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Materials Science and Engineering: An Introduction

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236 10.21 Two major differences between martensitic and pearlitic transformations are 1) atomic diffusion is necessary for the pearlitic transformation, whereas the martensitic transformation is diffusionless; and 2) relative to transformation rate, the martensitic transformation is virtually instantaneous, while the pearlitic transformation is time-dependent. 10.22 Two important differences between continuous cooling transformation diagrams for plain carbon and alloy steels are: 1) for an alloy steel, a bainite nose will be present, which nose will be absent for plain carbon alloys; and 2) the pearlite-proeutectoid noses for plain carbon steel alloys are positioned at shorter times than for the alloy steels. 10.23 There is no bainite transformation region on the continuous cooling transformation diagram
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Unformatted text preview: for an iron-carbon alloy of eutectoid composition (Figure 10.17) because by the time a cooling curve has passed into the bainite region, the entirety of the alloy specimen will have transformed to pearlite. 10.24 This problem asks for the microstructural products that form when specimens of a 4340 steel are continuously cooled to room temperature at several rates. Figure 10.20 is used for these determinations. (a) At a cooling rate of 10 ° C/s, only martensite forms. (b) At a cooling rate of 1 ° C/s, both martensite and bainite form. (c) At a cooling rate of 0.1 ° C/s, martensite, proeutectoid ferrite, and bainite form....
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