Nanoscale Precipitation-Strengthened Steels

From Northwestern University Center for Atom-Probe Tomography
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Copper precipitation strengthened steels are of interest to the transportation, infrastructure, and defense industries. The steels are also referred to as copper steels, Fe-Cu steels, copper precipitation-hardened steels, copper age-hardenable steels and copper-bearing steels. Applications can be as diverse as rail tank cars, oil pipelines, offshore oilrigs, ship hulls, bridges, building support columns, automotive and truck frames, and naval applications. These steels are commonly classified in the family of high-strength-low-alloy (HSLA) steels. These alloys contain less than 0.25 wt.% C, making them less responsive to martensite formation during quenching. The strength of Fe-Cu steels is derived from both solid-solution strengthening and precipitation strengthening. The lack of martensite also prevents the formation of a brittle heat-affected-zone (HAZ) during welding. The ferritic microstructure means better cryogenic fracture toughness and possible blast resistance at low temperatures. Lastly, the presence of Cu improves the atmospheric corrosion resistance of the alloys.

Precipitation strengthening affects the mechanical properties of Fe-Cu steel alloys in a much more dramatic manner than solid-solution strengthening. Solution treating followed by thermal aging between 450 degrees Celsius and 600 degrees Celsius, increases the yield and tensile strengths and also increases hardness. The heat treatment also reduces impact toughness, and lowers elongation and area reduction. The undesirable loss of impact toughness can be reduced by hot rolling prior to solution treating and aging. Recent studies have achieved high strength and good fracture toughness by optimizing the precipitation strengthening and microstructure in Fe- Cu steels.