The practice of heating carbon steel components to a temperature above the upper critical (recognised by blacksmiths as a certain colour change) and then quenching in a liquid has long been used to achieve ‘hardness’. It is now appreciated that the degree of hardness achieved will vary according to the rate (speed) of cooling, and that different steels require different cooling rates to achieve full hardness. This is referred to as the steel’s ‘Critical Cooling Rate’. In practice the optimum cooling rate for a component is usually decided on the basis of experience or trials, and will be determined to a large extent by the section and size of the component. THERMISOL AQ 123S fluid shows accelerated cooling characteristics which help achieve correct hardness