Pearlite/bainite phases

The change of crystal structure which occurs in some materials in which different crystal structures are stable over different temperature (and pressure) ranges. In ferritic steels, the most important transformation is from the high temperature form, austenite, to lower temperature transformation products, such as ferrite, pearlite, bainite, martensite and, in weld metals, acicular ferrite and ferrite with aligned second phase. 

Finally, at even lower transformation temperatures, a completely new reaction occurs. Austenite transforms to a new metastable phase called martensite, which is a supersaturated solid solution of carbon in iron and which has a body-centred tetragonal crystal structure. Furthermore, the mechanism of the transformation of austenite to martensite is fundamentally different from that of the formation of pearlite or bainite in particular martensitic transformations do not involve diffusion and are accordingly said to be diffusionless. Martensite is formed from austenite by the slight rearrangement of iron atoms required to transform the f.c.c. crystal structure into the body-centred tetragonal structure the distances involved are considerably less than the interatomic distances. A further characteristic of the martensitic transformation is that it is predominantly athermal, as opposed to the isothermal transformation of austenite to pearlite or bainite. In other words, at a temperature midway between (the temperature at which martensite starts to form) and m, (the temperature at which martensite... 

The structures and phase transformations observed in steels have been dealt with in some detail not only because of the great practical importance of steels, but also because reactions similar to those occurring in steels are also observed in many other alloy systems. In particular, diifusionless transformations (austenite -> martensite), continuous precipitation (austenite -> pearlite) and discontinuous precipitation (austenite -> bainite and tempering of martensite) are fairly common in other alloy systems. 

When anstenite is cooled nnder more rapid conditions, a compound called bainite is produced. Bainite is a noneqnilibrinm prodnct that is similar to pearlite, but consists of a dispersion of very small FesC particles between the ferrite plates. Bainite formation is favored at a high degree of snpercooling from the austenite phase, whereas pearlite forms at low degrees of supercoohng, or more equilibrium cooling. 

The diffusion-dependent transformations of austenite (to ferrite, pearlite, and bainite) compete with the martensitic transformation such that the volume fraction available for the latter will decrease as the volume transformed by the former increases. This transformation kinetics of the diffusional phase transformations is strongly dependent on alloy composition. 

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. 

Microstructure of Metallic Matrix Phases. Ferritic, pearlitic, austenitic, bainitic (austempered). More details are presented in Fig. 3.1-119 and Table 3.1-79. 

Yet another microconstituent or phase called martensite is formed when austenitized iron-carbon alloys are rapidly cooled (or quenched) to a relatively low temperature (in the vicinity of the ambient). Martensite is a nonequilibrium single-phase structure that results from a diffusionless transformation of austenite. It may be thought of as a transformation product that is competitive with pearlite and bainite. The martensitic transformation occurs when the quenching rate is rapid enough to prevent carbon diffusion. Any diffusion whatsoever results in the formation of ferrite and cementite phases. 

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. 

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