Investigation of the surface behavior of 1.2709 maraging steel produced by SLM following post-processing treatment. Application of Mössbauer spectroscopy to materials used in additive manufacturing

Maraging steel is a high-strength material with enhanced mechanical properties. Mössbauer spectroscopy (MS), also known as recoilless nuclear gamma resonance, can be used to study the surface and bulk of a material. Conversion X-ray Mössbauer spectroscopy (CXMS) and conversion electron Mössbauer spectroscopy (CEMS) provide information about the structure of materials at different depths: 10 µm and 0.3 µm, respectively. This method can reveal information about the inhomogeneity of the material's surface layer. In this research, the study samples were made from 1.2709 maraging steel metal powder using selective laser melting (SLM). The initial samples were in the shape of a cylinder with 25 mm diameter and 5 mm height. After 3D printing, the surface of the studied samples was subjected to a post-production treatment process. The samples were studied under the following conditions: after sandblasting; after sandblasting and annealing at 500 °C in air, after polishing; and after polishing and annealing at 500 °C in air. Studies of the phase composition and phase transformations of 1.2709 maraging steel have revealed the relation between surface treatment methods and phase distribution. A detailed investigation of the surface of the sample revealed that the initial sandblasted and polished samples contained martensite and retained austenite phases. The phase composition in the surface layer is also affected by the surface treatment method. While the polished sample showed only the martensitic phase, sandblasting preserved both the austenitic and martensitic phases. The samples exposed to temperature also show the presence of iron oxide (α-Fe2O3) at a depth of around 10 μm. Studies of the thin surface layer at depth between 0.1 and 0.4 μm have revealed that the surface is inhomogeneous. A polished surface tends to create the α-Fe2O3 oxide when exposed to temperature, whereas a sandblasted surface is covered with γ-Fe2O3.