Influence of heat treatment on the fracture toughness and crack propagation in 5% Cr martensitic steel

The high strength of 5% chromium steels at elevated temperature is directly related to their complex microstructure generated by specific heat treatments that consist of thoroughly controlled austenitizing, quenching and tempering. Fracture toughness and fatigue crack propagation are studied for various heat treatments. It is shown that changing the austenitizing temperature, i.e. the grain size and morphology of primary carbides, does not change the fracture characteristics. However, modifications of tempering, directly impacting the alloy hardness, result in drastic changes of fracture toughness and crack propagation rates. As no change in grain or martenstic lath morphology is observed, the associated modification of the Paris law coefficients is attributed to a change in the distribution and volume fraction of nanometric carbides investigated by Small Angle Neutron Scattering.