The mechanical properties of two heat-treated steel forgings, manufactured from medium carbon Ni-Cr-Mo-V steels have been measured and compared. Microstructural characteristics such as martensite packet and lath size, dislocation density and precipitate size and distribution were measured and used in a detailed quantitative analysis of the relation between microstructural characteristics and proof stress. It was found that the microstructural differences identified could well account for the differences in tensile strength between the materials. Electron microcopy indicated that temper embrittlement leading to intergranular fracture may be responsible for the differences in transition temperature. (c) 2005 Springer Science + Business Media, Inc.