Aluminium nitride precipitation and grain structure of continuously cast billet corners
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The highly grain refined corners of continuously cast steel billets and the high levels of associated aluminium nitride precipitation were investigated. From experiments using steels grain refined by the addition of aluminium, it was found that aluminium nitride precipitated during recalescence after the temperature of the billet corners had fallen below the Ar3 temperature in the mould and secondary cooling zone. The aluminium nitride was found to persist and prevent austenite grain growth during reheating to the straightening temperature and this produced the refined grain structure and high aluminium nitride content of the cooled billet.MST/1170Keywords:
Aluminium nitride
A new nitriding method has been devised which requires only a simple vacuum furnace and enables direct nitridation of solid aluminium without any prior surface treatment. It can be used to produce thick aluminium nitride surface layers on aluminium, under nitrogen at atmospheric pressure. A critical element of the process is the use of a magnesium vapour source that reduces/disrupts the natural, protective oxide film on the aluminium surface and facilitates nitriding. The nitride surface layers form through two distinct modes, one growing outward from the aluminium plate surface and the other growing into the aluminium. Studies of the nitride layers utilizing optical microscopy, TEM, SEM, XRD and XPS have been conducted. Details of the composition, structure and growth as well as possible mechanisms for the nitride formation are presented. Understanding of the reaction may have important implications for the production of wear resistant coatings on bulk Al as well as for the production of Al/AlN composites.
Aluminium nitride
Aluminium oxides
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Hot pressing
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Aluminium nitride (AlN) transparent ceramic was fabricated by a spark plasma sintering (SPS) technique. Samples were processed at 1800°C, 25 MPa pressure, and 6 Pa vacuum without sintering additives, and at different heating rates and sintering durations ranging from 4 to 20 min. X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and transmission electron microscopy (TEM) investigation indicated that the ceramics prepared by SPS had good transparency, because of its high purity, fine particle size, and uniform microstructure. Dislocations and pores in crystals were observed by TEM, which adversely affect the transmittance of AlN transparent ceramics. Experiments showed that the SPS technique is an effective method for the fabrication of transparent ceramics.
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Transparent ceramics
Aluminium nitride
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Aluminium nitride
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Influence of the addition of silicon on mechanical properties and corrosion resistances of P/M austenitic stainless steels was investigated by means of microstructual examinations, calorimetric analysis and X-ray analysis. Silicon addition to P/M austenitic stainless steels makes the sintering in the duplex structure of austenite(γ) and ferrite(α) possible by the liquid phase sintering with a eutectic liquid. For example, addition of 4mass% Si results in the formation of about 40vol% of ferrite. Tensile strength of sintered steels increases with increased silicon content, and the maximum strength (940MPa) was obtained in the steel with 4mass%Si which was sintered for 3.6ks at 1623K. On the other hand, elongation of sintered steels tends to increase with rising sintering temperature. The maximum elongation (47.5%) was obtained in the steel with 2mass%Si which was sintered for 3.6ks at 1673K. The sintered steel with 2mass%Si was found to have excellent corrosion resistance in a boiling solution of 65% HNO3 : The corrosion rate of the steel was very small in comparison with the sintered SUS304L steel, but it was three times larger than that of a SUS304L steel produced by ingot metallurgy. However, further addition of silicon causes a decline in corrosion resistance probably due to a decrease in the green density.
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The present investigation was carried out to know the effect of alloying elements on austenite grain size of high purity iron alloys. The single or combined effects of alloying elements i.e. aluminium, nitrogen, carbon, silicon, and (or) manganese were examined. The results obtained are summarized as follows.1) A ustenite grain size of pure iron-nitrogen alloys was somewhat refined as the content of acid soluble nitrogen was increased up to about 0.02% while that of pure iron-aluminium alloys was not infl uenced with an increase of acid soluble aluminium of less than 0.5%.2) The austenite grain size of pure iron alloys was not affected by the increase in AlN and the coarse grains of nearly the same size were observed.3) Carbon had a strong effect on the austenite grain refinement in the range of concentration of less than about 0.05%, while in the range from 0.1 to 0.5% the fine grained austenite was not affected by a furtheri ncrease in carbon content, and there was a trend for the grains to be coarsened when the content of carbon was increased beyond 0.5%.4) Carbon did not show any function to prevent or disturb the austenite grain coarseni ng.5) The grain size of the fine grained austenite in iron-carbon alloys was not influenced by the presence of nitrogen, aluminium or by the precipitated AlN. It was controlled mainly by the carbon content itself.6) The austenite grain was refined by the addition of silicon in the range less than 0.2%, while it was coarsened by the presence of silicon more than 0.2% in the range less than 2.1%.7) The austenite grain was slightly refined by the addition of manganese in the range less than 13.7%.8) The austenite grain was finest and the grain size number was 8 to 10 in the alloys containing more than two of carbon, AlN, silicon, and manganese. Carbon played a main role for the grain refinement also in these alloys.9) The presence of AlN was effective to raise the coarsening temperature, though it was not effective to to the austenite grain refinement.
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AbstractAbstractThe highly grain refined corners of continuously cast steel billets and the high levels of associated aluminium nitride precipitation were investigated. From experiments using steels grain refined by the addition of aluminium, it was found that aluminium nitride precipitated during recalescence after the temperature of the billet corners had fallen below the Ar3 temperature in the mould and secondary cooling zone. The aluminium nitride was found to persist and prevent austenite grain growth during reheating to the straightening temperature and this produced the refined grain structure and high aluminium nitride content of the cooled billet.MST/1170
Aluminium nitride
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