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
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Iron-Carbon Isothermal Transformation • Martensite – Nonequilibrium singlephase – From diffusionless transformation of austenite – Rapid quenching – Occurs instantaneously – Time-independent – Competitive with pearlite and bainite – Body-centered tetragonal (BCT) – Supersaturated solid solution
Materials Science
Lath
Lenticular
(less than 0.6 wt% C)
(more than 0.6 wt% C)
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Iron-Carbon Isothermal Transformation
Materials Science
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Iron-Carbon Isothermal Transformation 1.
2.
3.
Rapidly cool to 350 oC, hold for 104 s, and quench to room temperature. Rapidly coo to 250 oC, hold for 100 s, and quench to room temperature. 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
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Iron-Carbon Isothermal Transformation
Materials Science
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Continuous Cooling Transformation Diagram Shift to longer time and lower temperature
Materials Science
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Continuous Cooling Transformation Diagram
Materials Science
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Critical Quenching Rate •
The minimum rate of quenching to generate 100% martensite.
Materials Science
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Mechanical Behavior of Pearlite • Cementite is very hard and brittle.
• Fine pearlite is harder and stronger than coarse pearlite. Materials Science
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Mechanical Behavior of Spheroidite • •
Less boundary area per unit volume in spheroidite Spheroidized steels are extremely ductile.
Materials Science
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Mechanical Behavior of Bainite •
Bainite is stronger and harder than pearlite.
Upper bainite
Pearlite vs. Bainite
(300 ~ 540 oC)
•Competitive
Lower bainite (200 ~ 300 oC)
Materials Science
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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
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Tempered Martensite •
Heat treatment martensite between 250 ~ 650 oC. • Internal stress is relieved. • Ductility and toughness are enhanced. martensite (BCT, single phase) Æ tempered martensite (ferrite + cementite)
