US2750279A - Process of treating cast iron with iron-fluorine compounds - Google Patents

Description

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PROCESS OF TREATING CAST IRON WITH IRON-FLUORINE COMPOUNDS No Drawing. Application May 24, 1952, Serial No. 289,902

14 Claims. (CI. 75-53) This invention relates to the manufacture of cast iron and more particularly to the treatment of cast irons while in their molten state to improve the physical properties of the final cast metal.

Cast irons are generally ditferentiated from steels in that steels have a carbon content of less than 1.7%, whereas cast irons are those iron compositions containing more than this amount of carbon. The carbon content of most cast irons is between 1.7 and 4.5%, the carbon usually being present in amounts such that it cannot all be retained in solid solution, as austenite at the eutectic temperature. Graphitization occurring in cast irons produces graphite or free carbon dispersed throughout the solid solution when the iron is cooled. The amount of this graphite, the form in which it is present, and its degree of distribution all have a substantial eilect on the physical properties of the iron casting obtained.

It is generally considered that the graphite in commercial cast iron results from the dissociation of cementite (FesC) and that the amount formed depends in part on the rate of cooling of the cast metal. Graphitization may be almost complete when cast irons containing 3.5 to 4.5% total carbon are cooled very slowly. Graphitization is also affected by the presence of other materials. For example, silicon (which is nearly always present in foundry irons) in concentrations up to 3% will accelerate the dissociation of the cementite into graphite and iron. Since other materials such as silicon, phosphorus, sulfur, etc., when present in the cast iron composition, have a substantial effect on the physical properties of the final casting, considerable work has been done on the modification of castings by additions of agents for controlling the amount of these materials in the cast iron composition. The present invention, however, is not primarily concerned with the treatment of iron-carbon compositions for the purpose of changing the over-all chemical composition of the metal treated, but is directed more to the treatment of the iron-carbon compositions so as to modify or change the form of the materials therein and thereby modify the physical properties of the final casting.

The method of treating cast irons of the present invention is particularly beneficial with respect to gray iron.

Gray iron covers a series of eutectic ferrous alloys that offer a wide selection of mechanical properties together with economy in the production of cast shapes and in subsequent machining. Composition and processing are adjusted so that usually the matrix structure is largely pearlitic with many graphite flakes dispersed throughout. The presence of graphite flakes imparts the characteristic gray fracture of the gray iron. Also, graphite being a natural lubricant, its presence promotes ready machinability, reduces sensitivity to notch efiect and imparts useful damping properties and resistance to wear.

Gray irons are usually made within the composition ranges of 1.00 to 2.75% silicon and 2.70 to 3.60% total carbon. Other elements present in typical compositions and their amounts are: 0.50 to 0.80% manganese, 0.10 to 0.40% phosphorus, 0.07 to 0.10% sulfur, 0.00 to 2.75%

2 nickel, 0.00 to 0.80% chromium, and 0.00 to 1.00% molybdenum.

Since the specific gravity of graphite is low as compared to iron, its per cent by volume is substantially greate than its per cent by weight. For example, iron having a 3.0% graphite by weight would have about 9.6% graphite by volume. In general, graphite flakes account for about 6 to 10% of the volume in typical gray cast iron. The graphite thus affects the continuity of the matrix to such an extent that it exerts a very pronounced effect on the mechanical properties of the final casting. The form in which the graphite appears as well as the manner in which it is distributed are, therefore, vitally important in determining the physical properties of the metal.

The presence of graphite, though very beneficial in many respects, tends to soften the casting and decrease its tensile strength particularly where the graphite is not well dispersed and occurs in relatively large flake form. For this reason gray iron is classified, depending on the amount, form and distribution of the graphite, into types which are most suited for particular uses.

It has now been discovered that adding a small amount of an iron-fluorine compound to the molten metal substantially modifies the form in which the carbon appears in the final casting and greatly improves its physical properties. The treatment of the molten metal with the iron-fluorine compound hould not be confused with the addition of fluorine-containing compounds for fluidizing the flux or for the purpose of cleansing the metal and emoving impurities such as sulfur and phosphorus. Thus, for example, fluorides such as calcium fluoride, sodium fluoride, sodium fluosilicate and cryolite have been added to iron alloys with beneficial results; however, the addition of these fluorides to cast iron melts does not have the same beneficial etlect on the microstructure of the cast metal as does the addition of iron-fluorine compounds.

The addition of iron-fluorine compounds to the molten metal has a marked effect on the manner in which the graphite is dispersed and thus a marked effect on the final physical properties of the casting. Its primary effect, apparently, is to act as a graphite-refining element, i. e., to inhibit the growth of the graphite flakes or stringers. The result is that the graphite particles are widely separated and not interlaced. In addition, the combined carbon content is increased. However, this increase in com bined carbon apparently has little eflfect insofar as increasing the Brinell hardness which is surprising since iron carbide is an extremely hard material. There is apparently little or no effect on the over-all chemical composition. This is readily illustrated by the following table.

(The iron-fluorine compound in each case was the reaction product of aqueous HF and siderite, 5 pounds of the reaction product being added per ton of metal treated.)

The castings obtained from gray iron melts to which small amounts of an iron-fluorine compound had been added were generally found to have the following characteristics: The castings were sound. The tensile and transverse strengths were increased. Good machinability was observed in gray cast irons of high tensile and transverse strength; thus, castings having the tensile strength of a class 40 iron (40,000 to 49,000 pounds p. s. i.) were obtained by adding iron-fluorine compounds to melts that would normally result in class 30 irons (30,000 to 35,000 pounds p. s. i.) while still maintaining the better machinability of the class 30 iron. The appearance of the castings was improved. The hardness of the as-cast condition was not increased and hardenability on heat treatment was good. The resistance to wear was increased. The castings were found less likely to spell on the application of load or heat. Also, sub stantially no increase in dimensions was observed after repeated heating and cooling up to 1600 F. (871 C.).

Most of the gray iron produced in this country is melted in the cupola furnace. Metal, fuel and flux comprise the cupola charge. The fuel is coke and the flux limestone. Fluorspar, soda ash, or proprietary fluxes such as Purite (fused soda ash), or Cornell Flux are sometimes also used. The molten iron is run from the cupola into ladles from which the metal is poured into molds. In practicing our present invention, the iron-fluorine compound is, therefore, preferably added in the cupola, substantially no fuming being observed. (Though this is the preferred practice, beneficial results are also obtained by adding the iron-fluorine compound in the ladle.) The addition is generally made in the amount of 3.7 to 6 pounds of the iron-fluorine compound per ton of metal treated though amounts as low as 2.5 pounds per ton of metal treated may be used with beneficial results. Amounts substantially in excess of 6 pounds of the iron-fluorine compound per ton of metal treated apparently have no harmful efiects on the metal treated and may be used if desired.

When iron-fluorine compounds were added to cast iron melts and the castings then chilled in metal molds, the castings were found to have a substantially greater ferric carbide content than castings obtained without the addition of an iron-fluorine compound. After annealing these castings, the carbon was found to be present in a substantially nodular form. The castings from melts treated with an iron-fluorine compound were readily annealed. Also, particularly with respect to gray iron, the castings treated with the iron-fluorine compound were found, after annealing, to have a substantially greater impact strength than similar castings obtained from melts to which an iron-fluorine compound had not been added.

Though iron fluorine compounds, in general, produce the above described desirable results, the preferred ironfiuorine compounds are those obtained through the reaction of an aqueous solution of hydrogen fluoride with siderite. This iron-fluorine product is a new product of manufacture and is claimed as such in our copending application Serial No. 289,901 filed May 24, 1952.

In order to better illustrate the practice of the present invention, the following examples are given. The examples, however, are given for purposes of illustration only and the invention is not to be limited thereto.

Example 1 Fifty net tons of gray iron were processed in a 54-inch cupola using a charge made up of 50% pig iron, 45% cast scrap, and 5% steel rails. A closed can containing 5 pounds of reaction product of siderite and aqueous hydroan iron-fluorine compound had not been added. The following is a comparison of the castings:

Using an l8-inch experimental basic-line cupola, 2250 pounds of charge was melted. The iron charge was made up of 33% pig iron, 57% cast scrap and 10% steel. Two and one-half pounds of ferrosilicon were added with each 150 pounds of metal charged.

Seven 150 pound charges of metal constituted the base charge or untreated metal. These seven charges were melted first. Immediately following, eight charges of the same composition were melted, each 150 pound charge containing a six-ounce can of siderite-aqueous hydrogen fluoride reaction product. The cans were punctured to permit vapor escape. Regular cupola practice was followed in melting the metal. The molten metal was tapped into 250 pound ladles and tensile strength bars (A. S. T. M., A4848-Type B) were cast from the treated and untreated metal. The bars compared as follows in composition and tensile strength.

, Addition of Composition, Percent by Weight i i fi Siderite-HF l Product Total C 3. 58 3. 47 Si 1. 2. 06 Mn. 0. 59 0. 60 S 0. 072 0. 072 Tensile Strength, p. s. i 26, 710 36, 050

Hardness as Cast, BEN.-- 217 217 Example 3 Example 4 Two hundred pounds of a metal mix composed of 84% pig iron, 12% steel, 2% ferrosilicon, 2.4% ferrophosphorus and 0.2% sulfur were melted in a crucible in an induction furnace at a temperature of approximately 2600 F. The temperature of the melt was then lowered to about 2475 F. and an 85 pound pipe (length 44 inches, inside diameter 6 inches and thickness 0.37 inch) was cast centrifugally in a metal mold. The melt tempera ture was then increased to approximately 2575 F. and to the pounds of remaining metal was added 4.6 ounces of the reaction product of siderite and aqueous hydrogen fluoride, the addition being made in a cast iron container. Five minutes after the addition, a second 85 pound pipe section, identical to the first, was cast in the same manner. The pouring temperature of the melt for the second pipe was 2500 F.

On comparing the two pipes, it was observed that the pipe from the melt to which the siderite-HF product had W "III Second Pipe, siderite- Composition, Pereeut First PIDQINO Addition HF Reaction by Weght duct Addition As Cast Chill Medium to light chill. Very dense chill- Maximum depth 54 wall. Scattered earofpipewail. bide patches remainder of wall. 10 mins 20 mins.

Annealing Time at 20 Corrected Impact, FtJ. Annealed Microstructure.

Predominantly dend- 40% temper carbon ritic. Some temper type graphite. 60 carbon type graphdendritic graphite. ite. Steadite structure more discontinuous.

1 Drop-hammer test. Water under 35-50 p. s. 1. is in pipe. Hammer raised by 2-inch increments.

In the heretofore mentioned table and in the examples, the practice of the present invention has been illustrated through the use of iron-fluorine compounds obtained by the reactionof hydrogen fluoride on siderite in an aqueous medium and by the thermal decomposition of ferrous fluosilicate (a new product of manufacture claimed in our aforesaid copending application Serial No. 289,901). These particular iron-fluorine compositions contain, in chemical combination, the elements iron, fluorine and oxygen and can, it is believed, be generally described by the respective empirical formulae Fagin, Fef' F (0.25-1.9) HF.( 1.5-5.0) H

and

F i F4-11-20H However, the invention is not limited to these since all iron-fluorine compounds appear to have, in varying degree, the heretofore described beneficial effects when added to cast iron melts. These include both the complex and simple iron fluorides including the reaction products of hydrogen fluoride with iron compounds in general, and in particular with siderite, hematite, limonite, magnetite, scrap iron, forging scale and red mud, the reaction product of hydrogen fluoride with siderite being preferred.

Having thus described our invention, we claim:

1. In the production of cast iron, the step comprising adding to the molten iron an iron-fluorine compound.

2. In the production of cast iron, the step comprising adding to the molten iron at least 2.5 pounds of an ironfluorine compound per 2000 pounds of metal treated.

3. In the production of cast iron, the step comprising adding to the molten iron a compound containing combined iron, fluorine and oxygen.

4. In the production of cast iron, the step comprising adding to the molten iron an iron-fluorine compound obtained by reacting an aqueous solution of hydrogen fluoride with an iron compound.

5. In the production of cast iron, the step comprising adding to the molten iron the reaction product of an aqueous solution of hydrogen fluoride with at least one of the materials of the group consisting of siderite, hematite, limonite, magnetite, scrap iron, forging scale and red mud.

6. In the production of cast iron, the step comprising adding to the molten iron the reaction product of aqueous hydrogen fluoride with siderite.

7. In the production of cast iron, the step comprising adding to the molten iron at least 2.5 pounds of the reaction product of aqueous hydrogen fluoride with siderite per 2000 pounds of molten metal treated.

8. In the production of cast iron, the step comprising adding to the molten iron an iron-fluorine compound obtained by heating ferrous fluosilicate to a temperature of to C.

9. In the production of gray cast iron, the method of modifying the graphite dispersion comprising adding 3.7 to 6 pounds of an iron-fluorine compound per 2000 pounds metal, the addition being made to the cupola.

10. In the production of gray cast iron, the step comprising adding a compound containing in chemical combination ferrous fluoride, ferric fluoride, hydrogen fluoride and water to the molten iron in amounts of 3.7 to 6 pounds of said compound per 2000 pounds of metal treated.

11. The process of claim 1 in which the iron-fluorine compound contains 10 to 36% ferrous iron, 8 to 32% ferric iron and 22 to 42% fluorine.

12. The process of claim 1 in which the iron-fluorine additive can be expressed by the empirical formula where a=0.4 to 3.5, b=1.0, 0:3.8 to 10 and d=0.25 to 1.9 and e=1.5 to 5.0

13. The process of claim 9 in which the iron-fluorine additive can be expressed by the empirical formula aFeFabFeFacHEdHzO where a=1.6 to 2.5, b=1.0, c=0.4 to 0.9 and d=3.0 to 5.0.

14. The process of claim 9 in which the iron-fluorine additive can be expressed by the empirical formula aFeFabFeFacHFdHzO where a=0.9 to 1.2, b==1.0, c=0.8 to 1.9 and d=3.5 to 4.0.

References Cited in the file of this patent UNITED STATES PATENTS 48,483 Everett June 27, 1865 1,098,346 Goldmerstein May 26, 1914 2,306,976 Pedersen Dec. 29, 1942 FOREIGN PATENTS 2,683 Great Britain 1860

Claims (1)

1. IN THE PRODUCTION OF CAST IRON, THE STEP COMPRISING ADDING TO THE MOLTEN IRON AN IRON-FLUORINE COMPOUND.