Comparative study for physical properties and corrosion mechanism of synthetic and in situ MgAl2O4 spinel formation zero cement refractory castables

Abstract

Magnesium aluminate spinel (MgAl₂O₄) is an excellent castable refractory product due to its high temperature thermal, chemical and mechanical properties. Alumina spinel castables are produced by addition of synthetic spinel or in situ spinel formation during the firing process. In the first part of the experimental studies, alumina rich MgAl₂O₄ spinel castable was produced using a solid state reaction technique. Tabular alumina and sea water magnesia (<100 μm) were used as starting raw materials. In the second part of the experimental studies, commercial synthetic spinel added castables were produced. In order to compare experimental results, both parts of the experimental study involved compositions with the same proportions of MgO. α-500 hydratable alumina was used as binder. Castables were sintered at 1500 and 1600°C. Water absorption, apparent porosity, bulk density and cold crushing strength values were considered and the optimum sintering temperature, proportions of synthetic spinel and sea water magnesia were determined. The XRD patterns confirm the phase formation of MgAl₂O₄. Moreover, the physical properties of the castables were supported by this XRD analysis. Scanning electron microscopy investigations of the fired samples were carried out to compare the effect of synthetic spinel addition and in situ phase formation on the physical properties of the castables. The mechanism of slag penetration to two types of zero cement castables for steel ladles was examined and the penetration layer chemically analysed by energy dispersive X-ray analysis studies.     

Corrosion fatigue characterisation of sintered multiphase stainless steels

Abstract

Multiphase stainless steels are produced for their attractive properties of mechanical strength and corrosion resistance relative to their austenite–ferrite structure. The manufacture of these steel by powder metallurgy technology presents some advantages in terms of low cost and formability of complex shapes. Mechanical and corrosion resistances are not at the level of the wrought steels due to their porous nature. In this work the fatigue and corrosion fatigue behaviour of some sintered steels obtained by sintering from 316L and 434L base powders has been studied for characterisation and comparison. The sintered steels were fatigue tested in two different environments: air and NaCl aqueous solution. The tests performed indicate that the chemical and microstructral composition has no great influence on fatigue behaviour in comparison with the manufacturing technology (sintering). This is most evident in the more aggressive environment, like seawater, in which these steels could be advantageously used. The analysis of fracture surfaces using SEM microscopy shows a peculiar crack propagation characterised by cleavage, stress intensification due to porosity, and features of localised ductility on sintering necks and base powder particles.     

Synthesis and wear of Al based,free standing functionally gradient materials: effects of different reinforcements

Abstract

In the present study, three aluminium based functionally gradient materials (FGMs), reinforced with different ceramic particulates (silicon carbide, aluminium oxide, and titanium carbide), were successfully synthesised using the innovative gradient slurry disintegration and deposition (GSDD) technique. The results for Al/SiC and Al/Al₂O₃ revealed, in common, an increase in the weight percentage of reinforcement along the direction of deposition, to result in an increase in porosity and microhardness. However, for Al/TiC, the reverse trend was observed, with porosity and microhardness decreasing with increasing distance from the base of the ingot. The porosity levels for Al/TiC were also found to be significantly lower than those ofthe other two FGMs. Thermomechanical analysis of the FGMs showed thatthe average coefficient of thermal expansion of the high reinforcement end was reduced, as compared to the high aluminium end. Sliding wear test results also revealed that the high reinforcement end was more wear resistant than the high aluminium end, except for the case of Al/Al₂O₃. An attempt is made to interrelate the effects of different types of particulates, with microstructural development, microhardness and wear rate results obtained in the present study.     

Microstructural evolution of Inconel* 718 during ingot breakdown: process modelling and validation

Abstract

A computer model is presented that describes microstructural evolution during the ingot breakdown of nickel base superalloy Inconel 718 via the open die cogging operation. To support the development of the model, a compression testing programme has been carried out which covers the ranges of temperatures, strains, and strain rates experienced during thermomechanical processing. Analysis of the flow curves has allowed the identification of the regimes in which the various deformation mechanisms take place. Logic based rules have been incorporated into the model, and this has allowed predictions of the microstructural evolution to be made. Where possible, the results have been compared with the available experimental data and it is shown that theory and experiment are in reasonable agreement. A number of computational experiments have been carried out, to study the effects of changing the forging procedure.     

Utilisation of silicon rubber to characterise tool surface quality during die compaction

Abstract

A new methodology was developed to observe and measure tool wear and tool surface quality during the die compaction process. The newly developed method is a non-destructive test that relies on silicon rubber to transcribe the inner surface profile of the compaction die. After verification of the method, aluminium and iron alloy powders were compacted to quantify tool wear and tool surface quality with two die materials, tungsten carbide and tool steel. The tool surface quality was quantified by recording surface roughness of the die replicas on a surface profilometer.     

Direct observation of ternary solid state transformation in Bi–Cd–In alloy system

Abstract

Cast or solution treated specimens of a Bi–9·0Cd–26·7In (wt-%) alloy were observed to form a fine, three phase microstructure on aging at room temperature, replacing a single phase formed at a higher temperature. The three phases resulting from this solid state reaction were found to grow with a lamellar morphology into the high temperature phase, with a growth rate of 0·5–1·0 μm h^-1 at room temperature. The equilibrium temperature for the transformation was found to be ～25°C. Using a Hitachi S-4500 field emission SEM, the phase transformation was followed in progress at magnifications of 3000 and 10 000 times. It was noted that a volume change was associated with the transformation. It was concluded that the transformation is of the ternary eutectoid type.     

Stellite 21 coatings on AISI 410 martensitic stainless steel by gas tungsten arc welding

Abstract

Degradation of AISI 410 martensitic stainless steel, a typical alloy for many applications such as steam turbine blade, could impair its efficiency and lifetime. To overcome this problem, critical surfaces could be modified by weld cladding via gas tungsten arc welding technique. In the present research, a comparative study of Stellite 21 weld overlays deposited in three different thicknesses, i.e. dilutions, at various preheat and post-weld heat treatment temperatures on the surface of AISI 410 martensitic stainless steel, has been made. The surface of coatings has been examined to reveal their microstructures, phase characterisation and mechanical properties using XRD, microhardness tester and metallographic techniques. The results showed that the deposition of Stellite 21 coating on AISI 410 martensitic stainless steel improved its corrosion resistance. Moreover, the volumetric dilution had a considerable effect on the hardness, microstructure and electrochemical corrosion behaviour of Stellite 21 weld overlays.     

Synergistic effects of formaldehyde and alcoholic extract of plant leaves for protection of N80 steel in 15%HCl

Abstract

The synergistic effects of formaldehyde and an alcoholic extract of plant leaves have been studied by weight loss measurements at temperatures up to 363 K using various concentration ratios of the two inhibitors to protect N80 steel against corrosion in 15% hydrochloric acid. The inhibition efficiency was found to decrease with increasing temperature for all except two of the various concentration ratios that were studied. After identifying these two most promising mixtures, their corrosion prevention effects for N80 steel in hydrochloric acid were studied in more detail by weight loss and potentiostatic polarisation measurements. The corrosion rate of the steel was decreased by the presence of small additions of the inhibitors. The extent of decrease was found to depend on the nature of the corrosion inhibitor and its concentration. The inhibition efficiencies of the two plant based inhibitor mixtures were compared with those of two commercially available oil industry corrosion inhibitors. In all cases the adsorption of the inhibitors on the steel appeared to follow the Frumkin or Langmuir adsorption isotherm. The inhibition efficiencies of the two plant based inhibitors and the two commercial inhibitors were evaluated at a concentration level of 0.8% for temperatures in the range 303 K to 363 K) and exposure times of between 1 h and 24 h. Thermodynamic parameters including the free energy of adsorption, activation energy, enthalpy and entropy were calculated in both the absence and presence of inhibitors. Potentiostatic polarisation tests have revealed that inhibitors are primarily of the anodic type.     

Comparative study of three different types of dental diamond burs

Abstract

Wear mechanisms of three different types of dental burs were studied by means of cutting experiments performed on machinable glass ceramic using a laboratory system designed for this purpose. The dental handpiece used for this research was subjected to a constant feed rate in order to better simulate the actual working conditions of a dental bur. The new and the worn-out burs were studied by optical and scanning electron microscopy. Diamond particle wear-out was found to be the dominant wear mechanism in all cases; a quantitative analysis was performed on the optical micrographs of the new and worn burs. In situ force measurements showed that the forces exerted by the bur increase with the blunting process in order to keep the required feed rates; each bur type seems to have a different characteristic curve of force versus the number of cuts.     

Solving corrosion problems in biofuels industry

Abstract

Ethanol is increasingly popular as fuel. It is renewable, cleaner than gasoline, and during the 2008 oil price peak, its costs of production without subsidies actually rivalled those of gasoline. The fuel grade ethanol producers are large consumers of stainless steel, in particular for cookers, fermentation tanks, and distillation columns as well as storage tanks for the finished product. The main reason is that cleanliness is important for the enzymes. However, contaminants play an important role in production as well as storage and distribution. Contaminants are causing a phenomenon called stress corrosion cracking. Biodiesel fuel is a mixture of fatty acid alcohol esters. Modern diesel engines are very sensitive to fuel quality and purity. Biodiesel is unstable, just like butter, which is going rancid in a matter of weeks. Biodiesel degradation is an oxidation process, which among others is speeded up by tiny amounts of metal ions leaching from production and storage vessels. Biodiesel vessels need to be designed in non-degradable materials of construction, as stainless steel. In addition, pretreatment of low cost feedstock and methyl recycling operations use strong acids, which also necessitate stainless steels. Biomass is gaining importance as a source for renewable fuels and hydrogen in addition to heat and electricity. Corrosion issues are numerous and include high temperature corrosion under ash and alkali salt deposits and metal dusting at gasification. Fuel grade ethanol, biodiesel and biomass represent three different corrosion issues, which can be solved using the correct stainless steel.