Backqround of the Invention Very specific demands have to be met by piston rings that are intended for use in for instance marine diesel engines, particularly as concerns strength, anti-corrosive properties, wear strength, and material resilience. The piston ring is intended to abut on the one hand against an associated piston groove, on the other against an engine cylinder-bore. Consequently, the ring must be wear-resistant, particularly at the interface towards the cylinder bore, where high friction is generated when the engine is in operation. It therefore also must possess an inherent tension or

resilience, whereby the piston ring will constantly be forced outwards, into abutment against the cylinder bore. In addition, upon each explosive stroke of the engine, the piston ring is urged with considerable force radially outwards, into abutment against the cylinder bore, with consequential increase of stress. In operation, the piston ring is also exposed to high temperatures, to considerable temperature differences, and to the effects of a highly corrosive environment.

In order to withstand the effects of these stress-inducing causes, the piston ring therefore also must exhibit considerable wear strength, ductility and toughness. By ductility is to be understood herein the maximum possible deformation of the material before dislocations occur and cracking begins. By toughness is to be understood herein the maximum possible deformation of the material from the instance of incipient cracking until a fracture occurs in the material.

Today, piston rings generally are manufactured from a cast-iron blank, which meets the requirements imposed on the material as regards strength and resilience but not on wear resistance on the surface thereof that faces the cylinder bore. Cast iron does not either possess the required toughness. A cast-iron piston ring blank therefore usually is provided with a wear-resistant wear layer on the surfaces most exposed to wear.

The wear layer, which usually is formed by a chromium-compound material, generally is applied to the piston ring blank in an electrolysis process as described e. g. in EP Patent Specification

0 668 375. In accordance with the teachings of this specification the piston ring blank (substrate) is given a hard chromium layer in an electrolysis process.

However, difficulties do arise in achieving a sufficiently strong bond between the material of the blank and the material of the wear layer, which causes problems, because of the risk that the material of the wear layer be torn away from the material of the blank. When this happens, the comparatively soft material of the blank-material surface is exposed to wear in the area of contact against the cylinder bore, with resulting considerable shortening of the life of the piston ring.

Another problem is that the wear layer gradually wears away, even if the bond between the surfaces is comparatively strong. The wear on the piston ring progresses slowly as long as the wear layer is intact but very rapidly, once that layer has disappeared. As a result, it may be difficult to determine in time when a piston ring change should be made.

A method in accordance with the introduction is briefly described in JP4002743.

Summary of the Invention The object of the present invention is to provide a material, particularly intended for piston rings, that meets the requirements necessary in piston rings as regards wear resistance, resilience, anti-corrosiveness, hardness, toughness, and ductility. Another object of the

invention is to provide a method of producing the material, and a piston ring, which does not suffer from the above drawbacks found in the prior art.

The first object of the present invention is achieved by means of a metallic composite material as described in the introduction, the pressure and heat treatments of the production method being simultaneous and the metallic powder materials, after mixture of the two powders, including from 2.1 to 2.8 percent by weight of C.

The second object of the present invention is achieved by means of a method as set forth in the introduction, according to which method the pressure and heat treatments are simultaneous and are carried out until the austenitic temperature has been reached, whereupon the material is cooled to form a martensitic matrix structure, followed by annealing, when the material has formed a martensitic structure. The composite powder comprises from 2.1 to 2.8 percent by weight of C.

The third object of the present invention is to provide a piston ring manufactured from such a material.

It is already known to compress an essentially compact material from a powder mixture by subjecting it to pressure and heat as described in e. g. EP-0 785 289. In accordance with this publication, the object is to produce a steel that is able to withstand severe abrasion, a capacity necessary in e. g. tools that are used to crush stone. The disclosed material is formed essentially from two components, the first one being an austenitic powder, more precisely a so called

Hadfield manganese steel powder, and the second one an iron-based, essentially martensitic powder which, when transformed, produces segregated particles in the material. The martensitic powder comprises a total of at least 0.8 percent by weight of C and N, and at least 8% of alloying elements which form the segregated particles in the finished material. After the transformation process a material is obtained having a microstructure consisting of a ductile austenitic steel in combination with a martensitic structure comprising hard segregated particles. The martensitic structure and the associated segregated particles are the cause of the abrasive effect aimed at for stone-crushing applications. The austenitic structure is tougher than the martensitic and essentially forms a matrix around the harder and more brittle martensitic areas. In this way the toughness and the ductility of the material are increased, and cracking of the material is prevented.

A material of the kind described in EP-0 785 289 is, however, too abrasive to be used in piston rings. A piston ring made from such a material would, in operation of the engine, risk tearing of the associated cylinder-lining material. In addition, the ductility and the toughness are not sufficient to make the material adequate for use for piston rings and consequently, hard particles would risk being torn away from the surface of the ring, which would increase further the abrasive friction against the rest of the ring when the engine is in operation.

In several papers issued by Helsinki University of Technology, among them a paper entitled "Microstructure and mechanical properties of hot work tool steel matrix composites produced by hot isostatic pressing", Powder Metallurgy 1997, Volume 40 No. 1, and a paper entitled"Processing and properties of particulate reinforced steel matrix composites", 1998 Elsevier Science S. A., Materials Science and Engineering A 246 (1998) 331-234, it is suggested that a steel matrix be reinforced by a ceramics powder in order to produce wear-resistant parts. The latter are primarily intended to be used for cutting tools. Several ceramics powders are suggested, among them Cr3C2, which is mixed with a tool steel-powder. Following a HIP procedure (High Isostatic Pressing) to compact the material, the ceramics material is found to form a protective network around the steel matrix. Like the material described in EP-0 785 289, the material thus produced has been found to possess satisfactory abrasiveness, which is required from cutting tools but which also makes the material unfit for use in the manufacture of piston rings. In addition, the material does not possess sufficient ductility to serve as a material for a piston ring used in e. g. diesel engines.

Thus, no metallic powder materials have been used to date for the manufacture of piston rings, since the properties of known materials would have had a devastating effect on the cylinder associated with the piston ring in a diesel engine. A piston ring manufactured from any of the hitherto known compact metal powder materials would have been far

more abrasive than both the piston and the cylinder bore, causing these two parts to wear out quicker than the piston ring. This is a considerable disadvantage, considering that exchange of these parts is difficult and expensive.

In view of the above, the present invention therefore suggests a metallic powder material, which is particularly intended for piston rings and which is produced by mixing two different powder materials and transforming said materials into an essentially compact material by means of pressure and heat, one of said powders being a metallic powder and the other one a ceramics powder chosen to ensure that after transformation of the material a ceramics-material network completely or partly extends through a metal matrix likewise formed in the transformation. The pressure and heat treatments are simultaneous and after mixing of the two powders, the metallic powder material contains from 2.1 to 2.8 percent by weight of C.

In accordance with the teachings of the invention, the transformation of this material comprises austenisation, cooling to form a martensitic phase, and annealing. After the transformation, the material has a microstructure similar to that of an annealed martensitic steel structure through which extends a ceramics-material network.

The presence of ceramics-material particles imparts to the material a comparatively high degree of abrasiveness. However, this property is counteracted by the ceramics-material network that insures the cohesion/bond between the ceramics

material and the martensitic structure, which increases the ductility and the toughness of the material. The properties of wearability, ductility and resilience, otherwise difficult to combine, thus are achieved in one and the same material.

As a result, a piston ring manufactured from the inventive material, when properly lubricated, does not wear out piston grooves or lining materials. At the same time, the hardness of the material is of a nature making it superfluous to provide the piston ring with further external wear layers. Thus, the piston ring may be manufactured throughout from one single material as disclosed above.

As previously mentioned, the finally formed compact material in accordance with the invention has the structure of an annealed martensite with the ceramics material forming a cohesive/bonding network structure in the martensite. The martensite, which is hard but brittle, thus is protected by the ceramics-material network, which acts as reinforcement of the martensite. At the same time, the discrete ceramics-material particles form hard, in themselves abrasive particles.

The ceramics-material to be chosen as a reinforcement material should exhibit tendencies to form microstructures in the form of networks and useful diffusion with the matrix without the formation of new powder-material phases caused by chemical reactions.

The simultaneous pressure and heat treatments employed to achieve the compact nature of the material result in a more compact material. One

such suitable treatment method is the HIP (Hot Isostatic Pressing) method. The inventors have found that by employing this method it becomes possible to produce materials that contain considerably larger amounts of carbon than by hitherto known methods and exhibit excellent strength properties.

Preferably, the reinforcing ceramics material consists of a carbide material, and preferably of Cr3C2. This material has proved to possess suitable properties inasmuch as it forms networks in the material in the HIP process but does not subsequently diffuse further in the material. In this manner a network is formed throughout the entire material and not only e. g. on one surface thereof. It is likely that this affects also the elasticity and the strength of the material.

Another suitable ceramics material is Cr2C3, which possesses the advantage of being thermally stable.

The metal powder could consist of e. g. a hard chrome steel.

Preferably, the powder materials obtained when the two powders are mixed comprise the following substances in the proportions indicated, viz.: 2.1- 2.8 percent by weight of C, 0.7-1 percent by weight of Si, 0.1-0.5 percent by weight of Mn, 20-24 percent by weight of Cr, 1-3 percent by weight of Mo, and 2-4 percent by weight of V.

The inventive material preferably exhibits a Brinell hardness in the range of 270-440. In addition, the elasticity module preferably amounts to at least 200 000 N/mm2. The tensile strength

advantageously exceeds 700 N/mm2, and preferably exceeds 1 100 N/mm2. The elongation should exceed 1%. All these requirements are set in consideration of the usability of the material for piston rings.

The invention also provides a method of producing a metal matrix composite material of the kind referred, according to which a composite powder is formed by mixing two different powder materials, whereafter the powder mixture thus obtained is subjected to pressure and heat until it reaches an austenitic temperature, and then it is cooled to form a martensitic matrix structure. The first material powder is a metal powder, and the second material powder is a ceramics powder, and after the formation of the martensitic structure the material is annealed to form a material that completely or partly exhibits a network of a ceramics material throughout the essentially martensitic matrix structure. The pressure and heat treatments are effected simultaneously, and the metallic powder material resulting from the mixture of the two powders contains 2.1-2.8 percent by weight of C.

The inventors have found that the use of such a simultaneous pressure and heat treatment makes it possible to produce a material that possesses sufficient strength to be used e. g. for piston rings.

Preferably, the treatment involving subjecting the material simultaneously to pressure and heat employs the so called HIP (Hot Isostatic Pressing) technique. Preferably, following the formation of

the martensitic form, the material is annealed to reduce the hardness to a desired value.

Annealing advantageously is performed by repetition of the annealing process at least once.

The material in accordance with the invention likewise makes possible the manufacture of a serviceable piston ring in accordance with the invention, wherein the material is a metallic composite material, which forms a steel matrix and a reinforcement structure from a reinforcement material. A piston ring of this nature meets the demands on strength, anti- corrosiveness, wear resistance and resilience, thus obviating the problems caused by separation on or wear of a separate wear layer. At the same time the material is not so hard that, when the piston cylinder system is lubricated, it damages the piston groove or the cylinder bore.

Brief Description of the Drawincrs Fig 1 is a diagram showing the rate of wear relative to pressure-exposure in tests of a previously known material, vis-a-vis an alloy known as tark alloy.

Fig 2 is a diagram showing the rate of wear relative to pressure-exposure in tests of a material in accordance with one embodiment of the invention vis-a-vis an alloy known as tark alloy.

Fig 3a is an OM picture shown in 25x magnification, of a material in accordance with the invention that has been etched to make the microstructure appear more clearly.

Fig 3b is an OM picture shown in 500x magnification, of the material in Fig 3a.

Fig 3c is an OM picture shown in 1300x magnification, of the material in Fig 3a.

Fig 3d is an OM picture shown in 3000x magnification, of the material of Fig 3a.

Fig 4 shown one embodiment of a piston ring in accordance with the invention.

Fig 5 is a partial view of two piston rings fitted into their respective piston groove in a piston-cylinder unit.

Description of Preferred Embodiments In accordance with a preferred embodiment of the invention a material is produced using the HIP method, which material comprises a high-chrome steel matrix reinforced with 1-10 percent by weight of Cr3C2. The grain size of Cr3C2 is 20-100 micrometers. As the matrix material is used a powder material comprising the following substances in the proportions indicated, viz.: 1.7 percent by weight of C, 0.8 percent by weight of Si, 0.3 percent by weight of Mn, 18 percent by weight of Cr, 1.0 percent by weight of Mo, 3.0 percent by weight of V, the balance of the material essentially consisting of Fe. The powder material used should be of a very pure quality.

Cr3C2 was chosen as the reinforcement material on account of its tendency to form microstructures in the form of networks and its useful diffusion with the matrix without new phases of the powder materials being formed through chemical reactions.

The mixture of metal and composite powders was effected in a conventional Turbula mixer, initially in a dry state and subsequently using a conventional tacking agent, in order to prevent particles having different densities from separating during the handling of the powder mixture. After mixing, the powders were put in a steel container and were dried for 18 hours. The containers were sealed, evacuated and introduced into conventional HIP equipment for transformation.

The transformation parameters were the following, viz.: a temperature of 1180-1215°C, a pressure of 100 MPa, and a heat-holding period of 3 hours. The HIP-treatment was followed by a heat treatment involving slow heating up to 850°C for 45 minutes and quick heating to 1050°C for 40 minutes, followed by rapid cooling to 500°C and finally cooling in air at room temperature. After this procedure, the material exhibit an essentially martensitic structure, with a comparatively high degree of hardness. The annealing process consisted of double annealing, at 600°C and thereafter at 650°C, each for a length of 2 hours. The annealing process serves to reduce the abrasiveness and hardness of the material to values that make the material suitable for piston rings.

The resulting material possesses excellent workability and resistance against wear and abrasive wear, and it possesses excellent strength in combination with high elasticity. The Brinell hardness is between 270 and 350 and the elasticity module is 200 kN/mm2. The tensile strength is less

that of the non-annealed material but still exceeds 700 N/mm2 and the elongation exceeds 1%. Thus, the material exhibits sufficient toughness to be used for the manufacture of piston rings.

If the same procedure is used with 10 percent by weight of Cr3C2 instead of 4 percent, a Brinell hardness of 360-440 is obtained, and an elasticity module of 200 kN/mm2, a tensile strength of mores than 1100 N/mm2, and a breaking strength exceeding 1%. Also this material is useable for piston rings.

In comparison could be mentioned that the tensile strength of the material suggested in the articles above amounts to only 400 N/mm2 with 10% of reinforcing Cr3C2, which thus is substantially below that of the inventive material.

Figs 1 and 2 show the unique properties of the inventive material as found in another comparison.

The diagrams of Figs 1 and 2 show the results of the rate of pressure-induced wear found in tests carried out vis-a-vis so called tark alloy, which is a commonly used cylinder-bore material. Fig 1 shows a commercially available cast material. As indicated, the rate of wear is about 100 mm/lOOOkm (totally) at pressures between 5 and 10 MPa. At pressures exceeding 10 MPa, the rate of wear rises drastically, causing scuffing of the material, and the figure shows the functional-strength limit of the material. The corresponding diagram with respect to the inventive material shows a completely different pattern. The rate of wear increases very slowly, and not even at a pressure of 22 MPa does it attain the lOOmm/lOOOkm (totally)

that the cast material exhibits at pressures between 5 and 10 MPa. There is no scuffing.

Fig 3a is an OM picture, shown in a 25 times magnification, of an inventive material, which has been etched to make the microstructure of the material appear more clearly. The ceramics-material particles are clearly visible against the background of a metal matrix. These particles contribute strongly to the excellent wear resistance of the material.

Fig 3c shows the same material as Fig 3a but magnified 1300 times. Also from this figure does appear the bonding ceramics-material network, which extends at least partly through the metal matrix.

The ceramics-material network contributes to retaining the larger ceramics-material particles, preventing the latter from separating from the matrix and in consequence thereof form discrete, strongly abrasive particles.

In Fig 3d, finally, is shown the ceramics- material network of Fig 3c, magnified 3000 times.

This figure also shows the structure of the small particles of the ceramics network that have aggregated during the manufacturing process so as to form a network.

Figs 4 and 5 are part of the present description in order to explain in more detail the appearance and function of the piston ring proper.

Figs 4 and 5 show a piston ring (1) intended for marine diesel engines of a conventional nature. The shape of the piston ring (1) is not completely round, and the ring (1) proper is provided with a through-slit (2) to allow the ring to be compressed

when being mounted in the associated piston groove (3). When the piston ring (1) is received in the piston groove (3) its inherent resilience will force it slightly outwards, into abutment against the cylinder bore (5). During operation of the diesel engine, the piston ring (1) is exposed to wear primarily at the surface thereof that abuts against the cylinder bore (5) but also at the surfaces (4) that face the piston groove (3).

As will be appreciated, the present invention is not limited to the embodiment described herein.

For example, further substances may be added to the powder material in order to modify its properties in some respect. A lubricant could for instance be added to the powder mixture in order to produce a material that is suitable for the manufacture of a so called self-lubricating ring.

Instead of Cr3C2, the ceramics material used as a reinforcing material could be e. g. A1203, Cr203 or any other ceramics material possessing the necessary network-forming properties. Different ceramics materials could also be used simultaneously, such as for example one ceramics material could make up the larger-size granules having the abrasive effect whereas another ceramics material could be the one that forms the essentially thinner network structure. However, together these ceramics materials should form a stable network capable of reinforcing the martensite structure and of retaining the larger ceramics-material particles.