Chapter 10 Phase Transformations in Metals hfang2/MECE3345/Lectures/ Iron-Carbon Isothermal...

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Transcript of Chapter 10 Phase Transformations in Metals hfang2/MECE3345/Lectures/ Iron-Carbon Isothermal...

  • Materials Science Chapter 10 1

    Chapter 10 Phase Transformations in Metals

  • Materials Science Chapter 10 2

    The Kinetics of Solid-State Reactions • Obstacles in solid-state

    transformation – Diffusion

    – Energy increase

    • Phase transformation stages – Nucleation: at grain boundaries

    – growth

    • Kinetics (time-dependent)

    y = 1 – exp(-ktn)

    T = constant

    • Rate of transformation

    • Characterization of transformation

    S-Curve

    5.0

    1 t

    r =

  • Materials Science Chapter 10 3

    Temperature Dependence RTQAer /−=

    Percent recrystallization for pure copper

  • Materials Science Chapter 10 4

    Iron-Carbon Isothermal Transformation • Pearlite

    )%7.6()%022.0()%76.0( 3 CwtCFeCwtCwt +⇔αγ

  • Materials Science Chapter 10 5

    Iron-Carbon Isothermal Transformation • T-T-T diagram (isothermal

    transformation diagram) • Eutectoid temperature • Supercooling • Phase regions

    • Transformation rate • Pearlite nucleation

    • This curve only valid for eutectoid composition

  • Materials Science Chapter 10 6

    Iron-Carbon Isothermal Transformation

    • Thermal treatment curve

  • Materials Science Chapter 10 7

    Iron-Carbon Isothermal Transformation

    • Coarse pearlite • Fine pearlite

  • Materials Science Chapter 10 8

    Iron-Carbon Isothermal Transformation

  • Materials Science Chapter 10 9

    Iron-Carbon Isothermal Transformation

    • Coarse pearlite • Fine pearlite

  • Materials Science Chapter 10 10

    Iron-Carbon Isothermal Transformation • Bainite Upper bainite

    (300 ~ 540 oC) Pearlite vs. Bainite

    •Competitive

    Lower bainite

    (200 ~ 300 oC)

  • Materials Science Chapter 10 11

    Iron-Carbon Isothermal Transformation

    • Spheroidite – From pearlite or bainite

    – Heated about 700 oC for 18 ~24 hours

    – Driving force • Reduction of α - Fe3C

    phase boundary area

  • Materials Science Chapter 10 12

    Iron-Carbon Isothermal Transformation

    • Martensite – Nonequilibrium single-

    phase – From diffusionless

    transformation of austenite – Rapid quenching – Occurs instantaneously – Time-independent – Competitive with pearlite

    and bainite – Body-centered tetragonal

    (BCT) – Supersaturated solid

    solution

    Lenticular

    (more than 0.6 wt% C)

    Lath

    (less than 0.6 wt% C)

  • Materials Science Chapter 10 13

    Iron-Carbon Isothermal Transformation

  • Materials Science Chapter 10 14

    Iron-Carbon Isothermal Transformation

    1. Rapidly cool to 350 oC, hold for 104 s, and quench to room temperature.

    2. Rapidly coo to 250 oC, hold for 100 s, and quench to room temperature.

    3. Rapidly cool to 650 oC, hold for 20 s, rapidly cool to 400 oC, hold for 103 s, and quench to room temperature.

  • Materials Science Chapter 10 15

    Iron-Carbon Isothermal Transformation

  • Materials Science Chapter 10 16

    Continuous Cooling Transformation Diagram

    Shift to longer time and lower temperature

  • Materials Science Chapter 10 17

    Continuous Cooling Transformation Diagram

  • Materials Science Chapter 10 18

    Critical Quenching Rate

    • The minimum rate of quenching to generate 100% martensite.

  • Materials Science Chapter 10 19

    Mechanical Behavior of Pearlite

    • Cementite is very hard and brittle.

    • Fine pearlite is harder and stronger than coarse pearlite.

  • Materials Science Chapter 10 20

    Mechanical Behavior of Spheroidite • Less boundary area per unit

    volume in spheroidite • Spheroidized steels are

    extremely ductile.

  • Materials Science Chapter 10 21

    Mechanical Behavior of Bainite • Bainite is stronger and harder than

    pearlite. Upper bainite

    (300 ~ 540 oC) Pearlite vs. Bainite

    •Competitive

    Lower bainite

    (200 ~ 300 oC)

  • Materials Science Chapter 10 22

    Mechanical Behavior of Martensite

    • Martensite is the hardest and strongest and most brittle (negligible ductility).

    • Related to interstitial carbon atoms.

    • Volume increase during phase transformation.

  • Materials Science Chapter 10 23

    Tempered Martensite • Heat treatment martensite

    between 250 ~ 650 oC. • Internal stress is relieved. • Ductility and toughness are

    enhanced. martensite (BCT, single phase)

    tempered martensite (ferrite + cementite)

    9300 X

  • Materials Science Chapter 10 24

    Temperature and Time Influence on Tempering

  • Materials Science Chapter 10 25

    Iron-Carbon Transformation

  • Materials Science Chapter 10 26

    Iron-Carbon Transformation

  • Materials Science Chapter 10 27

    Mechanical Behavior

  • Materials Science Chapter 10 28

    Mechanical Behavior

  • Materials Science Chapter 10 29

    Mechanical Behavior

  • Materials Science Chapter 10 30

    Mechanical Behavior

    Chapter 10Phase Transformations in Metals The Kinetics of Solid-State Reactions Temperature Dependence Iron-Carbon Isothermal Transformation Iron-Carbon Isothermal Transformation Iron-Carbon Isothermal Transformation Iron-Carbon Isothermal Transformation Iron-Carbon Isothermal Transformation Iron-Carbon Isothermal Transformation Iron-Carbon Isothermal Transformation Iron-Carbon Isothermal Transformation Iron-Carbon Isothermal Transformation Iron-Carbon Isothermal Transformation Iron-Carbon Isothermal Transformation Continuous Cooling Transformation Diagram Continuous Cooling Transformation Diagram Critical Quenching Rate Mechanical Behavior of Pearlite Mechanical Behavior of Spheroidite Mechanical Behavior of Bainite Mechanical Behavior of Martensite Tempered Martensite Temperature and Time Influence on Tempering Iron-Carbon Transformation Iron-Carbon Transformation Mechanical Behavior Mechanical Behavior Mechanical Behavior Mechanical Behavior