Investigation on Fracture Toughness Values for Various Martensite Volume Fractions

In this study the effect of martensite volume fraction on hardness and fracture toughness properties of AISI 4140 and AISI 4340 steels were investigated. After determining of intercritical boundary temperatures for steels, the water quenching processes were performed at three intercritical annealing temperatures. The martensite volume fractions were determined from microstructure images for each condition by using image processing method. The martensite volume fraction increased with increasing intercritical temperature. The hardness and fracture toughness tests were conducted on the specimens. Increasing of martensite content increased the hardness values whereas decreased the fracture toughness values. Although AISI 4340 had higher hardness than AISI 4140, it had lower fracture toughness properties