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Chapter 10: Problem 28

Briefly describe the simplest continuous cooling heat treatment procedure that would be used in converting a 4340 steel from one microstructure to another. (a) (Martensite + bainite) to (ferrite + pearlite) (b) (Martensite + bainite) to spheroidite (c) (Martensite + bainite) to (martensite + bainite \(+\) ferrite)

Short Answer

Expert verified
Answer: (a) To convert to (ferrite + pearlite), heat the steel above the A1 point (around 727°C), hold for complete austenite grain growth, and cool at a moderate rate. (b) To transform to spheroidite, heat the steel just below the A1 point (around 700°C), hold for an extended period, and slowly cool to room temperature. (c) To achieve a mix of (martensite + bainite \(+\) ferrite), heat the steel above the A1 point (around 727°C), hold for complete austenite grain growth, and cool at a rate between slow and moderate.

Step by step solution

01

(a) (Martensite + bainite) to (ferrite + pearlite)

To convert the microstructure of a 4340 steel from (Martensite + bainite) to (ferrite + pearlite), follow these steps: 1. Heat the steel to a temperature above the A1 transformation point (around 727°C) to enter the austenite phase. 2. Hold the steel at this temperature for a sufficient amount of time to allow for complete austenite grain growth and stabilization. 3. Gradually cool the steel at a moderate rate, allowing carbon diffusion to occur, leading to the formation of ferrite and pearlite.
02

(b) (Martensite + bainite) to spheroidite

To transform the 4340 steel microstructure from (Martensite + bainite) to spheroidite, perform the following heat treatment procedure: 1. Heat the steel to a temperature just below the A1 transformation point (around 700°C). 2. Hold the steel at this temperature for an extended period of time, promoting the diffusion of carbon atoms and the transformation of the initial microstructure into the stable spheroidal cementite (spheroidite). 3. Slowly cool the steel back down to room temperature.
03

(c) (Martensite + bainite) to (martensite + bainite \(+\) ferrite)

To achieve a mixed microstructure of (martensite + bainite \(+\) ferrite) in a 4340 steel, perform the following steps in the heat treatment process: 1. First, heat the steel to a temperature above the A1 transformation point (around 727°C) to form austenite. 2. Hold the steel at this temperature, allowing for complete austenite grain growth and stabilization. 3. Afterwards, cool the steel at a rate that is between slow and moderate. This cooling rate will allow the formation of ferrite and maintain some martensite and bainite present in the initial microstructure. 4. Continue cooling the steel to reach room temperature. As a result, the final microstructure will be a mix of martensite, bainite, and ferrite within the 4340 steel.

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Most popular questions from this chapter

Using the isothermal transformation diagram for a \(0.45 \mathrm{wt} \%\) 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 time-temperature treatments. In each case assume that the specimen begins at \(845^{\circ} \mathrm{C}\left(1550^{\circ} \mathrm{F}\right)\) 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 \(700^{\circ} \mathrm{C}\left(1290^{\circ} \mathrm{F}\right)\), hold for \(30 \mathrm{~s}\), 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 s, rapidly cool to \(400^{\circ} \mathrm{C}\left(750^{\circ} \mathrm{F}\right)\), hold for \(10 \mathrm{~s}\), then quench to room temperature. (f) Rapidly cool to \(450^{\circ} \mathrm{C}\left(840^{\circ} \mathrm{F}\right)\), hold for \(10 \mathrm{~s}\), then quench to room temperature. (g) Rapidly cool to \(625^{\circ} \mathrm{C}\left(1155^{\circ} \mathrm{F}\right)\), hold for \(1 \mathrm{~s}\), then quench to room temperature. (h) Rapidly cool to \(625^{\circ} \mathrm{C}\left(1155^{\circ} \mathrm{F}\right)\), hold at this temperature for \(10 \mathrm{~s}\), rapidly cool to \(400^{\circ} \mathrm{C}\left(750^{\circ} \mathrm{F}\right)\), hold at this temperature for \(5 \mathrm{~s}\), then quench to room temperature.

Cite two important differences between continuous cooling transformation diagrams for plain carbon and alloy steels.

Make a copy of the isothermal transformation diagram for an iron-carbon alloy of eutectoid composition (Figure 10.22) and then sketch and label time- temperature paths on this diagram to produce the following microstructures: (a) \(100 \%\) fine pearlite (b) \(100 \%\) tempered martensite (c) \(50 \%\) coarse pearlite, \(25 \%\) bainite, and \(25 \%\) martensite

In terms of heat treatment and the development of microstructure, what are two major limitations of the iron-iron carbide phase diagram?

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: $$ \begin{array}{lc} \hline \text { Fraction Transformed } & \text { Time (s) } \\ \hline 0.2 & 12.6 \\ 0.8 & 28.2 \\ \hline \end{array} $$
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