EP3007842B1 - Method for producing heat- and wear-resistant molded parts, in particular engine components - Google Patents

Description

The invention relates to a process for the production of 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, highest demands are made in terms of thermal stability, temperature and wear resistance. But even high-temperature-stressed, rotationally symmetrical engine components such as valve guides, bearing bushes, shaft seals, valve seat inserts (VSR) and engine components for exhaust gas recirculation (EGR components) must meet these requirements.

For example, a valve seat ring (VSR) as a tribological partner to the valve must ensure a secure sealing of the combustion chamber for proper combustion, a faultless charge exchange, the heat transfer from the valve and the combustion chamber to the cylinder head, as well as a lossless guidance of the fresh and exhaust gas flow.

A tailored to the respective applications design 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 on the VSR leads to the loss of valve clearance and thus to leaks and compression losses. The further development of internal combustion engines and thus the increase in power density and reduction of emission values are therefore limited, among other things, by the functionality of the tribological system valve / VSR.

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

In addition, depending on the application, additional technical requirements are made of the components, such as sufficient relaxation and corrosion resistance. Relaxation is a thermally activated process that occurs under mechanical stress at high temperatures and causes components to release internal stress. In short, relaxation is the transformation of elastic strain into plastic creep at constant total elongation. In a VSR pressed into the cylinder head with a defined excess, the so-called overlap, high compressive stresses occur. Depending on the level of stress and the relaxation resistance of the respective material, the VSR reduces these stresses by relaxation. The initially purely elastic elongation caused by the press-fitting is partially converted into plastic creep. The VSR has a lower coverage after removal. Consequently, valve seat rings must have sufficient relaxation resistance.

Furthermore valve seat rings are subjected to corrosive application due to application. For example, the tribochemical load on VSR is exacerbated by the measures under the new emission regulations. In commercial vehicle applications, exhaust gas recirculation (EGR) can lead to condensate formation in the inlet to reduce nitrogen oxide emissions and, as a result, corrosion to the VSR. Critical states do not occur at high loads, but at standstill when the engine cools down. In many commercial vehicle applications, therefore, the use of corrosion-resistant VSR is required, at least on the inlet side.

The prior art discloses a variety of methods for producing the above-described engine components; For example, by casting techniques on the processes of centrifugal and sand casting, as well as powder metallurgy via pressing and sintering. Also, special manufacturing processes in which material is applied by a build-up welding process are known.

So revealed the WO 2005/012590 A2 a method for producing valve seat rings which are formed by thermally sprayed layers of a Co or Co / Mo base alloy and a Lichtbogendrahtspritzverfahren with one or more metallic cored wires, the jacket contains the essential portion of the Co to be deposited.

Further, the WO 2001/049437 A2 a powder metallurgy 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, substantially 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 producing rigid molds from an Fe-based sintered material by preparing the sintered material by water atomization, drying and agglomeration with an organic binder, followed by pressing and sintering.

Cast-formed VSR 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 are generally no longer able to meet the highest demands in heavy commercial vehicle and stationary engines and in alternative fuel operation. This requires special alloys based on Ni and Co. These are characterized by high hot compressive strength, high temperature resistance and excellent wear properties. In addition, these materials show sufficient resistance to relaxation and corrosion.

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

Two production methods are known for the casting production of VSR, namely the centrifugal and sand casting. Centrifugal casting is particularly suitable for large quantities. However, there are technological and economic limitations due to the high need for reworking. Thus, the production of high-strength Co or Ni-based materials via centrifugal casting is only partially possible or due to the heavy machinability usually uneconomical. Sand casting, on the other hand, is only suitable for small and medium quantities. The individual steps of post-processing are the same in spin casting and single casting. In both cases, the VSR must be completely processed to remove the casting skin.

Finally, the reveals EP 1 536 027 B1 a sintered body of a raw or granulated powder having a relative density of 97% or more. According to the invention, a powder with an average particle size of 8.5 μm or less is to be provided for this purpose. Since the average grain size of the high powder for filtering according to the information in the document is comparatively small, the filtration diffusion rate is improved and the quality of the filtering increased to a great extent.

Due to the high content of expensive alloying elements and the problem described in the conventional production, in particular of such a finely ground powder, it is necessary to enrich the state of the art in order to meet the market requirements and the divergent objective described above.

The object of the invention is to provide a process for the production of heat-resistant and wear-resistant molded parts, in particular engine components, which circumvents the above-mentioned disadvantages in the prior art. In particular, engine components for highly stressed 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 end customers. Furthermore, should be specified by the process produced resistant to wear and wear resistant moldings.

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

The present invention therefore turns away from the prior art, which calls for achieving high densities, in particular of> 97% grain size for the sintered powder of <8.5 microns. Accordingly, according to the invention leads, contrary to the prior art, an increase in the particle sizes to> 10 microns, preferably to> 30 microns and more preferably to> 50 microns in conjunction with a water atomization at comparable high densities of the sintered body.

The following process steps are to be carried out according to the invention: First, a Fe or Co based sintered material having an average particle size of> 10 .mu.m, preferably of> 30 .mu.m and more preferably of> 50 .mu.m, is produced by water atomization, i. high pressure water is used to smash the molten metal stream; then the agglomeration of the sintered material is carried out by spray drying to 10 to 400 microns with an organic binder, optionally solid lubricants and / or hard phases are added; then a cold or warm isostatic pressing of the sintered material is carried out with a pressure of 400 to 900 MPa to densities of 5 to 7 g / ccm; then the sintering of the green compacts takes place 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, the sintering material used is an iron-based alloy having 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 residue Fe; as well as 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, the sintering material used is an iron-based alloy having the following composition in% by weight:

• 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; Remainder Fe; as well as 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, the sintering material used is an iron-based alloy having 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; Remainder 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, the sintering material used is an iron-based alloy having the following composition in% by weight:

• 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; as well as max. 3.0 unavoidable impurities.

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

In a further preferred embodiment of the invention, the sintering material used is a cobalt-based alloy having the following composition in% by weight: max. 3.0 Fe; 2.0-2.8 C; 27.0 - 32.0 Cr; 0.5-1.5 Si; Rest of Co; as well as max. 3.0 unavoidable impurities.

As a protective atmosphere, an N ₂ -H ₂ atmosphere is used with a mixing ratio of about 80 - 20% or vacuum. It has been found that the temperatures during sintering are between 1,000 to 1,300 ° C, with an Fe-based sintered material in the lower temperature range, ie, from 1,000 to 1,200 ° C and a Co-based material in the upper temperature range, ie 1,100 to 1300 ° C must be sintered. The preparation of the Fe- or Co-based sintered material is carried out by a so-called. Water atomization.

The powder metallurgical production process makes it possible to produce high-precision molded parts, which require fewer post-processing steps compared to the classical production by casting. In addition, materials with special structural properties can be produced. In the context of the invention, it has been found that the components produced have an improved microstructure and distribution of the hard phases compared to the prior art. Thus, the microstructure of the alloys according to the invention is composed of a matrix having a particle size of 5-10 μm, in which uniformly finely divided carbides or intermetallic phases having a particle size of likewise 5-10 μm and optionally solid lubricants, such as, for example, MoS ₂ , are stored. The heterogeneous microstructure is very promising from a tribological point of view: heterogeneous microstructures with hard carbides or intermetallic phases, for example, have proven effective against the wear mechanisms of adhesion, surface distortion and abrasion (see Sommer, Heinz, & Schöfer, 2011).

In addition, dense microstructures can be produced, which leads to increased corrosion resistance, compared to conventional sintered materials from conventional sintered powders with an initial particle size greater than 50 microns. These structural properties can not be produced by the hitherto known production methods. 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 engine component moldings are based on base iron and shellac base powders and also have a heterogeneous microstructure with embedded hard phases and solid lubricants. Experience has shown that inlet components with low to medium requirements can be produced, among other things, materials with open porosity. For higher requirements and outlet components, the porosity is usually filled with Cu via an infiltration process. As a result, on the one hand the susceptibility to oxidation is reduced and on the other hand the thermal conductivity is increased. Various attempts to produce the above-described high-alloy Fe base and in particular Co base alloys with in some cases sufficient corrosion resistance by means of classical powder metallurgical production methods have failed. It has thus been found that the materials described can be produced exclusively by the production method 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 valvetrain and turbocharger components for use in high-temperature applications or powder metallurgy production of high-alloyed alloys. and especially co-base materials.

The invention is based on the finding that the powder-metallurgical production of engine components, such as valve seat rings, for high-temperature applications is characterized by a low post-processing requirement, high cost-efficiency and resource-saving production. Various experiments have shown that classic cast materials with equivalent property profile can be produced exclusively via the described manufacturing process. It was found that materials with very heterogeneous structures and positive wear properties can be produced

The moldings, in particular engine components, must be post-processed, in particular machined, since the green compacts shrink by 10 to 20% during the sintering process. A finishing step is understood to mean turning (outside and inside), plan, round and vibratory grinding. Which or which post-processing step (s) are used depends on the respective engine component to be produced. The Nachbearbeitungsbedarf is significantly reduced in the inventive method compared to the classical casting technique, whereby the economic production of high-alloy Fe and especially co-base materials is possible.

In addition, solid lubricants, which are resistant up to temperatures of 1100 ° C, possibly with a very high proportion, be added. The solid lubricants and optionally hard phases are formed as a heterogeneous structure and evenly distributed in the molding. For example, molybdenum disulfide (MoS ₂ ), manganese sulfide (MnS) and calcium fluoride (CaF) can be used as the solid lubricant. The addition of solid lubricants leads on the one hand to increased wear resistance in dry environments such as those present in internal combustion engines and, on the other hand, to improved machinability during the finishing of the engine manufacturer.

Furthermore, hard materials, for example intermetallic phases or ferro-molybdenum (FeMo), can be admixed. 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 may also contain hard, fine-grained and fine-grained carbides as well as other heterogeneously distributed carbides. This leads to increased wear resistance and relaxation resistance, since grain boundary slip 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 produced economically from materials which are exclusively known by casting technology. This opens up wear-technological and economic advantages. To date, no manufacturing process is known with which, with equal high efficiency such material concepts can be realized.

The parameters for further processing of the granulated sintered material according to conventional powder metallurgical production methods 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 produced by the process according to the invention, in particular engine components, have a wide field of application. The focus is on the application in the field of engine components which have a dense, heterogeneous microstructure and uniformly distributed solid lubricants and hard phases, and / or in the area of components which have a high corrosion resistance through a dense structure. These include, in particular, valve seat rings for highly loaded internal combustion engines, in particular utility vehicle and stationary engines and bearing bushes for turbochargers and other components of the exhaust gas line, which must have excellent tribological properties with the lowest solid lubrication; In addition, they must withstand high stress 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 known as PL 510.

A water atomized sintered material having the following composition in wt .-%: 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 remainder Fe, with an average grain size of> 30 microns was sintered according to the inventive method to form a molding, wherein a first Cold isostatic presses with a pressure of 800 MPa, debindering at 750 ° C, and finally a sintering under inert gas (H ₂ - N ₂ ) at 1135 ° C for 40 min and a subsequent heat treatment, namely a hardening at 920 ° C and tempering at 670 ° C was carried out.

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

The microstructures of the two materials PL 510 and PLS 510 are in the Fig. 1 and FIG. 2. This shows in comparison that in both micrographs, the gray hard phase is well and evenly formed, but in the PLS 510 finely divided, is present. These very finely divided carbides prove to be particularly advantageous for the molding according to the invention in relation to the described high-temperature applications. In particular, from a tribological point of view, significant advantages compared to the classic material of the casting technique can be seen.

Example 2:

Example 2 describes the production according to the invention of a material which is known by casting technology known as PL 860.

A water atomized sintered material having the following composition in wt .-%: 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; Residue Co, with an average grain size of> 30 microns was sintered according to the inventive method to form a molding, initially a cold isostatic pressing with a pressure of 800 MPa, debindering at 750 ° C, and finally sintering under vacuum at 1250 ° C. for 3 h took place.

The density of the molding obtainable by the process according to the invention, hereinafter referred to as PLS 860, is 8.47 to 8.56 g / cc; the hardness is 53.8 HRC. The molding is particularly suitable for high temperature applications and has a high wear, relaxation and corrosion resistance. It was proven on a relaxation test bench that the material PLS 860, a improved relaxation resistance, in comparison to the material PL 860. This is also clear from the microstructural characteristics of the two materials, which are represented by the micrographs of Figs. 3 and 4. It shows that the material has a heterogeneous microstructure with finely divided hard phases. As a result, grain boundary sliding is prevented on the one hand. Furthermore, experience has shown that the microstructure has a very positive effect on wear resistance.

Example 3:

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

A water atomized sintered material having the following composition in wt .-%: 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; Residual Fe, with an average particle size of> 30 microns was sintered according to the inventive method to form a molding, initially a cold isostatic pressing with a pressure of 800 MPa, debindering at 750 ° C, and finally sintering under inert gas (H ₂ - N ₂ ) was carried out at 1120 ° C for 1 h. The density of the molding obtainable by the process according to the invention, hereinafter referred to as PLS 26, is 7.35 g / cc; 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 microstructures of the two materials PL 26 and PLS 26 are shown in Figs. 5 and 6.

Example 4:

Example 4 describes the production according to the invention of a co-base material which is known by casting technology under the designation PL 840.

A water atomized sintered material with the following composition in wt .-%: max. 5.0 Fe; Max. 1.0 C; 15.0 - 30.0 Mo; 11.0 - 25.0 Cr; 1.0-2.5 Si; Residual Co, with an average particle size of> 30 microns was sintered according to the inventive method to form a molding, wherein initially a cold isostatic pressing with a pressing pressure of 800 MPa, debindering at 750 ° C, and finally sintering under vacuum at 1,250 ° C for 3 hours.

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

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

Claims (9)

1. Method for producing heat-resistant and wear-resistant moulded parts, in particular engine components, using a sintering material based on Fe or Co, according to the following procedure:

   - production of the sintering material based on Fe or Co 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 or hot isostatic pressing of the material with a pressure of from 400 to 2000 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 according to Claim 1, characterised in that 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.
3. Method according to Claim 1, characterised in that 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.
4. Method according to Claim 1, characterised in that 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.
5. Method according to Claim 1, characterised in that 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.
6. Method according to Claim 1, characterised in that 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; 1 1.0 - 25.0 Cr; 1.0 - 2.5 Si; remainder Co; and at most 3.0 unavoidable impurities.
7. Method according to Claim 1, characterised in that a cobalt-based alloy having the following composition in wt.% is used as the sintering material: at most 3 Fe; 2.0 - 2.8 C; 27 - 32 Cr; 0.5 - 1.5 Si; 10.0 - 14.0 W; remainder Co; and at most 3.0 unavoidable impurities.
8. Method according to any one of Claims 1 to 7, 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.
9. Method according to any one of Claims 1 to 7, characterised in that solid lubricants, which are stable up to temperatures of 1100°C, are admixed.

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