High Temperature Austenitizing of Low Alloy Steels: What Is Left After 40 Years?

In 1974 it was discovered that quenching AISI 4340 steel from 1200 °C leads to much higher fracture toughness, in the as quenched state, than conventional austenitizing at 870 °C. Further researches have ascertained that the apparent toughness increase is limited to fracture toughness tests (K_Ic), whereas Charpy-V impact tests do not show any betterment due to high temperature austenitizing, in respect to conventional austenitizing. Various explanations of these contradicting results were given on the basis of the then existing theories (notably by Ritchie et al. and by Firrao et al.), and it was further ascertained that the betterment of fracture toughness was limited upon tempering to a maximum temperature of 250 °C, making it useless for most applications. The puzzling phenomenon has been recently reconsidered for the validation of new Blunt Notch Brittle Finite Fracture Mechanics theories. Results are given and possible future applications to industrial cases presented.