Papers by Author: Johanne Laigo

Abstract: It is here demonstrated that combinations of various modeling techniques can be used to predict the service behaviour of heat resistant alloys, and sometimes to design “made-to-measure” alloys for specific high temperature applications. A first example is given, where an affordable creep-resistant nickel-base superalloy for operation around 750°C has been designed without any experiment. Based on the analysis of huge databases on existing alloys, Gaussian processes were used to predict its thermomechanical properties. Thermo-Calc allowed to design its fabrication process and to assess its weldability and its thermodynamical stability at service temperature. The alloy has then been tested and the validity of the modeling approach verified a posteriori, in particular a creep rupture life of 100 000 h at 750°C under 100 MPa. A second example is given, in the case of high-carbon Fe-Ni-Cr based alloys for reformer tube applications (HP steels). Their mechanical properties are predicted through the analysis of existing data with artificial neural networks. Parallelly, their thermodynamical stability in operating conditions is assessed using Thermo-Calc in combination with Dictra, to simulate the precipitation of carbides in the austenite matrix during service. It is therefore tried to understand microstructural evolution in service, damage mechanisms, and durability.

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.