Quenching Steel Essay

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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
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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…show more content…
The rate of cooling is immaterial except for some steels which are susceptible to temper brittleness. As the tempering is increased, the martensite of hardened steel passes through stages of tempered martensite and is gradually changed into a structure consisting of spheroids or cementite in a matrix of ferrite, formerly termed as sorbite. These changes are accompanied by a decreasing hardness and increasing toughness. The tempering temperature depends upon the desired properties and the purpose for which the steel is to be used. If considerable hardness is necessary then the tempering temperature needs to be low. On the contrary if considerable toughness is needed then the tempering temperature is to be

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