Spheroidite

Pearlite is coarsened by the transition of the cemen-tite lamellae into spherical particles, thus reducing the interfacial free energy per unit volume. The process is called spheroidization and consequently the resulting microstructural constituent is termed spheroidite. 

Figure 3.20. Optical micrograph of a pearlitic steel (note the lamellar regions) that has been partially transformed to spheroidite. Image taken at 2,000 x resolution. Photograph courtesy of US Steel Corporation.

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. 

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. 

Figure 10.19 Photomicrograph of a steel having a spheroidite inicrostructure. The small particles are cementite the continuous phase is a-ferrite. 1000 X.

Concept Check 10.1 Which is more stable, the pearlitic or the spheroiditic microstructure Why ... 

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. 

Other elements of the microstructure relate to the shape and distribution of the phases. In this respect, the cementite phase has distinctly different shapes and arrangements in the pearlite and spheroidite microstructures (Figures 10.15 and 10.19). Alloys containing pear-litic microstructmes have greater strength and hardness than do those with spheroidite. This is demonstrated in Figure 10.30a, which compares the hardness as a function of the... 

The microstructure of tempered martensite consists of extremely small and uniformly dispersed cementite particles embedded within a continuous ferrite matrix. This is similar to the microstructure of spheroidite except that the cementite particles are much, much smaller. An electron micrograph showing the microstructure of tempered martensite at a very high magnification is presented in Figure 10.33. 

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—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. 

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. 

Pearlite (coarse and fine) Bainite Spheroidite Martensite Tempered martensite... 

Briefly cite the differences among pearlite, bainite, and spheroidite relative to microstructure and mechanical properties. 

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

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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