Steels- Pearlite Reaction Flashcards
Describe the structure of pearlite
A lamellar mixture of ferrite and Fe3C
What is the temperature range in which pearlite forms?
From T sub R (around 550C) up to the A1 temperature (eutectoid temperature).
Pearlite reaction
Eutectoid reaction.
γ->α+Fe3C
Transformation is a function of temperature and time
What is the interlamellar spacing?
Maybe symbol S sub 0. Distance between midpoints of two nearest lamellae of the same phase. E.g shortest distance between midpoints of two ferrite lamellae
How does carbon concentration in austenite around growing pearlite vary?
Next to Fe3C lamellae the carbon concentration in austenite is low. Next to ferrite lamellae it is high. Assume it’s to do with ferrite requiring more carbon from the adjacent austenite for it to grow
Assumptions for Johnson-Mehl equation and what it is valid for
Valid for any diffusional transformation.
Random nucleation
Constant nucleation rate
Constant growth rate
Standard fraction transformed vs time graph shape
Time on log scale. Fraction transformed (Xr) from 0 to 1. Like cumulative frequency curve. Start at some x intercept. Starts slow due to small number of nuclei. Gets faster. Near y=1 gets slower as untransformed volume of γ is small
How does Xr vs logt graph change with nucleation and growth rates?
Reducing nucleation rate makes whole curve shallower gradient so reaches Xr=1 at longer time but x intercept the same.
Reducing growth rate shifts x intercept to longer time and shallower curve so finishes at longer time. For the same changes growth rate has a bigger impact than nucleation rate so is the more dominant factor.
How does Xr vs logt graph change with temperature?
Doesn’t affect the shape but higher temperatures shift whole graph to longer times because transformation rate decreases as the T sub R decreases (reaction temperature?)
How do hardness and strength vary with temperature?
They both decrease as temperature increases. Don’t know if this is the transformation temperature or just the temperature of the final part.
Johnson-Mehl equation
X(t)=1-exp((-πNG^3t^4)/3)
X(t) is fraction transformed
N is nucleation rate
G is growth rate
t is reaction time
How does interlamellar spacing vary with T subR and graph that shows this?
High TR gives larger S. Graph of S vs 1/ΔT where ΔT is A1-TR. So ΔT increases with decreasing TR and so 1/ΔT decreases. Plot x axis going right to left for increase. Graph is straight line negative gradient with high TR at top left for large S and low TR at bottom right for small S
What are colonies?
Regions of different orientations of lamellae within a pearlite nodule
How does TR affect the number and size of pearlite nodules?
At high TR, large S and G/N ratio meaning few large pearlite nodules.
At low TR, small S and G/N ratio meaning many small pearlite nodules.
