Feeding and risering of high-alloy steel castings

A more accurate, less conservative set of feeding distance (FD) and riser sizing rules is developed for high-alloy steel castings produced from alloy grades CF-8M, CA-15, HH, HK, and HP. These rules are designed to produce radiographically sound castings at 2 pct sensitivity. By comparing results between plate casting trials and the corresponding simulations of those trials, a relationship is shown to exist between a local thermal parameter known as the Niyama criterion and ASTM shrinkage X-ray level. This relationship is then used in an extensive set of casting simulations to numerically determine FDs for a wide range of casting conditions. It is shown that the FD rule developed in an analogous earlier study for carbon and low-alloy (C&LA) steels can also be used for these high-alloy grades, provided that the FD is modified by a multiplier that accounts for the high-alloy steel grade. In addition, it is shown that multipliers for superheat, sand mold materials, and the use of chills developed in the earlier work are also valid with these high-alloy steel grades. In comparison with previously published high-alloy FD rules, the present rules are shown to provide longer FDs (and hence higher casting yields) in most casting situations. This study also investigates riser sizing rules. It is determined that for open top risers, the previously published C&LA riser sizing rule is also valid for high-alloy steels. This rule is less conservative than existing high-alloy riser sizing rules, specifying smaller risers that produce higher casting yields. In addition, for vented blind top risers, it is shown that the previously published rules are also overly conservative.     

Development of new feeding-distance rules using casting simulation: Part I. Methodology

A methodology is developed to relate measured shrinkage porosity levels in steel castings to predictions from casting simulations, in order to determine feeding distances. Low-alloy steel casting trials were conducted to acquire a statistically meaningful set of experimental data for top-risered cast steel sections having various ASTM shrinkage X-ray levels. Simulations of the casting trials were then performed, using casting data recorded at the foundries during the trials. The actual casting soundness resulting from these trials, measured in terms of the ASTM shrinkage X-ray level, is quantitatively compared to the soundness predicted by simulations, measured in terms of a local thermal parameter known as the Niyama criterion. A relationship is shown to exist between the X-ray level and both the minimum Niyama criterion value as well as the area (in the plane of the X-ray) with Niyama values below a threshold value. Once the correlations developed in Part I of this article were established, an extensive set of additional casting simulations was performed to determine the feeding distances for castings with a wide variety of casting parameters. These data were then used to develop a new set of feeding-distance rules, which are given in Part II of this article.     

Feeding of high-nickel alloy castings

Feeding of the nickel-based alloys CZ-100, M-35-1, and CW-12MW, as well as of the austenitic stainless steel CN-7M, is investigated, using a combination of casting experiments and simulation. Casting trials are performed at five foundries, to produce a total of 55 plates of varying lengths and radiographic soundness levels. In order to develop the property databases necessary to simulate the casting of these alloys, temperature data are recorded for each alloy during the casting trials. These measured data are used in conjunction with material property simulation to develop the necessary property data for each alloy, including the solidification path. These property data are used to simulate the casting trials. Good agreement between the simulation results and the radiographic testing (RT) results for the castings is obtained. A quantitative relation between the measured ASTM X-ray levels and the predicted minimum Niyama criterion value is established for all but the CZ-100 alloy, which does not appear to suffer from shrinkage defects. A large number of additional simulations are used to develop general feeding distance (FD) rules. The new rules are shown to provide accurate FDs for the casting trial plates. The FDs of the high-nickel alloys (except CZ-100) are found to be at least 25 pct shorter than those for typical low-alloy steels.     

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.     

Feeding and Distribution of Porosity in Cast Al-Si Alloys as Function of Alloy Composition and Modification

Unmodified, Na-modified, and Sr-modified castings of Al-7?pct Si and Al-12.5?pct Si alloys were cast in molds in which it was possible to create different cooling conditions. It is shown how solidification influences the distribution of porosity at the surface and the center of the castings as a function of modification and Si content in sand- and chill-cast samples. Eutectic modification, Si content, and cooling conditions have a great impact on the distribution of porosity. Unmodified and Na-modified castings are more easily fed with porosity tending to congregate near the centerline of the casting, while Sr-modified castings solidify in a mushy manner that creates a more homogeneous distribution of porosity in the casting. The amount of porosity was highest in the Sr-modified alloys, lower in the Na-modified alloys, and lowest in the unmodified alloys. The size of the porosity-free layer and the effectiveness of the feeders were greater in the castings made with the steel chills due to the increased thermal gradients and consequent increase in the directionality of solidification.     

采煤机摇臂壳体铸造工艺

从保证表面质量、内在质量、尺寸公差以及方便操作、节约成本等方面入手,根据采煤机摇臂壳体铸件结构特点和技术要求,利用铸造模拟软件,进行铸造工艺设计,包括合理设置冒口、补贴、冷铁的形状和规格,合理划定型芯分界等,满足了技术条件要求.     

基于FLOW-3D仿真的横浇道集渣效果研究

夹杂缺陷对铸件产生严重破坏作用．在浇注过程中，横浇道具有一定捕获夹杂物的功能，即集渣作用．为了探索横浇道的集渣作用，以含碳量为0．45％的碳钢为例，利用FLOW-3D模拟软件研究了横浇道的形式、浇注系统的类型、集渣包等对集渣效果的影响．仿真结果表明：横浇道充满可以明显提高集渣效率；横浇道横截面的变化及弯曲不利于提高集渣效果；横浇道上设置集渣包有利于提高集渣效率．     

Improvement of Castability and Surface Quality of Continuously Cast TWIP Slabs by Molten Mold Flux Feeding Technology

An innovative continuous casting process named POCAST (POSCO’s advanced CASting Technology) was developed based on molten mold flux feeding technology to improve both the productivity and the surface quality of cast slabs. In this process, molten mold flux is fed into the casting mold to enhance the thermal insulation of the meniscus and, hence, the lubrication between the solidifying steel shell and the copper mold. Enhancement of both the castability and the surface quality of high-aluminum advanced high-strength steel (AHSS) slabs is one of the most important advantages when the new process has been applied into the commercial continuous casting process. A trial cast of TWIP steel has been carried out using a 10-ton scale pilot caster and 100-ton scale and 250-ton scale commercial casters. The amount of mold flux consumption was more than 0.2 kg/m² in the new process, which is much larger than that in the conventional powder casting. Trial TWIP castings at both the pilot and the plant caster showed stable mold performances such as mold heat transfer. Also, cast slabs showed periodic/sound oscillation marks and little defects. The successful casting of TWIP steel has been attributed to the following characteristics of POCAST: dilution of the reactant by increasing the slag pool depth, enlargement of channel for slag film infiltration at meniscus by elimination of the slag bear, and decrease of apparent viscosity of the mold slag at meniscus by increasing the slag temperature.     

近年稀土钢研究进展与加速研发新思路

通过对近几年来稀土钢发展相关文献的调研,统计了稀土相关论文、专利增长数量和应用领域分布,分析了稀土钢的研发进展。结果表明,无论是基础研究还是应用领域拓展,稀土钢近年来都得到迅速发展,稀土电工钢和稀土TWIP钢成为最热门的研发钢种,但稀土添加工艺等关键技术的不足,以及有待发展的稀土钢研发方法,延缓了稀土钢的发展与应用。稀土钢生产实践表明各种稀土添加工艺中连铸结晶器喂线法是钢铁连铸生产最有效的稀土添加方法,稀土电渣重熔工艺对生产重大装备制造用大型铸件具有重要应用价值。介绍了有关引入材料基因组工程先进研究理念,发展高通量计算与模拟、高通量制备与表征研究方法,建立稀土钢数据库,加快稀土钢研发的新思路。     

A Modified Cast-on Method for the Reinforcement of Aluminum Castings with Dissimilar Metals

A modified cast-on method has been developed to reinforce aluminum castings with steel insert. Defect-free bond between the steel insert and the aluminum casting has been consistently obtained. Data obtained from a push-out experiment indicated that the bond strength was much higher than that obtained using the Al-Fin approach. This paper introduces this modified method in four sections: the coating of the steel pins, the cast-on method, microstructure characterization, and the bond strength. The section on the coating of the steel pins contains coating material selection, electroplating technique for plating Cu and Ni on steel, and diffusion bonding of the coatings to the steel. The section on cast-on method deals with factors that affecting the quality of the metallurgical bond between the coated steel and the aluminum castings. The results of microstructure characteristics of the bonding are presented in the microstructure characterization section. A push-out experiment and the results obtained using this method is described in the section of bond strength/mechanical property.     

Rayleigh Number Criterion for Formation of A-Segregates in Steel Castings and Ingots

A Rayleigh number-based criterion is developed for predicting the formation of A-segregates in steel castings and ingots. The criterion is calibrated using available experimental data for ingots involving 27 different steel compositions. The critical Rayleigh number above which A-segregates can be expected to form is found to be 17 ± 8. The primary source of uncertainty in this critical value is the dendrite arm spacing. The Rayleigh number criterion of the current study is implemented in a casting simulation code and used to predict A-segregates in three case studies involving steel sand castings. By comparing the predictions with observations made in the actual castings, the Rayleigh number criterion is shown to correctly predict the regions where no A-segregates form. However, the regions where A-segregates do form are somewhat over-predicted. Based on the results of the three case studies, the primary reason for this over-prediction is persumed to be the presence of a central zone of equiaxed grains in the casting sections. A-segregates do not form when the grain structure is equiaxed.     

Application of insulation padding in a heavy turbine guide vane casting

To replace metal padding by insulation padding for castings can save the melt and reduce cleaning work of castings.The design of insulation padding was investigated.The equation of the modulus extension factor for insulation padding and the ratio of its thickness over the modulus of a casting were improved to deter-mine the thickness of insulation padding.The insulation padding was designed for a turbine guide vane casting weighing 3.5 t.A sound casting was obtained with 750 kg steel saved.On the other side, the cast-ing obviously expanded at the interface with the insulation padding, which is perhaps the reason that the use of insulation padding has been suspended for many years.To avoid the expansion of insulation pad-ding, a shielding layer made of a kind of material of good fire resistance was adopted to prevent the insula-tion layer from touching the melt.The shielding layer serves as a cushion of heat and expansion during so-lidification process so as to resist the expansion of castings and guarantee the feeding effect at the same time.Furthermore, insulation padding can be placed by a certain offset into the mold cavity so as to coun-teract the expansion of castings.     

Mathematical modeling of porosity formation in solidification

Shrinkage porosity and gas porosity occur simultaneously and at the same location when conditions are such that both may exist in a solidifying casting. Porosity formation in a solidifying alloy is described numerically, including the possible evolution of dissolved gases. The calculated amount and size of the porosity formed in Al-4.5 pct Cu plate castings compares favorably with measured values. The calculated distribution of porosity in sand cast Al-4.5 pct Cu plates of 1.5 cm thickness matches experimental measurements. The decrease of the hydrogen content by strong degassing and the increase of mold chilling power are recommended to produce sound aluminum alloy castings. The calculated results for steel plate castings are in agreement with the experimental work of Pellini. The present modeling has clarified the basis of empirical rules for soundness and suggests that the simultaneous occurrence of shrinkage and gas evolution is an essential mechanism in the formation of porosity defects.     

A model of the interfacial heat-transfer coefficient for the aluminum gravity die-casting process

Interfacial heat-transfer coefficients were measured during the solidification of Al-Si alloys against coated die steel chills with varying chill temperature, coating thickness and coating type. Two principal resistances to heat transfer across the casting-chill interface were identified, namely, (1) the resistance to heat transfer of the coating itself and (2) the resistance to heat transfer of a layer of gas, (assumed to be air), trapped between the coating and casting surfaces by virtue of their roughness. These thermal resistances were evaluated by measurement of the coating thermal conductivity and determination of the thickness of the applied coatings and the thickness of the layer of air between the coating and casting surfaces. This produced a simple equation to predict the interfacial heat-transfer coefficient during the solidification of Al alloy die castings, which produced values that were found to agree well with the experimentally determined results. This equation was used to interpret the experimentally measured heat-transfer coefficients and to explain their variation with the different experimental conditions employed. A simple modification of the equation can also take into account the formation of an air gap, where the casting locally retreats away from the die surface, leading to a local reduction in the heat-transfer coefficient.     

Control of the Molten Steel Level in the Top Side-Pouring Twin-Roll Casting Process Based on Fuzzy Rules Optimized by Particle Swarm Optimization Algorithm

Twin-roll strip casting is a near-net-shape casting technology that can produce thin steel strips directly from molten steel. Stably controlling the molten steel level is regarded as an important issue to ensure strip quality and casting process stability. As the control of the molten steel level is a time-varying, nonlinear, and multidisturbance complex system, it is difficult to establish an accurate process model for designing a model-based controller. Top side-pouring twin-roll casting is a new kind of twin-roll strip casting technology. This study introduces the control system of the top side-pouring twin-roll casting process. A fuzzy logic controller (FLC) with its fuzzy rules optimized by particle swarm optimization (PSO) is developed to regulate the molten steel level. Simulation results show that the performance of the FLC can be improved while its fuzzy rules are optimized by PSO. The objective function of PSO has a great influence on the optimization of the fuzzy rules. The top side-pouring twin-roll casting experiments are carried out using the FLC with its fuzzy rules optimized by PSO; the results show that strip quality and casting process stability are guaranteed.     

Mechanisms of Soldering Formation on Coated Core Pins

Die soldering is one of the major casting defects during the high-pressure die casting (HPDC) process, causing dimensional inaccuracy of the castings and increased downtimes of the HPDC machine. In this study, we analyzed actually failed core pins to determine the mechanism of soldering and its procedures. The results show that the soldering process starts from a local coating failure, involves a series of intermetallic phase formation from reactions between molten aluminum alloys and the H13 steel pin, and accelerates when an aluminum-rich, face-centered cubic (fcc) phase is formed between the intermetallic phases. It is the formation of the aluminum-rich fcc phase in the reaction region that joins the core pin with the casting, resulting in the sticking of the casting to the core pin. When undercuts are formed on the core pin, the ejection of castings from the die will lead to either a core pin failure or damages to the casting being ejected.     

The new rules of measurement.

Classical test theory, which the authors maintain applied psychologists are still too often exclusively taught, are contrasted with the new rules of measurement. In the newer, model-based version of test theory, called item response theory (IRT), some well-known rules of measurement no longer apply. Six old rules of measurement that conflict with the new rules are reviewed, and intuitive explanations of the new rules are provided. Readers are also directed to additional informational sources about IRT, which, it is argued, every psychologist should be familiar with. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Production of Steel Castings in Cylindrical Molds

The continuous casting of steel is simulated by means of Sn–Pb alloy on two models of the mold. One of the models, which has been patented, has a closed loop for two-phase coolant circulation. The following characteristics are compared: the cooling of the two castings; the temperature of the models’ cylindrical walls; the thickness of the crust formed; the structure of the castings; and their surface quality. The results are used in simulation of the thermal processes in a model with the continuous supply of an increasing heat flux and the correction of the processes for steel casting.     

Use of plastic bar molded casting in the modification of variably expanding investment to compensate for casting shrinkage of nickel-chromium alloy

Trial plastic bar molded castings were compared for accuracy with cast crowns by means of a nickel-chromium alloy, which was used to establish the liquid density of variably expanding investments. The plastic bars were invested to evaluate the change of expansion rate in a wide range of six liquid densities from 0% to 100%, and the distances between the sections were measured before and after casting. Wax crowns were cast to obtain a more detailed relation around 0% expansion at seven liquid densities, and the gaps between the base of the die and the margin of the crown were measured before and after casting. The expansion rates of both castings were calculated and thus were proportional to the liquid density; both regression curves indicated high correlation coefficients. As a result of the statistics of the Student's t-test, the difference between the two methods was not significant. The trial plastic bar was useful in establishing the variably expanding investment for precise casting of artificial crowns.