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SUMMARY OF PHASE DIAGRAM,TTT & CCT DIAGRAM, & HOT TREATMENT, PHASE…
SUMMARY OF PHASE DIAGRAM,TTT & CCT DIAGRAM, & HOT TREATMENT
PHASE DIAGRAM
- Phase transformation under equilibrium cooling(slow cooling)
- Temperature vs. composition
- Also tell about important of phases.
TTT
- Transformation at diffrent cooling rate
- have 2 additional phase due to non equilibrium which is Bainite&Martensite
- has 1 specific composition
- phases being predict with diffrent cooling rate
- Temperature vs. time
CCT
- TTT has little alteration becomes CCT
- Usually does not use Isothermal cooling
- single cooling rate throughout cooling stage using cooling medium.
- Martensite have the highest hardness.
- Large Martensite grain have lower hardness compared to small grain.
Spheroidite<coarse pearlite<fine pearlite<Bainite<Martensite tempered<Martensite
- When strength high,ductility will less
- There are 3 types of phase diagram which is unary,binary and ternary.
- Have unlimited solubility with single phase, limited with multiple phases and no solubility (oil&water region).
-Hume Rothery's condition for solid solubility:-
-> similar size
->same crystal structure
-> same valence
-> = electronegativity
- There are 2 types of solid solution, interstitial and substitutional.
- Fast rate of cooling have cored structure whereas slow cooling have equilibrium structure.
Phases in phase diagram;-
- Pure metal
- Solid solution
- Intermetallic compound
- Interstitial compound
- have congruent phase no compositional change due to transformatian.
- Meanwhile incongruent phase at least 1 phase will change.
- Possible reaction between elements are eutectic, peritectic,monotectic,eutectoid & peritectoid.
HEAT TREATMENT :fire:
- Heat treatment consist of annealing, normalize and spheroidize
Annealing
- Done in deform material to regain ductility.
- Process: Heating-hold-cool down
- 3 types of anneal-> Full annealing, process anneal, stress-relief anneal
- Full anneal is heated to austenization Temp. , have highest ductility,coarse pearlite and have low hardness.
- Process anneal no austenization, Only recovery and recrystallization.
- Recovery process only for stress-relief anneal. Then no change in microstructure. It relieve entanglement and dislocation and regain ductility.
Sperodizing
- To improve machinability,produce coarse spheroidal/globular cementite particles in ferritic matrix.
- Process: Heat below eutectoid-hold.
- Requires when lowest hardnest needed to make easy machinability.
- Done in medium or high C steel.
- Parameters similar to process annealing.
- Pearlitic structure and cementite network are dissolve.
Normalizing
- To refine grain and get uniform structure.
- Faster cooling rate = finer pearlite=stronger&harder steel.
- For TTT when cool down at diffrent rate , diffrent microstructure will form
- There is present of non-equilibrium phase which is Martensite and Bainite.
- At high temperature coarse pearlite is form due to faster diffusion.
- Meanwhile for low temperature it form fine pearlite due to slower diffusion.
- When cooling rate is faster Martensite is form .
- Martensite also not an Isothermal transformation.
- Martensite have hard & brittle structure.
- Martensite is form as the cooling rate is faster where does not have time to form Pearlite/ Bainite.
- Bainite is feather like shaped.
- There is also spheroidite structure when heating too long.
- For moderate cooling, Bainite, Pearlite & Pearlite + Bainite will form
- When TTT undergo alteration it will shift to CCT.
- Often Isothermal cooling is not being done.
- The time required for reaction to begin & end is delayed.
- TTT curves shifted to longer time and temperature. Where the TTT line will shift to right.
- Bainite will not form.
- Austenite-> pearlite transformation just below the nose in the graph.
- Iron-iron carbide is an example of phase diagram with equilibrium phases
- Iron&Carbon is 2 main element in steel where Iron is allotropic that can be exist in <1 phase.
- It consists of eutectic , eutectoid and peritectic reaction.
- Consists of Austenite,Ferrite and Cementite as present phases.
- Cementite is not an alloy element.
- Ferrite is alpha iron, BCC&magnetic.
- Austenite is gamma iron , FCC & non-magnetic.
- Steel must be taken to Austenite region for softening and hardening. Whenever slowly cooled it will transform from into Ferrite & Cementite.
- Austenite when transform at 0.8% C with 723 degree celcius will form Pearlite.
- Pearlite is eutectoid mixture of ferrite & Cementite that form lamellar structure .
- Pearlite present in hypereutectoid,hypoeutectoid & eutectoid steel.
- proeutectoid also present before eutectoid form.
- Austenite that have c content <0.8% become ferrite.
- Austenite with C content >0.8% become cementite.
Phases:-
1) Ferrite
2) Eutectic steel - Pearlite
3) High C steel - Pearlite + cementite4) Medium C steel - Pearlite+ferrite
Hardening: Quenching & Tempering
- Austenite is to gain ductility whereas hardening via quenching is to increase hardness of steel.
- HAVE 3 STEPS:-
Austenize until hot --> Quench--> Tempered if needed
- Medium of quenchng are water, air jet, polymer solution,salt & oil.
- Choice of medium to quench depending on final hardness value.
Tempering
- Tempered is to reduce residual stress & to gain some ductility & toughness.
- Terms in tempering are;-
- retained Austenite
-tempered Martensite
-Quench cracks
-Marquenching
AUSTEMPERING VS. MARTEMPERING
For Martempering;-
- Equalize surface and inner temperature of component.
- After tempered --> Martensite.
- Minimal tempered gradient & transformation stress to avoid distortion and cracking.
For Austempering;-
- Resemble Martempering except after holding temperature.
- Forming Bainite.
- Form between rapid cooling curve Martensite & slower cooling Pearlite.
- When high temperature tempering, elongation increase, but hardness and yield strength will reduce due high C content diffuse out.
Methods carrying Quenching & Tempering:
- Conventional tempering
- Martempering
- Austempering
- All have similar step
- But differ on how to quench
- Tempering at high temperature dispersed Ferrite & Cementite further
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