METHOD OF HEAT TREATING BEARING MATERIALS

Abstract: An improved method of producing a powdered metal aluminum base bearing material is provided. A bearing strip composed of three layers of sintered aluminum base part­ icles which has been roll clad to a rigid backing layer is sub­ jected to a heat treatment procedure in a continuous manner at a temperature of from about 700° F to about 900 °F for at least thirty seconds and then cooled at a rate of at least 100° F/hr. Bearings made from the resulting material show dram­ atic fatigue life improvement in comparison to that obtain­ able with currently available powdered metal aluminum bearing materials.

Description: Background of the Invention The present invention relates generally to an aluminum base bearing made by powder metallurgy techniques and, more particularly, to a bearing having a surface layer of pre-alloyed aluminum base particles. It has been known to make aluminum base bearings by powder metallurgy techniques containing a bearing phase of conventional materials such as lead, tin, copper, cadmium, etc. However, considerable difficulty has been experienced in the fabrication and use of such bearings, especially in imparting superior bearing load carrying capacity and anti-seizure properties to the bearing structure. One method used to achieve improved bearing properties was to have a bearing layer in which the particles of the bearing layer are in pre-alloyed powder form, particularly where the bearing phase is in an intra-particle position relative to the aluminum. This bearing with a fine dispersion of the bearing phase in the individual aluminum particles and method of manufacture are described in U.S. Patents 3,797,084, and 4,069,369 filed December 18, 1972 and May 4, 1973, respectively, and owned by the assignee of the instant application. The current trend toward higher output engines, such as turbo charged engines, has given rise to the need for even higher performance bearing materials. Presently, the only bearing materials which consistently meet the performance requirement of these higher output engines are overlay plated tri-metal bearings. These bearings while having good performance characteristics are expensive to produce, exhibit accelerated wear and provide clearance control problems. Accordingly, it is a principal object of this invention to provide a method of making an aluminum based bearing by powder metallurgy techniques which has improved bearing load carrying capacity and anti-seizure properties. Summary of the Invention A method of producing a powdered metal aluminium base bearing material as specified in the prior art portion of claim 1 is disclosed in the above-mentioned US-A-4 069 369. The present invention, as defined in claim 1, provides a method of making an aluminium base bearing material having superior fatigue and anti-seizure properties. Description of the Preferred Practice of the Invention The present invention relates to a method of producing bearing materials which exhibit properties not obtainable heretofore by prior art techniques. As below noted, the present invention represents a significant improvement over the method disclosed in US Patents Nos 3 797 084 and 4 069 369. Thus, in US-A-4 069 369, it is explained that the aluminium and bearing phase materials are pre-alloyed to formed a uniform dispersion of the bearing phase materials through the aluminium to provide an intra-particle bearing phase dispersion portion while an inter-particle bearing phase distribution is obtained when the bearing phase materials are intermixed with the aluminium without pre-alloying. Specifically, the present improvement is achieved via the unexpected discovery that a superior bearing material is produced when the thermal processing of the bearing material having a rigid backing layer clad thereto is controlled such that the material is heated at a temperature ranging from about 371°C (700°F) to about 482°C (900°F) for at least 30 seconds to effect alloy solutionising and then rapidly cooled. The cooling rate is dependent upon the solution treating temperature wherein this rate is more rapid for the higher portion of the solution heat treating range than for the lower portion but in all cases more rapid than the 28°C (50°F)/hour associated with standard full annealing and in fact more rapid than 56°C (100°F)/hour. That is, the cooling rate for the present invention is higher for materials heated to 482°C (900°F) than for those heated to 371°C (700°F). The techniques and materials utilised in the practice of the present invention are generally described in US Patent 3 797 084 except for the above-described critical thermal treatment. Thermal processing according to the present invention has been totally redefined over the prior art. Specific elements of this redefinition are as follows: a) Post thermal processing is mandatory, not optional. b) The thermal processing has been changed from full annealing to solution treating. This change has produced the unexpected result of obtaining the strengthening effect of the copper and/or other alloy additions without experiencing the potential bearing surface property degradation generally associated with solution treating of bearing materials. c) The thermal treating temperature has been redefined from 316°C-399°C (600° F - 750° F) to 371°C - 482°C (700° F - 900° F) to obtain effective solutionizing. d) The cooling rate has been changed from less than 28°C/hr (50° F/hr.) required for full annealing where material hardness is at a minimum and ductility is at a maximum to greater than 56°C/hr (100° F/hr.) to take advantage of the strengthening influences of the alloying elements. The preferred rate to maximize material properties is in excess of an average of 28°C/min (50° F/min.) during the first three minutes of cooling. Materials used in the practice of the present invention included: a) The bottom layer, i.e. the powder metal bonding layer, can consist essentially of more than 55 weight percent aluminum with the balance being selected from a first group of additives consisting of silicon, copper, manganese, magnesium, nickel, iron, zinc, chromium, zirconium, titanium and mixtures thereof. b) The intermediate layer, i.e. the powder metal bearing layer, can consist essentially of at least 55 and up to about 95 weight percent aluminum, with the balance selected from the first group of additive materials in an amount of 0 to about 20 weight percent and from a second group of bearing phase materials in the amount of 5 to 25 weight percent, the second group consisting of lead, tin, cadmium, bismuth, antimony and mixtures thereof. c) The surface layer, i.e., the sacrificial layer deposited on the powder metal bearing layer, can consist essentially of more than 50 weight percent of aluminum particles with the balance of additives being selected from the first and second groups. In addition, the aluminum and the bearing phase materials of the bearing layer are in prealloyed particle form to establish an intra-particle position relative to each other and the bearing phase particles in the sacrificial layer are formed so as to establish an interstitial position therein relative to the aluminum particles. The following is a detailed example showing the practice of the instant invention. (1) An air atomize bearing powder material was produced by the techniques described in U.S. Patent 3,797,084. The nominal composition in weight percent of the alloy was 7.5% lead, 1.5% tin, 0.9% copper, 4.0% silicon, with balance being aluminum. (2) A sacrificial layer material was produced which had a nominal composition in weight percent of 80% aluminum, and 20% of an 85/15 lead-tin solder powder. (3) A bonding layer material consisting of pure aluminum was produced. (4) The pure aluminum powder, bearing alloy powder, and sacrificial powder were simultaneously roll compacted to produce a green, three layered strip with the alloy powder interposed between the aluminum (bonding) layer and the sacrificial layer. (5) The compacted strip, in coil form, was sintered in an air furnace at a temperature of 524°C ± 14°C (975° F ± 25 °F) for a minimum of 12 hours. (6) Prior to roll bonding the above sintered strip to a steel substrate, it was heated for 2 hours at 204°C (400° F) followed by 2 hours at 427°C (800° F) to preclude moisture related blister formation. (This technique is preferred, but not mandatory). (7) The sintered and thermally treated strip was roll bonded to a dead soft steel backing in the following preferred manner: a) Alkaline clean and rinse the steel; b) Grind the steel surface to remove oxides and provide fresh, rough surface for bonding; c) Wire brush the pure aluminum side of sintered strip to remove oxides and provide active bonding surface; and d) Simultaneously pass the sintered strip with freshly prepared aluminum layer and ground steel backing, face to face, through a rolling mill, wherein the sintered strip is reduced in thickness a minimum of 55% and a metallurgical bond effected between the aluminum and steel. (8) In the preferred method, an additional cold reduction of the steel/aluminum alloy composition of about 5% is achieved in another rolling operation which is performed after roll bonding. (9) The finished rolled structure is thermally treated in a continuous manner wherein: a) The structure is heated to a temperature range of about 371°C (700° F) to about 482°C (900° F); b) The structure is soaked for a time of at least 30 seconds but no longer than the time required for the formation of brittle aluminum/iron intermetallic. For example, the maximum time limit at 482°C (900° F) would typically be about five minutes. c) Cooling the so heat-treated structure at a rate of at least 56°C (100° F) per hour, and d) In the preferred practice of the invention, the structure is heated to a temperature of about 399°C (750° F) to about 427°C (800° F) and soaked for a minimum of 2 minutes. The following is a detailed description of various tests conducted to show the benefit of the present invention. Specifically, Figure 1 illustrates the effect of post clad thermal treatments on bearing fatigue life as measured by the Underwood test. Bearings manufactured in accordance with this invention exhibited more than twice the life of those manufactured with the standard thermal process. Each data point represents the average of four test results. All tests were conducted at a unit load of 551 bar (8000 PSI) (theoretical peak film pressure of 8096 bar (117 500 PSI)) and terminated at the first sign of cracking (failure). All test bearings were made from material prepared in the manner described herein. This material came from the same source, i.e. a single coil. All processing except the final thermal treatment was performed in production. A laboratory furnace was used for the treatments Shown in Figure 1. The air cooling cycle involved removing material from the furnace after it had soaked at the desired temperature for 2 minutes and allowing it to cool in air. Under the above conditions, the following cooling cycles were recorded: Treatment Temp Cooling Rate °C/min (°F/min) 316°C (600°F) 399°C (750°F) 454°C (850°F) 1st Min 92 (165) 131 (235) 152 (274) 2nd Min 47 (85) 62 (111) 76 (136) 3rd Min 35 (63) 40 (72) 50 (90) 4th Min 24 (44) 35 (63) 33 (59) 5th Min 19 (35) 27 (49) 27 (48) Temp at 5 Min °C (°F) 98 (208) 104 (220) 117 (243) Av Cooling Rate for 5 min 43°C (78°F)/min 59°C (106°F)/min 67°C (121°F)/min The furnace cooling cycle was accomplished by means of a controller which was programmed to cool the furnace at a rate of 28°C (50°F) per hour after the material had soaked at the desired temperature for 30 minutes. Figure 2 illustrates the effect of the post clad thermal treatments on bearing alloy hardness as measured by the Knoop micro-hardness scale. Each point represents the average of 5 readings. Hardness is a fairly good indicator of the tensile and fatigue strength of the material. In Figures 1 and 2 the properties of material processed according to the instant invention are shown in curve A whereas those of material outside of the scope of the invention are illustrated by curve B. From the foregoing it is noted that superior bearing material can be produced via the practice of the present invention.

Claims: A method of producing a powdered metal aluminum base bearing material having superior fatigue and anti-seizure properties which method comprises: a) simultaneously roll compacting three distinct layers of aluminum base powder particles, in which the bottom layer of said layers constitutes a powder metal bonding layer consisting of more than 55 weight percent aluminum and the balance selected from a first group of additives consisting of silicon, copper, manganese, magnesium, nickel, iron, zinc, chromium, zirconium, titanium and mixtures thereof; the intermediate layer of said layers constitutes a powder metal bearing layer consisting of at least 55 up to about 95 weight percent aluminum, with the balance being selected from said first group of materials in an amount of 0 to about 20 weight percent and from a second group of bearing phase materials in the amount of 5 to 25 weight percent, said second group consisting of lead, tin, cadmium, bismuth, antimony and mixtures thereof; the surface layer of said layers constitutes a sacrificial layer deposited on said powder metal bearing layer and consisting essentially of more than 50 weight percent of aluminum particles and the balance of additives selected from said first and second groups, with said aluminum and said bearing phase materials of said bearing layer being placed in prealloyed particle form to establish an intra-particle position relative to each other and the bearing phase particles in said sacrificial layer being formed without prealloying to establishing a non prealloyed interstitial position therein relative to the aluminum particles; b) sintering the so-formed three-layered composite; and c) roll cladding the bonding layer face to face onto a rigid backing layer; characterised in that the roll clad composite material is heat treated in a continuous manner to a temperature from about 371°C to 482°C (700° to 900°F) for a period of at least 30 seconds and then the material is convection cooled at a rate greater than 56°C/hr (100°F/hr). A method according to claim 1, wherein the cooling rate is at an average of at least 28°C/min (50°F/min) during the first three minutes of cooling. A method according to claim 1 or 2, wherein the roll clad composite material is maintained at a temperature of from about 371°C to 482°C (700° to 900°F) for a period of time ranging from at least 30 seconds to a maximum less than the time required for the formation of a brittle aluminium/iron intermetallic. A method according to claim 3, wherein said heating takes place at a temperature ranging from about 399°C to 427°C (750° to 800°F) and is maintained at said temperature for about two minutes.

Inventors: BRYDA, KENNETH A., LANNER, HANS G., STUTZMAN, RODNEY W., YAHRAUS, WILLIAM A.
Applicants: J P I TRANSPORTATION PRODUCTS INC
Application year: 1987
Main classification: B22F7
IPC codes: F16C33, B22F7, B22F3