Phase Transformation

Phase Diagram

Information obtainable

phase present under certain composition and temperature

solid solubility of one element in another

range of temperature which melting happens for different composition

weight percentage of phases

Phases in phase diagram

Pure metal (in no solubility system)

Solid solution (when solute atoms diffuse into host element)
Type----------------------------------------------------------------------------
Substitutional - solute occupy regular lattice of solvent
Interstitial - solute occupy interstices of solvent

Intermetallic compound (form between metals as secondary phase) Type------------------------------------------------------------------------------------Stoichiometric - have definite atom ratio (represented by vertical line) Non-stoichiometric - have range of composition

Interstitial compound (form between metals and non metals as secondary phase)

Special phase transformation points when decrease in temperature Eutectic ------- L → α + β ----------------------------------------------------------- Peritectic ------ α + L → β ----------------------------------------------------------Monotectic ---- L1 → L2 + α ------------------------------------------------------Eutectoid ------- γ → α + β --------------------------------------------------------Peritectoid ------ α + β → γ ---------------------------------------------------------

a diagram that shows the transformation of phases under equilibrium or slow cooling in parameters of composition and temperature

Possible microstructure in Fe-C system

Ferrite, α-Fe - (BCC and magnetic)(soft)-----------------------------------------


Austenite, γ-Fe - (FCC and non-magnetic)-------------------------------------


Cementite, Fe3C - (secondary phase of Fe-C system)(hard)--------------


Pearlite - lamellar structure of α-Fe & Fe3C ----------------------------------
Coarse pearlite - forms at higher temp, softer --------------------------------
Fine pearlite - soft at lower temp, slightly harder than coarse pearlite -


Martensite - α-Fe with high saturated Fe3C trapped in BCT structure--
Tempered martensite - martensite that is tempered, slightly tougher ---


Bainite - plate-shaped α-Fe, with Fe3C particles-----------------------------
Upper bainite - feathery bainite forms at upper temp range --------------
Lower bainite - dispersed Fe3C in plated α-Fe, forms at lower temp --


Spheroidite - spherical Fe3c in the matrix of α-Fe------------------------------

TTT Diagram

-describe effects of actual cooling conditions
-shows non-equilibrium cooling transformation of Fe-C system

a diagram that shows the transformation of phases in steel of a specific composition in a temperature-time graph

TTT

CCT Diagram

while TTT shows isothermal cooling, CCT shows path of metal under constant cooling rate

Why need CCT?
Because in practical, heat treatments for steel involve continuous cooling

Heat Treatment

consist of:
Annealing

  • a series of process of heat, hold, then cooling down a processed metal in furnace to reduce it residual stress
    Full anneal - heat to γ-Fe temp, hold for an extended time, cool slowly through transformation range
    Process anneal - heat to temp below lower critical line, allowing recovery and recrystallization but not growth.
    Stress relief anneal - heating only to relieve some of the entangled dislocation & regain some ductility.

sphereodizing

  • heat just below eutectoid, hold for a long period so that spheroidite is formed


Quenching

  • immersion of metals in water, oil, polymer solution, salt or forced air to cool it rapidly
  • Usually start from γ-Fe


Tempering

  • is done after quenching to relieve some of its residual stress and gain some ductility
    Conventional tempering - to form tempered martensite
    Martempering - to reduce cracks on martensite surface after quenching
    Austempering - to form bainite

normalizing

  • similar to annealing but heat at temperature above upper critical temp, shorter holding time and cool under room temp to refine grains & get uniform structure


Alloying elements

effects

  • affect hardenability


  • alter shape of PD

  • reduce rate of tempering

Conclusion

  • PD mapped out possible phases that formed under equilibrium state
  • TTT provide possible phases formed when cooling at different rate for a specific composition
  • CCT provides a more practical cooling rate as it is in continuous cooling instead of isothermal
  • Different heat treatment is required to produced different microstructure with desired properties

a diagram that shows type of phase changes
of a material when is cooled at different rates