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

Abstract

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 electrodischarge machined parallel (P || ), perpendicular (P ⊥ ), 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 ⊥ 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.

Additive manufacturing of steels: a review of achievements and challenges

“…The processing parameters control the mechanical performance of H13 tool steels. As shown by Pellizzari et al [219] in an LPBF H13, the fracture toughness increases with volume energy density. This is because higher energy densities reduce the porosity and unmelted particles, i.e.…”

“…Other processing parameters such as building direction and different thermal cycles have a less pronounced effect compared to the energy density. An interesting point, mentioned in [219], is that the formation of secondary cracks perpendicular to the principal ones produces a decrease in the driving force for the main crack propagation, which results in improved fracture toughness. This effect is more pronounced in the tempered condition compared to the quench-tempered condition, due to more pronounced precipitation of carbides network at the prior melt boundaries which promotes the formation of secondary cracks, as well as due to the generally finer microstructures of tempered parts.…”

Effect of Different Post-Processing Thermal Treatments on the Fracture Toughness and Tempering Resistance of Additively Manufactured H13 Hot-Work Tool Steel

“…In plane strain fracture toughness testing, the stress concentration factor decreases with an increasing root radii; therefore, higher stress is needed to meet the "critical stress intensity." This results in a slightly higher apparent fracture toughness (K app ) compared with that of the plane strain fracture toughness (K IC ) tested with a fatigue pre-cracked specimen (i.e., ρ → 0) [15,19]. However, for notch radii below 100 µm, the K app do not appear to vary significantly as a function of notch radii [19].…”

“…However, few systematic studies have been performed to evaluate the effect of direct tempering on mechanical properties and tempering resistance at elevated temperatures. Deirmina et al [12,15] compared the effect of two heat-treatment scenarios for laser powder bed fusion-processed H13 and demonstrated that the direct tempering of samples, with the notch perpendicular to the deposited layers, from the as-built condition (ABT) resulted in better static fracture toughness (K IC ) compared with the quenched and tempered (QT) counterpart due to the secondary crack formation and crack deflection. On the contrary, for samples with the notch parallel to the deposited layers, the QT counterpart showed significantly higher fracture toughness compared with the ABT counterpart.…”

Effect of heat treatment and defects on the tensile behavior of a hot work tool steel manufactured by laser powder bed fusion

“…Instead, in both HTA and HTB conditions, the microstructure appeared comparable to the one of wrought steels of similar composition, with no evidence of the typical features of as-built LPBF components, except for the peculiar defects resulting from the LPBF process (lack of fusion defects and gas pores). As reported in literature, 20,22,23,25,37,41,43,45,51 the lack of the typical microstructural features of LPBF components comes from the austenitizing step, performed during HTA and HTB heat treatments. In fact, the high temperature during austenitizing promotes alloying diffusion, resulting in i) chemical homogenization, ii) removal of alloying segregation and solidification structure, and iii) recrystallization via nucleation of new austenite grains with homogeneous composition and equiaxed morphology.…”