The Effect of Time and Temperature of Nitridation-Oxidation Process on Properties and Corrosion Resistance of AISI 316L Steel

In this research, AISI 316L austenitic stainless steel has been subjected to plasma nitriding and oxidation- nitridation heat treatment at several temperatures for different times. Plasma nitriding of the samples was performed in N₂/H₂ = 1/3 atmosphere at temperatures of 425, 450, and 475°C for 5 h. To study the effects of the combined nitridation-oxidation process on mechanical and physical properties, the samples have been exposed in O₂/H₂ = 1/5 oxidating atmosphere at 425, 450, and 500°C for 15, 30, and 60 min, respectively. The mechanical and physical properties of the samples were studied after nitridation-oxidation heat treatment. The microstructural properties were examined by optical microscopy and scanning electron microscopy; the phases were analyzed by X-ray diffraction. The wear behavior of the oxidized-nitrided samples was studied using pin-on-disk tribotesting. The hardness and depth of the nitrided layer were measured by a Vickers hardness tester. The corrosion resistance of both untreated and treated samples was tested by the Tafel polarization and potentiodynamic polarization in 3.5% NaCl solution at ambient temperature. The results indicate that the combined nitridation-oxidation heat treatment improves both the pitting corrosion and wear resistances of AISI 316L steel and further increases its hardness.     

A new insight into the stability of variable viscosity diffusive boundary layers in porous media under gravity field

We study the stability of gravitationally unstable transient diffusive boundary layers with variable viscosity in porous media. The previous studies characterize the effect of viscosity variation only in terms of viscosity contrast and generalize their findings. However, conclusions of different studies seem contradictory. Our results demonstrate that stability of diffusive fronts is governed by the boundary layer viscosity and not solely by the viscosity contrast. In other words, the use of viscosity contrast to ascertain the stability of the system cannot be generalized. Nonlinear simulations are conducted based on a finite difference scheme to validate the results of linear stability analysis for which the amplification theory is adopted. We also revisit other available scaling approaches used to characterize the effect of viscosity variation on the onset of convective dissolution and explain why previously made conclusions are not inclusive and sometimes appeared to be contradictory. A critical Rayleigh number is found to predict stability of Rayleigh‐Darcy convection in a porous layer with variable viscosity. The results reveal that this critical value can differ highly from the conventional value of 4π². © 2017 American Institute of Chemical Engineers AIChE J, 64: 1083–1094, 2018     

Pattern classification of internal incipient faults during impulse tests using continuous wavelet analysis

In the case of a fault occurrence, the pattern of the fault currents obtained by the standard impulse tests contains a typical signature of the nature and the location of the insulation failure involved. This paper presents a new approach to classify the pattern of the arc discharge location as one of the important types of internal incipient faults in transformer windings. The continuous wavelet transform (CWT) has been used to calculate the most predominant frequency of each fault and its time of occurrence. The data obtained from the field tests of a 66 kV/25 MVA interleaved transformer winding and the computer simulations have been used for the classification.     

Application of bifurcation theory in dynamic security constrained optimal dispatch in deregulated power system

In a deregulated environment of power systems, the transmission networks are often operated close to their maximum capacity. Besides, the independent system operator must operate the system to satisfy its dynamic stability constraints under credible contingencies. In this paper, a novel technique based on iterative stability constrained Optimum Power Flow is proposed. Particle Swarm Optimization methodology is employed to maximize the social welfare with consideration of static and dynamic functional operating constraints and Dynamic Loading Margin (DLM) requirements with respect to normal condition and contingencies. New linear Hopf bifurcation (HB) index is used for fast detection of the DLM. Furthermore, since the pattern of load increase is difficult to be predicted in the new market environment, a method for determining sensitive loading direction associated with DLM is proposed. IEEE14 bus test system with both supply and demand bidding are used to illustrate and test the proposed technique.     

PE/Clay Nanocomposite with Bimodal Molecular Weight Distribution Produced Via In-Situ Polymerization

A Ti-based Ziegler–Natta catalyst supported on the clay was used for producing the polyethylene/clay nanocomposites through in situ polymerization. This catalyst showed high activity in the ethylene polymerization. The two-step polymerization approach, i.e. in the presence and absence of hydrogen, was laid out to broaden the molecular weight distribution of the polyethylene/clay nanocomposite. The molecular weights and molecular weight distribution of the nanocomposites were characterized by the gel permeation chromatography. It was found that the molecular weight distribution was remarkably widened towards bimodal distribution by using the above mentioned approach. The thermal properties of the produced nanocomposites were studied by differential scanning calorimetry and thermal gravimetric analysis. The microstructure of the resulting bimodal polyethylene/clay nanocomposite was investigated by X-ray diffraction and transmission electron microscopy. The thermal gravimetric analysis indicated an improved thermal stability of the produced nanocomposites. In addition, the studies proved the nanocomposite formation with the exfoliated structure of the clay in the polyethylene matrix.     

Microstructural,mechanical and thermal shock properties of triple-layer TBCs with different thicknesses of bond coat and ceramic top coat deposited onto polyimide matrix composite

In this study, a triple-layer thermal barrier coating (TBC) of Cu-6Sn/NiCrAlY/YSZ was deposited onto a carbon-fiber reinforced polyimide matrix composite. Effects of different thicknesses of YSZ ceramic top coat and NiCrAlY intermediate layer on microstructural, mechanical and thermal shock properties of the coated samples were examined. The results revealed that the TBC systems with up to 300 µm top coat thicknesses have clean and adhesive coating/substrate interfaces whereas cracks exist along coating/substrate interface of the TBC system with 400 µm thick YSZ. Tensile adhesion test (TAT) indicated that adhesion strength values of the coated samples are inversely proportional to the ceramic top coat thickness. Contrarily, thermal shock resistance of the coated samples enhanced with increase in thickness of the ceramic coating. Investigation of the TBCs with different thicknesses of NiCrAlY and 300 µm thick YSZ layers revealed that the TBC system with 100 µm thick NiCrAlY layer exhibited the best adhesion strength and thermal shock resistance. It was inferred that thermal mismatch stresses and oxidation of the bond coats were the main factors causing failure in the thermal shock test.