Metallurgical Considerations

Steels are alloys of iron and carbon that contain the BCC crystalline structure at room temperature. Since steels contain the BCC structure, their formability is not as good as metals with an FCC structure but better than those with the HCP structure. In general, most forming operations can be conducted at room

Fig. 5.1. Mass and Complexity ofF/A-18 Landing Gear

temperature as long as the material has not been heat treated to high strength and hardness levels.

As shown in the iron-carbon phase diagram of Fig. 5.2, when steel alloys are heated sufficiently above the A3 temperature, they transform to the FCC austenite (y) structure. On slow cooling, the structure transforms back into the BCC ferrite (a) structure, along with cementite (Fe3C), to form a structure called pearlite. This transformation forms the basis for heat treatment by quenching and tempering. If the steel is austenitized at a temperature sufficiently above

Table 5.1 Compositions of Select High Strength Steels


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