Microstructural effects on discrepancy between toughness under sharp cracks and blunt notches in Ti-added 8Ni-13Co secondary hardening steel

In this study, the toughness under sharp cracks and blunt notches is investigated in terms of the prior austenite grain size (PAGS) and inclusion particles as a function of the austenitizing treatment in a Ti-added 13Co-8Ni secondary hardening steel. For the quantitative analyses of the inclusion and precipitate particles, small-angle neutron scattering analyses are conducted under austenitizing conditions of 1050 °C, 1200/1050 °C, and 1200/1200 °C; the impact toughness values are 32, 30, and 24 J for each austenitizing condition, respectively. In contrast, the fracture toughness values under the same conditions are 66, 78, and 103 MPa·m^1/2. Thus, the fracture toughness significantly improves under 1200/1200 °C austenitizing conditions with coarse PAGS; however, the impact toughness deteriorates. The adverse effect of the grain size on the toughness under sharp cracks and blunt notches is elucidated in terms of the effective microstructural factors that control the fracture process inside the plastic zone, the size of which varies with the notch sharpness. In particular, through considering the density of the slip bands as a function of the grain size in the small confined plastic zone before the sharp crack, the complicated problem regarding an increase in the fracture toughness with an increasing grain size is described from micromechanical and microstructural perspectives.