EP3007842B2 - Method for producing heat- and wear-resistant molded parts, in particular engine components - Google Patents

Description

The invention relates to a method for producing heat-resistant and wear-resistant molded parts, in particular engine components.

Basically, there is always interest in heat-resistant and wear-resistant molded parts. In particular, engine components such as valve train and turbocharger components are subject to the highest demands with regard to heat resistance, temperature and wear resistance. However, rotationally symmetrical engine components that are subject to high temperatures, such as valve guides, bearing bushes, shaft seals, valve seat rings (VSR) and engine components for exhaust gas recirculation (EGR components) must also meet these requirements.

For example, a valve seat ring (VSR) as a tribological partner to the valve must ensure that the combustion chamber is sealed securely for perfect combustion, faultless gas exchange, heat transfer from the valve and the combustion chamber to the cylinder head, and loss-free guidance of the fresh and exhaust gas flow.

A design tailored to the respective applications and the right material for this engine component, or the combination of the components VSR and valve, are therefore essential to ensure a perfect combustion process. A high wear rate at the VSR leads to a loss of valve clearance and thus to leaks and loss of compression. The further development of internal combustion engines and thus the increase in power density and reduction in emissions are therefore limited, among other things, by the functionality of the tribological system valve / VSR.

The main stresses in the valve / VSR tribological system are, for example, high relative movement and high surface pressure on the valve seat due to high combustion pressures from 200 to 250 bar, high thermal stress with temperatures at the valve seat of the valve seat ring of 300 to 500 ° C for highly stressed commercial vehicle and stationary engines and low solid lubrication when using alternative fuels and new exhaust gas treatment concepts. This results in extreme tribological stress on the valve and VSR components, which must be countered above all with appropriate material concepts.

In addition, depending on the application, additional technical requirements are placed on the components, such as sufficient relaxation and corrosion resistance. Relaxation is a thermally activated process that occurs under mechanical stress at high temperatures and leads to components relieving internal stresses. In short, relaxation is the transformation of elastic strain into plastic creep strain with constant total strain. High compressive stresses occur in a VSR that is pressed into the cylinder head with a defined oversize, the so-called overlap. Depending on the level of stress and the resistance to relaxation of the respective material, the VSR relieves these stresses by relaxation. The initially purely elastic stretch caused by the pressing is partially converted into plastic creep strain. The VSR shows less coverage after expansion. Consequently, valve seat rings must have sufficient relaxation resistance.

Valve seat rings are also subject to corrosive stresses due to the application. For example, the tribochemical load on VSR has been tightened as a result of the measures in the course of the new exhaust gas regulations. In commercial vehicle applications, exhaust gas recirculation (EGR) to reduce nitrogen oxide emissions can lead to condensate formation in the inlet and consequently to corrosion on the VSR. Critical conditions do not occur at high loads, but at a standstill when the engine cools down. In Many commercial vehicle applications therefore require the use of corrosion-resistant VSR at least on the inlet side.

A large number of processes for producing motor components described above are known in the prior art; for example, in terms of casting technology using the centrifugal and sand casting methods, and powder metallurgy using presses and sintering. Special manufacturing processes in which material is applied by a build-up welding process are also known.

So the reveals WO 2005/012590 A2 a process for the production of valve seat rings which are formed by thermally sprayed layers of a Co or Co / Mo-based alloy and an arc wire spraying process with one or more metallic cored wires, the sheath of which contains the essential part of the Co to be deposited.

Furthermore, the WO 2001/049437 A2 a powder-metallurgically produced sintered molded part for tribological parts in the automotive industry with high temperature and wear resistance, which consists of a powder mixture with molybdenum-phosphorus-carbon steel powder and at least one other, essentially phosphorus-free steel powder in a weight ratio of 5:95 to 60:40, carbon powder and at least one solid lubricant is available.

The US2003 / 177866 discloses a method for manufacturing rigid dies from an Fe-based sintered material by manufacturing the sintered material by water atomization, drying and agglomeration with an organic binder, followed by pressing and sintering.

VSR castings are mainly used in the commercial vehicle sector. Typical materials for medium to high loads are high-alloy, modified high-speed steels and high-alloy Fe-Cr steels. However, these materials generally no longer meet the highest demands in heavy commercial vehicle and stationary engines and operation with alternative fuels. This requires special alloys based on Ni and Co. These are characterized by high heat resistance, high temperature resistance and excellent wear properties. In addition, these materials show sufficient resistance to relaxation and corrosion.

In the course of the increasing emission requirements, as well as the increase in performance and the extension of the maintenance intervals, such material concepts are increasingly required for commercial vehicle and stationary engines.

Two manufacturing processes for the casting of VSR are known, namely centrifugal and sand casting. Centrifugal casting is particularly suitable for large quantities. However, due to the high post-processing requirements, there are technological and economic limits. For example, the production of high-strength Co or Ni-based materials by centrifugal casting is only possible to a limited extent or is generally uneconomical due to the difficult machinability. Sand casting, however, is only suitable for small and medium quantities. The individual post-processing steps are the same in centrifugal and single casting. In both cases, the VSR must be completely machined to remove the cast skin.

Finally, the EP 1 536 027 B1 a sintered body made of a raw or granular powder with a specific gravity of 97% or more. According to the invention, this is a powder with an average grain size of 8.5 μm or less provided. Since the average grain size of the high powder for the filtering is comparatively small according to the information in the publication, the filtering diffusion rate is improved and the quality of the filtering is markedly increased.

Due to the high content of expensive alloying elements and the problems described in conventional production, in particular such a finely ground powder, an enrichment of the prior art is required in order to meet the market requirements and the divergent objective described above.

The invention has for its object to provide a method for producing heat-resistant and wear-resistant molded parts, in particular engine components, which avoids the above-mentioned disadvantages in the prior art. In particular, engine components for heavy-duty commercial vehicle and stationary engine applications are to be created, which have sufficient wear resistance, low part costs, and good machinability on the production lines of the end customers. Furthermore, heat-resistant and wear-resistant molded parts produced using the method are to be specified.

To achieve the object, it is proposed to produce a water-atomized sintered material based on Fe or cobas, which has a grain size of> 10 µm, preferably of> 30 µm and particularly preferably of> 50 µm, and this material according to the further process steps according to claim 1 or to treat claim 2.

The present invention accordingly turns away from the prior art, which requires, in order to achieve high densities, in particular of> 97% grain size for the sinter powder of <8.5 μm. According to the invention, therefore, contrary to the prior art, increasing the grain sizes to> 10 μm, preferably to> 30 μm and particularly preferably to> 50 μm in conjunction with water atomization leads to comparable high densities of the sintered bodies.

According to the invention, the following process steps are to be carried out: First, an Fe- or Co-based sintered material with an average grain size of> 10 µm, preferably of> 30 µm and particularly preferably of> 50 µm is produced by water atomization, i.e. high pressure water is used to break up the molten metal jet; the agglomeration of the sintered material then takes place by spray drying to 10 to 400 μm with an organic binder, solid lubricants and / or hard phases optionally being added; then a cold isostatic pressing of the sintered material takes place with a pressing pressure of 400 to 900 MPa to densities of 5 to 7 g / ccm; then the green compacts are sintered at temperatures of 1,000 to 1,350 ° C and finally a post-processing step.

• 1,5 - 2,0 C; 5,0 - 13,0 Mo; 5,0 - 10,0 Cr; 0,8 - 1,8 Si; max. 1,0 Mn; 1,5 - 4,0 V; 0-10,0 Co Rest Fe; sowie max. 3,0 nicht vermeidbaren Verunreinigungen.

In one embodiment of the invention, an iron-based alloy with the following composition in% by weight is used as the sintered material:

• 1.5 - 2.0 C; 5.0 - 13.0 Mo; 5.0 - 10.0 Cr; 0.8-1.8 Si; Max. 1.0 Mn; 1.5 - 4.0 V; 0-10.0 Co balance Fe; and max. 3.0 unavoidable impurities.

• 1,9 - 2,2 C; 6,5-8,5 Cr; 1,1 - 1,4 Si; <0,5 Ni; 0,6-0,8 Mn; 2,3-2,7 V; 0,1 - 0,3S; 0 - 10,0 Co; Rest Fe; sowie max. 3,0 nicht vermeidbaren Verunreinigungen.

In a further embodiment of the invention, an iron-based alloy with the following composition in% by weight is used as the sintered material:

• 1.9 - 2.2 C; 6.5-8.5 Cr; 1.1 - 1.4 Si; <0.5 Ni; 0.6-0.8 Mn; 2.3-2.7 V; 0.1 - 0.3S; 0-10.0 Co; Balance Fe; and max. 3.0 unavoidable impurities.

• 0,2 -1,0 C; 20,0 -30,0 Cr; 14,0 - 23,0 Ni; 1,0 -3,0 Mo; < 2,0 Mn; 1,8 - 3,5 Si; 2,0 - 4,0 W; 1,0 - 3,0 Nb; 0,2 - 1,0 S; Rest Fe sowie max. 3,0 nicht vermeidbaren Verunreinigungen.

In a further embodiment of the invention, one is used as the sintered material Iron-based alloy used with the following composition in% by weight:

• 0.2 -1.0 C; 20.0 -30.0 Cr; 14.0-23.0 Ni; 1.0 -3.0 Mo; <2.0 Mn; 1.8-3.5 Si; 2.0 - 4.0 W; 1.0 - 3.0 Nb; 0.2-1.0 S; Rest of Fe and max. 3.0 unavoidable impurities.

• 0,6 - 1,1 C; 1,5 - 3,5 Mo; 21,0 - 28,0 Cr; 14,0 - 23,0 Ni; 2,0 - 3,3 Si; 2,0 - 3,5 W; 1,0-3,0 Nb; 1,0 - 3,5 Cu; Rest Fe; sowie max. 3,0 nicht vermeidbaren Verunreinigungen.

In a further embodiment of the invention, an iron-based alloy with the following composition in% by weight is used as the sintered material:

• 0.6-1.1 C; 1.5-3.5 mo; 21.0 - 28.0 Cr; 14.0-23.0 Ni; 2.0-3.3 Si; 2.0 - 3.5 W; 1.0-3.0 Nb; 1.0-3.5 Cu; Balance Fe; and max. 3.0 unavoidable impurities.

In a further embodiment of the invention, a cobalt-based alloy with the following composition in% by weight is used as the sintered material: max. 5.0 Fe; Max. 0.7 C; 15.0 - 25.0 Mo; 14.0-23.0 Cr; 0.7 - 1.4 Si; Balance Co; and max. 3.0 unavoidable impurities.

An N2-H2 atmosphere with a mixing ratio of approx. 80 - 20% or vacuum is used as the protective atmosphere. The temperatures during sintering were found to be between 1,000 to 1,300 ° C, with an Fe-based sintered material in the lower temperature range, i.e. from 1,000 to 1,200 ° C and a Co-based material in the upper temperature range, i.e. 1,100 to 1,300 ° C must be sintered. The Fe or Co-based sintered material is produced by a so-called water atomization.

The powder metallurgical manufacturing process makes it possible to produce high-precision molded parts that require fewer post-processing steps compared to conventional casting. In addition, materials with special structural properties can be manufactured. In the context of the invention, it was found that the components produced have an improved structural structure and distribution of the hard phases compared to the prior art. The structure of the alloys according to the invention is composed of a matrix with a particle size of 5-10 μm, in which evenly finely distributed carbides or intermetallic phases with a grain size of likewise 5-10 μm and possibly solid lubricants, such as MoS2, are stored are. The heterogeneous structure is very promising from a tribological point of view: For example, heterogeneous structures with hard carbides or intermetallic phases have proven themselves against the wear mechanisms of adhesion, surface disruption and abrasion (see Sommer, Heinz, & Schöfer, 2011).

In addition, dense structures can be produced, which leads to increased corrosion resistance compared to classic sintered materials made from conventional sintered powders with an initial grain size greater than 50 µm. These structural properties cannot be produced using the manufacturing processes known to date. Thus, the process is an enrichment of the state of the art both for the Fe-based Tool Steel materials and in particular for the Co- and Ni-based special materials.

Typical powder metallurgically produced molded parts for engine components are based on iron and fast-working steel base powders and also have a heterogeneous structure with embedded hard phases and solid lubricants. Experience has shown that inlet components with low to medium requirements can be made from materials with open porosity. For higher requirements and outlet components the porosity is usually filled with Cu via an infiltration process. This reduces the susceptibility to oxidation and increases the thermal conductivity. Various attempts to manufacture the above-described high-alloy Fe base and in particular Co-base alloys, some of which have sufficient corrosion resistance, using conventional powder metallurgy manufacturing processes have failed. It was found that the materials described can only be manufactured using the manufacturing process described.

The invention is based on the general idea of combining the advantages of powder metallurgy with the advantages of casting technology, with the aim of economically producing highly wear-resistant engine components, in particular valve train and turbocharger components for use in high-temperature applications or for the powder-metallurgical production of high-alloy alloys. and especially co-based materials.

The invention is based on the knowledge that the powder-metallurgical manufacture of engine components, such as valve seat rings, for high-temperature applications is characterized by a low need for reworking, high cost efficiency and resource-saving manufacture. Various tests have shown that classic cast materials with an equivalent property profile can only be produced using the manufacturing process described. It was found that materials with very heterogeneous structures and positive wear properties can be manufactured

The molded parts, in particular engine components, have to be reworked, in particular machined, since the green compacts shrink by 10 to 20% during the sintering process. A post-processing step means turning (outside and inside), face grinding, cylindrical grinding and slide grinding. Which or which post-processing step (s) are used depends on the particular engine component to be manufactured. The need for post-processing is significantly reduced in the method according to the invention in comparison to conventional casting technology, which enables the economical production of high-alloyed Fe and in particular Co-based materials.

In addition, solid lubricants that are resistant up to temperatures of 1,100 ° C, possibly with a very high proportion, can be added. The solid lubricants and possibly hard phases are designed as a heterogeneous structure and evenly distributed in the molded part. For example, molybdenum disulfide (MoS2), manganese sulfide (MnS) and calcium fluoride (CaF) can be used as the solid lubricant. The admixture of the solid lubricants leads on the one hand to increased wear resistance in dry environments as found in the internal combustion engine and on the other hand to improved machinability in the finishing process of the engine manufacturer.

Hard materials, for example intermetallic phases or ferro-molybdenum (FeMo), can also be added. The hard phases have a grain size of at least 50 to max. 500 µm, preferably from 150 µm to 300 µm. The densely sintered base materials can also contain hard, fine-grained and finely divided carbides as well as other heterogeneously distributed carbides. This leads to increased wear resistance and relaxation resistance, since grain boundary sliding and material deformation are hindered.

It has also been found that a sintered molded part produced according to the invention has increased corrosion resistance in comparison to conventional sintered materials with open or Cu-filled pores.

Overall, according to the invention, components for internal combustion engines can be economically produced from materials which are known only in terms of casting technology. This opens up wear-related and economic advantages. To date, no manufacturing process is known with which such material concepts can be implemented with the same high level of efficiency.

The parameters for the further processing of the granulated sintered material in accordance with conventional powder metallurgical manufacturing processes can be determined by a small number of tests, since the person skilled in the art is familiar with the effects which occur in principle.

The heat-resistant and wear-resistant molded parts, in particular engine components, produced by the method according to the invention have a wide field of application. The main application is in the area of engine components that have a dense, heterogeneous structure and evenly distributed solid lubricants and hard phases, and / or in the area of components that have high corrosion resistance due to a dense structure. These include, in particular, valve seat rings for highly loaded internal combustion engines, in particular commercial vehicle and stationary engines and bearing bushes for turbochargers and other components of the exhaust system, which must have excellent tribological properties with minimal solid lubrication; they also have to withstand high temperatures and pressures of> 210 bar.

The invention is described in more detail by the following process examples.

Example 1:

Example 1 describes the production according to the invention of a material which is known by casting technology and is known under the name PL 510.

A water-atomized sintered material with the following composition in% by weight: 1.5-2.0 C; 5.0-13.0 Mo; 5.0-10.0 Cr; 0.8-1.8 Si; Max. 1.0 Mn; 1.5-4.0 V; 0-10.0 Co rest Fe, with an average grain size of> 30 μm, was sintered into a shaped part in accordance with the method according to the invention, firstly cold isostatic pressing with a pressure of 800 MPa, debinding at 750 ° C., and finally sintering under protective gas (H2-N2) at 1135 ° C for 40 min and a subsequent heat treatment, namely hardening at 920 ° C and tempering at 670 ° C.

The density of the molded part obtainable by the process according to the invention, hereinafter referred to as PLS 510, is 7.51 g / ccm; the hardness is 55.3 to 58.7 HRC. The molded part is particularly suitable for high-temperature applications and has a high resistance to wear and relaxation.

The structural characteristics of the two materials PL 510 and PLS 510 are in the Fig. 1 and 2 shown. The comparison shows that the gray hard phase is good and even in both micrographs, but more finely distributed in the PLS 510. These very finely divided carbides prove to be particularly advantageous for the molded part according to the invention in relation to the high-temperature applications described. From a tribological point of view in particular, there are considerable advantages compared to the classic material used in casting technology.

Example 2:

Example 2 describes the production of a material which is not produced according to the invention and which is known under the designation PL 860.

A water-atomized sintered material with the following composition in% by weight: 0.1-0.3 Fe; 2.0-2.8 C; 27.0 - 32.0 Cr; 0.5-1.5 Si; 10.0 - 14.0 W; The rest of Co, with an average grain size of> 30 μm, was sintered into a shaped part in accordance with the method according to the invention, firstly cold isostatic pressing with a pressure of 800 MPa, debinding at 750 ° C., and finally sintering under vacuum at 1250 ° C. for 3 h.

The density of the molded part obtainable by the process according to the invention, hereinafter referred to as PLS 860, is 8.47 to 8.56 g / ccm; the hardness is 53.8 HRC. The molded part is particularly suitable for high-temperature applications and has a high resistance to wear, relaxation and corrosion. It was demonstrated on a relaxation test bench that the PLS 860 material has improved relaxation resistance compared to the PL 860 material. This is also evident from the structure of the two materials, which is shown by the micrographs of the Fig. 3 and 4 are shown. This shows that the material has a heterogeneous structure with finely distributed hard phases. On the one hand, this prevents grain boundary sliding. Experience has shown that the structure also has an extremely positive effect on wear resistance.

Example 3:

Example 3 describes the production according to the invention of a material which is known by casting technology and is known under the name PL 26.

A water-atomized sintered material with the following composition in% by weight: 0.2-1.0 C; 20.0 - 30.0 Cr; 14.0-23.0 Ni; 1.0-3.0 Mo; 0.5-1.0 Mn; 1.8-3.5 Si; 2.0 - 4.0 W; 1.0 - 3.0 Nb; 0.2-1.0 S; The rest of Fe, with an average grain size of> 30 μm, was sintered into a molded part in accordance with the method according to the invention, firstly cold isostatic pressing with a pressure of 800 MPa, debinding at 750 ° C., and finally sintering under protective gas (H2-N2 ) at 1,120 ° C for 1 h. The density of the molded part obtainable by the process according to the invention, hereinafter referred to as PLS 26, is 7.35 g / ccm; the hardness is 265 HV 10. The molded part is particularly suitable for high temperature applications and has a high resistance to wear, relaxation and corrosion.

The structural characteristics of the two materials PL 26 and PLS 26 are in the Fig. 5 and 6 shown.

Example 4:

Example 4 describes the production of a Co base material according to the invention, which is known by casting technology and is known under the name PL 840.

A water-atomized sintered material with the following composition in% by weight: max. 5.0 Fe; Max. 1.0 C; 15.0 - 30.0 Mo; 11.0-25.0 Cr; 1.0 - 2.5 Si; The rest of Co, with an average grain size of> 30 μm, was sintered into a shaped part in accordance with the method according to the invention, firstly cold isostatic pressing with a pressure of 800 MPa, debinding at 750 ° C., and finally sintering under vacuum at 1,250 ° C. for 3 h.

The density of the molded part obtainable by the process according to the invention, hereinafter referred to as PLS 840, is 8.62 g / ccm; the hardness is 49.2 to 51.1 HRC. The molded part is particularly suitable for high-temperature applications and has a high resistance to wear and relaxation.

The structural characteristics of the two materials PL 840 and PLS 840 are in the Fig. 7 and 8 shown. The comparison shows that the PLS 840 material has a heterogeneous structure with evenly distributed intermetallic phases and solid lubricants, while the PL 840 material shows large, coherent hard phases, which means that increased resistance to wear and relaxation can be expected. It was also found that the machinability of the PLS 840 material is significantly improved compared to the PL 840. The production of the PLS 840 material is significantly increased compared to its casting counterpart

Claims (4)

1. Method for producing heat-resistant and wear-resistant moulded parts, in particular engine components, using a sintering material based on Fe in which,
   an iron-based alloy having the following composition in wt.% is used as the sintering material: 1.5 - 2.0 C; 5.0 - 13.0 Mo; 5.0 - 10.0 Cr; 0.8 - 1.8 Si; at most 1.0 Mn; 1.5 - 5.0 V; 0 - 4.0 Ti; 0 - 10.0 Co; remainder Fe; and at most 3.0 unavoidable impurities,
   an iron-based alloy having the following composition in wt.% is used as the sintering material: 1.9 - 2.6 C; 6.5 - 8.5 Cr; 1.1 - 1.8 Si; <0.5 Ni; 0.6 - 0.8 Mn; 2.3 - 2.7 V; 0.1 - 0.3 S; 1.0 - 3.0 W; 7.0 - 14.0 Mo; 0 - 10.0 Co; remainder Fe; and at most 3.0 unavoidable impurities,
   an iron-based alloy having the following composition in wt.% is used as the sintering material: 0.2 - 1.0 C; 20.0 - 30.0 Cr; 14.0 - 23.0 Ni; 1.0 - 3.0 Mo; < 2.0 Mn; 1.8 - 3.5 Si; 2.0 - 4.0 W; 1.0 - 3.0 Nb; 0.2 - 1.0 S; remainder Fe; and at most 3.0 unavoidable impurities, or
   an iron-based alloy having the following composition in wt.% is used as the sintering material: 0.6 - 1.1 C; 1.5 - 3.5 Mo; 21.0 - 28.0 Cr; 14.0 - 23.0 Ni; 2.0 - 3.3 Si; 2.0 - 3.5 W; 1.0 - 3.0 Nb; 1.0 - 3.5 Cu; remainder Fe; and at most 3.0 unavoidable impurities
   according to the following procedure:

   - production of the sintering material based on Fe with a particle size > 10 µm, preferably > 30 µm, and particularly preferably > 50 µm, by water atomisation;

   - agglomeration of the sintering material by spray drying to from 10 to 400 µm with an organic binder, optionally admixture of solid lubricants and/or hard phases;

   - cold isostatic pressing of the material with a pressure of from 400 to 900 MPa to densities of from 5 to 7 g/ccm;

   - sintering of the green compacts at temperatures of from 1000 to 1300°C;

   - finishing of the moulded parts.
2. Method for producing heat-resistant and wear-resistant moulded parts, in particular engine components, using a sintering material based on Co in which
   a cobalt-based alloy having the following composition in wt.% is used as the sintering material: 5.0 Fe; at most 1.0 C; 15.0 - 30.0 Mo; 11.0 - 25.0 Cr; 1.0 - 2.5 Si; remainder Co; and at most 3.0 unavoidable impurities
   according to the following procedure:

   - production of the sintering material based on Fe with a particle size > 10 µm, preferably > 30 µm, and particularly preferably > 50 µm, by water atomisation;

   - agglomeration of the sintering material by spray drying to from 10 to 400 µm with an organic binder, optionally admixture of solid lubricants and/or hard phases;

   - cold isostatic pressing of the material with a pressure of from 400 to 900 MPa to densities of from 5 to 7 g/ccm;

   - sintering of the green compacts at temperatures of from 1000 to 1300°C;

   - finishing of the moulded parts.
3. Method according to claim 1 or 2, characterised in that hard substances having a particle size of from at least 50 µm to at most 500 µm, preferably from 150 µm to 300 µm, are admixed.
4. Method according to claim 1 or 2, characterised in that solid lubricants, which are stable up to temperatures of 1100°C, are admixed.

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