Pearlite formation

Johnson and Mehl analyzed the kinetics of pearlite formation by assuming that the growth rates, G, in three dimensions and the nucleation rate, N, are constant. With a constant growth rate, the volume, V, of a spherical particle nucleated at a time, r, at a time, t, is... 

Sha2] Microscopic examination Hydrogen influence on the pearlite formation and on the eutectoid temperature... 

Malleable Irons. The melt treatment of malleable cast irons involves Mg, Ca, Bi, or Te additions. But malleable irons have an as-cast stmcture consisting of FcbC in a pearlitic matrix. By heat treatment in the range of 800-970 the cementite phase is transformed into graphite (TG). The cooling is controlled in such a way as to promote pearlite formation, ferrite formation, or a mixture of the two. 

At lower transformation temperatures (<770K approx.) a second reaction, the formation of bainite intervenes. Like pearlite, the bainite constituent in steels consists of a mixture of ferrite and an iron carbide and is formed by... 

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... 

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 solubility of carbon in iron is reduced by the addition of phosphorus, but the temperature of formation of the eutectoid pearlite is not influenced by the presence of the phosphide. P. Goerens and W. Dobbelstein gave for the composition of the ternary eutectic E, Fig. 27, at 953°, l-96 per cent, of carbon, 6-89 per cent, of phosphorus, and 9145 per cent, of iron and J. E. Stead, respectively 1 92, 6 89, and 9149. In Fig. 26, A represents the iron-phosphorus eutectic, and B, the iron-carbon eutectic. They showed that when sat. solid soln. of iron phosphide in iron are heated or cooled they show no critical point at Ars, and the structure is not broken up even... 

When austenite is transformed to ferrite and pearlite below 727 °C, the composition of the pearlite and the amount of proeutectoid ferrite depend on the transformation temperature. The reason for this can be understood by extrapolating below 727 °C the line that represents the solubility of carbon in austenite, as shown in Figure 7.7. In a steel that contains less than 0.77% C, proeutectoid ferrite must form before any pearlite forms. Ferrite formation enriches the carbon content of the austenite. Pearlite can form only when the austenite has been enriched enough so that it is saturated with respect to carbon. This happens at 0.77% C if the transformation occurs at 727 °C. At temperatures below 727 °C,... 

No special strict requirements are put forward with respect to cast iron composition because in this case adhesion of enamel is of a mechanical nature. The carbon content is usually 3.4 —3.7%, of which 2.5 —3.0% is in graphite form. The base material should be mostly pearlitic, since free cementite in the form of larger crystals is responsible for bubble formation by oxidation. The quality of enamels is adversely affected by sulphur. 

The jS constituent f of the aluminium bronzes can exhibit two formations, firstly, a cellular or honeycombed network and, secondly, a considerably finer, lamellar, structure, analogous to the pearlite in annealed steels. 

Mehl, R. F. The structure and rate of formation of pearlite. Trans. Amer. 

The next step after formation of adsorbed hydrogen is absorption of the hydrogen. The ensuing chemical effect of hydrogen on steel is marked by decarburization of pearlite and formation of hydrocarbons, usually methane (CH4) ... 

For normalising, a soaking of the castings above the critical temperature followed by air cooling is necessary. Again a soaking temperature of 900 - 925 C is usually used, to ensure that the carbides are broken down. Forced air cooling is used to form pearlite. The type of heat treatment furnace available and the size of the load determines the process cycle that is possible. It may be necessary to adjust the metal composition with tin or copper to help the formation of fully pearlitic structures. 

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