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Fig. 5.7. Static Fatigue of 4340 Notched Bars4

strength levels (220-280 ksi). The 4340 can be modified with vanadium (4340V) that forms a stable high melting point carbide that helps in pinning grain boundaries and preventing grain growth during hot working operations. The vanadium addition also serves as a grain refiner that increases toughness.

When the silicon content of4340 is increased to 2%, the strength and toughness increases in the manner shown in Fig. 5.8. The increased silicon content provides deeper hardenability, increases solid solution strengthening, and provides better higher temperature resistance. The increase in toughness is attributed to silicon retarding the precipitation of cementite from retained austenite during tempering and to the stabilization of carbides. Silicon added to the basic 4340 composition forms the alloy 300M, which nominally contains 1.6% silicon. Vanadium is added for grain refinement, and the sulfur and phosphorus levels are kept very low to reduce temper embrittlement and increase toughness and transverse ductility. The 300M is also vacuum arc remelted to lower the hydrogen and oxygen contents. The lower oxygen content minimizes the formation of oxide inclusions and increases toughness. However, due to 300Ms high silicon and molybdenum contents, it is extremely prone to decarburization during heat treatment, and when heat treated to strength levels above 200 ksi, it is also susceptible to hydrogen embrittlement.

Tempering Temperature (° F)

Fig. 5.8. Effects of Silicon Content on 4340 Steel5

Tempering Temperature (° F)

Fig. 5.8. Effects of Silicon Content on 4340 Steel5

High strength steels are available in a variety of quality levels depending on the type of melting practice used. While many of these steels were originally air melted, the trend has been to move to more advanced melting techniques such as vacuum degassing, electroslag remelting (ESR), VAR, and double vacuum melting (vacuum induction melting followed by vacuum are remelting (VIM-VAR)) for improved cleanliness and higher quality. These methods reduce both the quantity of dissolved gases (hydrogen, oxygen, and nitrogen) and the non-metallic inclusions. As the high strength steels have evolved since the mid-1970s, improvements in melting process control and inspection have steadily increased fracture toughness, ductility, and fatigue resistance. A comparison of air and vacuum melted 300M, shown in Fig. 5.9, illustrates the property advantages imparted by vacuum processing. Both VAR and ESR are acceptable

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