RELATIONS BETWEEN MICROSTRUCTURE AND MECHANICAL-PROPERTIES IN SECONDARY HARDENING STEELS

Significant developments in the evolution of secondary hardening steels over the past 75 years are reviewed. It is shown that by the beginning of World War II the key solid state changes responsible for the unusual tempering resistance of secondary hardenning steels were becoming clear. In the two decades following the war transmission electron microscopy provided the high resolution tool needed to study the strengthening mechanisms and apparently intrinsic brittleness of secondary hardening steels. The chemical and microstructural factors known to cause the poor toughness are described and progress made in the last decade to improve the toughness is discussed. In particular, the role of the non-carbide-forming elements is examined. The beneficial and detrimental effects of silicon are described in detail. Finally, the enhancement of toughness that might be achieved from the partial substitution of silicon by aluminum is considered. © 1979.