Pearlite coarse

Coarse pearlite Fine pearlite Upper Bainite... 

To summarize, the relative hardness of the various phases discussed thus far (Brinell hardness values in parentheses) martensite (300-700) > tempered martensite (300-450) > bainite ca. 400) > fine pearlite (100-300) > coarse pearlite (100-220) > spheroidite (90-180). The hardness and brittleness of cementite is much greater than ferrite, whereas the latter has significantly greater ductility. 

CP, coarse pearlite M, martensite A, austenite F, ferrite Can be produced from a malleable-iron base composition Copper can replace all or part of the nickel Such as Durion, Durichlor 51, Superchlor Such as Ni-Resist austenitic iron (ASTM A 436)... 

In curve E, the temperature is lowered to just below the eutectoid temperature and held imtil it is 100% converted to coarse pearlite. 

Briefly describe the microstructure for each of the following microconstituents that are found in steel alloys fine pearlite, coarse pearlite, spheroidite, bainite, martensite, and tempered martensite. 

Photomicrographs of (fl) coarse pearlite and (h) fine pearlite. 3000X. 

We now discuss the mechanical behavior of iron-carbon alloys having the microstructures discussed heretofore—namely, fine and coarse pearlite, spheroidite, bainite, and martensite. For all but martensite, two phases are present (ferrite and cementite), and so an opportunity is provided to explore several mechanical property-microstructure relationships that exist for these alloys. 

Figure 10.30 (a) Brinell and Rockwell hardness as a function of carbon concentration for plain carbon steels having fine and coarse pearlite as well as spheroidite microstructures, (b) Ductility (%RA) as a function of carbon concentration for plain carbon steels having fine and coarse pearlite as well as spheroidite microstructures. 

Stronger than coarse pearlite, as demonstrated by the upper two curves of Figure 10.30a, which plots hardness versus the carbon concentration. 

As might be expected, spheroidized steels are extremely ductile, much more than either fine or coarse pearlite (Figure 10.30h). In addition, they are notably tough because any crack can encounter only a very small fraction of the brittle cementite particles as it propagates through the ductile ferrite matrix. 

Coarse pearlite a-Ferrite + Fe3C Alternating layers of a-ferrite and FesC that are relatively thick Harder and stronger than spheroidite, but not as ductile as spheroidite... 

Coarse and fine pearlite—the alternating a-ferrite and cementite layers are thinner for fine than for coarse pearlite. Coarse pearlite forms at higher temperatures (isothermally) and for slower cooling rates (continuous cooling). 

Spheroidite—this is composed of spherelike cementite particles that are embedded in a ferrite matrix. Heating fine/coarse pearlite or bainite at about 700°C for several hours produces spheroidite. 

Fine pearlite is harder, stronger, and more brittle than coarse pearlite. 

For iron-carbon alloys, in addition to discussions of the heat treatments that produce the several microconstituents (fine/coarse pearlite, bainite, martensite, etc.) and their mechanical properties, correlations were made between mechanical properties and structural elements of these microconstituents. These correlations are indicated in the following concept map ... 

On the basis of diffusion considerations, explain why fine pearlite forms for the moderate cooling of austenite through the eutectoid temperature, whereas coarse pearlite is the product for relatively slow cooling rates. 

Briefly explain why fine pearlite is harder and stronger than coarse pearlite, which in turn is harder and stronger than spheroidite. 

Medium- and high-carbon steels having a microstructure containing even coarse pearlite may still be too hard to machine or plastically deform conveniently. These steels, and in fact any steel, may be heat-treated or annealed to develop the spheroidite structure as described in Section 10.5. Spheroidized steels have a maximum softness and ductility and are easily machined or deformed. The spheroidizing heat treatment, during which there is a coalescence of the Fc3C to form the spheroid particles, can take place by several methods, as follows ... 

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