Using the isothermal transformation diagram for a 1.13 wt %...

Using the isothermal transformation diagram for a $1.13$ wt $\%$ C steel alloy (Figure $10.39$ ), determine the final microstructure (in terms of just the microconstituents present) of a small specimen that has been subjected to the following timetemperature treatments. In each case assume that the specimen begins at $920^{\circ} \mathrm{C}\left(1690^{\circ} \mathrm{F}\right)$ and that it has been held at this temperature long enough to have achieved a complete and homogeneous austenitic structure. (a) Rapidly cool to $250^{\circ} \mathrm{C}\left(480^{\circ} \mathrm{F}\right)$, hold for $10^{3} \mathrm{~s}$, then quench to room temperature. (b) Rapidly cool to $775^{\circ} \mathrm{C}$ (1430^\circ), hold for $500 \mathrm{~s}$, then quench to room temperature. (c) Rapidly cool to $400^{\circ} \mathrm{C}\left(750^{\circ} \mathrm{F}\right)$, hold for $500 \mathrm{~s}$, then quench to room temperature. (d) Rapidly cool to $700^{\circ} \mathrm{C}\left(1290^{\circ} \mathrm{F}\right)$, hold at this temperature for $10^{5} \mathrm{~s}$, then quench to room temperature. (e) Rapidly cool to $650^{\circ} \mathrm{C}\left(1200^{\circ} \mathrm{F}\right)$, hold at this temperature for $3 \mathrm{~s}$, rapidly cool to $400^{\circ} \mathrm{C}$ $\left(750^{\circ} \mathrm{F}\right)$, hold for $25 \mathrm{~s}$, then quench to room temperature.

Using the isothermal transformation diagram for a $1.13$ wt $\%$ C steel alloy (Figure $10.39$ ), determine the final microstructure (in terms of just the microconstituents present) of a small specimen that has been subjected to the following timetemperature treatments. In each case assume that the specimen begins at $920^{\circ} \mathrm{C}\left(1690^{\circ} \mathrm{F}\right)$ and that it has been held at this temperature long enough to have achieved a complete and homogeneous austenitic structure.
(a) Rapidly cool to $250^{\circ} \mathrm{C}\left(480^{\circ} \mathrm{F}\right)$, hold for $10^{3} \mathrm{~s}$, then quench to room temperature.
(b) Rapidly cool to $775^{\circ} \mathrm{C}$ (1430^\circ), hold for $500 \mathrm{~s}$, then quench to room temperature.
(c) Rapidly cool to $400^{\circ} \mathrm{C}\left(750^{\circ} \mathrm{F}\right)$, hold for $500 \mathrm{~s}$, then quench to room temperature.
(d) Rapidly cool to $700^{\circ} \mathrm{C}\left(1290^{\circ} \mathrm{F}\right)$, hold at this temperature for $10^{5} \mathrm{~s}$, then quench to room temperature.
(e) Rapidly cool to $650^{\circ} \mathrm{C}\left(1200^{\circ} \mathrm{F}\right)$, hold at this temperature for $3 \mathrm{~s}$, rapidly cool to $400^{\circ} \mathrm{C}$ $\left(750^{\circ} \mathrm{F}\right)$, hold for $25 \mathrm{~s}$, then quench to room temperature.

Key Concepts

Phase Transformation Kinetics

Phase Transformation Kinetics refers to the study of the rates at which austenite transforms into various microconstituents under different temperature conditions. It encompasses concepts such as nucleation, growth, and the incubation period required for phase changes, all of which are essential for interpreting TTT diagrams and predicting the final microstructure.

Cooling and Quenching

Cooling and Quenching are heat treatment processes that control the rate of temperature drop from the austenitic state. Rapid cooling (or quenching) can bypass diffusional transformations and favor the formation of martensite, while slower cooling rates allow sufficient time for diffusive transformations to produce phases like pearlite or bainite. The specific cooling path chosen determines the balance and distribution of phases in the final microstructure.

Bainite

Bainite is another microstructure resulting from the transformation of austenite, forming at intermediate temperatures between the pearlite and martensite regions. It is characterized by a fine microstructure with features distinct from those of pearlite, reflecting a different nucleation and growth process that occurs more rapidly than pearlite formation but requires diffusion.

Martensite

Martensite is a supersaturated, body-centered tetragonal (BCT) phase formed by a diffusionless transformation when austenite is rapidly cooled below a critical temperature. This transformation preserves the high-temperature structure in a distorted form, resulting in a hard and brittle microstructure. Quenching from the austenitic region typically leads to martensite formation if no other diffusive transformations have occurred.

Pearlite

Pearlite is a two-phase lamellar microstructure comprising alternating layers of ferrite and cementite. It typically forms during slower cooling of austenite at temperatures where diffusion is active, and its formation is depicted on TTT diagrams as a diffusive transformation that occurs over a relatively long time scale.

Austenite

Austenite is a high-temperature, face-centered cubic (FCC) phase of iron in which carbon is uniformly dissolved. It serves as the parent phase for many transformations during cooling in steels. The assumption of a homogeneous austenitic structure prior to cooling ensures that the ensuing phase transformations are controlled solely by the subsequent heat treatment parameters.

Isothermal Transformation Diagram

An Isothermal Transformation Diagram, also known as a TTT diagram, is a graphical representation that shows the time and temperature conditions under which various phase transformations occur in steels that are initially fully austenitic. It maps out transformation start and finish times for phases such as pearlite, bainite, and martensite, and is a critical tool for predicting the final microstructure after specific heat treatments.

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