The effect of retained austenite on the fracture toughness of high-speed steels

The aim of this work was to find the quantitative dependences between fracture toughness K_lc and the volume fraction of retained austenite in the matrix of quenched high-speed steels. The tests were carried out on three model alloys of a different content quotient of Mo: W which, after quenching, were gradually supercooled up to ? 196°C and then tempered at 450°C. Also the measurements of the content of retained austenite in the vicinity of the surface of a sample fracture were carried out. It was determined that after tempering at 450°C the fracture toughness of the matrix of high-speed steels is directly proportional to the content of retained austenite in it. Every 1 % by volume of retained austenite increases the fracture toughness K_lc of the matrix by about 5%, despite the fact that most probably it is completely transformed into fresh martensite in front of a propagating crack. Higher fracture toughness of the matrix of high-speed steels rich in molybdenum should be explained exlusively by a larger content of retained austenite. Transformations in the martensitic part of the matrix of the alloys richer in molybdenum clearly reduce the advantageous effect of retained austenite on this steel feature.