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Heat treatment and metallic materials

Which statement about the martensitic transformation in steel is correct?

  1. AIt proceeds slowly as carbon atoms diffuse over long distances and precipitate cementite at the austenite grain boundaries
  2. BIt is a diffusionless shear transformation producing body-centered tetragonal (BCT) martensite supersaturated with carbon
  3. CThe slower the cooling rate, the more time the carbon atoms have to diffuse and rearrange, so the more martensite forms in the steel
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Answer: B. It is a diffusionless shear transformation producing body-centered tetragonal (BCT) martensite supersaturated with carbon

When austenite is quenched faster than the critical cooling rate, carbon has no time to diffuse, and a shear-type transformation occurs in which atoms shift cooperatively by small amounts, producing body-centered tetragonal (BCT) martensite supersaturated with carbon. Because the carbon distorts the lattice along the c axis, martensite is very hard and brittle. The transformation starts at the Ms temperature and progresses as the temperature falls, with almost no dependence on holding time. Pearlite and bainite are the structures that separate into ferrite and cementite with carbon diffusion, and it is pearlite that forms more readily the slower the cooling.

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More Heat treatment and metallic materials questions

  1. Which correctly describes the difference between a TTT (isothermal transformation) diagram and a CCT (continuous cooling transformation) diagram?
  2. Adding alloying elements such as Mn, Cr and Mo to steel improves its hardenability. Which is the correct reason?
  3. What is the main purpose of tempering a quenched steel?
  4. The yield stress of a steel follows the Hall–Petch relation σy = σ₀ + k/√d with σ₀ = 70 MPa and k = 0.74 MPa·√m. Which value is closest to the yield stress for an average grain size d = 25 µm?
  5. The yield stress of a metal was 150 MPa at an average grain size of 100 µm and 250 MPa at 25 µm. Assuming the Hall–Petch relation σy = σ₀ + k/√d, which value is closest to the yield stress at a grain size of 4 µm?