FRACTURE-BEHAVIOR OF C-MN STEEL AND WELD METAL IN NOTCHED AND PRECRACKED SPECIMENS .2. MICROMECHANISM OF FRACTURE

The microscopic fracture behaviors of a C-Mn steel plate and of two types of weld metal in Charpy V, crack tip opening displacement (COD), and precracked impact specimens have been studied by observations of the size distribution of the cleavage-initiating particles, the morphology of the fibrous crack zones, the microstructure in the region initiating the cleavage crack, and the size distribution of microcracks remaining in the specimens. It has been found that the different values of the microscopic cleavage fracture stress, σf, measured in notched and precracked specimens of the same metal result from the change in the critical event (critical length) in the fracture processes. The critical event in the Charpy V-notched specimens at -45 °C to -65 °C is the propagation of a ferrite grain-sized microcrack into the ferrite matrix; however, in the precracked COD specimens at -110 °C, it is the propagation of a second-phase particle-sized microcrack into the neighboring ferrite grain. The change of the critical event is considered to be related to the difference of effective shear stress ahead of a notch root or a crack tip. The different sensitivities to the notch acuity of the base and weld metal result from their micro-structures. Finally, the prerequisite for establishing a correlation between Charpy V and COD tests is analyzed. © 1990 The Minerals, Metals & Materials Society - ASM International - The Materials Information Society.