This work investigates the use of powder metallurgy to create innovative duplex composite steels, called COMPLEX. This method uses pressure-assisted techniques, in particular SPS (Spark Plasma Sintering) and HIP (Hot Isostatic Pressing), to develop new material grades from powders. This approach offers several advantages, including a fine microstructure in the final material, guaranteeing the best properties for use, independent control of the chemical composition and proportion of each phase, and the possibility of producing complex-shaped parts close to the final dimensions. A first study was conducted using SPS, chosen for its rapidity, due to fast heating and cooling rates. Austenitic and ferritic stainless steel powders (a ductile 316L and a corrosion resistant 410L, respectively) were mixed in varying mass fractions of 410L ranging from 10% to 90%, followed by sintering via SPS. The aim is to optimize the compromise between mechanical strength and ductility, intrinsically linked to the proportion of austenite and ferrite in duplex steels. The microstructural characterizations revealed that martensite forms at the austenite/ferrite interfaces, and the width of the martensitic regions is influenced by Cr and Ni diffusion between 316L and 410L. Additionally, it was observed that the 410L (50%) - 316L (50%) blend had better homogeneity and hardness than the other blends. Consequently, 410L (50%) - 316L (50%) blend was selected for sintering via HIP. A comparative study was conducted on samples elaborated by SPS and HIP, focusing on morphology, composition/nature of phases, and grain orientation. The objective was to assess the impact of the change in the time scale on diffusion phenomena (several hours for a HIP cycle compared to several minutes for an SPS one) and the resulting microstructure. COMPLEX steels are unique in that the characteristic size of the phases is that of the initial powder particles (down to a few hundred microns), which is larger than the metallurgical grain size of conventional duplex steels.