EP0435019A1 - Workpiece, especially cam from sintered powder metal alloy and process for preparing same - Google Patents

Abstract

Shaped articles, in particular cams for camshafts of internal combustion engines, are exposed to high stresses causing wear. To make them wear-resistant, they are produced from a sintered alloy prepared by powder metallurgy. The alloy has a hardened matrix with incorporated copper and consists of 0.5-16% by weight of molybdenum, 1-20% by weight of copper, 0.1-1.5% by weight of carbon and optionally of added amounts of chromium, manganese, silicon and nickel in a total amount of max. 5% by weight, the remainder being iron.

Description

The invention relates to a molded part, in particular a cam made of a sintered, powder-metallurgically produced alloy for a modular camshaft for internal combustion engines, and to a method for its production.

The camshafts of camshafts for internal combustion engines are exposed to very heavy wear. In order to fulfill their task of engine control, the wear must not exceed a few µm during their entire service life. Load cycles with insufficient lubrication must also be endured. The usual method in literature and technology is the use of high-carbide alloys, which are either produced by powder metallurgy from appropriate materials or by rapid quenching of cast iron. This means that both abrasive and adhesive wear can be kept within limits.

In addition to mechanical, the cams are also exposed to thermal stress. For this reason, the cams have to be so hard that they can maintain them even after a long start. This can be achieved by hardening and then tempering at a temperature above the operating temperature. The cams should also exhibit excellent operating behavior under operating conditions in which there is insufficient lubrication and which promote adhesive wear.

For several years, especially since the modular camshafts for internal combustion engines have emerged (1, 2), the discussion of wear in the cam-counter body system has been intensified. In addition to indications that the wear in this system depends on the lubrication (3) and the finishing by grinding or superfinishing (4, 5), there are a large number of publications that try to solve the problem on the basis of material development .

For a promising approach, the wear problems of this system must first be analyzed. Numerous publications (3, 6, 7) indicate that the wear and tear phenomena are primarily polishing wear, pitting and fretting.

The polishing wear is an appearance of the abrasive wear, in which there is very little abrasion with a small corrugation width due to the appropriately fine abrasive materials. The cam which has been worn out in this way appears to be polished to a polished finish, the roughness of the worn areas usually being much smaller than that of the undamaged (ground) areas. The polishing wear can be caused as three-body wear by quartz dust in the oil. Sand is one of the most common abrasives that occur in engineering. Since the polishing wear also occurs under test conditions in which contamination of the oil can be ruled out, there must also be another mechanism. Obviously, the polishing wear can also be promoted by a hard, rough counterpart that contains no carbides.

Pitting is the result of surface fatigue. The pressure threshold stress on the cam surface, which is determined by the kinematics, can lead to local crack propagation. The cracks run below the surface and either merge with other cracks or emerge from the surface again. The result is the formation of relatively large wear particles and pits on the surface. This wear can be promoted by additives in the oil (3) if the additives cause the crack to spread, e.g. by lowering the surface energy.

Eating is a result of adhesive wear, i.e. the mutual welding of the surfaces. It is favored by the use of martensitic base and counter bodies (8) and by the use of unalloyed oil. Experiments with increased spring force of the valve spring also favor seizure. While in forty-three pairs twenty-six failed to eat when unalloyed oil was used, not a single pair failed to eat when using alloyed oil (8). However, the failure due to pitting in alloyed oil increased from seventeen pairs to thirty-five pairs (8).

Despite the frequent occurrence of pitting, this wear and tear is paid less attention in the examinations than the other two. The pitting itself basically does not affect the function of the cam (6). But it reduces the load-bearing surface, so that the surface pressure increases, which can cause failure due to seizure. In addition, the tendency to pitting can be clearly identified in short-term tests with increased load (7), while extrapolation can only be carried out with extreme caution in the event of polishing and seizure wear (8, 9). Pitting is not critical as long as it occurs to a small extent. It can also be easily simulated in terms of tests.

Most publications deal with the avoidance of seizure and polishing wear. All attempts are aimed at producing materials with a high carbide content (2, 6, 8, 9, 10, 11, 12, 13). By their great Hardness reduces the depth of penetration of the counter body. This reduces the size of the wear particles and thus the possible wear rate (14). The second effect is the low tendency to adhesion that the carbides have. Adhesion wear is completely avoided by the carbides with a sufficiently large volume fraction. Attempts to reduce the cam wear by a solid lubricant embedded in the cam are not known.

The embedding of lubricants in sintered alloys has long been used to produce self-lubricating bearings (15). Is used for. B. lead, which is introduced by soaking in a relatively complex alloy (Fe-Co-Mo-Ni-Cr-Si-C). This alloy has proven itself when used for valve seats in internal combustion engines (16).

There has been a lot of discussion in the literature about copper as an alloying element because it is an easy-to-process element (its oxygen potential is much lower than that of iron). Mechanical properties (17, 18) or dimensional behavior (19, 20) and homogenization (21) are often discussed. In conventional steel technology, copper is known as a steel pest because it promotes the tendency to break red (22). This failure does not play a role in powder metallurgical production, as long as the molded parts are not to be formed by sinter forging.

The influence of. Copper on the wear of sintered iron is significantly less than the influence of density, at least with copper admixtures of 0 to 2% (23). Samples of different densities were examined in the Amsler tribometer (two rollers roll against each other with a slip percentage of 10%). The atmosphere (air, argon or oxygen) has a decisive influence on the amount of wear, the wear under oxygen is 72 times greater than that in air. Since the wear under argon lies between the two values, an influence of water vapor is probable in the tests. The sintering conditions (1120 C) suggest that the copper is completely dissolved in the matrix.

The influence of copper admixtures in the range of 0 to 4% to various sintered steels containing phosphorus was also investigated (24). In the higher alloy variants (4% Mo, 4% Ni or 4% MCM, a master alloy made of molybdenum, chromium and manganese), the addition of copper causes a decrease in wear in the pin-washer test. In the case of the lower alloyed variants, the copper additive has a relatively unsystematic influence. The effect of copper is based on matrix hardening. Although the sintered density decreases with increasing copper content, the hardness increases continuously with the copper content. Because of the increase in hardness, it can be assumed that the copper is completely dissolved in the matrix. The decrease in density is also an indication of this. Copper leads to a decrease in density during sintering if it dissolves in the matrix and pores remain where it originally existed.

The combination of a binder phase made of copper, manganese or nickel or combinations thereof with very hard HSS particles has also been investigated (25). The structure produced in this way is more ductile than pure HSS and has proven itself well for wear applications.

In many other studies (26, 27, 28, 29) copper is used as a model material for basic studies. The finding that the wear rate of copper when dry sliding against iron is 5 times lower than that of nickel appears remarkable (28). This result indicates the low adhesion tendency of copper-iron combinations and the associated good emergency running properties of copper.

Molybdenum is found in many P / M steels. The reason for the frequent use of 0.5% molybdenum is certainly of a purely practical nature. Commercial base iron powder contains 0.5% molybdenum. A conscious admixture only happens in the rarest of cases. Fe-P-Cu-Mo alloys with Cu contents up to 4% and Mo contents between 2% and 4% have also been investigated (17). All alloying elements were mixed in elementally. The samples with 2% Mo and 4% Cu have an irregular two-phase structure after sintering at 1200 C for one hour. This inhomogeneity becomes even clearer when the Mo content is increased to 4%. Carbon slows down the diffusion of Cu into Fe, but it does not prevent complete dissolution.

Numerous tests are known for controlling the wear of the cam-counter body system, all of which have hitherto been based on the generation of a carbide-rich structure.

The invention is based on this prior art, which aims to improve the emergency running properties of a cam, which is achieved according to the invention in that the alloy has a hardened matrix with embedded copper and from 0.5-16% by weight molybdenum , 1 - 20% by weight of copper, 0.1 - 1.5% by weight of carbon and optionally from admixtures of chromium, manganese, silicon and nickel of a total of max. 5 wt .-% and consists of the rest of iron. The additives are used to adapt the alloy to the application in terms of secondary hardness, strain hardening and hardenability. The procedure for The production of such a cam is now characterized in that a sinter powder made of 0.5-16% by weight of molybdenum, 1-20% by weight of copper, 0.1-1.5% by weight of carbon and optionally from admixtures of Chromium, manganese, silicon and nickel of a total of max. 5 wt .-% and from the rest of iron to a cam molding with a green density above 7 g / cm ^{3 is} pressed and sintered at temperatures below 1150 ° C for a sintering time of 10 to 60 minutes and then tempered.

• Auf der Basis eines vorlegierten Pulvers aus Eisen und Molybdän wurde eine Pulvermischung mit 1,5 Gew.-% Molybdän, 10 Gew.-% Kupfer, 0,8 Gew.-% Kohlenstoff und dem Rest Eisen hergestellt. Durch Pressen mit einem Druck von 1500 MPa wurden Nocken mit einer Gründichte von 7,2 g/cm³ hergestellt. Durch Sintern bei 1120 0 C für 30 min wurde das Gefüge konsolidiert. Ein anschließendes Vergüten durch Glühen bei 930 ° C für 60 min, Abschrecken in Öl und Anlassen bei 150 C für 60 min wurde ein Gefüge erzeugt, das eine Oberflächenhärte von 44,4 HRC (793 HVI) besitzt. Obwohl über 7 Vol.-% des Gefüges aus elementarem Kupfer bestanden, wurde die hohe Härte erreicht. Die Nocken erwiesen sich im Prüfstand als außerordentlich verschleißfest. Auch unter Bedingungen, bei denen Mangelschmierung auftritt und die daher adhäsiven Verschleiß fördern, wiesen die Nocken ein ausgezeichnetes Betriebsverhalten auf.

Example:

• On the basis of a pre-alloyed powder of iron and molybdenum, a powder mixture with 1.5% by weight of molybdenum, 10% by weight of copper, 0.8% by weight of carbon and the rest of iron was produced. Cams with a green density of 7.2 g / cm ³ were produced by pressing at a pressure of 1500 MPa. The structure was consolidated by sintering at 1120 ° C. for 30 minutes. Subsequent quenching and tempering by annealing at 930 ° C for 60 min, quenching in oil and tempering at 150 C for 60 min resulted in a structure that had a surface hardness of 44.4 HRC (793 HVI). Although over 7% by volume of the structure consisted of elemental copper, the high hardness was achieved. The cams proved to be extremely wear-resistant on the test bench. The cams also showed excellent operating behavior under conditions in which insufficient lubrication occurs and which therefore promote adhesive wear.

The accompanying illustrations show a micrograph of a cam according to the invention, which is manufactured according to the example explained above. Fig. 1 is an enlargement on a scale of 200: 1; Fig. 2 is an enlargement of the same micrograph on a scale of 500: 1.

• 1. Martensit (grau)
• 2. Kupfer (hell)
• 3. Poren (schwarz)

The micrograph clearly shows three phases:

• 1. martensite (gray)
• 2.Copper (light)
• 3. pores (black)

The martensite has a very even appearance. No inhomogeneities can be seen. This corresponds to the expectations, since a pre-alloyed, i.e. already homogeneous powder was used.

The copper is present in irregular spots evenly distributed over the structure. The size of the copper grains is 10 to 30 µm. The pores are well rounded. They are distributed bimodally. One size range is around the value normally observed for steels of 5 µm, the second is around 50 µm. The large pores are secondary pores that result from the dissolution of copper.

Individual phases were identified with the help of microhardness measurements. For the bright areas, the microhardness was less than 50 HV0.01. Since the phase was very finely divided, the diagonals of the impressions were almost as large as the areas themselves, so that an exact specification of the microhardness is not possible. The hardness of pure copper is 34 HV (38).

It is therefore certain that the light areas are copper and not carbides or an alloy of copper and iron or an intermetallic phase of iron and molybdenum. In any case, there should be no doubt about the identity of the pores and the martensite. The martensitic areas in the core had a hardness of just under 400 HV0.01. The macro hardness HV10 was determined to be 372. The hardness values were measured in the core.

With the help of quantitative stereology (point analysis (30)) the volume fraction of the undissolved copper was determined. The copper content was 7.8% by volume. The chemically analyzed copper content was determined to be 7.4% by weight. The density of copper is somewhat greater than that of iron, so that a larger proportion would be derived from stereological analysis. Within the framework of the measurement errors that are always present, the results from the two analyzes can be regarded as the same. This means that the copper is completely undissolved, the matrix is probably completely free of Cu.

The volume fraction of the pores was also determined stereologically and using gravimetric density measurements. The corresponding value was 6.5%.

The alloy Fe / 1.5Mo / 10Cu / 0.8C consists, in addition to a small proportion of pores, of elemental copper and martensite, in which only a negligible proportion of copper is dissolved. While the pores on the surface improve the lubrication somewhat, the copper content serves as a solid lubricant to improve the emergency running properties. The martensite causes the resistance to abrasive wear.

When using alloyed powder, the pressing forces decrease, the wear on the tools during the production of the molding is reduced, and different alloy contents can also be set more easily. However, it is also conceivable to use a mixed alloy powder, but taking into account the diffusion properties of copper and molybdenum, it cannot be ruled out that a significantly different microstructure could result, since copper should then partially dissolve in the iron, so that the proportion of the free elemental copper in the structure drops dramatically.

The results achieved by the proposals according to the invention are surprising even for the relevant expert. According to previous experience, a large part of the copper contained in the alloy would have to dissolve even after relatively short sintering times and low sintering temperatures. In a basic work, Bockstiegel (20) has shown that the dissolution process at 1150 C is already completed after less than 30 min. Bockstiegel gives 7.5% of the solubility in accordance with (36). With a copper content of 10%, only 2.5% undissolved Cu should be found after sufficient sintering. However, the quantitative analysis of the alloy has shown that practically all of the copper is present in the matrix undissolved.

Only molybdenum can be held responsible for this. The insolubility of copper in molybdenum (34) suggests that molybdenum greatly reduces the solubility of copper in iron. If one looks at a phase diagram of Fe-Mo, it is noticeable that the transition from gamma to a-iron takes place at 2.6% by weight (1.5 at.%) Mo in the range around 1100 ° C. Molybdenum is therefore a very strong α-opener, i.e. the steel is preferably in the krz structure.

However, the solubility of copper in iron is much lower in the a phase than in the automotive gamma phase: While up to 7.5% by weight dissolves in gamma iron, the maximum solubility in the a phase is only 1.4% by weight (36). The fact that the a-phase is largely stabilized by the molybdenum (1.5% by weight) largely prevents the copper from diffusing in. However, copper is obviously not completely insoluble in Fe-Mo. In the Fe-1% Mo system, the diffusion coefficient of copper was measured (37), which suggests that finite solubility for copper exists at least at these small molybdenum concentrations.

The test results indicate that the molybdenum content was chosen in the correct order. A content of 0.5% is probably not sufficient to lower the solubility of copper to the extent observed here. 0.5% therefore appears to be a reasonable lower limit.

The upper limit is rather set by economic considerations. The Mo content will therefore be limited to around 16%. At 16% by weight Mo, the a region is left at the sintering temperature (1120 C), which can lead to a change in the behavior of the alloy. This limit could therefore be called the upper limit.

The copper content must be selected so that it guarantees the required emergency running properties. The lower limit can be set at around 1%, since the effect of copper as a solid lubricant will hardly suffice. A value must be chosen as the upper limit, in which a sufficient part of the structure is still present in the form of the hard martensitic matrix, in order to guarantee that the supporting surface remains sufficiently large. In terms of magnitude, an upper limit of 20% can be assumed.

The alloy according to the invention can only be produced by powder metallurgy. The special structure, which consists of a martensitic matrix and elementary copper, is created directly by the sintering process. The extremely low solubility of copper in Fe-Mo is exploited, which means that the copper content is practically completely available as a solid lubricant and does not lead to swelling as is the case with Cu-alloyed materials. It can be assumed that a comparable structure is obtained when using a mixed or diffusion alloy powder.

As the state of the art shows, various metals are known as solid lubricants in sintered materials, but the use of copper as a problem solution for the cam / counter body system is new. Compared to some processes in which the solid lubricant, e.g. Lead, which is introduced into the matrix by impregnation, has the advantage of the alloy according to the invention that the copper is contained in the material from the outset. However, it is also possible to introduce the copper by impregnating a molded part of low density. In addition, a uniform copper distribution and a fixed copper content can be guaranteed, whereas the volume fraction and the distribution are imposed by the distribution and size of the open pores when soaking. However, this is much more difficult to influence than the size, amount and distribution of the copper in the powder mixture, so that the process reliability in the system presented here is increased.

Molybdenum very effectively prevents the copper from dissolving in the matrix, so that the copper can be available as a solid lubricant. By using a solid lubricant, a major problem of wear in the cam-counter body system, the adhesion, is successfully solved. As a positive additional effect, the molybdenum prevents the swelling otherwise observed in copper-alloyed materials. This increases the working accuracy and improves the mechanical properties.

If the advantages according to the invention are achieved in particular by using pre-alloyed powder, it is also conceivable to produce a comparable structure differently: a ge mixed alloy Fe-C-Mo powder is first solidified by sintering and homogenized. By choosing a very low green density, open pores remain in the structure, which are closed by impregnation with copper. A comparable structure can also be created in this way. A pre-alloyed powder can also be assumed in this process variant.

The above considerations, insofar as they relate to the wear properties of cams, are also relevant for other molded parts that are subject to wear, for example rocker arms, rocker arms etc., i.e. molded parts that are subject to sliding wear.

literature

• (1) Kimura T .: Development of Ferrous Sintered Parts in Japan, Modern Developments in Powder Metallurgy, ed. P. U. Gummeson, D. A. Gustafson 21 (1988) 551 - 561
• (2) Tanase T., Mayama 0., Matsunaga H .: Properties of Wear-Resistant Alloys Having High Volume Fraction of Carbides, Modern Developments in Powder Metallurgy, ed. P. U. Gummeson, D. A. Gustafson 21 (1988) 563-573
• (3) Jahanmir S .: Examination of Wear Mechanisms in Automotive Camshafts, Wear 108 (1986) 235-254
• (4) Isakov A.E. et al: The Optimal Hardness of Camshaft Journals and Cams for Improved Wear-Resistance, Russian Engineering Journal 54, 7 (1974) 38-40
• (5) Isakov A.E. et al: Manufacturing Technology Reduces Camshaft Journal and Cam Wear, Russian Engineering Journal 54, 5 (1974) 49-51
• (6) Eyre T. S., Crawley B .: Camshaft and Cam Follower Materials, Tribology International 13, 4 (1980) 147-152
• (7) Riccio G: The Camshaft, Metallurgical Science and Technology 4, 3 (1986) 96-103
• (8) Reinke F .: Structure of ledeburitic surface layers by remelting of cams and cam followers, VDI report, No. 506 (1984) 67-74
• (9) Werner G. D., Ziese J .: Improvement of wear resistance on camshafts through targeted nitriding and oxidizing conditions, VDI reports, No. 506 (1984) 59 - 62
• (10) Arnhold V., Wähling R: High Performance Components Produced via Vacuum Sintering, Modern Developments in Powder Metallurgy, ed. P. U. Gummeson, D. A. Gustafson 21 (1988) 183 - 195
• (11) Thümmler F., Oberacker R., Klausmann R .: Sintered Steel With High Carbide Content, Modern Developments in Powder Metallurgy ed P. U. Gummeson, D.A. Gustafson 20 (1988) 431 - 441
• (12) Reinke F .: Local remelting to build up wear-resistant ledeburitic surface layers on workpieces made of cast iron, in particular camshafts and cam followers, AEG-Elotherm, company report 2 - 15
• (13) Beiss P., Duda D., Wähling R .: Highly wear-resistant molded parts - production - properties - possible uses, cancer eyes Information
• (14) Archard J. F .: Contact and Rubbing of Flat Surfaces, Journal of Applied Physics, 24, 8 (1953) 981-988
• (15) Boyer H.E., Gall T. L .: Metals Handbook - Desk Edition, ASM Metals Park, Ohio 1985, Oct. 25
• (16) Suzuki K., Ikenoue Y., Endoh H., Uchino, M .: New Sintered Valve Seats for Internal Combustion LPG Engines, Modern Developments in Powder Metallurgy, ed. P. U. Gummeson, D.A. Gustafson 21 (1988) 157-170
• (17) Hamiuddin G., Upadhyaya G.S .: Effect of Copper on Sintered Properties of Phosphorus - Containing Ternary Iron Powder Premixes, PMI 14, 1 (1982) 20-24
• (18) Lindskog P., Carlsson A .: Sintered Alloys Based on Sponge Iron Powder with Additions of Ferrophosphorus, PMI 4, 1 (1972) 39-43
• (19) Dautzenberg N., Dorweiler H.J .: Dimensional Behavior of Copper-Carbon Sintered Steels, PMI 17, 6 (1985) 279-281
• (20) Bockstiegel G .: Appearance and causes of changes in volume when sintering compacts from iron-copper and iron-copper-graphite powder mixtures, steel and iron 79, 17 (1959) 1187-1201
• (21) Esper F.J., Friese K.H., Zeller, R .: Sintering Reactions and Radial Compressive Strength of Iron-Tin and lron-Copper-Tin Powder Compacts, International Journal of Powder Metallurgy 5, 3 (1969) 19-31
• (22) Wegst W .: StahlKey, Verlag StahlKey Wegst GmbH, Marbach 1989, 4
• (23) Biggiero G., Borruto A., Ercolani D .: Influence of the Addition of Copper on the Wear Resistance of Sintered Ferrous Materials, Horizons of Powder Metallurgy, ed. W. A .: Kaysser, W. J. Huppmann (1986) 1289 - 1295
• (24) Hamiuddin M .: Wear Behavior of Sintered Phosphorus Containing Ternary Iron Alloy Powder Compacts, PMI 17, 1 (1985) 20-22
• (25) Fischmeister H .: Improvements Relating to Tough Material for Tools and / or Wearing Parts, Patent GB2157711A, UK April 4, 1985
• (26) Chen L.H., Rigney D.A .: Transfer During Unlubricated Sliding Wear of Selected Metal Systems, Wear of Materials, ed. K. C. Ludema ASME (1985) 437-446
• (27) Sheasby JS, Mount GR, Eider JE: Direct Observation of the Wear of Copper, Wear of Materials, ed. KC Ludema ASME (1985) 545-549
• (28) Sasada T., Norose S .: The Dependence of Wear Rate on Sliding Velocity and Sliding Distance for Dry Cu / Fe and Ni / Fe, Wear of Materials, ed. K. C. Ludma ASME (1985) 432 - 436
• (29) Reid J.V., Schey J.A .: Adhesion of Copper Alloys, Wear of Materials, ed. K.C. Ludema ASME (1985) 550-557
• (30) Underwood E.E .: Quantitative Stereology, Addison-Wesley (1970)
• (31)
• (32)
• (33)
• (34) Hansen M .: Constitution of Binary Alloys, Mc Graw-Hill New York 1958, 600
• (35)
• (36) as (34), p. 580
• (37) N.N .: Diffusion Data 4 (1970) 424
• (38) Samsonov G. V .: Handboo-k of the Physiochemical Properties of the Elements, IFI / Plenum New York 1968, 303

Claims (6)

1. molded part, in particular cams made of a sintered, powder-metallurgically produced alloy for a modular camshaft for internal combustion engines, characterized in that the alloy has a hardened matrix with embedded copper and from 0.5 to 16% by weight of molybdenum, 1 to 20 % By weight of copper, 0.1-1.5% by weight of carbon and optionally from admixtures of chromium, manganese, silicon and nickel of a total of max. 5 wt .-% and consists of the rest of iron. 2. A method for producing a molded part, in particular a cam according to claim 1, characterized in that a sintered powder of 0.5 - 16 wt .-% molybdenum, 1 - 20 wt .-% copper, 0.1 - 1.5 wt .-% carbon and possibly from admixtures of chromium, manganese, silicon and nickel of a total of max. 5 wt .-% and from the rest of iron to a cam molding with a green density above 7 g / cm ^{3 is} pressed and sintered at temperatures below 1150 * C for a sintering time of 10 to 60 minutes and then tempered. 3. The method according to claim 2, characterized in that a pre-alloyed iron-molybdenum powder is used. 4. The method according to claim 3, characterized in that the copper is added by mixing in the pre-alloyed iron-molybdenum powder. 5. The method according to claim 2 or 3, characterized in that an alloyed or mixed-alloyed iron-molybdenum powder is solidified and homogenized by sintering and then the copper is introduced by soaking in the open pores of the structure of the workpiece. 6. The method according to any one of claims 2 to 5, characterized in that the carbon is introduced by adding graphite to the sinter powder or in whole or in part by a carburizing atmosphere during sintering and / or during hardening.

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