Segregation Zone Research Articles

Study on the Macrosegregation Behavior for the Bloom Continuous Casting: Model Development and Validation

With the aid of a coupling electromagnetic-thermal-solute transportation model validated by the industrial investigation, a three-dimensional (3-D) plus two-dimensional (2-D) hybrid modeling method has been presented for the exploration of the macrosegregation and macroscale transport phenomena in the bloom continuous casting (CC) processes of high-carbon GCr15-bearing steel. The evolution and characteristics of solute distribution and its influence on the porosity formation in the strand during CC process have been revealed. Solute segregation degree changes from a positive to a negative value with distance from strand surface in the region of initial solidification shell within thickness of 25 mm, which can be attributed to the circulation flow ahead the solidification front and the floatation of solute-rich molten steel at the upper part of the mold. The discontinuous, nonfrozen band induced by the zigzag solute distribution is proven to be the main reason that leads to the porosity formation in the final solidification stage of the CC strand. As the solidification proceeds, the segregation degree of C at the strand center is increased from 1.0 to 1.2, while the melt liquidus temperature is reduced from 1726 K (1453 °C) to 1706.91 K (1433.91 °C) during the CC process. Moreover, with the action of gravity and thermosolutal convection, a negative segregation region in the concave shape and an irregular positive segregation zone are produced in the fixed and loosened side of shell, respectively.

Spatial Distributions of Alloying Elements Obtained from Atom Probe Tomography of the Amorphous Ribbon Fe75C11Si2B8Cr4

Spatial distributions of alloying elements of an Fe-based amorphous ribbon with a nominal composition of <TEX>$Fe_{75}C_{11}Si_2B_8Cr_4$</TEX> were analyzed through the atom probe tomography method. The amorphous ribbon was prepared through the melt spinning method. The macroscopic amorphous natures were confirmed using an X-ray diffractometer (XRD) and a differential scanning calorimeter (DSC). Atom Probe (Cameca LEAP 3000X HR) analyses were carried out in pulsed voltage mode at a specimen base temperature of about 60 K, a pulse to base voltage ratio of 15 %, and a pulse frequency of 200 kHz. The target detection rate was set to 5 ions per 1000 pulses. Based on a statistical analyses of the data obtained from the volume of <TEX>$59{\times}59{\times}33nm^3$</TEX>, homogeneous distributions of alloying elements in nano-scales were concluded. Even with high carbon and strong carbide forming element contents, nano-scale segregation zones of alloying elements were not detected within the Fe-based amorphous ribbon. However, the existence of small sub-nanometer scale clusters due to short range ordering cannot be completely excluded.

Effect of Asynchronous Adjustments of Clamping Cylinders on Triangular Crack of Slab Castings Under Application of Soft Reduction

Soft reduction is known to be one of the best ways to improve the internal quality of slab castings such as center segregation, center porosity, centerline or triangular zone cracks, which is based on a proper adoption of the amount of reduction upon the given final solidification zone through roll gap adjustments. The synchronization of the clamping cylinders for roll gap adjustments should be very important to the application of soft reduction, including the synchronization of the clamping cylinders adjustments in the same and different segments. The synchronization of clamping cylinders adjustments is mainly affected by the adjustable accuracy of the four position-controlled clamping cylinders mounted in the upper frames of the segments according to a predetermined transformation relationship between the signals of displacement sensors and aimed roll gap, which, however, is also influenced by the installation accuracy, the precision of displacement sensors, the deformation of the segment frames and/or its bearing pedestals. Due to the actual asynchronous adjustments of the four clamping cylinders, the dynamic soft reduction operation is normally applied at non-ideal mechanical conditions. Here 7 possible situations of asynchronous adjustments of the local segments which may induce gap deviation have been presented. The roll gap deviation in the soft reduction region of a slab casting has been studied by a 3-D visco-elastic plastic FEM model, through which the additional inter-roll bulging, the related triangular cracks induced by one kind of the possible asynchronous adjustment situation and the effectiveness of soft reduction have been analyzed. A critical tolerance for the gap adjustments has been proposed for better contribution of soft reduction to the internal quality of slabs.

Ingot molds for obtaining long cylindrical ingots and features of the macrostructure of the metal

It is shown that it is possible to cast long ingots with a height/diameter ratio as large as 9.0. The ingots’ macrostructure is free of both nonaxial and axial segregation zones. The physical basis for the production of ingots with such unusual features is the creation of the conditions required for “horizontal” crystallization of the steel as it solidifies in newly designed molds. The main design innovation is the variation in the thickness of the mold walls. Wall thickness is equal to 0.55 of the radius of the internal cavity in the bottom part of the mold and 0.45 of that radius in the upper part. The new mold designs that have been developed have been patented by the Russian Federation.

Analytical Model for Zones Distributions in Non-Horizontal Miscible WAG Injection

The advantages of WAG (water-alternating-gas) injection compared to gas injection and waterflooding, from technical and economical points of view, are results of creating a mixed zone in which both water and gas are flowing simultaneously. Previously, an analytical model was developed in the literature to evaluate complete segregation distance and zone distributions for horizontal miscible WAG injection models. Even though this analytical model is derived using simplifying assumptions, it still gives good insight into the design of new projects when the lack of data does not allow a detailed full field reservoir simulation to be carried out. On the other hand, the advantages and disadvantages of applying WAG injection in dipped reservoirs, and up-dip or down-dip patterns are still in question while there isn’t any analytical model available for analyzing process in this type of reservoir. This paper develops analytical equations for a non-horizontal model and investigates the effect of dip angle on segregation effects in miscible WAG injection process. Up-dip versus down-dip WAG injection is another subject which will be discussed in this paper.

Failure Analysis of Commercial Vehicle Crankshaft: A Case Study

A failure analysis has been conducted on a diesel engine crankshaft used in a commercial vehicle, which is made from 42CrMo forging steel, and the crankshaft was after induction hardening process. The fracture occurred between the 4th journal and the 4th crankpin, fracture section indicates that fatigue is the dominant mechanism of failure for this crankshaft. In order to find the failure causes, the dimension of journal boss fillet, material chemical compositions, surface hardness and depth of hardness layer were evaluated. Chemical compositions of crankshaft material, surface hardness and the depth of hardness layer are within the specified range, however the dimension of journal boss fillet is less than specified, and the transition zone of hardness layer was found on the journal boss fillet. Heavy segregation and many non-metallic inclusions aggregate zones were found from metallographic, these would lead to the reduction of endurance bending strength. FEA analysis results suggest that the dimension of journal boss fillet have an important effect on the stress and stress amplitude of the crankshaft. So the disqualification machining and material property are the main causes for the crankshaft fatigue fracture.

Laves Phase Control in Inconel 718 Weldments

The detrimental laves formation in fusion zone during welding of Inconel 718 is controlled with compound current pulsing technique along with helium shielding gas. Also solid solution filler wire is used to minimize the niobium segregation. Welds were produced in 2mm thick sheets by GTA welding process and subjected to the characterization techniques. The results show, refined fusion zone microstructure, reduced amount of laves phase, minimum niobium segregation and softer fusion zone in the as welded condition.

Intergranular Fracture of Spoke Plate of a Large Gear

A crack occurred on the spoke plate of a large marine gear apparently when the gear was being moved at the fabrication site. Fractographic analysis of the fracture surfaces in this study revealed that the crack initiated from the core of spoke plate, and propagated circumferentially toward the surfaces of two holes on the spoke plate and simultaneously to the upper and low ends of the gear. Intergranular fracture was found in crack origin and in regions where the fracture developed slowly. The intergranular facets were associated with ductile shearing between the facets. Transgranular cleavage fracture was apparent in the fast crack propagation region. Metallographic observations also revealed that a type of macro-casting defect, general spot segregation, was generally present on the gear material. A large size macro-spot segregation zone was present near the crack origin and presented a favorable metallurgical condition for hydrogen-induced fracture. The conditions conductive to hydrogen embrittlement include the presence of excessive amounts of S and P, and the appearance of bainite. Therefore, it was concluded that the crack of the spoke plate of the gear was hydrogen induced. The probable source for hydrogen was introduction during the melting and casting process.

Analysis of Powder Segregation in Powder Injection Molding

The powder injection molding (PIM) combines the thermoplastic and powder metallurgy technologies to manufacture intricate parts to nearly shape. The powder segregation is a special effect arising in PIM different from than the pure polymer injection. The two-fluid flow model is used to describe the flows of binder and powder so as to realize the prediction of powder segregation effect in PIM injection. To take into account binder–powder interaction, the mixture model of inter-phase exchange term is introduced in the two-fluid model. The two-fluid equations largely resemble those for single-fluid flow but are represented in terms of the mixture density and velocity. The volume fraction for each dispersed phase is solved from a phase continuity equation. As the key to calculate the phase exchange term, the drag coefficient is defined as a function of mixture viscosity. The effective viscosity of binder and powder are agreed with the additive principle. The volume fractions of binder and powder give directly the evolution of segregation during the injection course. Segregation during PIM injection was simulated by software CFX and results were compared with experimental data with good agreement. The basic reasons that caused segregation are identified as boundary effect, differences in density and viscosity of binder and powder. The segregation zones are well predicted. This showed that the two-fluid model is valid and efficient for the prediction of the segregation effects in PIM injection.

Study on microstructure of squeeze casting AZ91D alloy

The present paper reports the investigation of the microstructure distribution of squeeze cast AZ91D alloy. The microstructure of squeeze cast AZ91D alloy is not uniform and is composed of four zones, which are chilled layer, segregation zone, pressured crystallisation area and hot spot area respectively. Moreover, in the pressured crystallisation area, the microstructure sequence in the transverse section from the outside to the inside could be divided into four sublayers, such as fine equiaxial dendrite area, dendrite area with a high directivity, confusion dendrite area and disorder dendrite area. The volume fraction of the intermetallic compound Mg17Al12 also varied with the location. The volume fraction of Mg17Al12 in the pressured crystallisation area is the largest except in the segregated zone.

The Unlikely Young Cosmopolitans of Cairo

Cairo, a divided city home to many numerous worlds within, is part of the global condition which characterizes cities of the developing South. While a new network of 'global spaces' (Oncu &amp; Weyland, 1997: 1) occupied by five-star hotels, multinational companies and world-class shopping malls has invaded its metropolitan life, beyond the sleek facade of Cairo's urban skylines, an altogether different story subsists. Although its modern 'global spaces' have become zones of "first world" sophistication and global belonging for some, for the vast majority, they are unattainable zones of exclusion and segregation which bar them from taking part in these acts of belonging. Thus, Cairo looked at from ‘below’ reveals a gloomier yet increasingly more dynamic and unpredictable narrative of struggle: a daily struggle for space, identity and recognition. Such juxtapositions and inconsistencies, characteristic of contemporary cosmopolitan cities, have become a backdrop against which fresh possibilities for new and unusual forms of imagination and belonging are constantly being created and re-created. By understanding cosmopolitanism not as westernization, but as a form of internal heterogeneity, where through personal strategies and performance we incorporate the global into our own local repertoires, this paper is arguing that it is lower middle class Egyptian youth who are more deserving of the cosmopolitan label owing to their ability to undergo a careful and inclusive negotiation between alternative cultural repertoires. It is important to note that in the Egyptian context, the lower middle class have many characteristics that the west would associate with the working class.

Three‐dimensional Simulation of Thermosolutal Convection and Macrosegregation in Steel Ingots

AbstractThermosolutal convection and macrosegregation formation during the solidification of steel ingots are numerically simulated in three dimensions. The simulation is based on a fully coupled model for mass, momentum, energy, and species conservation equations. The interdendritic flow in the mushy zone is governed by Darcy's law, and the permeability term is discretized using an interpolated liquid fraction method. The numerical results for a benchmark test of macrosegregation in a Pb‐Sn alloy are compared with experimental data taken from the literature. The present model is applied to simulate the solidification of industrial steel ingots. Preliminary predictions are obtained, including the positive segregation in the hot top, and the conically shaped negative segregation zone at the bottom of the ingot. The predicted variation of the segregation ratio in carbon along the vertical centreline of an ingot is compared with measurements, and generally good agreement is observed. Future attention should be paid to the precision of prediction by considering complex solidification issues, such as the sedimentation of free equiaxed grains and the formation of shrinkage cavity.

Devising a set of measures to improve the quality of rolled plates in the Z-direction

This article is devoted to determining the reasons for the fracture of rolled plates in the Z-direction. Technological recommendations are developed to improve plate quality and reduce the rate of rejection of cast and rolled steel. A method is proposed for determining the mechanical properties of the matrix and the axial segregation zone separately on one specimen.

Hot Cracking Susceptibility of Twin Roll Cast Al-Mg Alloy Welds

The hot cracking susceptibility of two different Al-Mg alloys during welding at various applied strains was investigated. At each strain value, the amount of hot cracking susceptibility increased with increasing magnesium content. Thermal analysis results verified that hot cracking susceptibility indeed can be related to the solidification range. As suggested in the solidification range approach, the results of the present study confirm that the extent of solidification and liquation cracking depend on the magnitude of solidification range and the strain imposed during welding. The fracture surfaces of liquation and solidification cracks were investigated with EDS. Liquation crack surfaces were found to have high Mg content whereas solidification crack surfaces revealed iron rich phases. A quench test was designed to observe the effect of mid-plane segregation zone. It was observed that high segregation at the mid-section of the alloy having higher magnesium can increase the liquation cracking susceptibility during welding since it decreases the solidus temperature of the intergranular liquid.

Grain size control in Al–Si alloys by grain refinement and electromagnetic stirring

The present study concerns the directional solidification of grain-refined and non-refined Al–7 wt.% Si alloys under the influence of a travelling magnetic field (TMF). Upwards and downwards travelling fields have been applied to provide a forced convection within the solidifying melt. The formation of a fine equiaxed structure is favoured by both the addition of grain refining AlTi5B1-particles and electromagnetic stirring as well, whereas the addition of grain refiners into the melt appears to be more efficient for achieving a reduction of the mean grain size. A minimum grain size has been observed of the electromagnetic agitation of a grain-refined alloy. A melt stirring by a sufficiently high magnetic field provides a homogeneous grain size distribution in the sample volume, but, gives rise to the formation of segregation zones.

Modeling of eutectic macrosegregation in centrifugal casting of thin walled ZA8 zinc alloy

The maximum segregation zone and microstructure formation during the solidification of thin walled ZA8 zinc–aluminum alloy produced by centrifugal casting are investigated. From the results obtained, it is seen that the maximum segregation zone of the eutectic through the part section corresponds to the zone of final solidification point. The concentration of eutectic through the section changes depending on the initial mold temperature, pouring temperature, and cooling rate. A high cooling rate reduces the rate of change in eutectic concentration across the section. The distance separating the maximum segregation zone from the inner and outer faces of the casting can be controlled by controlling the ratio between the speeds of the solidification fronts advancing from opposite sides. The microstructure obtained becomes finer as the cooling rate increases. The structure of eutectic changes according to the cooling rate, and may be granular or lamellar.

NETmix®, a new type of static mixer: Modeling, simulation, macromixing, and micromixing characterization

AbstractNETmix® is a new technology for static mixing based on a network of chambers connected by channels. The NETmix® model is the basis of a flow simulator coupled with chemical reaction used to characterize macro and micromixing in structured porous media. The chambers are modeled as perfectly mixing zones and the channels as plug flow perfect segregation zones. A segregation parameter is introduced as the ratio between the channels volume and the whole network volume. Different kinetics and reactants injection schemes can be implemented. Results show that the number of rows in the flow direction and the segregation parameter control both macro and micromixing, but the degree of micromixing is also controlled by the reactants injection scheme. The NETmix® model enables the systematic study of micromixing and macromixing for different network structures and reaction schemes, enabling the design of network structures to ensure the desired yield and selectivity. © 2009 American Institute of Chemical Engineers AIChE J, 2009

Modelling of macrosegregation in large steel ingot with considering solid movement

Based on the Eulerian and the volume averaging methods, a mathematical model of macrosegregation formation with considering solid movement during solidification of large steel ingot is developed. The cases with and without solid movement are studied and compared. The result shows that the A segregation band results from a narrow channel that forms around the position where the solidified shell in the horizontal direction meets the one in the vertical direction, which serves as a barrier to the fluid flow, leading to the accumulation of solute. Effects of critical solid fraction and density of solid on evolution of macrosegregation are investigated. The results indicate that both the positive segregation in the A segregation band and the bottom negative segregation become more pronounced with the critical solid fraction increasing. As the density of solid increases, stronger macrosegregations occur at the upper and bottom parts of the ingot. However, the length of A segregation band is not increased with density of solid. Calculated results show good agreement with experimental ones, except a larger bottom negative segregation zone.

Recovery of metals from waste printed circuit boards by a mechanical method using a water medium

Research on the recycling of waste printed circuit boards (PCB) is at the forefront of environmental pollution prevention and resource recycling. To effectively crush waste PCB and to solve the problem of secondary pollution from fugitive odors and dust created during the crushing process, a wet impacting crusher was employed to achieve comminution liberation of the PCB in a water medium. The function of water in the crushing process was analyzed. When using slippery hammerheads, a rotation speed of 1470 rpm, a water flow of 6 m 3/h and a sieve plate aperture of 2.2 mm, 95.87% of the crushed product was sized less than 1 mm. 94.30% of the metal was in this grade of product. Using smashed material graded −1 mm for further research, a Falcon concentrator was used to recover the metal from the waste PCB. Engineering considerations were the liberation degree, the distribution ratio of the metal and a way to simplify the technology. The separation mechanism for fine particles of different densities in a Falcon concentrator was analyzed in detail and the separation process in the segregation and separation zones was deduced. Also, the magnitude of centrifugal acceleration, the back flow water pressure and the feed slurry concentration, any of which might affect separation results, were studied. A recovery model was established using Design-Expert software. Separating waste PCB, crushed to −1 mm, with the Falcon separator gave a concentrated product graded 92.36% metal with a recovery of 97.05%. To do this the reverse water pressure was 0.05 MPa, the speed transducer frequency was set at 30 Hz and the feed density was 20 g/l. A flow diagram illustrating the new technique of wet impact crushing followed by separation with a Falcon concentrator is provided. The technique will prevent environmental pollution from waste PCB and allow the effective recovery of resources. Water was used as the medium throughout the whole process.

Grain boundary diffusion and wetting in the analysis of intergranular penetration

Intergranular penetration of liquid bismuth has been analysed in two pure metals, Cu at 500 °C and Ni at 700 °C, used either as polycrystals or as oriented bicrystals. At the liquid/solid interface, large grooves have developed in Cu–Bi, while micrometer-thick films were observed in Ni–Bi. The bismuth concentration measurements obtained by Auger electron spectroscopy indicate a zone of monolayer Bi segregation followed by a diffusion-type profile over a distance of the order of 100 μm for Cu–Bi and a nanometer-thick film followed by similar diffusion-type profile for Ni–Bi. In both cases the kinetics of intergranular penetration and embrittlement has been shown to be parabolic. It is concluded that no wetting occurs in Cu–Bi system at 500 °C while Bi wets Ni at 700 °C. It is postulated that the mechanism of intergranular penetration operates at a very tip of the penetration front, as opposed to the tip of liquid Bi film observed by scanning electron microscopy, and must be based on diffusion rather than wetting phenomena. Some suggestions are formulated for the future research in the area of intergranular penetration that can be split in two phenomena: grain boundary wetting above the wetting transition temperature and grain boundary diffusion below.