Abstract In this research a variety of heat treatments were performed on AISI 4340 steel, the regimes consisted of quenching and tempering at a range of different tempering temperatures. Heat treatments were performed as a method to alter the mechanical properties via manipulation of the material microstructure. AISI 4340 steel was initially austenitised then quenched and/or tempered at tempering temperatures of 200°C, 400°C and 600°C, which produced a variety of mechanical and microstructural changes. The mechanical properties of toughness and hardness were characterised using the Charpy Impact test, measuring the impact energy absorbed prior to fracture, and Vickers hardness test, measuring the resistance to deformation, respectively. The results indicated that an increase in toughness also correlates with a decrease in hardness. Microstructural characterisation of the specimens was carried out using Scanning Electron Microscopy (SEM) which produced microscopic images at a number of magnifications. The images obtained showed a correlation between the microstructural evolution produced via the increase in tempering temperature which induced increased carbide precipitation, and the improvement of material toughness of 4340 steel. Furthermore, fractography was performed using SEM. From the images obtained the nature of fracture was observed and distinguished for the different heat treatment regimes. This further indicated the extent at which the mechanical properties were altered as a result of the different heat treatment tempering temperatures.