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Chapter 10: Problem 22

Make a copy of the isothermal transformation diagram for a 1.13 wt\% \(C\) iron- carbon alloy (Figure 10.39 ), and then on this diagram sketch and label time- temperature paths to produce the following microstructures: (a) \(6.2 \%\) proeutectoid cementite and \(93.8 \%\) coarse pearlite (b) \(50 \%\) fine pearlite and \(50 \%\) bainite (c) \(100 \%\) martensite (d) \(100 \%\) tempered martensite

Short Answer

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Question: Sketch and label the time-temperature paths on an isothermal transformation diagram for a 1.13 wt% C iron-carbon alloy to produce the following microstructures: (a) 6.2% proeutectoid cementite and 93.8% coarse pearlite, (b) 50% fine pearlite and 50% bainite, (c) 100% martensite, and (d) 100% tempered martensite. Answer: Using the isothermal transformation diagram for a 1.13 wt% C iron-carbon alloy, sketch and label the time-temperature paths for each microstructure as follows: (a) start the transformation from the upper left of the region before the proeutectoid cementite line and pass through the coarse pearlite region, (b) cool the alloy to the temperature range between the end of the fine pearlite and beginning of the bainite region and hold until complete transformation occurs, (c) cool the alloy quickly from the austenite region to below the martensite start (Ms) temperature and then follow a path below the martensite finish (Mf) temperature, and (d) first follow the path for 100% martensite formation, then reheat the alloy to the tempering temperature above the Mf temperature and hold until the transformation occurs.

Step by step solution

01

Understand the Isothermal Transformation Diagram

The isothermal transformation diagram, also referred to as a Time-Temperature-Transformation (TTT) diagram, represents the transformations of microstructures in an alloy over time when cooling at different temperatures. For a 1.13 wt% C iron-carbon alloy, we have phase regions for proeutectoid cementite, pearlite (coarse and fine), bainite, martensite, and tempered martensite in the diagram.
02

Locate Microstructure Regions on Diagram

On the isothermal transformation diagram, identify the regions of microstructures present for each case: (a) proeutectoid cementite & coarse pearlite (b) fine pearlite & bainite (c) martensite (d) tempered martensite
03

Sketch Time-Temperature Paths for Each Microstructure

For each of the given microstructures, sketch the time-temperature paths on the isothermal transformation diagram: (a) For 6.2% proeutectoid cementite and 93.8% coarse pearlite, begin the transformation from the upper left of the region before the proeutectoid cementite line and pass through the coarse pearlite region. (b) For 50% fine pearlite and 50% bainite, start cooling from the austenite region to the temperature range between the end of the fine pearlite and beginning of the bainite region, and hold this temperature until complete transformation occurs. (c) For 100% martensite, cool the alloy quickly from the austenite region to below the martensite start (Ms) temperature and then follow a path below the martensite finish (Mf) temperature. (d) For 100% tempered martensite, first follow the path for 100% martensite formation and then reheat the alloy to the tempering temperature above the Mf temperature and hold until the transformation occurs.
04

Label the Paths on the Diagram

For each of the paths sketched in the previous step, label them with the microstructure they correspond to: (a) 6.2% proeutectoid cementite and 93.8% coarse pearlite (b) 50% fine pearlite and 50% bainite (c) 100% martensite (d) 100% tempered martensite Now, the isothermal transformation diagram with the required time-temperature paths for the given microstructures is complete.

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Key Concepts

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

Proeutectoid Cementite
Proeutectoid cementite refers to the cementite (an iron carbide, Fe3C) that forms before the eutectoid transformation in steel alloys. In the context of the isothermal transformation diagram for a 1.13 wt% C iron-carbon alloy, the formation of proeutectoid cementite occurs as the alloy is cooled down from the austenite phase region (where iron is in a face-centered cubic structure combine with carbon).
Starting above the eutectoid temperature, the cooling must be at a rate that allows carbon to diffuse out and form cementite before crossing the line for the pearlite transformation. The exercise requires 6.2% proeutectoid cementite which means that a small proportion of cementite will form before the remainder of the alloy transforms into pearlite.
The region specifically designated for proeutectoid cementite on an isothermal transformation diagram allows us to visualize and control the cooling rate to ensure the correct proportion. By carefully cooling the material through the correct path on the diagram, we can create a desired balance of strength and ductility in the alloy.
Pearlite Microstructure
Pearlite is a common microstructural feature in steel alloys, created during the slow cooling of austenite past the eutectoid temperature. It derives its name from its pearly appearance under microscopic examination. Pearlite is a lamellar, or layered, structure composed of alternating layers of ferrite (a body-centered cubic form of iron) and cementite that form simultaneously due to the eutectoid reaction. The relative thickness of these layers and the amount of pearlite in an alloy can significantly alter its mechanical properties.
In the specified exercise, coarse pearlite is mentioned. This is produced at higher temperatures within the pearlite region on the isothermal transformation diagram, where diffusion occurs more readily, hence forming wider layers of ferrite and cementite. Coarse pearlite offers better ductility but lower strength compared to its fine counterpart. The diagram provides insights into the conditions required to achieve either a coarse or fine pearlite, depending on the intended application of the material.
Martensite Transformation
Martensite refers to a supersaturated solid solution of carbon in a body-centered tetragonal (BCT) form of iron which is formed by the rapid cooling (quenching) of austenite. Unlike the gradual transformations of proeutectoid cementite and pearlite, the martensite transformation does not involve diffusion - it’s a diffusionless transformation that occurs almost instantaneously when the material is cooled below the martensite start temperature (Ms). The result is a very hard, but brittle, microstructure.
For students, the critical point to understand is that the formation of martensite is heavily dependent on the rate of cooling. The isothermal transformation diagram highlights the Ms and martensite finish (Mf) temperatures. To achieve a microstructure of 100% martensite, as mentioned in the exercise, the alloy must be cooled rapidly through these temperature points without allowing time for other transformations to occur. It's this martensitic transformation that gives many steel alloys their high-strength characteristics, making it invaluable for numerous tools and structural applications.
Tempered Martensite
Tempered martensite forms when martensite, which is initially very hard and brittle, is reheated to a lower temperature—below the eutectoid temperature and usually above the martensite finish (Mf) temperature. This process, known as tempering, is done in order to achieve a balance between the hardness, strength, and ductility of the steel. During tempering, some of the stresses and distortions within the martensitic structure are relieved, and carbon atoms precipitate out of the solid solution, forming fine carbide particles.
Tempering is an essential step after quenching, meant to reduce brittleness and achieve the desired mechanical properties necessary for practical applications. In the context of the exercise, obtaining 100% tempered martensite means that the entire alloy undergoes quenching to form martensite first, followed by controlled reheating which modifies the microstructure and improves toughness. The isothermal transformation diagram assists in selecting the precise tempering temperature and time to ensure a homogenous structure.

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Most popular questions from this chapter

Compute the rate of some reaction that obeys Avrami kinetics, assuming that the constants \(n\) and \(k\) have values of 2.0 and \(5 \times 10^{-4}\), respectively, for time expressed in seconds

Name the microstructural products of eutectoid iron-carbon alloy \((0.76 \mathrm{wt} \% \mathrm{C})\) speci mens that are first completely transformed to austenite, then cooled to room temperature at the following rates: (a) \(1^{\circ} \mathrm{C} / \mathrm{s}\) (b) \(20^{\circ} \mathrm{C} / \mathrm{s}\) (c) \(50^{\circ} \mathrm{C} / \mathrm{s},\) and (d) \(175^{\circ} \mathrm{C} / \mathrm{s}\)

Name the two stages involved in the formation of particles of a new phase. Briefly describe each.

The kinetics of the austenite-to-pearlite transformation obey the Avrami relationship. Using the fraction transformed-time data given here, determine the total time required for \(95 \%\) of the austenite to transform to pearlite: $$\begin{array}{cc} \hline \text {Fraction Transformed} & \text {Time (s)} \\ \hline 0.2 & 280 \\ 0.6 & 425 \\ \hline \end{array}$$

Rank the following iron-carbon alloys and associated microstructures from the hardest to the softest: (a) 0.25 wt\% C with coarse pearlite, (b) 0.80 wt\% \(C\) with spheroidite, (c) \(0.25 \mathrm{wt} \% \mathrm{C}\) with spheroidite, and (d) 0.80 wt\% C with fine pearlite. Justify this ranking.
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