WEB Microstructural characterization of high-alloy quenching and partitioning TRIP steel manufactured by electron beam melting
The powder-bed additive manufacturing technology Electron Beam Melting (EBM) is suitable to produce metallic components with complex geometries. However, especially alloys with a cubic crystal structure synthesized by EBM tend to form columnar grains of preferred orientation. Moreover, processing induced defects detrimentally influence static and cyclic mechanical properties. Austenitic transformation-induced plasticity TRIP steels show a high damage tolerance due to their phase transformation and, thus, excellent mechanical properties less sensitive to process-induced inhomogeneities. The disadvantage of these steels, however, is their low yield strength of about 180-300 MPa. Alloyed with a significant amount of carbon and nitrogen, a heat treatment consisting of quenching and a partitioning (Q&P) following the EBM process stabilizes the austenite and increases the yield strength. In the current study the EBM processability of a high-alloy TRIP steel, developed for Q&P treatment, is investigated. For this purpose, cuboids were produced from which tensile test specimens were extracted. Subsequently, the tensile specimens were solution annealed at 1150°C and then cooled to -196°C followed by tempering at 450°C. In Q&P state, tensile tests were performed and compared to the as-built as well as the annealed state. The chemical composition was determined to investigate the effects of EBM processing on interstitial alloying elements such as carbon and nitrogen which are relevant for the partitioning. Additionally, these investigations were supplemented by microstructural characterization using electron backscatter diffraction and transmission electron microscopy. Compared to the as-built state, the tensile specimens which have undergone Q&P treatment show a significantly increased yield strength of about 1100 MPa with no significant reduction in ductility. As a result of the work it was shown that despite large defect structures resulting from the manufacturing process, material with extraordinarily high strength and good ductility were achieved.