Abstract:
A method for preparing an austempered, work-hardening steel casting. A melt with certain chemical ranges including relatively high carbon and silicon content is poured, heat treated, cooled, austenitized, quenched and austempered, before final cooling to room temperature. This process provides a steel casting with increased wear life, characterized by a duplex microstructure containing ferrite plus carbon in solution in austenite known as ausferrite. The austenite will transform during abrasive service to a hard, wear-resistant martensite on the surface.

Description:
PRIORITY CLAIM 
       [0001]    This application claims priority to and the benefit of provisional patent application No. 61/235,592, filed Aug. 20, 2009, and entitled Ausferritic Wear-Resistant Steel Castings. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention is in the technical field of material science. More particularly, the present invention is in the technical field of wear resistant cast or forged steel. 
       BACKGROUND 
       [0003]    During the manufacturing of wear resistant steels produced using the steel casting sand molding process, limitations exist that affect the overall wear life of the final product. These limitations are due to the hardness that can be achieved using conventional processing and heat treating methods. With low alloy steels, a relationship exists between carbon and hardness such that increasing the carbon content of the steel produces higher final hardness and greater wear resistance. However, as the carbon content increases, the steel becomes more brittle, and the risk of cracking during the manufacturing processor during use increases. These factors limit the hardness, toughness, and wear life of the product. 
         [0004]    Conventional heat treatment of carbon low alloy material to achieve maximum hardness consists of converting austenite to martensite by rapid water quenching. See  FIG. 1 , Cooling Rate Curve A. This transformation increases stresses in the part due to a volumetric phase change as well as geometry-related factors which increase in proportion to the carbon content. Therefore, it would be desirable to manufacture steel castings with a chemistry and heat treatment that would allow much higher carbon content without risk of cracking, resulting in higher hardness and longer wear life. 
         [0005]    The present invention satisfies these needs by providing a method for production of a cast or forged steel product with increased wear life, characterized by a duplex microstructure containing ferrite plus carbon in solution in austenite known as ausferrite. The austenite will transform during abrasive service to a hard, wear-resistant martensite on the surface. 
       SUMMARY 
       [0006]    One embodiment of the present invention comprises a method of preparing an austempered, work-hardening steel casting, comprising melting an alloy mixture containing (i) from about 0.6 to about 1.0 percent by weight carbon; (ii) from about 0.5 to about 1.0 percent by weight manganese; (iii) from about 1.5 to about 2.5 percent by weight silicon; and (iv) the remainder iron. If the casting is greater than about one-inch thick, then it is advantageous to add at least one hardening agent to the melt, selected from the group consisting of from about 0.4 to about 2.5 percent by weight chrome; from about 0.0 to about 0.75 percent by weight nickel; or from about 0.0 to about 0.5 percent by weight molybdenum. After the alloy is melted, it is poured into a mold of desired shape at a temperature of about 2700°-2800° F. and cooled in the mold until it reaches a temperature of less than about 1000° F., followed by shake-out and cooling to ambient temperature. Then, the casting is austenitized at a temperature of about 1600°-1800° F. until an austenitic matrix is achieved. Next, the casting is quenched in a medium capable of providing a quench rate sufficient to transform the entire casting to the desired ausferric matrix to a temperature of about 550°-725° F., and that temperature is maintained for about 1-6 hours. The resulting ausferric matrix casting is then cooled to ambient temperature. It is preferable, between the melting and pouring steps to deoxidize the alloy mixture by adding aluminum in the range of 0.02-0.04 percent by weight. It is also preferable between the first cooling step and the austenitizing step to homogenize the casting by heat treating at a temperature of about at least 1800° F. for a sufficient length of time to reduce the number and size primary carbides. This time usually is a minimum of about 30 minutes per inch of section thickness, followed by air cooling to ambient temperature. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The present invention will be explained, by way of example only, with reference to certain embodiments and the attached Figures, in which: 
           [0008]      FIG. 1  is a time-temperature-transformation diagram of a typical, prior art method of producing a steel casting; 
           [0009]      FIG. 2  is a time-temperature-transformation diagram of one embodiment of the method of the present invention; and 
           [0010]      FIG. 3  is a time-temperature process chart of one embodiment of the method of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    In one embodiment of the present invention, steel castings are produced within the chemical ranges shown in Table 1 below. Specifically, a high carbon and silicon content are used to produce the desired work hardening results. In heavier sections, the formation of upper transformation ferrite and carbides is prevented by enhancing the hardenability of the chemistry with the addition of hardenability agents, such as chromium, molybdenum, nickel or combinations of these, in combination with the specialized austempering heat treatment. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Element 
                 Percent by Weight 
               
               
                   
                   
               
             
             
               
                   
                 Carbon 
                 0.6-1.0 
               
               
                   
                 Manganese 
                 0.5-1.0 
               
               
                   
                 Silicon 
                 1.5-2.5 
               
               
                   
                 Chrome 
                 0.4-2.5 
               
               
                   
                 Nickel 
                 0.0-0.75 
               
               
                   
                 Molybdenum 
                 0.0-0.5 
               
               
                   
                 Iron 
                 Remainder 
               
               
                   
                   
               
             
          
         
       
     
         [0012]    Depending on casting size, it may be necessary to add hardenability agents, such as molybdenum, chrome, or nickel, either singly or in any combination thereof, for aiding austemperability. For example, a small casting up to an inch thick usually does not require alloying with the above mentioned hardenability agent(s) because the part is so small that sufficient quenching severity can be experienced throughout the bulk of the casting without the hardening agent(s). On the other hand, heavier castings (generally greater than about one-inch thick) usually require the addition of such agents to allow through quenching of the thicker casting components to achieve through hardening of the desired severity. Ranges of the hardenability agents change with casting size, but ranges of molybdenum 0-0.5% by weight, chrome 0.4-2.5% by weight, and nickel 0-3.5% by weight have been found particularly effective for larger castings having thicknesses greater that one inch. The amount of these elements to be incorporated is dependent upon the quenching equipment and the quenching mediums being used, as will be understood by those skilled in the art. 
         [0013]    As shown in  FIG. 3 , preparing austempered, work hardening steel castings in accordance with one embodiment of the present invention comprises (a) melting the cast steel mixture to form a melt and preferably deoxidizing the steel by adding aluminum in the range of about 0.02-0.04% by weight to reduce harmful oxygen and to minimize grain growth; (b) pouring the melt into a mold of desired shape at a temperature of about 2700°-2800° F. (depending on the exact chemistry and section thickness) to form a near net shape casting; (c) allowing the casting to cool in the mold until it reaches a temperature less than about 1000° F., and then shakeout the casting and continue cooling to ambient temperature (i.e., below about 90° F.); d) preferably homogenizing the castings by heat treating at about 1800° F. minimum for a sufficient length of time to reduce the number and size of primary carbides, followed by cooling to ambient temperature; (e) austenitizing the casting at a temperature of about 1600°-1800° F. until an austenitic matrix is achieved; (f) quenching the austenitic casting to a temperature of about 550°-725° F. (and maintaining that temperature preferably between about 1 to 6 hours depending on section thickness, chemistry, and desired properties) in a medium such as, but not limited to, molten salt or a medium capable of providing a quench rate sufficient to transform the entire casting to the desired ausferritic matrix; and (g) cooling the ausferritic matrix casting to ambient temperature (e.g., below about 90° F.). 
         [0014]    As shown in  FIG. 2 , heat treatment consists of first homogenizing at a temperature of 1800° F. minimum (with 1850°-1900° F. being preferred) for a sufficient length of time to help dissolve primary carbides formed during the casting process (see area “a” on  FIG. 2 ). As a practical matter, all (or in some cases even the majority) of primary carbides cannot or need not be dissolved given temporal process constraints, chemistry, and desired material properties. However, the homogenization reduces the number and size of primary carbides, resulting in an increased amount of carbon in the microstructure during the remaining process steps. The homogenization time is normally about 30 minutes per inch of section thickness, followed by air cooling to ambient temperature prior to normal steel foundry cleaning operations. After cleaning, austenitize at a temperature of 1600°-1800° F. (see area “b”), then holding until the entire casting reaches temperature in order to achieve an austenitic matrix. The specific temperature and hold time is determined by the final chemistry and section thickness, and a range of 1650°-1750° F. is preferred. Then austempering by quenching in a medium such as, but not limited to, molten salt or a medium capable of providing a quench rate sufficient to transform the entire casting to the desired ausferritic matrix, at a temperature of about 550°-750° F. (with about 650°-725° F. being preferred) and maintaining that temperature usually 1 to 6 hours depending on section thickness, chemistry, and desired properties (with a range of 2 to 4 hours being preferred), until the casting is substantially transformed to an ausferritic matrix, as shown by line segment “cd” in  FIG. 2 . The casting is then cooled to ambient temperature as indicated by line segment “de” in  FIG. 2 . 
         [0015]    As shown at line segment “cd” on  FIG. 2 , during the nucleation and growth phase of austempering a silicon alloyed cast steel at 550°-725° F., austenite directly decomposes into a carbide-free ausferrite microstructure. Due to its effect on carbide formation, high silicon not only stabilizes a carbon saturated austenite in the austempered structure but also retards the formation of unwanted carbides. This is characteristic of the ausferrite microstructure, which consists of a carbide free blend of acicular ferrite and austenite. Ausferrite has a combination of high strength and toughness; also, due to the presence of carbon saturated austenite, it has the ability to work harden on the surface under low and high stress abrasive environments. Deformation induced by high-impact, abrasive wear or sliding abrasion converts to fine martensite thus producing a surface with a much higher converted hardness (about 600+Brinell) resulting in excellent wear-resistance qualities. 
         [0016]    In one preferred embodiment, for a casting having a three-inch thickness, the following specific chemistry was found to produce a wear-resistant steel in accordance with the present invention: carbon 0.9% by weight, manganese 0.6% by weight, silicon 2.0% by weight, chrome 0.6% by weight, nickel 0.25% by weight, and molybdenum 0.25% by weight. For this chemistry, forty to fifty-percent improvements in wear resistance, as compared to a conventional wear-resistant alloy, have been achieved. The specific chemical elements vary from the foregoing specifications within the ranges provided of Table 1, depending on casting size, thickness and effectiveness of the quenching medium; as will be understood by those skilled in the art. 
         [0017]    Another embodiment comprises using the principles the present invention with the forging process, as follows: (a) melting the cast steel mixture as described by Table 1 to form a melt; (b) pouring the melt into a mold to form a billet or forging blank, at a temperature of about 2600°-2800° F.; (c) after shakeout, allowing the billet to cool to ambient temperature for normal cleaning operations; (d) re-heating the billet to the desired forging temperature as required by the forging process normally in the range of 2000°-2400° F.; (e) forging to near a desired net shape; (f) austempering as shown in  FIG. 2  by first austenitizing the forging at a temperature of 1600°-1800° F. and holding the forging at temperature until an austenitic matrix is achieved, the specific temperature and hold time being determined by final chemistry and section thickness; (g) quenching the austenitic forging to a temperature of about 550°-725° F. (and maintaining that temperature usually about 1-6 hours depending on section thickness, chemistry, and desired properties) in a medium such as, but not limited to, molten salt to transform the entire casting to a desired ausferritic matrix; and (h) cooling the ausferritic matrix forging to ambient temperature. 
         [0018]    Therefore, utilizing the combination of the steel casting sand molding or forging processes, a chemistry as defined by Table 1 consisting of high carbon, high silicon chromium molybdenum steel along with the austempered heat treatment as shown in  FIG. 2 , a superior higher hardness, work-hardening, wear-resistant material can be produced without the risk of cracking or distortion as compared to conventional methods. 
         [0019]    Although the present invention has been described and shown with reference to certain preferred embodiments thereof, other embodiments are possible. The foregoing description is therefore considered in all respects to be illustrative and not restrictive. Therefore, the present invention should be defined with reference to the claims and their equivalents, and the spirit and scope of the claims should not be limited to the description of the preferred embodiments contained herein.