Kinetics – time dependence of transformation rate

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Kinetics – time dependence of transformation rate

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Kinetics of Solid-State ReactionsMost reactions involve impedance Formation of a new phase Composition different from parent Atomic rearrangement via diffusion required Energy increase at new boundaries Processes in microstructural transformation Nucleation is first process in phase transformation Occurs at imperfection sites, grain boundaries Growth (until equilibrium)

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Typical kinetic behavior for most solid-state reactions

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Fraction of transformation (Avrami equation)Rate of transformation

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For most reactions rate increases with temperature:

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Isothermal Transformation Diagrams (Pearlite)(0.76 wt% C) (0.022 wt% C)+Fe3C(6.70 wt% C) Upon cooling  (intermediate carbon content) transforms to  (much lower carbon content) and Fe3C (much high carbon content) Pearlite =  + Fe3C

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Pearlite (Eutectoid Composition) Reaction v. Log of TimeTemperature kept constant throughout course of experiment

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More Convenient AnalysisOnly valid for eutectoid composition

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Note from Figure 11.4Derived from series of S-curves Plot of temperature (y-axis ) v. log time in seconds (x-axis) Austenite transformation only upon cooling below eutectoid temperature Beginning curve, 50% transformation, and completion curve Austenite to left: pearlite to right Start and finish curves are nearly parallel, and they approach eutectic line asymptotically

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Reaction Rate At temperature just below eutectic line rate is very slow Apparent contradiction to r = Ae-Q/RT – increase in temperature causes increase in rate of reaction Between about 540 0C and 727 0C – transformation is controlled by pearlite nucleation Nucleation rate decreases with rising temperature (less supercooling) Activation energy (Q) of nucleation increases with temperature But, at lower temperatures austenite decomposition – transformation is diffusion controlled (as predicted by equation 11.3)

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Isothermal Transformation Diagram (Time–Temperature–Transformation, T-T-T)Very rapid cooling (AB) Isothermal (BCD) C (3.5s is beginning) D (15s is completion)

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Compute the mass fractions of  ferrite and cementite in pearlite

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This problem asks that we compute the mass fractions of ferrite and cementite in pearlite. The lever-rule expression for ferrite is And, since = 6.70 wt% C, Co = 0.76 wt% C, and C = 0.022 wt% C Similarly, for cementite

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Morphology of PearliteFerrite to cementite (approximately 8:1) At temperature just below eutectoid – relatively thick  and Fe3C layers (coarse pearlite) Diffusion rates are relatively high and carbon diffuses over long distances With decreasing temperature, carbon diffusion rate decreases and layers become thinner (fine pearlite)

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Bainite – another product of austenite transformationNeedles or plates – needles of ferrite separated by elongated particles of the Fe3C phase Bainite forms as shown on the T-T-T diagram at temperatures below those where pearlite forms Pearlite forms – 540 to 727 0C Bainite forms – 215 to 540 0C Bainite and pearlite are competitive with each other – once some portion of an alloy is transformed to either pearlite or bainite, transformation to the other microconstituent is not possible without reheating to form austentite Unlike pearlite – kinetics of bainite obey Arrhenius equiation – why?

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Maximum rate of transformation Pearlite – nucleation controlledBainite – diffusion controlled

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SpheroiditeSteel alloy having either pearlite or bainite microstructure heated to and left at a temperature below eutectic line for long period of time (18 to 24h) Microstructure formed is sphere like particles embedded in a continuous -phase Transformation due to additional carbon diffusion with no change in composition Driving force- reduction of -Fe3C boundary line

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MartensiteFormed when austenite cools rapidly (or is quenched) to a relatively low temperature (near ambient) – instantaneous Diffusionless transformation of austenite Competitive with pearlite and banite Occurs when quenching rate is rapid enough to prevent carbon diffusion Must be formed from austenite; cannot be formed from pearlite or bainite

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Martensite, (cont’d)FCC austenite experiences polymorphic transformation to BCT – diffusionless transformation from austentite – almost instantaneous since not dependent on diffusion Martensite structure typically maintained indefinately at room temperatures Supersaturated solid solution capable of rapidly transforming to other structures if heated to temperatures at which diffusion rates become appreciable

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Needle-shaped Portion is Martensite – Rest is austensiteMartensite does not appear on iron-rich phase diagram because it is metastable

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Last Updated: 8th March 2018

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