AbstractA new approach is suggested whereby the mechanical properties of commercially available 0·4C–Cr–Mo–Ni ultrahigh strength steel can be significantly improved whether the testing orientation is longitudinal or transverse. The new approach consists of combining the decreased hot rolling reduction treatment with a short time isothermal transformation treatment. This approach, compared with conventional heat treatment, resulted in significant improvements of the plane strain fracture toughness KIcat increased strength and of the Charpy V-notch impact energy, for either testing orientation. Steel treated using the new approach also had an improved combination of mechanical properties compared with steels treated using high austenitisation and controlled rolling, which are possible approaches to improving the values of KIcor other mechanical properties of steels. The favourable effect on the mechanical properties is discussed in terms of metallography, X-ray measurements, and fractography.MST/984