‘Transitional’ steels have an unusual combination of mechanical properties. They may have a low yield stress (20–40 kg/sq. mm=28, 400–56, 900 psi) and a high tensile strength (100–200 kg/sq mm=142, 000–284, 000 psi). The mechanical properties of these steels depend mainly on the extent to which austenite decomposes into martensite in deformation, and on the fracture strength [resistance to failure under the action of normal stresses], of the martensite formed. When martensite produced by quenching is present in the initial steel, its rupture resistance also determines the mechanical properties of the steel. In tensile tests on transitional nickel steels containing 0. 26% carbon, initially purely austenitic, there was a considerable plastic deformation (elongation up to 60%); fracture occurs without necking, and the elongation exceeds the reduction of area. In spite of considerable preliminary plastic deformation, the failure is of the brittle type. 3. It has been confirmed that in steels containing unstable austenite, a combination of strength and ductility can be achieved, unattainable in steels of other structural categories.