Segregation Effects and the Toughness of Untempered Low-Alloy Steels

ZACKAY et al.1 have discussed the increase in fracture toughness of untempered ultra-high strength steels resulting from the use of high austenizing temperatures for solution treatment. Their results indicate that the toughness is increased suddenly and dramatically, by a factor of more than two, when the steels are austenized at temperatures greater than 1,100° C (Fig. 1). The effect is promoted by fast quenching rates. They show further that a two-step quenching procedure from high austenizing temperatures (for example, 1,200° C→870° C→quench) can eliminate any danger of quench cracking, whilst causing only a small (7%) reduction in fracture toughness. This improvement in toughness, obtained by using high austenizing temperatures, was attributed to the retardation of grain boundary nucleation of a second phase when high energy grain boundaries, associated with small grains, are eliminated by grain growth.