US1953180A - Cast iron annulus and method of manufacturing the same - Google Patents

Description

Patented Apr. 3, 1934 UNITED STATES CAST IRON ANNULUS AND METHOD OF MANUFACTURING THE SAME Frederick C. Langenberg, Edgewater Park, and Herbert G. Reddick, Burlington, .N. J., assignors to United States Pipe and Foundry Company, Burlington, N. J., a corporation of New Jersey No Drawing. Application August 20, 1932,

Serial No. 629,742

.8Claims.

Si. S. Mn. P.

w 3.00-3. 1.50-2.50 .o5-.1o .35-.80 .so-rs A typical example of an iron mixture ordinarily used for the casting of pipes in the way above indicated is as follows:

T.O. 0.0. St. S. Mu. P.

The object of our invention is, primarily, to provide an annealed, centrifugally cast cast iron pipe of materially increased ductility and, consequently, less brittleness than pipes as heretofore manufactured by the described process from irons of compositions coming within the above stated specification and to provide an improved method of manufacturing such pipes by means of which pipes of the improved quality and characteristics may be commercially produced.

Pipes as cast by the described method from irons coming within the specification given above have usually what we might call a triplex crystalline structure. In the outer stratum the structure is columnar and normal to the surface. In the middle stratum the structure is dendritic with more or less of a tendency of the dendrites to lie normal to the surface. In the inner stratum the structure is graphitic, the graphite existing as medium sized plates or flakes. The three structures will now be described in detail.

The principal component of the outer layer of this triplex metal structure is, to the best of our present knowledge, an eutectic of iron carbide 50 with austenite in various stages of decomposition through which is interspersed fine particles of phosphide of iron;

The central area, which is dendritic, is composed of patches 'of graphite which tend to form nests in a matrix of silico ferrite, pearlite and phosphide eutectic. The dendritic structure gradually disappears as the inside structure is approached. At the same time the phosphide eutectic becomes more pronounced and, when the inside structure is reached, there exist plates of graphite in a matrix of silico ferrite, pearlite and phosphide eutectic. It should be understood that the demarc tion between the three structures is not clearly efined but there is a gradual transition from one representative structure to the other. In castings, such as those described, it is possible, by chilling the metal very rapidly, particularly when silicon is in the lower limits of the range described, to practically eliminate the inside or graphitic structure. This is particularly true when the above conditions exist in conjunction with a thin casting. Under such a condition the outside structure and the structure described as the central structure remain unchanged in their characteristics.

Theexterior and middle structures previously described are characterized after casting by their hardness and brittleness, the outside structure being much harder and more brittle than the middle structure. It is also a well known fact and recognized in the arts that a cast iron casting made by any process in which the cooling is relatively rapid has present therein internal strains which are often objectionable. I

In order to diminish the hardness and brittleness of the cast pipes as well as to eliminate or modify any internal strains therein, it has been the practice to anneal the pipes substantially as described inthe patent to de Lavaud 1,280,418, the effect of this annealing treatment being to substantially eliminate the combined carbon, the result of which is to diminish the hardness of the outer and middle structure and to very greatly increase the ductility of the pipe but while "pipes cast and annealedas above describedhave been made, marketed, and used in very large quantities and have given satisfactory service in use, it would obviously be very desirable if their ductility could be further increased and we have discovered that this can be accomplished by sochanging the annealing conditions as to effect the elimination from combination of the carbon without at the same time bringing about the solution of the iron phosphide and iron phosphide eutectic to more than a minimum degree and we believe we are the discoverers of the fact that such castings can have their ductility materially increased if the iron phosphide and iron phosphide eutectic components of the casting are not taken into solution during the annealing of the casting to a greater extent than will bring about the presence in the finished casting of 65% or less of the phosphorus content of the casting in solution and, generally speaking, it is true that the smaller percentage of the phosphides dissolved in the ferrite the greater will be the ductility of the annealed casting. It must be understood that what we have said refers to annealed pipe in which the combined carbon has been reduced to a percentage of recognized tolerance, say, not to exceed 0.15% of the mass of the casting.

While our experiments have shown that the higher the percentage of undissolved iron phosphide and iron phosphide eutectic in the annealed casting, the greater the degree of duc tility, it is not possible in any cycle of heating and cooling, such as cooling down a casting in the mold or of annealing, to avoid the passing into solution of some percentage of the undissolved iron prosphide and iron phosphide eutectic existing in the unannealed casting and the percentage unavoidably brought into solution will vary with the thickness of the casting and with the duration of the heat treatment to which the ,casting is exposed and with the temperature of the annealing heats which we have found effective to eliminate the combined carbon without at the same time bringing all or an unduly large portion of phosphides into solution. An annealed casting which preserves a considerable portion of its phosphorus as iron phosphide or iron phosphide eutectic constituent in undissolved condition will show material increase in ductility in comparison with a similar annealed casting in which substantially all of the phosphides have been dissolved. In one specific experiment it was observed that a casting having 37% of its phosphorus as iron phosphide or iron phosphide eutectic had a resistance of impact approximately twice as great as a similar casting in which only 25% of its phosphorus as iron phosphide'or iron phosphide eutectic remained in the undissolved condition. In the case of comparatively thin castings of, say, a pipe with a wall thickness of .34, we have found it quite practicable to substantially eliminate combined carbon from the casting without effecting the solution of more than 50% of the phosphorus in the iron phosphide or iron phosphide eutectic constituent of the iron.

While it is true that the iron phosphide and iron phosphide eutectic present in undissolved form in a pipe centrifugally cast in a chilled mold from an iron coming within the specification which we have given above will dissolve to a greater or less extent during the process of annealing at all temperatures above that of the critical point below which carbon is not eliminated from its combination with the iron, we have found that the rate of solution diminishes rapidly with decreasing temperature and that by properly regulating the temperature to which the cast pipe is exposed during the process of annealing it is entirely practicable to eliminate substantially all of the combined carbon without effecting the solution of more than 60% of the phosphorus in the phosphides. A practical an nealing process must, however, have due regard to cost and time of treatment and these considerations, as well as the elimination of the combined carbon and the maintenance in the annealed casting of not less than 35% of the phosphorus in the phosphides in undissolved condition. have been in our minds in devising and perfecting our improved annealing method, which may be described as follows.

We heat the pipes as rapidly as is practicable with avoidance of bringing about such inequalities in temperature in different parts of the pipe as would produce injurious strains, to a temperature between 1650 F. and 1725 F. In practice, we hold this high temperature at approximately 1700 F. Heating to this temperature will in a short time, initiate the decomposition of even the most refractory carbides which occur in the outer stratum of the casting, thus making possible the substantially complete elimination of combined carbon from the casting by treatment at lower heats and within practically permissible time limits. By limiting this high heat treatment to temperatures not exceeding 1725 F. we accomplish the necessary initiation of decomposition of the more refractory carbides without at the same time taking in solution more of the iron phosphide and/ or iron phosphide eutectic components of the casting than is consistent with the production of a finished casting in which not more than 65% of the phosphorus will be found'in solution. Having effected the necessary initiation of decomposition of the refractory carbides, we then reduce the rate at which phosphorus is being taken into solution by reducing the temperature of the casting as rapidly as practicable and at a rate which will not bring about dangerous inequalities of temperature in different parts of the casting, to a range of temperatures between 1450 F. and 1350 F. and we hold the casting within this range of temperatures for a suflicient length of time to bring about the elimination of combined carbon down to a percentage not exceeding 0.15% of the mass of the casting. This practically concludes the annealing process, though we would add that the pipe should be progressively cooled from the range of temperatures between 1450" F. and 1350" F. to a temperature of approximately 1200 F. to avoid possible mischievous inequalities in temperature in different parts of the casting and after having reached a temperature of approximately 1200 F. a pipe can be cooled in air without mischievous results.

In developing our method as a practical process of manufacture, we have annealed our pipes in a furnace of the general type illustrated and described in the patent to Clark, Number 1,856,863, granted May 3, 1932. The length of the furnace with which our development work has been done was 62 feet, 6 inches, and the furnace used, of course, supplied with oil burners and ventilating apertures for regulating the temperature of the different zones through which the pipes passed in moving from the entrance to the exit openings at the ends of the furnace. Operating upon pipes having the distinctive crystalline strata characteristics of pipes cast in the manner previously described and of a diameter of 4 inches and wall thickness of .32 inches and feeding the pipes to the annealing furnace at a temperature approximating 1200" F., we find it practical to raise the temperature of the casting to approximately 1700 F. in 13 minutes and to effect a suflicient initiation of decomposition in the more refractory carbides by holding the casting at or about this temperature for a period of 5 minutes. The progressive cooling of the casting from approximately 1700 F. to 1450" F. was satisfactorily accomplished in 7 minutes and by holding the casting point below 0.15% of the we: the casting. The progressive cooling of the pipe from 1350 F.

to 1200 F. was satisfactorily effected in 5 minutes and the pipes were out injurious results.

Operating upon pipes of 24 inches in diameter and wall thickness of 0.8 inches, the preliminary heating of the pipes from an entrance temperature of 1200 F. to approximately 1700 F. required, under the conditions with which we were working, 27 minutes and to insure an initiation of decomposition in the more refractory carbides we found it advisable to maintain the casting at temperatures between 1650 F. and 172'5" F. for a period of 10 minutes. The cooling from1650 F. to 1450 F. progressed through a period of 16 minutes and to effect a substantial elimination of combined carbon we held the pipes between 1450 F. and 1350 F. for a period of 10 minutes and the cooling of the casting from 1350 F. to 1200 F. was progressive through a period of 9 minutes.

Working with pipes of larger size and thicker then cooled in air withwalls the times of exposure to the different heats tory results can be obtained with the largest and heaviest castings by heating the castings up to the range between 1650 F. and 1725 F. in a period not to exceed 40 minutes and by holding the castings between 1650 F. and 1725 F. for a period not to exceed 15 minutes, then progressively cooling the castings to 1450 F. through a period not to exceed 24 minutes and then holding the casting at temperatures between 1450 F. and

1350 F. for a period not to exceed 16 minutes.

With very heavy pipes the period of cooling from 1350 F. to 1200 F. may be as long as 14 minutes.

' From the facts given above, it will be obvious that our process for producing from a cast pipe of the character specified, an annealed pipe from which the combined carbon has been practically eliminated and which will not have dissolved phosphorus in excess of 65% of the total phosphorous content of the metal, can be carried on under thoroughly practical commercial conditions and that. the points to be had in view for the production of the best results, that is, of pipes containing dissolved phosphorus in the smallest possible percentage,- are, first, that the pipe should not be held at the range of temperature between 1650 F. and 1725 F. for a longer period than that necessary to initiatethe decomposition of the more refractory carbides; second, that the progressive heating of the pipes up to 1650 F. and the progressive cooling of the pipes from 1650" F. to approximately 1450 F. should be carried on as rapidly as practicable so as to avoid as much as possible the taking into solution-of phosphorm during these stages of the annealing pracess, and; third, that the casting should be held at the temperatures between 1450 F. and 1350 F. only for such length of time as will result in the elimination of combined carbon to a point not exceeding 0.15% of the mass of the casting. a

The distinguishing characteristics of our improved pipe are, in the first place, the composition of the iron of which it is formed, that is to say, composition coming within the specification given above, of which the important considerations are that the phosphorus should exceed .30% and the silicon should not exceed 2.50%. The second distinguishing characteristic is that the annealed pipe preserves in outline at least the distinguishing crystalline structure which we have described as peculiar to the castingas it comes from the mold. The third distinguishing characteristic is that our improved casting re tains in its structure not less than 35% of its phosphorous contents in the form of undissolved 8 iron phosphide and/or iron phosphide eutectic. The fourth distinguishing characteristic is that our finished casting, while containing not more than 65% of its phosphorous content in solution, has a combined carbon content not to exceed 0.15% of the mass of the casting and the fifth distinguishing characteristic is the material increase in ductility of the casting as compared with annealed castings made from, similar iron compounds and treated by previously known and practised annealing methods.

Apart from'the materially increased ductility of pipes manufactured by our improved process, the process has the imp artant advantage of having the tensile strength of the iron at a temperature under 1725" F., very much greater than is the case when the casting is heated to ahigher degree, say to 1800 F. as was customary and this enables the pipe to be handled in the annealing furnace much more freely and with less pre-.

C. Si. S. Mn.

3.00-3.85 1.50-2.50 .05-.10 .3&.80

said annealed annulus having a wall structure made up of concentric annuli, each retaining in outline the distinctive crystalline formation successively occurring at different distances from the outer surface of the annulus when irons coming within the above specification are cast in an externally cooled centrifugal metal mold, having,

not more than 65% of its phosphorous content in solution and having not less than 35% of its phosphorous content present in the form of undissolved iron phosphide and/or iron phosphide eutectic, said annealed annulus having not more than 0.15% of combined carbon with its uncoxnbined carbon in graphitic form.

2. An annealed cast iron annulus as called for in claim 1, in which the phosphorous component of the metal of the casting in solution does not exceed 60% of the total phosphorous component. 3. An annealed cast iron annulus as called for in claim 1, in which the phosphorous component of the metal of the casting in solution does not exceed 50% of the total phosphorous component. 4. The method of manufacturing annealed,

centrifugally cast, cast iron annuli from iron compounds coming within the following specification: v j V 0. 'sl. '8. r.

which consists in casting said iron compound in the form of an annulus in an externally cooled metal centrifugal mold, thereby'producing a cast anulus having a wall structure made up of concentric annuli, each having the distinctive crystalline structures successively occurring at different distances from the outer surface of the annulus characteristic of annuli so cast and an annular casting in which the phosphorous component of the metal is, to a large extent, present in the casting in the form of iron phosphide and/or iron phosphide eutectic; then annealing said cast annulus by raising its temperature to a point above 1650 F. and not to exceed 1725 F., for a sufiicient time to initiate but not complete the decomposition of even the most refractory carbide constituents of the casting, then progressively reducing the temperature of the casting until said temperature reaches a range between 1450" F. and 1350 F., to reduce the rate at which phosphorus is taken into solution and then holding the casting within said last mentioned range of temperatures until the combined carbon does not exceed 0.15% of the mass of the casting and then progressively reducing the temperature of the casting, so as to avoid annulus to the range of temperatures between 1650 F. and 1725 F. in a period not to exceed 40 minutes.

6. In the method of manufacturing annealed, centrifugally cast, cast iron annuli, as called for in claim 4, the step which consists in maintaining the annulus within a range of temperature between 1650 F. and 1725 F. for a period not to exceed 15 minutes.

7. In.the method of manufacturing annealed, centrifugally cast, cast iron annuli, as called for in claim 4, the step which consists in progressively lowering thetemperature of the annulus from the temperature of 1650 F. to a temperature of 1450 F. in a period not to exceed 24 m1nutes.

8. In the method of manufacturing annealed, centrifugally cast, cast iron annuli, as called for in claim 4, the steps which consist in progressively raising the temperature of the annulus to the range of temperatures at or above 1650 F. in a "period not to exceed 40 minutes, holding the annulus within a range of temperatures from' 1650 F. to 1725 F. for a period not to exceed 15 minutes, progressively reducing the temperature from a point at or above 1650" F. to a point within the rangeof 1450 F. and 1350 F. during a period not to exceed 24 minutes.

- FREDERICK C. LANGEN'BERG.

HERBERT G. REDDICK.