, as the thread is too short to take the full load-bearing capacity of the material. The specifications 025, 035 and 040 are common. 035 means a test load of 350 N / mm2.


The following materials, among others, are used for screws:

Nominal tensile strength
[N / mm2]
Stretch limit
Re / Rp0.2
[N / mm2]
Nominal tensile strength Rm, [N / mm2]
Yield strength Re / Rp0.2 [N / mm2] (d
Nominal tensile strength Rm, [N / mm2]
Yield strength Re / Rp0.2 [N / mm2] (d

Note: The minimum tensile strength of quenched and tempered steels depends on the diameter - hence two values.

Product info:

As there are consequently a number of differences in strength, this property should always be taken into account when using screws. If you are only looking for a screw for household use, you often do not necessarily find out about this property and this usually does not have any consequences. The question becomes a little more important when you erect a WPC privacy fence in your private garden that has to withstand wind and weather. However, when it comes to large constructions, the material used should also meet the required properties. One way of examining materials is to etch a polished ground surface under a reflected light microscope with subsequent assessment. The structures of the structure become visible, from which a person skilled in the art can draw conclusions about the material used or its condition.

Author: Uwe Koerbitz

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