Microstructure and Microhardness of 3D-Printed Samples of Steel X45CrSi 9-3

The main aim of this study is to investigate the microstructure and microhardness of Wire Arc Additive Manufactured (WAAM) samples produced under different layer deposition strategies and corresponding interlayer temperature conditions. Experimental samples were produced using the WAAM process with X45CrSi9-3 steel. During the experiments, both the number of layers and the thermal conditions (heating and cooling) were systematically varied. This was achieved by fabricating samples consisting of five layers with three beads per layer. The layer deposition procedure was implemented in two different ways: (i) with a waiting period after each layer to allow cooling to room temperature, and (ii) without such a waiting period. Thermal cycles at selected locations within the samples were calculated using simulation modeling. By combining these thermal cycles with the continuous cooling transformation (CCT) diagram, the expected microstructures in the vicinity of these locations were determined. These predictions were supplemented by microstructural analysis and hardness measurements. Particular emphasis was placed on the influence of interlayer temperature and repeated heating and cooling cycles. The analyses enabled the identification of process parameters that facilitate control over microstructure, microhardness, and property gradients. It can be concluded that the interlayer holding time provides an effective means of controlling the microstructure of the workpiece, ranging from predominantly austenitic to predominantly martensitic. Depending on the thermal cycles, the measured microhardness varied within the range of 360–900 HV. Metallographic examination revealed a wide spectrum of non-equilibrium microstructures, including martensite with varying degrees of tempering, retained austenite, pearlite, and bainite. The application of a thermal model to the conducted experiments, combined with the CCT diagram, indicated that the expected microstructures consist predominantly of martensite with varying degrees of tempering, retained austenite, carbides, and, in some cases, up to 5% pearlite.