Cracking in M2 high speed steel

AbstractCracked carbides (non-propagating microcracks) were observed in M2 type high speed steels deformed in four-point bending in the temperature range 17–600°C to ∼0·7% of the yield strain, i.e. to below the stress for macroscopic yielding and ductile or brittle fracture. Although the sintered M2 steel underwent a brittle–ductile transition at ∼300°C, no associated significant change in the fracture mechanism was detected. In all samples studied, cracking of individual carbides (nucleation of failure) was followed by the subcritical linking of these microcracks through the matrix, i.e. growth, and only then by catastrophic propagation, as a Griffith crack from an approximately semielliptical edge (or corner facet) fracture initiating region. Fracture mechanics analyses indicate the depth of this region to be in excess of the Griffith–Irwin flaw size, except for the brittle specimens. The region of subcritical crack growth is compared to the process zone reported for ceramics.