Grain Size Dependence of Fracture Toughness in an Ultrahigh Strength Maraging Steel

Abstract

The effect of prior austenite grain size on fracture toughness (K_IC) in 2 750MPa grade 13Ni-15Co-10Mo maraging steel has been investigated. The prior austenite grain size was varied in the range of 760Oμ using both thermomechanical treatment and solution treatment.
K_IC increased with increasing the grain size. For example, K_IC increased from 2OMPa·m^1/2 for 8μ to 30MPa·m^1/2 for 500μ and varied linearly with the reciprocal square root of the grain size. The acoustic emission measurement during fracture toughness test and fracture surface observation at the tip of fatigue precrack revealed that subcritical crack growth of intergranular fracture mode occurred as the grain size increased and K_IC was apparently increased due to the microbranching effect of the crack front. On the basis of the above observation, it was suggested that true grain size dependence of K_IC in 13Ni-15Co-10Mo steel might be less than the experimentally obtained result.