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Compared to hard metals, high-speed steels exhibit a good combination of high strength with reasonable toughness. These mechanical properties enable the excellent performance of high-speed steels as tool materials in metalworking applications. The composite microstructure with primary- and secondary carbides and the surrounding metallic matrix provides this key feature. To increase efficiency parameters such as tool life, HSS are subject to continuous development regarding heat treatments and their effect on the variation of the microstructure. For tool steels, knowledge of the effect of carbide coarsening heat treatments of high-speed steel on carbide spacing is still incomplete. Therefore, the analysis of the microstructure of tool steels resulting when varying the heat treatment is necessary. In the current work, specimens made from commercially available high-speed steel were subjected to several distinct heat treatments, such as long-term and high-temperature annealing in vacuum. Specimens quenched and tempered to industrial standards are used as a reference. Differences regarding carbide volume fraction, diameter, and spacing were observed with a scanning electron microscope. Furthermore, changes in matrix hardness were studied quantitatively using nanoindentation. Due to the above-described heat treatments, significant carbide coarsening and an increase in carbide spacing were achieved. These changes in microstructural features such as carbide spacing and matrix hardness were then correlated with the applied heat treatment parameters.