Steel Hardening by Quenching and Tempering...

 Steel Hardening by Quenching and Tempering Hardening is carried out by quenching steel, which consists of cooling it rapidly from a temperature above the transformation temperature (A?).  The quenching is necessary to suppress the normal breakdown of austenite into ferrite and cementite (pearlite), and to cause a partial decomposition at such a low temperature to produce the new phase called martensite. To achieve this, steel requires a critical cooling velocity, which is greatly reduced by the presence of alloying elements. In such case hardening of steel occurs with mild quenching. Martensite is a supersaturated metastable phase and has body centered tetragonal lattice (bct) instead of bcc. After steel is quenched, it is usually very hard and strong but brittle. Martensite looks needle like under microscope due to its fine lamellar structure. Steel is quenched in water or brine for the most rapid cooling, in oil for some alloy steels, and in air for certain higher alloy steels. Water is one of the most efficient quenching media where maximum hardness is required, but it is liable to cause distortion and cracking of the work piece. Where hardness can be sacrificed, whale, cotton seed and mineral oils are used. These tend to oxidize and form sludge with consequent lowering of efficiency. The quenching velocity of oil is much less than water. To minimize distortion, long cylindrical objects should be quenched vertically, flat sections edgeways and thick sections should enter the bath first. To prevent steam bubbles forming soft spots, a water quenching bath should be agitated. Steel can be hardened by the simple expedient of heating to above the A? transformation temperature, holding long enough to insure the attainment of uniform temperature and solution of carbon in the austenite, and then cooling rapidly (quenching). Complete hardening depends...