Tempering Heat Treatment Process- Classification Of Tempering
If high carbon steel is quenched for hardening in a bath, it becomes extra hard, extra brittle and has unequal distribution internal stresses and strain and hence unequal harness and toughness in structure. These extra hardness, brittleness and unwanted induced stress and strain in hardened metal reduce the usability the metal. Therefore, these undesired needs must be reduced for by reheating and cooling at constant bath temperature. In tempering, steel after hardening, is reheated to a temperature below the lower critical temperature and then followed by a desired rate of cooling. Reheating the of hardened steel is done above critical temperature when the structure is purely of austenite and then quenching it in a molten salt path having temperature in the range of 150-500°C. This is done to avoid transformation to ferrite and pearlite and is held quenching temperature for a time sufficient to give complete formation to an intermediate structure referred to as bainite then cooled to room temperature. The temperature should not be held less than 4 to 5 minutes for each millimeters of the section. After tempering structure is changed into secondary structure like martensite, troostite, sorbite and spheroidised.
Fig. shows different tempered states of martensite, troosite, sorbite and spherodite. Depending upon the temperature of reheat, the tempering process is generally classified in to three main categories.
Classification Of Tempering Process
Low Temperature Tempering
Hardened steel parts requiring tempering are heated up to 200°C and then quenched in oil. Tempering is used to retain hard micro-structure of martensite which increases brittleness.
Fig a. represents the microstructure of martensite.
Hardened steel parts requiring tempering are heated in the temperature range of 200-350°C. This process gives troosite structure. Troosite structure is another constituent of steel obtained by quenching tempering martensite. It is composed of the cementite phase in a ferrite matrix that cannot be resolved by light microscope. It is less hard and brittle than martensite. It is also produced by cooling the metal slowly until transformation begins and then cooling rapidly to prevent its completion. It has a dark appearance on etching. It is weaker than martensite. Fig b. represents the microstructure of troosite.Medium Temperature Tempering
High Temperature Tempring
Hardened steel parts requiring tempering are heated in the temperature range of 350-550°C. This process gives sorbite structure. Sorbite structure is produced by the, transformation of tempered martensite. It is produced when steel is heated at a fairly rapid rate from the temperature of the solid solution to normal room temperature. It has good strength and is practically pearlite. Its properties are intermediate between those of pearlite and troosite.
Parts requiring tempering are heated in the temperature range of 550-750°C. This process gives spheriodite structure. Fig (c) represents the microstructure of sorbite. However there are other special kinds of tempering also which are discussed as under.