Thursday, November 13, 2008

Carbon Microstructure

Carbon Microstructures

Metallic materials take the form of a crystalline structure in the solid state (with the exception of amorphous metals that have been formed under radical cooling conditions, unlike those that occur in normal processing). Steel that already make composite have crystalline structure, carbon microstructure already changed chemically make the strength of steel change.

The crystalline structure and the alloying elements added to pure iron give carbon steel the ability to have a wide range of properties, which make it one of the most useful materials in industry today. The crystalline structure of carbon steel might include body-centered cubic (ferrite), facecentered cubic (austenite), or body-centered tetragonal (martensite) forms.

The crystalline structure forms in many directions during solidification from the molten state of the material. Solidification starts from initiation points and continues until the crystalline structure that is formed runs into another island that started from a different point. Each of these islands of a single orientation is a grain that exists as a singular structure. The size of these grains also contributes to the properties of the material and as will be discussed, also affects the ability of the material to form certain microstructures. As the material cools, carbon steel crystalline structures are forced to change from one structure to another—these are called phase transformations. The different structures have different limits of solubility of the alloying elements, primarily carbon in carbon steels. The microstructure can also contain other compounds, such as metallic carbides, interspersed with the crystalline form. The complex microstructure of carbon steel includes the crystalline structure, the grain size, and the size and frequency of the interspersed metallic compounds.

Carbon steels can exist in different microstructures or combinations of microstructures. The microstructures of carbon steels include not only the crystalline structure but also various metallic carbides or compounds in different arrangements.

Pearlite is an arrangement of thin alternating and roughly parallel lamellar platelets of ferritic (body-centered cubic) structures with iron carbides (Fe3C) called cementite. The lamellar platelets can be coarse or fine, but they are often recognizable with optical microscopy. Bainite is an arrangement of aggregates of ferrite with distributions of precipitated carbide particles. However, the arrangement can take different forms, thus the terms upper bainite and lower bainite. Upper bainite consists of small ferrite grains that form in plate-shaped sheaths. These grains are interspersed with the cementite that forms at relatively high temperatures.

Lower bainite consists of needlelike ferrite plates containing a dispersion of very small carbide particles. (Note that the 0.77% carbon material used to illustrate these structures is not the low- to medium-carbon steels of this report).

These different microstructures or crystalline structures have significantly different properties that are determined by alloy content (again, primarily carbon) and the various thermal cycles that can exist during fabrication and heat treatment.

Next article: Carbon Steel Corrosion Control

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