Grade / Class and Fastener Strength
Fastener Grade (imperial) or Class (metric) refers to the mechanical properties of the fastener material. Generally, a higher number indicates a stronger, more hardened (but also more brittle) fastener.
MILD STEEL / LOW TENSILE / COMMERCIAL GRADE
The lowest grade is 4.6, commonly known as mild steel, low tensile or commercial grade.
High tensile bolts have improved hardness (strength) after cold working, by controlled heat treatment and quenching.
High tensile bolts are available in imperial and metric sizes.
Imperial sized bolts are available in either UNC (a coarse thread) or UNF (a fine thread).