Case Study: Ductility Transition of Airplane Landing Gear

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Concepts Shown:

brittle-to-ductile transition temperature, toughness

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Materials with which airplanes are constructed must be able to tolerate extreme atmospheric and environmental changes and still perform at an acceptable level. For example, when a plane flies, its temperature drops to -40�C. This fact becomes important when engineers must choose a material to build the landing gear. During flight, the landing gear drops in temperature and remains there until after the plane has landed. If the landing gear was made out of a steel with a sharp ductile to brittle transition, it would shatter upon the impact of landing. For this reason, engineers have chosen to use a 4340 steel in landing gear. 4340 steel has the following composition ranges: C: .36-.44 Mn: .55-.80 Si: .15-.30 Cr: .60-.90 Ni: l.65-2.00 Mo: .20-.30 and an ultimate strength that varies from 1500-1900 MPa depending on the heat treatment. But, more importantly, 4340 steel does not have a sharp ductile to brittle transition. The percentages of Ni and Cr add elasticity to the material, and the Si and Mo additions retard the formation of the brittle phases in steel by retaining the austenite phase (ductile and FCC) and suppressing the formation of brittle carbide decomposition phase [austenite -> ferrite + carbide]. As shown in the figure below, the toughness or ductility is approxiamately the same over a wide range of temperatures. Even though the landing gear is at -40�C, it can tolerate the forces due to landing because it is still ductile. Graphically, the slope of the transition line of 4340 steel is less than that of an ordinary Fe-C steel indicating the 4340 steel is more ductile at lower temperatures.

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Author:
Dave Goodman
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