Untempered Ultra-high Strength Steels of High Fracture Toughness

Abstract

ULTRA-HIGH strength steels now in use were developed decades ago by trial and error methods, and they all have some undesirable characteristics, such as low fracture toughness at high levels of yield strength. So far there has been little effort to use the new concepts of alloying and micromechanics of fracture to improve existing alloys, or to find new ones with better combinations of properties. We are studying the factors that contribute to notch brittleness in high strength steels, and we have learned how to increase the fracture toughness of steels having yield strength in excess of 200,000 pound inch⁻² by as much as 70%. We use treatments that differ significantly from the normal commercial practice for quenched and tempered low alloy steels, which involves heating to the lower end of the austenite temperature range (to minimize grain size), quenching fast enough to produce martensite, and tempering at a temperature that will optimize mechanical properties.