Micromechanism of the transition of fibrous cracking to cleavage of C-Mn base and weld steel

Abstract

In this present work, crack opening displacement (COD), four-point bending (4PB), Charpy V, and three-point bending (3NB) tests of specimens of C-Mn base and weld steel were carried out in the brittle-ductile transition temperature region. Some specimens were fractured and some specimens were unloaded prior to fracture after fibrous cracking occurred and extended to variouslengths. Through detailed observation of the variation in the shapes of microcavities located at the tip or on the two sides of fibrous cracks in unloaded specimens and the variations of shapes of dimples located at various lengths of cracks on fracture surfaces, the micromechanism of the change from fibrous cracking to cleavage was analyzed. It was revealed that no matter whether a specimen was notched or precracked, as long as a fibrous crack initiated and propagated in it, the critical event for cleavage fracture is the unstable extension of a ferrite grainsized crack. The main factor promoting the transition from the fibrous crack to cleavage was the increase of the local tensile stress ahead of the crack which was caused by the increase of the triaxiality of stress and the apparent normal stress in the remaining ligament. The considerable scattering of toughness values in the transition temperature region is due to the random variation of the widths of the tips of the fibrous cracks during their extension and the random distribution of the weakest constituents in the microstructure.