Fracture Behavior of AISI M-2 High Speed Tool Steel

A fracture toughness study was made on M-2 high speed steel. The major experimental variables included: melting processes (air-melted, ESR and AOD), austenitizing (single and dual) temperature, tempering temperature, and carbide distribution and morphology. Microstructural characterization was performed by using X-ray diffraction, optical microscopy, scanning electron microscopy and transmission electron microscopy of carbon extractive replica. It appears that undissolved carbides are the major microstructural constituents that control crack initiation and hence toughness. Retained austenite is believed to be responsible for the increasedKIc values observed in dually austenitized specimens. The large grain size resulting from this process apparently impaired impact toughness. Despite the variety of melting processes and heat treatments investigated, fracture toughness remained in the range of 11 to 27.5 MPa m^1/2, inversely proportioned to the hardness in the range of 55 to 66 Rc.