Ausferritic Wear-Resistant Steel Castings

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.

1 . A method of preparing an austempered, work-hardening steel casting, comprising:
 (a) 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; 
   (b) pouring the melt into a mold of desired shape at a temperature of about 2700°-2800° F.;   (c) allowing the casting to cool in the mold until it reaches a temperature of less than about 1000° F., then shaking out said casting and allowing it to cool to ambient temperature;   (d) austenitizing the casting at a temperature of about 1600°-1800° F. until an austenitic matrix is achieved;   (e) quenching the austenitic casting to a temperature of about 550°-725° F. and maintain that temperature between about 1-6 hours in a medium capable of providing a quench rate sufficient to transform the entire casting to the desired ausferric matrix; and   (f) cooling said ausferric matrix casting to ambient temperature.   
     
     
         2 . The method of  claim 1 , wherein said casting is greater than about one-inch thick, and wherein said alloy mixture melted in step (a) further comprises at least one hardening agent selected from the group consisting of:
 (v) from about 0.4 to about 2.5 percent by weight chrome;   (vi) from about 0.0 to about 0.75 percent by weight nickel; or   (vii) from about 0.0 to about 0.5 percent by weight molybdenum.   
     
     
         3 . The method of  claim 1 , further comprising, between said melting step (a) and said pouring step (b), deoxidizing said mixture by adding aluminum in the range of 0.02-0.04 percent by weight. 
     
     
         4 . The method of  claim 1 , further comprising, between said cooling step (c) and said austenitizing step (d), homogenizing said 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. 
     
     
         5 . The method of  claim 4 , wherein said homogenizing comprises holding the castings at said temperature for a minimum of about 30 minutes per inch of section thickness, followed by air cooling to ambient temperature. 
     
     
         6 . The method of  claim 5 , wherein said temperature is about 1850°-1900° F. 
     
     
         7 . The method of  claim 1 , wherein in said austenitizing step (d), said temperature is about 1650°-1750° F. 
     
     
         8 . The method of  claim 1 , wherein the quenching medium in said quenching step (e) is molten salt. 
     
     
         9 . The method of  claim 1 , wherein in said quenching step, said temperature is about 650°-725° F. 
     
     
         10 . The method of  claim 1 , wherein in said quenching step, said casting is held at said temperature for about 2 to 4 hours. 
     
     
         11 . The method of  claim 1 , wherein said casting is about three inches thick, and wherein in said melting step (a), said alloy mixture contains 0.9 percent carbon by weight, 0.6 percent manganese by weight, 2.0 percent silicon by weight, and further comprises 0.6 percent chrome by weight, 0.25 percent nickel by weight, and 0.25 percent molybdenum by weight. 
     
     
         12 . A method of preparing an austempered, work-hardening steel forging, comprising:
 (a) 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; 
   (b) pouring the melt into a mold to form a billet at a temperature of about 2600°-2800° F.;   (c) allowing the billet to cool in the mold until it reaches a temperature of less than about 1000° F., then shaking out said casting and allowing it to cool to room temperature;   (d) heating the billet to the desired forging temperature as required by the forging process;   (e) forging the billet to near a desired shape;   (f) austenitizing the casting at a temperature of about 1600°-1800° F. and holding the casting at said temperature until an austenitic matrix is achieved;   (g) quenching the austenitic casting to a temperature of about 550°-725° F. and maintaining that temperature for about 1-6 hours in a medium capable of providing a quench rate sufficient to transform the entire casting to a desired ausferric matrix; and   (h) cooling said ausferric matrix casting to ambient temperature.   
     
     
         13 . The method of  claim 12 , wherein said forging is greater than about one-inch thick, and wherein said alloy mixture melted in step (a) further comprises at least one hardening agent selected from the group consisting of:
 (v) from about 0.4 to about 2.5 percent by weight chrome;   (vi) from about 0.0 to about 0.75 percent by weight nickel; or   (vii) from about 0.0 to about 0.5 percent by weight molybdenum.   
     
     
         14 . The method of  claim 12 , wherein in said heating step (d), said forging temperature is about 2000°-2400° F. 
     
     
         15 . An article made of a steel alloy cast in accordance with the method of  claim 1 . 
     
     
         16 . An article made of a steel alloy forged in accordance with the method of  claim 12 .