FR2995615A1 - PROCESS FOR PREPARING A SET OF SINTERED ELEMENTS - Google Patents

Abstract

The invention relates to a method for preparing an assembly consisting of a first sintered element and another sintered element or a solid material element, in which the sintered element (s) is are prepared from an iron-based sintered material, in particular steel, and the solid material element is prepared from an iron-based material, in particular steel, wherein in addition, in the first and optionally the second sintered element, before the interconnection thereof or the connection with the solid material element, the carbon content is increased by carburation, and in which before the connection, at least in the first sintered element, a layer with an increased carbon content is removed at least at the junction point. Before carburization, the zone to be carburized, in particular the entire sintered element, is at least partly provided with a diffusion barrier for the carbon.

Description

The present invention relates to a process for the preparation of an assembly composed of a first sintered element and another sintered element or of a solid material element, in which the sintered element (s) ( s) is / are prepared from an iron-based sintered material, in particular steel, and the solid material element is prepared from an iron-based material, in particular the steel, wherein in addition, in said first and optionally the second sintered element, before their interconnection or connection with the solid material element, the carbon content is increased by carburation, and wherein before the connection, at least in said first sintered member, an increased carbon-containing layer is removed at least at the junction point. For iron-based elements that are welded together in order to prepare a larger assembly from them, a carbon equivalent as low as possible must exist at the junction area, otherwise the elements have poor capability. welding. In material science, the carbon equivalent is a measure of the weldability of unalloyed and low alloyed steels. The carbon content and a plurality of other alloying elements in the steel affect its behavior. In order to evaluate the weldability, the carbon equivalent thus groups together in a single numerical value the carbon content and the weighted part of the elements having a similar influence on the weldability of the steel, as we could wait for the carbon. In this case, a carbon equivalent value of less than 0.45% implies good weldability. Depending on the machining thickness, higher values require preheating of the material, as this does not always lead to the desired result. From a value greater than 0.65, the workpiece is only -2 difficult to weld since the martensitic transformation may cause cracks cold or hot in the workpiece. The carburizing of steel elements, made of solid material or sintered material, by means of carburization is a usual method for improving the mechanical characteristics of such elements. During cementation, carbon is introduced into the material in the surface areas where a concentration gradient is formed through the layer thickness. Although this procedure is in principle the same for the elements of solid material, so for example the forged elements, that for the sintered elements, the result is however very different.

This is due to the fact that due to the porosity of the sintered elements, the carbon diffuses much more rapidly inside them than inside the elements made of solid material. This results in differences in the limiting resistance since the carbon concentration gradient is flatter due to the faster diffusion of the latter. For cemented steel elements, in order to improve the weldability, the increased carbon content marginal layer is usually mechanically removed in the welding zone. However, for the sintered elements, this process leads to the desired result only to a certain extent, since the carbon, as explained above, penetrates very deeply into the sintered elements and so a very large layer thickness would have to be removed to to reach a layer with a lower carbon content. This in turn causes a decrease in the load capacity of the assembly. It is therefore the object of the invention to improve the welding ability of cemented sintered elements to prepare an assembly. This object is achieved by the method mentioned at the beginning, in which before carburization, the zone to be carburized, in particular the entire sintered element, is at least partially provided with a diffusion barrier for the carbon. In this case, it is advantageous that in the subsequent carburization stage the diffusion barrier prevents the carbon from advancing to deeper layers, that is to say also up to the central zone, or although it significantly reduces the carbon content in the central area. It is thus possible to prepare a sintered element in which it is possible to achieve a clear concentration jump with respect to the carbon at the transition from the marginal zone to the central zone of the sintered element. Therefore, a sintered element prepared according to said method may have a hard outer martensitic layer and a soft perititro-ferritic core area thereon. With such a pronounced jump in carbon concentration, it is possible to produce a hardness gradient generated by a sharp transition from the martensitic structure to the equilibrium structure. It is thus possible to limit the high carbon portion to a relatively small marginal area that can be removed to prepare the assembly without significantly deteriorating the mechanical characteristics of the assembly by this weakening of material. As a side effect, the dynamic behavior of the sintered element can thus be improved in comparison with the sintered elements known according to the state of the art. Preferably the diffusion barrier is created by oxidation from the material of the element itself. It is thus possible to obtain a corresponding simplification of the process since it is not necessary to apply additional materials to the sintered element. In addition, it is thus also possible to better avoid the introduction of foreign bodies and subsequently -4 a possible modification of the property profile of the sintered element which can also be manifested only in use following various changes phase. However, an essential advantage is then due to the fact that the oxides are at least partly reduced again during carburation by the carbon which diffuses inside, so that this carbon is consumed and thus the depth of penetration carbon can be reduced overall because of too high diffusion rate or the concentration jump at the interface between the marginal zone and the central zone can be prepared in a more pronounced manner. According to the preferred embodiment of the process, the oxidation is carried out with steam. Currently, the oxidation under water vapor is implemented as a reprocessing process to obtain an improvement in the utility of the sintered parts. It is used both to increase corrosion resistance and to increase wear resistance. Likewise, it is possible to adjust an increase in the hardness of the element, a modified appearance, a better layer adhesion of the subsequently applied layers and an increased density of the elements. However, the elongation at break of the elements treated only with steam drops by about 50%. In the process according to the invention, the steam treatment has the advantage of making it possible to use a technology already known and repeatedly proven so that the execution of the process does not require any substantial modification with regard to equipment.

For the same reasons, the oxidation preferably takes place at a temperature between 400 ° C. and 800 ° C., in particular between 550 ° C. and 620 ° C. On the other hand, it is possible to combine the oxidation with a burning operation. It is thus possible to configure the preparation of the sintered element more efficiently, in that at the same time as the oxidation, impurities can be removed from the sintering process as such. As a side effect, it may then happen that combinations by oxidation, formed during the burning process, may also serve as a diffusion barrier. For a better understanding of the invention, this will be explained in more detail with the aid of the following description. The preparation of sintered elements proper is sufficiently described in the state of the art so that thereafter only a brief overview of a possible alternative embodiment of a sintering process will be provided. Essentially, the preparation can include the steps of powder mixing, compression, dewaxing and sintering. If necessary, the sintering may be followed by thermal reprocessing and / or mechanical reprocessing. 1) Mixing powders The iron powder mixtures used may have a total of up to 10% by weight of non-ferrous metal alloy elements, optionally up to 5% by weight of carbon in the form of graphite, up to 3% by weight of compression aid and up to 0.5% by weight of an organic binder. These mixtures are, for example, conventionally prepared from pure iron powder or pre-alloy or partially alloyed iron powders as base material and the addition of alloying elements as well as compressing aids. . Alternatively, so-called masterbatches in highly concentrated form, where appropriate also using temperature and / or solvents, are premixed and then mixed with the iron powder or mixed directly with the iron powder by addition of the components. individual. Binders that can be used include resins, silanes, oils, polymers or adhesives. Compression additives include waxes, stearates, amides and polymers. The pre-alloy elements may be Mo, V, Si, Mn.

Other non-ferrous alloy elements, such as for example chromium, copper, nickel, manganese, silicon, molybdenum and vanadium, make it possible to improve the properties of these powder-based sintered elements accordingly. iron, as is known for steels according to the state of the art. For example, the fact of adding molybdenum alloy allows to avoid the brittleness of income chrome steels. In this case, quenching penetration and viscosity are improved. In addition, it is possible to increase the creep resistance at elevated temperatures. Nickel improves the cold workability. Manganese makes it possible to improve the breaking strength and the yield strength. With the aid of silicon, it is possible to prevent the deposition of cementite resulting from the martensite during the income. Since the effect of principle of these alloying elements is known per se from the state of the art, an additional discussion can be omitted here.

The part of the nonferrous alloying elements can also be selected in a range having a lower limit of 0.2% by weight and an upper limit of 8% by weight, in particular in a range having a lower limit of 1% by weight. weight and an upper limit of 6% by weight. Typical mixtures are, for example: Fe (with pre-alloy of 0.85% by weight of Mo) + 0.1% by weight to 0.3% by weight of C + 0.4% by weight 1.0% by weight of a compression aid and optionally a binder - Fe + 1% by weight to 3% by weight of Cu + 0.5% by weight to 0.9% by weight of C + 0.3% by weight to 0.8% by weight of a compression aid and optionally a binder - Astaloy CrM (iron powder with Cr + Mo pre-alloy) + 1% by weight to 3% by weight of Cu + 0.1% by weight to 1% by weight of C + 0.3% by weight to 1.0% by weight of a compression aid and optionally a binder. However, all conventional compositions of the sintering technique are also possible. Examples of suitable mixtures are: 18% by weight of Mn + 2.5% by weight of Al + 3.5% by weight of Si + 0.5% by weight of V + 0.3% by weight of B the remainder being Fe or 24% by weight of Mn + 3% by weight of Al + 2.5% by weight of Si, the remainder being Fe or 14% by weight of Mn, 5% by weight of Ni + 3% by weight of Al + 3% by weight of Si, the remainder being Fe. These mixtures are mixed and homogenized by appropriate mixing processes resulting from powder metallurgy. It is also possible to use the technique of the binder process known from the state of the art and to use the known process of the alloy by diffusion for a more homogeneous distribution, especially parts of fine powder. 2) Compression Pretreated iron powder mixtures according to the above process can be densified and shaped by coaxial compression methods. Care must be taken to take into account changes in shape and configuration caused during subsequent process steps in the manufacture of compression tools. The use of suitable lubricants and binders promotes densification. Depending on the bulk density and the theoretical density of the powder mixtures, compression pressures of 400 MPa to 1200 MPa are applied for this purpose. The tablets (also called blanks) obtained in this way are the starting point for the following process steps. Instead of the coaxial compression methods, it is also possible to apply other compression methods, as are usual in sintering technology, for example also isostatic compression methods, etc. In order to achieve a reproducible dimensional behavior during sintering, it is appropriate during compression to ensure a distribution of the density as homogeneous as possible or at least reproducible within the sintered element. Since a liquid phase can occur during sintering (depending on the process conditions and the composition of the compression powder), it is advantageous during compression to choose the density to achieve a density distribution. as homogeneous as possible. Typical compression densities are thus between 6.4 g / cm 3 and 6.6 g / cm 3, but may also be chosen to be higher depending on the chemical composition and the compressibility of the powder. The necessary lubricants, if any, may be applied to the element either by conventional immersion processes or preferably by means of spraying methods before or during compression. 3) Dewaxing and sintering -9 The dewaxing of the tablets can be carried out by heat treatment, preferably under the effect of atmospheric gases with partial carburization or easily oxidizing, that is to say that they can be at least partially released by burning the organic binder and lubricants (insofar as this is not done in a subsequent process step, as described hereinafter) and sintered, preferably in continuous sintering furnaces. In this case, reducing atmospheres are obtained by using nitrogen / hydrogen mixtures with a hydrogen content of up to 30% by volume. Optionally, it is also possible to use carburizing gases, such as, for example, methane, propane or the like, or the dewaxing can be promoted by the easily oxidizing nature of the process gas (where appropriate only in partial areas of the furnace sintering), for example using an endothermic gas, moistened nitrogen or the like. However, the sintering can also take place under vacuum which makes it possible to obtain stabilization of the liquid phase during sintering. The temperatures during sintering are between 1050 ° C and 1350 ° C depending on the alloy system used, the sintering bearing time being between about 2 minutes and 1.5 hours. The process control during sintering is chosen so as to produce as little deformation as possible of the element in a liquid phase created if necessary. Reproducibility of the result is favored by adequate process control, such as atmospheric control, temperature control, dew point measurement, etc. The sintered element is preferably cooled to a selected cooling rate in a range having a lower limit of 0.5 K / s, in particular 1 K / s, and 20 K / s, preferably 15 K / s. If necessary, to obtain higher densities, the tablets can also be presintered by heat treatment at a temperature below 1100 ° C. under the effect of the reducing atmospheric gases mentioned above, and can be recompressed if necessary. , in particular with simultaneous dewaxing. It is thus possible to obtain the preparation of a weak sintered bond between the particles. 4) Thermal reprocessing Various heat treatments known from the state of the art are applicable. Thermal processes can be used to modify the ratio of the ferrite / perlite structural components. 5) Mechanical treatment All known methods of mechanical reprocessing or coating are possible. To improve the mechanical properties of these sintered elements based on iron, they are usually hardened, for example cemented. During carburizing, the carbon portion in the sintered element is increased by carburizing at the surface as well as in the surface layers. According to the process of the invention, it is then provided before the dipping to apply or to achieve a diffusion barrier for the carbon on or in the sintered element. An increase in the properties of the ultimate strength of cemented sintered parts is thus obtained. The increase in the ultimate strength can be produced by the introduction of a high carbon gradient and hardness.

In this regard, it should be noted that a high carbon gradient refers to a carbon concentration gradient in which the concentration decreases abruptly, i.e. in the range of a layer thickness between 400 μm and 1500 μm. pm, up to at least 80% of the initial concentration at the surface of the sintered element. At the same time, a high hardness gradient is achieved in which the hardness decreases through this 80% layer thickness range with respect to the hardness at the surface of the sintered element. In general, during cementation of the sintered parts or sintered elements, the carbon diffuses very rapidly through the pore channels and can thus penetrate rapidly inside the element. The introduction of a diffusion barrier makes it possible to reduce the rate of penetration of carbon. As a diffusion barrier, for example, a copper-based overlay paste may be applied, for example by spreading, spraying or immersing the sintered element or sintered elements. However, in the preferred embodiment, the diffusion barrier is formed by the sintered element itself, that is, from the material of the sintered element. More preferably, the iron is oxidized, particularly magnetite (Fe3O4). In particular, this oxidation can be carried out under steam.

Inserting a "damping process" before the carburizing operation causes a magnetite layer (Fe304) to be formed on the sintered element and / or in the pores of the sintered element. in the surface areas treated with water vapor, and can thus create a diffusion barrier. Before entering the iron particles, the carbon must reduce the oxide layer. Thus, the carbon diffusion rate is slowed down and the penetration depth of the carbon atoms decreases. Consequently, it is only at the edge of the element, ie in the marginal layer, that a martensitic zone is formed, whereas the interior of the element, that is, ie the central area, retains the original balance structure. The higher the hardness gradient, the higher the resistance of the element. With this method, it is possible to obtain resistances of the tooth roots exceeding 880 MPa (0F0.50 measured according to DIN 3990). During the steam treatment, the sintered ferrous material is heated in an atmosphere with water vapor and nitrogen such that on 3 Fe + 4H 2 O Fe 3 O 4 + 4H 2 on the surface accessible to water vapor (also in open pores) forms a dense, bluish-black, magnetite (Fe304) reaction layer that closes the pore channels as the thickness increases. Its rate of formation decreases with the increase of the layer. The temperature of the treatment under steam can then be between 400 ° C and 800 ° C, in particular between 500 ° C and 700 ° C, for example 600 ° C. The duration of the oxidation by means of the steam is preferably selected in a range of 60 minutes to 420 minutes, in particular in a range of 90 minutes to 200 minutes. Preferably, the oxidation under water vapor, i.e., said preoxidation process, is carried out in a continuous process in which the finished sintered elements are transported on a strip through the vapor of water. water or an atmosphere containing water vapor. The bandwidth that affects, as the temperature or the duration of the oxidation, the layer thickness of the oxide layer formed, can be between 25 mm / min and 80 mm / min, in particular between 30 mm / min. min and 80 mm / min. It is therefore possible with these parameters to regulate and / or control the oxidation process, in particular the layer thickness of the formed oxide layer. Subsequently, this layer thickness affects the concentration gradient achieved, as will be explained below. As an additional parameter, the proportion of water vapor in the oxidation atmosphere can be used.

This part of water vapor in the oxidation atmosphere may be between 75% by volume and 90% by volume, in particular between 80% by volume and 90% by volume, preferably between 85% by volume. volume and 90% by volume. The remainder up to 100% by volume is then air respectively. In particular, a volumetric flow rate of water vapor is between 40 kg / h and 100 kg / h, preferably between 75 kg / h and 90 kg / h. Instead of being carried out under water vapor, the oxidation can also be carried out with carbon dioxide or air or oxygen or mixtures thereof or with water vapor.

Thanks to a long duration of the oxidation treatment and / or the application of a high reaction temperature and / or a high proportion of oxidant and / or a low strip speed, a relatively high layer thickness of iron oxide, and vice versa. However, it is also possible for example that a high temperature and a high band speed are used to adjust the layer thickness. This list of oxidation control options should not be considered exhaustive.

Instead, it serves only to demonstrate that different combinations make it possible to adjust the layer thickness of the oxide layer to be formed, and hence subsequently the intensity of the carbon gradient or the hardness gradient.

The layer thickness of the oxide layer is preferably between 1 μm and 5 μm, in particular between 2 μm and 4.5 μm. It should be noted here that the oxide layer is not necessarily made entirely, ie continuously, but areas without an oxide layer or with a thinner oxide layer are allowed. in the course of the oxidation process, although in order to achieve as homogeneous carburation as possible, a layer thickness as homogeneous as possible over the entire surface to be carburized is preferable. Following this oxidation process, the sintered element is brought to carburizing. The carburization can be carried out for example in the form of carburization or carbonitriding. Both methods are well known from the state of the art. This increases the carbon fraction in the marginal layer.

For this purpose, in a carburizing aggregate, the sintered elements treated under steam are exposed to a reducing atmosphere. The gaseous composition is controlled using methanol and nitrogen. The proportion of methanol can then be selected in a range from 50% by volume to 80% by volume, in particular in a range from 50% by volume to 70% by volume. The nitrogen portion can then be selected in a range from 20% by volume to 50% by volume, particularly in a range from 35% by volume to 45% by volume. For example, a composition of 40% of H 2, 40% of N 2 and 20% of CO is used. It is also possible to use other gases, for example an endothermic gas (mixture of natural gas and atmospheric oxygen).

During the carburization, that is to say the increase of the carbon portion, in the marginal layer of the sintered element, the iron oxide layer, in particular the magnetite layer, is reduced to the surface of the element as well as on the inner surfaces of the open porosity, probably according to Fe3O4 (s) + 2 CO + 2H2 (g) 4 3 Fe (s) + 2 CO (g) + 4H2O (g). The carbon from the atmosphere thus decomposes the oxide slowly and can then diffuse into the iron grains. Since the carbon must progress at low speed, it is possible to ensure a certain homogeneity in the batch using the constant or slightly reduced process time.

The process time of the carburation can then be between 60 minutes and 640 minutes, in particular between 60 minutes and 350 minutes. The temperature of the carburization can then be between 800 ° C. and 1000 ° C., in particular between 830 ° C. and 950 ° C., preferably between 850 ° C. and 900 ° C. By varying the carburizing temperature and the carburization time, it is possible to control the depth of the martensitic marginal zone. The diffusion rate or diffusion coefficient increases exponentially with temperature and with the time root. This allows us to estimate how far carbon can enter the element. Carburization can be performed as a pure carburization. However, it is also possible to introduce other elements during the carburization or enrich the sintered element with at least one other element, so for example perform a carbonitriding. The introduction of the diffusion barrier with the consecutive carburization of the sintered element makes it possible to obtain a clearly defined martensitic marginal zone on the forgings. With a pronounced hardness gradient, generated by a sharp transition from the martensitic structure to the equilibrium structure, ie a perlito-ferritic structure, one can increase the dynamic performances since the sintered elements thus have on the one hand a hard marginal layer and on the other hand a relatively soft central zone with respect thereto. On the other hand, for sintered elements prepared without producing a diffusion barrier before the carburization, and therefore in particular without an oxidation step, the carbon penetrates deeply inside the element, because of the high diffusion coefficient. After quenching, a martensitic structure is thus produced in most of the volume of the element, so that the sintered element becomes brittle and the boundary strength properties degrade. In the case of subsequent machining by turning or removal of chips, as well as in the case of grinding operations, it is possible to take into account the removal of material by a margin of the marginal layer; that is, the marginal layer is prepared during the carburizing step with an upper layer thickness corresponding to the surge. In an alternative embodiment of the method, it is also possible to combine the step of the treatment under steam or oxidation with a burning operation so that it is possible to omit the additional process step, as explained previously. The temperature is then regulated at least in two phases, the burning operation being carried out in a first step at a low temperature in comparison with the oxidation temperature, and thereafter the temperature is increased to ) values mentioned above for oxidation. In the context of the invention, the following tests, grouped by way of example in Table 1, were carried out inter alia. For this purpose, a sintered powder gear has been prepared. In order to better verify the effects of the individual parameters described above, the same composition of the sintered powder was used for all the tests. However, it should be noted that in the context of the invention, it is also possible to use other ferrous or iron-based compositions. Examples 1 to 3 were prepared according to the process of the invention. Example No. 4 was prepared according to a method of the state of the art. For the comparative test with a sintered pinion prepared according to the state of the art, prepared following the same process path as the examples of the invention, but without treatment under water vapor, the same composition of the powder was been used. Composition of the sintered powder: 0.85% by weight of Mo + 0.3% by weight of C + 0.9% by weight of a compression aid, the remainder being Fe. preparation of the blank: 6 t / cm2 Temperature during sintering: 1250 ° C Sintering time: 45 minutes Composition of the reducing atmosphere: N2 / H2 (60% by volume / 40% by volume) Table 1 1 2 3 4 (Comparative example) Oxidation time [minutes] 90 90 60 Oxidation temperature [° C] approx. approx. approx. 600 600 600 Part of water vapor in the atmosphere 80 80 80 of oxidation [% by volume] Speed of tape [m / s] 50 50 80 Thickness of layer 4.1 4.1 3.5 layer of oxide [} gym] T oil [° C] 110 110 110 80 Carburation time [minutes] 30 + 30 + 30 + 90 + 90 60 60 60 Temperature of 830 860 860 900 carburation [° C] Carbon level [ %] 0.5 + 0.5 + 0.5 + 0.8 + 0.65 0.7 0.7 0.7 GFO, 50 [MPa] 658 898 862 7181 1: after heat treatment and honing been carried out at two different carbon levels. Therefore, in Table 1, two times are indicated for the carburization time, i.e. Sample 1 was carbureted for 30 minutes with a carbon level of 0.5% in volume and for 60 minutes with a carbon level of 0.7% by volume.

The carbon level then refers to the carbon share in the carburizing atmosphere. The oil temperature (T oil) in Table 1 indicates the temperature of the quench oil in which the sintered element is quenched.

The composition of the atmosphere during carburation was chosen according to the state of the art. The finished sintered gears have shown in the dynamic comparison tests of better limiting strength properties that sintered gears of the same geometry which have been prepared according to a method without the realization of a diffusion barrier, in particular a layer of 'oxide. With the aid of the sample bearing the number 1, Table 1 also shows that at a lower carburization temperature, a lower limit resistance is obtained. With this process it is possible to prepare sintered elements in which the carbon fraction decreases abruptly at the transition from the marginal layer to the central zone, in particular from a value of 0.65% by weight to a value of less 0.1% by weight, so that a clearly visible phase boundary on the micrograph is made between the marginal layer and the central zone.

In principle, carbon concentration gradients from 0.8 wt.% To less than 0.1 wt.%, In particular from 0.7 wt.% To 0.1 wt.%, May be set. With the process according to the invention, it is possible to prepare sintered elements having a hardness HV 0.3 in the marginal layer between 600 and 750, in particular between 650 and 750. The hardness HV 0.3 in the -19- central zone can be between 100 and 200, in particular between 120 and 180. In addition to any type of pinion, the method according to the invention also makes it possible to prepare other sintered elements, for example synchronization rings, sleeves sliding, coupling bodies, etc. In the following process of preparing the assembly, at least one of the sintered elements thus prepared is connected by welding to another sintered element, which may also have been prepared in the manner described above, or to an element solid material composed of an iron-based material, in particular steel.

For example, the assembly may comprise a pinion welded to a shaft or a welded coupling body or a welded member of a synchronization device, or be formed therefrom. The solid material element, for example the shaft, can also be cemented in a manner known from the state of the art. Thus, this element may have a higher carbon content in the marginal zone than in the central zone. Before the interconnection of the elements, therefore the first sintered element with the second sintered element - of course, it is also possible to interconnect more than two elements in a set, all or some of which can be prepared according to the method described above - the layer with the increased carbon content is removed at least at the junction point, at least in the first sintered element. The removal takes place in particular by means of mechanical methods, for example by removal of chips, as known from the state of the art.

Preferably, the layer with the increased carbon content is removed to a layer thickness at which the carbon content has decreased by at least 80% relative to the carbon content at the surface of the carbon. sintered element. Since a larger number of elements are used which have an outer marginal layer with an increased carbon content, it is preferably on all these elements at least at the junction point that this layer is removed before welding. The expression "at the junction point" means that the area removed may be slightly larger, for example up to 5%, than the area of the junction area itself. However, this does not mean that this layer is removed on the entire element.

In comparison with the state of the art, the method according to the invention makes it possible for example to obtain that it is not necessary to remove a layer up to a layer thickness of 2 mm, but only up to at a layer thickness of not more than 0.5 mm.20

Claims (5)

REVENDICATIONS1. Process for the preparation of an assembly composed of a first sintered element and another sintered element or of a solid material element, in which the sintered element (s) is / are prepared (s) ) from an iron-based sintered material, in particular steel, and the solid material element is prepared from an iron-based material, in particular steel, in which, in addition, in the first and optionally the second sintered element, before the interconnection thereof or the connection with the solid material element, the carbon content is increased by carburation, and in which before the connection, at least in the first sintered element, a layer with increased carbon content is removed at least at the junction point, characterized in that before carburizing, the zone to be carburized, in particular the entire sintered element, is at least partially provided a diffusion barrier for carbon. 2. Method according to claim 1, characterized in that the diffusion barrier is produced by oxidation from the material of the element. 3. Method according to claim 2, characterized in that the oxidation is carried out with steam. 4. Method according to any one of claims 2 to 3, characterized in that the oxidation is carried out at a temperature between 400 ° C and 800 ° C. 5. Method according to any one of claims 2 to 4, characterized in that the oxidation is combined with a burning operation.