Blue Steel

Steel can change colour due to oxidation during heat treatment. I don’t know if this depends only on the thickness of the oxide, or if there is also some chemical difference. I tempered a high carbon alloy for 30 minutes at 300 Celsius (carbide free bainitic steel), and it changed into this purple colour.

Purple Steel Tensile Sample

There are a few web pages which give details of the Temper Colours – this one has a table with colours and hardnesses of various steel grades.

The surface colour depends on the oxide thickness, so is dependent upon alloy chemistry, temperature, time, surface finish and furnace environment.

There is also a page on wikipedia on the blueing of steel.