Group 6: Non-equilibrium cooling

What is Non-equilibrium Cooling?

During a rapid cooling, the precipitated solid molecules in the liquid have not diffused uniformly, resulting in non-uniform concentration of solid molecules in the crystallisation.

- Phase Change at temperatures not at phase diagram boundary lines

- nucleation, growth and transformation rates as a function of time (not temperature)

Martensite:

• • Formed when when Austenite is rapidly cooled (e.g. quenched)

  • Fast cooling prevents Carbon diffusing out of Iron (can’t form Fe3C)

Pearlite:

• Alternating layers of ferrite (α) and cementite (Fe₃C)

• Formed during slow cooling

• Forms Body-Centred Tetragonal crystal structure. Elongated BCC

• Most real world cooling scenarios are not fully at equilibrium

• Martensite is a ‘supersaturated’ solution

• Forms needle shaped grains and transformation occurs almost instantaneously

• Austenite is denser than martensite → net volume increase → causing internal stresses that may cause crack

Tempered Martensite

• • Heat treatment that enhances ductility and toughness of martensite.

  • Heating martensite to a temperature below the eutectoid for a specific time period.

  • Normally between 250℃ to 650℃.

  • Temperature raise allows C to diffuse and form Fe₃C. (Has ⍺ and Fe₃C phases).

  • BCT transforms to BCC.

  • As hard and strong as martensite, but with enhanced ductility and toughness.

Bainite

• • Formed when autensite is cooled past a critical temperature (e.g. 727℃).

  • Bainite consists of needle-like particles, cementite (Fe3C) in a ferrite matrix.

  • The transformation begins with nucleation of ferrite plates (displacive growth) at the autensite boundaries.

  • The temperature becomes high enough that Carbon diffuses out of the lattice and the FCC structure transforms into a BCC structure.

  • Eventually cementite precipitates in the remaining austenite layers between the ferrite platelets.

Time - Temperature Transformations

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