Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C33/00—Making ferrous alloys
  • C22C33/02—Making ferrous alloys by powder metallurgy
  • C22C33/0257—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements
  • C22C33/0264—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements the maximum content of each alloying element not exceeding 5%
  • C22C33/0271—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements the maximum content of each alloying element not exceeding 5% with only C, Mn, Si, P, S, As as alloying elements, e.g. carbon steel
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C33/00—Making ferrous alloys
  • C22C33/02—Making ferrous alloys by powder metallurgy
  • C22C33/0207—Using a mixture of pre-alloyed powders or a master alloy

Definitions

• the invention relates to a method for producing sintered steel according to the preamble of the main claim.
• the strength properties of sintered steels are essentially determined by the space filling, apart from the composition.
• the pore shape is also important.
• Sintered steels with a space filling> 94% and rounded pores are desirable.
• the most economical way to achieve this goal is the so-called simple sintering technique, in which pressing and sintering is carried out only once.
• the simple sintering technique customary today the sintering of the powder particles takes place essentially by solid phase diffusion, it being possible for it to be supported by the appearance of liquid phases. In the case of two- or three-substance systems, this generally leads to a space filling of around 92%.
• the simple sintering technique with the appearance of liquid phases is used today in the production of components from high-speed steel powders.
• high-speed steels sintered densities of at least 97% are achieved, but the starting powders have very complicated compositions, since they consist of at least five alloy components.
• the particles of the starting powder have a completely homogeneous composition since the alloy is made before pressing and sintering.
• each powder particle is in a solid and in a liquid state of aggregation within a certain temperature range, which leads to the fact that the proportion of pores can be reduced to: g 3%, but the temperature range in which this phenomenon is used for the purpose of Compacting can be exploited by sintering, with 2 to 3 ° C very narrow, which places very high demands on the temperature constancy during the sintering process and is one of the reasons why the sintering process for the high-speed steel powders has to be carried out in a vacuum.
• DE-A-2 625 212 describes a process whose aim is to produce sintered steels with good mechanical properties, the sintering temperature being intended to be as low as possible. For optimal use of the alloying elements, they should be distributed as homogeneously as possible. This goal is achieved by liquid phase sintering with the help of carbon, because diffusion goes much faster over a liquid phase than over a solid phase.
• One difficulty is that a high carbon content of approximately 4% is required because of the desired low sintering temperatures, which would lead to a powder which is difficult to press. Therefore, two different powders are used, which have a different carbon content.
• powder 1 is a atomized, pre-alloyed powder made of iron with other alloy components such as manganese, molybdenum, chromium and nickel and with 4.3 to 4.5% carbon. So this is a hypereutectic alloy.
• the powder 2 which is present in the mixture in at least nine times the amount, is an atomized pre-alloyed powder which contains only 0.1 to 0.8% by weight of carbon and thus largely determines the properties of the powder mixture.
• This powder mixture is then pressed to press densities of, for example, 6.7 g / cm 3 . If this pressed part is now heated for sintering, the carbon-rich phase would lose carbon during the 10 to 20 minute heating, since this readily diffuses away from the surface of the corresponding particles.
• a coating for example made of copper, with a thickness of 0.01 to 200 ⁇ m is placed around the particles of the powder 1 in order to prevent the diffusion of the carbon from the inside of the particle to the outside.
• This also applies just above the melting point of copper, namely 1082 ° C; at the sintering temperature of about 1120 to 1150 ° C, which corresponds approximately to the eutectic temperature of powder 1, the powder 1 is partially liquefied, and the alloying elements including the carbon quickly diffuse out of the powder 1 into the powder 2, since the copper jacket is now destroyed diffusion is no longer slowed down.
• the individual particles of powder 1 are completely homogeneous in themselves, so that above the sintering temperature, ie between the solidus and the liquidus curve, the particles of powder 1 in the copper shell are completely in a two-phase form, namely solid and liquid .
• the copper does not serve as an alloy component here, but only as a diffusion barrier for the carbon of the carbon-rich particles, as long as the copper is still solid.
• the method according to the invention with the characterizing features of the main claim has the advantage over normal sintered steels, which are generally produced by mixing the element powders as a two- or three-material system, that it enables a sintered steel to be used by simple sintering technology to fill a space> 94%. bring to.
• This is done by two-phase sealing sintering, but in the process according to the invention an inhomogeneous starting powder containing several components is used, which, without reactions of the components with one another, only during the entire sintering process is in an aggregate state.
• the two-phase state is achieved during the sintering by reactions of two or more starting components, an initially non-existing new phase being formed, which is then simultaneously present in the solid and liquid physical state.
• the distribution of these components in the compact must be such that the reactions occur at as many locations in the compact as possible during sintering. Furthermore, the two-phase state must be maintained as long as possible so that the pores can largely migrate outwards.
• the composition of the starting powder must be selected so that the component, which occurs simultaneously in the solid and in the liquid state, is available in sufficient quantities.
• the core consists of pure iron
• the alloy components e.g. silicon and phosphorus in the form of ferrosilicon and ferrophosphorus
• the temperature range during the sintering is not so critical, since a range of approximately 30 ° C. is available here.
• the sintering process does not need to be carried out in a vacuum; here, sintering is preferably carried out under hydrogen.
• the sintering temperature for high-speed steel powders is relatively high at more than 1250 ° C.
• the method according to the invention has proven particularly useful, for example, in the production of the technically interesting Fe-Si-P sintered alloy, which basically has the advantage that silicon and phosphorus are inexpensive, readily available elements which cause very little difficulty in the eventual reprocessing of sintered steel parts .
• the two-phase state occurs for alloys up to about 40% Ni above 1450 ° C.
• spherical iron powder was provided with a nickel layer by vapor deposition, the layer thickness of the nickel being chosen so that a gross content of about -5% nickel was reached.
• the powder obtained in this way was shaped at a pressure of about 7 Mp / cm 2 into a compact, which was then first tempered at 1000 ° C., so that an alloy was already formed in the boundary zone between iron and nickel. The temperature was then raised to approximately 1470 ° C.
• Fe-Si-P alloy is a technically interesting alloy.
• Fe-Si-P there are various two-phase areas that can be used for the production of high-density sintered parts.
• Iron, ferrosilicon, ferrophosphorus and graphite powders were used as starting materials. It has been shown that it is also necessary in this case to provide a diffusion brake between the iron and the alloy powders.
• the iron powder first coated with graphite powder by mixing the graphite powder with 5 cm 3 per 1000 g of iron powder of a 35% aqueous dextrin solution as a binder and applying it to the iron powder particles.
• the gross carbon content was between 0.05 and 0.3% by weight.
• this powder was subjected to a heat treatment at 700 ° C. for one hour. Then a mixture of ferrosilicon and ferrophosphorus was applied in the same way and again a heat treatment was carried out as above.
• the powder which is now in the form of a panate, in which each powder grain consists of a core which is encased in a layer of another material, was then pressed in a conventional manner and sintered in hydrogen at 1150-1180 ° C. for one hour.
• iron, ferrosilicon and ferrophosphorus powders were mixed thoroughly in the customary manner, 0.7% by weight of a synthetic wax being added as a pressing aid to some of the samples. These powder mixtures were then processed as above.
• An alloy that is particularly favorable in terms of its properties has the following composition:
• Panat or mixed powder with wax were pressed at 6.5 to 8.5 Mp / cm 2 . After sintering at 1150 ° C. for one hour in hydrogen, the sintered densities were between 7.25 and 7.40 g / c m 3 .
• the main properties of this alloy are:

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Powder Metallurgy (AREA)

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• 1982
  • 1982-07-14 DE DE19823226257 patent/DE3226257A1/de not_active Withdrawn
• 1983
  • 1983-06-25 DE DE8383106223T patent/DE3366712D1/de not_active Expired
  • 1983-06-25 EP EP83106223A patent/EP0099015B1/de not_active Expired
  • 1983-07-08 JP JP58123559A patent/JPS5923841A/ja active Granted

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