A model for predicting the thermal conductivities of bentonite-bonded molding sands at high temperatures

The effective thermal conductivities of bonded molding sands vary with the dry density, binder content, initial moisture content, temperature as well as the types of sand and binder clay. In this study, a theoretical model for predicting the effective thermal conductivities of bentonitebonded molding sands was developed. The results of measurement of the effective thermal conductivities of molding sands at temperatures up to 750°C were used. The binder thermal conductivities of both western bentonite and southern bentonite were suggested as a function of dry density, binder content and initial moisture content and were assumed not to vary with temperature. The radiation model proposed by Vortmeyer was also incorporated. The model developed in this study was proved to predict well the effects of binder content, initial moisture content, dry density and temperature.     

A model for prediction of the effective thermal conductivity of granular materials with liquid binder

A model is developed to predict the effective thermal conductivity of a three-component system containing sand particles bonded with liquid binders. The effective thermal conductivities of the bonded and unbonded sands are measured by the line-heat-source method at room temperature. The parameters of a two-component model are determined from the measurements for unbonded sands. Finally the model for the three-component systems such as bonded sand is developed using the assumption of the coalescence of the liquid binder at the particle contact points. Comparison between the experimental results and the model predictions shows that the model developed in this study predicts well the effective thermal conductivities of fluid-saturated sand or sand bonded with liquid binders. Also, this model can be used to determine the effects of the parameters such as the thermal conductivities and the volume fractions of components and the geometrical characteristics of solid particles on the effective thermal conductivity of granular materials with or without the liquid binder.     

防止铸钢粘砂的黄壤土型砂性能与工艺研究

运用黄壤土型砂的烧结产生可剥离烧结层,可以有效防止铸钢件表面的粘砂类缺陷,提高铸钢件表面质量。调整黄壤土型砂的石英砂质量分数及粒径,进行不同温度下的烧结试验,研究适合不同温度下烧结的型砂配比。通过模拟烧结层试样的高温与室温力学性能测试,研究烧结层力学性能。依据型砂烧结试验和烧结层力学性能试验结果,将黄壤土和粒径为0.15～0.30 mm(50/100目)石英砂以3∶7配制的黄壤土型砂运用在铸钢件浇注试验,发现砂型与铸件界面产生一层致密的烧结层,并且在室温下可以轻易地从铸件表面剥离,获得表面光洁的铸钢件。研究表明,调整黄壤土型砂的成分可以调节型砂耐火度,使其满足在不同浇注温度下,在铸型—铸件界面处生成适度烧结的烧结层。该烧结层在高温下呈现良好塑性,粘附在铸钢件表面,抵挡金属液的冲刷和渗透;当铸钢件冷却至室温时,烧结层呈现出明显的脆性,可以很容易地剥离铸钢件表面,从而极有可能防止铸钢件的粘砂类缺陷,获得表面质量高的铸钢件。     

型砂对空气冲击造型砂型紧实效果的影响

使用空气冲击造型机（砂箱尺寸：600mm×450mm×390mm）和空气冲击制样机（样筒尺寸：φ50mm×120/250mm）研究了型砂对空气冲击造型砂型紧实效果的影响。砂型湿压强度随型砂紧实率变化过程中在型砂紧实率为30%左右时有一极大值。随膨润土加入量的增加砂型湿压强度增加,但在膨润土加入量较低时,强度增加的幅度较大;膨润土继续增加,强度增加的幅度减小。煤粉和重油加入量增加,淀粉加入量减少以及死粘土含量增加,均使砂型湿压强度增加。对上述试验结果进行详细的分析,并结合空气冲击造型的特点,推荐空气冲击造型型砂选择的一些基本原则。     

型砂的高温力学性能及应力-应变关系

本文对国内中、大型铸钢件生产中常用的几种型砂的高温力学性能进行了实验研究。通过测取型砂在不同温度下单轴压缩的应力-应变曲线,求得型砂在不同温度下的形变模量E,抗压强度σ p以及型砂软化后的粘度系数φ。通过测定试样的破坏角求出型砂的摩擦角ψ和粘聚力C。应用实验中获得的力学参数,建立了型砂在高温下的弹塑性或粘性的应力-应变模型。     

型砂砂粒间粘土膜的流变特性

用一台自己设计的仪器对型砂砂粒间粘土膜进行了测试。结果分析显示：粘土膜的流变特性可以用宾汉体－开尔文体流变模型，亦即Ｂ－Ｋ模型表达；同时观察到，在其变形过程中有应力松弛及触变现象。文中讨论了它们在型砂紧实过程中可能起作用的程度。     

Powder injection molding 440C stainless steel

In this work, the processing steps for producing 440C stainless steel parts by means of powder injection molding technique were investigated. The molded specimens were debinded by solvent debinding followed by thermal debinding methods and were sintered under vacuum atmosphere. Effective densification took place in the temperature range 1,230–1,240°C in the sintering. After heat treatment, specimens sintered at 1,240°C for 30 min had the tensile strength of 876.3 MPa, the hardness of 57.7 HRC. Pitting mainly occurred in injection molding 440C stainless steel specimens in NaCl solution. The content of carbon has serious effect on the shape retention. Some methods, such as preventing from oxidation, are presented to avoid the as-sintered specimens from deformation.     

TiH2粉注射成形用新型粘结剂的研制及其性能

研发了一种适合TiH2粉注射成形用新型石蜡基粘结剂,对粘结剂组元间的相容性进行了计算,并分析了该粘结剂的流变性能和脱脂性能。结果表明:该新型粘结剂各组元相容性好,能够与TiH2粉实现良好混合,装载量(体积分数)达到70%以上;粘结剂流变性能佳且稳定,在注射温度为160℃和剪切速率为125s-1的条件下,黏度为0.081Pa.s,粘流活化能为36.85kJ.mol-1;与常规石蜡基粘接剂相比,在同样工艺条件下使用正庚烷脱脂时相同时间下脱脂率提高20%～30%;脱脂坯缺陷少、保形性好;与原料粉相比,经过热脱脂后坯样的碳含量仅增加了0.02%,氧和氮含量均降低。     

Tribological performance of A206 aluminum alloy containing silica sand particles

The dry-sliding tribological behavior of A206 aluminum alloy containing silica sand was investigated using a three pin-on-disk tribometer against an SAE 1045 steel counterface. The worn surfaces of the pins were then analyzed by optical microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy (EDS). The test results showed that the addition of silica sand particles decreased the friction coefficient of Mg modified A206 alloy. The wear rate of the composites increased with increases in the applied pressure from 0.35 to1.75 MPa and with increases in the silica sand content from 0% to 13%. The wear rate variation with the applied pressure is attributed to the shift in the dominant wear mechanisms from oxidation and mild abrasive wear at applied pressures at and below 0.35 MPa to delamination accompanied by severe abrasive wear at applied pressure levels above 0.35 MPa. The high wear rate may be as a result of an overall decrease of the fracture toughness of the composites containing silica particles. The temperature near the counterface surface increased with increases in both silica content and the applied pressure due to the lower thermal conductivity of silica sand and greater abrasion that occurs at higher silica contents. A T6 heat treatment did not significantly decrease the friction coefficient or the wear rate of either the A206 matrix alloys or the composite containing silica sand.     

Oscillating sliding wear behaviour of SiC,TiC, TiB2, 59SiC–41TiB2 and 52SiC–24TiC–24TiB2 materials up to 750°C in air

The tribological behaviour of different monolithic and composite ceramics was evaluated in the temperature range between room temperature and 750°C. The test method was oscillating sliding with a ball‐on‐disk arrangement in an SRV machine. Alumina balls were used as counter body. The friction behaviour was determined on‐line, and the wear behaviour was determined from calculations on the basis of wear scar dimensions and profilometric measurements. The friction depends on temperature and shows an increase for most materials for increasing temperature; the smallest friction at all temperatures is found for monolithic TiC. The wear behaviour shows different trends for the different materials. In tests against SiC a maximum of wear is found at 500°C, for TiC at 200°C and for TiB₂ at 750°C. The composite ceramics suffer the smallest wear of all materials in the range from 200°C to 500°C. Copyright © 2006 John Wiley & Sons, Ltd.     

Effects of silica addition on the particle characteristics and phase transition of flame-synthesized γ-Al2O3 nanoparticles

The effects of silica addition on the thermal stability of flame-synthesized γ-Al₂O₃ nanoparticles are investigated in this study. Based on the electron microscope images of pure alumina, nanoparticles with diameters ranging from 20 nm to 30 nm were synthesized. No significant changes were observed after heat treatment at 900°C, 1100°C, and 1180°C. After heat treatment at 1260°C, sintered particles larger than 100 nm were observed. All images of the nanoparticles doped with silica, including those taken after heat treatment at 1260°C, showed similar particle sizes regardless of treatment temperature. Based on the X-ray diffraction patterns and specific surface areas, the onset temperature of the phase transition to α-Al₂O₃ for the flame-synthesized pure alumina was near 1180°C. The addition of silica greatly increased the thermal stability of the synthesized γ-Al₂O₃ nanoparticles.     

Assessment of the erosion resistance of steels used for slurry handling and transport in mineral processing applications

The wear environment of steels used for containing, transporting and processing erosive mineral slurries is often such that fluid borne particles form a layer moving at high speed across the wearing surface. Information on the performance ranking of such materials is limited, particularly with respect to the influence of steel hardness and microstructure on the resistance to erosion. This is particularly important for the oil sands industry of Northern Alberta where handling and processing of essentially silica-based solids results in extremely severe wear conditions. This paper presents slurry erosion data obtained on 11 commercially available wear resistant plate and pipeline steels with hardness values up to 750 HV. These data were obtained using a Coriolis erosion tester operated at 5000 rpm with an aqueous slurry containing 10 wt.% of 200–300 μm silica sand particles.

The Coriolis erosion tester was selected because it provides a low-angle scouring action that simulates the erosive conditions encountered in oil sands and tailings pipeline transport and in some related processing operations. Results show that this test method is able to discriminate clearly between the erosion resistance of these steels, expressed in terms of specific energy (the energy necessary to remove unit volume of test material), with the most erosion resistant steel being more than five times superior to the least resistant. A graphical relation between steel hardness and erosion resistance is given. A comparison is also made between slurry erosion data and the performance of the materials in the ASTM G65 dry sand rubber wheel (DSRW) sliding abrasion test. Comments on the influence of the macro- and microstructures of the steels on their wear behaviour are included.     

基于两级闭环控制的型砂水分在线测控系统

提出了一种基于两级闭环控制的新型型砂水分控制系统,阐述了温度补偿系数的设定方法.该系统以单片机为控制核心,通过检测旧砂温度和含水量控制砂冷却系统的加水量,以实现旧砂含水量的初步控制;通过测量检测冷却后的旧砂含水量,控制混砂机的加水量.系统采用了自行研制的水分传感器实现型砂水分的在线检测,选用了美国Dallas公司最新推出的DS18B20数字式温度传感器检测砂温及环境温度,以实现系统的温度补偿.本系统具有运行可靠,灵敏度高,控制精度高等特点.     

Vacuum Tribological Properties of Titanium Enhanced via Ultrasonic Surface Rolling Processing Pretreatment and Plasma Nitriding

For the purpose of enhancing the vacuum friction-reducing and antiwear performance of UNS R50400 Ti, ultrasonic surface rolling processing (USRP) has been introduced as pretreatment of plasma nitriding (PN). Various nitriding temperatures from 650 to 950°C have been adopted to investigate the effects of USRP pretreatment on plasma nitriding of Ti. The results of experiments show that the friction-reducing and antiwear performance are enhanced by USRP pretreatment when the PN temperature is below 750°C, resulting from the formation of thicker and harder nitriding layer with a reasonable gradient structure. Nevertheless, USRP pretreatment results in a rough surface and a large-sized columnar crystal gradient structure when the PN temperature is above 750°C, leading to poor vacuum tribological performance. In aggregate, USRP pretreatment is an effective pretreatment to decrease the optimized plasma-nitriding temperature from 850 to 750°C.     

<Emphasis Type="Bold">Laser Sintering of Silica Sand – Mechanism and Application to Sand Casting Mould</Emphasis>

Silica sand is commonly used in the foundry industry. With a high melting point of 160°C, the silica sand is normally sintered in a high-temperature furnace. However, silica with contents of calcium, aluminium, magnesium, and chlorine, etc. can form low-melting point eutectics. Therefore, a relatively low-power laser can be used to sinter the silica sand directly. The investigation of the mechanism and process for direct laser sintering of the silica sand, without any binder, is presented in this paper. Combined with rapid prototyping (RP) technology, the laser sintering of the silica sand can be used to directly fabricate a sand casting mould, called a rapid sand casting mould. By avoiding the time-consuming process of fabricating a pattern, the rapid sand casting mould process has the potential of further reducing the lead time for producing a casting product. Some important issues, such as the lead time of producing a sand mould, its accuracy, and surface finish, etc., are discussed.     

型砂质量在线检测与控制模型的研究

型砂质量的在线控制基于两大基本前提:1.实现型砂性能的自动检测;2.寻找并建立型砂成分与性能之间的相互关系.为此,常规试验中既能反映型砂成分变化又适于在线快速检测的紧实率、水分、砂温、透气性与壁裂强度五性能被选作为在线检测项目.文中详细讨论了型砂性能在线检测仪的研制过程、诸项目的检测原理以及仪器所能达到的检测精度.由于采用了80C552微处理器的控制,本仪器可在1min内完成全部五项目的检测与结果的打印.以此为基础,本文进一步探讨了利用人工神经网络(ANN)原理建立型砂制备这一非线性系统中成分与性能相互关系的原理与具体实现过程.研究表明,采用由非线性变换单元组成的前馈网络(B一P网络)方法建立的型砂成分与性能关系的网络模型(CPN),可用于型砂的预测,且预测精度高于由传统的回归方法建立的模型,从而为实现型砂质量的在线控制打下了基础.     

铸造用型芯砂无机粘结膏的制备试验

介绍了一种用水玻璃为粘接剂的铸造用型芯砂无机粘结膏的研制试验。以膨润土为悬浮剂,测试了在石英粉、石墨粉和滑石粉等不同耐火填料情况下粘接膏的悬浮性能,并用水玻璃砂8字试样测试了各类试验无机粘结膏的粘接强度。     

铸件鼠尾形成机理的分析

鼠尾的形成是由于铸型型腔局部表面干砂层受热膨胀，当膨胀力大于滑移阻力时，干砂层产生了剪切滑移。滑移的阻力主要来自相邻的干砂层。阻力随干砂层长细比的增加而急剧减小。提高型砂的热强度，减少干砂层的长细比，能有效增加鼠尾产生的临界应力σc，有效防止鼠尾的产生。当干砂层长细比小于1/2或热压应力不大于水分饱和凝聚区的湿压强度时，绝不可能产生鼠尾。     

Miscibility and stability of synthetic lubricants with HFC refrigerants

The phase-out of CFC production and further regulations on HCFC are required because of their association with the depletion of stratospheric ozone. HFCs and their mixtures have evolved as long-term replacements for CFCs and HCFCs. For air conditioning and refrigeration systems, new synthetic lubricants having miscibility with HFC refrigerants, similar to that of mineral oils with CFCs, have been developed. Data on the miscibility ofR-32/125 and R-125/143a with polyol ester lubricants are presented in this paper. In the temperature range of interest (−40°C to 60°C), we observed the upper immiscible region, lower immiscible region, as well as the coalescence of the two regions, for polyol ester lubricants with these HFC refrigerant mixtures. HFC-143a is least miscible among the three pure HFC refrigerants. The stability of R-32/125 and R-125/143a with dry polyol ester lubricants is very good at 204° C. With high moisture content, hydrolysis of the lubricant occurs at high temperatures. The onset of significant hydrolysis of lubricant (B) takes place between 175°C and 200°C. Care must be exercised to maintain the dryness of polyol ester lubricants and refrigeration systems to prevent this occurring.     

Microstructural,Mechanical, and Tribological Properties of Rice Husk–Based Carbon: Effect of Carbonizing Temperature

In this study, we investigated the microstructural, mechanical, and tribological properties of rice husk (RH)-based carbon carbonized at various carbonizing temperatures under dry conditions. All samples exhibited amorphous carbon structures and the X-ray diffraction spectra of the samples carbonized at 1300 and 1400?°C indicated the presence of a polymorphic crystals of silica. The hardness increased with temperature due to the densification of the structure and the presence of the hard crystalline silica. At low normal loads, the mean friction coefficient of the material decreased as the carbonizing temperature was increased from 600 to 800?°C and slightly decreased as the carbonizing temperature was further increased from 800 to 1400?°C. At the highest load, all samples, except for that carbonized at 600?°C, exhibited extremely low friction coefficients (around 0.05). The wear rates of the all samples were smaller than 10^?5 mm³/N·m, indicating that RH carbon exhibits sufficient wear resistance. A Raman spectroscopic analysis of the worn surface of a steel ball revealed that the transfer layer at 600?°C had a less graphitic structure compared to the other carbonizing temperature. Based on these findings, we recommend an optimal carbonizing temperature for applications of sliding materials exposed to dry sliding contact.