Microstructure development in finely atomized droplets of copper-iron alloys

The solidification behavior of Cu_(100-X)FeX (X = 15, 20, 30, and 40) alloys was investigated by gas atomization technology. The effects of the size and composition of the atomized droplet on the microstructure development during cooling through the metastable miscibility gap have been discussed. A smaller atomized droplet achieves a finer dispersed microstructure. Alloys of composition close to the critical composition of the alloy system are relatively easy to undercool into the miscibility gap. The forces acting on the Fe-rich sphere during the liquid-liquid phase transformation were analyzed. The formation of an Fe-poor layer on the powder surface is the result of the common action of the Fe-rich sphere’s Marangoni migration and the repulsive interaction between the cellular solid/liquid interface and the solidified Fe-rich sphere. The Fe-rich spheres in the center part of the powder are entrapped between the equiaxed grains of Cu-rich phase and finally distributed at the grain boundaries and triple junctions.     

Rapid solidification of martensitic stainless steel atomized droplets

The microstructure of martensitic stainless steel powders produced by inert gas atomization was investigated. Depending upon the powder particle size, the microstructure was found to exhibit a cellular, dendritic, or martensitic morphology. Relationships between the microstructure scale and the particle diameter were identified. It was found that at a critical particle diameter of 25 to 30 μm, the structure changed from cellular/dendritic (96.5 vol pct bcc and 3.5 vol pct fcc) to martensite. The solidification path of the powder particles below and above 25 to 30 μm in size was considered. High-temperature X-ray diffraction (HTXRD) measurements revealed that there is a delay in the appearance of the fcc phase for the small particle size. The delay in the appearance of the fcc phase is a result of different nucleation sites for the fcc phase between the large and the small particle size.     

Interaction mechanisms between ceramic particles and atomized metallic droplets

The present study was undertaken to provide insight into the dynamic interactions that occur when ceramic particles are placed in intimate contact with a metallic matrix undergoing a phase change. To that effect, Al-4 wt pct Si/SiC_p composite droplets were synthesized using a spray atomization and coinjection approach, and their solidification microstructures were studied both qualitatively and quantitatively. The present results show that SiC particles (SiC_p) were incor- porated into the matrix and that the extent of incorporation depends on the solidification con- dition of the droplets at the moment of SiC particle injection. Two factors were found to affect the distribution and volume fraction of SiC particles in droplets: the penetration of particles into droplets and the entrapment and/or rejection of particles by the solidification front. First, during coinjection, particles collide with the atomized droplets with three possible results: they may penetrate the droplets, adhere to the droplet surface, or bounce back after impact. The extent of penetration of SiC particles into droplets was noted to depend on the kinetic energy of the particles and the magnitude of the surface energy change in the droplets that occurs upon impact. In liquid droplets, the extent of penetration of SiC particles was shown to depend on the changes in surface energy, ΔE_s, experienced by the droplets. Accordingly, large SiC particles encoun- tered more resistance to penetration relative to small ones. In solid droplets, the penetration of SiC particles was correlated with the dynamic pressure exerted by the SiC particles on the droplets during impact and the depth of the ensuing crater. The results showed that no pene- tration was possible in such droplets. Second, once SiC particles have penetrated droplets, their final location in the microstructure is governed by their interactions with the solidification front. As a result of these interactions, both entrapment and rejection of SiC particles occurred during droplet solidification. A comparison of the present results to those anticipated from well-established kinetic and thermodynamic models led to some interesting findings. First, the models proposed by Boiling and Cisse^[24] and Chernovet al.^[58] predict relative low critical interface velocities necessary for entrapment, inconsistent with the present experimental findings. Second, although the observed correlation between the critical front velocity and droplet diameter was generally consistent with that predicted by Stefanescuet a/.’s model,^[27] the dependence on the size of SiC particles was not. In view of this discrepancy, three possible mechanisms were proposed to account for the experimental findings: nucleation of α-Al on SiC particles, entrapment of SiC particles between primary dendrite arms, and entrapment of SiC particles between secondary dendrite arms.     

The electromagnetic force field,fluid flow field,and temperature profiles in levitated metal droplets

A mathematical representation has been developed for the electromagnetic force field, the fluid flow field, the temperature field (and for transport controlled kinetics), in a levitation melted metal droplet. The technique of mutual inductances was employed for the calculation of the electromagnetic force field, while the turbulent Navier-Stokes equations and the turbulent convective transport equations were used to represent the fluid flow field, the temperature field, and the concentration field. The governing differential equations, written in spherical coordinates, were solved numerically. The computed results were found to be in good agreement with measurements reported in the literature regarding the lifting force and the average temperature of the specimen and carburization rates, which were transport controlled.     

Behavior of liquid metal droplets in an aspirating nozzle

Measurements of particle size, velocity, and relative mass flux were made on a spray field produced by aspirating liquid tin into 350 °C argon flowing through a venturi nozzlevia a small orifice in the throat of the nozzle. Details of the aspiration and droplet formation process were observed through windows in the nozzle. The spatial distribution of droplet size, velocity, and relative number density was measured at a location 10 mm from the nozzle exit. Due to the presence of separated flow in the nozzle, changes in nozzle inlet pressure did not significantly effect resulting droplet size and velocity. This suggests that good aerodynamic nozzle design is required if spray characteristics are to be controlled by nozzle flow. This article is based on a presentation made in the symposium “Spray Processing Fundamentals: Coating and Deposition” presented as part of the 1990 TMS Fall Meeting, October 9, 1990, in Detroit, MI, under the auspices of the TMS Synthesis and Analysis in Materials Processing Committee.     

Distribution of metal droplets in top slags during ladle treatment

Abstract

The investigation focused on the mixing of the metal and slag phases during ladle refining from the point of tapping the EAF to casting. Steel droplet distributions were determined for slag samples taken at different stages in the ladle refining process at two different steel plants in Sweden. The droplet distributions were determined using light optical microscopy and classification according to the standard SS111116. Sample analysis results showed the slag samples taken before vacuum degassing to contain the greatest concentration of steel droplets. The total interfacial area between the steel droplets and slag was determined to be 3–14 times larger than the projected flat interfacial area between the steel and slag. The effects of slag viscosity and reactions between steel and slag on metal droplet formation in slag were also considered.     

Production of atomized metal and alloy powders by the rotating electrode process

Translated from Poroshkovaya Metallurgiya, No. 9(333), pp. 11–22, September, 1990.     

Microstructural features and heat flow analysis of atomized and spray-formed Al-Fe-V-Si alloy

Microstructural features of rapidly solidified powders and preforms of Al₈₀Fe₁₀V₄Si₆ alloy produced by spray forming process have been studied. The atomization and spray deposition were carried out using a confined gas atomization process and the microstructural features were characterized using scanning electron microscopy and transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques. The microstructure of a wide size range of atomized powders invariably revealed cellular and dendritic morphology. The extent of dendritic region and the dendritic arm spacing were observed to increase with powder particle size. The TEM investigations indicated the presence of ultrafine second-phase particles in the intercellular or interdendritic regions. In contrast, the spray deposits of the alloy showed considerable variation in microstructure and size and dispersion of the second-phase particles at specific distances from the deposit-substrate interface and the exterior regions of the deposit. Nevertheless, considerable homogeneity was observed in the microstructure toward the center of the spray deposit. The formation and distribution of a cubic phase α-Al(Fe,V)Si has been characterized in both atomized powders and spray deposits. A one-dimensional heat flow model has been used to analyze the evolution of microstructure during atomization and also during spray deposition processing of this alloy. The results indicate that thermal history of droplets in the spray on deposition surface and their solidification behavior considerably influence the micro-structural features of the spray deposits.     

Heat flow in copper converters

A mathematical heat flow model has been developed to quantify the influence of out-of-stack time and other variables on the temperature distribution in the refractory wall of copper converters. Factors such as diameter of the converter, size and position of the converter mouth, and the use of a mouth cover have been studied with the model, in order to relate converting practice to operating problems. The results of the model indicate that when the converter is out of the stack, heat losses through the mouth of the converter cause the internal refractory surface to cool rapidly which may lead to freezing at the tuyere line and tuyere blockage when blowing is resumed. The temperature gradient, localized to within 60 to 80 mm of the refractory inside wall, changes markedly within the first minutes of the converter being out of the stack. This may generate thermal stresses in the converter wall and contribute to refractory erosion at the tuyere line. Covering the converter mouth during out-of-stack periods significantly reduces the change in temperature gradient at the inside wall as well as heat losses from the converter. Formerly Graduate Student     

Pore formation in atomized powders

Conclusions Experiments have demonstrated that contamination of an alloy with argon in the manufacture of an atomized powder takes place during two stages of the production process: during contact between argon and the melt in the furnace and during atomization. Contamination during the first stage is characteristic of centrifugal atomization, and during both stages, of gas atomization. The porosity of particles can be reduced by shortening the time of contact of argon with the melt in the furnace before atomization. The centrifugal atomization process enables the porosity of powder partieles to be decreased by a factor of 10 and the weight fraction of argon by a factor of 7–9 compared with the gas atomization method.Translated from Poroshkovaya Metallurgiya, No. 1(265), pp. 10–14, January, 1985.     

Heat flow in blast furnace hearths

Abstract

Heat flow and temperature distribution in iron blast furnace hearths has been investigated using a digital computer implemented simulation. Furnace hearths, with and without underhearth cooling, have been simulated to provide dynamic thermal response characteristics and to explore refractory material requirements for improved hearth design.

A digital computer program utilizing numerical approximations to the heat flow equations in three dimensions (two dimensions with radial symmetry in the third) was used to calculate unsteady state heat transfer characteristics. Stave cooling, carbon and ceramic refractories, refractory thermal property variations, salamander formation, underhearth cooling systems, and other design and operating parameters were considered in evaluating the steady and unsteady state temperature distributions in blast furnace hearths.

Résumé

On fait l' étude du flux de chaleur et de la distribution de température dans le creuset d'un haut fourneau par une simulation à l'aide d'un ordinateur. Nous considérons le creuset sans ou avec refroidissemen t afin d'obtenir l' évolution des proprietes thermiques et de sonder les besoins en matériaux réfractaire pour ameliorer le design du creuset.

Un programme d'ordinateur basé sur la méthode d'approximations numeriques aux équations tridimentionnelles de transfert de chaleur (deux dimensions rectilignes avec symétrie radiale dans la troisième direction) a été employé dans le calcul du comportement de transfert de chaleur en régime transitoire. Nous avons considéré le refroidissement des douves, l'u tilisation des réfractaires à carbone et à ceramique, la variation des propriétés thermiques des réfractaires, la formation d'un loup ferreux, d'autres designs et les paramètres d' opération dans cette évaluation des distributions des températures en régime transitoire et en régime permanent des creusets des hauts-fourneaux.     

Analysis of interfacial area changes during spontaneous emulsification of metal droplets in slag

In some metal/slag reactions involving spontaneous emulsification, there is a significant increase of interfacial area, which in turn affects the global rate. In previous work by the authors, the reaction between Fe-Al alloy droplets and CaO-SiO₂-Al₂O₃ slag was investigated. Re-evaluation of the data has shown that at an initial reaction rate above 9 × 10⁻⁷ mol min⁻¹ mm⁻², the maximum change in interfacial area increases linearly with the initial rate and with the change of free energy due to chemical reaction. There were found to be two sources of interfacial area increase: (a) flattening of the original droplet, which was independent of initial rate; and (b) separation of smaller droplets, which was dependent on the initial rate.