LUBRICATION METHOD FOR ROTARY-PRESS TOOLS

Abstract

A new method for lubricating the tools of a pelletizing press is described, based on the use of dry lubricants mixed with polystyrene balls as a carrier. The solid lubricant is introduced into the die, applied to the walls of the compacting chamber by the movement of the punches, and then ejected following the normal pelletizing cycle. Hence, a very thin film of lubricant remains on the walls and makes it possible afterwards to pelletize a uranium dioxide powder. The process has been applied to a double rotary press in which one half is used to “pelletize” the lubricant and the other half to obtain the required production of UO₂ pellets. Pellets have been fabricated from a loose virgin powder which had not been granulated.

The process does not lead to contamination of the UO₂ pellets because the deposit of lubricant is very thin and uniform. It is applicable whatever the shape and the dimensions of the pellets may be, owing to the good flow rate of the polystyrene balls. Moreover, the polystyrene compact splits as soon as the applied pressure is released since the balls are elastic; a closed cycle can therefore be operated. Other advantages of the process are also described. The reproducibility of the UO₂ pellets fabricated by this technique indicates its efficiency.     

Study of the reduction of UO2 by magnesium or calcium dissolved in molten chlorides

The reduction of solid UO₂ to uranium by magnesium or calcium dissolved in their molten chlorides has been studied. The rate of reduction per unit area of UO₂ surface, at constant temperature and concentration of reductant in the molten chloride, was found to increase with time to a constant value. The rate of reduction per unit area was observed to be proportional to the concentration of reductant in the molten salt. The small increase observed in the reaction rate over the temperature range 750° to 850°C, suggests that the reduction is controlled by transport of the reductant to the reaction site. Solidified salt, containing UO₂ pellets which had been partially reduced, was sectioned, polished, and examined microscopically. The products of the reduction reaction form concentric layers around the UO₂ pellets. Layers of metallic uranium and oxide containing small amounts of dispersed salt alternated with layers of salt containing small amounts of metallic uranium and oxide. The layers ruptured, presumably because the volume of the products, uranium and oxide, is greater than the volume of the UO₂. Therefore, an impervious layer did not form on the oxide surface to inhibit the reduction reaction.     

AN INVESTIGATION INTO METHODS FOR THE PARTICLE-SIZE ANALYSIS OF UO2 POWDER AND THE EFFECT OF BALL-MILLING ON SIZE DISTRIBUTION

Abstract

Techniques for the particle-size analysis of “ceramic-grade” UO₂ in the 4–40 μm size range are assessed, with a view to selecting the most suitable method for analysis of UO₂/PUO₂ mixtures under glove-box conditions.The β-backscatter method is considered to show some advantages over the Coulter counter or optical microscopy and the mean coefficient of variation is ± 1.2% at the 1σ level.

The effects of various forms of pretreatment, e.g., high-speed stirring, manual mixing, and ultrasonic shaking, are examined and discussed with relation to the agglomerative properties of UO₂. Ultrasonic shaking is the most successful form of dispersion and the necessity of standardizing the pretreatment stage is emphasized.

β-backscatter is used to compare particle-size distribution after wet and dry ball-milling. Whilst little difference is observed in the ultimate particle size produced by the two treatments, the effect of agglomeration in dry milling is clearly shown. The increased effectiveness of milling UO₂ powder in a suitable dispersing medium, rather than water, is demonstrated.     

Influence of Lubricants on Dimensional Changes and Mechanical Properties of Sintered Ferrous Compacts

Abstract

The influence of admixed zinc stéarate on the shrinkage of uniaxially pressed iron powder compacts has been studied. For pressing conditions which caused inhibition of compaction the removal of the stéarate during sintering produced an increase in shrinkage parallel to the pressing axis and in direct proportion to lubricant content. Additions of stearic acid (varying particle size), zinc stearate, lithium stearate, stearamide, and Cosmic 64 wax were used to investigate the influence of lubricant on mechanical properties of green and sintered iron powder compacts. Green strength was reduced relative to unlubricated material only by lubricants whose physical and chemical properties enabled them to produce and maintain extensive interparticle films during pressing. Vapour from the rapid initial decomposition of lubricants which reduced green strength could have a deleterious physical influence on the tensile strength of dewaxed or sintered Fe compacts. Decomposing lubricants also produced undesirable chemical effects. These arose from reactions between lubricant decomposition products and the matrix or by these products interfering with reactions between matrix and sintering atmosphere.     

THE FABRICATION AND PROPERTIES OF URANIUM OXIDE–IRON CERMETS

Abstract

Cermets of iron–uranium dioxide, containing up to 30 vol.-% UO₂ have been rolled to rod. Although the cores rolled centrally along the length, the core was distorted in section owing to stiffening of the core relative to the sheath as the temperature dropped during rolling. The UO₂ particles were fragmented during rolling and formed as “stringers” along the direction of rolling, seriously weakening the structure transversely.

By extrusion at 1200° C., cermets containing up to 50 vol.-% UO₂ were satisfactorily fabricated. The extrusion pressure increased sharply above 35 vol.-% of ceramic and when large reductions in area were taken. The extrusion characteristics do not conform to isotropic plastic extrusion nor to a pure viscosity effect; the influence of the dispersed particles on metal flow is explained by a strong interference effect between particles and the metal during flow. The properties of the extruded rods were good; the core density was 90–95% of theoretical, decreasing slightly with increasing ceramic content; there were no obvious directional properties in the core, nor was the ceramic fragmented as in rolling. The characteristic “tail-end defect” of the extrusions was partly corrected by using shaped sealing plugs.     

THE PRODUCTION OF CERMETS CONTAINING A RELATIVELY LARGE AMOUNT OF DISPERSED PHASE

Abstract

The preparation and related properties of dispersions of ceramics in metallic matrices are reviewed, with particular reference to recent British work in this field. There have been substantial advances in the fabrication of cermets by powder metallurgy, particularly those based on UO₂ or PUO₂ and stainless steel, it being now possible to manufacture almost perfect dispersions in bar or strip form. Some closer understanding of the mechanical properties of cermets or porous matrices has been achieved. These theoretical and technological developments are capable of wide application to non-nuclear materials.     

Reduction swelling behaviour of haematite/magnetite agglomerates with addition of MgO and CaO

Abstract

The influence of three kinds of CaO and MgO additives (dolomite, burnt lime and serpentine) on the reduction swelling behaviour of haematite–magnetite (H–M) concentrates pellets was studied. Burnt lime and dolomite increased the reduction swelling index of H–M oxidised pellets, while the reduction swelling index was able to be reduced when serpentine was added. CaO accelerated the formation and growth of metallic iron whiskers and led to abnormal swelling of the magnetite briquettes, while MgO was able to be dissolved in wüstite and reduced the migration rate of Fe²⁺; therefore, there was no catastrophic swelling in either the haematite or magnetite briquettes. As far as H–M concentrate pellets were concerned, because the solubility of CaO in magnetite was greater than that in the primary haematite and the secondary haematite generated from magnetite during the oxidation was easy to be reduced to wüstite, there was abnormal swelling in the reduced H–M pellets with CaO addition.     

FACTORS CONTROLLING THE SHAPE REGULARITY OF SINTERED U02 PELLETS

Abstract

As explained mathematically, the magnitude of the ‘diabolo’ effect in sintered nuClear-fuel pellets is a function of the slope and position of the sinterability curve and the density gradient existing in the green pellets. If sinterability is representedby a straight line passing through the origin, the diabolo effect is negligible; this condition can be fulfilled when using a deactivated powder.

In any other case, the density gradient must be kept as low as possible to produce sintered pellets of regular shape. This is discussed as a function of powder roughness, lubricant quality, lubrication technique, and compaction characteristics.

When the sinterability curve cannot be adapted and powder quality and lubrication conditions are fixed, the possibility still exists of minimizing irregularities in sintered diameter by using a single-action press and compacting the powder in an adapted conical die with a moving lower punch.     

Electric arc furnace dust-coal composite pellet: effects of pellet size,dust composition,and additives on swelling and zinc removal

Abstract

Electric arc furnace (EAF) dust-coal composite pellets were heated from room temperature to 1423 and 1468 K under flowing argon by means of two heating patterns (non-isothermal tests). Apparent volume variation, compressive strength after heating, and zinc removal efficiency were evaluated, the last as a function of the additives used in the present work. A decrease of pellet size (from 14 to 7 mm in diameter) as well as the presence of Portland cement contributed towards avoiding abnormal swelling caused by growth of iron whiskers, and, as a consequence, there was no significant decrease of compressive strength at ~1323 K. At 1397 K, highest zinc removal was obtained for pellets with 12 wt-%KCl, and reasons for this result are proposed.     

CONSIDERATIONS ON THE PRACTICAL EFFECTS OF LUBRICANTS AND BINDERS COMMONLY USED IN COMPACTING METAL POWDERS

Abstract

Data acquired from practical industrial experience with several commonly used compacting lubricants have been investigated from the viewpoints of: (1) thermal decomposition in relation to sintering conditions (atmospheres, vacuum, temperature); (2) influence on the compactibility of iron powder (improved density) and the reduction of ejection force.

On the basis of the results obtained, the paper concludes with several points of guidance regarding the use of lubricants in iron powder metallurgy.     

Recovery of metallic iron from high phosphorus oolitic hematite by carbothermic reduction and magnetic separation

Abstract

This study aims to provide theoretical and technical basis for economical and rational use of high phosphorus oolitic hematite. Following physical, chemical and microscopic characterisation of high phosphorus oolitic hematite ore the feasibility of separation of phosphorus and metallic iron by reduction roasting and magnetic separation process were investigated. The results indicate that such a process is a feasible and efficient method for iron and phosphorus separation of high phosphorus oolitic hematite. The recovery of metallic iron and dephosphorisation rate is relatively low without additives but is significantly improved by appropriate CaO and Na₂CO₃ addition. With 8%CaO and 3%Na₂CO₃ the recovery of metallic iron and dephosphorisation rate reach 95.1 and 94.0% respectively.     

LUBRICATION EFFECTS IN THE COMPACTION OF SPONGE-IRON POWDER AT LOW AND HIGH SPEEDS

Abstract

The effects of the amount and method of lubrication have been investigated when compacting Höganäs sponge-iron powder, NC100-24, at both low and high speeds. Pressing characteristics, ejection loads, and the final properties of the sintered compacts were markedly affected by both the amount and the method of lubrication. From the results obtained, an optimum amount of admixed zinc stearate is recommended for both low and high-speed conditions.     

润滑剂对Fe基粉末冶金材料温压工艺的影响

通过扫描电子显微镜观察和性能测试研究了硬脂酸锌、乙烯基双硬脂酰胺(ethylene bis stearamide,EBS)、复合润滑剂以及压制温度对Fe基粉末冶金材料温压工艺的影响规律。结果表明:当润滑剂加入量(质量分数)超过0.4%后,Fe基粉末的流动性和松装密度均随润滑剂加入量的增加而降低,其中加入单一EBS润滑剂的影响更大。添加润滑剂后增加了Fe基粉末冶金生坯的致密度,其中添加硬脂酸锌和复合润滑剂的Fe基粉末冶金生坯断口颗粒间结合更为紧密。润滑剂对提高Fe基粉末冶金试样生坯密度、烧结密度及抗弯强度的作用顺序为复合润滑剂硬脂酸锌EBS,Fe基粉末冶金材料的密度和力学性能均随温压温度的升高而增加。在最佳润滑剂加入量0.4%时,120℃温压Fe基粉末冶金试样密度比室温压制Fe基粉末冶金试样的密度提高了0.14~0.21 g/cm~3,硬度和抗弯强度提高了40%~65%。     

PILOT STUDY OF PREFORM DESIGN FOR SINTER FORGING

Abstract

An important difference between sinter forging and conventional forging is that of consolidation of the PM preform. It therefore follows that sinter forging would seem to be a hot-compaction process with some bulk metal plastic flow. The permitted amount of plastic flow is considerably less for a PM preform than for a preform produced from wrought material of equivalent composition, since PM preforms exhibit poor ductility and are therefore more prone to cracking. The purpose of this study is to produce some guidelines to assist in designing preforms for sinter forging and to attempt to minimize the ‘trial and error’ approach. This has been undertaken by the production and study of a component suitable for PM forging, which at present is produced by conventional forging.

The lateral flow of metal was observed and investigations undertaken as to how density variations across the preform might assist the consolidation process. An iron powder containing 1% graphite and 1% zinc stearate was used in the production of components.     

润滑剂对温压FeSiAl磁粉芯性能的影响

采用温压成形工艺将水雾化Fe Si Al粉末制备成磁粉芯;用X射线衍射对原始粉末和经过绝缘包覆及热处理的粉末进行物相分析;采用软磁交流测试仪测量磁粉芯的磁损耗;利用精密磁性元件分析仪测量样品的磁导率。混合不同质量分数的硬脂酸锌和聚乙二醇(PEG)作为温压润滑剂,并研究其对Fe Si Al磁粉芯性能的影响。结果表明,1 100 MPa/100℃的温压成形条件下,当硬脂酸锌和PEG质量比为2:3,添加温压润滑剂的质量分数为1.3%时,磁粉芯生坯密度达到最大值5.75 g/cm3,热处理后为5.74 g/cm3。660℃×1 h热处理后,100 k Hz下,相应的有效磁导率?e达到137.9;350 k Hz/50 m T下磁损耗Ps为81.78 W/kg。     

DIMENSIONAL AND MICROSTRUCTURAL ANALYSIS OF COEXTRUDED UO2-STAINLESS STEEL CERMET PINS

Abstract

Coextrusion has been used to prepare UO₂ (35 vol.-%)-stainless steel cermet fuel-pins (5·6 mm dia. × 1000 mm long), which could be used as fuel elements in fast-flux test reactors. The procedure required to obtain a nearly homogeneous distributionof the fissile phase in the matrix is described. The influence of billet design on the dimensions of the fuel core and cladding is analysed with a view to its optimization. The extent of particle deformation as a function of extrusion parameters has been measured quantitatively by means of image analysis on particles having different (12·7%; 5·8%) inner porosities. Thermal expansion and conductivity as well as tensile strength of the cermets are reported and compared with existing data.     

Reactive hot pressing of nonstoichiometric uranium dioxide

High density UO_2+x pellets have been produced by reactive hot pressing uranyl oxalate at temperatures up to 700°C. Rapid densification occurred during the decomposition reactions resulting in densities in the range 90 to 92 pct of the theoretical. A density of 98 pct of the theoretical value was achieved by further hot-pressing at 650° to 700°C for 30 min. This densification behavior can be related to the nonstoichiometry and submicron sized particles of UO_2+x produced in the decomposition reactions. The kinetics of hot-pressing of powder compacts of this UO_2+x were studied in the temperature range 500° to 700°C. The results were analyzed utilizing models proposed by Fryer. The activation energy of 53 kcal per mole, obtained from this analysis is the same as that for creep of nonstoichiometric urania in the temperature range 975° to 1400°C, suggesting that the mechanism controlling the rate of the final stage of densification may be a creep process.     

Uranium use in CANDU reactors—review and prospect

Abstract

A co-operative governmentindustry development program launched in the late 1950s has generated in Canadian industry the capability to produce high-quality uranium oxide fuel bundles at all-inclusive prices of less than $50 per kg U. The direct consequence is that fuel cycle costs in heavy-water-moderated CANDU power reactors are the lowest of any reactor type now in operation. Even in competition with fast breeder reactors the CANDU systems are likely to retain the lowest fuelling costs for years to come.

The Canadian requirements for sintered UO₂ pellets will grow by nearly tenfold to some 1000 tons per year during the 1970s. Within that period, however, several alternative fuel cycles may offer stiff economic competition for natural UO₂ once-through fuel. These include (a) the ‘valubreeder’ cycle which is based on a mixture of uranium and thorium; (b) natural uranium silicide, U₃Si; and (c) plutonium recycle. All three alternatives become more attractive relative to natural UO₂ as uranium prices rise.

Résumé

Un programme de développement organisé conjointement par le gouvernement et l'industrie dans les dernières années 1950 a donné à l'industrie canadienne la compétence voulue pour fabriquer des faisceaux combustibles de haute qualité à un prix global inférieur à $50 par kilogramme d'uranium. La conséquence directe est que le coût du cycle de combustible des réacteurs CANDU modérés par eau lourde est inférieur à celui de tous les types de réacteurs actuellement en service. Même en concurrence avec les réacteurs surgénérateurs la ftlière CANDU continuera probablement d'avoir pendant des années le cycle de combustible le plus économique.

Au cours de la prochaine décennie, les besoins du Canada en matière de pastilles d'UO₂ augmenteront d'environ 1000 tonnes par an. Au cours de cette période, cependant, plusieurs autres cycles de combustible pourraient concurrencer le combustible à passe unique qu'est le bioxyde d'uranium naturel. Il y aurait, entre autres: a) le cycle quasi-surgénérateur employant un mélange d'uranium et de thorium; b) le siliciure d'uranium naturel, U₃Si; et c) le plutonium recyclé. Ces trois possibilités gagneront de l'intérêt par rapport au bioxyde d'uranium naturel à mesure que le prix de l'uranium augmentera.     

DIRECT REDUCTION OF MIXED MAGNETITE AND COAL PELLETS USING INDUCTION HEATING

A low-temperature process for iron-making, using mixed pellets of magnetite and domestic coal with an organic binder was investigated. Reduction tests performed in an induction furnace resulted in a product direct reduced iron (DRI) containing 75–80% metallic iron. The coal energy consumption of 23.71?GJ/ton DRI compares favorably with current DRI processes, although the total energy consumption of 33.65?GJ/ton is relatively high for iron-making. The use of low-grade coal as a reductant may improve the economics of the process as high-grade coking coal supplies dwindle worldwide.     

FRICTION COEFFICIENTS BETWEEN IRON POWDER COMPACTS AND DIE WALL DURING EJECTION USING VARIOUS ADMIXED ZINC STEARATE LUBRICANTS

Abstract

Laboratory compaction and ejection studies have been made using a reduced iron powder mixed with a number of zinc stearates having median particle sizes between 4 and 22μm. Comparable experiments were carried out on a fully instrumented production press, which was operated at compacting pressures between 300 and 500 MN/m² to produce compacts with true densities ranging from 5·90 to 6·70 g/cm³. Determination of ejection forces by the two methods enabled calculations of the coefficients of friction between compact and die wall to be made for mixtures containing 0·5–2·0 wt.% zinc stearate. These showed that the behaviour during compaction and ejection was comparable on both laboratory and production scales and gave very similar results. An interpretation of the results is given and values of coefficients of friction are presented which show that these are dependent on the type of zinc stearate used.