Steel casting by diffusion solidification

A new process for casting and welding carbon steels is described in which carbon diffuses isothermally or adiabatically within an intimate mixture of solid low carbon steel and high carbon liquid iron to effect solidification and subsequent homogenization with respect to carbon. Advantages over conventional casting processes and products result from 1) 150 to 200‡C lower casting temperature; 2) reduced solidification shrinkage, obviating the need for risers in most cases; and 3) more rapid solidification, especially for castings with large ratios of volume to area. In its most versatile form the process involves low pressure forced infiltration of a mold filled with preheated spherical low carbon steel particles by a higher-carbon liquid. The process can reliably produce castings with greater than 99 pct of theoretical density; solidification times typically range from a few seconds to several minutes; and tensile strengths as high as 185 ksi with 15 pct reduction of area to break have been attained. The ductility of such castings is approximately one order of magnitude more sensitive to total oxygen content than the ductility of wrought steels, probably because of cavitation nucleated by oxides during solidification of the pools of liquid trapped between the shot particles. An analysis of the kinetics of the infiltration and solidification is per-formed for steel casting by diffusion of carbon, manganese or heat in iron. The iron-carbon system is most tractable; steel casting by thermal diffusion has also been demonstrated but no attempt was made to test the iron-manganese system. GEORGE LANGFORD, formerly with the Monsanto Triangle Park Development Center, Inc.     

Steel casting by diffusion solidification

A new process for casting and welding carbon steels is described in which carbon diffuses isothermally or adiabatically within an intimate mixture of solid low carbon steel and high carbon liquid iron to effect solidification and subsequent homogenization with respect to carbon. Advantages over conventional casting processes and products result from 1) 150 to 200°C lower casting temperature; 2) reduced solidification shrinkage, obviating the need for risers in most cases; and 3) more rapid solidification, especially for castings with large ratios of volume to area. In its most versatile form the process involves low pressure forced infiltration of a mold filled with preheated spherical low carbon steel particles by a higher-carbon liquid. The process can reliably produce castings with greater than 99 pct of theoretical density; solidification time typically range from a few seconds to several minutes; and tensile strengths as high as 185 ksi with 15 pct reduction of area to break have been attained. The ductility of such castings is approximately one order of magnitude more sensitive to total oxygen content than the ductility of wrought steels, probably because of cavitation nucleated by oxides during solidification of the pools of liquid trapped between the shot particles. An analysis of the kinetics of the infiltration and solidification is performed for steel casting by diffusion of carbon, manganese or heat in iron. The iron-carbon system is most tractable; steel casting by thermal diffusion has also been demonstrated but no attempt was made to test the iron-manganese system. GEORGE LANGFORD, formerly with the Monsanto Triangle Park Development Center, Inc.     

Isothermal reduction kinetics of self-reducing mixtures

Isothermal reduction of haematite carbon mixtures was investigated at temperatures 750–1100°C under inert atmosphere. Mass loss curves proved the stepwise reduction of haematite to metallic iron. The non-linear feature of haematite to magnetite reduction kinetics was observed and an activation energy of 209?kJ?mol^?1 was calculated. Irrespective of carbon-bearing material type, reduction rate of magnetite was linear. Activation energy values were calculated to be 293–418?kJ?mol^?1. Significant increase in the reduction kinetics in the last step (Wustite reduction) was observed and explained by the catalytic effect of freshly formed metallic iron. During the initial stages of wustite reduction, the activation energy values were calculated to be in the range of 251–335?kJ?mol^?1 for all carbon-bearing materials.     

Interface attachment kinetics in alloy solidification

The current status of our understanding of nonequilibrium interface kinetics during solidification is reviewed. Measurements of solute trapping and kinetic interfacial undercooling during rapid alloy solidification are accounted for by the continuous growth model (CGM) without solute drag. Disorder trapping has been predicted and observed in the rapid solidification of ordered intermetallic compounds. In systems that undergo either solute or disorder trapping, a transition from short-range diffusion-limited to collision-limited growth occurs, which originates in the reduced driving free energy for the formation of such metastable materials, resulting in three orders of magnitude change in the interface mobility. Applications to cellular and dendritic growth are discussed. A correlation is presented for estimating the diffusive speed—the growth rate necessary for substantial solute trapping—for alloy systems in which it has not, or cannot, be measured. The raw data for Si(Bi) solute trapping measurements to which many models have been compared are presented in the Appendix.     

Effect of grain boundaries on isothermal solidification during transient liquid phase brazing

The influence of liquid penetration at grain boundary regions on the rate of advance of the solid-liquid interface during isothermal solidification of transient liquid phase (TLP) brazed nickel joints has been examined. The test samples used in this study were Ohno-cast nickel with a grain size of >4 mm and a fine-grained nickel with a grain size of around 40 μm. Both Ni-base materials had the same chemical composition. The rate of isothermal solidification was greater when fine-grained nickel was employed during TLP brazing using Ni-11 wt pct P filler metal at 1200 °C. Liquid penetration at grain boundaries accelerates the isothermal solidification process by increasing the effective solid-liquid interfacial area and increasing the rate of solute diffusion into the base material. An analysis of electron channeling patterns has confirmed that random high-angle boundaries have a greater influence on the rate of isothermal solidification than ordered boundaries including small-angle or twin boundaries. Formerly Visiting Scientist, Department of Metallurgy and Materials Science, University of Toronto. Formerly Postdoctoral Fellow, Department of Metallurgy and Materials Science, University of Toronto     

Efficient estimation of diffusion during dendritic solidification

A very efficient finite difference method has been developed to estimate the solute redistribution during solidification with diffusion in the solid. This method is validated by comparing our computed results to the results of an analytical solution derived by Kobayashi^[4] for the as-sumptions of a constant diffusion coefficient, a constant equilibrium partition ratio, and a par-abolic rate of the advancement of the solid/liquid interface. The flexibility of our method is demonstrated by applying it to the dendritic solidification of a Pb-15 wt pct Sn alloy, for which the equilibrium partition ratio and diffusion coefficient vary substantially during solidification. The fraction eutectic at the end of solidification is also obtained by estimating the fraction solid, in greater resolution, where the concentration of solute in the interdendritic liquid reaches the eutectic composition of the alloy. TURBO PASCAL is a trademark of Borland International, Scotts Valley, CA. IBM PC is a trademark of International Business Machines Corporation, Armonk, NY.     

Heat transfer-solidification kinetics modeling of solidification of castings

A close examination of the recent developments in the field of computer simulation of solidification process reveals that a combination of both macroscopic and microscopic models is necessary in order to accurately describe the solidification of castings. Currently available macroscopic models include models that describe heat transfer from metal to mold, fluid flow of liquid metal during mold filling, and stress field in the casting. At the microscopic level, the models should include more intricate issues such as solidification kinetics and fluid flow in the mushy zone. Although significant progress has been accomplished over the years in each field, the task of including all of these models into a comprehensive package is far from being complete. This paper describes the state of the art on coupling the macroscopic heat transfer (HT) and microscopic solidification kinetics (SK) models and introduces thelatent heat method as a more accurate method for solving the heat source term in the heat conduction equation. A new method for calculation of fraction of solid evolved during solidification based on computer-aided cooling curve analysis (CA-CCA), as well as a method based on nucleation and growth kinetics laws, is discussed. A new nucleation model based on the concept of instantaneous nucleation, which is used to describe equiaxed eutectic solidification of commercial alloys, has been introduced. It is demonstrated that the instantaneous nucleation model agrees well with the experimental results in terms of cooling curves and of evolution of the fraction of solid during solidification. Validation results are also shown for SK models that are based on CA-CCA coupled with HT models for eutectic Al-Si and gray cast iron alloys.     

Coarsening kinetics during solidification of Ni-Al-Ta dendritic monocrystals

Several dendritic monocrystals of nickel-rich Ni-Al-Ta alloys were directionally solidified at about 0.25 m/h⁻¹ under a gradient of 8 × 10⁻³ K/m⁻¹. The solid-liquid interface was fossilized at a given moment by rapidly quenching the remaining liquid. In some specimens crystal pulling was interrupted for various lengths of time prior to quenching. The quenched solid-liquid interfaces were used for a convenient and rapid evaluation of: 1) isothermal coarsening kinetics of the dendritic solid at a temperature between the liquidus and the eutectic temperatures and; 2) dendrite coarsening kinetics during solidification. It was found that extension to the ternary Ni-Al-Ta system of a model previously developed for binary systems predicted isothermal dendrite coarsening kinetics in close agreement with experimental results. Agreement for coarsening kinetics during solidification was less good. An increase in tantalum or aluminum contents slowed down coarsening, yielding finer microstructures. At equal atomic percental increase in concentration, the effect of tantalum was more significant than that of aluminum.     

Interfacial transport kinetics during the solidification of silicon

Studies of the temperature dependence of interfacial reaction kinetics have recently been made in experiments on crystal nucleation and growth in undercooled liquid Si, allowing estimation of the apparent activation energy over a fairly wide range in temperature. The activation energy inferred from growth experiments, at temperatures near the equilibrium melting temperature (T_m), is ∮.2eV while nucleation experiments at temperatures near 2T_m/3 indicate a value of 1.09 eV. This discrepancy can be resolved if the interfacial kinetics are described by a Fulcher-Vogel expression rather than an Arrhenius one, consistent with interfacial atomic mobility being free-volume limited. The ideal glass transition temperature indicated by a free-volume analysis is 1040 ± 100 K for liquid Si; however, experimental measurements of the solidification rate for amorphous Si indicate a slightly lower value.     

高磷铁矿石含碳球团等温还原动力学

为了研究高磷铁矿石含碳球团等温还原动力学在温度为1 173、1 273、1 323、1 373、1 423和1 473K时,采用界面化学反应模型、Jander方程、Ginstling-Broushtein方程、G Valensi-R E Carter方程等固-固/气反应机理函数对反应过程进行拟合,并采用XRD、SEM、EDX等对样品的物相组成、微观形貌和元素分布进行表征分析。研究结果表明,随着还原程度提高,反应速率由0迅速增至最大值,随后逐渐减小并趋于平缓;当温度为1 173～1 373K时,反应过程符合界面化学反应,表观活化能为70.02kJ/mol,线性相关系数为0.948 1;当温度为1 373～1 473K时,反应过程符合Jander方程,限制步骤为铁离子固相扩散,表观活化能为215.36kJ/mol,线性相关系数为0.991 2。     

Measurement of liquid metal diffusion coefficients from steady-state solidification experiments

Steady state solidification experiments were carried out on Sn?Bi alloys of compositions from zero to 3.5 at. pet Bi with tube diameters from 1.9 to 5 mm. Solute profiles were determined utilizing a radioiostope, Bi²⁰⁷, and a microtome slicing technique. Results indicate the technique offers a relatively precise method for determining liquid metal diffusion coefficients at the solidus temperature. Convection does not appear to be a problem for tube diameters under 3 mm. It is shown that neglect of thermal transport may give errors in the measured diffusion coefficients as high as 10 to 30 pet.     

Thermal analysis of isothermal solidification kinetics duirng transient liquid-phase sintering

A technique for experimentally measuring the kinetics of isothermal solidification during transient liquid-phase sintering (TLPS) has been successfully developed using differential scanning calorimetry (DSC). Comparison of these data with a model based on diffusion of Sn solute into an array of spherical Pb particles reveals very good agreement between predicted and measured solidification rates. The DSC technique and the solid-state diffusion model represent valuable tools in investigating the parameters that control TLPS, such as powder size, solder composition, process temperature, and heating rate.     

Diffusion-based model for isothermal solidification kinetics during transient liquid-phase sintering

Results from a diffusion-based model for transient liquid-phase sintering (TLPS) were used to predict the influence that the solute diffusivity (D), base-metal particle size (a), base-metal grain size (d), alloy partition coefficient (k), and the extent of solute saturation (X _max/X _o) had on the rate of isothermal solidification (where both X _max/X _o and k determine the initial liquid weight fraction in the system W _{A o}). The solidification rate increases with an increase in D and a decrease in a, d, X _o, and W _{A o}, but decreases with an increase in k. Model predictions are close to, but slightly underestimate, results for the solidification rate measured in a Pb-Sn TLPS system.