Hypoeutectoid Steel Fe 3 C (cementite) 1600 1400 1200 1000 800 600 400 0 1 2 3 4 5 6 6.7 L (austenite) +L + Fe 3 C + Fe 3 C L+Fe 3 C (Fe) C o , wt% C 1148°C T(°C) 727°C C 0 0.76 proeutectoid ferrite pearli te 100 m R S w = S /( R + S ) w Fe 3 C =(1- w ) w pearlite = w pearlite r s w = s /( r + s ) w =(1- w )
T (°C). 1600. d. L. 1400. g. + L. g. g. g. 1200. L +Fe 3 C. 1148°C. (austenite). g. g. g. 1000. g. g. + Fe 3 C. g. g. Fe 3 C (cementite). r. s. 800. a. g. g. 727°C. a. a. a. g. g. R. S. 600. a. + Fe 3 C. w. =. s. /(. r. +. s. ). a. w. =. (1-. w. - PowerPoint PPT Presentation
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Hypoeutectoid Steel
Fe 3
C (
cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
(austenite)
+L
+ Fe3C
+ Fe3C
L+Fe3C
(Fe) Co , wt% C
1148°C
T(°C)
727°C
C0
0.76
proeutectoid ferritepearlite
100 m
R S
w =S/(R+S)wFe3C =(1-w)
wpearlite = wpearlite
r s
w =s/(r+s)w =(1- w)
Proeuctectoid Ferrite – Pearlite
0.38 wt% C: Plain Carbon – Medium Carbon Steel
Hypereutectoid Steel
Fe 3
C (
cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
(austenite)
+L
+Fe3C
+Fe3C
L+Fe3C
(Fe) Co , wt%C
1148°C
T(°C)
0.7
6 Co
Adapted from Fig. 9.33,Callister 7e.
proeutectoid Fe3C
60 m
pearlite
R S
w =S/(R+S)wFe3C =(1-w )
wpearlite = wpearlite
sr
wFe3C =r/(r+s)w =(1-w Fe3C )
Fe3C
Proeutectoid Cementite - Pearlite
1.4 wt% C: Plain Carbon – High Carbon Steel
Phase Transformations• We just studied Phase Diagrams which are thermodynamic
maps which tell us the equilibrium phases present at any specific combination of temperature, pressure, and composition
• These phase diagrams are based on the concept of Gibbs Free Energy, G, which we have briefly introduced before: G is the thermodynamic driving force for a reaction If G is negative then there is a probability that a reaction will
occur. The more negative G becomes, the more driving force there is for
the reaction Thermodynamics tells us the probability of a reaction but not the
rate – the rate of a reaction is determined by Kinetics
Now we are going to shift perspectives and discuss the details of how we transform from one phase to another
Phase Transformations
Let’s categorize with 3 types:
1. Simple diffusion-dependent transformations in which there is no change in the number or composition of the phases present
Examples: Solidification of a pure metal Allotropic transformations Recrystallization and Grain Growth
2. Diffusion-dependent transformations in which there is a change in the phase compositions and or number of phases present
Examples: Eutectoid reaction Peritectic reaction
3. Diffusion-less transformations, in which a metastable phase is producedExamples: Martensitic and Bainitic transformations
Phase transformations involve some form of change in the microstructure
Nucleation
– nuclei (seeds) act as template to grow crystals– for nucleus to form, rate of addition of atoms to nucleus must
be faster than rate of loss– once nucleated, grow until reach equilibrium
Driving force to nucleate increases as we increase T– supercooling (eutectic, eutectoid reactions)
Small supercooling few nuclei - large crystals
Large supercooling rapid nucleation - many nuclei, small crystals
During Phase transformation – new phase formed with different physical/ chemical characteristics than the parent phase
Diffusion based Phase Transformations do not occur instantaneously – nucleated
Solidification: Nucleation Processes
• Homogeneous nucleation – nuclei form in the bulk of liquid metal– requires supercooling (typically 80-300°C max)
• Heterogeneous nucleation– much easier since stable “nucleus” is already present
• Could be wall of mold or impurities in the liquid phase– allows solidification with only 0.1-10ºC supercooling
Consider Solidification First
Let’s assume spherical nuclei
Why? Sphere has the smallest surface area/ surface energy for a given volume
Let’s Determine the equations that define behavior