Fundamentals and applications on in situ spinel formation mechanisms in Al2O3–MgO refractory castables

Although the physical expansion associated with the in situ formation of magnesium–aluminate spinel (MgAl₂O₄) is well-reported, some questions related to this behavior, such as the different volume change values experimentally attained when compared to theoretical one and the pore generation after the reaction, remain open. Thus, the main objective of this work is to shed some light on these questions by evaluating a cement-bonded alumina–magnesia castables, designed using dead-burnt magnesia of different particle size ranges. Microstructural observations suggested that the faster Mg²⁺ migration during the spinel formation led to vacancy accumulation and, consequently, to pore generation, as a direct result of the Kirkendall effect. Additionally, the overall expansion of alumina–magnesia castables seemed to be ruled by two main factors: its sintering efficiency and the different possibilities of the Al₂O₃ and MgO interactions in the mixture. Those consequences, however, do not usually affect the castable corrosion behavior in industrial applications, due to the benefits imposed by the structural constraint.     

A BREAKTHROUGH IN PRODUCTION GRINDING WITH BN WHEELS -A RESULT OF SYSTEMATIC SCIENTIFIC WORK

Several parallel paths are being pursued in the further development of grinding processes. On the one hand, since higher material removal rates are to be realized, machines of greater rigidity with more efficient drives and tools are required. On the other hand, increasing demands are placed on the quality of machined parts. One trend is towards the utilization of super hard abrasives such as cubic boron nitride (CBN). Advantages of this grinding material as opposed to the conventional corundum are significantly greater hardness and, therefore, longer profile life, higher material removal rates, and lower grinding temperatures. One disadvantage is the price of the tool, which is several magnitudes higher than for corundum. Considering the high initial costs for CBN cutting material, destruction of the wheel caused by faulty operation of the machine or undue allowance must have the aim of transferring supervision and control tasks from the operator to the machine control. Fundamental for this concept are suitable sensor systems for monitoring the grinding process. A constant normal grinding force achieved by regulating the feed rate avoids clogging and destruction of the wheel. In addition, the grinding time is reduced, since the bending of the spindle can be compensated.     

Observational Study of Clogging Specimens from the Tundish Well Showing Origin and Growth of a Clog in an Al-Killed Ti-Alloyed Steel Cast

Metallurgical and Materials Transactions B - Post-mortem tundish skull samples of Ti-alloyed, Al-killed IF steel have been studied regarding the formation of clogging. By using modern...     

Slag Resistance of Al2O3–MgO Refractory Castables in Different Environmental Conditions

Although the corrosion performance of spinel‐containing castables has been extensively investigated in recent years, no previous studies accessed the different conditions present in the ladle bottom. In this region, strong variations in the atmospheric environment are often detected, which could drastically change the interactions between refractory and molten slag. In the present work, the main corrosion mechanisms of an alumina–magnesia castable in two environmental conditions (oxidizing – pO₂ = 0.21 atm—or reducing – pO₂ = 10^?15 atm— atmosphere) were evaluated by means of scanning electron microscopy and EDS analyses of the corroded samples and thermodynamic simulations. The attained results showed that the slag penetration was suppressed in the presence of oxygen due to the precipitation of a great amount of calcium monoaluminate (CA) crystals as the refractory interacted with slag. Conversely, the CA phase was not stable under reducing conditions and, therefore, many more refractory components (Al₂O₃, MgO, and MgAl₂O₄) had to be dissolved to precipitate calcium dialuminate (CA₂) by reacting with infiltrating slag. Thus, besides providing a suitable and more realistic understanding of the castable performance in service conditions, the results also indicated that the prediction of the environmental conditions is of utmost importance for the design of high performance refractories.     

Mould Powder Requirements for High‐speed Casting

Mould powders play an important role in the stability of the continuous casting process of steel. The main functions of mould slag (i.e. molten powder) are to provide sufficient lubrication and to control the heat transfer between the developing steel shell and the mould. Sufficient lubrication requires an undisturbed melting of mould powders and uniform infiltration of mould slag. Based on the casting practice in IJmuiden, it is found that these demands become even more important for the applied high casting speeds in thin slab casting at 5 to 6 m/min. At Corus RD&T, mould powders were characterised by X‐ray diffraction and subsequent fully quantitative Rietveld analysis. Additionally, the melting of mould powders has been studied in‐situ using high‐temperature X‐ray diffraction, to gain crucial knowledge about melting relations. Slag rims obtained from the thin slab caster mould were characterised using extended microscopic techniques in order to describe the mechanisms of rim formation and growth. Finally, slag films obtained after casting were characterised. As a result, not only the melting process of mould powder, but also the mechanism of formation and growth of slag rims is much better understood. This knowledge will be applied to define the demands on the composition and properties of mould powder for even higher casting speeds.