MXPA01002879A - Warm compaction of steel powders - Google Patents

Abstract

The present invention concerns a process of preparing high density, warm compacted bodies of a stainless steel powder comprising the steps of providing a mixture of a low carbon, low oxygen stainless steel powder including 10-30%by weight of Cr, optional alloying elements and graphite and inevitable impurities, mixing the powder with a high temperature lubricant and compacting the mixture at an elevated temperature. The invention also concerns a composition of the stainless steel powder, optional additional alloying elements and a high temperature lubricant.

Description

COMPACT HOT ATTACHMENT OF STEEL POWDERS FIELD OF THE INVENTION The present invention relates to a hot compaction process of steel powder compositions, as well as the compacted and concreted bodies obtained therefrom. Specifically, the invention relates to hot compaction of stainless steel powder compositions.

BACKGROUND OF THE INVENTION From the beginning of the industrial use of the powder metallurgical processes, that is to say the pressing and concreting of the metallic powders, great effort has been made in order to increase the mechanical properties of the components of the P / M and to improve the tolerances of the finished parts, in order to expand the market and achieve the lowest total cost. Recently, much attention has been paid to hot compaction as a promising method to improve the properties of material / powder components. The hot compaction process gives the opportunity to increase the level of density, that is, the level of porosity in the finished parts decreases. The hot compaction process is applicable to most material / powder systems. Normally, the hot compaction process leads to greater strength and better dimensional tolerances. A possibility of raw machining, ie machining in the "as pressed" state, is also obtained by this procedure. Hot compaction is considered to be defined as compaction of a particulate material consisting mostly of metal powder above about 100 ° C to about 150 ° C according to the currently available powder technologies, such as Densmix, Ancorbond or Flow-Met. A detailed description of the hot compaction process is described for example in the document presented at the World Congress of PM TEC 96 in Washington, in June 1996, which is incorporated herein by reference. Specific types of lubricants used for the hot compaction of iron powders are described for example in the patents of E.U.A. 5,154,881 and 5,744,433. However, in the case of stainless steel powders, it has now been found that the general advantages of hot compaction have been negligible since only minor differences have been demonstrated for example in density and strength. Additional and important problems, which are encountered when compacted in hot stainless steel powders, are high ejection forces and high internal friction during compaction.

BRIEF DESCRIPTION OF THE INVENTION It has now unexpectedly been found that these problems can be eliminated and that a substantial increase in the strength and density of the raw material can be obtained; provided that the stainless steel metallic powder is distinguished by very low carbon, oxygen and silicon contents. More specifically, the oxygen content should be less than 0.20 preferably below 0.15 and more preferably below 0.10 and the carbon content should be less than 0.03, preferably less than? 0.02 and more preferably below 0.01% by weight. The experiments also indicate that the silicon content is an important factor and that a silicon content should preferably be less than about 0.5%, more preferably less than 0.3% and most preferably less than 0.2% by weight, in order to eliminate the problems encountered when stainless steel powders are hot compacted. Another discovery is that the hot compaction of this stainless steel powder is much more effective at high compaction pressures, that is to say that the density differences of the hot and cold compacted bodies of this powder increase with increasing compaction pressures, which is completely contrary to what happens with steel powders and standard powders.

DETAILED DESCRIPTION OF THE INVENTION Preferably, the powders subjected to hot compaction, are prealloyed water-atomized powders, which include, in weight percent weight, 10-30% chromium, 0-5% molybdenum, 0-15% nickel, 0-0.5% silicon 0-1.5% manganese, 0-2% niobium, 0-2% titanium, 0-2% vanadium 0-5% Fß3P 0-0.4%, graphite and at most 0.3 % of unavoidable impurities and more preferably 10-20% of chromium, 0-3% of molybdenum, 0.1-0.3% of silicon, 0.1-0.4% of manganese, 0-0.5% of niobium, 0-0.5% of titanium, 0 -0.5% vanadium, 0-0.2% graphite and essentially no nickel, or alternatively 7-10% nickel, the remainder being iron and unavoidable impurities. The preparation of said powders is published in the PCT patent with application SE98 / 01189, which is incorporated herein by reference. The lubricant can be of any type while it is compatible with the hot compaction process. More specifically, the lubricant must be a high temperature lubricant selected from the group consisting of metal stearates, such as lithium stearates, paraffins, waxes, natural and synthetic fat derivatives. Polyamides of the type described, for example, can also be used in the U.S.A. ,154,881 and 5,744,433, to which reference was made above and which are incorporated herein by reference. The lubricant is normally used in amounts between 0.1 and 2.0% by weight of the total composition. According to one embodiment, the mixture including the iron powder and the high temperature lubricant may also include a binder. This agent can be selected, for example, from cellulose esters. If present, the binder is usually used in an amount of between 0.01-0.40% by weight of the composition. Optionally but not necessarily, the powder mixture including lubricant and an optional binder is heated to a temperature of 80-150 ° C, preferably 100-120 ° C. The heated mixture is then compacted in a tool heated to 80-130 ° C, preferably 100-120 ° C. The obtained raw bodies are then concreted in the same way as the standard materials, ie at temperatures between 1100 ° C and 1300 ° C, the most marked advantages are obtained when the concretion is carried out between 1120 ° C and 1170 ° C since in this temperature range, the compacted material will maintain a significantly higher density compared to the standard material. In addition, the concretion is preferable to carry out it in a standard non-oxidative atmosphere for periods of between 15 and 90, preferably between 20 and 60 minutes. The high densities according to the invention are obtained without the need of recompaction, reconcretion and / or concretion in an inert atmosphere or under vacuum. The invention is illustrated by means of the following non-limiting examples.

EXAMPLES EXAMPLE 1 This experiment was carried out with a standard material 434 LHC, available from Coldstream, Belgium, as a reference, and water-atomized powders having low carbon, silicon and oxygen contents (designated as powder A and powder B respectively), prepared in accordance with PCT patent application No. SE 98/01189 referred to above. Six stainless steel blends, which had the composition shown in Table 1, were prepared according to Table 2. The compaction was made in the samples of 50 g to 400, 600 and 800 Mpa and the density of the raw material of each sample was calculated The hot compaction was carried out with 0.6% by weight of a polyamide type lubricant and the cold compaction was carried out with a standard ethylene-bis-steramide lubricant (Hoechst wax available from Hoechst AG, Germany). ). The results are presented in table 3.

TABLE 1 TABLE 2 TABLE 3 * Only two cylinders were compacted due to the immersion in the wall of the die. This example shows that the hot compaction of standard 434 LHC reference powder does not work properly due to high friction during ejection. It is also shown that the compressibility (density of raw material) of stainless steel powder with low oxygen / carbon content that had a low silicon content, according to the present invention, it is increased at an elevated temperature and that this effect is especially pronounced at high compaction pressures.

EXAMPLE 2 The purpose of this investigation was to verify that the hot compaction of the stainless steel powder is also possible under conditions similar to those of production. 30 kg of each of the aforementioned powders was mixed. The standard 434 LHC powder was mixed with an ethylene-bis-stearamide lubricant and the hot compaction powder was mixed with a high temperature lubricant of the polyamide type. 500 parts of each powder sample were compressed in a 45 tonne Dorst mechanical press, equipped with a heater for heating the powder and electrically heating the tools. The powder was heated to 110 ° C and subsequently pressed in the form of rings in tools heated to 110 ° C. The rings were pressed at a compaction pressure of 700 MPa and concreted at 1120 ° C in a hydrogen atmosphere for 30 minutes. In these specific parts, the dimensions, density and resistance to radial crushing were measured. The results of the compaction and concreting experiments in an automatic press gave the results shown in table 4.

TABLE 4 * Only 4 rings could be pressed before the tool has to be polished. Therefore, no concretion was carried out and no values were obtained. The hot compacted rings showed lower elastic return compared to the standard compacted rings. The resistance of the raw material increased by 30%, from 16 to 21 PMa. The resistance to radial crushing increased by 80% after concretion, which is strongly related to the density of the concreted material of 6.59 g / cm3 for the standards and 6.91 g / cm3 for the hot compacted. The height dispersion decreased during the concretion of both series of compaction. The height dispersion for the standard material was 0.34% for the cold compacted material and 0.35% for the hot compacted material. This result indicates that the tolerances after concreting are the same for hot compacted material as for standard compaction. The results also indicate that hot compaction of 434 LHC powder is not possible.

Claims (19)

NOVELTY OF THE INVENTION CLAIMS

1. - A process for preparing high density hot compacted bodies of a stainless steel metallic powder, comprising the steps of; provide a mixture of stainless steel powder with low content of oxygen, silicon and carbon that includes 10-30% by weight of chromium, optional elements of alloy, graphite and unavoidable impurities; mix the powder with a high temperature lubricant; and compacting the mixture at elevated temperature.

2. The process according to claim 1, further characterized in that the oxygen content of the stainless steel powder is below 0.20, preferably below 0.15 and more preferably below 0.10% by weight, the silicon content. is less than 0.5, preferably less than 0.3 and preferably less than 0.2% by weight and the carbon content is below 0.03, preferably below 0.02 and preferably below 0.01% by weight.

3. The process according to claim 1 or 2, further characterized in that the powder includes at least one lubricant of high temperature.

4. The method according to claim 3, further characterized in that the lubricant is selected from the group consisting of metal stearates, such as lithium stearate, paraffins, waxes, natural and synthetic fat derivatives, and polyamides.

5. The method according to claim 4, further characterized in that the amount of lubricant is between 0.1 and 2.0 of the total composition.

6. The process according to any of claims 1-5, further characterized in that the mixture also includes elements of alloy and / or graphite.

7. The process according to any of claims 1-6, further characterized in that the powder includes at least one binder in an amount of 0.01-0.40% by weight of the composition.

8. The process according to any of claims 1-7, further characterized in that the powder is preheated to a temperature of between 80 and 130 ° C before compaction.

9. The process according to any of claims 1-8, further characterized in that the powder is compacted in a die preheated to a temperature of between 80 and 150 ° C.

10. The process according to any of claims 1-9, further characterized in that the powder is compacted at a pressure of between 400 and 1000 MPa.

11. The method according to any of the preceding claims which further includes the steps of concreting the raw bodies obtained at a temperature between 1100 ° C and 1300 ° C preferably between 1120 to 1170 ° C in a standard non-oxidative atmosphere, for periods of between 15 and 90, preferably between 20 and 60 minutes.

12. A powder composition for hot compaction comprising a powder of stainless steel with low content of silicon, oxygen, essentially free of carbon, atomized with water, annealed, which in addition to iron, comprises 10-30% in chrome weight, optional alloy elements, 0-0.4% by weight of graphite, and no more than 0.5% by weight of impurities, and 0.2-2.0%, preferably 0.4-1.5% by weight of a high temperature lubricant.

13. The powder composition according to claim 12, further characterized in that the oxygen content of the stainless steel powder is below .2, preferably below 0.15 and more preferably below 0.10, the silicon content. it is less than 0.5, preferably less than 0.3 and more preferably less than 0.2% by weight and the carbon content is below 0.03, preferably below 0.02 and preferably below 0.01% of the weight of the powder.

14. The composition according to claim 13 comprising, in percent by weight, 10-30% chromium, 0-5% molybdenum, 0-15% nickel, 0-1.5% manganese, 0-2% niobium, 0-2% titanium, 0-2% vanadium, 0-5% Fe3P, 0-0.4% graphite and when much 0.3% of unavoidable impurities, the rest being iron.

15. The composition according to claim 14, comprising, in percent by weight, 10-20% chromium, 0-3% molybdenum, 0.1-0.4% manganese, 0-0.5% niobium, 0- -0.5% titanium, 0-0.5% vanadium and essentially no nickel, the rest being iron.

16. The composition according to claim 14, comprising, in percent by weight, 10-20% chromium, 0-3% molybdenum, 0.1-0.4% manganese, 0-0.5% niobium, 0-0.5% titanium, 0-0.5% vanadium and 7-10% nickel, the rest being iron.

17. The composition according to any of claims 12-16 further characterized in that the lubricant is a high temperature lubricant selected from the group consisting of stearates of metals such as stearates, paraffins, waxes, natural and synthetic fat derivatives and polyamides.

18. The composition according to claim 17, further characterized in that the amount of lubricant is between 0.1 and 2.0% by weight of the total composition.

19. The composition according to any of claims 12-18, further characterized in that the composition includes at least one binding agent in an amount of 0.01-0.40% of the weight of the composition.