US2490818A - Cast-iron pipe - Google Patents

Description

Dec; 13, 1949 M, KUNIANSKY 2,490,818

I CAST IRON PIPE Filed Aug. 28, 1947' 5 She'ets-Sheet 1 MOLTEN FERROUS ALLOY HAVING A GRAPHITIZING CONTENT SUFFICIENT TO'PRODUCE A SUBSTANTIALLY GREY IRON PRODUCT HAVING NOT MORE THAN A SLIGHT CHILL IN A TEMPERA- TURE CONTROLLED MOLD AND A TOTAL CARBON CONTENT SUFFICIENT TO PROVIDE PROPER FLUIDITY FOR CENTRIFUGAL CASTING ADDITION OF ZIRCONIUM OR ZIRCONIUM CONTAINING ALLOY IN CARBIDE STABILIZINGPROPORTIONS CASTING CENTRIFUGALLY IN TEMPERATURE CONTROLLED MOLDS PIPE AT LEAST A' SUBSTANTIAL PORTION OF THE WALL SECTION OF WHICH IS OF WHITE IRON Dec. 13, 1949 KUNIANSKY 2,490,818

CAST IRON PIPE Filed Aug. 28, 1947 3 Sheets-Sheet 2 WW /A Dec. 13, 1949 M. KUNIANSKY CAST IRON PIPE s Sheets-Sheet 3 Filed Aug. 28, 1947 :z N mm .3. E a Q. 3 :2 Q.

0 m N wv m 6:. 23 35 $5 15; 2835 m3: 252 M95 Patented Dec. 13, 1949 UNITED STATES PATENT OFFICE CAST-IRON PIPE Max Kuniansky, Lynchburg, Va.

Application August 28, 1947, Serial No. 771,131

3 Claims. 1

This invention relates to the casting of pipe and the resulting product, and more especially to the late addition of zirconium to molten ferrous mixtures, containing a substantial percentage of a graphitizing agent, to prevent excessive formation of flake graphite. More specifically, the present invention contemplates the addition of zirconium to a molten ferrous alloy in amounts sufflcient to stabilize iron carbide.

The present application is a continuation-inpart of my co-pending application Serial No. 501,681 filed September 9, 1943, now abandoned.

At present in the casting of white iron pipe, it is necessary to limit the amount of silicon (or equivalent graphitizing constituent) in the metal in order to prevent the formation of primary graphite, during solidification, which would result in the production of a, gray iron product which would be inferior when annealed. The prolonged annealing period, which is required to convert white iron pipe into a machinable and sufficiently ductile product is the direct result of this low percentage of graphitizing constituent. The required annealing time could, therefore, be substantially reduced if the content of the graphitizer could be increased without the formation of excessive primary (flake) graphite. This prolonged annealing period is a serious disadvantage in the production of white iron pipe because of the expense involved in constructing and maintaining the spacious furnaces required for annealing articles of such dimensions.

It is known that an increased cooling rate during solidification will stabilize iron carbide and thereby permit the presence of an increased con tent of graphitizer. Therefore, the advent of centrifugal casting in temperature-controlled metal molds permitted an increase in the graphitizing constituent, especially in castings of thin section. However, the permitted increase is not appreciable in casting pipe by such methods "because the thick wall section required in most pipe lessens the cooling efiect of the mold on the inner poi"- tions of such sections.

A reduction in the total carbon content will also make possible an equivalent increase in the graphitizing constituent, but such a reduction is unsatisfactory, particularly in the centrifugal" casting of pipe, because of a corresponding decrease in the fluidity of the molten metal. Therefore, under the most favorable conditions, it has been found necessary, in casting white iron pipe in metallic molds to have a silicon content that will not generaly run above 1.5%. The total carbon content will depend upon the g'raphitizing content of the ferrous alloy but must be adequate to give sumcient fluidity to the molten meter (usually above 2%).

One of the objects of the present invention is to overcome the above mentioned disadvanf tages of the prior art.

Another object of this invention is toprovidea process for casting a pipe at least a substantial portion of the wall section of which is of white iron, from ferrous alloys containinga graphitizing constituent sufficient to produce a substantially gray iron, and a total carbon content which is adequate to render the molten metal suflicientl fluid for centrifugal casting. A further object of this invention is to provide a cast iron pipe, having at least a substantial chill, which is characterized by the presence ofzirconium in the cast product in carbide stabilizing proportion, and a percentage of graphitizing constituent suificient to produce at least a substantially gray product, if the zirconium were omitted. With these and other objects in view which will" be apparent'from the subsequent description, the" present invention embraces broadly the discovery that zirconium, if added to the molten metal in the proper proportions, and just prior to casting,- will act as a stabilizer of iron carbide during so-f lidification of the ferrous alloy, thereby prevent-- ing excessive precipitation of primary (flake) graphite. This discovery is advantageous because it permits the use of graphitizing agents and a percentage of total carbon which is adequate to render the alloy sufliciently fluid for casting, in the production of a pipe having at least a substantial chill which, without the addition of zirconium, would be either a gray iron pipe or one having only a, slight chill.

The metallographic structure of the pipe will depend upon a number of variable factors including the raw materials selected, the melting and pouring temp ratures, types of furnace used,

conditions present during solidification (prif marily the desired thickness of the pipe and the rate of cooling), the amount of zirconium added, etc. For these reasons, the pipe may be entirely of white iron (which term is intended to include both a true white iron which is substantially without flake graphite and aniron of white fracture containing appreciable, but not excessive quantities of flake graphite), or the White iron may merely constitute a substantial surface chill If the white iron is merely in the form of a chill,

such a chill will be of greater depth (because of the stabilizing property of the zirconium) than would be the case if zirconium had not been added in carbide stabilizing proportions.

The white portion of the product, when cast to give any one of the forms just described, may be more rapidly annealed because of the increased content of graphitizer. Moreover, the annealed casting will have definitely superior properties to those obtained if zirconium in stabilizing proportions was omitted because of the increased amount of carbon in temper form.

In the drawings:

Figure 1 is a flow sheet disclosing one method of practicing the invention.

Figures 2, 3 and 4 consist of photomicrographs at 100 diameters disclosing views of the outer, center and interior portions, respectively, of a section of a centrifugally cast pipe of white fracture containing an increased content of graphitizer.

Figures 5, 6 and 7 show comparative photographs of sections of the walls of three pipes that have been cast centrifugally in a metal mold at controlled temperature. These photographs illustrate the eiiect of silicon, and zirconium in carbide stabilizing proportions. on the depth of chill, when the percentage of total carbon is substantially constant and the conditions of casting are identical.

In practicing the invention, the zirconium in carbide stabilizing proportions is added to the molten ferrous alloy prior to pouring and thereby furnish a proper control for the process. In practice, the ferrous alloy is composed principally of scrap iron and is normally heated to temperatures encountered in good foundry practice.

The percentage of zirconium necessary to stabilize the iron carbide during solidification is relatively small but, for the reasons previously mentioned, depends primarily on the ratio of pig iron to scrap iron which is used, the conditions under which the casting is made (including the rate of cooling), the thickness of the wall section of the desired casting and the final metallographic structure of the casting which is contemplated. For example, the addition of zirconium to the molten metal in amounts producing residual concentrations of zirconium as low as 01% of the total weight of the ferrous alloy has an appreciable chilling effect when a water cooled mold is employed and the silicon and total carbon content are not excessive, although sufilcient to result in the formation of a gray iron product if the zirconium were omitted.

In general, the amount of zirconium required will vary with the percentage of silicon (or equivalent graphitizer) and total carbon present in the iron. An increase in one of these without an eouivalent decrease in the other requires an additional amount of zirconium. Moreover, zirconium is a deoxidizer and, therefore, must be added in sufficient quantities to leave a stabilizing residue after any deoxidizing activity has been completed. It is believed that it is this deoxidizing tendency which has caused zirconium to be previously considered a graphitizer.

In practice, amounts in excess of 1.5% by weight of zircon um are seldom reouired in order to obtain a sufiicient residue for stabilizing purposes. It has been found desirable to add the zirconium in the form of an alloy. The amount of alloy required will vary for the reasons previously mentioned, but will ordinarily fall within a range of from .5% to 2.0%, by weight, of'

the molten metal being treated. An efiicient alloy which has been employed is composed of 35-40% zirconium, 47-52% silicon, and the balance iron. Best results have been obtained if zirconium free of aluminum and calcium is added. An alloy free of aluminum and calcium and containing 12-15% zirconium, 51% iron and 34 37% silicon has been found to be especially satisfactory for carbide stabilizing.

The photographs shown in Figures 5, 6 and 7 illustrate the eiiect of varying the chemical analysis of the ferrous alloy before casting. The pipes from which the photographed sections were taken were all centrifugally cast under identical conditions in a metallic water cooled mold, but as illustrated, the depth of chill varied considerably in these sections which were selected with care so as to be representative of the entire pipe. For example, the section shown in Figure 5 has an appreciable chill because the silicon content is relatively low (1.28%). The section shown in Figure 6, on the contrary, is substantially without chill due to the fact that the silicon content has been increased to 1.52%. This photograph is in sharp contrast to the photograph shown in Figure 7, which discloses a substantial chill at least one-half Way through the wall section, although the ferrous alloy was poured from the same ladle of base metal and had the same analysis as that employed in obtaining the pipe disclosed in Figure 6. This difference arises from the presence of zirconium in carbide stabilizing amounts (069%) which was added just prior to pouring. It is also interesting to note that the chill is substantially greater than that shown in Figure 5, although a higher percentage of silicon was present in the zirconium-treated metal.

Although a true white iron (substantially Without flake graphite) or a deeply chilled iron can be obtained by the addition of zirconium in carbide stabilizing proportions, in practice the applicant employs conditions which produce an iron of white fracture throughout the section, such as disclosed in Figures 2, 3 and 4. As illustrated in these photographs, this pipe is characterized by an appreciable amount of flake graphite which is uniformly distributed throughout the wall section, but also contains an appreciable percentage of graphite in carbide form as illustrated by the white areas. This product is obtained by the use of ferrous alloys composed principally oi iron scrap and containing the following ingredients within the specified range by weight:

Iron (principally scrap) Silicon 1.5% to 3.0%

Carbon 2.50 to 3.50%

Manganese 11% to 1.30%

Sulphur in the usual amounts Phosphorus in the usual amounts Traces of the usual impurities This mixture is melted and superheated in a cupola furnace to temperatures encountered in good foundry practice and is then withdrawn to a ladle where zirconium is added just prior to pouring. The mixture is then poured into a rotating water cooled mold where it solidifies rapidly in the form of a pipe. Preferably this newly formed pipe is withdrawn from the mold while still hot and is subjected to a subsequent annealing process.

Normally, the percentage of zirconium required to stabilize the iron carbide suificiently to produce the illustrated product will vary greatly because of the constant change in the analysis of the molten ferrous mixture due to the use of scrap metal. In practice, a sufficient control is maintained to prevent the percentages of the ingredients from falling outside the range that has been previously indicated, but within these ranges the ferrous mixture can vary greatly without substantially effecting the final product if the amount of zirconium is varied sufficiently to effectively control these conditions. The amount of wild metal present in the particular molten metal is an important factor to be considered in determining the quantity of zirconium which must be added, as zirconium is a deoxidizer and, therefore, sufficient quantities must be added to leave a stabilizing residue after this deoxidizing activity has been completed, In practice, the percentage of residual zirconium which is required is relatively small, being as low as .01% by weight where little deoxidizing activity is required and where iron pipe of relatively thin section is being cast. Amounts in excess of 1.5% are seldom required unless unusual conditions are present.

Actually, it is desirable in this process to add the zirconium in the form of small pellets and preferably as an alloy. The alloy which has been previously described and which is composed of -40% zirconium, 47-52% silicon and the balance iron, has been employed and has proved to be an effective carbide stabilizer. The amount of alloy will vary within a range of from 1.5 to 2.0% by weight of the total mixture. This zirconium alloy in pellet form provides an excellent control of the relatively small proportions of the metal which is required. The absence of aluminum and calcium from the alloy makes it especially desirable since the best results have been obtained when the ferrous alloy is free of those elements.

The following examples are given to illustrate the application of this process to obtain a pipe of white fracture of the type which has been described.

Example I An alloy containing 2.90% total carbon, 2.22% silicon, .79% manganese, 0.072% sulphur, and .16% phosphorus and the balance iron was cast in a rotating water cooled 8 inch mold. After solidification the structure of the pipe was gray with the exception of a slight chill on the outside surface. A zirconium alloy containing 37.53% zirconium, 50.12% silicon and 9.15% iron was then added, just prior to pouring, to a ferrous alloy of the same analysis in the amount of 1.0% by weight of the alloy and cast in the same mold. The resulting product was a pipe of white fracture which contained an appreciable amount of flake graphite, distributed uniformly through the wall section. The pipe was completely annealed within three hours at temperatures not exceeding 1700 F. The product was of gray fracture and contained a high percentage of temper carbon.

Example II The same experiment was repeated and the same results obtained with a ferrous alloy containing 3.49% total carbon, 1.83% silicon, 35% manganese, .051% sulphur, .73% phosphorus and the balance iron. The same zirconium-silicon alloy was employed but was decreased to 0.75% by weight.

As has been previously mentioned the product obtained by the addition of zirconium in stabilizing proportions (whether in the form of true white iron, 'a pipe of white fracture through the 6 section, or apipe having an appreciable chill), is readily annealable because sufficient amounts of silicon or equivalent graphitizer may-be present without causing the excessive formation of flake graphite. Moreover, the total carbon content is sufficient to provide adequate fluidity for centrifugal casting purposes. The carbon content may vary between rather wide limits, but generally will be between 2.5 and 3.75%. The period of time required to anneal this product will, of course, depend upon the depth of chill,

pipes of white iron sections throughout requiringamore extended period. However, a pipe of white fracture, such as shown in Figures 2, 3 and 4 can be annealed completely by the precipitation of all the iron carbide in the form of temper carbon, within two or three hours, at a temperature which need, not exceed 1700 F. The present invention is not, however, limited to any specific annealing cycle and, therefore. a more extended treatment can be employed if desired.

In any event, the annealed product will be definitely superior to heat treated iron, cast gray, because such a product contains no temper carbon. The product will, of course, be less malleable than malleable iron (except that in the case of pipe the wall section of which is a true white iron throughout), because of the presence of flake graphite in the other products contemplated by this invention.

The product 'disclosed in Figures 5, 6 and 7 when annealed is characterized by the presence of a mixture of flake and temper carbon. The percentage of ferrite and pearlite present in this product, after annealing, will vary with the particular type of annealing cycle which is employed.

The annealed white portions of these products is a tough, easily machinable metal. Wall tensile strengths of pipe having these metallographic structures have ranged from 45,000 lbs. per square inch to 77,500 lbs. per square inch. Elongations have ranged from 0% to 3%. Moduli of rupture and elasticity as determined from the Talbot strip test have ranged as follows: Modulus of rupture from 70,000 to 115,000 lbs. per square inch and modulus of elasticity from 4,500,000 to 10,000,000 lbs. per square inch. The hardness range on the Rockwell B scale is from approximately to 89.

While for purposes of illustration the adaptation of this invention to the casting of pipe |by centrifugal methods in temperature controlled metallic molds has been disclosed, it is believed obvious that other types of rotatable molds may be employed. Moreover, any suitable ferrous mixture can be selected and pretreated in any suitable manner .before the later addition of the zirconium.

I claim:

1. A chill cast iron of thin wall section cast in water cooled molds, at least a substantial section of the wall of the cast iron being of white iron comprising iron, 2. total carbon content of 2-4%, silicon in concentrations suflicient to produce a gray iron wall section in the absence of zirconium, and sufiicient zirconium to promote the formation of a casting having at least a substantial section of its wall of white iron, the residual zirconium in the casting ranging from 0.01 to 1.5%.

2. A centrifugally cast pipe of ferrous alloy, at least a substantial section of the wall of which 7 of at least l /z% and from .0l% to 1.5% zirconium in .the cast product.

3. A centrifugally cast pipe at least a substantial portion of the wall section of which is of white iron, said portion being characterized :by the 5 REFERENCES CITED The following references are of record in the file of this patent:

* UNITED STATES PATENTS Number Name Date 1,640,674 Schwartz Aug. 30, 1927 1,683,714 Emmel et a1 Sept. 11, 1928 OTHER REFERENCES Alloy Cast Irons, :pages 143 and 196. Published in 1939 by the American Foundrymens Association, Chicago, Illinois.

Cast Metals Handbook, pages 327, 4'71, and 472. Published in 1940 by the American Foundrymens Association, Chicago, Ill.

The Journal of The Iron and Steel Institute, No. 1, 1941, pages 2071, and 240 to 249R Published loy the Iron and Steel Institute, Grosvenor Gardens, London, England.

Images