Papers by Author: Jader Furtado

Abstract: Fretting fatigue is one of the major factors in the design of hydrogen equipment. The effect of internal hydrogen on the fretting fatigue strength of austenitic stainless steels was studied. The internal hydrogen reduced the fretting fatigue strength. The reduction in the fretting fatigue strength became more significant with an increase in the hydrogen content. The reason for this reduction is that the internal hydrogen assisted the crack initiation. When the fretting fatigue test of the hydrogen-charged material was carried out in hydrogen gas, the fretting fatigue strength was the lowest. Internal hydrogen and gaseous hydrogen synergistically induced the reduction in the fretting fatigue strength of the austenitic stainless steels. In the gaseous hydrogen, fretting creates adhesion between contacting surfaces where severe plastic deformation occurs. The internal hydrogen is activated at the adhered part by the plastic deformation which results in further reduction of the crack initiation limit.

Abstract: The present work focuses on the experimental multi-scale characterization of fracture of an AISI 4135 steel by using the Disk Pressure Test (DPT). In order to precise the specific features of hydrogen embrittlement, comparison was made between disks burst under helium and hydrogen gas. SEM - EBSD analysis of disks samples before and after the test allowed to analyze and to compare the main microstructural mechanisms of the failure process. The location of the main crack initiation was consistent with Finite Element (FE) simulations of the DPT.

Abstract: Heat-resistant steels of HP series (Fe-25Cr-35Ni) are used as reformer tubes in petrochemical industries. Their composition includes Nb and Ti as strong carbide formers. In the ascast condition, alloys exhibit an austenite matrix with intergranular MC, M23C6 and/or M7C3 eutectic carbides. During exposure at high temperature, phase transformations occur: chromium carbides of M7C3 type transform into the more stable M23C6 type, intragranular M23C6 carbides precipitate, and a silicide, the G-phase (Ni16Nb6Si7), forms due to the instability of MC carbides (NbC). Thermodynamic simulation is of great help for understanding precipitate formation and transformations. Thermo-Calc and Dictra are used to simulate the precipitation of carbides in the austenite matrix during service. However, from an experimental point of view, M23C6 and M7C3 are not easy to distinguish in bulk alloys. Indeed, backscattered scanning electron microscopy does not bring any contrast between the two phases, and energy dispersive spectroscopy (EDS) analysis does not lead to carbon content and consequently to the distinction between M23C6 and M7C3. With transmission electron microscopy (TEM), sample preparation is difficult and the observed area is extremely small. In the present work, HP alloys are investigated by electron backscatter diffraction (EBSD) coupled to EDS. Carbides are identified on the basis of crystal structure, in the bulk, within their microstructural context, and the experimental procedure is both simpler and cheaper than TEM. Precipitates (M23C6, M7C3) could be identified by orientation mapping and single spot analysis.