Capillary absorption kinetics of concrete–ethylene glycol system was studied with respect to concrete matrix porosity and liquid viscosity. Porosity of specimens was altered by air-entraining agents ...and superplasticizers. Liquid which doesn’t react with cement gel was chosen for the experiment in order to study the reasons for deviation from
Lucas–Washburn equation observed in concrete–water system. Viscosity of ethylene glycol changes from ∼23 to 2
mPa s in the temperature range from 20 to 100
°C. The values of the capillary coefficient were determined at 20, 60 and 100
°C using Neutron Radiography and were found to be in the range from ∼1.5 to 4.9
mm
h
−1/2. The results show that the
Lucas–Washburn equation in concrete–ethylene glycol system is valid only for ∼25
h, which indicates that swelling and rehydration of cement gel are not the main reasons for deviation observed in concrete–water system.
Improved sample wettability was obtained by oxygen plasma functionalization of pyrolytic graphite. The samples were exposed to highly dissociated oxygen plasma with the density of 1 x 10(16) m(-3), ...the electron temperature of about 5.5 eV and the density of neutral oxygen atoms of 8 x 10(21) m(-3) for 20 s. The surface wettability was measured by a contact angle of water drop. The contact angle dropped from original 112 down to about 1 deg .The functional groups were detected by XPS analyses. The survey spectrum showed a substantial increase of oxygen concentration on the surface, while high-resolution analyses showed additional oxygen was bonded onto the graphite surface in the form of C-O polar functional group responsible for the increase of the surface energy.
Delayed ettringite formation (DEF) is a chemical reaction with proven damaging effects on the mechanical properties of hydrated cementitious composite (concrete). Ettringite crystals can cause cracks ...and the widening of cracks due to pressure on the crack walls caused by the positive volume difference in the reaction. In this paper, we investigated the potential to utilise the positive volume difference in DEF in order to improve the mechanical properties of hydrated fine grained aerated concrete. Fine dispersed crystallisation nuclei, achieved by adding air-entraining agent (AEA) and by short vibration of specimens, are presented as the main requirement for such improvements. Control tests of expansion and mechanical properties were performed on samples of concrete with and without AEA by inducing DEF. The microstructure of fine grained aerated concrete was examined with an optical microscope and scanning electron microscope. We found that the controlled DEF, which is guaranteed by adding AEA and with the formation of uniformly dispersed air bubbles, which are crystallisation sites for ettringite crystals, improves the mechanical properties. The specimens with induced DEF were measured and found to have a 6.8% increase of compressive strength.
Repair welding of cold-work tool steels in cold is very risky and almost impossible by conventional processes. The application of pulse shaping in laser cladding with wire to avoid the solidification ...problems in relevant steel is demonstrated. The results show that sound remelting and/or cladding can be achieved by the right selection of laser parameters and pulse shape, i.e. long pulse duration, moderate pulse peak powers and ramped-down pulse shape. Despite the defects and softening in the cladding due to the formation of retained austenite, the cladding shows better wear resistance at lower loads compared to the heat-treated base material.
The paper gives the results of our research into the technique of manufacture of the laddered chape from the sword scabbard with openwork fitment from the River Ljubljanica. In addition to ...characterisation of the alloys by proton-induced X-ray emission spectrometry (PIXE; cf. Šmit, Istenič, Perovšek 2010) the study included observation of fragments of bridges from the laddered chape using scanning electron microscopy (SEM), semi-quantitative chemical analysis of minute areas under an electron microscope (SEM/EDS) and metallographic research. Results of the study indicate that the laddered chape was made of forged steel and that the very thin layers (lamellae) of bronze in the front bridges acted as solders, thus giving an idea of how the laddered chape was constructed.
•We examine transformation kinetics of austenite during isothermal annealing of heat-resistant stainless steel.•Long-time exposure to high temperatures 600–900°C leads to microstructural ...changes.•Sigma phase particles occurred in the form of network of plates.•After fracture occurred intercrystalline brittle mode of fracture surfaces.•The increase in hardness was associated with annealing duration and content of sigma phase.
In this work, the results of the transformation kinetics of austenite heat-resistant stainless steel (AISI 310S) with 2.27wt.% silicon were presented. The results of microstructural and fractography analysis, as well as the results of hardness after the isothermal heat treatment of the steel in the temperature range from 600 to 900°C at different annealing times (from 1 to 956h) are shown. It was found that the microstructure of steel after isothermal annealing consisted of austenite, carbide (M23C6) and σ-phase. The number of σ-phase precipitates increases with higher annealing times. In longer annealed steel samples the significant coarsening of precipitates was observed, as well as a large amount of σ-phase particles in the form of a chain and a network of plates. Precipitates were both observed at grain boundaries and within austenite grains. σ-phase precipitation involves two mechanisms: transformation γ→Cr23C6→σ and transformation γ→α′→σ. The hardness began to increase after 48h of annealing at a temperature of 600°C, while in the temperature range of 700–900°C the hardness increased with annealing times higher than 8h. The slope of hardness curve is caused by microstructural changes. The fracture surface mode was intercrystalline brittle. Relatively large dimples can be related to large particles of precipitates which occurred after long annealing times.
•A hot cracking of austenitic stainless steel were identified.•On the grain boundaries oxide and carbide films were found.•The basic reason of hot cracking is non-grinding oxidized metal surface.•For ...hot cracking prevention an oxidized layer must be removed.
In fillet welds made of 18Cr–9Ni–7Mn austenitic stainless steel centerline cracks were established along the connection line of two solidification fronts in the weld. On the grain boundaries oxide and carbide films were identified. Hot cracks resulted from a combined activity of a large gap between plates and the resulting high tensile stresses occurring during cooling, free root surface of the weld in the large gap, and intergranular oxide and carbide films in the weld. Since the parent metal was cut with highly oxidating plasma and atmospherically corroded surfaces was not ground prior to welding, remelted surface oxides passed into the weld pool. The increased Cr content identified in intergranular oxide films results from Cr oxidation in the filler material.
► An analysis of the catastrophic corrosion of the hot water pipeline system was made. ► The inbuilt pipes in a hot water system are made from an appropriate material. ► An oxygen in the water caused ...three types of surface corrosion of the pipe wall. ► Uniform corrosion, corrosion under the deposit and tuberculation was observed. ► For corrosion prevention a regular control of the heated water is essential.
An analysis of the catastrophic corrosion of the hot water pipeline system was made. The cause was oxygen, dissolved in water due to the malfunctioning of the degassing device in the water preparation phase. Because of differential aeration, cathodic and anodic areas formed underneath the oxide deposit on the bottom side of the pipeline. The consequence of the thus activated pitting corrosion was pipeline perforation. It is imperative to regularly test the water for dissolved oxygen and monitor its pH value.
Failure analysis of a motor-car coil spring Kosec, Ladislav; Nagode, Aleš; Kosec, Gorazd ...
Case studies in engineering failure analysis,
October 2015, 2015-10-00, Letnik:
4
Journal Article
Recenzirano
Odprti dostop
•A failure analysis of a motor-car coil spring was made.•A damage of protective layer was the reason for corrosion occurrence.•A combination of corrosion and fatigue caused the failure of the spring.
Amelogenesis imperfecta (AI) is an inherited tooth disorder with widely varying phenotypes. The aim of this study was to determine the microhardness and microstructure characteristics of the enamel ...in AI teeth. The AI phenotypes examined were hypoplastic (pitted and smooth form), hypomaturated, and hypocalcified. Six AI patients were diagnosed according to clinical characteristics. The microhardness of the enamel was measured on axial cuts of AI teeth acquired from the patients. The measurements were done on several sites from the enamel surface towards the dentine-enamel junction using the Vickers scale. Values of microhardness were compared to corresponding control teeth. The microstructure of AI enamel types was evaluated using scanning electron microscopy. The values of microhardness in pitted hypoplastic AI samples were, on average, lower compared to the control enamel and dropped markedly towards the dentine-enamel junction. The smooth hypoplastic enamel was not only extremely thin but also much softer than control enamel. The values for hypomaturated AI fluctuated, but the palatal sites were markedly softer than in the control tooth. Hypocalcified enamel was the softest, with values resembling those of dentin. Microstructural changes varied from altered orientation of enamel prisms in pitted hypoplastic AI to lack of normal prismatic structure and severe porosity in hypocalcified AI. The present results suggest different microhardness profiles and microstructures in each phenotype. Variations among phenotypes are expected with larger case selection in this genetically heterogeneous disease.
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