High-speed steels combine the properties of hot strength and cold work steels. With these properties, high-speed steel displays, compared to hard metals, 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, such as cutting or shaping. The combination of high strength and toughness is provided by the composite microstructure made of primary and secondary carbides and the surrounding metallic matrix. High-speed steels are continually being developed concerning heat treatments and their impact on the variation of the microstructure to increase efficiency factors such as tool life. For tool steels, knowledge of the effect of austenitization time, austenitization temperature, and tempering temperature heat treatments on high-speed steels is still incomplete. Thus, the analysis of the microstructure of tool steels resulting when varying the heat treatment is essential. In the current work, specimens made from commercially available high-speed steel were subjected to a variety of different heat treatments in vacuum and atmosphere environments. Specimens hardened and tempered according to industrial standards are used as a reference. Divergences regarding carbide spacing, carbide size, and carbide volume fraction were observed with a scanning electron microscope. Additionally, variations in matrix hardness were studied quantitatively using nanoindentation. As a result of the above-described heat treatments, significant carbide coarsening and an increase in carbide spacing was accomplished. Furthermore, modifications in matrix hardness were noticeable as well. The applied heat treatment parameters, such as carbide coarsening and matrix hardness, were then connected with these modifications in microstructural features.