Regarding the microstructural features of SLM steels, their grains elongate along the building direction through the solidification [12]. This elongation results in different crack propagation routes in horizontal and vertical specimens, as shown in Fig. 9. According to this figure, there is a small resistance against the crack growth in horizontal specimens, and this lack of pinning effect results in smaller ΔKth values for these specimens [58–61]. In addition, according to [59], another reason for the lower ΔKth values was easier propagation paths along the cellular boundaries and laser tracks in the horizontal specimens, as shown in Fig. 10. Laser track generally refers to the trail of laser beam scanning on the build plane and normal to the build direction, which sometimes leaves behind a visible mark.