AbstractHeat treatments have been carried out on an annealed low alloy Cr–Mo–3·5Ni–V steel (J-steel) to investigate the effects of various normalising and hardening temperatures, before tempering, on the mechanical properties. It was observed that the 0·2% proof stress increased, whereas the impact toughness, as measured by the increase in transition temperature, decreased with increasing hardening temperature for both normalised and non-normalised material. For a constant hardening temperature, the 0·2% proof stress passed through a minimum and the Charpy upper shelf energy passed through a maximum, at a normalising temperature of 890°C. The variation in properties has been correlated with the observed variation in martensite packet size, dislocation density, precipitate size, and volume fraction. It was found that a 0·2% proof stress above 1100 MN m−2, and a Charpy impact energy of greater than 50 J at−40°C located within the upper shelf energy region, was attainable with the following heat treatment: normalising from 870°C and hardening from 850°C, followed by tempering at 620°C and oil quenching to room temperature.MST/1870