The present investigations focused on the thermal oxidation of two variants of MAR-M246 alloy having the same contents of Ta and Nb in at. pct, considering the effects of total replacement of Ta by Nb. The alloys were produced by investment casting using high purity elements in induction furnace under vacuum atmosphere. The alloys were oxidized pseudo-isothermally at 800°C, 900°C and 1000°C up to 1000 hours under lab air. Protective oxidation products growing on the surface of the oxidized samples were mainly Al 2 O 3 , Cr 2 O 3 . Other less protective oxide such as spinels (NiCr 2 O 4 and CoCr 2 O 4 ) and TiO 2 were also detected as oxidation products. The conventional alloy exhibited slight internal oxidation at 800°C and an enhanced resistance at 900°C and 1000°C. The Nb-modified alloy presented an exacerbated internal oxidation and nitridation at 900°C and 1000°C and an enhanced resistance at 800°C. At 1000°C, Nb-modified alloy was particularly affected by excessive spalling as the main damage mechanisms. From a kinetic point of view, both alloys exhibit the same behavior at 800°C and 900°C, with k p values typical of alumina forming alloys (2 9 10 À14 to 3.6 9 10 À13 g 2 cm À4 s À1 ). However, Ta modified alloys exhibited superior oxidation resistance at 1000°C when compared to the Nb modified alloy due to better adherence of the protective oxide scale.
The focus of this work was to determine microstructural features in commercial Ni-Cr alloys which could be used to identify indirectly the presence of beryllium. Thus, eight commercial alloys were characterized by chemical analysis, thermal analysis, X-ray diffraction (XRD), scanning electron microscopy -back-scattered electron images (SEM/BSE), energy-dispersive spectroscopy (EDS). The results indicate that the presence of beryllium can be inferred from microstructural analysis via XRD and SEM/BSE. The X-ray diffractograms of the beryllium-containing alloys showed clearly the existence of the NiBe intermetallic phase. SEM/BSE images of these alloys show a very characteristic eutectic microstructure which also indicates the presence of this element. These characteristics are not observed in the beryllium-free alloys.
Solution and aging heat-treatments play a key role for the application of the superalloys. The aim of this work is to evaluate the microstructure of the MAR-M246 nickel-based superalloy solutioned at 1200 and 1250°C for 330 min and aged at 780, 880 and 980°C for 5, 20 and 80 h. The c¢ solvus, solidus and liquidus temperatures were calculated with the aid of the JMatPro software (Ni database). The as-cast and heattreated samples were characterized by SEM/EDS and SEM-FEG. The c¢ size precipitated in the aged samples was measured and compared with JMatPro simulations. The results have shown that the sample solutioned at 1250°C for 330 min showed a very homogeneous c matrix with carbides and cubic c¢ precipitates uniformly distributed. The mean c¢ size of aged samples at 780 and 880°C for 5, 20 and 80 h did not present significant differences when compared to the solutioned sample. However, a significant increasing in the c¢ particles was observed at 980°C, evidenced by the large mean size of these particles after 80 h of aging heat-treatment.
Some new pre-salt fields at Santos Basin in Brazil are located in water depths as deep as 2200m and about 300 km away from the coast. There is variable level of contaminants in the produced fluid, mainly CO2 that affects the material selection of the infield flowlines and risers. Based on these constraints, Petrobras has selected UNS N06625 clad or lined steel linepipes to develop the first fields in Pre-Salt area and also the module 3 of Roncador a post-salt field in Campos Basin. Several challenges have arisen during design, construction and installation of these facilities related to pipeline welding procedures, NDT inspection and Engineering Critical Assessment (ECA). Firstly weld overmatching condition may not be fully achieved due to differences in mechanical properties between UNS 06625 and API X65, and concern increases when reel-lay installation method is chosen. Another welding issue is the maximum interpass temperature of nickel alloys (DNV and PETROBRAS standards limit that to 100°C) and this impacts pipeline installation productivity. Thirdly, back purging characteristics and number of passes protected with purging gases affects the possibility of root oxidation. Also, the inspection of the weld overlay in the pipe end of lined linepipes is other point of great concern. As defect sizing is mandatory for ECA, lined pipes have been designed with a weld overlay length which allows the inspection of the final girth weld by AUT. However, AUT solutions are normally more efficient in rolled or extruded materials than in weld overlaid ones. Additionally, the ECA methodology for both girth welds and weld overlay has complexities that are not usually addressed in a regular ECA for carbon steel pipelines (e.g. internal misalignment of girth welds in risers has stringent requirements because of its effects on fatigue performance and, consequently, the ECA girth weld criteria). Also CRA clad/lined pipelines and risers qualification program may include additional testing when compared with usual carbon steel welding qualification process (namely pitting and intergranular corrosion, full scale fatigue, spooling trials of lined linepipes, segment testing for ECA, and others). Finally, the contribution of clad/lined layer in pipeline design strength is also discussed. This work presents challenges PETROBRAS has faced at design and construction phases of on-going Guaré and Lula-NE pre-salt fields and Roncador field projects, as well as the solutions proposed by the project team in order to overcome the issues raised during project execution.
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