Method of producing large components from austempered ductile iron alloys

Abstract

A method by which properties of a component ( 18 ) formed of a ductile iron alloy and having thick sections can be promoted with an austempering process. The method entails casting a ductile iron alloy containing iron, carbon, silicon and alloying constituents. The casting is solidified at a rate that inhibits segregation of the alloying constituents to grain boundaries of the casting, and so that the casting contains graphite nodules having a count of greater than 100 nodules per mm 2 . The casting is then austempered by heating to an austenitization temperature to yield a microstructure having a single-phase matrix of austenite that contains carbon, and then quenching the casting to an austempering temperature. The casting is held at the austempering temperature for a duration sufficient to yield a microstructure whose matrix is mostly ausferrite and essentially free of martensite and pearlite.

1 . A method of producing a component ( 18 ), the method comprising:
 casting a ductile iron alloy containing iron, carbon, silicon and alloying constituents to produce a casting having a section thickness, the casting being solidified at a rate that inhibits segregation of the alloying constituents and so that the casting contains graphite nodules characterized by a graphite nodule count of greater than 100 nodules per mm 2 ; and then   austempering the casting by heating the casting to an austenitization temperature for a sufficient duration to yield a microstructure having a single-phase matrix of austenite that contains carbon, and then quenching the casting in a quenching medium to cool the casting from the austenitization temperature to an austempering temperature that is above a martensite start temperature of the ductile iron alloy to avoid formation of martensite and pearlite in the casting, the casting being held at the austempering temperature for a duration sufficient to yield a microstructure that has a matrix of mostly ausferrite and is essentially free of martensite and pearlite.   
     
     
         2 . The method according to  claim 1 , wherein the casting step is a rapid solidification casting technique. 
     
     
         3 . The method according to  claim 2 , wherein the casting step is a centrifugal casting technique that comprises dispensing a melt of the ductile iron alloy inside a cylindrical-shaped mold rotating about an axis thereof, and the casting is axisymmetric. 
     
     
         4 . The method according to  claim 1 , wherein the casting has a nodule count of about 150 to about 300 nodules per mm 2  following the casting step. 
     
     
         5 . The method according to  claim 1 , wherein the casting has a section thickness of at least ten centimeters. 
     
     
         6 . The method according to  claim 1 , wherein the austenitization temperature is in a range of about 815° C. to about 980° C. 
     
     
         7 . The method according to  claim 6 , wherein the casting is held at the austenitization temperature for a duration of about one to about three hours. 
     
     
         8 . The method according to  claim 1 , wherein the austempering temperature is in a range of about 230° C. to about 400° C. 
     
     
         9 . The method according to  claim 8 , wherein the casting is held at the austempering temperature for a duration of about one to about four hours. 
     
     
         10 . The method according to  claim 1 , wherein the casting is quenched at a rate of at least 5.5° C./minute during the austempering step. 
     
     
         11 . The method according to  claim 1 , further comprising the step of annealing the casting at a temperature of about 450° C. to about 750° C. for a duration sufficient to eliminate any carbides or pearlite in the casting. 
     
     
         12 . The method according to  claim 1 , wherein the ductile iron alloy contains, by weight, about 3.0% to about 3.9% carbon, about 1.9% to about 2.7% silicon, up to 0.3% manganese, up to 0.8% copper, up to 0.2% nickel, up to 0.05% chromium, up to 0.02% vanadium, up to 0.01% sulfur, and up to 0.3% molybdenum, and the balance iron and incidental impurities. 
     
     
         13 . The component ( 18 ) produced by the method of  claim 1 . 
     
     
         14 . The method according to  claim 1 , wherein the component ( 18 ) is a main shaft ( 18 ) of a wind turbine ( 10 ). 
     
     
         15 . The main shaft ( 18 ) produced by the method of  claim 14 . 
     
     
         16 . The method according to  claim 14 , wherein the main shaft ( 18 ) is hollow and has an outer diameter of at least 135 centimeters. 
     
     
         17 . The main shaft ( 18 ) produced by the method of  claim 16 . 
     
     
         18 . The method according to  claim 14 , further comprising the step of installing the main shaft ( 18 ) in a wind turbine ( 10 ) to couple a rotor blade assembly of the wind turbine ( 10 ) to a generator of the wind turbine ( 10 ). 
     
     
         19 . The wind turbine ( 10 ) produced by the method of  claim 18 . 
     
     
         20 . The method according to  claim 1 , wherein the component ( 18 ) is an automotive component, or a construction component, or a mining component, or a railroad component, or an agricultural component.