Download - TTT notes
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Objectives
1. Describe how nucleation and growthprocesses are affected by supercooling
and superheating
2. Use a TTT diagram to design a coolingprocess to produce a desired
microstructure
3. Explain the microstructure produced on aTTT diagram in terms of nucleation and
growth mechanisms
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Formation of New Phases FollowsFormation of New Phases Follows
A Nucleation + Growth ProcessA Nucleation + Growth Process
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In SolidificationIn Solidification
This Results inThis Results in
GrainsGrains
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Can Model Growth:Can Model Growth:
y = 1 exp (-k ty = 1 exp (-k tnn))
y - fraction transformedy - fraction transformed
k, n - constantsk, n - constants
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By Convention Rate
r = 1
t 0.5
t 0.5 = time for 50% conversion
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Rate is Temperature Dependent
r = A eQ / RT
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The resulting microstructure give
evidence of this process:
Dendrites
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Square Mold Solidification:
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Three cast structures of metals solidified in a square mold
(a) Pure metals (b) Solid-solution alloys ( c) Structure obtained
using nucleating agents
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In solidification undercooling affects rates
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Equilibrium Phase Diagram will give
-Accurate identification of phases at a
given temperature.
-Phase breakdown for slow processes
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Equilibrium Phase Diagram will NOT tell you
-Non-equilibrium structures
-Phases present in rapidly cooled parts
For this you need a TTT diagram
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Time Temperature Transformation Diagram
-Identifies non-equilibrium structures
produced in processing-Realistic cooling rates
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Iron -
Carbon
TTT
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Austenite: FCC iron w/ carbon
Pearlite: Layers of ferrite and Fe3C
Coarse- Produced just below eutectoid-thick layers
Fine- Produced @540 C, thin layers
Bainite: 200 500 C Transformation
Martensite: Rapid Cooling
Four Microstructures inFour Microstructures in
SteelSteel
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Heat Treatment forhardening begins
with Austenite
-FCC Iron
-High Carbon Solubility
Softening Heat Treatments just
require reheating.
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Pearlite
Austenite
Boundaries
Cementite
Ferrite
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Bainite
Acicular
Upper at 330 500 C
Lower at 200 300 C
Cementite
Ferrite
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Martensite
-Body Centered Tetragonal
-Carbon Trapped
-Very Hard + Brittle
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Tempering (Tempered Martensite)Reheat Martensite
-Improves toughness
-Small reduction in hardness
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Tempered MartensiteTempered Martensite
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Principles of TTT Diagram-Time is time at that temperature,
earlier temperatures do not count
- The Etch-A-Sketch principle-Fractions must total 100%
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Cool Rapidly to 350 C, hold for 10 ^3 s, then quench to room temp.
A. 100% Pearlite
B. 100% Bainite
C. 100% Martensite
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Rapidly cool to 625 C, hold for10 s, then quench to room temp.A. 100% Coarse Pearlite
B. 100% Medium Pearlite
C. 100% Coarse Bainite
Rapidly cool to 600 C hold for 4 s rapidly cool to 450 C hold for 10 s then quench to
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Rapidly cool to 600 C, hold for 4 s, rapidly cool to 450 C, hold for 10 s, then quench to
room temp.
A. 50P/25B/25M
B. 25P/50B/25M
C. 50P/50B
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Reheat the specimen in part C
to 700 C for 20 hrs.
Rapidly cool to 300 C hold for 20 s then quench to room temp in water Reheat to 425C
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Rapidly cool to 300 C, hold for 20 s, then quench to room temp in water. Reheat to 425 C
for 10 ^3 s and slowly cool to room temp.
A. Martensite
B. Fine Bainite
C. Tempered Martensite
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H. Rapidly cool to 350 C, hold for 150 s, then quench to room temp.
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MicrostructureMicrostructure
DeterminesDetermines
HardnessHardness
E ect oec o
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E ect oec oAlloyingAlloying
on TTTon TTT
DiagramDiagram4340 Steel4340 Steel
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It is often easier
to quench at a
fixed rate:
Continuous
Cooling
Diagram
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Effect of AlloyingEffect of Alloying
ElementsElements Most alloying elements added toMost alloying elements added to
control rate of FCCcontrol rate of FCCBCCBCC
transformationtransformation
Some elements add corrosionSome elements add corrosion
resistanceresistance
The alloy itself does not affectThe alloy itself does not affect
properties greatly at lowproperties greatly at low
concentrationconcentration
F ll U H t T t
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Follow- Up Heat Treats
Stress Relieving
Annealing
Normalizing
Spheroidizing
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Ferrous Materials
IronWrought (low % C)
Cast (high % C)
Plain Carbon
Low Alloy Steels
Alloy Steels
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Annealing1. Soften Material for Machining
2. Relieve Stresses
3. Alter Properties4. Condition for Subsequent work
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Normalizing1. Relieve Stress
2. Refine Grains
3. Homogenize Part
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SpheroidizingConvert carbide inclusions
to a more spherical shape
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Spheroidite
(1000X)
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Problems of
Quick Cooling
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Rapid QuenchingRapid Quenching
ProblemsProblems
1.1. Distortion (ThermalDistortion (Thermal > S> SYY))
2.2. CrackingCracking
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Tricks
Marquenching
Quench to MS
Slow Cool to MF in Air
Austempering
Make Bainite Just Above MS
Slow Cool to Room Temp.