Around 1970 it was discovered that quenching AISI 4340 steel from 1200 °C leads to much higher fracture toughness, in the as quenched state, than by conventional austenitizing at 870 °C. Further researches have ascertained that the apparent toughness increase is limited to fracture toughness tests (KIC), whereas Charpy-V impact tests do not show any betterment due to high temperature austenitizing, in respect to conventional heat-treating. Various explanations of these contradicting results were given on the basis of the then existing theories. 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.

Ti-6Al-4V alloy is characterized to be sensitive to heat treatment and deformation. This paper focuses on microstructural evolution and variation in mechanical properties with respect to the deformation and change in the heat treatment cycle. Different heat treatment cycles such as mill annealing, solution treatment and beta solution treatment followed by annealing were carried out on deformed and undeformed Ti-6Al-4V samples. Heat treated samples were studied using optical and scanning electron microscopy. Also different mechanical tests (i.e. tensile test, fracture toughness test) were conducted and results were analyzed. Large variation in mechanical properties and microstructures were found out with different heat treatment cycles. Fracture toughness was found to be high for beta solution treatment samples than the mill annealed and solution treated samples and the reason for the same has been analyzed.