The effect of double austenitization and quenching on the microstructure and mechanical properties of CrNiMoWMnV ultrahigh-strength steels after low-temperature tempering

With the aim of improving the strength and impact toughness combination of two ultrahigh-strength quenched and tempered steels, the effect of high-temperature austenitization and quenching prior to conventional austenitization, quenching and tempering at 200 °C has been investigated. The CrNiMoWMnV steels concerned had carbon contents of 0.18 and 0.32 wt% C and tensile strengths 1370 and 1840 MPa. Microstructures were characterized using laser scanning confocal microscopy, field emission scanning electron microscopy combined with electron back scattering diffraction and X-ray diffraction. Carbide characteristics were studied using transmission electron microscopy on carbon extraction replicas. Mechanical properties were characterized in terms of hardness, tensile and Charpy-V impact testing. The microstructure of the investigated steels after all treatments consisted of tempered martensite with small fractions of precipitates and retained austenite. The volume fraction of retained austenite was increased through the use of the double austenitization and quenching plus tempering route as compared to the conventional quenching and tempering route. Three main precipitate types were observed in all the investigated steels: complex carbides M_xC_y, aluminium nitride AlN, titanium-vanadium carbonitride (TiV)(CN) and complex AlN precipitates with a core of (TiV)(CN). The size of the largest precipitates was reduced as a result of the double austenitization treatment. In both investigated steels, the energy absorbed and the percentage ductile fracture in the CVN impact test were improved as a result of the extra austenitization and quenching: in the case of the 0.18C steel this was achieved without a loss of hardness or tensile properties, but with the 0.32C steel there was a slight decrease in the hardness and tensile properties.