Mathematical model of kinetics of filling the model during casting by gasified models

Based on an analysis of the thermal destruction of cellular polystyrene (CPS), an investigation into filling the mold during casting by gasified models is performed. A mathematical model of the process developed on the displacement of gaseous products of the thermal destruction of CPS from the region of gasifying the model by excess pressure in a gas gap between the CPS and liquid metal is made. The results of calculations are compared with the available experimental data on the time dependence of the pressure in the gas gap and measurements of the duration of the pouring time of actual castings. The satisfactory similarity of the calculated dependences and characteristics with experimental ones is shown. The formula for the calculation of the pressure in the thermal destruction region is suggested. This pressure determines the stability of the form and conditions of obtaining suitable castings, as well as the method for calculation of the filling time of the mold.     

煤气化气用于气基竖炉生产DRI技术的进展

介绍了煤气化气用于Midrex和HyL技术的发展和工业化应用现状。同时介绍了煤气化气技术的工业化现状及不同工艺的特点,对适用于Midrex和HyL工艺的煤气化技术进行了分析。Midrex与HYL技术都对煤气化气有一定要求,相比来说,HYL要求得更严格一些。Lurgi法制得的煤气化气经过脱CO2后可直接用于Midrex和HYL竖炉;用Texaco、Undok和Shell技术制得的煤气化气都需经过处理和转换才可用于Midrex和HYL竖炉。     

Deoxidation of molybdenum during vacuum sintering

Molybdenum is usually fabricated through the powder metallurgy (P/M) process, using fine powders with a relatively high oxygen content. Oxygen, however, is one of the main elements causing embrittlement during the deformation processing of molybdenum, such as rolling, extrusion, and forging. Thus, how to deoxidize the compact as completely as possible is critical in the P/M process. This study shows that, as an alternative to hydrogen reduction, molybdenum oxides can be reduced by adding organic lubricants to the compact and by sintering the compact under high vacuum with long sintering times. After 10 hours of sintering at 1750 °C and a 0.03 torr vacuum, the oxygen content decreased from 0.927 wt pct of the green compact to 0.017 wt pct. The ductility also improved significantly compared to compacts sintered for 5 hours, which contained 0.218 wt pct oxygen. The morphology evolution, weight changes, and the X-ray analysis indicated that the oxide was first present in the form of MoO₃. It was then transformed into MoO₂ before deoxidation was completed. Two deoxidation mechanisms were identified: evaporation and decomposition of MoO₃ and MoO₂, with evaporation being dominant in the early-stage sintering and decomposition being dominant in the later stage.     

The effect of high vacuum on the low cycle fatigue law

Push-pull fatigue tests have been conducted on several materials at various frequencies and temperatures in air and high vacuum (10⁻⁸ torr) and the fatigue life determined in terms of the cyclic plastic strain. In contrast to a changing exponent of the Coffin-Manson law with increasing temperature in air, in high vacuum this exponent is found to remain nearly constant at a value of about 0.5. Further, the temperature sensitivity of this exponent and of life at a specific plastic strain range in high vacuum is slight. Pronounced plastic instability (specimen shortening and fattening) was observed for the ductile metals investigated and crack nucleation was retarded. In all cases crack propagation was transgranular in vacuum. It is concluded that for the materials, temperature, and frequencies investigated, the degradation of fatigue life at elevated temperature is due to environmental enhancement of intergranular fracture. Materials investigated include A286 at room temperature and 593°C, nickel A at 550°C, 304 stainless steel at 816°C and 7075T6 aluminum alloy.     

The effect of solution heat treatment and quenching rates on mechanical properties and microstructures in AlSiMg foundry alloys

Four common AlSiMg foundry alloys have been solution heat treated at 813 K, quenched, and immediately aged at 423 K for up to 240 minutes. The mechanical properties are found to be related to the amount of Mg and Si in the alloys. A high strength is obtained after only 60 minutes of solution heat treatment, indicating that the solid solution is rapidly saturated on Mg and Si. The ductility is very much related to the amount of silicon present and the refinement of the silicon crystals within the eutectic areas, since silicon crystals are observed to crack when load is applied. Thus, a well-modified structure is the best way to obtain high ductility. Reduced quencing rates after solution heat treatment lead to a lower strength, since a lower number of hardening β′-Mg₂Si precipitates are formed. The ductility of alloys with 0.6 wt pct Mg is increased with a reduced quenching rate. A more ductile matrix corresponding to the lower amount of hardening precipitates can explain this. Alloys with 0.2 wt pct Mg remain relatively unchanged. A hypothesis that may explain this phenomenon is the precipitation of brittle silicon or formation of coarse Mg2Si within the dendrites.     

The cognate facilitation effect: Implications for models of lexical access.

Do nonselected lexical nodes activate their phonological information? Catalan-Spanish bilinguals were asked to name (a) pictures whose names are cognates, in the 2 languages (words that are phonologically similar in the 2 languages) and (b) pictures whose names are noncognates in the 2 languages. If nonselected lexical nodes are phonologically encoded, naming latencies should be shorter for cognate words, and because the cognate status of words is only meaningful for bilingual speakers, this difference should disappear when testing monolingual speakers. The results of Experiment 1 fully supported these predictions. In Experiment 2, the difference between cognate and noncognate words was larger when naming in the nondominant language than when naming in the dominant language. The results of the 2 experiments are interpreted as providing support to cascaded activation models of lexical access. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Model-based melt rate control during vacuum arc remelting of alloy 718

Vacuum arc remelting (VAR) is used widely throughout the specialty metals industry to produce superalloy and titanium alloy cast ingots. Optimum VAR casting requires that the electrode melting rate be controlled at all times during the process. This is especially difficult when process conditions are such that the temperature distribution in the electrode has not achieved, or has been driven away from, steady state. This condition is encountered during the beginning and closing stages of the VAR process, and also during some process disturbances such as when the melt zone passes through a transverse crack. To address these transient melting situations, a new method of VAR melt rate control has been developed that incorporates an accurate, low-order melting model to continually estimate the temperature distribution in the electrode. This method of model-based control was tested at Carpenter Technology Corporation. In the first test, two 0.43-m-diameter alloy 718 electrodes were melted into 0.51-m-diameter ingots. Aggressive start-up and hot-top procedures were used to test the dynamic capabilities of the control technique. Additionally, a transverse cut was placed in each electrode with an abrasive saw to mimic an electrode crack. Accurate melt rate control was demonstrated throughout each melt. The second test used an electrode size and grade proprietary to the host company. Because it was not stress relieved after the primary casting process, the electrode was known to possess multiple cracks that make accurate melt rate control impossible using standard VAR controller technology. This electrode was also successfully melted with good melt rate control using the model-based controller.     

高速钢粉末真空高温处理时组织结构之变化

研究了水雾化M 2和M 4高速钢粉末在 7× 10 -2 Pa真空下加热后组织结构的变化。当粉末加热至固相线以下 15～ 2 0℃ ,即 12 2 0～ 12 2 5℃时 ,M6C及MC型碳化物都由 1～ 2 μm长大到 3～ 4 μm ;当粉末加热至固相线以上时 ,碳化物尤其是M6C型碳化物颗粒快速长大 ,并且有不均匀长大现象 ;当温度升至 12 5 0℃后 ,贫碳型的M4C3 转变为MC型碳化物 ,其中大部分Fe被V所取代 ;当进一步加热至 12 6 0℃以上时 ,M6C型碳化物将转变为M2 C碳化物。经真空脱氧后高速钢粉末的氧含量降低至 10 0× 10 -6以下。     

The effect of leukotriene synthesis inhibitors in models of acute and chronic inflammation

To define the risk factors related to the occurrence of fungemia in children infected with human immunodeficiency virus (HIV), we performed a matched case-control study. During a 6-year period (1987-1993), fungemia developed in 22 (6.3%) of 347 HIV-infected children observed at the Pediatric Branch of the National Cancer Institute. Each of these 22 cases was matched by age and gender with three controls. Multiple logistic regression indicated that the best predictor of fungemia in this population was the presence of a central venous catheter placed for > 90 days (P < .00001), followed by a group of risk factors composed of 10 independent variables adjusted for a CD4 cell count of < 100/MicroL (P < .045). Those variables included treatment with more than three antibiotics, treatment with more than three parenteral antibiotics, > 30 days of antibiotic treatment, bacterial infections, > 30 days in the hospital, hypoalbuminemia, C3 (Centers for Disease Control and Prevention) classification of HIV infection, and malnourishment. We conclude that prolonged placement of central venous catheters is the most important risk factors for fungemia in HIV-infected children and that the risk of fungemia is further influenced by antibacterial therapy, catheter manipulation, and host response.     

Obtaining casts of aluminum alloys by foundry in loose molds fabricated on installations of three-dimensional printing

Three-dimensional printing is used more and more often at Russian enterprises in preparing the production of cast articles for fabricating models and casting molds. A ZPrinter 310 Plus and a ZCast molding sand are used at the Moscow Institute of Steel and Alloys to obtain molds by this method in the production of pilot casts of aluminum and magnesium alloys. Because of the absence of reference data on the strength and heat-transfer properties of these mixtures, it is difficult to optimize the thickness of the mold wall during their design. In connection with this, the physical and production properties of the ZCast mixture were determined as applied to different conditions of formation of the mold wall before it was poured with the aluminum alloy. The results showed that the method of rapid prototyping the models and molds can be effectively used in foundry for obtaining casts of aluminum alloys, especially at the stage of preparing the production of cast articles.     

The scattering of ultrasound by human tissues--some theoretical models

Scanning electron microscopy studies of decalcified human secondary osteons were conducted to determine correlations in fiber orientations in these osteons with their appearance under crossed polaroids. Using a new sample preparation technique, osteons were prepared to expose many of their lamellar interfaces. By studying many such samples, it was concluded that osteons which appear 'dark' under crossed polaroids contain fibers with orientations having little or no transverse component, whereas 'bright' osteons possess fiber orientations with transverse and longitudinal components. 'Intermediate' osteons have been found to contain fiber orientations richer in longitudinal component than in transverse.     

不锈轴承钢真空制备过程洁净度及碳化物变化

 为了提高G102Cr18Mo高碳不锈轴承钢的洁净度、细化碳化物组织,采用真空感应熔炼、两次真空自耗重熔、大锻压比锻造的工艺路线,研究了真空处理及大锻压比锻造对化学成分、气体含量、夹杂物分布、二次枝晶间距及碳化物颗粒度的影响。研究结果表明,真空感应熔炼过程(VIM)中,随着铝含量的增加,碳的脱氧能力大幅降低,即使铝质量分数为0.003%也对碳的脱氧能力有明显的阻碍作用;真空自耗重熔过程(VAR)由于高的真空度、高的重熔温度等热力学条件以及反应动力学条件的改善,氧含量显著降低,第一次自耗重熔后氧质量分数从0.001 49%降低至0.000 57%,降低了61.7%,第二次自耗重熔后氧质量分数降低至0.000 50%。真空感应熔炼、真空自耗重熔过程,夹杂物的成分变化不大,主要以Al-Si夹杂为主,其次为Al₂O₃夹杂,再次为MnS夹杂、Mg-Al-Ca、Mg/Ca-Al夹杂。双真空冶炼后,钢中夹杂物主要为0～5 μm的细小夹杂物,未发现大于20 μm的夹杂,含有少量10～20 μm的夹杂,钢的洁净度大幅度提高。在真空自耗锭横断面上,从边部向芯部二次枝晶的形貌变化不大,二次枝晶间距逐渐增大,但是变化趋势缓慢,二次枝晶间距为85～95 μm,这主要得益于低的自耗重熔速度。对真空自耗锭进行大变形处理,最终锻造成40 mm的圆棒,碳化物颗粒的最大尺寸不大于20 μm,平均尺寸为15 μm,且没有碳化物聚集的现象。低的自耗重熔速度和大锻压比锻造是碳化物细化的关键。