Lecture - Chapter 10

Lecture - Chapter 10 - Chapter 10 Phase transformations A...

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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? Metal-alloys 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 iron-carbon 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 time-independent 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 solid-state 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 iron-carbon alloy of eutectoid composition (0.76 wt% C), isothermal fraction reacted versus the logarithm of time for austenite-to- 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 e-Q/RT Where: A = constant independent of temperature R = Universal gas constant T = temperature in K Q = activation energy needed for transformation Isothermal transformation diagrams (Time-Temperature-Transformation 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 250-750 taught by Professor Signer during the Summer '10 term at Wichita State.

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Lecture - Chapter 10 - Chapter 10 Phase transformations A...

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