José de Anchieta Rodrigues

Growth of nitrided layers on Fe–Cr alloys

Abstract Chromium is an important alloying element present in numerous commercial steels. A systematic study on the nitriding behavior of Fe–Cr alloys is helpful in predicting the properties of nitrided Cr-alloyed steels. Aspects such as microstructural evolution, growth kinetics, and mechanical properties should be particularly emphasized. Fe–Cr alloys containing 5, 10, and 20 wt.% Cr have been arc melted and subsequently plasma nitrided under a N 2 –80% H 2 atmosphere. The microstructure of the resulting nitrided layers was characterized and the microhardness profiles evaluated. Thicker layers developed on low chromium alloys. Differences in hardness profiles were also observed as a function of chromium contents. Nitriding Fe–5% Cr alloys resulted in two discrete fronts, refereed to as the diffusion front and the transformation front. Transformed regions sustained a decrease in hardness from 1000 down to 600 HV, associated with the conversion of homogeneously dispersed fine precipitates into coarser needle like particles immersed in the ferritic matrix. Similar behavior was not observed for the other alloys, where both fronts developed simultaneously.

Correlation between fracture toughness, work of fracture and fractal dimensions of Alumina-mullite-zirconia composites

The purpose of this work is to show the correlation between the fractal dimension, D, and mechanical properties such as work of fracture, gwof, and fracture toughness, KIc. Alumina-mullite-zirconia composites were characterized by the slit-island method, SIM, to obtain values of D and its fractional part, D*. The fracture surface roughness was also evaluated using a cyclic voltametric method. It will be shown that there is a positive experimental dependency of gwof on D* and that there is not an evident correlation between KIc and D*.

Insights on the fractal-fracture behaviour relationship

The fractals theory has been increasingly applied in the field of materials science and engineering. Models of fractal lines and surfaces have been generated to describe the microstructural features of materials. Special interest is placed upon a description of the fracture surface based on a fractal geometry in order to understand the crack path in materials. Several papers have demonstrated the relationship between the fractal dimension of a fracture surface and the values of roughness and fracture toughness. In this work an extension of the theory of fractals for ceramic materials is proposed, to which the crack deflection toughening mechanism is thought to be related. In order to accomplish this objective, a review describing the concept of fractals and its relationship with the fracture toughness is presented. In the following part, a correlation between fractal dimension, total energy of fracture and the average resistance to crack propagation is proposed; all these parameters being dependent on the history and on the complexity of crack propagation path.

Alumina-mullite-zirconia composites obtained by reaction sintering Part II. R-Curve behavior

Applying Linear Elastic Fracture Mechanics equations and sample compliance variation to quantify the instantaneous crack length, the R-curve behavior of alumina-mullite-zirconia composites obtained by reaction sintering, was evaluated as a function of zirconia and mullite content. Changes in the R-curve profile as a function of the notch geometry (Chevron and straight-through notch) was observed and discussed, based on the analysis of the y(α) function applied to each notch type. The influence of the y(α) function in the R-curve shape was observed in both the initial and the final crack propagation region where, in the latter, the R-curves presented a sharp increase. In order to suppress these effects, the R-curve values for pure alumina were deducted from those obtained for the different composites produced. The analysis of the resulting curves highlights the influence of the amount of zirconia and mullite inclusions in these composites.

A study about the contribution of the α-β phase transition of quartz to thermal cycle damage of a refractory used in fluidized catalytic cracking units

The deterioration of refractories used in fluidized catalytic cracking units (FCC-units) is responsible for high costs of maintenance for the petrochemical industry. This is commonly associated with coke deposition during the production of light hydrocarbons. However, other mechanisms responsible for causing damage may also occur, such as the generation of cracks by expansive phase transition. The aim of the work herein was to study the contribution of the a-b phase transition of quartz particles to the deterioration of a commercial aluminosilicate refractory used in a riser by the means of slow thermal cycles. Such damage may occur if the working temperature of the equipment fluctuates around the a-b transition temperature (573 °C). The current study considered the material with and without coke impregnation to evaluate the combined effect of coke presence and phase transition. To evaluate the damage, it was used the Youngs modulus as a function of temperature by applying the Impulse Excitation Technique under controlled atmosphere. An equipment recently developed by the authors research group was applied. Specimens were prepared and submitted to slow thermal cycles of temperatures up to 500 °C and up to 700 °C, with a heating rate of 2 °C/min. Part of the specimens was previously impregnated with coke by a reactor using propen. To complete the evaluation, characterization by X-ray diffraction, as well as by dilatometry and scanning electron microscopy were performed. The findings of this study showed that the presence of quartz particles determine the thermo-mechanical behaviour of the material, as well as the thermocycling damage resistance. In spite of the fact that the a-b phase transition stiffens the material during the heating stage, it increases the damage by slow thermal cycling. The coke impregnation increases the resistance to slow thermal cycles, however it decreases the resistance to the damage evolution.

Preparation of refractory calcium aluminate cement using the sonochemical process

Calcium aluminate cements (CAC) were prepared using the sonochemical process, followed by heat treatment. A study was made of the action of ultrasonic waves and the influence of thermal treatment conditions on two initial molar compositions of 1:1 and 1:2 of calcia:alumina. The aqueous suspension containing the raw materials (A-50 alumina and CaO) was subjected to an ultrasonic bath, followed by drying and burning at 1000, 1200 and 1300 oC. These cements were characterized by SEM, XRD and the mechanical strength was evaluated by splitting tensile tests, using commercial cement as a reference. Furthermore, the phases were semi-quantified using the Rietveld method. The results show that hydration and sonochemical action increased the reactivity of the raw materials during firing and that phase formation is dependent on the thermal treatment conditions. The CAC cements were obtained at temperatures at least 200 oC lower than those used in conventional methods, indicating the potential of this route of synthesis.

Efeito do tempo de exposição a uma atmosfera coqueificante na microestrutura e nas propriedades de um concreto refratário usado na indústria petroquímica

Refractory castables used in Fluidized Catalytic Cracking Units (FCCU) are said to deteriorate due coke formation during the production of light hydrocarbons, causing a shortening in the operating time of the reactor. Consequently, a significant financial loss for the petrochemical will occur. Several studies have been carried out, but none of them showed clearly how much is the contribution of the coke for the concrete final deterioration. It still remains the doubt if the coke is the responsible for the damage observed macroscopically in a FCCUs riser. In this way, this work aimed to study the effect of the time in a cokemaking atmosphere on an anti-erosive class-C refractory castable, seeking for microstructural changes or on physical properties that indicate degradation mechanisms and give support to the understanding of the phenomenon. Samples of an industrial refractory castable used in petrochemical units were prepared and subjected to a forced cokemaking process in a simulation reactor. The temperature and the heating rate were kept constant at 540 oC and 50 oC/h, respectively. The values of 10, 60, 120, 240 and 480 h were used for the time of exposition to the propene gas. The microstructure of the samples was characterized through optical and scanning electron microscopy and its mineralogical phases through X-ray diffraction. Complementary analyses were necessary to a better understanding of the phenomenon. The results show that the surface and the microstructure are gradually impregnated by coke, which fills up pores, microcracks and cracks. Evidences of microcracking around the coke filled pores were not found. However, many aggregates present some type of deterioration related to the time of exposition to propene. Those damages are not necessarily caused by coke directly.

Synthesis and Mechanical Characterization of Iron Oxide Rich Sulfobelite Cements Prepared Using Bauxite Residue

The bauxite residue (BR) is the solid waste of largest generation by the aluminum industry, with a generation estimated in 10 million ton/year in Brazil. Its high alkalinity demands elevated costs to store it safely. Due to the high content of Al2O3 and Fe2O3 in BR, the present study evaluated its application for the synthesis of iron rich sulfobelite clinkers. Because of its composition, the cement made with this clinker presents environmental (30% to 62% reduction of CO2-equivalent emission), economic and even technical advantages over Portland cements. Formulations containing: limestone, gypsum, BR and clay, were fired at 1230°C to synthesize the clinkers. Variations in BR and clay content were studied to obtain three formulations, F-15, F-18 and F-21, of different Al2O3/Fe2O3 ratio values. Cements containing more than 10.5 wt-% of BR achieved mechanical resistance comparable to Portland cements (CP-II-Z32 and CP-V-ARI) for 7 and 28 days of curing age.

Effect of Specimen Size on the Resistance to Thermal Shock of Refractory Castables Containing Eutectic Aggregates

Thermal shock is one of the most severe conditions to which a refractory lining can be subjected to during its industrial application. Thus, there are several methods available for testing thermal shock damage resistance of refractories. Among them, a very common method is the quench test, which consists of quenching a hot refractory bar into water. After that a retained mechanical property is determined. Considering this, the aim of the work herein is to compare the thermal shock damage resistance between two specimen sizes among several materials. The dimensions 150 mm × 25 mm × 25 mm and 160 mm × 40 mm × 40 mm were used. The small bars are generally used for mechanical characterization in the refractory field (recommended by ASTM C1171-05). The large bars, on the other hand, are a requirement of DIN 196-1, which regards procedures for testing cement materials. Experimental results have indicated that the thermal shock damage is bigger for the large bars, as predicted by theoretical aspects. Although the size difference between the specimens was not so big, it was possible to observe the size effect using a statistical treatment. Five different castable formulations, of which three contained eutectic aggregates, were tested. The highest variation found in thermal shock damage resistance because of the size was about 15%.

Potential Use of Colloidal Silica in Cement Based Composites: Evaluation of the Mechanical Properties

The properties of cement based composites depend not only on the properties of their individual components but also on their interfacial characteristics and transition zone between fiber and matrix. There has been a renewed interest in the use of cellulosic pulp as micro-fiber reinforcement in cement based composites. The addition of nanoparticles, such as colloidal silica, to fiber-cement could allow a better control of its microstructure and the enhancement of the matrix/fiber interface. The objective of this work is to evaluate the effects of colloidal silica on the microstructure and mechanical performance of cementitious matrices and fiber-cement composites. These cementitious materials were prepared with 0%, 1.5%, 3%, 5% and 10% w/w colloidal silica suspension content. Cementitious matrices without fibers were produced by vibration. Fiber-cement composites with unbleached Eucalyptus kraft pulp as a micro-fiber reinforcement were produced by the slurry dewatering technique followed by pressing. All composite materials were cured by water immersion. A splitting (Brazilian) test was carried out to determine the tensile strength of cementitious matrices. Mechanical behavior of the fiber-cement composites was evaluated via modulus of rupture and fracture toughness based on load-displacement curves (L-d curves) under continuous loading and 3-point bending arrangement. The energy of fracture was measured through a stable crack propagation test with SENB (single-edge notched bending) configuration also under a 3-point bending arrangement. The matrix with highest content of colloidal silica suspension (10% w/w) presented high values of water absorption and consequently presented the lowest splitting tensile strength. The average values of modulus of rupture and fracture toughness of fiber-cement tend to decrease with increasing colloidal silica content. However, the pullout mechanism increased significantly in the fiber-cement composites with additions between 3% and 10% w/w of colloidal silica suspension as compared to that without any addition, noted by degree of improvement in the energy of fracture and by scanning electron microscopy micrographs (SEM). These findings show the potential use of colloidal silica to improve the transition zone between the cellulosic fiber and the cementitious matrix. The results of this study show an important way to engineer and control the fracture process of the composites.