A grade 8.8 fastener can be manufactured from any material whose mechanical properties meet or exceed the requirements set forth in the relevant harmonised standards.
The two-digit nomenclature is not used to describe individual steel grades and only applies to metric fasteners defined under ISO 965 (as well as derived standards)
The designation system is based on two numbers e.g 8.8 . The first number is the tensile strength of the bolt material (N/mm2 )/100. The second number is = 1/100.(the ratio of the Proof (or Yield ) stress and the Tensile strength expressed as a percentage = 100.[Yield (Proof stress) /Tensile strength] /100
meaning that a grade 8.8 bolt has a nominal ultimate tensile strength (UTS) of 800N/mm2 (or Mpa) and a nominal yield strength of 640N/mm2 (or Mpa).
These values are not expressed in Kg/mm2 or PSI as the first is an expression of mass by surface-area, the second is an expression of weight by surface-area and neither are true units under the SI system.
Lastly, the actual steel grade used by manufacturers may vary but in most cases, a grade 8.8 fastener (self-colour, black-oxide or zinc plated) will be manufactured from a medium-alloy medium carbon steel, typically with 0.30 - 0.50 % Carbon and alloyed with other elements such as Molybdenum, Vanadium, Manganese and sometimes Boron or Cobalt in small quantities. These alloying elements provide the required mechanical properties to reach the minimum strength set forth by the harmonised standards.
Bare in mind that people sometimes refer to "high-tensile bolts" which are typically grade 8.8 or superior under the metric fastener system. Grade 8.8 and superior bolts should always have their grade permanently marked on the head to differentiate them from "low-tensile bolts" as inadvertantly replacing the former with the latter could have disastrous effects.
Grade 4.6 bolts are typically refered to as "low tensile" and are often used in the construction industry as these bolts offer higher ductility and resilience compared to "high-tensile" bolts.
Hope this helps.
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