CS209604B1 - Mechanical powder iron - Google Patents

Description

FIELD OF THE INVENTION The present invention relates to powdered iron intended primarily for the production of diffuse pre-alloyed powders and sintered alloy articles by powder metallurgy processes which achieve higher properties.

For the production of shaped articles by iron-based powder metallurgy, except in the case where full pre-alloyed powders produced by melt spraying are used, iron powder is used as the starting material. Most commonly, sponge iron powder is produced by reducing various types of iron oxides and sprayed iron powder is produced by spraying a metal melt with air, inert gases or water. In order to meet the requirements of powder metallurgy for its properties, with respect to its use in the production of sintered shaped parts, iron powder is usually still heat-treated and often some of its properties, such as e.g. granulometric composition by sieving.

If there are any differences in the physical or technological properties of these types of powdered iron, e.g. in the moldability or mechanical properties of the sintered parts produced therefrom, these differences are attributed to minor variations in their physico-chemical properties due to the manufacturing processes. In terms of applicability, both types of powdered iron for particular applications are considered equivalent. One method of producing alloyed sintered articles is to make them from blended powders. The essence of this method is to add powder alloys to the iron powder, preferably with a lubricant.

After mixing from such a powder mixture, the shaped articles are pressed in closed tools or the powder mixture is rolled. Such cold compaction is always followed by sintering in protective atmospheres. During the sintering process, which is carried out at temperatures below the melting point of the parent metal, a solid metal body is formed on the one hand and, on the other hand, alloyed iron powder particles are alloyed in the molding by the added alloying elements.

The main drawback of the sprayed and spongy types of powdered iron, when used for the production of sintered alloyed articles by means of mixed powders, is that the alloying of the individual powdered iron particles in the compact, which takes place by diffusion of the alloying elements in the solid state, takes a long time. High temperatures, often above 1200 ° C, are used to shorten the alloying time, and hence the sintering, to achieve a sufficiently homogeneous structure of such materials and satisfactory properties. This is mainly due to the fact that these types of powdered iron, their particles, have a relatively coarse-grained structure, characterized by a mean metallographic grain size of ferrite that is greater than 20 µm, often around 35 µm.

This means that under these conditions, at a commonly used particle size of less than 100 or 160 µm, there is a small number of metallographic grain boundaries in each powder particle, the number of which decreases as the size of the starting particle decreases. of powders in the compact by the added alloying elements takes place predominantly in the form of volume diffusion, which is then the determining factor for the entire sintering and alloying process. The volume diffusion proceeds up to several orders of magnitude slower than the grain boundary diffusion. A partial reduction of the adverse consequences of this, manifested by the inhomogeneity of the structure and the lower properties of the articles, would require an increase in the sintering temperatures and an increase in the sintering time, requiring high production costs.

The abovementioned drawbacks are overcome by the use of mechanical powdered iron produced by milling low carbon steel pieces, preferably in eddy impact mills, by annealing and for the production of alloyed powders by diffusion or for the production of alloyed articles.

The iron powder produced by this process, if it is annealed after cold milling at temperatures lower than 900 ° C, preferably at temperatures in the range of 550 to 800 ° C, has a fine-grained structure characterized by a metallographic grain size of less than 20 µm, preferably based on an annealing temperature of less than 10 µm, which is a new discovered property of this iron powder. In this case, in each particulate iron powder being proportional to the reduction in its metallographic grain size, the number of grain boundaries increases. As a result, the alloying of such powder particles in the compact by sintering with the alloying element is preferably done by grain boundary diffusion, which is several times faster than that of coarse-grained powders by volume diffusion.

The advantage of using this powdered iron 8 with a newly discovered fine-grained structure is that under the same sintering conditions as in the case of the sprayed and spongy powdered iron, substantially more uniform flooding of the sintered articles is achieved, which results in higher physical properties. It is also possible to carry out the above-mentioned reasons. sintering at lower temperatures, not only at 1100 ° C, but also at temperatures below 900 ° C, which has hitherto not been used for alloyed materials in powder metallurgy.

A further advantage is that under the given processing conditions the properties of the alloys are better utilized, which in selected cases also leads to a reduction in their additions compared to the sprayed sponge iron powder. Reducing the sintering temperatures and possibly shortening the time will favorably save energy. These disadvantages are also confirmed by the following example.

Example

For the production of sintered manganese steel, in one case, the mechanical iron powder of the invention with a particle size of less than 160 microns and a metallographic ferrite grain size of 12 μη was used, and in the other case a sprayed iron powder of a particle size of . To these types of iron powder was added in a concentration concentration (6 Z) of carbon ferro-manganese and 1% zinc stannate as a lubricant.

From these powder mixtures, after cold mixing, standardized tear rods were pressed at a pressure of 589 MPa, which was then sintered in a retort furnace in a split plug, at one time at 1120 ° C for 3 hours and in the other at 875 ° C. for 1 hour.

After such a treatment at a temperature of 1120 ° C, a density of 7.12 g.cra, a tensile strength of 886 MPa, an elongation of 4% and a hardness were obtained with the samples of mechanical iron powder according to the invention.

179 HV10. Samples also treated with sputtering powdered iron achieved a density of 6.95 g.cm, a breaking strength of 639 MPa, an elongation of 3.4% and a hardness of 150 HV10. Sintering at a temperature of 875 ° C in the case of mechanical powdered iron gave a density of 6.73 g.cm, a breaking strength of 496 MPa and a hardness of 205 HV10. In contrast, moldings prepared on the basis of sprayed 33 powdered iron obtained a density of 6.73 g.cra, a strength of 138 MPa and a hardness of 142 HV10. These results clearly show the advantages provided by the use of mechanical powdered iron in the production of alloyed sintered articles.

The mechanical iron powder according to the invention can also be used for the production of diffusion-pre-alloyed powders, with a more uniform alloying effect than the other types of powders. Such pre-alloyed powders are still characterized by good compressibility as compared to the complete pre-sprayed powders produced by spraying and can be successfully used to produce sintered articles with high properties. Since the diffusion pre-alloying of the powders is carried out at temperatures below 900 ° C to avoid sintering, the sprayed powders only achieve very circumferential deposition, so that certain advantages of the process are lost.

Mechanical powder iron and its use in powder metallurgy is known. This kind of iron powder is in fact one of the oldest. However, it was used only for the production of iron, unalloyed sintered parts, where the newly discovered property, fine-grained, could not be manifested, therefore it was not even investigated and all types of powdered iron, including mechanical, were considered equivalent. Because the production of mechanical powdered iron in whirl mills is less economical than the production of sprayed and spongy powdered iron, and because of the reasons mentioned above, its high structural activity, due to the fine grain structure, was not known in most countries. With the new, now discovered property, mechanical powdered iron presents new possibilities for the production of sintered alloy steels and articles without increasing the content and changing the type of alloying, only by using existing advantageous structural properties.

Claims (1)

3,209,604 at a pressure of 589 MPa which was then co-sintered in the retort furnace in a cleaved flask, in one case at 120 ° C for 3 hours and in the second case at 875 ° C for 1 hour. After this treatment at a temperature of 1120 ° C, a density of 7.12 g.cra, a breaking strength of 886 MPa, a ductility of 4% and a hardness of 179 HV10 were obtained for mechanical iron iron samples according to the invention. For samples equally processed from sprayed iron powder, -3. density 6.95 g.cm, breaking strength 639 MPa, ductility 3.4% and hardness 150 HV10. Sintering at 875 ° C in the case of mechanical iron powder yielded a density of 6.73 g.cm, a breaking strength of 496 MPa and a hardness of 205 HV10. In contrast, compacted iron-powdered compacts produced a density of 6.73 g.cra, a breaking strength of 138 MPa and a hardness142 HV10. These results clearly show the benefits of using mechanical powdered iron in the production of alloyed sintered articles. The mechanical iron powder according to the invention can also be used for the production of diffusion-preformed powders, with a more uniform application of these powders than with other types of powders. Such pre-alloyed powders are still well compressible compared to complete sprayed powdered powders and can be successfully used to produce sintered articles with high properties. Since diffusion pre-glueing is carried out at temperatures below 900 ° C to avoid caking, only very roughly fused glue is achieved in sprayed powders, so certain advantages of this process are lost. Mechanical iron powder and its use in powder metallurgy is known. This kind of powdered iron actually belongs to the oldest. However, it was used only for the production of iron, unalloyed sintered moldings, where the newly discovered property, fine-grained, could not be seen, therefore it was not considered and all kinds of iron powder, mechanical porters, considered equal. Since the production of mechanical iron powder in swirling mills as economical as the production of sprayed and sponge-like iron powders and the aforementioned products was not known for its high structural activity, given the fine-grained structure, in most countries it was no longer manufactured. With the new, now discovered feature, mechanical iron powder presents new possibilities for the production of sintered alloy steels and articles without increasing the content and changing the type of alloy, using only the existing advantageous structural properties. OBJECT OF THE INVENTION The use of mechanical iron powder, produced by grinding low carbon cold pieces, preferably in vortex impact mills and annealing to produce diffuse pre-alloyed powders and sintered alloyed articles. Severografia. n. p .. race 7. Most