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Heat treatment and metallic materials

What is the main purpose of tempering a quenched steel?

  1. ATo make the martensite even harder and raise its wear resistance
  2. BTo increase retained austenite, stabilizing the microstructure and preventing dimensional change
  3. CTo restore toughness and reduce internal stresses at the cost of some hardness
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Answer: C. To restore toughness and reduce internal stresses at the cost of some hardness

As-quenched martensite is hard but very brittle and carries large internal stresses. In tempering, the steel is reheated to a temperature below the A1 temperature (727 °C); the supersaturated carbon precipitates as fine carbides and internal stresses relax, so hardness falls but toughness recovers substantially. The higher the tempering temperature, the softer and tougher the structure; low-temperature tempering is used for tools and high-temperature tempering for machine parts (the combination with the preceding quench is called quenching and tempering). An increase in hardness on tempering is limited to secondary hardening in certain alloy steels and is not the general purpose. Tempering tends to decompose retained austenite; increasing it is not the aim.

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