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Chapter 10Chapter 10Phase Phase

TransformationsTransformationsin Metals (2)in Metals (2)Development of Development of

Microstructure and Microstructure and Alteration of Mechanical Alteration of Mechanical

PropertiesProperties

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10.5 Isothermal 10.5 Isothermal Transformation DiagramsTransformation Diagrams

BainiteBainite Consists of ferrite and cementite phase Consists of ferrite and cementite phase

in which elongated particles of Fein which elongated particles of Fe33C are C are in ferrite matrix. in ferrite matrix.

Form only when cooling down to < Form only when cooling down to < 540540ooC, above 540C, above 540ooC only pearlite phase. C only pearlite phase.

See Fig. 10.7 ~ Fig. 10.9See Fig. 10.7 ~ Fig. 10.9 The formation of Bainite obey kinetic The formation of Bainite obey kinetic

equation:equation: )(RT

Q

eAr

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10.5 Isothermal 10.5 Isothermal Transformation DiagramsTransformation Diagrams

For 1.13 wt% C iron-carbon steel

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Formation of BainiteFormation of Bainite

Nose,Max rate of transformati

on

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Formation of BainiteFormation of Bainite

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Formation of BainiteFormation of Bainite

200-300200-300ooC: lower BainiteC: lower Bainite 300-540300-540ooC: upper BainiteC: upper Bainite Above 540Above 540ooC, transformation will be C, transformation will be

pearlite.pearlite. There is max. rate of transformation at There is max. rate of transformation at

540540ooC, denoted as C, denoted as ““NN”” point. point. Pearlitic and bainitic transformation are Pearlitic and bainitic transformation are

competitive with each other. Once form, competitive with each other. Once form, can not transform to the other phase. can not transform to the other phase.

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10.5 Isothermal 10.5 Isothermal Transformation DiagramsTransformation Diagrams

SpheroiditeSpheroidite If pearlite or bainite is heated to If pearlite or bainite is heated to

temperature below eutectoid temperature temperature below eutectoid temperature but keep for a long time.but keep for a long time.

FeFe33C phase become sphere-likeC phase become sphere-like Diffusion of Carbon but no change in Diffusion of Carbon but no change in

composition nor amount of phases. (ferrite, composition nor amount of phases. (ferrite, cementite)cementite)

Driving force for this transformation is the Driving force for this transformation is the reduction in α-Fereduction in α-Fe33C phase boundary area. C phase boundary area.

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10.5 Isothermal 10.5 Isothermal Transformation DiagramsTransformation Diagrams

Which is the more stable

microstructure ? The pearlite

or the spheroidite

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10.5 Isothermal 10.5 Isothermal Transformation DiagramsTransformation Diagrams

MartensiteMartensite Is a nonequilibrium single-phase structure Is a nonequilibrium single-phase structure

that results from a that results from a diffusionless diffusionless transformationtransformation of austenite after rapid of austenite after rapid cooling to room temperature. cooling to room temperature.

F.C.C. austenite change to a F.C.C. austenite change to a B.C.T. (body-centered tetragonal) ) martensite.martensite.

Phase change instantaneously, time-Phase change instantaneously, time-independent process.independent process.

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10.5 Isothermal 10.5 Isothermal Transformation DiagramsTransformation Diagrams

MartensiteMartensite The cooling temperature must sufficiently low The cooling temperature must sufficiently low

in order to prevent carbon diffusion.in order to prevent carbon diffusion. Martensite transformation start at certain low Martensite transformation start at certain low

temperature range, the transformation is temperature range, the transformation is termed termed athermal transformation, i.e. athermal transformation, i.e. temperature-dependent transformationtemperature-dependent transformation,, more lower temperature, more martensite more lower temperature, more martensite forms. (M(start), M(50%), M(90%) in forms. (M(start), M(50%), M(90%) in Fig. 10.13))

As long as the temperature is maintained, there As long as the temperature is maintained, there will be no further martensite transformation. will be no further martensite transformation.

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B.C.T. structureB.C.T. structure

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Martensite Martensite MicrostructureMicrostructure

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athermal transformationathermal transformation

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Martensite Martensite TransformationTransformation

The presence of alloying elements The presence of alloying elements (e.g., Cr, Ni, Mo, and W)(e.g., Cr, Ni, Mo, and W) Shifting to longer times the nose of the Shifting to longer times the nose of the

austenite-to pearlite transformation)austenite-to pearlite transformation) The formation of a separate bainite nose. The formation of a separate bainite nose.

See Fig. 10.14See Fig. 10.14 Steel contains only carbon is called plain Steel contains only carbon is called plain

carbon steel.carbon steel. Steel contains other elements than Steel contains other elements than

carbon is called alloy steel. carbon is called alloy steel.

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Martensite Martensite TransformationTransformation

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Ex 10.2 Ex 10.2

Specify the microstructure upon Specify the microstructure upon coolingcooling Rapidly cool to 350Rapidly cool to 350ooC, hold for 10C, hold for 1044s, and s, and

quench to room temperature.quench to room temperature. Rapidly cool to 250Rapidly cool to 250ooC, hold for 100s, C, hold for 100s,

and quench to room temperature.and quench to room temperature. Rapidly cool to 650Rapidly cool to 650ooC, hold for 20s, C, hold for 20s,

rapidly cool to 400oC, hold for 10rapidly cool to 400oC, hold for 1033 s, s, and quench to room temperature. and quench to room temperature.

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Ex Ex 10.210.2

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End of section (2)End of section (2)

You have learned:You have learned: Formation of Bainite phase.Formation of Bainite phase. Formation of Martensite phase.Formation of Martensite phase. Isothermal phase transformation Isothermal phase transformation Athermal phase transformation Athermal phase transformation Cooling curve effect on phase Cooling curve effect on phase

transformation.transformation.