The atmosphere in powder-based additive manufacturing influences the level of impurities inside the build volume as well as the phases formed during cooldown. We use a thermodynamic model to investigate trends for steels commonly considered for fusion reactors and power plants, namely 316L, F91, F92 and Eurofer97. The atmospheres considered are vacuum (similar to electron beam methods) and gaseous atmospheres based on nitrogen and argon, with impurity levels of 10 and 100 ppm O2, as well as a “worst case scenario” of air.