Tempering Effects for Lower Bainite, Martensite, and Mixed Microstructures onImpact, Fracture, and Related Mechanical Properties of ASTM A723 Steel

摘要

Mixed microstructures were produced by cooling ASTM A723 steel from 830 deg C(1526 deg F) to lower temperatures, notably at 210 deg and 250 deg C (410 deg and 482 deg F), which are below the start of the martensite transformation (M sub s) temperature of 286 deg C (547 deg F), and held for 2-1/4 hours. This resulted in the isothermal formation of 25 or 66 percent lower bainite as one part of the mixed microstructure with martensite. Tempering studies were conducted from 260 deg to 704 deg C (500 deg to 1300 deg F). Optimum tempering occurred at 593 deg C (1100 deg F), where differences between strength and hardness levels were minimized and the yield strength-to-ultimate tensile strength ratio was maximized. This allowed impact and toughness tests to be performed on an equal basis. Impact tests, conducted from -100 deg to 88 deg C (-148 deg to 190 deg F), revealed that mixed microstructures could produce higher impact energy values compared to all-martensite. When the mixed microstructure containing 25 percent lower bainite was tested at -40 deg F), the impact energy was increased by about 18 ft-lbs, compared to quenched and tempered martensite, and had the lowest ductile-brittle transition temperature of -73.3 deg C (-100 deg F) versus -68.3 deg C (-91 deg F) for martensite. Fracture toughness tests at -40 deg C revealed only a slight improvement in toughness for mixed microstructures at 188 Ksi (check) in., compared to all-martensite at 183.8 Ksi (check) in.