AbstractMeasurements of 0·2% proof stress, ultimate tensile strength, elongation, and impact energy are presented for low Mn, Cu and low Mn, Ni–Cu irons for various austempering conditions and related quantitatively to microstructural features. It is shown that the formation of more than 20% pearlite in the austempered structure causes an unacceptable reduction in mechanical properties. The pearlite affects ductility in upper bainitic irons and strength in lower bainitic irons. The 0·2% proof stress is shown to be dependent on the bainitic ferrite particle size and a Hall–Petch relationship exists between proof stress and particle size. The impact energy of irons austempered within the processing window is shown to increase with increasing volume fraction of retained austenite and then to reach a saturation level. The saturation level is discussed in terms of the occurrence of a transformation induced plasticity mechanism.MST/1900