Defect and Diffusion Forum Vols. 283-286

Abstract: It is known that additions of reactive elements such as Ce, La or Y improve the properties of protective oxide-scales on Ni and Fe based alloys [ - ] by increasing oxide adhesion, decreasing the transient time until a continuous Cr2O3 layer is formed and decreasing the parabolic rate constant. Nevertheless, the precise roles played by these reactive elements to improve scales and the precise mechanisms by which they are incorporated into the scale during the surface treatment processes are unknown. Although they are believed to be associated with transport properties in the scale, it is not clear how this occurs or why it improves oxidation resistance. This project is aimed to gain understanding of the scale evolution in Fe-22 wt.% Cr alloys at 800 oC in dry air during the transient stage after 15 minutes of oxidation. The effect of La (120 and 290 ppm) and Ce (270 and 610 ppm) additions added during melt-stage processing are investigated. The surface oxidation process was imaged in-situ through a Confocal Scanning Laser Microscope (CSLM) and the results were correlated with post-experiment characterization through FEG-SEM and FIB-SEM combined with 3D reconstruction. The roles of rare-earth oxide particles on nucleation of Cr2O3 and blockage of short-circuit diffusion paths in the oxide scale and underlying metal are discussed.

Abstract: During the in-line coating process of Mn-strengthened-Interstitial-Free-steels, selective oxidation under low dew-point atmospheres may result in surface oxides that are not wetted by the coating alloy. In the current study, we investigate the formation of external and internal oxides in the metal in order to characterize the influence of fast path diffusion. The short-time oxidation in the metal was carried out in a gold-image furnace and the resulting external and internal oxides were characterized through scanning-electron microscopy. An IF steel sample containing 1.5wt.% Mn was used and the effects of dew point (DP) ranging from –75 to –15oC on oxidation was investigated at 800oC by varying the PH2/PH2O ratio in the oxidizing gas. Simulations were carried out by considering the combined effect of diffusion and mass transfer represented by a system of partial differential equations. The model equations were solved using finite element method in a Multiphysics modeling software COMSOL. The experimental and simulated results were found to be in agreement, and showed that, for DP up to –30oC, gas phase mass transport of oxidant gas controls the oxidation which results in exclusively external oxide nodules distributed uniformly on the surface. At –15oC > DP > –30oC, the oxidation evolves as ridges along grain boundaries and the simulations indicate that this is due to solid state diffusion control and the controlling mechanism is fast path diffusion through the alloy. At this point, internal oxides also start to appear.

Abstract: The objective of this study is to determine the mechanism of the dramatic increase of impact toughness at low temperatures after post-weld heat treatment on weld joints. In this study, weld joints using two semi-automatic welding consumables were fabricated by flux cored arc welding with subsequent PWHT at 660°C for 65 min and 195 min, respectively. Tests of the tensile and yield strength, microhardness and impact toughness, were carried out. The microstructure was inspected by optical, scanning electron, and transmission electron microscopy in addition to compositional analysis using energy dispersive spectrometry. PWHT was observed to result in grain coarsening, sub-grain structure formation and decrease of the dislocation density. The increase of impact toughness is attributed to the relieved thermal stress, the inclusions and precipitations, softening of the structure, dislocation recovery and sub-grain structure.

Abstract: In this study, the microstructural behavior of two kinds of 9-12% Cr steels (specimen A and B) used for power plants with and without W, Co, B which were subjected to aging during various time and temperature were investigated by electron microscope and related analytical techniques. The results indicate that there are four kinds of precipitates in these materials, i.e., M23C6, Nb-rich and V-rich MX, W-rich and Mo-rich Laves phase, and Z-phase. Upon aging, the area fraction of M23C6 increased whereas that of Laves phases decreased despite an increase in size. The area fraction of W-rich Laves phase was much higher than that of Mo-rich Laves phase, indicating that W addition compared to that of Mo addition is more powerful in the formation of Laves phase precipitation (specimen A). Specimen A aged at 700°C exhibited the formation of Z-phase. The formation of Z-phase is known to have a tendency of dissolving MX precipitates in similar alloys. The high activity of Cr at 700°C is believed to be the cause of Z-phase formation.

Abstract: In Zr-Cu-Ni-Al bulk metallic glasses where there are no dislocations, localized plastic deformation in shear bands occurs largely by the formation and migration of defects such as voids, micropores, shear bands and local variations in composition. Thus, the investigation on defects is critical for the understanding and improvement of plastic deformation in metallic glasses. In this study, microstructures and nano defects in the Zr-Cu-Ni-Al BMGs are characterized by variety of techniques, such as X-ray diffractometry, high resolution transmission electron microscopy, scanning transmission electron microscopy and electron holography.

Abstract: It was reported that superplastic boronizing process (SPB) provides a much faster boronizing rate than the conventional boronizing process (CB). This process was conducted on duplex stainless steel (DSS) which exhibit superplasticity. The study concentrated on the effect of strain rate and compression strain on SPB. The process was conducted under four different strain rates and three diferent strains condition. Boronizing was successfully conducted with the best result obtained under the high strain rate range of 5 x 10-5 s-1 to 1 x 10-3 s-1 which is associated with the superplastic region. Through SPB, movement of atoms into the specimen was highly accelerated by the grain boundary sliding process leading to a formation of thick and hard boronized layer in extraordinarily short period of time.

Abstract: Before hot-dip galvanizing, the steel sheets are annealed in an atmosphere of N2 and H2, containing only traces of water (about -30°C dew point). The main purposes of this heat treatment are to recrystallize the steel substrate after cold rolling and to reduce the iron oxides in order to improve the wettability by liquid zinc. At the same time, the less-noble alloying elements (Mn, Si, P, Cr, Al, B) of the steel preferentially oxidize and segregate to the surface. The steel surface is then partly or wholly covered by oxide particles, sometimes resulting in problems of wettability by liquid zinc. The present study focuses on the characterization of the oxide particles formed on the surface of a low alloyed ferritic steel. The steel samples are annealed by means of a laboratory furnace with a temperature profile relevant to galvanizing line practice. The parameters characteristic of the oxide particles (size, surface coverage, chemical composition, depth of oxidation) are then determined using several analysis techniques.

Abstract: The residual element copper in recycled steels embrittles grain boundaries, causing a surface cracking phenomenon known as hot shortness. Embrittlement is caused by a copper-rich liquid phase that forms at the oxide/metal interface during steel oxidation. Another residual element, nickel, enriches along with copper and reduces hot shortness cracking. The mechanisms by which nickel affects copper enrichment behavior have not yet been adequately studied. This work examines the effects of nickel and copper on the oxidation behavior and oxide/metal interface microstructure of iron. Iron-0.3 wt% copper alloys containing 0.1 wt% nickel and 0.05 wt% nickel were compared. Pure iron was used as a reference material. Alloy samples were oxidized in air at 1150°C. The parabolic oxidation rates for both alloys were found to decrease by a factor of two from that of pure iron. Both alloys had perturbed oxide/metal interfaces consisting of alternating solid/liquid regions. The interface development is due to stabilization of perturbations in the solid/liquid interface. The interface morphology can also explain the observed decrease in oxidation rate.

Abstract: In this paper, we propose a hybrid method coupling a Lattice Boltzmann Method (LBM) and a Finite Volume Method (FVM), to study melting and solidification problems. The LBM is used to determine the dynamics field while the FVM is applied to discretize the energy equation. This model is validated by comparison to available literature results concerning a square cavity heated without phase change then for the melting of Gallium in an enclosure commonly used as benchmark test case.

Abstract: The present work investigates the influence of the synthesis conditions of specific ferrites (Ni0,30Cu0.07Zn0,63)Fe2O4, prepared with the mixed oxide process, on their microstructural homogeneity. Segregation was found inside the grains, in the form of clustering or compositional modulation. Both types of segregation, which are not observed in all grains or all samples, seem to be related to the stress of the material as well as the specific orientation of the individual grains, where the segregation appeared. The reason for this assumption, which will be discussed, is that after annealing, where stress relaxation takes place, the segregation completely disappeared.