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PHYSYICAL METALLURGY - Coggle Diagram
PHYSYICAL METALLURGY
Phase Diagram 
Phases
Austenite
Bainite
- Form at between nose & Ms T
- Diffusion controlled
- Nucleation start with ferrite crystal
- Superior ductility & toughness but has lower strength & hardness
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Martensite
- Hard & brittle
- Form when rapidly cooled from austenite.
- Nucleation start with carbide crystal.
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Pearlite
- Form at eutectoid temperature (727 C)
- Eutectoid mix of ferrite & cementite.
- Average carbon content is 0.76% C
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Cementite
- Orthorhombic
- Intermetallic compound of iron & carbon
- Very hard & brittle (can strengthen steel)
- Metastable phase
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Ferrite
- BCC structure
- Soft & ductile
- Stable at T between 1400C & 1539C
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Binary Phase Diagram
Reaction
Eutectoid
- 1 solid phase transform into
2 phase of different T
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Peritectic
- solid phase + liquid phase =second solid phase
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Eutectic
- A liquid transform into 2 solid phase
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- Form from 2 compenent.
- Variables involved:Temperature,Chemical composition,Pressure.
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Single Phase Diagram
- Form form 1 component.
- Single phase area
- Co-existence curve->boundary line
between 2 phase
Phase Transformation
Diffusion mechanism
- At low temperature,slow rate of diffusion increase
time need or the process to start & complete spheroidite
- Diffusion dependant
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Nucleation and Growth
- Below TE = nucleation low, growth high.
- Way below TE = nucleation high, growth low.
- Factor effect: microstructure finer when transformation rates increase,acceleration of transformation
possible if transfer & matter fast.
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Time Temperature Transformation (TTT)
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Alloy
Function
- Increase hardenability reduced
- Rate of tempering than other plain-carbon steel
- Change microstructure which is effect phase stability(phase diagram is altered)
- Allows for transformation deeper into the product than hard surface & soft core
Factor of Change
- Austenite grain size and chemical composition
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Heat Treatment
Annealing
Process anneal
- Heating to T lower critical line allow recovery & recrystallization
- Remove entangled dislocation/slip
- Not austenize & no phase change but cange in grain morpholog
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Full anneal
- Heat to austenize T & slow cooling
- Often in low or medium steel
- Most ductile & toughness
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Stress-relief anneal
- No change in microstructure
- Recovery only ,no recrystallizaton
- Regain some ductility
- Heating to temp lower critical line to relieved some entangled slip
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Normalizing
- Provide desired mechanical properties
- Improve machinibility & hardness
- Heat above critical T(more than annealing)
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Spheroidizing
- Heat below eutectoid
- Hold for a long period
- Cementite (pearlite & proeutectoid network) in steel is spheroidizing
- Used for steel to maximize toughness
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Steel Hardening
Quenching
- Cooling rate necessary to obtain full martensite (hard)
- Quenching medium:water,forced air(air jet) oil,polymer sol
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Tempering
Coventional Tempering
- Quench T below Ms
- Reheat to temper to desired hardness & relieved internal stress
- Autenize for min 0.5 hours
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Austempering
- Hold at 370 C for a long period
- While it pass through bainite
- Bainite:very ductile & tough but lower strength & hardness than martensite
- Once bainite form steel,quench lower to RT
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Martempering
- Incubation period above Ms to equalize T
- Held in salt bath to reduce thermal stress
- Equalize surface & inner T of component
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