Ductility spheroidite

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. 

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. 

Spheroidite a-Ferrite + Fe3C Relatively small FesC spherelike particles in an a-ferrite matrix Soft and ductile... 

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... 

Spheroidite is the softest and most ductile of the microstructures discussed. Embrittlement of some steel alloys results when specific alloying and impurity elements are present and upon tempering within a definite temperature range. 

D5 Is it possible to produce an iron-carbon alloy that has a minimum tensile strength of 620 MPa (90,000 psi) and a minimum ductility of 50% RA If so, what will be its composition and microstructure (coarse and fine pearlites and spheroidite are alternatives) If this is not possible, explain why. 

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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