1. Weldable high-strength steels can be divided into two groups: those with a ferrito-pearlitic structure and those consisting of acicular upper troostite. Steels of the first group are used either as-rolled or normalized. Their tensile strength is 55–71 kg/sq. mm; their weldability and impact toughness are good. Steels consisting of acicular lower troostite or martensite have a maximum TS of 87–110kg/sq. mm whereas the optimum impact toughness lies at a TS of 71–87kg/sq. mm. The best combination of properties is achieved in both groups after tempering. 2. In steels with an acicular-troostite structure, the transformation temperature depends little on the cooling rate (cross-section of part) and the mechanical properties are uniform across the section. Acicular lower troostite is the most desirable structure in weldable steels. Such steels do not require a hardening treatment.