Alloying element additions retard the formation rate pearlite and bainite

FIGURE 11.36 Specific volume versus temperature, upon cooling from the liquid melt, for totally amorphous
(curve A), semicrystalline (curve B), and crystalline (curve C) polymers.

mers. The glass transition temperature may also define the upper use temperature for glassy amorphous materials. Furthermore, Tm and Tg also influence the fabrica-tion and processing procedures for polymers and polymer-matrix composites. �These issues are discussed in other chapters.�
The temperatures at which melting and/or the glass transition occur for a polymer are determined in the same manner as for ceramic materials—from a plot of specific volume (the reciprocal of density) versus temperature. Figure 11.36 is such a plot, wherein curves A and C, for amorphous and crystalline polymers, respectively.1For the crystalline material, there is a discontinuous change in specific volume at the melting temperature Tm. The curve for the totally amorphous material is continuous but it experiences a slight decrease in slope at the glass transition temperature, Tg. The behavior is intermediate between these extremes for a semi-crystalline polymer (curve B), in that both melting and glass transition phenomena are observed; Tm and Tg are properties of the respective crystalline and amorphous phases in this semicrystalline material. As discussed above, the behaviors repre-sented in Figure 11.36 will depend on the rate of cooling or heating. Representative melting and glass transition temperatures of a number of polymers are contained in Table 11.1 and Appendix E.

Several microconstituents are possible for steels, the formation of which de-pends on composition and heat treatment. These microconstituents include fine and coarse pearlite, and bainite, which are composed of ferrite and cementite phases and result from the decomposition of austenite via diffusional processes. A spheroidite microstructure (also consisting of ferrite and cementite phases) may be produced when a steel specimen composed of any of the preceding microstructures is heat treated at a temperature just below the eutectoid. The mechanical characteris-tics of pearlitic, bainitic, and spheroiditic steels were compared and also explained in terms of their microconstituents.

Martensite, yet another transformation product in steels, results when austen-ite is cooled very rapidly. It is a metastable and single-phase structure that may be produced in steels by a diffusionless and almost instantaneous transformation of austenite. Transformation progress is dependent on temperature rather than time, and may be represented on both isothermal �and continuous cooling�transformation diagrams. Furthermore, alloying element additions retard the formation rate of pearlite and bainite, thus rendering the martensitic transforma-tion more competitive. Mechanically, martensite is extremely hard; applicability,