Adding nitrogen to stainless steel may be beneficial to the bulk properties of the material, such as increased surface hardness and optimized microstructures that retain the corrosion resistance. Nitrogen can be added in the liquid phase during fabrication of the steel, but this is usually an expensive and cumbersome process as it requires high N2 pressures and there is the risk of formation of nitrides during cooling and subsequent processing. An alternative is “High Temperature Solution Nitriding” (HTSN), where nitrogen is introduced from the gas phase to the solid state—a process which is highly analogous to classical carburizing of non-stainless steels. Nitrogen is typically added to stainless steels at temperatures ranging from 1,050–1,150°C and using pressures of N2 ranging from 0.1 to 3 bar (Refs. 11,12). The process entails gas quenching in N2 to suppress formation of chromium nitrides. The entire process is carried out in a clean environment and will provide improved corrosion resistance, higher hardness and wear resistance and enhanced fatigue performance. The process is particularly interesting for martensitic stainless steels but can also be applied to austenitic and duplex stainless steels. Depending on a wide variety of process conditions, some being the alloy content, nitrogen pressure and the subsequent cooling rate, the resulting final microstructure can vary quite significantly. Additionally, the total amount of nitrogen taken up by the material will also impact the final microstructure, for example the phases developed (viz. martensite) and the morphology of martensite (Ref. 13). Adding nitrogen through HTSN to martensitic steels can come at a cost. Since nitrogen itself is a strong austenite stabilizer, this can produce higher amounts of retained austenite following quenching, which many times are undesirable (Refs. 14,15). The complexity of diverse alloying elements and process conditions means that process optimization can be tedious and require many iterations. The interplay between retained austenite and solution hardening associated with martensitic stainless steels is considered through the analysis of sample microstructures of the investigated materials. In-situ methods offer the benefit of completing lab scale iterations, achieving fast results in a clean and controlled environment. As mentioned previously, the HTSN treatment can produce “case-hardening-like” results. The discussed in-situ techniques can be applied to other traditional gaseous case hardening processes such as carburizing of steels, where similar uptake behavior is present. 17-4 Precipitation Hardening Steel Precipitation hardening (PH) steels, such as 17-4PH, are martensitic stainless steels that harden through forming precipitates during an aging treatment. The standard heat treatment for 17-4PH for peak hardness is the 900H treatment, involving a solution treatment around 1,040°C, quench, then age around 480°C (≈900°F) for one hour (Ref. 16). 17-4PH primarily forms Cu precipitates in the martensite phase, so when the quenching fails to complete the martensite transformation, retained austenite will limit the hardness of the final material (Ref. 17). In AM, LPBF 17-4PH can have a primarily austenitic structure due to N2 gas-atomized powders and N2 cover gas used during processing. In addition to the austenite stabilizing effects of nitrogen, LPBF 17-4PH tends to have a cellular or dendritic structure associated with microsegregation of the alloying elements. This further suppresses the martensite transformation, leading to a primarily austenitic microstructure after heat treatment. Like other AM metals, the unique composition and microstructure of LPBF 17-4PH requires a modified heat treatment to achieve the desired material properties. Other studies have found success in lowering the solutionizing temperature (Ref. 17), extending the aging time (Ref. 18), or incorporating sub-zero Celsius treatments (Ref. 19).