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Development of AF 1410 ultrahigh-strength steel
March 13, 2018
Source: ASM International
In the late 1970s, the U.S. Air Force sponsored development work for improved submarine hull steels. From this work, AF 1410 evolved as an ultrahigh-strength steel with particularly high fracture toughness. AF 1410 is typically manufactured via vacuum induction melting (VIM) followed by vacuum arc remelting (VAR) to achieve the required low levels of impurities. For improved or finer grain size, producers often recommend forging reductions of 40% below 900°C (1650°F). This grade is typically supplied in normalized and overaged condition for best machinability. The steel is then renormalized and austenitized or double austenitized, air cooled/quenched, cooled to –75°C (–100°F), and aged to attain maximum properties.
The microstructure of AF 1410 consists of iron-nickel lath martensite with carbides from age precipitation for strengthening. Quenching from the austenitizing temperature produces a highly dislocated lath martensite that has a high toughness, as measured by the Charpy V-notch impact test. Aging produces a complex series of changes in carbide structure. At approximately 425°C (800°F), Fe3C is precipitated. At 455°C (850°F), Fe-Cr-Mo M2C carbide is obtained, which at 480 °C (900 °F) will begin to produce a pure molybdenum-chromium M2C carbide. By raising the temperature to 510 °C (950 °F), the M2C will begin to coarsen; at 540 °C (1000 °F) M2C will begin to be replaced by M6C, which has little strengthening effect.
The secondary hardening, which is due to the aging, produces a maximum tensile strength when aged at 480 °C (900 °F) using a 5 h aging time and a minimum impact energy when aged at 425 °C (800 °F). When aged in the temperature range between 425 and 540 °C (800 and 1000 °F), the impact energy exhibits a maximum at approximately 508 °C (947 °F), as shown in the graph. At aging temperatures above 540°C (1000°F), both the tensile strength and the impact energy decrease rather rapidly.
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