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Heat treatment and metallic materials

Adding alloying elements such as Mn, Cr and Mo to steel improves its hardenability. Which is the correct reason?

  1. AThe nose of the TTT diagram moves to shorter times, so austenite transforms faster and the steel hardens more easily
  2. BThe nose of the TTT diagram moves to longer times, so martensite forms more easily even with slow cooling
  3. CThe Ms temperature rises, so martensite starts to form earlier in cooling
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Answer: B. The nose of the TTT diagram moves to longer times, so martensite forms more easily even with slow cooling

Hardenability is the depth to which martensite can be obtained by quenching (how readily the steel hardens). Alloying elements such as Mn, Cr and Mo retard the pearlite and bainite transformations of austenite and move the nose of the TTT and CCT diagrams to longer times. This lowers the critical cooling rate, so martensite forms more easily at the center of thick sections or with slower cooling such as oil quenching. If the nose moved to shorter times, the steel would harden less readily. Most alloying elements actually lower the Ms temperature, so that is not the reason for improved hardenability. Hardenability is evaluated with the Jominy end-quench test.

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