EP1085953A1

EP1085953A1 - A process for preparing a sintered article

Info

Publication number: EP1085953A1
Application number: EP99917804A
Authority: EP
Inventors: Ole Huusmann
Original assignee: DTI Industri
Current assignee: DTI Industri
Priority date: 1998-05-11
Filing date: 1999-05-05
Publication date: 2001-03-28
Also published as: AU3595599A; WO1999058269A1; NO20005644D0; DK173647B1; NO20005644L

Abstract

A process for preparing sintered articles on the basis of particles covered with a layer of water glass is disclosed. Neighbouring particles of the article adhered by means of a direct physical contact obtained by heating to a temperature equal to or above the sintering temperature of the particles. Said sintering of the article results in enhanced strength and improved resistance to water or steam.

Description

A process for preparing a sintered article

The present invention relates to a process for preparing a sintered article. Furthermore, the inven- tion concerns such articles as well as the use of same as a core or a mould for iron and metal moulding or cast moulding.

Articles containing particles adhered together by a binder are used within many technical fields of application. Especially when casting iron or plastic objects, such articles have been used as moulds or cores. Usually, particles of quartz sand and binders of water glass are used.

The use of water glass as a binder to adhere quartz sand was introduced in foundries about 1920. In said process, sand is mixed with an aqueous solution of water glass, whereupon the mixture is teemed into a mould and cured by water evaporation. Said process was substantially improved about 1950 when the CC process was developed. By said process, C0₂ gas is led through the wet mixture of sand and water glass with a view to cure the binder. Cores or moulds obtained by the C0„ process are relatively porous and their strength is poor. In the late 1960's, self-curing binder systems were developed. The curing takes place by adding an organic ester to the wet mixture of sand and water glass prior to the mould being teemed.

In the late 1970' s, a dry curing process was developed in which energy from hot air, a hot mould or microwaves bring about curing of the water glass binder .

In the early 1990' s, the present inventor developed a curing process in which hot air was blown into a ventilated mould by means of a three-stage tempera- ture cycle, cf. "Miljøprojekt nr. 189, Miljøstyrelsen" .

However, all above-mentioned processes, in which water glass is used as a binder, have the disadvantage that the flowability of the mixture of sand, water glass and water is relatively poor when the mould is teemed. In consequence, the mixture tends not to flow into all mould cavities. Accordingly, the prior art processes, in which water glass is used, are limited to application in connection with moulds not having a detailed surface.

Realizing the shortcomings of the prior art processes, the present inventor invented a process for the preparation of particles covered with a layer of water glass and articles to be prepared from such water glass covered particles. Said invention has been disclosed in International Patent Application No. PCT/DK97/00575 , which had not yet been published on the date of priority of the present application.

The articles obtainable by PCT/DK97/00575 have sufficient strength for many purposes. For instance, the articles are excellent for moulds or cores for metal casting. However, for certain fields of application enhanced strength or improved resistance against water or steam, also at higher temperatures, is desired.

In JP 4325473 it is suggested to use an aqueous solution of water glass as casting and sintering aid when preparing a porous sintered article based on alumina powder. After simply mixing aqueous water glass and alumina powder, an article is dry cast and subsequently sintered. It is suggested to use the sintered porous article as a filter. Thus, it is impossible to work a green article before the sintering. Therefore, it is the object of the present inven- tion to provide a process for the preparation of a sintered article having a high degree of details and processable before the sintering. A further object of the invention is to provide a process for the prepara- tion of an article having enhanced strength and improved resistance to water or steam.

This object of the invention is met in providing a process for the preparation of a sintered article comprises the steps of - providing particles covered with a layer of water glass in a mould, curing the particle mass into a green article, and heating the green article to a temperature equal to or above the sintering temperature of the particles so that a direct physical contact between adjacent particles is formed.

In the present description and claims, the sintering temperature is defined as the temperature at which the surface of the particles with no layer of water glass would start melting so that a melting together of the adjacent particles is obtained. Preferably, a temperature at least 10 °C above the sintering temperature, especially a temperature 10-40°C above the sintering temperature, is used. A temperature at least 10°C above the sintering temperature is assumed to facilitate the penetration of the melted metal into the water glass layer, thus enabling the sintering of the surface of adjacent particles. In the present description and claims, the term

"water glass" is to be understood as lithium, sodium or potassium silicates. In proportion to the silicate component (SiC ) , the lithium, sodium or potassium component (M-O, M = Li, Na or K) may be present in variable amounts. The ratio Si0₂/M„0 is designated the weight module. A water glass having a low weight module is easily soluble in water and strongly basic due to a high content of the basic component MLO. Correspondingly, a water glass having a high weight module is less basic and less water soluble. In the present invention it is preferred to use water glass having a weight module between 0.5 and 4, especially between 1.8 and 3.5, for the covering of the particles. Furthermore, it is preferred that M = Na . Unless otherwise specified, in the following, the term "water glass" refers to water glass in which M = Na .

The particles to be covered with a layer of water glass may be any material to which water glass can adhere and that can be sintered. Examples of such materials are metallic materials and ceramics. Examples of usable metallic materials are aluminium, copper, tin, iron, wolfram, chrome, vanadium, molybdenum and manganese. Examples of ceramics are SiC>₂, l₂0₃, Fe₃0₄, Fe₂C>₃, MnO, NiO, ZnO, Zr0₂ or TiO„ . Furthermore, the material of the particles may be a mixture of two or more of the above .

The size and size distribution of the particles is not essential when preparing the covered particles. For mould and core boxes, particles having a size between 0.05 mm and 2.0 mm, especially between 0.10 and 0.60 mm, are often used. For use in connection with castings requiring a high degree of details and a reproduction of the design as precise as possible, particles having a diameter of 1 to 100 μm may be used. The selected particles may have a broad or narrow particle size distribution or the particle mass may consist of so- called double-sieved particles (double-sieved sand) showing two peaks on the grain size curve.

The article to be sintered may have been in- filtrated beforehand. It is particularly preferred that the article has been infiltrated with water glass and/or a metal or metal alloy prior to the sintering. Said infiltration may also take place after the sintering. In case the article is infiltrated with water glass, the water glass is chosen with a view to ensure the desired solubility. Water glass having a maximum weight module of appr. 3 may be dissolved immediately by water contact at ambient temperature within a foreseeable time frame, whereas water glass having a weight module above approximately 3.0, e.g. a weight module between 3.0 and 3.5, is sparingly soluble in cold water. Usually, a durable article that does not dissolve or disintegrate at high air humidity is desired, possibly combined with a temperature above normal ambient temperature. Therefore, a water glass having a weight module between 3 and 4, especially between 3.0 and 3.5, is often chosen.

For instance, articles relatively easily soluble in water are infiltrated with a water glass having a weight module of 1.8 to 3.0.

In case the article according to the invention is infiltrated with metal or metal alloy, said metal or metal alloy may be randomly chosen from among known metals or metal alloys. It is preferred to use copper or a cupriferous alloy for the infiltration of the article according to the invention. Especially, it is preferred to use a bronze, e.g. containing appr. 90% by weight copper and appr. 10% by weight tin. Examples of applicable metals to be used either alone or combined as alloys are aluminium, antimony, bismuth, cadmium, calcium, cobalt, copper, gold, iron, lead, magnesium, manganese, mercury, nickel, silver, thallium, tin and zinc . The article prepared according to the invention may have any outer shape. Since the particles covered with a layer of water glass are very easily flowing when dry and easy to pound into all cavities of a mould when a little humid, articles may be prepared as casts of original models having a very detailed surface. For instance, the article according to the invention may be a positive or a negative cast of a model. A negative cast may be used as a mould for preparing a copy of the original model, whereas the positive cast per se represents a complete or partial cast of the original model .

When used for iron or metal casting or for die- casting of plastics, the article according to the invention may constitute a mould and/or a core. The article according to the invention is applicable for other purposes as well, such as catalysts or carriers thereof, granulate filters, spark electrodes, electrodes for electrolysis etc.

The particles covered with a layer of water glass are preferably prepared by

(a) providing a mixture of particles to be covered, water and 0.1-5% by weight water glass, calculated on the basis of the weight of the particles wherein the water glass is present in dissolved form,

(b) stirring the mixture mechanically, optionally simultaneously supplying heat from an external heat source, and allowing the water to evaporate until at least so much water has evaporated from the mixture that it is no longer sticky.

The amount of water present in the mixture in step

(a) depends on several factors, such as the specific surface, the porosity and the electrostatic nature of the particles, but, preferably, calculated on the basis of the weight of the particles to be covered, it is at least 0.1% by weight. Typically, an amount of 1-3% by weight is chosen to ensure adequate humidity of the particles. Preferably, the amount of water does not exceed 5% by weight since a higher water supply does not contribute further to the humidity of the surface of the particles to be covered.

The amount of water glass in the mixture comprising particles to be covered, water and water glass depends on factors such as the desired thickness of the layer and the specific surface of the particles to be covered. On the basis of the weight of the particles, 0.1-5% by weight water glass may be used. Generally, it is preferred to use 1-3% by weight.

The mixture of step (a) is preferably obtained by (al) mixing water and particles to be covered,

(a2) stirring the mixture so as to have the water evenly distributed in the particle mass, (a3) adding 0.1-5% by weight water glass to the mixture, and (a4) continuing the stirring until the water glass is evenly distributed and dissolved.

In step (a3) , the water glass is mainly added to the mixture in a solid form as it is preferred to use particulate solid water glass prepared by spray drying. It is expedient to keep stirring during the entire preparation of the particles covered with a layer of water glass. Said stirring is made mechanically, mainly using rotating blades. The stirring speed is adjusted so as to ensure that no cured lumps of particles are formed, which would have to be subsequently crushed.

Since the stickiness of the mixture tends to increase significantly during the water evaporation, it has proven practical to use a container of plastic for the mixture as water glass hardly sticks to plastic. The stirring is mainly performed so vigorously that the mixture is heated and the water thus evaporated. If desired, the evaporation rate may be increased by supplying heat from an external source and/or evaporation may take place in vacuum. If desired, the par- tides used may be preheated before being mixed with water and water glass. Said procedure is particularly advantageous if the water glass is sparingly soluble, i.e. has a weight module between 3.0 and 4.0, in which case the particles may preferably be heated to a temperature not exceeding 100°C, preferably 80-90°C, before being mixed with water and water glass.

It must be ensured that the mixing leaves no lumps of either sand or water glass before essential evaporation of the water is allowed. During the evaporation of the water from the mixture, the viscosity of same increases, and after a while the mixture turns sticky so that the particles tend to stick both to each other and to the apparatus used. The stirring must be adapted to said stickiness tendency so as to prevent a too strong mutual binding of the particles. When the water content of the water glass layer has dropped below the lowest level allowing mutual binding of the particles, the viscosity decreases again. The stirring may continue until all water has evaporated and the dry particles covered with a layer of water glass have been obtained, however, the humid and nonsticky covered particles may be removed at an earlier stage if desired. Unexpectedly, it has turned out that the flowability of such particles covered with water glass is excellent, both when they are dry and slightly humid, but not when they are sticky.

Presumerably, said flowability is obtained both as a result of the layer of water glass being smooth and hard and because the particles^' covered interact during the drying so that the covered particles obtained are rounder than the uncovered particles.

A green articles may be prepared by (c) providing the particles covered with water glass in a mould, (d) ensuring the presence of water for the activation of water glass in the particle mass, and (e) curing the particles covered with water glass in the mould into a green article by supplying energy from a source thereof . The particles covered with water glass may be provided in the mould in an arbitrary way. In a first preferred embodiment, loose particles are teemed into a mould, and the mould is afterwards being slightly vibrated to ensure teeming of all mould cavities in order to obtain a tight and homogenous stuffing. Such a vibration will make the small particles move towards the surface of the particle mass and thus increase the density of the surface of the green article. After the vibration of the loosely stuffed particles, under the influence of the particles an after-vibration at a suitable pressure may be carried out, e.g. using a plumb, to enhance a tighter particle stuffing.

In another preferred embodiment, the particles covered with water glass are provided in the mould by being blown into same by means of an airflow. The airflow escapes though mould valves, and the particles will be stuffed in the mould under the influence of the pressure of the airflow.

In yet another preferred embodiment, the particles covered with water glass are provided in the mould by being extruded into same by a process designated "impact moulding". In said procedure, the particles are pounded into the mould under the influence of a high pressure, e.g. obtained by suddenly released pressure air. 10

In yet another preferred embodiment, slightly humid particles covered with a layer of water glass are provided in a mould, whereby it is ensured that the covered articles fill out all cavities of the mould, e.g. by pounding the particle mass or vibrating the mould containing the particle mass as described earlier. Said process entails the advantage that the amount of water necessary for the activation is already present in the particle mass. The presence of water in the particle mass and the supplying of energy from a source thereof will activate the water glass so that a coherent green article is formed. Therefore, the presence of water for the activation of the water glass in the particle mass must be ensured. Said water may for instance be present in the form of crystallization water, it may be added as water vapour or the particles used may be humidified by a small quantity of water prior to the teeming of the mould, e.g. by 0.1 to 0.7% by weight water, preferably appr. 0.3% by weight. The energy source for the curing may for instance be a source of microwaves or high- frequency waves, hot air, convection heat or steam.

In a first preferred embodiment vapour is conveyed through the dry particles covered with water glass provided in a mould to activate the water glass layer. Subsequently, pressure air at a temperature of 160- 200°C is added to cause a further heating of the covered particles and an initial evaporation of the water. Then the temperature is lowered to 80-160°C to remove the water from the green article prepared.

Possibly, a pressure air temperature of 0-80°C may be used at last to cool the green article and the mould. During the humidification of the particles covered with water glass in the mould, it is important to make' sure that an essentially homogenous humidification of all 11 areas of the mould takes place, without the water glass being flushed off the particles.

In a variant of said embodiment, particles covered with water glass humidified with water, e.g. up to 0.7% by weight, are used instead of vapour humidified particles. The pressure of the pressure air and the duration of the various temperature periods vary, dependent on the amount of water used for humidification, the size of the green article, the amount of used water glass etc., and said time periods may be determined by the person skilled in the art by routine tests. When preparing a 10 kg article, in which the particles are quartz sand having a medium particle size of 0.30 mm covered with a water glass amount of 0.8% by weight (module 2.0) and humidified by a water amount of 1% by weight at an air pressure of 700 kPa, the typical duration periods of the various temperatures would be: 10 sec of pressure air at a temperature of 160-200°C, 30 sec at a temperature of 80-160°C and 20 sec at ambient temperature. When changing the temperature, it has turned out to be advantageous to maintain the same pressure to prevent the green article from breaking.

In another preferred embodiment, the particles covered with water glass are cured by means of micro- waves or high-frequency waves. The water necessary to activate the water glass may be present as crystallization water in the water glass layer, it may be added by using humid particles having a water content of for instance 0.1-0.7% by weight or be provided by conveying water vapour. As far as the latter option is concerned, it has turned out to be possible to obtain curing of an article by placing a mould containing particles covered with water glass in a microwave oven, whereby the mould inlet is facing a moist blotting paper. A third preferred embodiment relates to the use of 12 moulds receiving heat by convection, e.g. by placing the moulds in an oven, placing the moulds on a heating plate, or a mould with a heating jacket may be used. If the mould containing a moist particle mass is heat- treated in an oven, an oven having air circulation is mainly used in order to ensure a more homogenous distribution of the heat and thus a more homogeneous curing of the green article. The oven is mainly heated to a temperature of 130-200°C. The duration of the heat-treatment in the oven depends on the size and wall thickness of the green article.

No matter which of the three above-mentioned embodiments is used for the curing a green article, an article comprising a large amount of particles bonded together by a layer of water glass is obtained. Surprisingly, it has turned out to be possible to produce green articles that do not contract essentially during the curing so that an essentially precise cast of the mould is obtained. Said property is particularly advantageous when casting a model with a view to produce an article to be used as a mould for preparing essentially identical copies of the model.

Furthermore, it is possible to produce a green article as a cast of a model, making the size of the cast larger or smaller than the surface of the model by means of adjusting the temperature of the model. Said property is particularly advantageous if the model to be copied is a spare part from which an oversize cast is desired. It has turned out that a green article produced from metal particles covered with water glass is not conductive, which is an indication of the completeness of the layers around the particles. Contrary hereto, the article is susceptible to microwave energy. Before the green article is sintered, if desired, 13 it may be worked so as to achieve the desired shape and surface quality.

After the sintering of a green article prepared from metal particles, the article is conductive and insusceptible to microwave energy. This indicates that, unexpectedly, the metal melting from the surface of the metal particles is penetrating the layer of water glass .

Heating to sintering the green article usually takes place in two or more temperature steps to avoid too much internal stress of the article due to the temperature difference between the surface and the inner article. Alternatively, the heating may take place continuously. Too much internal stress of the article may result in crack formation. Therefore, it is appropriate to preheat the green article in order to obtain a so high core temperature that the green article does not crack and, subsequently, raise the temperature to or above the sintering temperature. The duration of the preheating step inter alia depends on the wall thickness of the article and the particles chosen and can be determined by a person skilled in the art after appropriate routine tests.

The duration of the sintering period depends on many parameters, e.g. how much the temperature is raised above the sintering temperature and to which degree contact is desired between the particles. The temperature and the duration of the sintering may be determined by a person skilled in the art after appro- priate routine tests.

The heating and sintering is mainly carried out in an inert or reducing atmosphere to prevent a possible oxidizing effect of the oxygen in the air.

The green article may be infiltrated before, during or after the sintering to increase the strength 14 or improve the surface texture. Before or after the sintering, the green article may be infiltrated by a process comprising the following steps contacting the green article or the sintered article with a solution of water glass so that part of the solution is absorbed therein, heat-treating the green article or the sintered article, wherein a solution of water glass has been absorbed so that the water glass is solid- ified, and, optionally, repeating the preceding steps in order to obtain an infiltrated article.

The solution of water glass is usually aqueous, which makes it less detrimental to health, but other organic or inorganic solvents, in which water glass is soluble, are applicable. Since the solvent of the solution is to be removed in the subsequent step, a solution having a relatively high content of water glass is preferred. A solution having water glass and water in the weight ratio 1:2 has turned out to be suitable. The water glass may have a high or a low weight module. In case a low water solubility of the article is to be obtained, water glass having a weight between 3.0 and 4.0, particularly 3.0 to 3.5, has turned out to be appropriate.

The green article or the sintered article may be contacted with a solution of water glass in any suitable manner. For instance, the solution may be applied by spraying or brushing, or the article may be sub- merged into the solution.

The green article or the sintered article may be infiltrated in part or totally with the solution of water glass. By a partial infiltration, the solution of water glass will penetrate into the outer layer of the green article so that a reinforced shell is obtained 15 around the article. By a total infiltration all air is displaced from the article and replaced by the solution of water glass so that an article is obtained in which the entire article is reinforced. By partial infil- tration the penetration depth may be controlled, e.g. by adjusting the submersion time of the green article in the solution of water glass. If a partial infiltration is desired, the article may be submerged in the solution of water glass for 1-20 seconds, preferably 2- 10 sec. In case a longer submersion time of the article in the solution of water glass is desired, it is advisable that the water glass layer around the particles consists of water glass that is sparingly soluble in the solvent so that disintegration is prevented.

The green article or the sintered article, in which a solution of water glass has been absorbed, may be heat-treated in any suitable manner ensuring the solidification of the water glass in the green article. By the heat-treatment, the solvent evaporates and the water glass remains in the article and supports the article structure. By the heat-treatment possibly present crystallization water may be released as well. The heat treatment is usually carried out at a temperature of 100 to 250°C, preferably at a temperature of 120 to 180°C. The temperature to be chosen for the heat treatment, inter alia, depends on the water glass used since water glass having various weight modules may comprise crystallization water that is being released at various temperatures. It may be desirable to carry out the heat treatment at a higher temperature than mentioned above in order to release possibly present crystallization water.

When the green article is infiltrated with water glass prior to the sintering, an article of increased 16 strength is obtained. Primarily, because the water glass of the infiltration liquid is deposited within the porous structure of the green article. The increased strength results in an increased stability at the subsequent heat treatment by the sintering, which contributes further to the final sintered article maintaining the original shape of the green article.

A further strength enhancement may be obtained by mixing the solution of water glass for the infiltration with a metal powder having a diameter smaller than the pore diameter of the green article. When contacting the green article with the solution of water glass with suspended metal particles, metal particles and water glass will infiltrate the green article and solidify therein at the subsequent heat treatment.

It may be desirable to obtain an article having a non-porous surface. This may be obtained by carrying out an infiltration of the sintered article with water glass according to the process described above, poss- ibly repeated one or more times. The surface of the sintered and infiltrated article is smooth and non- porous and may thus advantageously be used as a mould, e.g. for injection moulding.

In another preferred embodiment, the green article or the sintered article is infiltrated with a metal or a metal alloy.

Preferably, the article is infiltrated by contacting the green article or the sintered article with a melted metal or a melted metal alloy so that melted metal or metal alloy is absorbed in the article, cooling the article comprising melted metal or metal alloy with a view to precipitate the metal or the metal alloy, and, possibly, - repeating the preceding steps in order to obtain 17 an infiltrated article.

Usually, the green article or the sintered article is infiltrated by being placed in an oven in physical contact with the metal in a solid form. It is preferred to use a briquette comprising pressed metal powder. The heat treatment in the oven usually consists of two steps, whereby, in the first step, the oven is preheated to a temperature below the melting point of the metal for infiltration to ensure the core temperature of the article being sufficiently high. In a second step, heating continues until the melting point of the metal or the metal alloy has been reached, whereby the metal will flow into the pores of the green article or the infiltrated article. The duration of the first and the second step inter alia depends on the wall thickness .

Preferably, the green article or the sintered article substantially have the same temperature as the melting temperature of the metal or the metal alloy, or, during the infiltration, a higher temperature. Accordingly, the metal or the metal alloy infiltrate the green article completely so that the total strength of the porous structure of the green article is enhanced. If only penetration of the outer layers of the green article is desired, the core temperature of the green article may be kept lower than the melting temperature of the metal or the metal alloy for infiltration.

It may be necessary to support the green article during the infiltration of same since water glass softens when heated to the melting point of many metals or the metal alloys for infiltration. For instance, water glass has a softening point at 640°C in case of a weight module of 3. However, the actual melting point of water glass is above the melting point or the 18 sintering temperature of most relevant metals or metal alloys. Therefore, to obtain a successful result it is usually a necessary prerequisite for the filtration to choose a metal or a metal alloy having a melting point below the melting point of the water glass covering the particles .

By complete infiltration of the green article with a metal or a metal alloy, a surplus of infiltrating metal or metal alloy is preferably used to ensure the complete infiltration. Before the cooling of the infiltrated green article with a view to obtain a solidification of metal or metal alloy, excessive metal/metal alloy is usually removed from the article.

The green article may be infiltrated with metal or metal alloy by any prior art method known to the person skilled in the art, e.g. as described in Metals Handbook, 9 edition, volume 7, pages 551-566.

The material for infiltration and for particles covered with water glass may be chosen so as to obtain simultaneous sintering and infiltration of the article so that one heating process is spared.

In the following the invention will be illustrated by way of examples . The examples are not to be considered limiting to the inventive scope defined in the claims.

EXAMPLES Example 1

Preparation of particles covered with water glass 6.0 kg particles of various materials and having various particle sizes were placed in a cylindrical plastic container having a diameter of 200 mm and a height of 190 mm. The plastic container was supplied with a stirrer comprising a central axis from where four blades extended. The length of the blades was 95 19 mm. The stirrer was started and adjusted to a rotating speed of 450 rpm.

During the stirring, 180 ml water was slowly admixed to the particles followed by a mixing for appr. 1 min to distribute the water within the particles. Subsequently, 180 g solid water glass having a weight module as stated in Table 1 was admixed to the mixture of particles and water, and the stirring continued until the water glass was dissolved. The stirring continued for appr. 45 min consisting of two periods, viz.

Period 1: 0-30 min after the admixture of water glass, the mixture was heated by the mechanical energy of the stirrer and water was allowed to freely evapor- ate;

Period 2: 30-45 min after the admixture of water glass, the water evaporation was so advanced that the water glass began to grow sticky and a tendency towards formation of loose agglomerates was noticed. Towards the end of the time interval, the stickiness of the mixture decreased again as the amount of water in the mixture had been reduced to a point lower than the lowest bonding level and the agglomerates were broken by the stirring. At the end of the period, the water content of the mixture amounted to appr. 0.7% by weight .

The product obtained was examined in a microscope and showed an even and smooth layer of water glass, which was assumed to be the reason for the easy flow- ability. 20

Table 1 :

Test Particle Particle Water Glass Water Glass No. Material Size Weight Amount

Module

1 iron, 50-180 μm 3.3 3.0

Hδganas ABC 100.30

2 bronze (89% by 300-400 μm 2.0 3.0 weight Cu, 11% by weight Sn)

3 tool steel, D2* < 40 μm 2.0 3.0

*) : 1.55% by weight C, 11.5% by weight Cr, 0.75% by weight Mo, 0.95% by weight V, balance: Fe .

Example 2

A sintered article containing particles of iron The water glass covered iron particles obtained in Example 1, Test No. 1, were teemed into a mould that was vibrated. The mould was cylindrical and having a diameter of 50 mm and a height of 50 mm.

The mould teemed with water glass covered particles was placed in an oven at 150°C for 40 min. Subsequently, the mould was removed from the oven and cooled to ambient temperature and the cured article taken out .

The cured article containing the covered particles was then placed in a sintering oven which had been preheated to 700°C in a reducing atmosphere of hydrogen. The sintering oven was of the continuous type. The article was kept at said temperature for 20 min in the reducing atmosphere of hydrogen.

Subsequently, the green article was conveyed to a zone having a temperature of 1,120°C and kept at same for 20 min. The sintered article obtained was cooled to 100°C in the course of 30 min. The weight of the obtained sintered article amounted to 344 g. 21

A microscope examination confirmed that a direct metallic contact had been established between the various particles present.

Example 3

Preparation of a sintered article containing bronze particles

The water glass covered particles of bronze obtained in Example 1, Test No. 2, were teemed into a mould that was vibrated. The mould was cylindrical having a diameter of 50 mm and a height of 42 mm.

The mould teemed with the water glass covered particles was placed in an oven at 150°C for 40 min. Subsequently, the mould was taken out and cooled to ambient temperature and the cured article removed from the mould.

The cured article containing the covered particles was then placed in a batch oven that had been preheated to 750°C in an inert atmosphere of nitrogen. The article was kept at said temperature for 30 min. Subsequently, the temperature was raised to 810°C in the course of 10 min, and said temperature was maintained for 15 min, which resulted in a sintering.

The sintered article was cooled to ambient tem- perature in the course of 30 min. The weight of the obtained sintered article amounted to 380 g.

A microscope examination confirmed that an immediate metallic connection had been established between the various particles present.

Example 4

A sintered article containing particles of tool steel

The water glass covered particles of tool steel obtained in Example 1, Test No. 3, were teemed into a 22 mould that was vibrated. The mould was cylindrical having a diameter of 50 mm and a height of 42 mm.

The mould teemed with water glass covered particles was placed in an oven at 150°C for 40 min. Subsequently, the mould was taken out and cooled to ambient temperature and the cured article removed from the mould.

The cured article containing the covered particles was then placed in a sintering oven that had been preheated to 300°C in a reducing atmosphere of hydrogen. The sintering oven was of the continuous type. The article was kept at said temperature for 30 min in the reducing atmosphere of hydrogen in order to liquate possibly present crystallization water. Then the article was conveyed to a zone having a temperature of 700°C, which temperature was maintained for 30 min.

Subsequently, the article was led to a zone having a temperature of 1,120°C, which temperature was maintained for 30 min. The sintered article was then cooled to ambient temperature in the course of 30 min.

Observations in a light microscope showed direct metallic contact between the particles. The weight of the article amounted to 390 g.

Claims

23P A T E N T C L A I M S

1. A process for preparing a sintered article comprising the steps of providing particles covered with a layer of water glass in a mould, curing the particle mass into a green article, and heating the green article to a temperature equal to or above the sintering temperature of the particles so that a direct physical contact between adjacent particles is formed.

2. A process according to claim 1, wherein the particles covered with a layer of water glass are provided by (a) providing a mixture containing particles to be covered, water and 0.1-5% by weight water glass, calculated on the basis of the weight of the particles wherein the water glass is present in dissolved form, (b) stirring the mixture mechanically, optionally simultaneously supplying heat from an external heat source, and allowing the water to evaporate until at least as much of the water has evaporated from the mixture that it is no longer sticky.

3. A process according to claims 1 or 2 , wherein the green article is prepared by

(c) providing the particles covered with water glass in a mould, (d) ensuring the presence of water for the activation of water glass in the particle mass, and (e) curing the particles covered with water glass in the mould into a green article by supplying energy from a source thereof .

4. A process according to any of the claims 1-3, 24 wherein the green article is heated to a temperature of 10-40┬░C above the sintering temperature.

5. A process according to any of the claims 1-4, wherein the particles comprise metal, such as alumin- ium, copper, tin, iron, wolfram, chrome, vanadium, manganese or molybdenum, or a mixture of two or more thereof .

6. A process according to any of the claims 1-5, wherein the particles comprise metal oxides, such as Si0₂, A1₂0₃, Fe₃0₄, Fe₂0₃ , MnO, NiO, ZnO, Zr0₂ , Ti0₂ or a mixture of two or more thereof .

7. A process according to any of the claims 1-6, wherein the water glass used for covering the articles has a weight module of 1.8 to 3.5.

8. A process according to any of the claims 1-7, wherein the sintered article is infiltrated by allowing admittance of a liquid material that is subsequently solidified.

9. A process according to claim 8, wherein the liquid material for infiltration is a solution of water glass .

10. A process according to claim 9, wherein the solution of water glass is aqueous and the water glass has a weight module of 3.0 to 3.5.

11. A process according to any of the claims 1-10, comprising the steps of contacting the sintered article with a solution of water glass so that the solution is absorbed therein, - heat-treating the sintered article, wherein a solution of water glass is absorbed so that the water glass is solidified, and, optionally, repeating the preceding steps to obtain an infiltrated sintered article.

12. A process according to claim 11, wherein the 25 solution of water glass comprises suspended metal powder having a diameter smaller than the pore diameter of the sintered article.

13. A process according to any of the claims 8-12, wherein the heat-treatment takes place at a temperature between 100 and 250┬░C, preferably at a temperature between 120 and 180┬░C.

14. A process according to claim 8, wherein the material for the infiltration is a metal or a metal alloy, such as copper, tin or a cupriferous and/or tinny alloy.

15. A process according to claim 8 or 14 comprising the steps of contacting the sintered article with a melted metal or a melted metal alloy so that the melted metal or metal alloy is absorbed in the sintered article, cooling the article containing melted metal or metal alloy for the solidification of the metal or the metal alloy, and, optionally repeating the preceding steps in order to obtain an infiltrated article.

16. A process according to claim 15, wherein the sintered article, when contacted with the melted metal or the melted metal alloy, essentially has the same temperature or a higher temperature than the melting temperature of the metal or the metal alloy.

17. A sintered article obtainable by any of the claims 1-16.

18. The use of the article according to claim 17 as a core or a mould for iron and metal casting or plastic moulding.