Various in-situ methods can track real-time heat treatment response in bulk materials. This can aid material engineers in designing the most effective heat treatment procedures and processing methods. In-situ methods such as thermogravimetry (TG), differential thermal analysis (DTA), and dilatometry, offer capabilities to examine heat treatment behavior in real time, giving insight into the thermochemical mechanisms and the thermal behavior of steel. This contribution specifically addresses different martensitic (stainless) steel where in-situ control during heat treatment and detailed understanding of their microstructure can pave the way for new applications. The examples presented are 1) heat treatment behavior of different classes of high carbon martensitic (stainless) steel powder for additive manufacturing application, i.e., stainless 440C, cold-work D2, hot work H13, and high speed steel (HSS) T15, 2) nitrogen alloying of wrought martensitic stainless steel AISI 420, and 3) heat treatment response of additively manufactured and conventional precipitation hardening maraging stainless steel 17-4PH. This work is not an in-depth treatise of these individual topics but is intended to show the importance of in-situ techniques for development of new materials (solutions). The individual topics are briefly introduced in the following. High Carbon Steel Powders for Additive Manufacturing Additive manufacturing (AM) is gaining popularity and experiencing rapid technological advancements. Recently, there has been a significant focus on the application of high-carbon steels (Ref. 1). AM methods, such as laser powder bed fusion (LPBF), binder jetting (BJ), and spray forming, offer several benefits, including the attainment of uniform microstructures which offer superior mechanical properties compared to conventionally manufactured metals. Currently, several specialized high-carbon powders are now commercially available for AM. For a successful adoption of high carbon steels for AM, more knowledge on the fundamental thermal behavior related to the processing is needed. To this end, powders and in-situ techniques are an excellent foundation for investigating heat-treating characteristics. 440C stainless steel (440C) is a high-carbon martensitic stainless steel known for its high hardness, wear resistance, and moderate corrosion resistance. According to Bang et al. (Ref. 2), LPBF addresses the limited industrial applications caused by 440C’s high hardness and low workability, while also improving overall mechanical properties such as ultimate tensile strength (UTS) and yield strength (YS) (Refs. 4,5). D2 cold-work steel (D2) is distinguished by its superior hardness, high strength, and excellent wear resistance, making it widely used in industrial applications like cutting and punching tools, as well as dies. This material is well-suited for AM techniques, particularly direct energy deposition (DED) (Ref. 5). H13 hot-work tool steel (H13), typically utilized in a quenched and tempered state, features a martensitic matrix with dispersed fine secondary carbides. Known for its high hardness and fracture toughness, H13 also offers excellent wear and erosion resistance, along with relatively high resistance to thermal shock and thermal fatigue. As noted by Park et al. (Ref. 5), metal deposited via the DED process exhibits different properties compared to wrought metal due to the rapid solidification rate and the high thermal gradient between the deposited metal and the substrate. The microstructure of deposited D2 and H13 has been shown to be highly uniform, with the hardness of deposited D2 comparable to conventional martensitic high-carbon stainless steel, and the hardness of deposited H13 exceeding that of wrought H13. T15 high-speed steel (T15) is well known for its high hardness and excellent wear resistance at elevated temperatures, attributed to its significant carbon and tungsten content, making it an ideal material for cutting tools, drills, blades, and knives. According to Zhang et al. the spray-formed T15 steel exhibits higher hardness, significantly enhancing its overall mechanical properties (Ref. 6). This increase in hardness is a crucial factor contributing to the superior mechanical performance of T15 steel in various applications (Refs. 8–11).