What is tempering?

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• Sravanthi    -Posted on 12 Nov 15
  - To produce martensite structures by heat treatment of steels by quenching and tempering involves three steps :
  
  1) Austenitizing: In austenitic phase, steel is heated to red hot temperature. It then forms a single phase mixture of FCC iron with carbon dissolved in the solution.
  
  2) Quenching: In this stage, high temperature austenite is cooled to room temperature to convert into martensite which is a new single phase mixture of carbon in iron. Crystal structure of martensite is BCT (Body Centered Tetragonal).
  
  3) Tempering: In this stage, martensite structure is decomposed by heating martensitic steel below its critical temperature. During this stage martensite is decomposed into tempered martensite, toughness of alloys is decreased to achieve greater toughness, stress is removed from the micro-structures and retained austenite is transformed into other products such as bainite and pearlite.
  
  - Martensite phase produced by quenching is hard, strong but brittle. Hence, it becomes difficult to use it for practical applications. Hence, the steel is tempered after hardening treatment. Depending upon the hardness required tempering temperature is decided because as tempering temperature increases toughness increases but hardness and strength decreases.
  

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