EP0565160A1 - Process for making sintered iron components having a pore-free zone - Google Patents

Abstract

The process for making a sintered component, which is pore-free in some zones or edge zones but porous in the remaining zones, from iron materials starts from a component brought by conventional powder-pressing and sintering processes to a residual porosity of about 10% by volume. By means of additional process steps, which essentially are the zonal introduction of additional materials or local mechanical recompaction, these zones are brought to a residual porosity of 5% by volume and less; this produces a closed pore structure. Under these set conditions, the sintering component can be brought in a final HIP process step or sintering-HIP process step in the pretreated zones to a material density of 100% or to complete freedom from pores. Essential advantages: local improvement of the material properties, ability of the finished sintered components to be calibrated.

Description

The invention relates to a method for producing sintered molded parts made of iron materials which are pore-free in individual zones or edge zones and are porous in the other zones.

Sintered molded parts made of iron materials are usually produced by compressing powders in green presses to form green compacts or powder compacts and then sintering them using largely standardized processes. Sintered densities of approx. 90% of the theoretical density are achieved. This density can only be improved to a limited extent by means of known additional processes, unless other significant disadvantages are accepted. Accordingly, the mechanical strength properties lag behind those of molded parts made of melted, 100% sealed materials.
The cost advantages of pure chipless production speak in favor of the use of sintering technology for the production of molded parts. In relation to the dimensions achieved in powder pressing, the finished parts have good dimensional stability and tight, reproducible dimensional tolerances. Furthermore, sintered moldings can be excellently calibrated due to the existing residual porosity after sintering, that is, they can be pressed to a limited extent very precisely to a predetermined target size.

A large number of methods have now become known in order to uniformly material-uniform sintered molded parts, which as usual have residual porosity, to at least approximately theoretical, i.e. Bring 100% material density. Powder forging is one of the proposed processes that does not quite reach full density. Hot isostatic pressing is another suitable method which, however, is very complex due to the necessary coating of the powder or sintered body and is therefore ruled out for mass parts. The sinter-HIP process is a modification of the HIP process, which can also be used to remove residual porosities in a sintered part under the restrictions mentioned.

All of these methods are used with the aim of improving the mechanical, but e.g. also to improve the corrosive properties of sintered molded parts. A disadvantage of all of these methods is that a sintered molded part refined in this way becomes a "blank" which has to be reworked mechanically and which in this respect differs significantly from conventionally produced sintered parts. Conventionally manufactured sintered parts, optionally recalibrated in presses, are usually ready-to-install components.

Methods are also known for making shaped bodies made of materials which differ in certain areas, at least one of which is a sintered body, as tight as possible in all areas and thus mechanically strong.

For example, DE-A1 22 58 310 entitled "Sintered iron molded part and method and sintered tile for its manufacture" describes a way in which a molded part pressed from ferrous material is brought into connection with a medium from which at least one of the following is connected during the sintering process the sintering temperatures diffuse austenite-forming elements into the surface of the molded part ".
This results in a material refinement in the surface area with the aim of improving the surface wear resistance. The finished sintered iron molded part has porosity in all areas, and also in the diffusion area the molded part has at least "closed porosity" with a maximum total of about 95% material density.

According to the teaching of DE-A1 23 10 536, "Process for the production of articles made of composite metal", a core of a molded part which has been produced by melt-metallurgy and is therefore completely sealed is placed in the center of a container and the space between the core and the container wall is filled with metal powder. The "known", that is to say enclosed in the container, is exposed in an autoclave to high pressures and temperatures on all sides that its density on all sides comes "in the range of 100% of the theoretical density". The composite thus obtained is then forged or rolled out, for example. According to claims, powder densities of more than 95% of the theoretical density are achieved by this method. The composite body becomes dense in its entirety.
This makes it possible to achieve composites, the core of which consists of relatively tough and easily machined metals, while the edge zones, for example for the mentioned application milling tool, teeth or other irregular cutting surfaces, consist of extremely hard material.

DE 30 07 008 describes a wear-resistant part for internal combustion engines which comprises a base body made of a melted iron or steel material and an iron-containing sintered body which is intimately connected to the base body by sintering. The essence of the invention is the iron alloy proposed for the sintered body.
This method also serves the purpose of producing parts "which are characterized by high toughness inside their body and a particularly high abrasion resistance, at least in a section of their surface".

According to DE-A2 20 50 276, a wear-resistant hard metal powder is pressed and sintered onto a steel base body in order to produce a workpiece with a wear-resistant surface.
In contrast to the sintering of iron materials, hard metal can be made almost 100% leakproof due to the binder phase which is molten during sintering. The finished composite body is uniformly tight. The disadvantage here is the strong sintering shrinkage, which precludes the manufacture of molded parts in narrowly tolerated target dimensions without post-machining post-processing - in addition to other disadvantageous factors such as material brittleness and material costs.

It is common to all of the previous publications mentioned that composite materials are created by joining together individual material areas using sintering technology. The finished material composites have a consistently high, ideally 100% density. Individual molded part areas have different mechanical properties, but always high wear and strength values in the area of surface zones.

In a further development of the state of the art mentioned, the object of the present invention is to achieve the high mechanical strength that can be achieved for 100% dense materials in the case of molded parts made of iron materials in appropriately stressed molded parts and yet to allow a final calibration of the sintered molded part.

In particular, the task consists in practically completely eliminating the residual porosity of approx. 10% by volume remaining in the usual production by means of sintering, that is to say at least approximately 100 in these zones, by means of a sequence of suitable, individually known method steps in individual predetermined zones of a sintered molded part % material density and correspondingly high mechanical strength and wear resistance.
At the same time, however, the approximately 10% by volume residual porosity must be maintained or increased in other zones of the sintered molded part.
This is to ensure that, as with the use of standard methods, the part dimensions achieved with the green compacting are consistently preserved and that the finished sintered parts are suitable for a final calibration.
The process to be used should also have sufficient economic viability for the production of mass parts.

According to the invention, the object described above is achieved in a process for producing a sintered molded part from ferrous materials of the type described at the outset, according to which a molded part is brought to about 10 vol zone-wise introduction of filler materials into the remaining pores and / or by means of locally effective mechanical densification of the molded part, in these zones to a residual porosity of 5% by volume or less and thus to a closed pore structure and then by means of the HIP or sinter-HIP process is further compressed in these zones. All other zones of the sintered molded part retain the usual residual porosity of approximately 10% by volume.

In the present invention, the term “dense, approximately pore-free sintered molding in individual zones or edge regions” by definition means that these zones are practically 100% dense, but at least have negligibly small residual porosity of less than 1% by volume.

The powder pressing and sintering processes for sintered molded parts made of ferrous materials, which are characterized as "usual", are described in a wide range of processes in the relevant standard literature.
The individual additional process steps that are essential to the invention also include processes of this type which are well established in sintering technology and are well known to those skilled in the art. Preferred design details are set out in the subclaims and in the examples.
The commonly used short name HIP process means the hot isostatic post-compression of sintered parts. In the sintering HIP process, the processes of sintering and hot isostatic recompaction take place simultaneously and side by side. Reference is made in detail to the description in the following exemplary embodiments.

The following configurations of the method according to the invention have proven particularly useful.

Among the filler materials that can be introduced into the basic matrix of the iron material, preference is given to those that become molten below the usual sintering temperature of iron materials. The group of such filler materials includes: copper, manganese, nickel, phosphorus and / or boron. These filler materials can be used infiltrate the capillary forces of the pores as a liquid phase into the pores of the base material during the molding.
The filler materials can be introduced into delimitable zones, for example also into surface marginal zones of predetermined thickness.

The filler materials can function as a pure pore filler, but they are e.g. according to a preferred embodiment of the inventive method with appropriate heat treatment at least partially alloyed with the iron base material.

It has proven itself in practice to allow a liquid phase, which forms within individual zones of a compact composed of different types of elements during the sintering, to migrate specifically into predetermined other zones of the sintered molded part.

The superficial post-compaction of sintered materials by means of mechanical pressing or rolling is known per se. For the production of sintered molded parts according to the present invention, it has proven to be particularly advantageous to recompress edge zones of sintered molded parts to a residual porosity of 5% by volume or less by means of tumble pressing.

The method according to the invention allows the production of sintered molded parts made of ferrous materials, in which the advantages of molded parts produced by conventional pressing and sintering processes are, above all, shape stability, calibration capability and economy, with the advantageous properties of a high material density and high mechanical strength in individual, highly stressed zones are combined. Of particular importance is the increase in mechanical and wear resistance, e.g. in the area of the tooth flanks of a gear wheel.

For the success of the overall process, which is essential to the invention, it is crucial to first bring the usual pore volume of the sintered base material to values of 5% by volume or less in zones and to produce a "closed" porosity in these zones. Only then can the corresponding zones be brought to 100% density by HIP-pen or sinter-HIP-en. The rest of the sintered molded part with a continuous, ie usual porosity of approx. 10 vol.% Remains unaffected by the post-compression measures.

The process according to the invention is explained in more detail below with the aid of individual examples.

example 1

A ring-shaped sintered body is produced as a composite body from two different powders.
Powder grade 1 is a commercially available iron powder, as is commercially available, for example, under the name ASC.
Powder grade 2 is an iron-copper alloy FeCu20, as is also commercially available.
A ring tool is filled inside, ie in the area close to the axis, with iron powder ASC, outside with an iron powder alloy FeCu20. The powder composite, initially pressed together with 6 t / cm², undergoes the following conversion during the subsequent sintering:
The outer ring area of the sintered body, originally containing FeCu20, is emptied of the Cu phase after sintering with the formation of liquid phases and is therefore highly porous, while the inner part of the ring is filled with copper when the copper becomes liquid due to the higher capillary forces occurring in the pores there Has. In the micrograph of the composite material, a closed porosity can be seen in the interior with only a low residual porosity overall. This residual porosity that is still present in the inner part of the ring is eliminated in a subsequent process step by sintering HIP-pen. The outer part of the ring remains highly porous. The sintered molded part is calibrated after the sintering HIP process.

Example 2

A ring-shaped sintered part is produced using commercially available iron powders according to the usual pressing and sintering processes and has a normal density of approx. 90% of the theoretical density. The surface zone of the ring, which is remote from the axis, is then compacted by rolling to a depth of 0.5 mm - 1 mm, increasing from the inside to the surface and approximately 95% on the surface Density. Subsequent HIP pens or sinter HIP pens are used to bring a narrow surface layer of the surface area to the desired 100% density.

In the event that the 100% dense zone of limited width that can be achieved by rolling is to be expanded, a defined amount of a liquid Cu phase is introduced into the sintered part in the pre-sintered and rolled sintered part blank using an impregnation process. The liquid phase is preferably stored in the edge area compressed by rolling, but not already 100% compressed, because there higher capillary forces occur due to the smaller pore dimensions. The infiltrated liquid phase still has a "closed residual porosity". HIP-pen compresses an enlarged edge zone 100%, while the normal porosity is retained inside the sintered part. The ring will then calibrate to size.

Example 3

A sintered molding produced by conventional pressing and sintering processes is compressed within defined zones by mechanical repressing to such an extent that during a subsequent sintering HIP process a liquid phase can be infiltrated, which initially accumulates in the redensified area of smaller pores due to the greater capillary forces there and then leads to densified zones with closed porosity via the liquid phase sintering process.
The subsequent sintering HIP process leads to molded parts with a pore-free zone. Outside the pretreated zones, the original, open porosity remains unchanged in the sintered part.
In a final calibration process, the sintered molded part is shaped into a dimensionally stable component, ie with narrow measurement tolerances.

Example 4

A gearwheel with approx. 90% density produced using commercially available, powdered iron base materials according to the usual pressing and sintering processes is coated in the area of the tooth contours with a boron or phosphorus base alloy mixed into a paste. These additional alloys serve as liquid phase formers. During the subsequent heating of the molded part to sintering temperature in a sintering HIP process, in a first sub-step the coated filler materials boron or phosphorus become molten and diffuse into the edge zones of the sintered molded part or, due to the capillary forces in the pores, become an edge zone of 0 , 5 to 1 mm thick. The composite obtained in this way has a closed porosity, ie at least 95% density, in the edge zone with deposits. This closed residual porosity is completely eliminated in a second step of the sintering HIP process.
The gears obtained in this way have a pore-free, 100% dense and high-strength surface zone in the tooth area, the strength of the surface coming close to that of corresponding molten steel materials or equaling this. The remaining zones of the gear maintain their original porosity. The gearwheel with the corresponding structure is calibrated in a final process step.

Claims (8)

Process for producing a sintered molded part made of ferrous materials which is pore-free in individual zones or peripheral zones and is porous in the other zones,
characterized,
that in a further process step a molded part brought to a residual porosity of about 10 vol Vol.% Or less and thus brought to a closed pore structure and then further compressed in these zones by means of the HIP or sinter-HIP process. A process for producing a sintered molded part according to claim 1, characterized in that filler materials are introduced which are molten below the usual sintering temperature of iron materials. A process for producing a sintered molded part according to claim 2, characterized in that Cu, Mn, Ni, P and / or B are introduced as filler materials. A process for the production of a sintered molded part according to claims 1 to 2, characterized in that, following pre-sintering of the iron base material during the sintering process, the filler materials are infiltrated into the pores of the base material in the liquid phase. A process for the production of a sintered molded part according to claims 1 to 4, characterized in that the filler material is applied in a metered amount to the iron material processed into the sintered blank, infiltrated into the iron material during the subsequent sintering process when the melting temperature is reached and is stored in the zones of smallest porosity. A process for producing a sintered molded part according to claims 1 to 2, characterized in that the iron materials are alloyed with the additional materials introduced in the sintered molded part. A process for producing sintered molded parts according to claim 1, characterized in that individual edge zones of the sintered molded part are post-compressed to 5% by volume residual porosity or less by means of tumble pressing. Sintered part produced by the method according to claims 1 to 7, characterized in that it has unchanged finished dimensions compared to the powder compact.