The effects of grain-refining precipitates on the development of toughness in 4340 steel

Abstract

Grain-refining precipitates have been identified as microstructural features that adversely affect the toughness of grain-refined, high-strength steels. For the case of an air-melt, aluminum-killed 4340 steel processed in a conventional manner, the presence of coarse A1N and TiN precipitates is shown to have a deleterious effect on both room-temperature impact toughness and fracture toughness. Coarse A1N precipitates, which evolve during the primary and secondary breakdown of the steel, are largely responsible for the development of quasicleavage fracture at ambient temperatures and the formation of fine-scale microvoids at upper-shelf temperatures. The refinement of grain-refining precipitates provides substantial increases in both lower-shelf and upper-shelf toughness for conditions associated with a critical damage mechanism of fracture. Improvements in upper-shelf toughness specifically result from a change in the species of grain-refining precipitate that nucleates secondary microvoids during the latter stages of fracture, viz. a refinement-induced transition from A1N to smaller TiN precipitates in the microstructure. This refinement-based mechanism of toughening is operative over a broad range of strength, although improvements in the transverse toughness of 4340 steel diminish with increases in sulfur content above 0.01 wt pct.