Fracture Toughness of a Hot Work Tool Steel Fabricated by Laser-Powder Bed Fusion Additive Manufacturing

The fracture toughness of AISI H13 tool steel, additively manufactured by laser powder bed fusion (L-PBF) technique, is studied. The influence of the building direction on fracture toughness is investigated on small notched bending samples heat treated according to two different thermal cycles, namely, quenching and tempering (QT) and only tempering (T). The notch is electro-discharge machined parallel (P-||), perpendicular (P-perpendicular to), and longitudinal (L) to the building direction. Both heat treatments, even if to a different extent, delete the as-built (AB) microstructure, producing secondary carbides precipitation in the martensitic matrix. The microstructure of the directly tempered parts is finer than the quenched and tempered ones. The fracture toughness increases moving from P-perpendicular to to P-||. The T samples show a higher apparent fracture toughness in the P-||, despite the higher hardness. Secondary cracks formation allows reducing the main crack opening driving force increasing the fracture toughness in P-|| samples. This effect is more pronounced in T samples where the stronger precipitation of carbides at the prior melt boundaries promotes secondary cracks. Moreover, in P-|| samples the laser tracks act as barriers to crack propagation and as stress dissipators.