US1614863A - Centrifugally-cast pipe - Google Patents

Description

Jan. 18 1927.

D. T. BEATTY CENTRIFUGALLY CAST PIPE Original Filed Dao. 2 1925 INVENTOR 0064 A6 71234-31 77? Patented Jan. 18, 1927.

UNITED STATES 1,614,863 PATENT OFFICE.

DOUGLAS T. BEATTY, OF BIRMINGHAM, ALABAMA, ASSIGNOB T SAND SPUN PATENTS CORPORATION, OF WILMINGTON, DELAWARE, A CORPORATION OF DELAWARE.

CENTBIFUGALLY-CAST PIPE.

Original application filed December 2, 1925, Serial No. 30,609, Divided and this application filed June 10, 1826. Serial No. 115,117.

This invention is an improvement in cast hollow bodies, and more particularly in centrifugally cast iron pipes.

In the usual process of casting hollow bodies, such as pipes, the molten metal is charged into the top of the vertically disposed stationary mold, flowing through gates in the head core, from four to six in number. The first of the metal falls the entire length of the mold through air, subject to the action thereof throughout its travel, and in large part rebounds in drops and splashes resulting in further exposure to the rapidly heating air. a

This action will decrease in violence as the mold fills, and the air is soon deprived of its oxygen. Hence the upper part of the mold is filled under comparatively favorable conditions. However, due to the above mentioned causes, the lower portion of the casting is peculiarly liable to casting flaws, such as blow-holes, cold shuts, sonims, nonmetallic inclusions, and the like.

When the metal ispoured into a stationary, refractory or metal mold, there is very little chance for mechanically trapped sand, facing, slag, oxides, or even gases, to escape by gravity from the lower part of the mold. The weight of the incoming metal acts to prevent, as does also the viscous and sluggish condition of the metal first flowing into the mold. This metal is cooled by its long journey over cold mold walls and through cold air.

Even such intrinsically inert gases as water vapor or carbon monoxide are frequently caught between the mold and wall surfaces freezing and setting inward from mold and core, to remain as blow holes in the finished castings. If it is sought to remedy this condition by raising the tem er-. ature of the molten metal, other trou les arise, such as the cutting of the mold and cores. From this results inclusions of sand, blacking, first formed oxides, and manganese sulphide, which is always arising in hot cast ll'Oll.

Hence the conditions which in the previous case caused trouble in the bottom of the mold, are simply transferred to the top of the mold, and no improvement is obtained. In practice a compromise is' reached, the iron being poured as hot as possible without serious damage to the top of the mold, but the inclusions are still present, though fairly well disseminated throughout the castings.

These conditions are remedied by centrifugal casting in refractory molds, wherein the lighter impurities are separated by gravity and removed to the inner surface of the casting. The product is further improved by the manner of pouring, wherein the mold is inclined during the pouring, so that the metal enters with practically no rebound or large body of metal so that the third phase of the operation, freezing,'takes place under conditions conducing to uniformity of the product.

When hollow bodies are cast in this manner the impurities, such as slag,'oxides, sand and the like, of lower specific gravity than the metal tend to deposit on the inner surface of the casting, usually near the ends, in the form of scruff. The union between the layer of scruff and the casting wall may be relatively intimate, thus marring and defacing the surface, and causingpitting of I the wall when the scrufi layer or coating is removed.

The conditions which result in the formation of scrufl may be remedied and the formation of anything approaching an intimate union between the scruff layer and the casting wall may be prevented by careful skim ming and surface slagging of the molten metal as it flows on to the mold wall, by the use of a suitable slagging material, such as soda ash. The impurities moved to the inner surface by centrifugal action and the crystallization of the metal, may be converted into a thin, fluid slag with a low fusing point, so that there is a clear separation between the casting wall and t e layer of scruff, to facilitate the easy removal of the scruff after setting of the casting.

After the metal is poured, a very.

shoi ttime, fromthree to six seconds, it isv in place on the wall of the mold, and the third phase of the process of casting, which is freezing or crystallization ensues.

freezing begins at the mold and proceeds in one direction only, that is from the outside to the inside. The 'process may be likened to the growth of trees,fpremising that their roots are inthe moldand their trunks and lbranches pushing into thefreezing meta..

c The trunks u sh into the molteifniass, the

branches deve op and interlock, until the whole: cross-section is filled; the trunks roceeding somewhat faster than the brane es. This structure-from. which originated the expression dendrites, is visiblcfm steel and white iron,- but in gray iron it isobstructed or obliterated by the formation of graphite.

This se uence of crystal formation tends to cleanse the metal by actually pushing the impurities and inclusions out to the .in-

ner wall of the pipe, where they are incor-- porated in the still fluid slag. 1 Iirprior processes, where the metal cools from both core and mold, the impurities are of necessity trapped between the surfaces. The cleans ing action of crystallization-Lie facilitated and aided by centrifugal forceacting in the same direction, during the second, casting, and third, freezing, hases. Both forces operate to eliminate s ag, gases and sonims.

The betterment ofthe production obtained by centrifugal casting may be greatly increased by using molds of refractory material, as for mstance sand.- The sand mold permits slow cooling, which facilitates the operations above mentioned. In addition because of the slow cooling and slow crystallization, the casting is suitable for machining without annealing, which is extremely injurious to the inside gray strata of the pipe, tending as it does to cause the iron to grow. Since the "rowth is preferential, occurring more rapidly in the gray iron interior than in the white, exterior strains are set up in the outside which tend to weaken this part of the casting.

A uniform distribution of free carbbn in the form of graphite is desirable, since the machinability of the casting is dependent .upon the quantity of residual combined carbon, The deposits ofgraphite seaming the metal in every direction promote the easy cutting of the tool, since they are'weak and also provide lubrication. The deposits of free carbon, that is, graphite, should, however, be of small size, 'sinceeach flake forms a plane of separation. :in the sound metal, and the reduction in size and quantity increases the strength of the casting.

One of the primary objects of the resent invention is the provision'of' a-oentri gally cast, sand-molded hollow casting, as for in- The,

stance, a pi e wherein everypart of the pipe possesses t e same qualities of strength against stresses of every character.

' Another object is the provision of a cast ing of'the character specified,substant1ally free from impurities and from flaws, due to whorls through thewall from the exterior to the interior surface.

With these and other objects in view, the

invention consists in the improved casting as an article ofmanufacture.

In the drawings is shown a reproduction of a micro-photograph of the section through the casting wall.

The present case is a. division of applicationNo. 30,609, filed December 2nd, 1925.

The casting forming the'subject matter of the present invention is produced by spinning or centrifugal casting in a refractory or sand mold. To facilitate charging with a minimum of rebound and splashing, the

pouring is done with the mold in inclined position, The charge of moltenmetal is all,

or substantially all, poured while the mold is inclined. During the pouring, the mold is rotated at a relatively low speed, and the sand lining is uniformly heated by fleetin contact with the mass of molten metal, whic remains in substantiallya compact mass during the operation, ofierlng a minimum surface to oxidation. 0

During the charging, or near the completion of the charging, the mold is moved quickly but quietly into level position, to uniformly distribute .the metal longitudinally of the mold, andat or near the attainment of the moldto level position the speed ,of rotation is sharply increased, so .that the molten metal is thrown upon the mold wall and is held thereon by centrifugal force. Before the metal flows from the ladle it is surface skinnned, and as it flows into the mold, or at any other preferred time, a suitable slugging material. such as soda ash, is added. This material in a finely puli 'erized condition fuses and blankets the surface of the metal with a thin, extremely fluid sla and reacting with thesilica, carbon dioxide is produced, which drives out the oxygen in the mold," and provides a non-oxidizing atmosphere. Thesla ging material, which may begone orlmore ofa number of materials, knownas Purification by centrifugal action is furthered by the crystallization which progresses from the exterior toward the interior of the casting, the process of crystallization assisting gravity by pushing 1m uritles to the interior of the casting. The reezing or cooling of the casting progresses radially inward, and a casting is produced wherein the free carbon is in the form of flakes, particles and whorls of relatively small size, but uniformly distributed throughout the structure, as clearly shown in the drawings.

To facilitate comparative consideration of the present casting and those produced in the usual manner in sand molds, the wall may be considered with reference to three zones, namely, those lying adjacent to the outer and inner mold walls, designated A and C, and that between, designated as B. In hollow castings made in sand molds by the usual process, zones A and C are substantially the same, containing primary graphite in a matrix of pearlite with a very little cementite at the extreme edges. Zone Bcontains primary graphite of much larger size than that in the other zones, in a matrix of pearlite and usually with some free ferrite. The gra bite is in the form of flakes and grains 0 relatively large size. The flakes and grains are of relatively large size, and from the center there is a fairly steady decrease in size toward both surfaces.

In the hollow castings of the present invention, zone A consists in fine primary graphite in a matrix of pearlite, with practically no trace of free cementite, except when extremely hard iron is used. In zone B the graphite is usually in whorls (far superior to flakes) in an almost wholly pearlite matrix. In zone C the flakes of graphite are somewhat larger, being of approximately the same size as those of zone C, in pipes cast by the usual processes. The flakes are in a pearlite matrix, with usually quite a bit of free ferrite.

Zone B of the present )ipe occupies normally three-quarters of t e total cross section, and the flakes in zone 0 often run to the very surface of the specimen, a phenomenon which is only possible in castings of the present invention.

When the casting phase commences in pipes forming the subject matter of the present invention, that is, the laying of the metal on the mold walls, said walls are hot and dry, and the molten metal is in comparatively heav masses, with no gases trapped by splatterm and covered by a thin, fluid blan et of c emically active slag of lower specific gravity than the iron, much lower freezing point, and fluid through the freezing range of the iron and beyond it.

The improved i cast in the manner described, is remarEa 1y free from impurities of every character. There is no chilling of the outer surface such as occurs in casting in metal molds, and the structure of the pipe is uniform throughout, from end to end, and from inner surface to outer surface.

What is claimed as new is:

1; IA sand molded pipe, cast centrifugally from skimmed, surface slagged metal in a non-oxidizing atmosphere.

2. A sand molded pipe cast centrifugally from skimmed, surface slagged metal in a non-oxidizing atmosphere, and cooled slowly from the exterior.

3. A pipe composed of iron substantially freed from impurities b skimming, surface slagging and centrifuga casting.

4. A sand molded pipe of metal, surface slagged and centrifugally purified during casting.

5. A sand molded pipe of ferrous metal, surface slagged and centrifugally purified during casting.

6. A sand molded pipe of ferrous metal surface slagged and centrifugally purified in a non-oxidizing atmosphere during castmg.

7. An iron pipe of homogeneous metal purified by centrifugal casting and surface slagging, with the free graphite in the form of minute flakes, particles and whorls, extending to the inner face of the casting.

8. An iron pipe of homogeneous metal purified by centrlfugal casting and surface slagging, with free graphite in the form of minute flakes, articles and whorls in a matrix of pea ite, uniformly distributed throu hout the structure.

9. n iron pi e of homogeneous metal DOUGLAS T. BEATTY.

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