91Ó°ÊÓ

Using the isothermal transformation diagram for an iron-carbon alloy of eutectoid composition (Figure \(10.22)\), specify the nature of the final microstructure (in terms of microconstituents present and approximate percentages of each) of a small specimen that has been subjected to the following timetemperature treatments. In each case assume that the specimen begins at \(760^{\circ} \mathrm{C}\left(1400^{\circ} \mathrm{F}\right)\) and that it has been held at this temperature long enough to have achieved a complete and homogeneous austenitic structure. (a) Cool rapidly to \(700^{\circ} \mathrm{C}\left(1290^{\circ} \mathrm{F}\right)\), hold for \(10^{4} \mathrm{~s}\), then quench to room temperature. (b) Reheat the specimen in part (a) to \(700^{\circ} \mathrm{C}\) \(\left(1290^{\circ} \mathrm{F}\right)\) for \(20 \mathrm{~h}\). (c) Rapidly cool to \(600^{\circ} \mathrm{C}\left(1110^{\circ} \mathrm{F}\right)\), hold for \(4 \mathrm{~s}\), rapidly cool to \(450^{\circ} \mathrm{C}\left(840^{\circ} \mathrm{F}\right)\), hold for \(10 \mathrm{~s}\), then quench to room temperature. (d) Cool rapidly to \(400^{\circ} \mathrm{C}\left(750^{\circ} \mathrm{F}\right)\), hold for \(2 \mathrm{~s}\), then quench to room temperature. (e) Cool rapidly to \(400^{\circ} \mathrm{C}\left(750^{\circ} \mathrm{F}\right)\), hold for \(20 \mathrm{~s}\), then quench to room temperature. (f) Cool rapidly to \(400^{\circ} \mathrm{C}\left(750^{\circ} \mathrm{F}\right)\), hold for \(200 \mathrm{~s}\), then quench to room temperature. (g) Rapidly cool to \(575^{\circ} \mathrm{C}\left(1065^{\circ} \mathrm{F}\right)\), hold for \(20 \mathrm{~s}\), rapidly cool to \(350^{\circ} \mathrm{C}\left(660^{\circ} \mathrm{F}\right)\), hold for \(100 \mathrm{~s}\), then quench to room temperature. (h) Rapidly cool to \(250^{\circ} \mathrm{C}\left(480^{\circ} \mathrm{F}\right)\), hold for \(100 \mathrm{~s}\), then quench to room temperature in water. Reheat to \(315^{\circ} \mathrm{C}\left(600^{\circ} \mathrm{F}\right)\) for \(1 \mathrm{~h}\) and slowly cool to room temperature.

Step by step solution

01

Part (a)

The initial sample is austenite held at \(760^{\circ}\mathrm{C}\). Cool it rapidly to \(700^{\circ} \mathrm{C}\) and hold there for \(10^4\mathrm{s}\). Locate this on the TTT diagram. Holding at this temperature lies in the pearlite region. Since we quench after this treatment, the final microstructure is roughly 100% pearlite.

02

Part (b)

Reheat the sample in part (a) to \(700^{\circ} \mathrm{C}\) for \(20 \mathrm{h}\). This is a very long time at the temperature ensuring that the whole sample will be transformed to austenite. So, the sample has 100% austenite.

03

Part (c)

Start at the initial austenite temperature. Cool it rapidly to \(600^{\circ} \mathrm{C}\) and hold there for \(4\mathrm{s}\). This lies in the bainite region. Now, cool it rapidly to \(450^{\circ} \mathrm{C}\) and hold there for \(10\mathrm{s}\). This lies in the pearlite region. Quench the sample to room temperature. The final microstructure will have bainite and pearlite, with the pearlite content being higher due to the long holding time.

04

Part (d)

Start at the initial austenite temperature. Cool it rapidly to \(400^{\circ} \mathrm{C}\) and hold there for \(2\mathrm{s}\). At this point, the austenite starts to transform into pearlite. Quickly quenching the sample doesn't give the pearlite enough time to completely form. The sample will have a mixture of pearlite and some remaining austenite.

05

Part (e)

Start at the initial austenite temperature. Cool it rapidly to \(400^{\circ} \mathrm{C}\) and hold there for \(20\mathrm{s}\). Holding at this point allows for more pearlite formation, thus having a higher percentage of pearlite with some remaining austenite.

06

Part (f)

Start at the initial austenite temperature. Cool it rapidly to \(400^{\circ} \mathrm{C}\) and hold there for \(200\mathrm{s}\). This is a longer holding time, which causes the sample to transform into pearlite entirely. The final microconstituent is 100% pearlite.

07

Part (g)

Start at the initial austenite temperature. Cool it rapidly to \(575^{\circ} \mathrm{C}\) and hold there for \(20\mathrm{s}\). This sample will transform into a bainite structure. Then, cool it rapidly to \(350^{\circ} \mathrm{C}\) and hold there for \(100\mathrm{s}\). This holding time allows the sample to transform into upper bainite. Quench the sample to room temperature. The final microstructure contains upper bainite and pearlite.

08

Part (h)

Start at the initial austenite temperature. Cool it rapidly to \(250^{\circ} \mathrm{C}\) and hold there for \(100\mathrm{s}\). This sample will transform into martensite. Quench the sample in water, then reheat to \(315^{\circ} \mathrm{C}\) for \(1\mathrm{h}\). This treatment causes the tempering of martensite, which slightly changes the microstructure. Slowly cool the sample to room temperature. The final microstructure is tempered martensite with some remaining austenite.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Iron-Carbon Alloy Microstructure

The microstructure of an iron-carbon alloy is crucial for understanding its mechanical properties. The iron-carbon system forms the basis for all steel and cast iron materials. At varying carbon contents and temperatures, the iron-carbon alloy displays different phases or microconstituents, including austenite, ferrite, cementite, pearlite, bainite, and martensite.

When heated to high temperatures, these alloys typically form an austenitic structure, which is a face-centered cubic (FCC) crystal structure that can dissolve more carbon than the body-centered cubic (BCC) structure of ferrite. Upon cooling, austenite can transform into several different microstructures depending on the cooling rate and heat treatment applied. Rapid quenching, for instance, can result in the formation of martensite, while slower cooling or isothermal holding at specific temperatures forms other microconstituents such as pearlite or bainite.

Eutectoid Composition

An iron-carbon alloy with eutectoid composition contains about 0.76% carbon by weight. At this specific carbon content, the alloy undergoes a transition from austenite to pearlite at a certain temperature in a stable manner. This specific reaction occurs isothermally – at a constant temperature – at approximately 727°C (1341°F) in a eutectoid reaction where the austenite transforms into two different microconstituents: ferrite and cementite, which together form pearlite.

This change in microstructure greatly affects the properties of the steel. When the steel is at eutectoid composition and is allowed to cool slowly from the austenite phase region, it will generally transform into a structure composed entirely of pearlite. The resultant microstructure is pivotal for educators and students to understand, as it determines the mechanical properties of the steel, such as hardness, strength, and ductility.

Austenitic Structure

An austenitic structure in iron-carbon alloys is characterized by a face-centered cubic (FCC) lattice which is stable at high temperatures. Austenite, the high-temperature form of iron, is nonmagnetic and highly ductile, capable of dissolving more carbon than ferrite. Upon cooling, the austenitic structure will transform into various other microconstituents through a series of reactions. The initial austenitic structure is essential for heat treating processes, as it acts as a precursor for microstructural transformations that improve mechanical properties.

Understanding the nature of the austenitic structure is vital because it is so responsive to subsequent cooling treatments. The cooling rate and temperature to which austenite is cooled can be used to control the final microstructure of the steel, thereby tailoring the steel's properties to meet specific requirements of toughness, strength, and hardness.

Quenching and Tempering

Quenching and tempering are heat treatment processes used to enhance the mechanical properties of steel. Quenching involves heating the steel to create an austenitic structure and then rapidly cooling it, typically in water or oil, to trap carbon atoms inside the austenitic matrix and form a very hard and brittle phase called martensite. However, because martensite is too brittle for most applications, tempering is typically performed after quenching.

Tempering involves reheating the quenched steel to a lower temperature, which reduces the hardness of the martensite while increasing ductility and toughness. The tempering temperature and time determine the resulting hardness and strength of the steel. When the process is done correctly, tempering develops a microstructure that is a more balanced compromise between hardness and ductility, crucial for components that must resist both wear and impact.

Microconstituents of Steel

The microconstituents of steel play a defining role in its overall properties. Pearlite, a lamellar mixture of ferrite and cementite, is common in slowly cooled eutectoid steels and provides a good balance of strength and ductility. Bainite is a steel microstructure that is formed at lower temperatures than pearlite and consists of a fine, acicular (needle-like) structure that offers more strength but less ductility than pearlite.

Martensite is the hardest and most brittle microconstituent that forms when austenite is quenched rapidly; it is also very strong. Ferrite, the basic microconstituent of all steel with a BCC structure at room temperature, is soft and ductile but does not hold much carbon. Cementite, or iron carbide, is a hard and brittle substance that increases with increasing carbon content. By manipulating the cooling rate and composition of the iron-carbon alloy, various microstructures can be achieved to meet the required mechanical properties for different applications.