Patent Description:
The term "easily sinterable powder" as used in present application refers to a powder that after cold forming at pressure of <NUM>-<NUM> MPa and free sintering (at atmospheric pressure) in a reducing atmosphere for a time no longer than <NUM> minutes at a temperature not higher than <NUM>° C allows to obtain sintered parts of the total porosity of less than <NUM>% by volume.

Factors for rendering the easily sintering nature to powders are:.

Low-alloy iron powders containing at least <NUM>% Fe by weight and sintered without the liquid phase allow obtaining sintered parts of the total porosity of less than <NUM>% by volume, while maintaining the fine-grained microstructure of the material. The two-phase microstructure of the material at the sintering temperature, composed of ferrite and austenite, is assured by the chemical composition of the alloy. Ferrite stabilizers, e.g.: P, W, Mo, Co, and austenite stabilizers, e.g.: Cu, Ni, are used in quantities and proportions assessed by means of experimental techniques (e.g. the high temperature X-ray diffraction phase analysis) or analytical techniques (e.g. the ThermoCalc software®) in order to obtain the ferrite-to-austenite volume ratio in the range from <NUM>/<NUM> to <NUM>/<NUM> within the possibly widest sintering temperature range. Furthermore, the increase in the phosphorus content of the powder contributes to the increase in the as-sintered hardness due to a strong solid solution strengthening effect.

The liquid phase sintered materials are characterized by a higher content of alloying elements such as Cu and Sn, which have to assure sufficient amount of persistent liquid phase at the sintering temperature in order to reach as-sintered porosity of less than <NUM>% by volume. Liquid phase sintering of high-alloy iron powders containing at least <NUM>% by weight of alloying elements makes it difficult to retain the fine-grained structure of the starting powder, but it promotes applicability of such powders whenever permanent bonding of the sintered part to another element must occur during sintering.

Easily sinterable iron-based powders are used for the production of sintered structural and tool products, in particular for the production of sintered metal-diamond composites obtained by the free sintering, but also by pressure sintering method. So far, the most commonly used material in these applications was a cobalt, high-alloy iron-based powders available on the market under the trade names Cobalite CNF® (Umicore, Belgium) and Next <NUM>® (Eurotungstène, France) and mixtures thereof with prealloyed tin bronzes, iron, nickel and tungsten carbide (e.g.: MX4885, MX4380, MX4590, MX4940, etc.). Cobalite CNF® powder is described in the publication by <NPL>. These raw materials, due to the high content of expensive alloying elements and the use of chemical production methods, are expensive to manufacture. They cause a serious health hazards to the persons vulnerable to prolonged exposure to fine powders containing cobalt and/or nickel, involving the frequent occurrence of cobalt lung, giant cell interstitial pneumonia, allergic and cancerous skin diseases. The above mentioned cost and environmental factors have caused a growing tendency to reduce the content of alloying elements, especially cobalt, in easily sinterable prealloyed iron-based powders, while maintaining their required technological and functional properties. Until recently research efforts have been directed toward seeking alternatives for cobalt, i.e. replacing it with other alloying elements, in consequence of that a total content of alloying elements still remained relatively high, for example in said powders Next400 and Cobalite CNF the total content of alloying elements is <NUM>% by weight and <NUM>% by weight, respectively.

Document <CIT> discloses a powder comprising Cr, from <NUM>,<NUM> to <NUM>,<NUM> wt. %; Mn, from <NUM>,<NUM> to <NUM>,<NUM> wt. %; P from <NUM> to <NUM> wt. %; Cu, from <NUM> to <NUM> wt. %; Si, from <NUM> to <NUM> wt. %; Mo, from <NUM> to <NUM> wt. %; C, from <NUM> to <NUM> wt. %; and the balance being Fe with less than <NUM> wt. % of impurities. The powder is used in a method for manufacturing an anti-wear slide member having improved anti-pitting and initial stage fit of its slide surface.

The previously known methods for manufacturing fine grain, especially easily sinterable prealloyed iron-based powders, using co-precipitation technology of metal hydroxides or oxalates from aqueous salt solutions, which, after filtration and calcination, are subjected to reduction with hydrogen, also proved unsatisfactory due to cost and environmental aspects. Such methods are known e.g. from the patent specifications <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>. Disadvantages of the known methods based on co-precipitation of hydroxides or salts are both their high cost as well as disposal problems with environmentally harmful wastes inevitable in those processes.

The object of the invention is to provide easily sinterable prealloyed iron-based powders that do not demonstrate the prior art disadvantages resulting from the high content of alloying elements. They are therefore more environmentally friendly, cheaper and easier to manufacture, while maintaining very good properties, both technological as well as functional.

The object of the invention is also to propose a method for manufacturing easily sinterable prealloyed iron-based powders, which method eliminates the disadvantages occurring in the prior art, namely it is environmentally friendly, less expensive to implement and demonstrates flexibility allowing the better adjustment of the properties of the resulting powder to a particular application.

The product of the invention is determined in independent claim <NUM> and dependent claims <NUM>-<NUM>.

The easily sinterable prealloyed iron-based powder, obtained by subjecting a reducible material, successively to milling, annealing and cooling thereof and grinding the cooled material to the powder, wherein the reducible material comprising oxides, carbonates, nitrates, metals and metal alloys and mixtures thereof, reducible by a technical purity hydrogen at a temperature not higher than <NUM>, wherein the powder consists of:.

at least one alloying element as alloying additives selected from the group consisting of Co, Ni, W and Mo in the total content of Co and/or Ni and/or W and/or Mo not more than <NUM>% by weight.

Preferably, the average particle size of the powder determined by the Fisher apparatus is not more than <NUM>.

Preferably, the powder comprises polycrystalline particles constituted of grains having average size not greater than <NUM>.

A method for producing the easily sinterable prealloyed iron-based powder according to claims <NUM> to <NUM> is determined in claims <NUM>-<NUM>.

A method for producing the easily sinterable prealloyed iron-based powder according to claims <NUM> to <NUM>, comprising the successive steps:.

Preferably, the annealing is carried out at a temperature of <NUM>-<NUM>, for a time of <NUM>-<NUM> hours, in a reducing atmosphere, which is hydrogen or a gas mixture containing hydrogen.

Preferably, the mechanical processing of the reducible material is performed in dry or wet condition.

Preferably, further drying is carried out after the mechanical processing in wet condition.

The easily sinterable prealloyed iron-based powder as described above is used for the manufacturing of sintered structural and tool components, in particular sintered metal-diamond composites.

The sintered product, especially the sintered metal-diamond composite, prepared from the easily sinterable prealloyed iron-based powder according to the invention is characterized in that the easily sinterable prealloyed iron-based powder is the powder as described above, wherein the total porosity of the sintered product at a temperature not higher than <NUM> is lower than <NUM>%.

Thus, the invention allows in a surprisingly simple and inexpensive manner to produce the easily sinterable prealloyed iron-based powder comprising at least <NUM>% of iron by weight, copper and phosphorus, and optionally at least one from the group of alloying elements including tin, cobalt,nickel, tungsten and molybdenum, as well as impurities, mainly in the form of hardly reducible oxides such, for example SiO<NUM>. The total content of alloying elements and impurities in the powders produced according to the invention does not exceed <NUM>% by weight, wherein the powders in which the minimum content of alloying elements is <NUM>% by weight are intended for applications in which it is required that permanent bonding of the sintered part to another element or elements, made of iron or its alloys, must occur during sintering, e.g. by brazing.

The easily sinterable prealloyed iron-based powder according to the invention, while maintaining properties similar to the known powders of this kind, has in comparison to them a number of economic, environmental and technological advantages such as:.

The method of the invention for producing the new prealloyed iron-based powders eliminates the expensive chemical method for obtaining mixtures of hydroxides, oxalates or other metal compounds hardly soluble in water, in which environmentally harmful waste (salts), that require utilization, are formed, and their later thermal decomposition to oxides occurs. It is replaced by a cheaper, mechanochemical synthesis of oxides, which, in comparison with the chemical method, gives greater freedom in selection the chemical composition of the powder. Mechanochemical synthesis consists in inducing chemical reactions preceded by grinding and mechanical activation of substrates. It enables receiving new materials characterized by a low level of chemical and structural heterogeneity, fine-grained microstructure and the most often desirable complex phase composition.

In the method according to the invention a reducible material is thus prepared through mechanical processing by milling, resulting in grinding, homogenization and activation of the reducible material. The crushed, homogenized and activated by grinding reducible material thus obtained is annealed in a reducing atmosphere, and then cooled to a temperature which prevents self-ignition of the material. Finally, the annealed and cooled material is ground to a powder having a predetermined average particle size. The application of mechanochemical processes, due to milling reagents in ball mills, simplifies synthesis procedures and eliminates the need of expensive waste utilization, which in turn significantly improves the ecology of producing high performance functional materials, thus meeting the principles of so called "Green Chemistry".

The method according to the invention has the following advantages in comparison to the traditional method of co-precipitation hydroxides or oxalates:.

The powders produced by the mechanochemical method of oxides synthesis are free of drawbacks, which intrinsically characterise commercial powders, while maintaining similar to them technological properties. For this reason, the presented invention has tremendous application potential.

The object of the invention is illustrated hereinafter in the embodiment and shown in the accompanying drawing, in which:.

A powder mixture containing <NUM> Fe<NUM>O<NUM>; <NUM> CuO and <NUM> of prealloyed Fe-P powder containing <NUM>% phosphorus by weight was prepared by mixing the ingredients in a Turbula type mixer for <NUM> minutes. The powders were placed together with grinding media in a <NUM> dm<NUM> roller ball mill drum. <NUM> diameter 100Cr6 bearing steel balls were used as the grinding media. The degree of filling of the mill was <NUM>% by volume, and the ball-to-powder weight ratio was <NUM>:<NUM>. Ethyl alcohol was poured into the drum in an amount required for the complete immersion of the grinding media together with the powder in the liquid. The drum was turned on to rotate at <NUM>% of the critical speed. After milling for <NUM> hours, the charge of the mill was dried in a laboratory drier at <NUM>. The powder was then subjected to reducing annealing for <NUM> minutes at <NUM> in a hydrogen atmosphere and cooled to below <NUM> in order to avoid self-ignition. The thus obtained metal sponge was ground in a ceramic mortar to powder (<FIG>) with a nominal content of <NUM>% Fe by weight, <NUM>% Cu by weight and <NUM>% P by weight, the hydrogen loss of <NUM>% by weight and Fisher sub-sieve size of <NUM>. The prealloyed nature of the powder was verified by X-ray diffraction phase analysis (<FIG>).

<NUM> portions of the powder obtained according to the procedure described in Example <NUM> were cold-pressed under <NUM> MPa in a carbide die having cavity dimensions <NUM> × <NUM>. The green densities of compacts was determined using the geometric method (Table <NUM>). The powder compacts were sintered in a laboratory tube furnace for <NUM> minutes at <NUM>, <NUM>, <NUM>, <NUM> and <NUM> in a hydrogen atmosphere. During heating the compacts were held for <NUM> minutes at <NUM> before proceeding to the sintering temperature. After sintering the samples were furnace cooled to room temperature. The sintered samples were tested for density by the water immersion technique (Table <NUM>).

A powder mixture containing <NUM> Fe<NUM>O<NUM>; <NUM> CuO and <NUM> of prealloyed Fe-P powder containing <NUM>% phosphorus by weight was prepared by mixing the ingredients in a Turbula type mixer for <NUM> minutes. The powders were placed together with grinding media in a <NUM> dm<NUM> roller ball mill drum. <NUM> diameter 100Cr6 bearing steel balls were used as the grinding media. The degree of filling of the mill was <NUM>% by volume, and the ball-to-powder weight ratio was <NUM>:<NUM>. Distilled water was poured into the drum in an amount required for the complete immersion of the grinding media together with the powder in the liquid. The drum was turned on to rotate at <NUM>% of the critical speed. After milling for <NUM> hours, the charge of the mill was dried in a laboratory drier at <NUM>. The powder was then subjected to reducing annealing for <NUM> minutes at <NUM> in a hydrogen atmosphere. After cooling to below <NUM> the obtained metal sponge was ground in a ceramic mortar to powder (<FIG>) with a nominal content of <NUM>% Fe by weight, <NUM>% Cu by weight and <NUM>% P by weight, the hydrogen loss of <NUM>% by weight and Fisher sub-sieve size of <NUM>.

<NUM> portions of the powder obtained according to the procedure described in Example <NUM> were cold-pressed under <NUM> MPa in a carbide die having cavity dimensions <NUM>. The green densities of compacts was determined using the geometric method (Table <NUM>). The power compacts were sintered in a laboratory tube furnace for <NUM> minutes at <NUM> in a hydrogen atmosphere. During heating the compacts were held for <NUM> minutes at <NUM> before proceeding to the sintering temperature. After sintering the samples were furnace cooled to room temperature. The sintered samples were tested for density by the water immersion technique (Table <NUM>). Selected sintered samples were then solution treated by reheating to <NUM>, holding for <NUM> minutes in nitrogen and quenching in water. Then they were aged for <NUM> minutes at <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM>. The sintered parts were tested for a Vickers hardness determined at a <NUM> kgf load (Table <NUM>).

<NUM> of dry powder mixture comprising <NUM> Fe<NUM>O<NUM>; <NUM> CuO; <NUM> of prealloyed Fe-P powder containing <NUM>% phosphorus by weight and <NUM> Sn was placed together with <NUM> of <NUM> diameter steel balls in a <NUM> dm<NUM> steel reactor of a laboratory, planetary ball mill (Activator <NUM>, Novosibirsk Corp. The charge was then subjected to high-energy milling for <NUM> hours at a rotational speed of <NUM> rpm in air atmosphere. During the milling process the reactor was water-cooled. In this manner, the composite oxide powder having a modified crystalline microstructure and characterized by a high susceptibility to reduction in hydrogen was obtained. The powder was then subjected to reducing annealing for <NUM> minutes at <NUM> in a hydrogen atmosphere and cooled to below <NUM> in order to avoid self-ignition. The metal sponge thus obtained was ground in a ceramic mortar to a powder (<FIG>) with a nominal content of <NUM>% Fe by weight, <NUM>% Cu by weight, <NUM>% Sn by weight and <NUM>% P by weight, the hydrogen loss of <NUM>% by weight and Fisher sub-sieve size of <NUM>. The prealloyed nature of the powder was verified by X-ray diffraction phase analysis (<FIG>).

<NUM> portions of the powder obtained according to the procedure described in Example <NUM> were cold-pressed under <NUM> MPa, in a carbide die having cavity dimensions <NUM>. The green densities of compacts was determined using the geometric method (Table <NUM>). The powder compacts were sintered in a laboratory tube furnace for <NUM> minutes at a temperature <NUM> in a hydrogen atmosphere. During heating the compacts were held for <NUM> minutes at <NUM> before proceeding to the sintering temperature. After sintering the samples were furnace cooled to room temperature. The sintered parts were tested for density by the water immersion technique (Table <NUM>) and for Vickers hardness determined at a <NUM> kgf load (Table <NUM>).

Claim 1:
An easily sinterable prealloyed iron-based powder, wherein the powder consists of:
- at least <NUM>% Fe by weight,
- alloying elements and impurities the total content of which is limited to <NUM>% by weight and as a balance,
- wherein the alloying elements are <NUM>-<NUM>% Cu by weight, <NUM> - <NUM>% P by weight and <NUM> - <NUM>% Sn by weight, and optionally
- at least one alloying element as alloying additives selected from the group consisting of Co, Ni, W and Mo in the total content of Co and/or Ni and/or W and/or Mo not more than <NUM>% by weight.