Estimation of Impact Toughness Transition Temperatures of As-Quenched Steels

High strength and sufficient toughness are key requirements for modern high-performance structural steels. In an attempt to develop a suitable estimation of impact toughness transition temperatures for as-quenched steels, we investigated the determiners of low-temperature toughness with a group of thermomechanically rolled direct-quenched steels with varying martensite contents. These were produced by altering chemical composition, finish rolling temperature and reduction below the non-recrystallization temperature, i.e. austenite pancaking, and characterised in terms of microstructural constituents, grain size distributions, texture and fractography. Provided the finish rolling temperature is high enough to avoid the formation of granular bainite on subsequent cooling, high levels of austenite pancaking yield the best combinations of low-temperature toughness and strength by effectively refining the size of the coarsest grains and randomizing the texture. While absolutely no direct correlation is found within as-quenched steels between the impact toughness transition temperatures and yield strength alone, T28J and T50 do closely follow a dynamic reference toughness, i.e. the opening stress intensity factor defined by yield strength and the size of the coarsest grains in the effective grain size distribution. This parameter reflects the transition temperatures – the lower the temperature, the lower the reference toughness needed to cause a local brittle fracture. Finally, we show that the impact toughness transition temperatures T28J and T50 of as-quenched steels can be accurately estimated, irrespective of the test specimen orientation, by utilizing just the dynamic reference toughness and the fraction of {100} cleavage planes within ± 15° of the specimen notch plane.