Patent Publication Number: US-4548643-A

Title: Corrosion resistant gray cast iron graphite flake alloys

Description:
This invention is concerned with gray cast iron alloys and more particularly with low cost, corrosion resistant gray cast iron alloys suitable for use in downhole environments. 
     BACKGROUND OF THE INVENTION 
     Downhole components such as the impellers, diffusers, and other parts of submergible pumps, are commonly formed of corrosion resistant, high nickel-copper gray cast iron alloys such as Ni-Resist. These alloys may comprise as much as 17.5% nickel and 7.5% copper, for example, and are quite expensive. Attempts to provide alloys with the corrosion resistance, high temperature strength, and other properties of alloys like Ni-Resist, but at lower cost, have not met with success. 
     SUMMARY OF THE INVENTION 
     This invention provides low cost, corrosion resistant gray cast iron alloys suitable as replacements for high nickel-copper alloys such as Ni-Resist in downhole environments. Advantageously, the alloys of the invention have substantially greater tensile strength than that of Ni-Resist. Moreover, the alloys of the invention can be used in as cast, annealed, or heat treated condition, dependent upon the application. 
     Aluminum is a principal alloying element employed in the invention. Aluminum is not usually found in cast iron because of its tendencies to cause porosity and embrittlement. However, through the use of appropriate amounts of aluminum in combination with other alloying elements in accordance with the invention, these problems have been overcome. 
     In one of its broader aspects, the present invention provides a corrosion resistant, gray cast iron alloy comprising, by weight, 2-9% aluminum, 1-5% nickel, 0.5-2.5% chromium, 0.25-1.5% molybdenum, 0.5-2% silicon, 0.25-1.2% manganese, 2.4-4% carbon, 0-2% copper, 0-0.75% tin, 0-0.5% vanadium, 0-0.3% boron, with the balance essentially iron. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The most important alloying element in the alloys of the invention is aluminum. The amount of aluminum employed is from 2-9%, by weight, 3-4% being preferred for non-hardenable alloys and 3-5% for hardenable alloys. If less than 2% aluminum is employed, castings formed of the alloys of the invention may suffer from pin-hole porosity. Above 9% aluminum, excessive embrittlement, rapid decrease in tensile strength, and large increase in hardness may result. The alloys of the invention contain 0.5 to 2.5% chromium, by weight. Above about 2.5%, chromium carbides that are formed may not be redissolved into the matrix, and the alloys may not be machinable. Some reduction in corrosion resistance, and increase in embrittlement, may occur if the carbides are formed in grain boundaries. 
     Molybdenum is employed in the invention for improved strength at operating temperature and to provide a slight increase in hardness. A range of 0.25 to 1.5%, by weight, is suitable. Above about 1.5% there is a little improvement in the properties of the alloy, but there is a significant increase in cost. 
     Nickel is employed in the invention to improve corrosion resistance. It also has effects similar to molybdenum, but to a lesser degree. A range of 1 to 5%, by weight, is suitable. Above about 5% improvement in corrosion resistance is not sufficient to justify the additional cost. 
     Manganese is used in the invention as a strengthener, a range of 0.25 to 1.2%, by weight, being suitable. Larger amounts increase work hardening and may be detrimental to machinability. 
     Silicon used in the invention improves casting fluidity and, to a limited degree, improves corrosion resistance. A range of 0.5 to 2%, by weight, is suitable. The addition of more silicon can cause embrittlement and undesirable matrix formation. An increase in the carbon equivalent leads to casting defects. 
     Carbon is used in the range of 2.4-4%, by weight. The carbon produces flake graphite cast iron and increases fluidity. In excess of about 4% carbon, undesirable amounts of carbides of chromium (and other elements such as vanadium and boron, if employed) and an iron-aluminum-carbide complex may result. 
     The alloys of the invention may also contain up to 2% copper, up to 0.75% tin, up to 0.5% vanadium, and up to 0.3% boron. Copper and tin serve as pearlite stabilizers, forming a hard, strong pearlite matrix upon solidification of the alloys. Addition of copper or tin above the specified limits can cause a very hard, brittle pearlitic matrix and degradation of the flake graphite. Vanadium and boron serve as hardeners and form carbides. However, unlike chromium carbides, the carbides of vanadium and boron can be redissolved into the matrix to avoid excessive hardness (assuming that vanadium and boron percentages are not greater than those noted). 
     The remainder of the alloys of the invention is essentially iron (and trace amounts of other elements commonly found in gray cast irons). 
     Preferred non-hardenable alloys in accordance with the invention have compositions as follows, by weight: 
     aluminum--3-4% 
     nickel--1.5-2% 
     chromium--0.5-1% 
     molybdenum--0.5-1% 
     silicon--0.75-1% 
     manganese--0.4-0.6% 
     carbon--3.2-4% 
     the balance being essentially iron (and the usual impurities in trace amounts). 
     Preferred hardenable alloys in accordance with the invention have compositions as follows, by weight: 
     aluminum--3-5% 
     nickel--1.5-2% 
     chromium--0.5-1% 
     molybdenum--0.75-1.5% 
     silicon--0.75-1% 
     manganese--0.4-0.8% 
     carbon--3.2-4% 
     copper--0.05-0.5% 
     tin--0-0.2% 
     vanadium--0-0.2% 
     boron--0-0.2% 
     The remainder of the alloys of the invention is essentially iron (and trace amounts of other elements commonly found in gray cast irons). 
     Alloys in accordance with the invention can be made in conventional furnaces, such as the high frequency induction furnaces, cupola furnaces, etc. conventionally employed in the manufacture of cast iron. Steel scrap, carbon (graphite powder), and pig iron are charged into the furnace in the usual manner and are heated until molten. The molten metal, at about 2550° to 2600° F., is then skimmed to remove slag. Alloying additions, including ferrochromium, nickel, ferromanganese, ferromolybdenum, ferrovanadium, ferroboron, copper, and ferrosilicon are then added (assuming alloys containing all of these constituents), with the ferrosilicon being added last. The molten metal is then heated to the tap temperature of about 3100° F. 
     Although aluminum may be added in the furnace, it is preferably placed in a preheated ladle covered with a small amount of steel scrap to retard floatation, and the 3100° F. molten metal is poured over the steel and aluminum. As the molten metal is being poured into the ladle, ferrosilicon is added to the molten stream as a post innoculation treatment. A quick stir with a carbon rod ensures good mixing of the aluminum. A covered ladle is preferred to reduce cooling of the metal and to retard surface oxidation. A tap temperature below about 3100° F., the molten metal is somewhat sluggish when poured, and poor mold filling, increased inclusions, and cold shunts may result. The pour from the ladle should be as fast as mold filling will allow, as the metal cools more rapidly than usual gray cast iron. Green sand molds commonly used for Ni-Resist gray cast iron work well in casting the alloys of the invention. Shake-out can occur when the metal is still hot, up to about 1500° F. Above this, some alloys may increase in hardness to a point where machining is difficult. 
     The corrosion resistance of alloys in accordance with the invention was tested both in downhole and laboratory controlled environments. The downhole environment contained a 26.6% brine solution at 209° F. The brine solution contained a gas fraction comprising 15% nitrogen, 35% methane, 21% carbon dioxide, and 29% hydrogen sulfide. The corrosion rate of an alloy of the invention comprising 3% aluminum, 2% nickel, 1% chromium, 0.5% molybdenum, 0.8% silicon, 0.61% manganese, 3.69% carbon, with the remainder essentially iron, was 0.06 mils per year. A Ni-Resist type 1-b alloy in comparison corroded at a rate of 1.9 mils per year. 
     In a laboratory test facility using an electrochemical corrosion analyzer, various alloy compositions were tested in an ASTM D1114 Substitute Sea Water and a NACE solution of 5% sodium chloride and 0.5% acetic acid. Typical corrosion rates varied from 12.6 mils per year for an alloy similar to the one above to 14.3 mils per year for the Ni-Resist in the NACE solution and 2.27 mils per year for the alloy of the invention and 2.36 mils per year for Ni-Resist in the ASTM D1114 Solution. 
     Mechanical properties were determined by standard ASTM tests in the as cast, annealed, and heat treated conditions. Tensile strength of the alloys of the invention varied from 35,000 lbs. per square inch to 59,000 lbs. per square inch, and hardness varied from Rockwell B-70 to C-49. The tensile strength of Ni-Resist is typically 22,500 lbs. per square inch. 
     Alloys in accordance with the invention can be hardened by heating to 1800° F., for example, and cooling in oil, water or air. This hardening process may be followed by a tempering process, between 200° F. and 1,000° F., for example, to increase toughness and slightly reduce hardness. The hardened alloys of the invention have substantially increased abrasion resistance, particularly desirable to improve part life in abrasive environments. The conventional gray cast iron alloys currently in use are non-hardenable. 
     Typical tensile strength and hardness of alloys of the invention are as follows: 
     
         ______________________________________                                    
                             Rockwell                                     
Alloy         Tensile Strength                                            
                             Hardness                                     
______________________________________                                    
non-hardenable                                                            
              49,000  psi as cast                                         
                                 C-34                                     
non-hardenable                                                            
              35,000  psi as cast                                         
                                 C-41                                     
non-hardenable                                                            
              41,000  psi annealed                                        
                                 B-84                                     
non-hardenable                                                            
              57,000  psi annealed                                        
                                 B-77                                     
hardenable    54,000  psi as heated                                       
                                 C-48                                     
                      treated                                             
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     The metallurgical microstructure of the alloys of the invention varies from an as cast pearlite to an annealed ferrite for the non-hardenable alloys. In the hardenable alloys, the structures vary from pearlite in the as cast to martensitic in the heat treated condition. Selected heat treatments may be employed to produce other structures. 
    
    
     The following are examples of alloys produced in accordance with the invention: 
     
         ______________________________________                                    
Heat No.                                                                  
        Al     Ni     Cr   Mo   Si   Mn   C    Cu                         
______________________________________                                    
007     2.93   2.09   1.00 1.00 1.04 0.72 3.07 0                          
002     2.97   2.05   0.78 1.06 0.57 1.19 3.35 0                          
013     2.99   2.25   0.98 1.05 1.91 0.27 3.12 0.54                       
005     4.11   2.25   0.98 0.46 0.97 0.86 3.07 1.74                       
001     4.07   2.45   1.06 0.97 0.98 0.73 3.21 0                          
011     4.01   2.77   1.03 0.60 0.97 0.40 3.00 0.47                       
015     4.06   1.99   1.06 1.05 1.04 0.45 3.10 0.94                       
017     2.99   3.43   2.21 0.96 0.97 0.41 3.15 0                          
025     4.00   2.95   1.21 0.64 1.97 0.55 4.08 0                          
019     4.01   2.98   2.15 1.23 0.96 0.50 2.60 0.19                       
024     4.99   4.84   2.45 1.12 1.0  0.87 3.60 0                          
010     4.01   1.24   1.09 0.52 0.97 0.58 3.10 0.34                       
004     3.03   1.74   0.77 0.98 0.61 0.54 3.25 0                          
003     2.99   1.89   1.03 1.06 0.52 0.43 3.35 0                          
016     3.00   2.72   1.02 0.51 0.98 0.26 2.93 0                          
  027A  2.02   2.10   1.14 1.09 1.01 0.69 2.88 0                          
  027B  8.97   2.44   1.06 1.02 1.02 0.64 2.88 0                          
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     The following are hardness values for the alloys of the foregoing examples: 
     
         ______________________________________                                    
Heat No.    Rockwell Hardness                                             
______________________________________                                    
007         B-87.3                                                        
002         C-41.1                                                        
013         C-40                                                          
005         B-73.3                                                        
001         C-34.2                                                        
011         C-34                                                          
015         C-43                                                          
017         C-44                                                          
025         C-16.5                                                        
019         C-38                                                          
024         C-36                                                          
010         C-34                                                          
004         C-48.5                                                        
003         C-42.8                                                        
016         C-39                                                          
  027A      C-33.5                                                        
  027B      C-35.8                                                        
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     In certain of the above examples, copper is an alloying element. If tin is employed as an alloying element, its effect is similar to that of copper, but, in general, less tin is required to afford the same structural change. If tin is added to the alloys of Heat Nos. 002 and 001 (as designated hereinafter by addition of a letter to the respective heat numbers), the hardness values for the alloys are as follows: 
     
         ______________________________________                                    
              Tin      Rockwell                                           
Alloy         Content  Hardness                                           
______________________________________                                    
002A          0.10%    C-35                                               
002B          0.25%    C-37                                               
002C          0.75%    C-41                                               
001A          0.10%    C-38                                               
001B          0.25%    C-39                                               
001C          0.75%    C-44                                               
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     Boron and vanadium (which act as carbide stabilizers and formers) increase the hardness very rapidly as their content increases. If boron or vanadium is added to the alloys of Heat Nos. 002 and 001 (as designated hereinafter by the addition of a letter to the respective heat numbers), the hardness values for the alloys are as follows: 
     
         ______________________________________                                    
             Vanadium or Rockwell                                         
Alloy        Boron Content                                                
                         Hardness                                         
______________________________________                                    
002D         0.2% V      C-34                                             
002E         0.4% V      C-38                                             
001D         0.2% V      C-32                                             
001E         0.4% V      C-41                                             
002F         0.1% B      C-30                                             
002G         0.2% B      C-34                                             
001F         0.1% B      C-40                                             
001G         0.2% B      C-42                                             
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     The preferred alloy content is with only small additions of tin, vanadium, or boron because in higher amounts the alloys may be too hard to machine. The tin bearing alloys can be heat treated to soften them, but the vanadium and boron bearing alloys usually stay hard even if given an annealing heat treatment, if the limits noted earlier are exceeded. There are times, however, when the unmachined casting is useful and the higher hardness is necessary. In these instances, larger percentages of tin, vanadium, or boron may be used. 
     The following are illustrative tensile strength values for alloys of the invention: 
     
         ______________________________________                                    
             Tensile                                                      
Heat No.     Strength Psi                                                 
______________________________________                                    
007          54,298                                                       
002          35,609                                                       
013          33,840                                                       
005          48,928                                                       
001          49,040                                                       
011          28,100                                                       
015          25,260                                                       
017          31,380                                                       
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     The alloys of the invention have superior strength and corrosion resistance, compared to other cast iron alloys used in impeller and diffuser stages of submergible pumps, for example. They have substantial corrosion resistance in water, brine, geothermal fluids and both sour and sweet crude oil. Tensile strength is at least double that of Ni-Resist, and downhole corrosion resistance is an order of magnitude better. Hardness and abrasion resistance can be made substantially higher than Ni-Resist, providing special utility in sandy wells. Cost is calculated at substantially less than Ni-Resist (probably one-third the cost). 
     While preferred embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that changes can be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims.