Fernando C. Rizzo Assunção

A comparison between the Warren-Averbach method and alternate methods for X-ray diffraction microstructure analysis of polycrystalline specimens

The fundamental parameters approach is used to simulate the instrument contribution to the X-Ray diffraction profile. This procedure eliminates the need to experimentally prepare a reference sample of the studied crystalline material when using the Warren-Averbach method to investigate microstrutural parameters. The Warren-Averbach method is also compared to the Balzar and Enzo methods, two other popular methods of size-strain analysis. The analysis was carried out using bohmite powder having two different nominal average crystallite sizes, 80 A and 200 A. A 50%-50% mixture of these materials was used as a third sample. The proposed simulation procedure provides good results and is much faster to implement than the usual procedure that includes the preparation of a reference. For larger crystallite sizes, the results calculated from the Warren-Averbach method for the volume-weighted average crystallite size differs significantly from the ones obtained from the Balzar and Enzo methods. The limitations of the Balzar and Enzo methods are also discussed.

Thermodynamic Evaluation of Carbide Precipitates in 2.25Cr ¾ 1.0Mo Steel for Determination of Service Degradation

The 2.25Cr¾1.0Mo steel is widely used in petroleum refining units, for example, in pressure vessels, working under severe operational conditions. Due to the high temperatures of some processes, many phenomena such as precipitation of carbides and their coarsening occur during service, resulting in changes of material microstructure. Such changes may modify the mechanical properties, which are responsible for the performance of the pressure vessels. Using a software for thermodynamic calculations, the thermodynamically stable carbides over the temperature range of interest (700-1040 K) were evaluated, supposing ferrite as the original microstructure. The theoretical carbide nucleation sequence was also established. Moreover, the theoretical amount of thermodynamically stable carbides and the fraction of Fe, Cr and Mo in the metallic sublattice of M23C6 were determined as a function of temperature. These theoretical calculations were compared to some experimental results on carbide precipitation in the 2.25Cr¾1.0Mo steel and two approaches for evaluating the service degradation were proposed. The first one is based on the amount of precipitated M23C6 and the other on the Fe/Cr ratio in the M23C6 carbide.

Processing of bulk Bi-2223 high-temperature superconductor

The Bi2Sr2Ca2Cu3 O10+x (Bi-2223) is one of the main high temperature superconductors for applications. One of these applications is the Superconductor Fault Current Limiter (SCFCL), which is a very promising high temperature superconducting device. SCFCLs can be improved by using bulk superconductors with high critical currents, which requires a sufficiently dense and textured material. In the present work, a process for improving the microstructure of Bi-2223 bulk samples is investigated. Pressed precursor blocks are processed by sintering with a further partial melting step, in order to enhance the Bi-2223 grain texture and to healing cracks induced by pressing. In order to improve the microstructure, the precursor is mixed with silver powder before pressing. Samples with and without silver powder have been studied, with the aim of investigating the influence of silver on the microstructure evolution. The phase contents and the microstructure obtained have been analyzed through XRD and SEM/EDS. The electromagnetic characterization has been performed by Magnetic Susceptibility Analysis. We present and discuss the process and the properties of the superconducting blocks. High fractions of textured Bi-2223 grains have been obtained.

Development of Bulk Bi2+xSr3-yCa yCu 2O8+delta Superconductors by Partial-Melting Route for Fault Current Limiters Application

The production of bulk Bi2+xSr3-yCayCu 2O8+D (Bi-2212) superconductors for fault current limiter application was developed via a partial-melting route. Aiming high Ic (critical current), which is the essential superconducting characteristic for application of this material in the construction of Fault Current Limiters (FCL), the produced blocks have predominance of Bi-2212 phase (83 wt%), which characterizes with high values of zero and onset transport critical temperature of 92K and 97.5K, respectively. A relatively low transition width, DT, from the superconducting to the normal state of 5.5K, revealed a good intergrain connectivity. Consequently, current measurements on the blocks of Bi-2212 show promising Ic values of 230A and 850A for direct and alternate current, respectively. It is expected that further increases in the Ic values will depend on the elimination of an observed amorphous phase and further reduction of amount and grain sizes of secondary phases, still present in the blocks obtained by the proposed partial-melting route. This may be achieved by a further optimization of the partial-melting processing parameters.

In Situ Observation of Phase Transformations in The Fe-Zn System

In this study, the MAXIM technique was used in an attempt to clarify the phase transformation sequence that occurs during in situ annealing of galvanized samples. A diffractometer equipped with a novel imaging system comprising a Micro-Channel Plate in front of a CCD camera was used. The galvanized samples were produced under typical industrial conditions, with effective aluminum content at 0.147wt.%. In situ experiments were performed and the phase evolution was recorded in real time. It can be concluded that, coupled to in situ thermal treatment, MAXIM is an efficient method to observe the evolution of the phases present in galvannealed samples. This technique has enough sensitivity to detect the evolution of the involved phases with good spatial resolution.

The effect of alloying elements on constrained carbon equilibrium due to a quench and partition process

Abstract The model proposed by Speer et al. to calculate the constrained carbon equilibrium is expanded to include any number of substitutional solutes and coded with the MatLab and Thermo-Calc programs. This model is used to evaluate the effect of Al, Cr, Cu, Mn, Mo, Ni and Si solutes on the final austenite carbon concentration of ternary Fe – X – C alloys. Comparison is also made with the carbon concentration measured experimentally in some Transformation Induced Plasticity steels.

Influence of g -phase on the high- temperature oxidation of NiAl-Fe alloys

The oxidation of NiAl, NiAl-20at.%Fe and NiAl-30at.%Fe at 1000-1100 °C in air has been studied. Pure NiAl shows excellent oxidation resistance due to the formation of an Al2O3 layer. NiAl-20Fe also shows good oxidation resistance due to the formation of an Al2O3 scale on a b-phase substrate. Moreover, some nodules consisting of mixed oxides of Fe and Ni grow over the ductile g-phase surface incorporated to the b-phase substrate. NiAl-30Fe alloy undergoes a much faster oxidation due to the formation of a non-protective Fe and Ni-rich scale, which is extremely susceptible to spallation. The addition of Fe to NiAl is detrimental to its oxidation resistance.

Wet Welding Field Trials in Shallow Waters for Structural Repairs in Floating Oil Production Units

Saving costs is a major attraction of wet welding application in structural repairs of offshore installations. Nevertheless, improve the quality of wet welding to get it as close as possible to plain structural steel quality and also qualify welding procedures in AWS D3.6 class ‘A’ have been challenges not consistently overcome. This paper describes wet welding trials in shallow waters (5m and 10m) with a rutile and an oxy-rutile type, both commercial electrodes. Two different base metal compositions were employed in the preparation of butt and fillet joints. The main objective is to amend new results of wet weds properties to those already published aiming the application of this welding technique under more reliable conditions and the qualification of welding procedures. The weldments were tested by Vickers hardness, Charpy V notch, tensile, shear strength, bending and fillet weld break tests, chemical and macrographic analysis. Some of these properties and diffusible hydrogen, obtained in laboratory with a mechanized gravity system, will also be presented in order to complement or explain the field testes results obtained. Both electrodes produced class AWS E 70XX weld metals and overall results according class “B” requirements of the AWS D3.6M:2010 code. Some good elongation results obtained encourage future trials to achieve class “A”. Barriers to the class “A” qualification of welding procedures in shallow waters are also discussed.Copyright