Cleavage initiation in steel: Competition between large grains and large particles

TiN particles in steel can limit austenite grain growth during reheating or welding. However, cuboidal TiN particles of large size (>1 μm) can act as cleavage fracture initiation sites and affect the low temperature toughness. Based on the assumption that the local principle stress acts uniformly everywhere within a grain, for a fixed particle size, a critical value of ferrite grain size can be obtained below which the grain boundary effectively retards the propagation of micro-cracks, which can be nucleated from TiN particles. In the present study, the relative effect of ferrite grain size and TiN particles on cleavage crack propagation has been assessed. Ti-containing high strength low alloy (HSLA) steels were processed using different thermo-mechanical schedules and their microstructures and Charpy impact properties were investigated. It has been found that in spite of the presence of large TiN particles, refinement in ‘effective ferrite grain size’ can significantly improve the impact toughness, provided the matrix strength and meso-texture (i.e. the intensity of low-angle boundaries) are restricted to low values.