配电网单相接地故障选线的一种新方法

为了提取故障暂态信号的相位谱特征以进行配电网单相接地故障选线，引入了零序电流解析信号的小波包分解。信号的解析表达形式可以更好地描述信号的幅值、相位和瞬时频率随时间变化的规律；它有着比原信号增大一倍的幅值谱、和原信号相同的相位谱和瞬时频率。小波包克服了小波变换不能对高频序列进一步分解的缺陷，在时域和频域上皆具有良好的局部化特性，可以聚焦被分析信号不同频带的时域特性。通过对构造的零序电流解析信号进行小波包分解，提取在适当频带的信号相位谱，依据故障线路和非故障线路相位谱的不同进行故障选线。理论分析和仿真结果表明：这种基于零序电流解析信号的小波包分解所提取暂态信号相位谱的选线判据具有很高的灵敏性和可靠性。     

Mechanisms of inclusion formation in low alloy steels deoxidised with titanium

Abstract

In the present investigation, the conditions for inclusion formation in two Ti deoxidised steels and one Al–Ca deoxidised steel have been examined by means of optical and electron microscopy, in combination with a thermo dynamic analysis of the phase relations involved. It is concluded that the Ti containing inclusions form as a result of a series of reactions occurring in the ladle, during solidification and in the solid state. The important solid state reaction products are MnS, TiN, and MnOTiO₂ . The presence of Mn rich compounds at the surface of the inclusions is consistent with the observation of a Mn depleted zone in the surrounding steel matrix. In contrast, the primary inclusions in the Al–Ca deoxidised steel are complex oxysulphides, which are thermodynamically more stable and can therefore form in the liquid state.     

New method to prepare iron particles with different morphologies: a way to get high green strength metal compacts

Abstract

Metaliron powders of well controlled size and morphology were synthesised by thermal decomposition under hydrogen of precipitated ferrous oxalates. Green compacts were prepared by uniaxial pressing of metal powders at 290 MPa. The bending green strengths of compacts were measured.

The precipitation of β-FeC₂O₄.2H₂O oxalate from ammonium oxalate gives rise to the formation of spherical particles by aggregation ofelongated grains. Thermal decomposition of this oxalate from 400 to 500°C under hydrogen permits metal iron particles with a rough surface to be obtained. Decomposition occurring above 500°C induces a smoothness of the particle surface. Metal particles synthesised at 500°C show both surface roughness and micrometer sized primary grains.This specific microstructure has allowed the highest value ofcompact green strength (31·7 MPa) to be obtained.

Acicular shaping of the β-FeC₂O₄.2H₂O particles precipitated from oxalic acid involves, after decomposition, an increase in the surface roughness and shape irregularity of the metal particles, owing to an entanglement of the elementary grains. An exceptional value (about 60 MPa) for the metal compact green strength was thus obtained for this type of powder.     

Effect of superimposed oscillations on creep behaviour of Al – 2 4·.5Cu and Al – 2 4·5Cu – 2 0·1In (wt-%) alloys containing θ′ precipitates

Abstract

The behaviour of both transient and steady state creep of Al – 4·5Cu and Al – 4·5Cu – 0·1In (wt-%) alloys was investigated using a constant stress where torsional oscillations of different frequencies and shear strain amplitudes were operated at various working temperatures. An augment in both transient and steady state creep rates was observed by increasing both frequency and shear strain amplitude of the applied oscillations. The mean values of the activation energy of both alloys for both creep stages were found to be equal to that quoted for dislocation intersection mechanism.     

European Union Policy: update New Chemicals

Abstract

In a previous article the EU New Chemicals Policy was introduced. With publication of draft legislation expected Spring 2003, more detail is provided about the policy and its likely effect on the PM industry; some technical and commercial issues underlying its implementation are discussed.     

Influence of cooling and reheating on the evolution of copper rich liquid in high residual low carbon steels

Abstract

This study investigates some effects of austenite microstructure on processes leading to copper hot shortness. Low carbon steels containing 0˙55 wt-% copper were subjected to two thermal profiles in an infrared image furnace with attached confocal scanning laser microscope: hold at 1150°C for 60 s; hold at 1150°C for 60 s, quench to 400°C, reheat to 1150°C. Heat treatments were conducted in dried/deoxidised argon to image microstructures. Subsequent samples were oxidised in air. The oxide/metal interface was studied in a scanning electron microscope. Additional confocal scanning laser microscope experiments involved melting copper directly on the steel. After quench/reheat, austenite grain size decreased by a factor of ～1˙7 and grain boundaries were redistributed. Copper evolved during the first heating was no longer found at boundaries. Results from direct copper exposure reveal an apparent effect of boundary character on copper penetration rate. Possible mechanisms by which hot shortness is affected are discussed.     

Coupled effect of heat input and beam oscillation on mechanical properties of alloy 718 electron beam weldments

Abstract

Autogenous full penetration electron beam welds were made on alloy 718 with and without beam oscillation technique. Weldments were subjected to two types of post-weld heat treatments: direct aging (DA) and solution treatment at 980°C followed by aging (STA). When the welds were prepared using different heat inputs in the welding processes with and without beam oscillation, the influence of beam oscillation could not be studied in isolation but the coupled effect of heat input and beam oscillation was studied. Laves particles were finer in size and lower in amount in unoscillated welds compared with those in beam oscillated welds subjected to DA condition. δ phase needles were observed around Laves particles in the welds subjected to STA condition. The amount of Laves particles was less and that of δ phase was more in unoscillated welds compared with those in beam oscillated welds subjected to STA condition. Unoscillated weldments exhibited longer fatigue lives compared with beam oscillated weldments in both DA and STA conditions owing to less amount of Laves in the former. Weldments in STA condition had longer lives compared with those in DA condition. The role of δ phase needles in fatigue life could not be identified.     

Influence of precursors on formation of TiO2–CrTaO4 rutile solid solutions

Abstract

Rutile solid solutions Cr_x Ti_1-2xTa_xO₂ (0 < x < 0·5) have been synthesised by: firing mixtures of Cr₂O₃ , Ta₂O₅ , and TiO₂ (anatase); firing colloidal gels obtained from TiCl₄ , TaCl₅ , and CrCl₃ . 6H₂O; and firing polymeric gels prepared from Ti⁴⁺ isopropoxide, Ta⁵⁺ethoxide, and Cr³⁺ acetylacetonate at 300–1300°C. Samples were characterised by XRD, differential thermal and thermogravimetric analysis, UV visible spectroscopy, Cielab colour parameter measurements, and electron microscopy. In samples synthesised from oxide mixtures, CrTaO₄ (rutile structure) appeared as an intermediate phase in the formation of the solid solutions and a positive departure from Vegard's law was observed. Ta₂O₅ segregation in gel samples seems to prevent the formation of material consisting only of rutile. The distortion of MO₆ octahedra varies as x increases. The chemical, structural, and thermal stability, and the colouration obtained on glazes containing 3 wt-%samples, indicate their potential for use as ceramic pigments. Under the experimental conditions used, the materials prepared by sol–gel synthesis were not found to be better than those prepared from oxide mixtures.     

Interaction between magnesia and graphite bricks in furnace producing vanadium nitride

Abstract

A cavity between magnesia bricks and graphite slabs was found in a commercial furnace produced vanadium nitride after a 13 months campaign. The magnesia, zirconia–corundum–mullite and hollow sphere Al₂O₃ bricks taken from the dismantled furnace were investigated by chemical analysis, XRD and SEM. It was found that the compositions of magnesia bricks, which contact with graphite slabs, were almost not changed but the thickness of remaining bricks was far lower than that of the original ones. Spinel was detected in used zirconia–corundum–mullite bricks and hollow sphere Al₂O₃ bricks, which were above MgO bricks. A mechanism of cavity formation was proposed. This result may be a new evidence of reaction mechanism between magnesia and graphite via gas phase.