ABRASION-RESISTANT SINTERED ALLOY

Abstract: Abrasion-resistant alloy sintered in a liquid phase, which contains, by weight, 1.5 to 4.0% C, 0.5 to 1.2% Si, 1.0% or less Mn, 2.0 to less than 8.0% Cr, 0.5 to 2.5% Mo, 0.2 to 0.8% P, and the balance of Fe.

Description: TECHNICAL FIELD: The present invention relates to a chromium-containing iron-base sintered alloy which is used as a sliding part material for a valve mechanism in an internal combustion engine. BACKGROUND ART Recently, the valve mechanism of an internal combustion engine is required to bear a heavy running load. In particular, the sliding parts, such as a camshaft and a rocker arm, are needed to stand against high plane pressures. Chromium-containing iron-base sintered alloys have been tried as a sliding part material not only to meet the aforementioned need but also to reduce the weight of the valve mechanism. Such alloys are known by JP A 54-62108, 56-12353 and 58-37158. The alloy of 54-62108, containing, by weight, Cr; 8.0 - 30.0%, C; 0.5 - 4.0%, P; 0.2 - 3.0%, the balance being Fe, has a problem that, where Cr exceeds 20.0%, chromium-carbide grows coarser and harder to give damage to the opposite sliding part. Another problem is that it is too hard to be machined. The second alloy of 56-12353, containing, by weight, Cr; 2.5 - 7.5%, Cu; 1.0 - 5.0%, C; 1.5 - 3.5%, P; 0.2 - 0.8%, Si; 0.5 - 2.0%, Mn; 0.1 - 3.0%, Mo; less than 3.0%, the balance being Fe, is less shrinkable even when sintered at a liquid-phase because of containing more than 1% of copper, so that it is unavailable for fabricating the fitting members of a camshaft, such as cam lobes and the like that are constrictively joined with the shaft after loosely mounted on the same. The third alloy of 58-375158, containing, by weight, Cr; 2.5 - 25.0%, C; 1.5 -3.5%, Mn; 0.1 - 3.0%, P; 0.1 - 0.8%, Cu; 1.0 - 5.0%, Si; 0.5 - 2.0%, Mo; less than 3.0%, S; 0.5 - 3.0%, Pb; 1.0 - 5.0%, the balance being Fe, has an advantage that copper is effective to prevent the growth of coarse chromium-carbide. However, it is relatively brittle because of containing sulphide and lead. US 4 388 114 relates to an anti-wear sintered alloy comprising 2.5 to 7.5 weight precent chromium, 0,10 to 3.0 % manganese, 0.2 to 0.8 % phosphorus, 1.0 to 5.0 % copper, 0.5 to 2.0 % silicon, 0 to 3 % molybdenum, 1.5 to 3.5 % carbon, balance iron and less than 2 % impurities. The present invention is intended to provide a chromium-containing iron-base sintered alloy that is superior in machinability and suitable for fabricating cam lobes and the like which are constrictively bonded to a shaft by a liquid-phase sintering after loosely mounted on the same shaft. DISCLOSURE OF INVENTION: Accordingly, the invention relates to an abrasion-resistant sintered alloy containing, by weight 1.5 to 4.0 % carbon, 0.5 to 1.2 % silicon, no more than 1.0 % manganese, 2.0 to less than 20 % chromium, 0.5 to 2.5 % molybdenum, 0.2 to 0.8 % phosphorus and, optionally, 0.5 to 2.5 % nickel, 0.1 to 5.0 % of at least one of boron, vanadium, titanium and tungsten, less than 300 ppm calcium, balance iron, said alloy being sintered at a liquide-phase. The reason for a range of 1.5 - 4.0% of carbon is that, where carbon exceeds 4.0%, chromium-carbide grows coarser and harder to produce large pores with the result that the alloy matrix is somewhat brittle after sintered, and that, where carbon is below 1.5%, the amount of chromium-carbide is insufficient to give the abrasion-resistant property to the alloy. The reason for a range of 0.5 - 1.2% of Si is that, where Si exceeds 1.2%, the alloy powders become less mouldable and more deformable when sintered, the sintered alloy matrix being brittle. Si is an important component to yield a liquid-phase in the case that C and P are relatively low in content, so that it should not be less than 0.5%. The reason for no more than 1.0% of Mn is that, where Mn exceeds 1.0%, the alloy powders become less mouldable and its sintering rate reduces to the extent that there remain large pores in the sintered alloy. The reason for limiting chromium to less than 20.0% is that more than 20.0% of chromium grows chromium-carbide coarser and harder to decrease the machinability of the alloy. The addition of less than 2.0% of chromium is also undesirable because it will result in insufficient formation of hard carbide, thereby deteriorating the anti-wearing property. It is preferable in the alloy used as a sliding parts subject to high plane pressures in the automobile engine under a heavy running load that the carbon content is increased with the chromium content although the former is usually decreased with the latter. Molybdenum is solid-solved in the matrix to increase the hardness as well as the wear resistance of the sintered alloy. The effect is saturated beyond the amount of 2.5%. However, it is too small if the amount is less than 0.5%. Thus, the amount of molybdenum is limited to 0.5 to 2.5%. Phosphorus contributes to the precipitation of Fe-C-P eutectic steadite, which has a high hardness and a low solidifying point of about 950 degree to promote the liquid-phase sintering. If the amount of phosphorus is less than 0.2%, the precipitation of steadite is too small to obtain a highly anti-wearing alloy. Besides, it is not easy to yield a liquid-phase. However, if the amount of phosphorus exceeds 0.8%, the alloy will have its machinability decreased by the steadite excessively produced. The purpose of adding nickel is to enlarge the amount of martensite and banite in the matrix and increase the tensile strength. However, if the addition of nickel exceeds 2.5%, the increase of residual austenite in the matrix decreasing the hardness and abrasion-resistance. The addition of less than 0.5%, by weight, of nickel is not effective to increase the tensile strength. The purpose of adding at least one selected from a group consisting of B, V, Ti, Nb and W is to promote yield of liquid phase as well as formation of carbide. The amount of addition is desirably limited to 0.1 to 5.0% in consideration of the hardness of the opposite sliding part. In preference, less than 300 ppm of calcium is added to improve the machinability of the alloy. The alloy of the invention is mostly used in slidable parts of camshafts and rocker arms and conveniently sintered at a liquid-phase yielding temperature. The reason for this is that the sinterable alloy powder preform, after loosely mounted on the shaft, is contracted and tightly joined with the same by the liquid-phase sintering. For example, cam lobes of sinterable alloy powders are loosely mounted on a steel shaft and then sintered at a liquid-phase yielding temperature in which the cam lobe is highly densed and firmly bonded to the shaft. BRIEF DESCRIPTION OF DRAWINGS: FIGS. 1 and 2 are photomicrographs of 200-fold magnifications showing the microstructure of the abrasion-resistant alloy of the invention, the reference marks A and B indicating matrix and carbide, respectively. THE BEST MODE CARRYING OUT THE INVENTION: The preferred examples of the present invention are illustrated below. EXAMPLE 1 Alloy powders are prepared to have the following composition, by weight, of 2.8% of C, 0.9% of Si, 0.2% of Mn, 0.5% of P, 15.5% of Cr, 1.9% of Ni, 1.0% of Mo, the balance being Fe, which are mixed together with zinc stearate. The mixture is compressed under a compression pressure of 5 to 7 t/sq. cm and then sintered at 1100 to 1200 degrees (average 1160 degrees) in cracked ammonia gas atmosphere furnace, thereby yielding a sintered alloy as micrographically shown in FIG. 2 in which white carbides are granularly distributed over the black matrix consisting of a martensite and banite mixture. The test results show that the alloy has a hardness of HRC 61.5, a density of 7.62 g/cu. cm, and a superior abrasion-resistant property. EXAMPLE 2 Other alloy powders are prepared to have the following composition, by weight, of 2.0% of C, 0.8% of Si, 0.15% of Mn, 0.45% of P, 6.0% of Cr, 1.6% of Ni, 1.0% of Mo, the balance being Fe, which are mixed together with zinc stearate. The mixture is compressed under a compression pressure of 5 to 7 t/sq. cm and then sintered at 1050 to 1180 degrees (average 1120 degrees) in cracked ammonia gas atmosphere furnace, thereby yielding a sintered alloy as micrographically shown in FIG. 1 in which white carbides B are granularly distributed over the black matrix A consisting of a martensite and banite mixture. The test results show that the alloy had a hardness of HRC 56.5, a density of 7.60 g/cu. cm, and a superior abrasion-resistant property. From the foregoing the ferrous sintered alloy of the invention has a structure composed of a martensite and banite mixture matrix yielded by a liquid-phase sintering and carbides granularly spread out in the matrix, therefore being superior in the anti-wearing property. The alloy is also superior in fitting property and productivity, because the powders is moulded and firmly bonded to the body by a liquid-phase sintering. The alloy advantageously contains less than 20% of chromium, so that coarse and hard chromium-carbide is prevented from growing to the extent that it gives damage to the opposite sliding part. Besides, the alloy is less brittle because of containing no sulphide nor lead. INDUSTRIAL APPLICABILITY: The anti-wear alloy of the invention is available as a material for fabricating sliding members in an internal combustion engine such as cams in a camshaft and tapets in a rocker arm.

Claims: An abrasion-resistant sintered alloy containing, by weight 1.5 to 4.0 % carbon, 0.5 to 1.2 % silicon, no more than 1.0 % manganese, 2.0 to less than 20 % chromium, 0.5 to 2.5 % molybdenum, 0.2 to 0.8 % phosphorus and, optionally, 0.5 to 2.5 % nickel, 0.1 to 5.0 % of at least one of boron, vanadium, titanium and tungsten, less than 300 ppm calcium, balance iron, said alloy being sintered at a liquid-phase. The alloy of claim 1, wherein the contents of carbon and chromium are as follows 1.5 to 3.0 % carbon and 2.0 to less than 8.0 % chromium. The alloy of claim 1, wherein the contents of carbon and chromium are as follows 2.0 to 4.0 % carbon and 8.0 to less than 20 % chromium.

Inventors: HIRAKAWA, OSAMU, URANO, SHIGERU
Applicants: NIPPON PISTON RING CO., LTD.
Application year: 1984
Main classification: C22C38
IPC codes: C22C38, B22F5, C22C33