Tensile behaviour of a hot work tool steel manufactured via Laser Powder Bed Fusion: effect of an innovative high pressure heat treatment

The combination of outstanding mechanical strength of tool steels and design freedom ensured by Additive Manufacturing (AM) processes is of a great interest for automotive applications. However, AM techniques produce peculiar defects, which affect the resulting mechanical behavior. For this reason, safety critical AM components are often subjected to hot isostatic pressing (HIP) to heal process defects. At the same time, tool steels require a proper quenching and tempering (QT) heat treatment to achieve high hardness and strength. In the present work, the effect of an innovative high pressure heat treatment (HPHT) on the tensile properties of a hot work tool steel produced via Laser Powder Bed Fusion (LPBF) was investigated. LPBF samples were subjected to two post-process heat treatments: a conventional quenching and tempering heat treatment performed in vacuum (CHT), and an innovative HPHT combining HIP e QT in a single step, to heal LPBF defects and obtain high mechanical properties. HPHT featured the same quenching and tempering cycle of CHT but it was performed under high pressure in a HIP furnace and with longer austenitizing time to promote defect closure. Tensile tests indicated no significant effect on proof and tensile strength for HPHT compared to CHT, but a significant reduction of elongation after fracture. Fractographic analyses and fracture mechanics calculations indicated that both CHT and HPHT specimens failed via crack propagation from large LPBF defects when the stress intensity factor reached the material fracture toughness. Fractographic analyses indicated an incomplete defect closure during HPHT due to the presence of an oxide film on the inner surface of defects, thus justifying the same failure mechanism and strength of CHT samples. Instead, it was proposed that the reduced elongation could arise from coarsened grains due to the longer austenitizing.