Selective Laser Melting (SLM) is a technique of Additive Manufacturing of metallic parts. There are still several barriers of acceptance of that process in the industry and particularly the lack of understanding of microstructures of materials obtained. One of the most common materials used in SLM is the stainless steel named “17-4PH”. However examination of this material after SLM shows a very unexpected microstructure. In this proposal we plan to study the phase transformations in that material at “very high” temperature (i.e. above 1400°C, just below the melting point) using neutrons diffraction in ISIS/HRPD with in-situ specimen heating. This study will allow better understanding of the microstructure and properties of stainless steel parts obtained from SLM.