BLUING

Bluing is originally the result of a heat treatment process known as tempering. After being hardened, many steel components are too brittle to be functional. A tempering treatment is then required to relieve internal stresses. Once quenched, the components are gently and evenly heated with a flame. As the temperature rises, the component changes colour progressively (straw yellow, yellow, brown, violet, dark blue, light blue). Dark blue indicates the optimal temperature at which the part (spring, screw, etc.) reaches the desired hardness and elasticity. The colour thus serves as an indicator of successful tempering rather than a purely decorative feature.

Today, the steels used for screws often no longer require such heat treatments. When screws appear blue nowadays, it is frequently due to PVD or CVD surface treatments, making the colouring purely aesthetic.

Useful links:
– Steels
– Heat treatments