Abstract:
A process for preparing a sintered article of a compacted iron-based metallurgical powder. The green compact is sintered at a closely held predetermined temperature in order to achieve desired density and dimensional stability.

Description:
[0001]    This application is a continuation of prior provisional application No. 60/348,140 filed Jan. 15, 2002. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates to processes for making parts using powder metallurgy manufacturing technology. In particular, this invention relates to the sintering of structural parts having tightly controlled mechanical and dimensional properties when using a fluidized bed furnace.  
           [0004]    2. Related Background Art  
           [0005]    The making and using of ferrous powders are well known, and are described in considerable detail in Kirk-Othmer&#39;s Encyclopedia of Chemical Technology, Third Edition, Volume 19, at pages 28-62. Ferrous powders can be made by discharging molten iron metal from a furnace into a tundish where, after passing through refractory nozzles, the molten iron is subjected to granulation by horizontal water jets. The granulated iron is then dried and reduced to a powder, which is subsequently annealed to remove oxygen and carbon. A pure iron cake is recovered and then crushed back to a powder.  
           [0006]    Ferrous powders have many applications, such as powder metallurgy (P/M) part fabrication, welding electrode coatings, flame cutting and scarfing. For P/M applications, the iron powder is often blended with selected additives such as lubricants, binders and alloying agents. A ferrous P/M part is formed by injecting iron or steel powder into a die cavity shaped to some specific configuration, applying pressure to form a compact, sintering the compact, and then finishing the sintered compact to the desired specifications.  
           [0007]    Other ferrous powders which can desirably be utilized in connection with this invention include the iron-graphite composite powders described in allowed U.S. Ser. No. 09/609,115, filed Jun. 30, 2000 which produce a malleable iron compact. The P/M powders and processes taught and claimed in the commonly-assigned U.S. Pat. Nos. 4,927,461, 5,069,714 and 5,682,591, and allowed Ser. No. 09/609,115 are hereby incorporated by reference.  
           [0008]    There are many instances of sintering non metallic materials and metallic products including powders, green compacts or other pressed iron powder structural parts. Commonly-assigned U.S. Pat. No. 5,876,481 (the P/M powders and processes taught and claimed in which are also hereby incorporated by reference) teaches sinter-hardening process by carefully controlling the rate of change of heating temperatures. Similarly, U.S. Pat. Nos. 3,249,662, and 5,796,018 refer to sintering in a fluid bed. The &#39;662 patent relates to ceramic articles and the &#39;018 patent relates to ferrous powder. Use of fluid beds is also mentioned in U.S. Pat. Nos. 4,317,676, 4,410,373, 4,415,527, 4,693,682, 5,271,891, 5,584,910, 5,745,834, 5,620,751 and 6,030,434.  
           [0009]    These methods, however, were never used to sinter ferrous green P/M compacts. Therefore the efficiency of the fluidized bed process for maintaining dimensional stability were not revealed in the prior art.  
         SUMMARY OF THE INVENTION  
         [0010]    It would be desirable to provide a P/M process that provides the advantages of producing sintered articles with excellent dimensional stability. Accordingly, the invention relates to a general method to improve the dimensional control and consistency of mechanical properties of parts produced from powder metallurgy.  
           [0011]    The present inventions have discovered that the sintering step of P/M processing unexpectedly critical to achieving improved dimensional tolerances. In particular, the present inventors have discovered it is necessary to strictly maintain sintering temperature during the sintering process to improve the dimensional control and consistency of mechanical properties. Conventional furnaces, due to their large temperature gradients across the width of their belts, radiant heat shadowing effects, and the complex relationship between belt speed, muffle temperature profile and part temperature, do not have the capability to sinter parts with today&#39;s necessary degree of dimensional control capability.  
           [0012]    These objects and others are attained by sintering iron or steel parts at a tightly controlled temperature for a predetermined period of time. This produces parts with a minimum of variation one to another in dimensional and mechanical performance properties.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 illustrates varying transverse rupture mechanical strengths obtained for a high carbon powder sintered at various temperatures in accordance with the present invention.  
         [0014]    [0014]FIG. 2 illustrates the temperature control maintained in a fluidized bed furnace during sintering 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    Currently, metal powders and more particularly iron and steel powdered metal parts, are pressed to shape in a die at relatively high compaction pressures to create a green part. P/M parts are first formed by injecting the metal powder into a die cavity shaped to some desired configuration, and applying pressure to form a compact. These compacts are then sintered. Sintering at temperatures typically in the range of 1100 to 1250° C. for a controlled period of time increases the strength of the bond between particles. Where alloys such as graphite (0-0.8%) and copper (0-2.5%) are present, this sintering results in the diffusion of the alloys throughout the metal matrix. When higher carbon levels are present, liquid phase sintering may occur in where a liquid phase forms between particles. The end result of this sintering process is an increase in mechanical properties of the part, an increase in part density and a change in the dimension of the parts themselves (a growth of +0.2% to a shrinkage in liquid phase sintering of −4.0%). Because of the inherent problems of controlling the temperature precisely in conventional radiant tube muffle furnaces, significant variation in dimensional and physical part properties occur.  
         [0016]    Essentially any ferrous powder having a maximum particle size less than about 300 microns can be used in the composition of this invention. Typical iron powders are the Atomet® iron powders manufactured by Quebec Metal Powders Limited of Tracy, Quebec, Canada.  
         [0017]    The iron powder of this invention may also be an iron-carbon-silicon alloy comprising about 2% to about 4.5% by weight carbon and about 0.05% to about 2.5% by weight silicon. Preferably, the composite powder comprises about 3% to about 4% by weight carbon and about 0.1% to about 2% by weight silicon. In one preferred embodiment, the composite powder comprises about 3% to about 4% by weight carbon and about 0.3% to about 2% by weight silicon. Exemplary iron-graphite composite powders according to this invention, having a microstructure comprised of carbon clusters embedded in a ferrous matrix, comprise about 3.2% to about 3.7% by weight carbon and about 0.8% to about 1.3% by weight silicon. The composite iron powder and/or resulting sintered articles of this invention may also contain at least one other alloying element conventionally used in the art.  
         [0018]    In the present invention, compacted parts are introduced into a batch or continuous fluid bed for a controlled period of time at a particular temperature or range of temperatures. In its preferred embodiment, this invention involves the placement of parts in a multi layer part container where each part is held in place on a ceramic or other type of high temperature resistant fixture. The fixtures are then introduced into a furnace with a tightly controlled temperature, preferably under a tightly controlled atmosphere. Fluidized beds have traditionally been used for batch heat treating of parts but not for the sintering of metal powder parts. Fluidized beds are commercially available, e.g., from Procedyne Corp., Newark, N.J.  
         [0019]    Fluidized beds in their simple form consist of a retort filled with an inert aggregate media through which heat is introduced in some manner, preferably through the walls and by preheating the fluidizing gas. The bed is fluidized through the introduction of a controlled volume of gas through the bottom. The continuous stirring and mixing that occurs as a result of the fluidization results in a isotherm condition throughout the bed and almost instantaneous heat transfer to any part introduced in the bed.  
         [0020]    The fluidized bed can operate in a continuous fashion in a rectangular configuration, with parts introduced at one end conveyed through the bed at a controlled rate of speed and then removed at the other. Nitrogen is the prime fluidizing gas with preferably at least 10% hydrogen added to achieve the best part properties. Hydrogen at elevated levels can be used as required.  
         [0021]    A steady state temperature profile is created from one end of the bed to the other and controlled by the rate and temperature of fluidizing gas that is introduced along the length of the bed as well as the amount of heat that is introduced to the bed through the metal shell or retort. The bed itself is composed of aluminum oxide ca.-80 mesh.  
         [0022]    As the parts move through the bed they are initially rapidly heated to approximately 600° C. where they are held for the vaporization and removal of lubricant (&lt;1% by weight within the green part). Delubing generally takes about 10 minutes. As the parts continue through the bed they are then rapidly heated to the final sintering temperature in the range of 1120 to 1160° C. and held at this constant temperature ±1° C. for 5 to 15 minutes, as evidenced in FIG. 2. At the end of the cycle they are rapidly cooled to 100° C. at which time the fixtures exit the bed and the parts are removed.  
         [0023]    The sintered article thus formed may then be subjected to post-sintering treatments, e.g., heat-treatment (such as quenching and tempering, and the like), coining, forging and cutting or machining, to produce a final article.  
         [0024]    The Examples which follow are intended as an illustration of certain preferred embodiments of the invention, and no limitation of the invention is implied.  
       REFERENCE EXAMPLE 1  
       [0025]    An iron powder was produced by water-atomization of a liquid iron containing 0.94% silicon and 3.29% carbon. The water-atomized iron powder was then thoroughly dried. Five samples of the powder were consecutively heated in a Lindberg tubular furnace under a vacuum atmosphere (less than approximately 30 mm Hg) at a temperature of 1020° C., maintained at that temperature for three hours, then cooled in a stepwise process for approximately 4 hours. The samples were cooled from 1020° C. to approximately 760° C. and were maintained at that temperature for approximately 1.25 hours, cooled to approximately 730° C. and maintained at that temperature for approximately 1.25 hours, then cooled to approximately 700° C. and maintained at that temperature for approximately 1.5 hours. The samples were thereafter cooled to room temperature. The degree of graphitization of the powder was determined by Computerized Image Analysis using conventional procedures. The five iron-graphite composite samples had an average graphite volume of approximately 10%.  
       EXAMPLE 1  
       [0026]    A part was pressed conventionally using a sample of the water-atomized iron powder described in Reference Example 1. The green compact was sintered at 1150° C. ±1° C. for 5 to 15 minutes under a nitrogen atmosphere with 20% hydrogen.  
         [0027]    Following sintering, the part was tested for transverse rupture strength (TRS) and was determined to have a TRS of 115,000 pounds per square inch (PSI) or 115 KSI.  
       EXAMPLE 2  
       [0028]    A part was pressed conventionally using a sample of the water-atomized iron powder described in Reference Example 1. The green compact was sintered at 1153° C. ±1° C. for 5 to 15 minutes under a nitrogen atmosphere with 20% hydrogen.  
         [0029]    Following sintering, the part was tested for transverse rupture strength and was determined to have a TRS of 134 KSI.  
       EXAMPLE 3  
       [0030]    A part was pressed conventionally using a sample of the water-atomized iron powder described in Reference Example 1. The green compact was sintered at 1156° C. ±1° C. for 5 to 15 minutes under a nitrogen atmosphere with 20% hydrogen.  
         [0031]    Following sintering, the part was tested for transverse rupture strength and was determined to have a TRS of 180 KSI.  
       EXAMPLE 4  
       [0032]    A part was pressed conventionally using a sample of the water-atomized iron powder described in Reference Example 1. The green compact was sintered at 1158° C. ±1° C. for 5 to 15 minutes under a nitrogen atmosphere with 20% hydrogen.  
         [0033]    Following sintering, the part was tested for transverse rupture strength and was determined to have a TRS of 170 KSI.  
       EXAMPLE 5  
       [0034]    A part was pressed conventionally using a sample of the water-atomized iron powder described in Reference Example 1. The green compact was sintered at 1160° C. ±1° C. for 5 to 15 minutes under a nitrogen atmosphere with 20% hydrogen.  
         [0035]    Following sintering, the part was tested for transverse rupture strength and was determined to have a TRS of 168 KSI.  
       CONCLUSION  
       [0036]    As a result of Examples 1-5, it is seen that a 6 degree change in sintering temperature resulted in a 57% increase in TRS. Moreover, it was determined that parts sintered at 1156° C.±1° C. exhibited 3.5% shrinkage and were fully dense. In contrast, parts sintered at 1130-1140° C.±1° C. retained some porosity but exhibited only 1.5% shrinkage.  
         [0037]    Other variations or modifications, which will be obvious to those skilled in the art through routine experimentation, are within the scope and teachings of this invention. This invention is not to be limited except as set forth in the following claims.