I was recently engaged to evaluate the static loading of a mount and the effects on the fasteners and components under preloaded conditions. I thought I would post a few tidbits from the research. Today I want to discuss the mechanical properties of metric fasteners (as related under ISO 898).
How Grades are Designated
Strength Grades for metric fasteners are designated by a symbol comprised of 2 numbers, separated by a decimal. For example, 4.6 and 8.8.
The first number represents 1/100 of the nominal tensile strength in N/mm^2.
(4.6) 4 x 100 = 400 MPa nom. tensile strength
The second number is a factor that is 10x the ratio between the nominal yield stress and the nominal tensile stress.
(4.6) [4 x 6] x 10 = 240 MPa Nom. yield stress
Additional Strength Grades
|Nom. Tensile Strength (MPa)||300||400||400||500||500||600||800||830||900||1040||1220|
|Nom. Yield Strength (MPa)||180||240||320||300||400||480||640||660||720||940||1100|
Grades shall be underlined when low carbon martensitic steels are used.
Information herein supplemented by Fastenal’s published strength tables. Torqueleader’s white paper noted that grades 8.8 and higher are represented as proof stress due to yield measurement problems, however Fastenal relates specific proof strengths.
Atmospheric Corrosion Resistant Materials
ASTM F568M-07 notes the inclusion of grades 8.8.3 and 10.9.3 which are not covered in the ISO standard. These are low carbon alloy that can in some cases withstand atmospheric conditions in an uncoated state.
Good Resources on Metric Fasteners
Fastenal’s charts which include the material treatment and detailed information
A great paper by Torqueleader
Wiki page on ASTM F568M