Determining role of microstructure on crack arrest and propagation phenomenon in low-carbon microalloyed steel

Abstract

The relationship between microstructure and crack arrest characteristics of a microalloyed steel with a yield strength of 460 MPa was elucidated in this study with particular focus on the effect of the microstructure on the propagation of brittle cracks. The study emphasized that the acicular ferrite (AF) content could be increased to 41% by lowering the final rolling temperature and final cooling temperature, which reduces the average effective grain size (EGS) to as small as ~4.1 μm and increases the high-angle grain boundaries (HAGB) proportion to as high as ~48.8%. The presence of AF in large numbers could reduce the ductile-to-brittle transition temperature to approximately −87 °C through refinement of the microstructure. The presence of HAGB between adjacent grains could significantly arrest cracks by changing the propagation direction of brittle cracks and was a decisive factor in determining the characteristics of brittle fracture. Moreover, the fracture roughness varied directly with the proportion of the HAGB and inversely with the average EGS. Martensite/austenite islands with low content (0.92%) and small size (~1.5 μm) reduced the stress concentration significantly at the crack tip. The excellent crack arrestability was attributed to AF, which minimized the expansion of cleavage facets by the split nail effect ahead of the growth direction, so that the cleavage facets were divided into two or more branches, considerably reducing the stress concentration and retarding brittle crack growth during crack propagation at low ambient temperature.