Wednesday, November 19, 2008

Transformation Behavior

The crystalline structure of pure iron is ferrite at room temperature. The room temperature form of ferrite is called alpha (α) ferrite. At higher temperatures, the ferritic structure is unstable and transforms into a facecentered cubic structure called gamma (γ) austenite. At even higher temperatures, the austenitic structure might again transform into a higher temperature form of ferrite; this is called delta (δ) ferrite. This structure change because of the crystalline structure is changed affected by temperature.

Iron-iron carbide phase diagrams (see Figure below) represent the crystalline structures, or phases, of the carbon steels in an equilibrium state that are determined by very slow cooling from molten material. This is not a realistic view of the microstructural phases that exist during normal fabrication processes because the heating and cooling rates significantly affect the temperatures at which the suggested phase transformations occur.

This effect can be seen in the temperature difference between A1, the equilibrium lower transformation temperature, and Ar1, the lower transformation temperature upon cooling. Although not shown, there is also a lower transformation temperature upon heating, Ac1, which is somewhat higher than A1. The Ac1 temperatures depict the start point of the transformation between the α ferrite and the γ austenite upon heating.

The phase diagram in Figure below also shows an equilibrium upper transformation temperature—A3. Similar to the variations noted for A1, there are also upper transformation temperatures upon heating and cooling (Ac3 and Ar3, respectively). The transformation temperatures indicate the points at which the structure becomes an unstable form and begins to undergo a transformation to a different crystalline structure. It can be seen that carbon steels, with a typical maximum carbon content of less than 0.35% for pressure-containing applications, will have a transformation temperature range that will vary with the carbon content and the rate of heating or cooling.

The ferritic structure at room temperature has a relatively low ability (probably less than 0.008%) to contain carbon atoms in the space between the iron atoms (interstitially). The face-centered cubic structure has a much higher affinity for carbon and can contain as much as approximately 2.1%. Carbon that cannot be contained interstitially can exist in other forms, such as iron carbides or carbides of other metal elements. In a carbon steel microstructure, iron carbides can appear as platelets or particles of cementite (Fe3C). A microstructure that has alternating platelets of ferrite and cementite is called pearlite. With certain rates of cooling, the carbon steel microstructure can also be bainite. Bainitic structures represent a variety of ferrite aggregates with a distribution of small iron carbide precipitates.

Upon heating the carbon steel microstructure through the transformation range, the ferrite will transform into an austenitic structure. Because the austenitic structure has a much higher solubility of carbon, the iron carbides dissolve and the carbon enters into solution with the austenitic iron microstructure. This is a time- and temperature-dependent mechanism that takes longer if the cementite particles or platelets are large. An increased rate of heating will also have the effect of requiring a higher temperature to complete the dissolution.

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