WO2001049436A2 - Powder metallurgy produced press-sinter shaped part - Google Patents

Abstract

The invention relates to a powder metallurgy press-sinter shaped part, more particularly a valve or valve part, exhibiting thermal and wear resistance and having the following composition by weight: 0.5 % to 2.0 % C; 5.0 % to 16 % Mo; 0.2 % to 1.0 % P; 0.1 % to 1.4 % Mn; 0 % to 5 % Cr; 0 % to 5 % S; 0 % to 7 % W; 0 % to 3 % V and < 2 % of other elements, the remainder being Fe.

Description

 Press-sintered molded part produced by powder metallurgy

The invention relates to a powder-metallurgically produced pressed-sintered molded part with high temperature and wear resistance, which is in particular a valve or valve part for internal combustion engines.

Intake and exhaust valves for internal combustion engines must meet high requirements for temperature resistance and wear resistance. In particular in the case of highly compressed modern engines with multi-valve technology and electronic control, it has become increasingly a problem to find materials which can permanently cope with the high temperatures prevailing at the outlet. Accordingly, valves have become more and more complex to manufacture, which has had an impact on the material and processing costs.

Various powder metallurgical processes have been proposed for the production of valve bodies or entire valves. Such powder metallurgical processes have found their way into the manufacture of valve seat rings in many cases, but have so far not been able to establish themselves in valve bodies or valves. The reasons for this were insufficient stability of the materials and insufficient temperature behavior.

To improve the performance of conventionally manufactured valves, particularly stressed areas - especially the valve seat - have been equipped with an inductive one

Seat hardening or armor. This should keep the wear within acceptable limits, taking into account that too With this technology, valve temperatures of 800 ° C - 900 ° C should not be exceeded. However, this is becoming increasingly difficult to maintain in modern engines.

The conventional manufacture of valves and valve bodies has become extremely complicated, particularly when armor is used. Starting from a rod section, the valve body is first produced by heating, compressing, calibrating and rotating, to which a rod section is added by friction welding. Other work steps include straightening, turning, grinding and cladding, grinding and heat treatment to the finished valve with seat armor. In particular in the area of seat armor, the build-up welding can lead to errors, which results in an undesirably high reject rate.

Solutions to carry out a suitable seat armor with the help of armor made from powder metallurgy have not achieved serial status. The application of the armor did not reduce the error rate. Rather, the armor made by powder metallurgy proved to be susceptible to crack formation in the subsequent process steps.

The materials and additional process steps required for seat armor or hardening make it seem desirable to manufacture valves or at least valve bodies from a uniform material in as few steps as possible, the material ensuring the necessary wear resistance, service life and heat dissipation.

Methods for producing at least the wear layer of heavy-duty sintered parts in connection with the valve control of an internal combustion engine are known from DE 41 04 909 A1. The sintered parts produced there by powder metallurgy are characterized by a high chromium and carbon content and are used for cams for valve control. The use of such sintered parts for valve bodies is not provided. In the production of complex molded parts, powder metallurgy often has the advantage over conventional techniques that the material properties can be optimized and the number of processing steps can be reduced. The invention is therefore based on the object of producing valves and valve parts, ie valve bodies and stems, by powder metallurgy from a suitable material, in particular taking into account the production outlay. Seat armor should be dispensed with as far as possible and the valve or valve part should have sufficient thermal conductivity for temperature control.

This object is achieved with a powder-metallurgically produced molded part of the type mentioned at the outset, which has the following composition by weight:

0.5% to 2.0% C; 5.0% to 16% Mo; 0.2% to 1.0% P; 0.1% to 1.4% Mn; 0% to 5% Cr; 0% to 5% S; 0% to 7% W; 0% to 3% V; <2% other elements and remainder Fe.

The metal powder used according to the invention is characterized in particular by a very high carbon, molybdenum and phosphorus content.

The carbon and phosphorus content cause the formation of temperature-resistant and wear-reducing carbide and phosphide phases, which give the material the necessary service life. Chromium, vanadium and tungsten can be added to vary the range of properties, but are not absolutely necessary in particular for the production of valves and valve parts. An appreciable sulfur content can serve as an internal lubricant, especially if it is MoS ₂ , but is generally not necessary for valves and parts.

The moldings produced by powder metallurgy according to the invention can be produced by conventional press-sintering processes. This also includes hot isostatic pressing, although this is not absolutely necessary. In general, compression to 7.5 g / cm ^{3 is} sufficient, although for numerous purposes a higher density, in particular about 7.7 g / cm ³ or more is very advantageous. By increasing the density and the associated reduction in the pore volume, there is also an improvement in the thermal conductivity and thus the temperature behavior. Furthermore, the stability is increased.

The molded parts according to the invention can be produced from the corresponding element powders. In most cases, however, it is expedient to use finished alloy components for the production, for example a finished alloy steel component, a phosphorus-molybdenum steel, optionally MoS ₂ and, if necessary, graphite, in each case in powder form. It is particularly preferred to use metal powders of irregular shape produced by atomization processes, which can give the pressed part produced therefrom a certain internal cohesion by means of teeth. To improve processability, reduce wear in the presses and improve cohesion, customary auxiliaries can be added, for example wax, in an amount of up to 1% by weight, based on the alloy powder.

Dendritic powders with an average diameter of less than 150 μm are preferably used, preferably less than 50 μm. Carbon is expediently admixed as graphite with an average size of 10 μm or less, if not already sufficiently represented in the finished alloy powder. The PMoFe steel powder, as can be used here, is described in WO-A-91/18123.

A powder composition with 0.5 to 2.0% carbon, 5.0 to 14% molybdenum, 0.2 to 1.0% phosphorus, 0.1 to 1 is particularly preferred for the production of valves, valve bodies or valve rods , 2% manganese, a maximum of 0.50% chromium and a maximum of 0.40% sulfur. In this case, other elements are represented with less than 2%, the rest is iron. The composition is based on weight percent. For valve bodies in particular, it is advisable to use the liquid phase sintering process. The finished valve body should have a density of at least 7.7 g / cm ³ .

Compared to the conventional manufacturing process for complete valves described at the outset, there is a significant reduction in the processing steps for the powder-metallurgically produced molded parts according to the invention. The steps in the manufacture of a valve according to the invention from a separately manufactured valve body and a rod section are as follows:

First pressing, sintering and tempering the valve body, then providing the rod section, joining the valve body and rod section, for example in a friction welding process, straightening, turning, grinding and heat treating the finished valve. The significant reduction in the number of production steps increases manufacturing accuracy and reduces the likelihood of errors. Furthermore, the reduced number of manufacturing steps allows more flexible reaction to changing system requirements.

The valves or valve bodies according to the invention show a high wear resistance even at the high temperatures and loads in the valve train, in particular for exhaust valves.

As for valves, the entire valve can be made from the materials described above. However, it is also possible and preferred to produce only the valve body from this material and to manufacture the stem from a conventional material. If the valve body and the valve rod are manufactured separately, the geometry for joining the valve head and valve stem can be varied further. All known welding processes can be used for positive joining processes (press fit). If the fit is blunt, a friction welding process is usually required. As far as the invention relates to valve bodies, these have the advantage over conventional valve bodies that they consist of a uniform material, ie they do not require local modification in order to adapt them to the particular circumstances of a piston outlet of an internal combustion engine. In addition to advantages in terms of production technology, this also means that the product is less susceptible to faults and damage, both in the manufacturing and in the operating phase.

It goes without saying that the press-sintered shaped bodies according to the invention can also be shaped bodies other than valves, valve bodies or valve stems. For parts that are designed to be self-lubricating, the powder composition used for manufacturing may contain a proportion of MoS ₂ that supplies the material with up to 5% by weight of sulfur. Sulfur contents of up to 3.0% by weight of the material are particularly suitable.

A particularly preferred variant of valves according to the invention are valve bodies which have a central recess into which the valve stem is inserted and welded in a form-fitting manner. Basically, any common form of welding can be used. Valves of this type make it possible to connect a valve body manufactured by powder metallurgy to a shaft which is manufactured conventionally or by powder metallurgy. This manufacturing process offers the advantage that the powder-metallurgically manufactured body can be easily connected to a conventional, third-party-equipped and equipped shaft. Naturally, this also applies to valve stems manufactured using powder metallurgy.

The press-sintered shaped bodies according to the invention are produced from the premixed or finished alloy powder as follows. First, the blank is pressed out of the powder with the aid of a conventional wax as a lubricant under customary pressing pressures to give moldings with a sufficient density. The pressure is expediently between 500 and 900 MPa. After pressing, the product is first dewaxed under a hydrogen-nitrogen protective gas atmosphere at a temperature of 500 to 750 ° C and then in an oven at a temperature of more than 900 ° C, preferably more than 1000 ° C, up to 1150 ° C, sintered. Pressures and temperatures essentially depend on the desired density of the molded part and on the composition of the metal powder. After cooling, the parts are left on and subjected to the necessary post-treatment steps.

As mentioned, it is particularly preferred for the production of valves for internal combustion engines to produce the valve body and valve stem in separate work steps and then to join them. The valve body is made by powder metallurgy, the stub shaft can be made conventionally or powder metallurgically. In this constellation, the body and the shaft can be connected to one another by friction welding, but preference is given to the form-fitting insertion of the shaft into a precisely fitting recess in the body by means of a transition piece or fitting piece which is formed in the lower region of the shaft with a precise fit. The stem and body are then welded to the complete valve and reworked.

Fig. 1 shows a valve body 1, which is made by powder metallurgy and is provided for butt connection to a stem 4.

Fig. 2 shows a valve body and a stem end as they can be added to a valve according to the invention. The valve body 1 is made by powder metallurgy and shows in its center the receptacle 2 for the fitting 3 of the stem 4. The stem and body are connected to one another by welding.

Example:

Metal powder of the following chemical composition by weight was used for a sintered body according to the invention:

0.9% carbon, 8.2% molybdenum, 4.8% tungsten, 1.4% vanadium, 0.42% phosphorus, 3.2% chromium and 1.2% sulfur.

Around 1.9% of other elements were represented, the rest was iron. A sintered body made of sintered molybdenum-phosphor steel with a density of 6.9 g / cm ^{3 was} obtained. The molded body showed good wear resistance and a finely divided structure of various carbides in a tempered martensitic matrix with embedded solid lubricant under high surface loads.

Claims

claims 1. Press-sintered molded part produced by powder metallurgy with high temperature and wear resistance, characterized by the following Composition by weight: 0.5% to 2.0% C; 5.0% to 16% Mo; 0.2% to 1.0% P; 0.1% to 1.4% Mn; 0% to 5% Cr; 0% to 5% S; 0% to 7% W; 0% to 3% V <2% other elements; Rest of Fe. 2. Press-sintered molding according to claim 1, characterized in that it has a density of at least 7.5 g / cm ³ . 3. Molding according to claim 1 or 2, characterized in that it is made from a metal powder which has been used at least partially fully alloyed. 4. Molding according to claim 3, characterized in that the starting powder contains finished alloy PMoFe powder. 5. Molding according to one of the preceding claims, characterized in that the starting powder was used in an atomized state. 6. Molding according to one of the preceding claims, characterized by the following composition by weight: 0.5% to 2.0% C; 5.0% to 14% Mo; 0.2% to 1.0% P; 0.1% to 1, 2% Mn max. 0.5% Cr max. 0.4% S <2% other elements rest Fe. 7. Molding according to one of the preceding claims, characterized in that the molding is a valve, a valve body or a valve stem. 8. Molding according to claim 7, characterized in that it is a valve body with a conventionally or powder metallurgically manufactured and attached stem. 9. Molding according to claim 8, characterized in that the valve body and valve stem are positively joined. 10. Molding according to claim 9, characterized in that the valve stem is fitted into the valve body. 11. Molding according to one of the preceding claims with a density of at least 7.7 g / cm ³ . 12. Shaped part according to one of the preceding claims, characterized in that it is compressed by liquid phase sinter.