Steels- Ferrite Formation Flashcards
Why is ferrite nucleation at GBs preferred?
As the destruction of GB energy partially compensated for creation of a new interphase boundary.
Balance between interfacial and strain energy for ferrite nucleation
Formation of a coherent interface can result in a low interfacial energy at the expense of an increase in strain energy.
Because ferrite nuclei have an orientation relation to austenite. This is Kurdjumov-Sachs (K-S) orientation relationship:
{111}subγ parallel to {110}subα
What does ferrite growth into allotriomorphs or idiomorphs require?
Long range carbon transport into γ.
Transport across the α/γ interface
Assumption when analysing ferrite growth
Assume equilibrium at α/γ interface
I assume this in relation to carbon concentration
Variation of carbon concentration over an α particle growing into the γ phase
Across the α particle is constant and very low at Cα. At interface vertical line up to much higher concentration Cγ. Exponential decay going further into γ phase down towards C0 which is about midway between. Particle growing in z direction
Fluxes for a moving α/γ boundary for ferrite growth
Requires removal of (Cγ-Cα)dz moles of C in dt. This must equal flux away from the interface times dt so:
(Cγ-Cα)dz = -Dc^γ(dC/dz)dt (ds actually curly)
Flux into interface=VCγ where V is growth velocity
Flux away from interface=VCα-Dc^γ(dC/dz)
Think Dc^γ is diffusion coefficient or diffusivity
Both fluxes remain balanced at the interface
Formula for growth velocity
V=(-Dc^γ(dC/dz))/(Cγ-Cα)
Assuming a linear concentration gradient at interface:
Means dC/dz=(Cγ-C0)/L so:
V=(Dc^γ(Cγ-C0))/(Cγ-Cα)L
Where L is distance between interface and where tangent to decay curve at the interface meets C0
Areas on carbon concentration profile graph for α particle growing into γ
A1 is square between Cα and C0 before interface. A2 is area between decay curve from Cγ and C0.
A1 proportional to solute rejected from α phase as it grows some distance dz. This solute is piled up directly in front of the α/γ interfaces. A1=A2. As α grows, A2 must increase for this to remain true. Does this by L increasing and so V slows over time
Dilatometry
Continuous method for following transformations. Keep heating sample and plot volume vs temperature. Always fairly linear. Starts α. Vertical small drop in volume when γ then linear increase. Vertical small increase when σ then linear increase
Constant temperature method for following transformations
Start with stable γ at austenitising temperature. Cool fast (causes thermal contractions) to temperature want to investigate and begin isothermal hold. Then an incubation period when unstable γ is present. Then transformation begins causing an increase in length until transformation ends. Plot graph of length vs logt. Starts high, steep drop, horizontal, fairly steep increase, levels off close to original high length
Discontinuous method for following transformations
Microscopy. Use very thin slices of steel (instantly equilibrated). Put in salt bath at austenitising temperature. Then move to salt bath at reaction temperature. Put into quench bath. Mount, polish, tech and observe so see how much has transformed to what. Repeat for different times at reaction temperature. Use salt bath instead of furnace for better uniformity of heating when submerged through conduction and not radiation.
