Abstract:
A compacted graphite iron brake component for use in a braking system having a final composition of: 3.0 to about 4.0 percent carbon; 2.15 to about 2.60 percent silicon; 0.40 to about 0.90 manganese; and the balance iron. The brake component having a frictionally engageable portion with a microstructure of compacted graphite disposed in a pearlitic matrix.

Description:
TECHNICAL FIELD 
     The present invention relates to an improved cast iron material, and particularly to an improved cast iron brake drum having a compacted graphite micro structure disposed within a primarily pearlitic metallurgical structure. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     Brake drums are widely employed in all vehicles having different types of cast iron brake drums. In service, these drums often times encounter strenuous work conditions, and thereby desirably employ materials that will exhibit excellent wear resistance, strength, and heat conductivity. In view of the complexities of the shape, and taking into account other material properties and processes requirements; it has been common to manufacture brake drums with a gray cast iron material. 
     The use of presently employed casting materials has presented limitations in improving the design of brake drums, especially in reducing part weight. By way of example, the trend has been towards reducing weight by casting gray iron against a steel shell to reduce the weight of the brake drums. These composite brake drums significantly increase production time and costs. Absent additional expensive, time consuming, and potentially inefficient heat treatments or finishing steps, significant volumes of high integrity brake drum castings are often not obtainable over short periods of time. 
     Accordingly, even though light weight castings are achieved, the manufacturer of high integrity iron brake drums require expensive and substantial time consuming post-casting finishing steps. A system is needed that permits the casting of an integrally shaped article, particularly a brake component, which has excellent machinability, wear resistance, and high strength characteristics, as well as heat transfer characteristics. Additionally, the brake components must also meet all required brake component properties which are required of brake components. The microstructure should include a generally uniform dispersion of compacted forms of graphite (vermicular) in a pearlitic iron matrix. 
     The present invention satisfies the above by providing an improved system for making a cast iron article, particularly a cast brake component. The gray cast iron brake component for use in a braking system has a final composition of: from about 3.0 to about 4.0 percent carbon; about 2.15 to about 2.60 percent silicon; about 0.40 to about 0.90 manganese; and the balance iron. The compacted graphite iron brake component has a frictionally engageable portion with a microstructure containing compacted graphite disposed in a primarily pearlitic matrix. Other advantages and objects of the present invention will become apparent to those skilled in the art from the subsequent detailed description, the drawings, and appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various advantages of the present invention will become apparent to one skilled in the art by reading the following specification and subjoined claims and by referring to the following drawings in which: 
     FIG. 1 is a perspective view of a brake drum; 
     FIG. 2 is a top view of the brake drum from FIG. 1; 
     FIG. 3 is a photomicrograph depicting a microstructure of a section (at a magnification of 100×; no etch) of a brake drum of the present invention; and 
     FIG. 4 is a photomicrograph depicting the microstructure of the section of the same portion (at a magnification of 400×; 4% nital etch) as FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While the material and system of the present invention may be suitable for manufacture of many different articles, for exemplary purposes it will be described herein for the manufacture of a brake component, and/or particularly, a brake drum. 
     The term “pearlite” (also known as pearlitic iron) shall mean an intimate mixture of ferrite and cementite. In particular, pearlite is a lamellar structure comprised typically of alternating layers of ferrite and cementite. As used herein, the term “ferritic” shall generally refer to a body centered cubic form of iron (also known as ferrite). 
     For ease of discussion, FIGS. 1 and 2 depict one illustrative example, without limitations, of a typical brake drum that can be employed in a vehicle braking system. These designs shown are cast as integral structures. FIGS. 1 and 2 illustrate views of a typical brake drum structure for a braking system. The function and operation of such a brake drum will be appreciated by the skilled artisan. The brake drum includes a first base portion having a first plate member, a wall depending from the first plate member, and a second plate member. 
     The skilled artisan will appreciate that FIGS. 1 and 2 are for illustration purposes only (to demonstrate the geometric intricacies of brake drums) and are not intended as limiting. The present invention contemplates its usefulness in many different configurations, other than those in FIGS. 1-2. 
     The ferrous base metal preferably is of a suitable composition to result, upon casting, in a compacted graphite cast iron. Thus the ferrous base metal preferably includes iron as a base material, carbon, silicon, manganese, magnesium, and alloys in predetermined amounts. Further, the melt is alloyed with a suitable pearlite stabilizer to achieve at least a predetermined amount of pearlite microstructure in the resulting article. These alloying elements can take the form of chromium, copper, tin, and all other pearlite stabilizing elements. Those skilled in the art would recognize that other alloys are available and suitably usable in their respective amounts. Compacted graphite iron is addressed in “Iron Casting Handbook,” 1981, pp. 381-397, herein expressly incorporated by reference. 
     For instance, for a preferred base material: 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Compacted Graphite Iron Composition 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 C 
                 3.30-3.70 
               
               
                   
                 Si 
                 2.15-2.70 
               
               
                   
                 Mn 
                 0.20-0.80 
               
               
                   
                 P 
                 0.01-0.04 
               
               
                   
                 S 
                 0.001-0.020 
               
               
                   
                 Cr 
                 0.01-0.06 
               
               
                   
                 Mo 
                 0.01-0.50 
               
               
                   
                 Cu 
                 0.10-0.90 
               
               
                   
                 Ti 
                 0.01-0.10 
               
               
                   
                 Sn 
                 0.001-0.15  
               
               
                   
                   
               
             
          
         
       
     
     The skilled artisan will appreciate that higher or lower contents than the above may be suitably employed. 
     The production of the present invention involves employment of a process having the steps of: 
     (a) preparing a melt of a ferrous base metal; 
     (b) alloying the melt in a suitable pearlite stabilizer to achieve the predetermined microstructure in a resulting article; 
     (c) providing alloying elements into the ferrous base metal to produce a compacted graphite structure; and 
     (d) pouring the melt to cast an article. 
     The ferrous base materials are prepared in any suitable manner. Upon preparation, it is maintained at a temperature of at least about 2600° F. in a suitable furnace. 
     The resulting material is a substantially homogenous microstructure of compacted graphite flakes dispersed relatively uniformly through a generally pearlitic to matrix. FIGS. 3 through 4 illustrate the microstructure and show how the graphite (the solid darker phase) is distributed throughout the pearlitic phase (the lighter phase in FIG.  3  and the darker phase in FIG.  4 ). The microstructures of FIGS. 3 and 4 extend through substantially the entire component (regardless of type), including the regions substantially adjacent the surface of the drum. 
     The amount of graphite present as a compacted structure in the present alloy should not exceed about 50% by volume. The preferred amount of graphite as compacted flakes is typically in the range of from about 0.05 to 20% by volume. The presence of graphite as flakes in amounts of up to 2% by volume of graphite present reduces the strength of the alloy. The compacted graphite iron is described in ASTM A-842-85 as a grade 300, 350, or 400. 
     Magnesium is alloyed with the base iron to produce a final magnesium content or 0.005-0.02 percent magnesium. A plurality of treatment processes exist to produce compacted graphite iron all of which also produce ductile iron. The treated iron is inoculated to reduce metallic carbides at a minimal level of inoculate additions. Ferrosilicon and derivatives of ferrosilicon inoculates are most often employed. The alloy has a microstructure of 50% to 100% pearlitic matrix. Most preferably, the microstructure is 65-85% pearlitic matrix. 
     While the above detailed description describes the preferred embodiment of the present invention, it should be understood that the present invention is susceptible to modification, variation, and alteration without deviating from the scope and fair meaning of the subjoined claims.