The limits of strength and toughness in steel

The ideal structural steel combines high strength with excellent fracture t oughness. In this paper we consider the limits of strength and toughness fr om two perspectives. The first perspective is theoretical. It has recently become possible to compute the ideal shear and tensile strengths of defect- free crystals. While the ferromagnetism of bcc Fe makes it a particularly d ifficult problem, we can estimate its limiting proper ties from those of si milar materials. The expected behavior at the limit of strength contains ma ny familiar features, including cleavage on {100}, [111] slip on multiple p lanes, "conditionally" brittle behavior at low temperature and a trend away from brittle behavior on alloying with Ni. The behavior of fee materials a t the limit of strength suggests that true cleavage will not happen in aust enitic steels. The results predict an ideal cleavage stress near 10.5 GPa, and a shear strength near 6.5 GPa. The second perspective is practical: how to maximize the toughness of high-strength steel. Our discussion here is l imited to the subtopic that has been the focus of research in our own group : the use of thermal treatments to inhibit transgranular brittle fracture i n lath martensitic steels. The central purpose of the heat treatments descr ibed here is grain refinement, and the objective of grain refinement is to limit the crystallographic coherence length for transgranular crack propaga tion. There are two important sources of transgranular embrittlement: therm al (or, more properly, mechanical) embrittlement at the ductile-brittle tra nsition, and hydrogen embrittlement from improper heat treatment or environ mental attack. As we shall discuss, these embrittling mechanisms use differ ent crack paths in lath martensitic steels and, therefore, call for somewha t different remedies.