10_11

Materials Science and Engineering: An Introduction

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237 (d) At a cooling rate of 0.01 ° C/s, martensite, proeutectoid ferrite, pearlite, and bainite form. 10.25 This problem asks that we briefly describe the simplest continuous cooling heat treatment procedure that would be used in converting a 4340 steel from one microstructure to another. Solutions to this problem require the use of Figure 10.20. (a) In order to convert from (martensite + bainite) to (ferrite + pearlite) it is necessary to heat above about 720 ° C, allow complete austenitization, then cool to room temperature at a rate slower than 0.006 ° C/s. (b) To convert from (martensite + bainite) to spheroidite the alloy must be heated to about 700 ° C for several hours. (c) In order to convert from (martensite + bainite) to (martensite + bainite + ferrite) it is necessary to heat to above about 720 ° C, allow complete austenitization, then cool to room temperature at a rate between 0.3 ° C/s and 0.02 ° C/s. 10.26 For moderately rapid cooling, the time allowed for carbon diffusion is not as great as for slower cooling rates.
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