RESUMEN
Among the materials that might be manufactured with laser powder bed fusion (LPBF), one can highlight maraging steels, with excellent weldability, strength and fracture toughness. However, the effects of the processing parameters and the mechanisms governing the as-built texture are not clear yet. A recent publication showed a low texture index in the prior austenite, in contrast to other alloys subjected to LPBF with the same strategy. Authors suggested several hypotheses, although no conclusions were drawn. This work aims to investigate these findings by using a 300 maraging steel processed under different conditions, i.e. different printer, powder layer thickness and laser emission mode. To do so, X-Ray Diffraction, Electron Backscattered Diffraction and Scanning Electron Microscopy have been used. Results show that the heat treatment intrinsic to the LPBF process does not affect the prior austenite grains, whose texture and morphology remain unchanged throughout the process. Also, for the studied ranges, the microstructure texture is not related to the powder layer thickness or to the laser emission mode, although it could be affected by the laser power or the scan strategy. Finally, a low degree of variant selection has been observed, where the selected variants are those that contribute to a martensite cubic rotated texture.
RESUMEN
Nano-scale investigations of bainitic structures formed at temperatures below 350 °C have shown that the bainitic ferrite lattice is super-saturated in carbon. A high density of intrinsic defects would be playing a part in the carbon-supersaturation levels detected. In this work, the role of C-vacancy complexes on carbon-supersaturation in low temperature bainite is investigated by means of Positron Annihilation Spectroscopy. Results reveal the presence of a significant amount of monovacancies in the structures that plays an important role on the formation of carbon clusters in the ferrite lattice of nano-scale bainitic structures.
RESUMEN
The ductile-to-brittle transition (DBT) behavior of two similar Fe-Cr-Al oxide dispersion strengthened (ODS) stainless steels was analyzed following the Cottrell-Petch model. Both alloys were manufactured by mechanical alloying (MA) but by different forming routes. One was manufactured as hot rolled tube, and the other in the form of hot extruded bar. The two hot forming routes considered do not significantly influence the microstructure, but cause differences in the texture and the distribution of oxide particles. These have little influence on tensile properties; however, the DBT temperature and the upper shelf energy (USE) are significantly affected because of delamination orientation with regard to the notch plane. Whereas in hot rolled material the delaminations are parallel to the rolling surface, in the hot extruded material, they are randomly oriented because the material is transversally isotropic.
RESUMEN
The recently developed nanobainitic steels show high strength as well as high ductility. Although this combination seems to be promising for fatigue design, fatigue properties of nanostructured bainitic steels are often surprisingly low. To improve the fatigue behavior, an understanding of the correlation between the nanobainitic microstructure and the fatigue limit is fundamental. Therefore, our hypothesis to predict the fatigue limit was that the main function of the microstructure is not necessarily totally avoiding the initiation of a fatigue crack, but the microstructure has to increase the ability to decelerate or to stop a growing fatigue crack. Thus, the key to understanding the fatigue behavior of nanostructured bainite is to understand the role of the microstructural features that could act as barriers for growing fatigue cracks. To prove this hypothesis, we carried out fatigue tests, crack growth experiments, and correlated these results to the size of microstructural features gained from microstructural analysis by light optical microscope and EBSD-measurements. Finally, we were able to identify microstructural features that influence the fatigue crack growth and the fatigue limit of nanostructured bainitic steels.
RESUMEN
The goal of this paper is to analyse the effect of adding Al on the non-steady pearlite growth occurring in a Fe-C-Mn system. The results are discussed in terms of the partitioning of elements across the austenite/ferrite and austenite/cementite interfaces, and the modification of the pearlite driving force related to the change in carbon activity in austenite.
