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Unformatted text preview: Chapter 10 Phase transformations A photomicrograph of pearlite steel that has partially transformed to spheroidite. Why study phase transformations? Metalalloys are heat treated to obtain desirable mechanical properties. When they are heat treated, their phases change. Knowledge on phase transformation by heat treatment is necessary in order to design a heat treatment. For example, the tensile strength of an ironcarbon alloy of eutectoid composition (0.76wt % C) can be varied between 700 to 2000 MPa depending on the heat treatment employed. Learning Objectives Know the equations which control transformation to a new phase. Learn about the microstructures that can be formed by heat treating steel alloys: fine pearlite, coarse pearlite, spheroidite, bainite, martensite and tempered martensite. Learn about the mechanical behavior of steel alloys when different microstructures are formed. Learning Objectives cont. Learn how to design a heat treatment to obtain a specific microstructure and mechanical properties in steel. The Kinetics of phase transformation A new phase is formed by nucleation (formation of very small particles) and growth of new phase. The formation of a new phase depends on time. The dependence of the formation of a new phase on time can be expressed as follows: y= 1 exp (kt n ) (Avrami equation) The Kinetics of phase transformation Y is fraction of transformation. K and n are timeindependent constants for the particular reaction. Fig 10.1 shows the dependence of the formation of a new phase on time. Fig 10.1 Plot of fraction reacted versus the logarithm of time typical of many solidstate transformations in which temperature is held constant. Fig 10.2 Percent recrystallization as a function of time and at constant temperature for copper. Fig 10.3 For an ironcarbon alloy of eutectoid composition (0.76 wt% C), isothermal fraction reacted versus the logarithm of time for austeniteto pearlite transformation. The Kinetics of phase transformation When the fraction of transformation y=0.5: The corresponding time of transformation = t 0.5 The rate of transformation to a new phase is defined as: r = 1/ t 0.5 The rate of transformation increases with temperature as: r = A eQ/RT Where: A = constant independent of temperature R = Universal gas constant T = temperature in K Q = activation energy needed for transformation Isothermal transformation diagrams (TimeTemperatureTransformation diagrams) Consider the eutectoid reaction in the Fe C system: γ , Austenite (0.76 wt% C) → Ferrite, α (0.022 wt % C) + Fe 3 C (6.7wt % C) Fig 10.4 Fig 10....
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This note was uploaded on 05/03/2010 for the course ME 250750 taught by Professor Signer during the Summer '10 term at Wichita State.
 Summer '10
 Signer

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