Modeling of solidification of molten metal droplet during atomization

A mathematical model to describe the solidification behavior of an atomized droplet during flight, in terms of nucleation temperature, recalescence temperature, nucleation position, solid fraction at nucleation temperature, and droplet temperature and velocity, is formulated. The concept of transient nucleation is applied to model the short nucleation event. A maximum droplet velocity exists, beyond which the droplet velocity shows an inflection phenomenon during the flight. The velocity of smaller droplets is higher at a shorter flight distance and lower at a longer flight distance. Variations of the gas flow patterns have more effects on smaller droplets, and the effects are more significant at a longer flight distance. A minimum surface heat-transfer coefficient exists as the droplet flies. Prior to nucleation or recalescence, smaller droplets have lower temperature at a given flight distance. Smaller droplets have lower nucleation temperature. Medium-size (around 80-μm) droplets fly over the shortest flight distance before the nucleation starts. Smaller droplets have a larger solid fraction at the end of recalescence. Atomization gas has more effects on the droplet momentum than on the heat content of the droplet.     

Thermodynamic Analysis and Experimental Study on Reaction of CO2 Gas with Hot Metal

The reaction of CO_2 gas with hot metal was investigated based on the thermodynamic analysis and experimental results.It shows that both silicon and carbon in hot metal can be oxidized by CO_2 gas in the temperature range of 1 300-1 500 ℃.When using graphite crucible,temperature has little influence on final mass percent of carbon w[C]because of the carburization effect.Decarburization degree rises significantly with increasing gas injection rate and w[C]can be reduced to 3.2% at most when using MgO crucible.Lower temperature or higher gas injection rate is propitious to promote desilication reaction,but only 5%-10% of desilication ratio could be obtained in 20 min.The final mass percent of silicon w[Si]when using MgO crucible is lower than that when using graphite crucible.Experimental results also demonstrate that CO_2 injection has no effect on the concentration of manganese,sulfur and phosphorus in hot metal.In view of the weak oxidation ability and temperature drop of hot metal,CO_2 gas is suggested to be used as carrier gas in desilication process rather than oxidizing agent.     

Analysis of Strip Temperature in Hot Rolling Process by Finite Element Method

 The program was developed by finite element method to calculate the temperature distribution in hot strip rolling. The heat transfer coefficient of air cooling, water cooling and thermal resistance between work roll and strip were analyzed. A new heat generate rate model was proposed according to the influence of source current density, work frequency, air gap and distance to edge on induction heating by finite element method (FEM). The heat generate rate was considered into the thermal analysis to predict the temperature distribution in the induction heating. The influence of induction heating on the strip temperature was investigated with different strip thicknesses. The temperature difference became more and more obvious with the increase of thickness. The strip could be heated quickly by the induction heating both in surface and center because of the thermal conductivity and skin effect. The heat loss of radiation has important influence on the surface temperature. The surface temperature could be heated quickly with high frequency when the strip is thicker.     

Preparation and Microstructure Evaluation of Directionally Solidified Nd-Fe-B Ingots by Electromagnetic Cold Crucible

In this paper Nd-Fe-B ingots with hyper-peritectic composition were prepared through continuous and directional solidification by a novel electromagnetic cold crucible approach.A group of experiments were carried out in order to investigate the effects of input power and withdrawal velocity on the microstructure and growth orientation of Nd-Fe-B phases.It was found that the peritectic Nd₂Fe₁₄B phase grows with a planar interface at lower withdrawal velocity and changed into the dendritic interface at higher withdrawal velocity.The result was explained by the theory of constitutional supercooling.Meanwhile the volume fraction of ferromagnetic T₁ phase was found to be increased first and then decreased with the increasing of withdrawal velocity during the growth process of the ingots.     

Influence of [La(EDTA)]~- on pore structure of activated alumina carrier at high-temperature

The activated alumina as a catalyst carrier were widely used in automotive exhaust catalysts under high temperature,such as,petroleum refining catalysts,hydrogenation and hydrodesulfurization catalyst carrier in China and abroad.γ-Al2O3 was the catalyst carrier that was the most widely used and the best carrier of improved specific surface area .However,the activation of the coating consisted of γ-Al2O3 were usually transformed into α-Al2O3 at 800 oC,causing increased density,reduced specific surface area,and the collapse result of pore structure.While the temperature reached 1200 oC,activation of coating detached from the carrier,the resistance of flowing gas increased,catalytic activity decreased.Addition of La2O3 stabilized the γ-Al2O3 crystal structure,which would keep the activation stability of coating at a high temperature and inhibit activity loss.The activated alumina carrier treated with the solid-pore-expanding agent containing [La(EDTA)]-that synthesized using solid and solid-hybrid approach with the thermal stabilizer lanthana and EDTA at the high-temperature had a 10-30 μm large pore porous network-like structure,after alumina was calcined at 1200 oC for 1 h.The specific surface area of four specimen were beyond 120 m2/g,while the sample specific surface area of x([La(EDTA )]-)=1% was up to 150.36 m2/g.     

Surface reactive species on MnOx（0.4）-CeO2 catalysts towards soot oxidation assisted with pulse dielectric barrier discharge

MnOx(0.4)-CeO2 was investigated for soot oxidation assisted with a pulse dielectric barrier discharge(DBD).The catalysts were evaluated and characterized with TPO(temperature programmed oxidation),X-ray diffraction(XRD),Raman and X-ray photoelectron spectroscopy(XPS).The ignition temperature Ti for soot oxidation decreased from 240.8 to 216.4 oC with the increase of the pulse DBD frequencies from 50 to 400 Hz,lower than that of the case without pulse DBD present(253.4 oC).The results of XRD,Raman and XPS agreed well with the TPO activities of MnOx(0.4)-CeO2 towards soot oxidation.More solid solution of ceria and manganese,and surface reactive species including O2–,O– and Mn4+ were responsible for the enhancement of soot oxidation due to pulse DBD injection in the present study.For solid solution favors to the activation and transformation of those species,which are believed to be involved in the soot oxidation in a hybrid catalysis-plasma.     

The Investigation of Round Billet Temperature and Magnetic Field Distribution and Variation During Induction Heating Process

Round billet temperature during induction heating was calculated with numerical simulation method in present work,the factors affect induction heating were studied,such as coil turns.using of magnetic material,the convective heat transfer between billet surface and surrounding environment,etc.It was found that coil turns played an important role in round billet temperature distribution,and it was necessary to choose reseaonable coil turns in order to get a relatively uniform temperature distribution.Using magnetic flux concentrator could greatly improve the billet end temperature,and the phenomena of low temperature in billet end would be elimiated.Besides,the billet temperature would be reduced by convective heat transfer in billet outsurface and air,longer time was cost to reache the target temperature.Meanwhile,the magnetic field during billet induction heating was calculated,it was used to explain billet temperature distribution and variation,the reasonable measures to control billet temperature during induction heating process were proposed.     

Refrigeration effect of La（FeCoSi）13B0.25 compounds and gadolinium metal in reciprocating magnetic refrigerator

The LaFe11.9–x Cox Si1.1 B0.25 with x=0.9 and x=0.82 compounds were synthesized from commercial purity raw materials.The magnetic property of LaFe11.9–x Cox Si1.1 B0.25 and Gd particles were tested on the reciprocating refrigerator at the same condition in order to compare the cooling capacity of the two materials.The results showed that the cooling velocity of Gd was obviously higher than that of LaFe11.9–x Cox Si1.1 B0.25.The maximum temperature span was 12.7 oC for LaFe11.0 Co0.9 Si1.1 B0.25,14.9 oC for Gd metal whose mass is the same as that of LaFe11.0 Co0.9 Si1.1 B0.25,8.1 oC for Gd metal whose volume is the same as that of LaFe11.0 Co0.9 Si1.1 B0.25.Series connection of LaFe11.0 Co0.9 Si1.1 B0.25 and LaFe11.08 Co0.82 Si1.1 B0.25 had the maximum cooling temperature span of 15.3 oC.     

Effect of rare earths on oxidation resistance of heat resistant steel

The application of rare earths(RE) in the Ni saving heat resistant steel was studied by metalloscopy,scanning electron microscopy(SEM),energy dispersive spectrometer(EDS),X-ray difference(XRD).Because the diffusion of chromium was promoted by RE,a dense and adhesive Cr2O3 layer could form rapidly at the early oxidation stage,which played a effective protection role;the pinning effect of silicon dioxide was enhanced by RE in the internal oxidation layer,which had a block effect on the diffusion of metal ions and oxygen ions at later stage of oxidation and resulted in that the high temperature oxidation rate of RE heat resistant steel was decreased.     

New Method for Evaluating Thermal Wear of Rolls in Rolling Process

A new method was developed by a thermal wear machine to evaluate the thermal wear of roils in steel rolling process. The steel strip and rolls were simulated by upper and lower heating disks. The upper heating disk could he kept at a temperature of over 900 ℃ by induction heating. The pressure between the disks as high as 323.2 MPa could be achieved and the slipping rate could be 12. 7 %. The thermal wear of high speed steel （HSS） roll material, the wear rate of the HSS roll, and the SEM morphology of a worn HSS roll surface were investigated. This method was useful and could be employed to simulate friction and wear between strip and roll during the strip rolling process.     

Die thermal behavior in machine casting of partially solid high temperature alloys

Experimental measurements and computer simulations of die thermal behavior during machine (die) casting of fully liquid and partially solid bronze alloy 905 were carried out. Ingots of the alloy were heated to temperatures above the liquidus and in the liquidsolid range in a reheat furnace. The partially solidified charge was previously made in a continuous slurry producer. Castings were made in both a low pressure laboratory machine and a high pressure commercial die casting machine. In both casting machines used, the maximum die temperature and the initial rate of temperature change in the die, at ∼0.014 in. from the casting-die interface, were significantly lower when a partially solid charge material was used. For example, measured die temperatures in the high pressure commercial machine were 475°C and 165°C above the initial die temperature when the bronze alloy was cast in the 100°C superheated state and the partially solid (volume fraction solid ∼0.57) state, respectively. Correlation of computer predictions and measured die temperatures have been used to calculate values of the heat transfer coefficient at the casting-die interface. Using these values, the maximum die surface temperature and die surface temperature gradients are calculated. The values of maximum surface temperature obtained are 800°C and 315°C for a superheated (100°C) liquid charge and a partially solid (volume fraction solid ∼0.57) charge, respectively. The corresponding surface temperature gradients are 5640°C/cm and 718°C/cm, respectively. The reduced thermal shock experienced by the die when a partially solid metal alloy is cast should improve die life considerably over conventional practice. Formerly Graduate Student at M.I.T.     

Determination of interface heat transfer coefficient between aluminum casting and graphite mold

To produce castings of titanium, nickel, copper, aluminum, and zinc alloys, graphite molds can be used, which makes it possible to provide a high cooling rate. No die coating and lubricant are required in this case. Computer simulation of casting into graphite molds requires knowledge of the thermal properties of the poured alloy and graphite. In addition, in order to attain adequate simulation results, a series of boundary conditions such as heat transfer coefficients should be determined. The most important ones are the interface heat transfer coefficient between the casting and the mold, the heat transfer coefficient between the mold parts, and the interface heat transfer coefficient into the environment. In this study, the interface heat transfer coefficient h between the cylindrical aluminum (99.99%) casting and the mold made of block graphite of the GMZ (low ash graphite) grade was determined. The mold was produced by milling using a CNC milling machine. The interface heat transfer coefficient was found by minimizing the error function reflecting the difference between the experimental and simulated temperatures in a mold and in a casting during pouring, solidification, and cooling of the casting. The dependences of the interface heat transfer coefficient between aluminum and graphite on the casting surface temperature and time passed from the beginning of pouring are obtained. It is established that, at temperatures of the metal surface contacting with a mold of 1000, 660, 619, and 190°C, the h is 1100, 4700, 700, and 100 W/(m² K), respectively; i.e., when cooling the melt from 1000°C (pouring temperature) to 660°C (aluminum melting point), the h rises from 1100 to 4700 W/(m² K), and after forming the metal solid skin on the mold surface and decreasing its temperature, the h decreases. In our opinion, a decrease in the interface heat transfer coefficient at casting surface temperatures lower than 660°C is associated with the air gap formation between the surfaces of the mold and the casting because of the linear shrinkage of the latter. The heat transfer coefficient between mold parts (graphite–graphite) is constant, being 1000 W/(m² K). The heat transfer coefficient of graphite into air is 12 W/(m² K) at a mold surface temperature up to 600°C.     

Relation between the Crystal Structure of 35KhGF Steel and the Viscous Flow Characteristics of the Forming Melt

The effect of the crystal structure of 35KhGF steel on the temperature dependence of the kinematic viscosity of the melt has been studied at temperatures of 1450–1780°C. The crystal structure of 35KhGF steel changes as a result of heat treatment, namely, normalizing and tempering. EBSD analysis is used to study the crystal structure of the steel. The kinematic viscosity of the liquid steel is measured by the oscillating crucible method during heating and subsequent cooling. The supercooling of the liquid metal before solidification and the activation energy of viscous flow are dependent on the heat-treatment conditions. This correlation is discussed in terms of metallurgical inheritance.     

Hillock-free aluminum thin films for electronic devices

The addition of an alloying element was proposed in order to suppress hillocks which grow on the surface of deposited aluminum conductors after they have been subjected to thermal cycling (200°C to room temperature) or high temperature heat treatment (400°C). The alloying element, tin, which has a small diffusion coefficient, a large binding energy with a lattice vacancy and a large atomic diameter, and manganese, which has a relatively small solid solubility into aluminum, were evaluated since they also have proper values of vapor pressure for ease of evaporation with aluminum. Alloy composition was determined to be just above the solid solubility of the element in aluminum at the deposition temperature of 350°C. It was proved experimentally that Al-0.06 wt pct Sn alloy and Al-0.1 wt pct Mn alloy, which had been selected for the abovementioned reason, had a marked effect in suppressing the growth of hillocks.     

Effect of Water Vapor During Secondary Cooling on Hot Shortness in Fe-Cu-Ni-Sn-Si Alloys

Residual Cu in recycled steel scrap can cause hot shortness when the iron matrix is oxidized. Hot shortness can occur directly after the solid steel is formed from continuous casting as the steel undergoes a cooling process known as secondary cooling where water is first sprayed on the surface to promote cooling. This is followed by a radiant cooling stage where the steel is cooled in air to room temperature. This investigation examines the roles of water vapor, Si content, temperature, and the presence of Sn in a Fe-0.2 wt pct Cu-0.05 wt pct Ni alloy on oxidation, separated Cu and Cu induced-hot shortness during simulations of the secondary cooling process. The secondary cooling from 1473 K (1200 °C) resulted in a slight increase in liquid quantity and grain boundary penetration as compared to the isothermal heating cycles at 1423 K (1150 °C) due to the higher temperatures experienced in the non-isothermal cycle. The addition of water vapor increased the sample oxidation as compared to samples processed in dry atmospheres due to increased scale adherence, scale plasticity, and inward transport of oxygen. The increase in weight gain of the wet atmosphere increased the liquid formation at the interface in the non-Si containing alloys. The secondary cooling cycle with water vapor and the effect of Sn lead to the formation of many small pools of Cu-rich liquid embedded within the surface of the metal due to the Sn allowing for increased grain boundary decohesion and the water vapor allowing for oxidation within liquid-penetrated grain boundaries. The presence of Si increased the amount of occlusion of Cu and Fe, significantly decreasing the quantity of liquid at the interface and the amount of grain boundary penetration.     

Effect of Bath Temperature on Cooling Performance of Molten Eutectic NaNO3-KNO3 Quench Medium for Martempering of Steels

Martempering is an industrial heat treatment process that requires a quench bath that can operate without undergoing degradation in the temperature range of 423 K to 873 K (150 °C to 600 °C). The quench bath is expected to cool the steel part from the austenizing temperature to quench bath temperature rapidly and uniformly. Molten eutectic NaNO₃-KNO₃ mixture has been widely used in industry to martemper steel parts. In the present work, the effect of quench bath temperature on the cooling performance of a molten eutectic NaNO₃-KNO₃ mixture has been studied. An Inconel ASTM D-6200 probe was heated to 1133 K (860 °C) and subsequently quenched in the quench bath maintained at different temperatures. Spatially dependent transient heat flux at the metal–quenchant interface for each bath temperature was calculated using inverse heat conduction technique. Heat transfer occurred only in two stages, namely, nucleate boiling and convective cooling. The mean peak heat flux (q _max) decreased with increase in quench bath temperature, whereas the mean surface temperature corresponding to q _max and mean surface temperature at the start of convective cooling stage increased with increase in quench bath temperature. The variation in normalized cooling parameter t ₈₅ along the length of the probe increased with increase in quench bath temperature.

     

The solubility ofRH2−x in Gd,Er, Tm,Lu and Y from ambient to 850°C

The solubility of hydrogen in Gd, Er, Tm, Lu and Y was determined from 25 to 850°C when the metal was in equilibrium withRH_2?x (x varies between 0.1 and 0.2 depending on the rare earth metal). The room temperature solubilities determined by the lattice parametric method were found to be <0.1, 3.6, 7.7, 20.6 and 19.0 at, pct H in Gd, Er, Tm, Lu and Y, respectively. The change in unit cell volume for each atomic percent hydrogen added was nearly the same for all metals. The solubility of hydrogen increases more rapidly with temperature in those metals with low solubility at room temperature. Thus the solubility of hydrogen at 850°C is nearly the same in all five of the metals studied, that is, 35.0, 36.2, 36.0, 36.0 and 37.3 at. pct H in Gd, Er, Tm, Lu and Y, respectively. The equilibrium pressure of H₂ in these studies was the equilibrium pressure of hydrogen in contact withRH_2?x at the temperature concerned. A change in slope was observed in the solubility curves of the Gd-, Er-, Tm- and Lu-H systems. The logC) (at. pct H inR) was plottedvs 1/T for each system. Straight lines were obtained at temperatures above and below the changes in slope of the solubility curves. A calculation of the approximate ΔH of solution ofRH_2?x in the metal sfrom the slope of the lines gave 4.35, 1.88, 1.28, 0.61 and 0.55 kcal/mole for Gd, Er, Tm, Lu and Y, respecitively in the low temperature portion. The change in slope which occurs at some point between 350°C and 650°C, depending on the metal, indicates a lower heat of solution ofRH_2?x in these metals at the higher temperatures. In Lu there appears to be yet another change in slope in the neighborhood of 250°C.     

Surface Phenomena in the Smelting Bath of an Oxygen Converter

The oxygen-converter production of steel is determined by processes in the converter’s reaction zone, which consists of primary and secondary regions. The primary region is the crater formed by the collision of a supersonic gas jet with the molten-metal surface. It is filled with metal droplets (diameter 0.1–2 mm). The surrounding secondary region consists of melt with an enormous quantity of gas bubbles (diameter 0.2–4 mm). The total surface area of the droplets and bubbles is four orders of magnitude greater than the surface of the quiescent melt. That indicates the important role of processes at phase boundaries in steel production. The structure of the reaction zone and the corresponding temperature distribution are studied by hot simulation, when the molten metal is blown by oxygen in a transparent quartz crucible. The transparent walls permit photographic and video recording of the processes in the crucible. Besides the temperature distribution, the hydrodynamics of the bath may be studied directly in the injection zone. The most unexpected result of hot simulation is the motion of the bubbles in the secondary region. They move normal to the crater surface. In other words, their motion is almost horizontal, rather than vertical, as in cold simulation in water. This may be attributed to nonuniformity of the melt’s surface tension, resulting in motion of the bubbles toward higher temperatures. In liquid with a temperature gradient, the surface tension will be different ahead of and behind the bubbles. The forces pushing the bubbles from behind are greater than the forces at the front. Accordingly, they move toward the region of lower surface tension. The nonuniformity of the surface tension is due to the temperature gradient (up to 1200°C within the secondary region) and the change in concentration of the melt components, especially oxygen. The surface tension of the ferrocarbon melt changes in a complex manner with increase in temperature. The surface tension rises on heating to 1550°C, but begins to decrease beyond 1550–1600°C. With decrease in carbon content in the melt, the maximum value of the surface tension increases. The motion of gas bubbles and other phases toward lower surface tension begins at the 1550°C isotherm, which is therefore the external boundary of the secondary region, separating it from the remainder of the bath. Within this boundary, the resultant vector of the surface forces pushes the gas bubbles and slag particles, together with the molten metal, horizontally toward the crater, at increasing speed. This determines the hydrodynamics of the smelting bath and the associated redistribution of oxygen over different parts of the bath and hence the refining process as a whole.     

The influence of gas atmospheres on the first-stage sintering of high-purity niobium powders

Niobium and tantalum surfaces easily absorb oxygen. With decreasing particle size the content of oxygen increases. The role of this surface oxygen and oxygen in the sintering atmospheres on the first-stage sintering is not well established. Therefore the sintering behavior of high-purity niobium powders was studied by annealing cylindrical powder compacts (particle size <63 μm) in the temperature range from 1000°C to 1600°C in ultra-high vacuum and under low oxygen partial pressures, as well as in inert gas atrnospheres with low oxygen contents. The specific surface of the samples was determined by metallographic methods, adsorption, and capacitance measurements. Low oxygen partial pressures (10^-3 Pa) lead to a slight enhancement of the surface diffusion which is controlling first-stage sintering. High heating rates (0T > 3000 min^-1) to temperatures above the melting point of Nb₂O₅ (Tm = 1495 °C) enhances the neck growth due to the formation of a liquid oxide phase on the surface of the powder particles. This paper is based on a presentation delivered at the symposium “Activated and Liquid Phase Sintering of Refractory Metals and Their Compounds” held at the annual meeting of the AIME in Atlanta, Georgia on March 9, 1983, under the sponsorship of the TMS Refractory Metals Committee of AIME     

Condensation of zinc vapor on solid media in Zn<Subscript>(<Emphasis Type="Italic">g</Emphasis>)</Subscript>-CO-CO<Subscript>2</Subscript>-Ar mixtures

The condensation experiments of zinc vapor on the solid media in Zn_(g)-CO-CO₂-Ar mixtures were conducted in a flow reactor at 800 °C to 1000 °C under a zinc partial pressure of 0.6 to 7.9 kPa. The condensates were weighed, and the zinc contents and their morphology were analyzed to investigate the effects of various factors on the condensation. It was found that the initial temperature of the media should be as low as possible in liquid condensation for the efficient recovery of zinc. The condensation is enhanced with zinc partial pressure of the gases. The medium made of silica with a smooth surface is favorable for the efficient condensation. In the gases not oxidizing to zinc vapor, metallic zinc can be obtained on the medium with an initial temperature over the range of 120 °C to 400 °C. For obtaining metallic zinc, it is necessary to raise the temperature of the gases, appropriately limit the zinc partial pressure, or maintain a sufficient CO/CO₂ ratio to avoid the oxidation of zinc vapor.