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MATSE 259 - Exam 3 Questions With Verified Solutions
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MATSE 259 - Exam 3 Questions With Verified Solutions Simple Diffusion Dependent Transformations No change in number or composition of phases present Diffusion Dependent Transformations Alterations in phase compositions and number of phases present Diffusionless Transformations metasta...

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  • June 7, 2024
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  • 2023/2024
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MATSE 259 - Exam 3 Questions With Verified
Solutions
Simple Diffusion Dependent Transformations
No change in number or composition of phases present


Diffusion Dependent Transformations
Alterations in phase compositions and number of phases present


Diffusionless Transformations
metastable phase, diffusion is very slow, a new phase


Two Types of Phase Transformation Stages
Nucleation (small particles) & Growth (increasing the size, parent phase disappears)


Two Types of Nucleation
Homogenous (uniform throughout parent phase) & Heterogenous (structural irregularities, container
surfaces and insoluble impurities)


What is the rate of transformation?
1/t(0.5)


Phase Transformations
1. crossing boundary in phase diagrams
2. during transformation - alloy proceeds to equilibrium
3. but most take time - heat treatment and microstructure
4. in reality we cool rapidly and get intermediate phases


Pearlite
1. austenite to alpha + Fe3C
2. Higher temp- coarse pearlite Lower temp- fine pearlite


Bainite
1. diffusion is involved
2. appear like plates and needles
3. thickness depends on temp
4. after nose on diagram


If some portion forms into pearlite or kainite, what must happen before transforming into another
microconstutuent?
Must reheat back to Austenite


Spheroidite
1. heating pearlite or bainite just below eutectoid for long period (24 hr)
2. Fe3C spheres

, Martensite
1. rapidly cooling to very low temperatures
2. not enough time for diffusion (diffusionless)
3. Cools without passing through transformation curves
4. Body Centered tetragonal (supersaturated, transform into another structure, diffusionless)
5. Appears of sharp plates and needles,
6. Retain Austenite can exist
7. DOES Not appear on phase diagram


Alloy Steels
1. Plain Steel - If carbon is only alloying element
2. Alloy Steel - If you add Cr, Ni, Si, or Mn
3. They shift isothermal curve and delay


Continous Cooling Diagrams
1. Time is required for reaction to begin and end
2. Isothermal curves are shifted to larger time and lower temps


Critical Cooling Rate
The minimum rate of quench that produces totally martensite structure


How to Increase Mechanical Behavior?
1. Increase carbon content (more Fe3C, stronger, yield and UTS)


Rank the microstructures that can be produced by quenching the eutectoid of Austenite
Martensite
Tempered Martensite
Fine Bainite
Coarse Bainite
Fine Pearlite
Coarse Pearlite
Spheroidite


Tempered Martensite
1. martensite after quenching is very brittle
2. increase in ductility and toughness by tempering
3. Almost as hard/strong as martensite but better ductility
4. If over tempered, becomes spheroidic


Temper Embrittlement
1. when impact test shows a reduction in toughness of the tempered steel
2. occurs when steel tempered @ 1070 & slow cooled or steel tempered from 700-1070 and found in
concentrations of Ni or Mg
3. result is duc to brit transition temp is shifted higher
4. crack propagation - intergranular (along GB)
5.Control by: compositional control OR tempering >1070 or <700 before quenching to room temp


Austenite Tree
Slow Cooled: Pearlite

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