Carbon and alloy steel fasteners, bolts class 8.8 , and nuts class 8 will all require heat treatment to achieve the mechanical properties specified by the standards authorities. British inch size fasteners grade R and above also require heat treatment.

The type of heat treatment is known as hardening and tempering. Hardening occurs when specific steels are heated to a temperature that changes the structure of the steel; this is followed by quenching (rapid cooling) by submerging the steel in oil. The temperature required for structural transformation is above 850˚C but may fluctuate depending upon the amount of carbon and alloying elements in the steel. The atmosphere of the furnace is controlled to reduce the amount of oxidation in the steel.

After hardening and quenching, the steel is hard and brittle and requires a process called Tempering to reduce both the hardness and brittleness. Heating to around 450˚C lowers the hardness, reduces the brittleness, and gives the fastener toughness. Tempering is the final process, the fastener has now achieved the desired properties. High tensile finished fasteners have a Black finish that is the result of hardening and tempering.