Fracture of nitrided and nitrocarburized blunt notch three-point bending die steel specimens

Results of studies on impact tested gas and plasma nitrided or nitrocarburized blunt notch die steel samples with varying notch root radii are presented. A detailed fracture mechanism sequence is proposed encompassing, (i) multiple mode-I radial cracks formation at the root of the notch within the N-diffusion zone; (ii) development of a cylindrical stress-free zone effectively creating a notch with a radius larger than the previous one; (iii) concurrent mode-II fractures running along logarithmic spirals of the slip line field forming at the new larger notch; (iv) race restricting to two of them, with one hitting the notched sample centerline before the other and further propagating into the region of minimum thickness with a mode-I fracture. Suggestions for improved steel compositions are given after developing relationships among impact absorbed energies and geometrical features of the fracture surfaces of plasma nitrided Charpy-U specimens.