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Machine Design

Crystal Structure

In the iron-carbon alloy system, an important phase transformation takes place between about 1,300 and 1,600°F. The exact temperature is determined by the amount of carbon and other alloying elements in the metal. Iron transforms from a face-centered cubic (FCC) structure -- called the gamma phase, or austenite -- at high temperature to a body-centered cubic (BCC) structure -- alpha phase, or ferrite -- at a lower temperature. In pure iron, this transformation (the A3 transformation) is marked by a distinct increase in length as the metal cools below the critical temperature because the body-centered lattice is less compact than the face-centered lattice.

High-temperature austenite, an FCC structure, allows enough space for carbon to squeeze in between the iron atoms. Iron atoms maintain their place on the lattice and carbon atoms become "interstitials." In the low-temperature ferrite, or BCC structure, however, there is no room for carbon atoms. What happens to these carbon atoms determines many of the properties of iron and steel.

For example, during the slow cooling of a low-carbon steel such as AISI 1020 (0.20% carbon), transformation begins as the metal reaches 1,555°F. The first metal to reach this temperature transforms to ferrite, the BCC structure, and expels the interstitial carbon into the remaining regions of austenite. As the metal cools further, more iron transforms into ferrite, leaving less austenite and more regions rich in expelled interstitial carbon.

Finally, at about 1,350°F, the lower end of the transformation temperature range for 1020 steel, the last remaining austenite tries to transform -- in spite of the rich carbon concentrations. At this point, two things occur: The carbon bonds with available iron atoms to form Fe3C, an intermetallic compound called cementite, or iron carbide, and it precipitates out as a discrete structure; the remaining austenite then transforms to ferrite.

The structure that results from this final transformation is a lamination consisting of alternating layers of ferrite and iron carbide. Of course, the portions of metal that transformed previously remain as large islands of pure ferrite. The laminated structure formed at the last moment is called pearlite. The combined structure of ferrite and pearlite is soft and ductile -- steel in its lowest-strength condition.

In contrast, when ferrous alloys are cooled rapidly, or quenched, expelled carbon atoms do not have time to move away from the iron as it transforms to ferrite. The steel becomes so rigid that, before the carbon atoms can move, they become trapped in the lattice as the iron atoms try to transform to the body-centered cubic structure. The result is a body-centered tetragonal structure in which the carbon atom is an interstitial member. Steel that has undergone this type of transformation is martensitic. Naturally, martensite is in a state of unequilibrium, but it owes much of its high strength and hardness (and lower ductility) to its distorted, stressed lattice structure.

A number of heat-treatment cycles have been developed to alter the structure of steel. For example, when martensite is tempered (heated below A3 temperature) some internal stresses are relieved, and the resulting structure has more ductility than as-quenched martensite.

Other heat treatments change the proportions of pearlite and martensite; some even entrap austenite at room temperature. Others alter or reduce the size of the grains or pattern of these structures, providing improved strength or toughness. And when other alloying elements -- including boron, nickel, chromium, manganese, silicon, and vanadium -- are added to the metal, the behavior of ferrous alloys, as they transform from one structure to another, is further complicated. But because the structure of steel -- and thus, the mechanical properties of steel -- can be altered in so many ways, ferrous alloys can be developed to suite an extremely wide variety of design needs.

TAGS: Metals
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