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