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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.