ABSTRACT
Carbon or alloy steel has a carbon content not greater than 1 per cent and this is mainly responsible for its high tensile strength. Alloying elements such as manganese, nickel and chromium may also be added to improve the mechanical properties of the steel, and various heat treatments have a beneficial effect; these treatments make use of the fact that if steel is heated to a temperature higher than about 850°C (1550°F) (termed the ‘transformation temperature’), its final structure and the extent to which its properties are improved depend on the rate of cooling. If the steel is cooled slowly from its transformation temperature, the treatment is termed ‘annealing’; if it is allowed to cool from the transformation temperature at its normal rate, the treatment is termed ‘normalizing’. If, on the other hand, the steel is suddenly cooled from above the transformation tempera ture to room temperature by immersion or ‘quenching’ in oil, the hardness and brittleness of the material are appreciably increased. Quenching is usually followed by a tempering process in which the steel is reheated to about 400°C (750°F) and allowed to cool in air; this reduces the brittleness of the steel. If the steel is rapidly cooled from above the transformation temperature to about 450°C (850°F) and then allowed to cool slowly to room temperature the process is termed ‘patenting’, and has an effect similar to that of quenching and temper ing. The term ‘stress relieving’ is used to describe heat treatment for a prolonged period at about 260°C (500 F) or a short period at about 500°C (950 F). The term ‘stabilizing’ denotes heat treatment at about 400°C (750°F) combined with a tensile stress of about 65 per cent of the ultimate strength of the steel.
