EP0099015A1 - Process for manufacturing high-density sintered steel by a simple sintering technique - Google Patents

Abstract

1. Method for the production of sintered steel with a space filling of at least 94% by a single-state sintering technique using a single type of powder, at least one layer consisting of one or more alloy components being applied to the surface of the iron powder particles, said power being moulded and sinterd at a temperature at which a liquid and a solid phase are produced by reaction of the powder particle granule with the applied layer material, and that simultaneously the diffusion of the alloy components applied as a layer into the powder particle granule and consequently formation of the liquid and solid phases are retarded either by the use of an iron powder granule with a low carbon content or by application of an additional layer of graphite between the granule and the layer consisting of the alloy components.

Description

State of the art

The invention relates to a method for producing sintered steel according to the preamble of the main claim. The strength properties of sintered steels, apart from their composition, are essentially determined by the space they fill. In addition, because of the notch effect of the pores, the pore shape is also important. Sintered steels with a space filling of ≧ 94% and rounded pores are desirable. The most economical way to achieve this goal is the so-called single sintering technique, in which the pressing and sintering is carried out only once. In 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%.

Thanks to the so-called double-sintering technology, in which a first sequence of presses and sintering is followed by a second, a space filling of almost 94% can be achieved. However, this technique is much more complex than the simple sintering technique. Finally, the sinter forging technology allows compression up to 99% and more, but it is very complex and in most cases uneconomical.

The simple sintering technique with the appearance of liquid phases, the so-called two-phase sealing sintering, is used today in the production of components from high-speed steel powders. With these 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. During 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 ü 3%, but the temperature range is in which this phenomenon is used for the purpose of compression can be exploited by sintering, with 2 to 3 0C very closely, which places very high demands on the temperature constancy during the sintering process and is one reason that the sintering process for the high-speed steel powders must be carried out in a vacuum.

Advantages of the invention

The method according to the invention with the characterizing features of the main claim has the advantage over the normal sintered steels, which are generally produced by mixing the element powders as a two- or three-material system, that a sintered steel can be achieved with a simple sintering technique on a spacer filling = bring 94%. 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, is only present in one aggregate state during the entire sintering process. 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 during sintering at as many locations in the compact as possible. 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 chosen so that the component, which occurs simultaneously in the solid and in the liquid state, is present in sufficient quantity.

Compared to the above-mentioned high-speed steel powders, there is the advantage that the temperature range during sintering is not so critical, since a range of approximately 30 ^° C. is available here. In addition, the sintering process does not need to be carried out in a vacuum, but rather is preferably sintered under hydrogen. In addition, the sintering temperature of the high-speed steel powders is relatively high at more than 1250 ^° C.

Advantageous further developments and improvements of the method specified in the main claim are possible through the measures listed in the subclaims. It is particularly advantageous to use the starting powder in this way to produce that the powder particles of the main component iron are coated with one or more layers of the alloy components, that this powder is then pressed and then sintered, as will be described in more detail below with the aid of examples. It is generally necessary to ensure that the alloy components applied as a layer do not diffuse too quickly into the iron during sintering. This can be done either by using an iron powder with a low carbon content or by applying a graphite layer as a diffusion brake to the iron powder particles as the first layer. The process 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 pose very little difficulty in the eventual reprocessing of sintered steel parts .

Description of the embodiments Example 1:

In the iron-nickel system, the two-phase state occurs for alloys up to about 40% Ni above 1450 ^° C. In order to be able to produce an Fe-Ni alloy with about 5% Ni and a space filling of ≧ 94% by simple sintering technology, 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 firstly annealed at 1000 ° C. was so that an alloy already formed in the border zone between iron and nickel. The temperature was then raised to approximately 1470 ° C. However, it turned out that this two-phase state did not last long enough, since the nickel diffused into the iron too quickly and the two-phase region was therefore left too quickly. In this case, the pores did not have enough time to migrate outwards. However, this difficulty could be overcome by using an iron powder containing about 0.2% by weight of carbon, which sufficiently delayed the nickel diffusion. In the sintered parts produced in this way, sintered densities between 7.4 and 7.5 g / cm ^{3 were} achieved, which corresponds to a compression to> 94% and to be attributed to the simultaneous and sufficiently long presence of an Fe-Ni phase in the solid and in the liquid state is.

Example 2:

It has already been indicated above that the Fe-Si-P alloy is a technically interesting alloy. In the ternary system 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. Therefore, the iron powder was 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. To ensure better adhesion of graphite to iron reached, this powder was subjected to a heat treatment at 700 ⁰ 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, now in the form of a panate, was then pressed in the customary manner and sintered in hydrogen at 1150-1180 ° C. for one hour. - Alternatively, 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 in some of the samples. These powder mixtures were then processed as above.

When testing the properties of the test specimens produced in this way, the samples produced from panat powder and from mixed powder with added wax largely showed the same results, while in the case of mixed powders without added wax, there were large variations in the property values, particularly at high pressing pressures and sintering temperatures.

Sintering densities between 7.25 and 7.40 g / cm ^{3 were} achieved at pressures between 6.5 and 8.5 Mp / cm ² . This corresponds to 94 to 96% room filling values. In this density range, the strength properties and the dimensional changes (shrinkage) during sintering are almost independent of the density.

• P 0,8 bis 1,4 Gew.-%
• Si 1,0 bis 2,0 Gew.-%
• C 0,05 bis 0,3 Gew.-% (bei Panat)
• Fe Rest

In the experiments described above, the components were used in such amounts that the proportions of the individual components in the alloys were in the following ranges:

• P 0.8 to 1.4% by weight
• Si 1.0 to 2.0% by weight
• C 0.05 to 0.3% by weight (for Panat)
• Fe rest

• P 1,0 Gew.-%
• Si 1,75 Gew.-%
• C 0,06 Gew.-% (bei Panat)
• Fe Rest

An alloy that is particularly favorable in terms of its properties has the following composition:

• P 1.0% by weight
• Si 1.75% by weight
• C 0.06% by weight (for Panat)
• Fe rest

Panat or mixed powder with wax were pressed at 6.5 to 8.5 _M p / cm ² . After sintering at 1150 ° C. for one hour in hydrogen, sintered densities between 7.25 and 7.40 g / cm ³ resulted.

• Bruchfestigkeit R_m = 570 bis 600 N/mm²
• 0,2 %-Dehngrenzen R_p0,2 (zyklisch) = R_{p0,2 (zügig)} = 500 bis 550 N/mm²
• Dauerschwingfestigkeit σ_D = 250 N/mm² bei α_k = 1
• (Biegung) σ_D = 140 bis 160 N/mm2 bei α_k = 3,2.

The main properties of this alloy are:

• Breaking strength R _m = 570 to 600 N / mm ²
• 0.2% proof stress R _p0.2 (cyclic) = R _{p0.2 (brisk)} = 500 to 550 N / mm ²
• Fatigue strength σ _D = 250 N / mm ² at α _k = 1
• (Bending) σ _D = 140 to 160 N / mm2 at α _k = 3.2.

• - Billige und leicht verfügbare Legierungselemente
• - Durch Einfachsintertechnik Raumerfüllung ≧ 94 %
• - Hohe Werte der mechanischen Eigenschaften bei niedriger Sintertemperatur von etwa 1150 °C
• - Im Dichtebereich 7,25 bis 7,4 weitgehende Unabhängigkeit der Festigkeit und der Maßänderung von der Dichte
• - Dauerschwingfestigkeit im gekerbten Zustand entspricht derjenigen von gekerbten Walzstählen, die im Bereich von 160 bis 180 N/mm² liegt
• - Trotz des hohen Si- und P-Gehaltes, der laut Phasendiagramm zu einer Abschnürrung des α-Bereiches führt, ist die Legierung in technisch verwertbarem Maße härtbar.

The advantages of this sintered alloy produced by the process according to the invention are summarized again below:

• - Cheap and readily available alloying elements
• - Space filling ≧ 94% thanks to simple sintering technology
• - High mechanical properties at a low sintering temperature of around 1150 ° C
• - In the density range 7.25 to 7.4, the strength and dimensional change are largely independent of the density
• - The fatigue strength in the notched state corresponds to that of notched rolled steels, which is in the range of 160 to 180 N / mm ²
• - Despite the high Si and P content, which according to the phase diagram leads to a constriction of the α range, the alloy is hardenable to a technically usable degree.

Claims (9)

1. A process for the production of sintered steel high space filling by simple sintering technology, wherein a powder is pressed from a metal alloy and sintered at a temperature at which the metal alloy is simultaneously in the liquid and in the solid state during sintering, characterized in that as the starting powder instead of one homogeneous metal alloy, a powder mixture consisting of several components with iron as the main component is used, which, without reaction of the components to one another, only exists in an aggregate state during the sintering process, the two aggregate states of which are solid and liquid, however, during the sintering process by reaction of at least two components of the starting powder with one another to form a corresponding alloy phase. 2. The method according to claim 1, characterized in that at least one layer of one or more alloy components is applied to the surface of the iron powder particles, the powder produced in this way is then pressed and then sintered. 3. The method according to claim 2, characterized in that a layer is first applied to the surface of the iron powder particles, which slows down the diffusion of the alloy components into the iron. 4. The method according to claim 3, characterized in that first a graphite layer is applied before the iron powder particles are provided with at least one layer of the alloy components. 5. The method according to claim 1, characterized in that iron powder is intimately mixed with one or more alloy components, that the mixed powder is then pressed and the pressed part is then sintered. 6. The method according to claim 5, characterized in that a wax is added to the powder as a pressing aid. 7. A method for producing an iron-silicon-phosphorus sintered alloy with a space filling of at least 94% according to one of claims 2 to 4, characterized in that the iron powder is first coated with a graphite layer by the graphite powder with a binder and then with the iron powder is mixed, that a layer of ferrosilicon and ferrophosphorus is then applied to the powder particles thus produced, by mixing the three types of powder with the addition of a binder, and that the entire powder is finally pressed and then sintered. 8. The method according to claim 7, characterized in that the components are mixed in such a ratio that the carbon content 0.05 - 0.3 wt .-%, the silicon content 1.0 - 2.0 wt .-%, the phosphorus content from 0.8 to 1.4 wt .-% is that the pressing pressure at 6.5 to 8.5 Mp / cm ² and the sintering temperature at 1150 - 1180 is ⁰ C and the sintering is carried out in a hydrogen atmosphere. 9. The method according to claim 7 or 8, characterized in that after the application of the graphite layer and after the application of the layer of the alloy components, a one-hour heat treatment is carried out at 700 ° C.