Abstract:
The alloy of ductile cast iron to which boron has been added is found to have improved annealability as compared to a non-boron treat ductile iron. The addition of boron in amounts between about 10 ppm and 150 ppm by weight effectively combines with nitrogen dissolved in the molten iron and thus serves to minimize the influence of nitrogen on the stability of carbides and pearlite and also forms BN, which can serve as nuclei for the precipitation of graphite.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This application claims priority to United States Provisional Patent Application No. 60/688,586, filed Jun. 8, 2005. 
     
    
     FILED OF THE INVENTION  
       [0002]     The invention relates generally to alloys of cast iron and, more specifically, to alloys of cast iron to which boron has been added and which increase the annealability of carbidic ductile iron in articles cast using the alloy and/or promote the formation of ferrite.  
       BACKGROUND OF THE ART  
       [0003]     Cast iron is an alloy of iron and carbon in which the carbon is in excess of the amount that can be retained in solid solution in austenite at the eutectic temperature. Carbon is usually present in the range of 1.8% to 4.5%, in addition, silicon, manganese, sulfur, phosphorus and other residual or specifically added alloying elements, all in varying amounts. Specific types of cast iron include gray, malleable, ductile and white irons. Magnesium is typically added to a low sulfur iron to produce ductile (spheroidal graphitic) iron. Because of the high carbon content, the structure of cast iron, as opposed to that of steel, exhibits a rich carbon phase. Depending primarily on composition, cooling rate and melt treatment, cast iron can solidify according to the thermodynamically metastable Fe-Fe 3 C system or the stable Fe-Gr system.  
         [0004]     In the Fe-Fe 3 C system, the rich carbon phase in the eutectic is iron carbide. In the Fe-Gr system, the rich carbon phase is graphite. An example of the Fe-Fe 3 C system is what is known as “white iron.” White iron exhibits a white, crystalline fracture surface because fracture occurs along the iron carbide plates; it is the result of metastable solidification (Fe 3 C eutectic). An example of the Fe-Gr system is what is commonly known as ductile iron, but has also been called spheriodal, nodular or SG iron. The graphite in this iron is present as nodules as compared to the graphite flakes in gray iron.  
         [0005]     However, the properties of ductile iron are controlled not only by the spheriodal shape of the graphite, but also by the metallurgical structure of the matrix. This matrix microstructure is controlled by the alloy content, whether deliberately added or as generally called “residuals,” and the cooling rate. Thus it is possible to have the graphite present with a spheriodal graphite morphology and also have a matrix that contains both primary carbides, ferrite and pearlite. Until the matrix structure is entirely ferritic, the impact strength and ductility of the casting will not be maximized. This maximization is often achieved by an annealing process in conjunction with minimizing the deleterious elements.  
         [0006]     Previous research efforts include Ball, D L, Nucleation of Euctectic Graphite in Cast iron by Boron Nitride, AFS Transactions, 1967 P 428-432; Ball, D L, Transactions of the Metallurgical Society of AIME, V 239 January 1967 P 31-36; Pehlke, R D, Wasa, H, Strong, G R Nitrogen in Malleable Iron Production, AFS Transactions 1978 P 125-134; Dawson, J V, Smith, W L, Bach, B B, Some Effects of Nitrogen in Cast Iron, Journal of Research and Development of the BCIRA, Research Report 355, 1953 and Sandoz, G, White Cast Iron Inoculation Effect on Graphitization AFS Transactions 1962 P 13-17 have shown that nitrogen can be present as dissolved (monatomic nitrogen) in cast iron that this nitrogen will affect both graphite morphology and matrix microstructure. Work also showed the boron will react with the dissolved nitrogen during solidification to form boron nitride (BN). Work by Ball in cast iron showed that the boron nitride formed a BN nucleus upon which graphite would form during solidification. This nucleus has a crystallographic structure similar to graphite.  
         [0007]     Other research work showed that boron additions during the processing of molten steel ties up nitrogen, again forming BN, and prevents problems associated with monatomic nitrogen.  
         [0008]     There is a need, accordingly, for a ductile iron that will readily respond to an annealing treatment and/or will exhibit a higher percentage of as-cast ferrite and a minimization of primary and intercellular carbide. It is known that nitrogen is an element that is present in molten iron and that nitrogen is a carbide stabilizer. Thus there is a need for an element or alloys of elements that can be added to molten ductile iron that will not only reduce the impact of dissolved nitrogen but will also promote the formation of ferrite, thus negating some of the influence of other carbide/pearlite stabilizing elements and/or to promote the annealability of the iron, which may be a necessary process to remove carbides from a rapidly cooled casting such as might be encountered when producing ductile iron pipe by the centrifugal casting process.  
         [0009]     SUMMARY OF THE INVENTION  
         [0010]     The invention consists of alloys used to promote the formation of ferrite and enhance the annealability of ductile iron. The alloys are characterized in that a source of boron is added to provide boron between about 10 and 150 ppm and preferably between about 35 and 85 ppm. The addition of boron is observed to increase the nodule count and enhance the annealability of the ductile iron such that the solutioning time (time to eliminate primary carbides) and the cooling rate (time to avoid the presence of pearlite in the final room-temperature structure) can be significantly reduced as compared to non-boron treated ductile iron. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a 100X photomicrograph of a chill cast section (½ inch thick) of non-boron treated ductile iron  
         [0012]      FIG. 2  is a 100X photomicrograph of a chill cast section (½ inch thick) of a boron treated (57 ppm) ductile iron.  
         [0013]      FIG. 3  is a 100X photomicrograph of a chill cast section (½ inch thick) of a non-boron treated iron after heat treatment at 1600° F. for three-quarters of an hour—open door furnace cool.  
         [0014]      FIG. 4  is a 100X photomicrograph of a chill cast section (½ inch thick) of a boron treated (80 ppm) ductile iron after heat treatment at 1600° F. for three-quarters of an hour—open door furnace cool. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0015]     Ductile iron as used in this disclosure is defined as an iron composition having the components and ranges as set out in Table 1.  
         [0016]     Non-chill cast iron as used in this disclosure is defined as iron cast in a sand mold, such mold not containing any type of insert that accelerates the rate of heat removal compared to sand.  
                         TABLE 1                           Composition of Ductile Iron            Weight Percent   Element               2.0-4.0   Carbon       1.5-4.0   Silicon        .1-1.5   Manganese       .005-.05    Sulfur       .01-.05   Magnesium       Balance   Iron and other incidental residual           elements                  
 
         [0017]     It is recognized that the spheriodal graphite shape in ductile iron is a result of treatment of the molten iron with magnesium. There are other treatments, such as with rare earths, that will also produce the desired spheroidal structure. The method of treatment to achieve the spheriodal graphite structure is not critical to this invention.  
         [0018]     The solubility of nitrogen (monotomic nitrogen) in molten iron is influenced significantly by both temperature and the composition of the molten iron. Carbon and silicon both reduce the equilibrium value of nitrogen in what would commonly be called molten cast iron. Uda and Pehlke (Uda &amp; Pehlke, Part 1 -Solubility of Nitrogen in Cast Irons, AFS Transactions, 1971, Paper No. 82) showed that the solubility of nitrogen in a molten alloy at 1500C (2732° F.) in an iron alloy with 3.5% C −2% Si is approximately 120 ppm. The paper thoroughly describes the mathematical relationships between the variables of carbon and silicon content and temperature. Numerous nitrogen analyses on gray and ductile irons show that the typical nitrogen content is between approximately 60 ppm and 110 ppm. It is also known that desulfurization processes and magnesium treatment processes will lower the dissolved nitrogen; but essentially never below 50 ppm.  
         [0019]     Boron has an atomic radii of 0.97A and nitrogen has an atomic radii of 0.71A and atomic weights of 10.82 and 14.08 respectively. As written in a paper by Gloria M. Faulring (Faulring, Nitrogen Scavenging with Boron, Electric Furnace Conference Proceedings, 1989, Pages 155-161): “The amount of boron required for scavenging nitrogen depends on the nitrogen content of the steel (iron). To optimize the effectiveness of the boron, the contained amount should be 0.8 to 1.0 times the nitrogen content, preferably about 0.8 i.e. % B/% N=0.8 to 1.0. The stoichiometric ratio of the amounts of boron and nitrogen in BN is 0.77.” 
         [0020]     However, boron is a strong carbide former and any amount in excess of the stoichiometric amount needed to tie up nitrogen as BN promotes the formation of very stable carbides, which can be difficult to remove during normal heat treatments. These carbides are typically present as intercellular carbides and are detrimental to impact strength of an annealed ductile iron.  
         [0021]     Trials have been run with boron levels above 150 ppm and intercellular carbides were observed as a result of this boron level. In cases, where trials were run with and without boron, with chemistries designed to promote carbides, boron additions in the 150 ppm plus range always generated more intercellular carbides than the same alloys without the boron addition.  
         [0022]     Thus one can conclude that boron levels must be kept below that which will cause the formation of boron containing carbides because these carbides will not be removed under the same annealing conditions as those alloys that do not exhibit boron-induced carbides. At those boron levels, even though a higher nodule count may be observed, the ductile iron will resist annealing. The intercellular carbides remaining after the annealing process will reduce the impact strength and ductility of the annealed ductile iron.  
         [0023]     Thus to successfully utilize this boron practice, the boron level must be kept below that which will create stable boron-alloy carbides. One would then conclude that the optimum desired practice of the art would include a nitrogen analysis and the boron level should not exceed this nitrogen level. In the absence of nitrogen analysis, and based on numerous published and published studies of nitrogen levels of ductile iron after magnesium treatment, the boron level in the alloy should not exceed about 60 ppm.  
       EXAMPLE I  
       [0024]     Boron was added to a ladle of molten iron in the form of FeB. The alloy used to make the boron addition is understood not to be critical to the observed results. The chemistry of the molten iron used in this Example I is provided in Table 2.  
                                           TABLE 2                           Composition of Alloy                Element   Percentage (%)                            Carbon   3.67           Silicon   2.88           Manganese   .33           Sulfur   .008           Magnesium   .049           Copper   .15           Chromium   .03           Nickel   .04           Molybdenum   .01           Vanadium   N.D.           Tin   .01           Aluminum   .032           Titanium   .006           Boron   .0057           Nitrogen   .0035           Iron and other   Balance           incidental           residual elements                      
 
         [0025]     Samples of non-boron treated and boron treated iron as represented by the chemistry in Table 2 were heat treated to remove primary carbides and minimize the amount of ferrite. Table 3 summarizes the results of a heat-treatment cycle of 1600° F. for three-quarters of an hour followed by open-furnace door cooling  
                         TABLE 3                           Summary of Microstructural Results After Heat Treatment            Sample Identification   Microstructural Analysis               Non boron Treated Chill Cast   3-5% carbide - 10-15% pearlite       Ductile Iron       Boron Treated Chill Cast Ductile Iron   &lt;1% carbide - 3-5% pearlite                  
 
         [0026]     Photomicrographs of these results are shown in  FIGS. 1-4 .  FIG. 1  (400X) shows the as-cast microstructure of the non-boron treated chill cast ductile iron.  FIG. 2  shows (400X) shows the as-cast microstructure of the boron treated chill cast ductile iron.  FIGS. 3 and 4  respectively show the two structures after a solutioning heat treat of 1600° F. for three-quarters of an hour followed by an “open-door” furnace cool. It is apparent that the boron greatly enhanced the annealability of the ductile iron and that the graphite nodule count of the boron treated ductile iron was greater than the non-treated iron.  
       EXAMPLE II  
       [0027]     Experimental samples of ductile iron, having the nominal composition and ranges set out in Table 4, were treated with boron.  
                             TABLE 4                           Composition of Alloy                Element   Percentage (%)                       Carbon   3.3-3.4           Silicon    2.0-2.15           Manganese   0.33-0.40           Sulfur   0.001-0.002           Magnesium   0.024-0.040           Copper   0.30-0.37           Chromium   0.17-0.20           Tin   0.009-0.012           Iron and other   Balance           incidental           residual           elements                      
 
         [0028]     The alloy used to make the boron addition is understood not to be critical to the observed results. The molten alloy was produced in a commercial pipe (deLavaud process) foundry. The metal had been cupola melted and dosed with 5% magnesium ferrosilicon to provide nominally 0.03% magnesium to provide sufficient magnesium to produce the residual magnesium concentration set out in Table 4 at the treatment temperature of the foundry. Samples of molten alloy were removed from the deLavaud machine with a ladle. The samples were treated with two different sources of boron, FeB and TiB 2 , in nominal amounts to provide 80 ppm of boron. The sources of boron were added to a pouring ladle, molten alloy was added to the pouring ladle and, after brief stirring with a steel rod, the samples were poured into chill molds to simulate the solidification rate of metal in the deLavaud molds. To dissolve any carbides and produce a ferritic matrix, the samples were then heat treated at between about 1700° F. and 1850° F. for 20-25 minutes, temperatures typical of the pouring temperatures for deLavaud pipe. Samples of untreated iron were also cast and subjected to heat treatment under the same conditions. Three castings of the untreated iron were prepared and two castings each of the FeB-treated the TiB 2 -treated iron were prepared. After heat treatment, the samples were allowed to cool and then cut into sections. The microstructures of twenty-five sections of each sample were examined and the nodules per square millimeter counted to determine if the boron-treated irons had increased nodule count, indicating that the boron treatments could be used to reduce the heat treatment time and energy required. The results are summarized in Table 5.  
                                                   TABLE 5                           Nodule Counts of Treated and Untreated Iron                    Addition of   Addition of           Control   FeB (80 ppm)   TiB 2  (60 ppm)                        Alloy Addition Type   None   FeB   TiB 2         Alloy Addition Amount    0   1 g   0.33 g       Ave. Nodule Count   426   565   516       Std. Dev.   165   140   110                  
 
         [0029]     The results show that the addition of nominally 80 ppm boron in the form of FeB or approximately 60ppm in the form of Titanium Diboride increased the nodule counts by between about 20 to 35%. No attempt was made to optimize the amount of boron added to the alloy. Those skilled in the art will recognize that higher nodule counts can be expected to substantially decrease the time required both for carbide dissolution and heat treatment time and energy.  
         [0030]     The foregoing description and drawings comprise illustrative embodiments of the present inventions. The foregoing embodiments and methods described herein may vary based on the ability, experience, and preference of those skilled in the art. Merely listing the steps of the method in a certain order does not constitute any limitations on the order of the steps of the method. The foregoing description and drawings merely explain and illustrate the invention, and the invention is not limited thereto, except insofar as the claims are so limited. Those skilled in the art who have the disclosure before them will be able to make modifications and variation therein without departing from the scope of the invention.