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  Piston ring having improved scuffing, cracking and fatigue resistances, and its production method, as well as combination of piston ring and cylinder blockUSPTO Application #: 20070187002Title: Piston ring having improved scuffing, cracking and fatigue resistances, and its production method, as well as combination of piston ring and cylinder block Abstract: A piston ring having improved scuffing resistance, cracking resistance and fatigue resistance, consists of a high-chromium martensitic stainless steel and a sliding nitriding layer formed on the surface of said steel. The stainless steel consists of C: 0.3 to 1.0%; Cr: 14.0 to 21.0%, N: 0.05 to 0.50%, at least one of Mo, V, W and Nb: 0.03 to 3.0% in total, Si: 0.1 to 1.0%, Mn 0.1 to 1.0%, P: 0.05% or less, S: 0.05% or less, the balance being Fe and unavoidable impurities. The sliding nitriding layer comprises on its surface hard particles mainly consisting of nitrides in a range of from 0.2 to 2.0 μm of average particle size, 7 μm or less of the longest diameter, and from 5 to 30% in area ratio. (end of abstract) Agent: Kubovcik & Kubovcik - Washington, DC, US Inventors: Junya Takahashi, Toru Onuki, Shigeo Inoue, Mitsutaka Sasakura USPTO Applicaton #: 20070187002 - Class: 148318000 (USPTO) Related Patent Categories: Metal Treatment, Stock, Carburized Or Nitrided, Nitrided, Ferrous (i.e., Iron Base) The Patent Description & Claims data below is from USPTO Patent Application 20070187002. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] This application is a continuation of U.S. patent application Ser. No. 10/333,326 which entered the U.S. national stage on Jan. 17, 2003, as a 371 of PCT/JP01/06127 filed Jul. 16, 2001, which claims priority based on Japanese patent application No. 2000-216255 filed Jul. 17, 2000, each of which is incorporated herein by reference. BACKGROUND OF INVENTION [0002] 1. Technical Field [0003] The present invention relates to a piston ring used in an internal combustion engine, particularly, a piston ring consisting of high chromium martensitic stainless steel with nitriding, having improved scuffing resistance (seizure resistance), cracking resistance (failure resistance) and fatigue resistance. The present invention is also related to a production method of the piston ring. [0004] 2. Background Technique [0005] Along with recent demands for low fuel consumption, weight reduction and high performance of internal combustion engines, the piston rings are thinned to reduce weight and to follow the high rotation of the engine. Material properties of the piston rings, such as wear resistance, scuffing resistance and fatigue resistance, and the like must be improved to enable thinning of a piston ring. The conventional cast-iron piston rings have, therefore, been replaced with steel piston rings particularly from a view point of the fatigue resistance and heat resistance. However, since the scuffing resistance of the steel piston-ring is inferior to that of the cast-iron piston-ring, any surface-treatment is usually applied to the sliding surface of steel piston ring. Steels for piston ring are roughly classified into carbon steel, silicon-chromium steel, and martensitic stainless steel. These classifications correspond to different kinds of surface treatments applied to the respective steels. Mainly, the chromium plating is applied to carbon steels and silicon chromium steels. Gas nitriding is applied to martensitic stainless steels. The chromium plating was the most frequent surface treatment of the steel piston ring previously, but has been mostly replaced at present with the nitriding, because the scuffing resistance of the chromium plating under high load is poor, and, further, the waste-liquid of the plating must be treated so as not to incur any environmental problem. [0006] High chromium martensitic stainless steel mainly used at present for the piston ring with nitriding is JIS SUS440B equivalent composition of C: 0.80-0.95%; Cr: 17.0-18.0%; Si: 0.25-0.50%; Mn: 0.25-0.40%; Mo: 0.70-1.25%; V: 0.07-0.15%; and Fe in balance. When the steel having this composition is subjected to nitriding, nitrogen atoms intrude and diffuse from the surface into the steel and form a nitriding layer. The nitrides in the nitriding layer are mainly compounds of Cr, V and Mo, which may contain the solute Fe. Chromium, which is the main component of this steel, is dissolved in the iron matrix, and is also present in the form of Cr carbides. [0007] Since the affinity of Cr for nitrogen is higher than that for carbon, when nitrogen diffuses from the surface by the nitriding, the reaction between the nitrogen and Cr carbides occurs to form the Cr nitrides. Since the Cr content of SUS 440B equivalent material is as high as 17.0-18.0%, hard Cr nitrides are dispersed in the nitriding layer in an appropriate area %. The nitriding layer is, therefore, relatively hard and improves the wear resistance and scuffing resistance. [0008] Recently published Japanese Unexamined Patent Publication No. 11(1999)-80907 proposes martensitic stainless steel with nitriding, having improved scuffing resistance, which contains Si: 0.25% or less, Mn: 0.30% or less; one or more of Mo, W, V and Nb: 0.3-2.5% or Cu: 4.0% or less; Ni: 2.0% or less, and Al: 1.5% or less. [0009] Japanese Unexamined Patent Publication 11(1999)-106874 discloses that when the quantity of M.sub.7C.sub.3 carbide in the microstructure is suppressed to 4.0% or less in area %, not only scuffing resistance but also workability of the piston ring steel material are improved. [0010] Although the wear resistance and scuffing resistance have been improved by the proposals as described above, when these piston rings are used in recent internal combustion engines operated under high revolution, and high power conditions, scuffing is liable to occur. [0011] Heretofore, liners are forced into the cylinder block of Diesel engines. These engines are changed to a monolithic block cast-iron of narrow bore distance without liners so as to attain weight reduction and cost saving. The combustion pressure tends to be increased from the viewpoints of waste gas purification and power increase. In the microstructure of the cast iron mono-block, from relatively large cooling rate difference in casting of the mono-block, graphite dispersion is not uniform and soft ferrite phase as the cause of scuffing is unevenly distributed. [0012] When the cylinder surface having the microstructure mentioned above is combined with the martensitic stainless-steel piston ring with nitriding, scuffing is liable to occur in the initial operation period for the following reasons. [0013] When the cylinder surface is finished by honing, the abrasives of a grinding wheel cause clogging due to ferrite phase, and the surface of the cylinder is liable to be roughened after the honing. The graphite is covered by the ferrite flowed plastically. As a result, lubrication and oil-reserving effects of the graphite are lowered since the area % of the graphite decreases. In the case of high combustion pressure, the back pressure applied to the piston ring becomes high. Scuffing frequently results from cracks on the outer peripheral surface of a piston ring, elongating in a direction perpendicular to the sliding direction. When the nitriding layer is inspected, cracks are detected along the lamellar compounds. The compounds are relatively coarse and are present along grain boundaries of the iron matrix, and is referred to as the gull phase in the field of the Japanese piston ring industry. The compound lamella distribute parallel to the surface of the piston ring. [0014] In order to solve the problems of the piston rings, the formation of TiN, CrN and the like is carried out by means of ion plating. The ion plating can improve the wear resistance and the scuffing resistance but the production cost is high. The reputation of ion plating by the users at present is not favorable in the light of the cost performance. [0015] It is, therefore, an object of the present invention to provide a high chromium martensitic stainless steel piston ring with nitriding, which ring is cost-effective and, which incurs neither wear, scuffing, cracking nor fatigue fracture even when used in a Diesel engine operated at high revolution and high combustion pressure, particularly, a cast-iron monolithic block Diesel engine, which is expected to be increasingly used in the future because of weight reduction. [0016] It is a specific object of the present invention to adjust the size and distribution of hard particles, mainly nitrides, in the nitriding layer of high chromium martensitic stainless steel and to reduce the size of grain boundary lamellar compounds to suppress initiation and propagation of cracks in connection with those compounds. [0017] It is also an object of the present invention to provide a method for producing a high chromium martensitic stainless piston ring. SUMMARY OF INVENTION [0018] According to explanation of "Automotive Piston Ring" edited by Editing Committee of Automotive Piston Ring, Sankaido Publisher, page 188, 1997, when load is concentrated on the convexities (especially of soft phases) of microscopic unevenesses on a sliding surface, the temperature rises there due to friction heat, and abnormal softening and melting occurs. This phenomenon results in scuffing of the piston ring. [0019] In the high chromium martensitic stainless steel with nitriding, the microstructure of the nitriding layer generally shows mainly hard nitrides dispersed in the tempered martensite matrix. The mechanism of scuffing is strongly dependent upon the microscopic unevenness on the sliding surface. In the nitriding layer, hard particles disperse in the relatively soft matrix. The microscopic unevenness is, therefore, defined by the size and dispersion state of the hard particles. When the cross section of the surface layer having such structure is observed, the following is apparent. The convex hard particles are brought into contact with an opposite sliding surface, while the relatively soft matrix is relatively concave. The lubricating oil retained in the concave portions is subjected to pressure during sliding. Frequency of the entire direct contact of the steel with nitriding and the opposite member is low, since the steel with nitriding has the microstructure as described above. As a result, the contact pressure between both sliding members is decreased. In addition, the oil is fed to the convex portions mentioned above. The scuffing can, therefore, be prevented. [0020] The hard convex particles can attain the effects as described above, provided that they are from sub microns to a few microns in size and dispersed in an amount of 5% by area or more. In the case in which the hard particles are extremely small and case in which they are small in quantity, the mechanism according to action and effect of the convex hard particles mentioned above cannot be expected. [0021] Meanwhile, these effects are influenced by the circumstances of the sliding surface of the opposite member. Specifically, in the case of the cast iron monolithic cylinder block having a non-uniform structure as described above, the surface of this block is liable to be roughened by the grinding. Frequently, the ferrite phase plastically flows and covers the graphite. [0022] The sliding surface of even such cast iron is modified by appropriate sliding referred to by experts as break-in or compatibility. That is, the following phenomenon occurs. When the rough inner surface of a cylinder is smoothened during the sliding, the ferrite is removed and the covered graphite is exposed. Until the break-in is progressed, an oil film on the sliding surface is frequently liable to be absent. When the oil film is absent, friction force applied on the outer peripheral surface of a piston ring is increased. The large friction force is repeatedly applied on the outer peripheral surface of a piston ring. The nitriding layer is, therefore, repeatedly subjected to large stress resulting in initiation and enlargement of cracks in a direction perpendicular to the sliding direction. Along with the progress of the adaptability phenomenon on the inner surface of a cylinder, the stress applied is lessened, while the cracks propagate with the lapse of time. As a result, the nitriding layer may locally peel in the surface, and the inner surface of a cylinder may be damaged. The scuffing is, therefore, liable to occur in the initial period of sliding. Since the grain boundary compounds in the nitriding layer are very brittle, the presence of them promotes the initiation and propagation of cracks. Continue reading... Full patent description for Piston ring having improved scuffing, cracking and fatigue resistances, and its production method, as well as combination of piston ring and cylinder block Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Piston ring having improved scuffing, cracking and fatigue resistances, and its production method, as well as combination of piston ring and cylinder block patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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