US2931744A - Method of grain refining centrifugal castings - Google Patents

Description

M. L. SAMUELS April 5, 1960 METHOD OF GRAIN REFINING CENTRIFUGAL CASTINGS Filed Aug. 26. 1957 5 Sheets-Sheet 1 k u P SAMUELS BY W ATTOR E April 5, 1960 M. L. SAMUELS METHOD OF GRAIN REFINING CENTRIFUGAL CAS TINGS Filed Aug. 26, 1957 5 Sheets-Sheet 2 U WM MM. 3 L m T R A M April 5, 1960 M. SAMUELS METHOD OF GRAIN REFINING CENTRIFUGAL CA STINGS Filed Aug. 26, 1957 5 Sheets-Sheet s 3mm MARTIN L. SAMUELS April 5, 1960 M. L. SAMUELS METHOD OF GRAIN REFINING CENTRIFUGAL CASTINGS 5 Sheets-Sheet 4 Filed Aug.

3mm MARTIN L; SAMUELS FIGx METHOD OF GRAIN REFINING CENTRIFUGAL CASTINGS Martin L. Samnels, Mount Holly, N.J., assignor to United States Pipe and Foundry Company, Birmingham, Ala, a corporation of New Jersey Application August 26, 1957, Serial No. 680,298

6 Claims. (Cl. 148-115) This invention relates to improving physical characteristics of certain metal products, and more particularly it relates to refinement of grain structure of ductile metal tubular castings and to the greatly improved resulting products.

In recent years centrifugal tubular castings have gained widespread acceptance for varied applications, while at the same time considerable progress has been made in techniques of casting and in casting quality. Such gains, however, have to a large extent been confined to castings of metal which undergo recrystallization, or allotropical conversion of crystal form, with change in temperature alone. This is primarily for the reason that there is considerable concern among most engineers as to the reliability and/or acceptability of castings of metals. The concern is based largely on the fact that a casting of any metal results in a productyhaving a coarse crystalline structure, either columnar in metal mold centrifugal castings or random in sand mold castings. It is generally believed that such coarse crystalline structure is deleterious as regards a number of important properties and, due to the coarse crystalline structure, castin gs do not possess tensile strengths sufiiciently high for compliance with the usual code specifications.

A considerable number of stainless cast metals can be subjected to heat treatment to bring about recrystallization and a final structure of fine grains, thus removing the objection based on grain size. Included in this group, and possessing critical points or allotropical conversion of crystal form with change in temperature, are the ferritic and martensitic steels. berv of applications, and particularly today when more and more tubing is required for high temperature and high pressure usage, the austenitic steels are required due to their superior ductility, high-temperature strength, and corrosion resistance.

Castings of austenitic steels have not, however, found' ready acceptance in a great number of applications due to the disadvantages mentioned, and instead austenitic forgings have been used when the properties of such metals are required for tubing. Forgings of austenitic steels are not without their disadvantages, however, the most noteworthy of which are the extremely high cost as compared to castings, and the directionality or anisotropic eifect for wrought steels, an effect not inherent with cast steel. As regards cost, and by Way of example, theonly source of seamless wrought cylinders of long lengths in size ranges of about 2.0 inches to 48 inches in diameter is from hot forging from the. ingot stage. and the resulting tubes must of necessity demand unusually high prices. Regarding directionality, this anisotropic efiect is reflected not so much on tensile strength;

For a large num-' Such operations are extremely involved and costly,v

2,931,744 Patented Apr. 5, 1960.

as on loss of ductility in tension tests and by loss of toughness in impact tests. Such effect, therefore, works contrary to the reasons for choosing austenitic steels over ferritic and martensitic steels in the first place.

A principal object of this invention is, accordingly, to

provide a method whereby the properties of tubular castings of ductile metals can be improved to overcome the objections noted for such castings;

Another principal object is to provide tubular shapes, of ductile metals without the disadvantages heretofore possessed by such shapes;

Still another object is the provision of a method for recrystallization on change of temperature alone, whereby the normal coarse crystalline structure thereof is converted into a fine grained crystalline structure; I An additional 'object is the provision of a method for changing the crystalline structure of centrifugally cast austenitic steel tubular castings from large columnar crystals to fine grained crystals, with the attendant improvement in physical properties, and to the resulting improved tubular product. a

With the above objects in view, and others which will become apparent from the following specification, the invention will now be 'described in detail, reference being made to the accompanying drawings in which:

Figure 1 is a view partly in section of an apparatus which may be employed in the method of this invention,- and in which the casting is in position to be expanded,

Figure 2 is a view similar to Figure 1, but after the casting has been expanded;

Figure 3 is a view of another apparatus similar to that illustrated in Figure l, but with a dimension-controlling envelope;

Figure 4 is a photograph of a macroetched stainless steel casting about actual size;

Figure 5 is a photomicrograph of the casting of Figure 4, etched, at diameters magnification;

Figure 6 is a photograph of the casting of Figure 4, macroetched, after expansion and annealing in accordance with this invention, about /2 actual size;

Figure 7 is a photomicrograph of the expanded and annealed casting of Figure 6, etched, at 100 diameters" magnification;

Figure 8 is an actual size photograph of a macroetched Figure 10 is a photomicrograph at 1000 diameters magnification of a section of hot rolled seamless tubing,

unetched;

Figure 11 is a photomicrograph at 1000 diameters" magnification of a section of a cold expanded and annealed tube;

Figure 12 is an actual size photograph showing the surface appearance of a stainless steel tube after cold expansion;

Figure 13 is a more detailed view of the surface shownin Figure 12, photographed at -2 cliameters"magnification.

I The objects are attained by this invention whereby tu-; bular castings are subjected to internal pressure to expand:

them appreciably and to a degree sufficient to substantially deform the cast crystalline structure, followed by annealing sufiicient to effect substantially complete recrystallization. The expansion of the tubular castings is essentially conducted under such conditions that the outer surface is free and unconfined. Preferably, the expansion is achieved by application of hydrostatic pressure to the inside of the casting, after closing both ends. The expanded tube is finally heated to the knownannealing temperature for the particular metal, and water quenched.

' Prior to this invention, there was no basis for a prediction that metal tubes could be treated in such manner to effect such drastic grain refinement. Known methods of cold deformation are entirely different. In this invention, for example, radial stresses are applied to the cylinder rather than the axial stresses of prior cold working methods. Furthermore, the outer surface of the cylinder is free and unconfined, and the internal pressure subjects every part of the cylinder, including any existing slightly defective areas, to rigorous stresses. In conventional methods the material is confined between two working surfaces at the point where cold plastic deformation is effected, e.g. between two rolls in cold rolling, between a mandrel and die in cold drawing, and between a die and hammers in cold swaging or rocking. This support between two working surfaces is, of course, much more .fav-

orable toward heavy cold working without rupture than,

Prior methods of hydraulic exsmaller than the inside diameter of the cylindrical casting.

3. The inside diameter of the remainder of the core piece, between the two ends, is less than at the ends. Grooves 4 are provided on the periphery of the core piece 2, near the ends, and O-ring gaskets 5 areplaced in each of the two grooves. The casting 3 is then slipped over the core piece 2. A tube 6 connects a source (not shown) of high pressure water to anopening 7 through the wall of core piece 2, communicating with the space 8 between the core piece and the casting. Heavy hold-down rings 9 are fastened around the outside of the casting opposite the grooves 4 and gaskets 5 to prevent expansion of the cylinder at the ends.

Water pressure is then applied to space 8 to effect expansion of the cylinder as shown in Figure 2. After expanding to the desired increase in diameter the pressure is released, the hold-down rings 9 are removed, and the core piece 2 taken out of the expanded casting. The expanded cylinder is then annealed and the unexpanded ends cropped off.

Pressure levels requirde can be estimated fairly closely through use of the formula:

PXD ST- 2t in which ST represents hoop stress in pounds per square inch, P represents gage pressure in pounds per square inch, D represents the outside diameter of the cylinder and t represents wall thickness.

Since the cylinder will not be expanded to its breaking point, and the increase in diameter partially counteracts cold work strengthening, actual gage pressuregrequired will be somewhat below the calculated pressure.

For 'many service applications, the expanded and annealed cylinder .can be used in that condition without;furtherrnachining. Inasmuch as the .operablemetalsdonot normally a room temperature operation ranging from about 50 F. to about 100 F. Lower temperatures tend to reduce the amount of cold expansion necessary to reballoon up, but rather strengthen in the most severely worked areas to then cause deformation to take place 'in' areas less severely stressed, the tendency is toward relatively uniform expansion. When high dimensional precision in the finished product is required, the expanded and annealed cylinder is machined.

For applications requiring high precision, a die 10 can be employed such as that illustrated in Figure 3. The die is bored to the required inside diameter, equal to the outside diameter to which the casting 3 is to be expanded, and placed around the casting. The diemerely prevents some parts of the cylinder from slightly exceeding the desired diameter while other parts are still slightly undersized; The die consequently permits less cut depth in final machining to specified sizes. The die or container is nota working surface, but is merely a dimension-controlling envelope. All useful work, as far as conditioning for .recrystallization is concerned, is completed before the expanding cylinder 3 contacts the inner wall of the container 10.

There are, of course, several ways of sealing the cylinders for cold expansion other than that specifically illustrated. Such sealing means, including those which permit expansion of the entire length of the casting, are well known.

Apparatus and means for accomplishing heat treatmen of cylinders are well known and are not specifically illustrated. Any heat treating arrangement which permits heating of cast cylinders to the required temperature, followed by water quenching, is satisfactory. Nevertheless,

the'heat treating or annealing step constitutes an essential feature of this invention, and is required in orderto relieve the stresses effected by cold expansion and to effect the re crystallization. The austeni-tic stainless steels allrequire ahigh'temperature solution anneal for, optimum corrosion resistance in any case. After cold expansion in accordance with this invention, the known and standard anneals do a completely satisfactory job of removing work-hardening effects and promoting recrystallization and grain refinement. Stand ard annealing temperatures for the various austenitic steels are listed in several reference handbooks, including, for example, The Making, Shaping and Treating of Steel," 6th edition, page 1322.

latter property is important in that it eliminates materials which will not work harden (become stronger. with increased cold plastic deformation) and which will balloon in the thinnest area causing rupture before muchchange is effected in areas where the wall thickness is slightly greater. the thinnest part of the wall will first expand and become strengthened, temporarily halting expansion at this point.' The thicker areas next expand beyond the yield strength,v resulting eventually in substantially uniform expansion over the entire tube.

Particularly suited for the invention are the AISI,300 series stainless steels, copper and copper alloys (the brasses and bronzes) and many nickel .base alloys.

Cold expansion in accordance with this invention is sult in recrystallization during the subsequent anneal but they also increase the tendency to rupture during the.

Speed of cold expansion is not particularly critical :ex cept that iexplosive rates should be avoided. .A .rnaxi- With materials which do work harden,'

mm rate of outside diameter increase of /2" per minute is safe and this varies the total time of expansion automatically with the diameter of the casting. For example, the fastest time in which a outside diameter casting should be expanded to 12" outside diameter increase) would be four minutes, whereas the fastest time for expansion of a casting to 48" diameter (also 20% increase) would be 16 minutes. Normally, the times involved will tend to be somewhat slower.

As already mentioned, one of the important features of this invention is the reduction in grain size of cast metals brought about by cold expansion and annealing. Figure 4 shows the very long columnar grains which are typical of metal mold castings of the type with which the invention is concerned. The illustration is an actual size photograph showing the maerostructure from a metal mold centrifugal casting of Type 304 stainless'stcel. Many of these individual crystals or grains run completely through the Wall thickness, and, in castings of greater wall thickness, the crystals are still longer. Another indication of the size of crystals can be seen by 5 a series of experiments utilizing identical metal compositions but with dilierent grain sizes. One portion of. a single 500 lb. electric furnace heat was poured into a dry sand static mold, producing the well known keel block type of casting, and then the other portion was poured into a heavy metal mold rotating at high speed to produce a typical centrifugal casting. A part of the centrifugal casting was subsequently subjected to a hot forging operation to produce rounds after a 4 to 1 reduction. This experiment thereby provided specimens possessing three different grain sizes: (1) coarsely columnar from the metal mold centrifugal casting, (2) coarse but randomly oriented grains of intermediate size from the static sand mold, and. (3) fine equiaxed grains from the hot forged rods. All 'three materials". originated directly from the same heat of steel s o va r ie, ations in minor compositional elements such as 'hydro-. gen, oxygen, or nitrogen, were virtually eliminated. Five steels were included in the experiments, and the mechanical properties are shownin Table I, in each case after water quenching from 1900 F.

Table I Yield Tensile Elon- Red. or V-notch Brlnell Steel Grain Strength, Strength, gation, Area, Charpy Hard- Type p.s.i. p.s.i. Percent Percent Impact, ness Ft. Lbs.

AC1 HF Alloy- 1 11,900 95,500 40.0 37.3 g 52.0 212 ACI HF Alloy- 2 49, 750 94,250 13. 5 15.9 19.0 223 AOI HF Alloy- 3 50,000 110,250 45.0 64.2 120.0 207 AISI Type 316;..- 1 28, 400 66, 750 57. 0 57. 5 120. 0 149 A181 Type 316- 2 39, 875 83, 500 42. 5 40. 1 79. 5 170 AISI Type 31 3 34, 400 80, 750 55.0 76. 3 120. 0 143 AC1 HT Alloy- 1 39, 100 85, 000 11. 0 9. 2 5. 5 V 201 A01 HT Alloy- 2 45,500 70,000 3. 5 4.7 5. 5 207 A01 HT Alloy 3 43, 750 100,000 29.0 44. 3 45.0 186- .AISI Type 304 EC 1 ,250 250 58.0 71. 8 120.0 137. 11151 Type 304 ECL- 2 33, 500 72, 000 .53. 5 67. 5 120.0 134 A181 Type 304 ECL. 3 40, 750 89, 250 56. 5 74. 4 120. 0 159 A01 Hl-I Alloy 1 46,250 79, 750 8.0 8. 5 3. 5 217 A01 HH Al1oy 2 47, 370 70, 000 3.0 2. 3 5. 5 217 A01 HH Alloy 3 56, 750 113,000 35.0 47.8 62.5 217 1=centricoarse; 2=static-coarse; 3=forging-flue.

reference to Figure 5, which is a photomicrograph of the casting of Figure 4, etched, at 100 diameters magnification. By way of comparison, wrought items are normally' purchased by reference to grain sizes (Nos. 1-8) as specified by the American Society for Testing 'Materials. ASTM Grain Size No. 1 refers to a structure having from -14, to 1% grains per square inch as viewed at 100 diameters, while Grain Size No. 8 refers to a structure having from 96 to 192 grains. Thus the ratio between grains shown in Figure 4 and ASTM No. 8 size is about 1000 to 1 or more. The ratio between grains shown in Figure 4 and ASTM No. 1 size is about 100 to 1. The decrease in grain size effected in accordance with the method of this invention is shown by reference to Figure 6 which is an actual size photograph (macroetched) of the casting shown in Figure 4, but after expanding the outside diameter about 15.5% and annealing. It will be observed that the coarsely columnar crystals evident in Figure 4 have been completely replaced by much smaller equiaxed crystals; This fact is still further illustrated in Figure 7, a photomicrograph of the structure of Figure 6, talren at 100 diameters magnification. The degree of crystal size reduction is clearly evident from a comparison of Figures 5 and 7.

Grain size is important only because of the properties affected thereby. The effects of coarse grain size are manifested in several ways: (1) somewhat lower mechanical properties, (2) increased susceptibility to grain boundary corrosion, (3) inferior machining characteristics, and (4) rough or orange peel surface developing upon severe cold forming or deep drawing a finished machined article. 1

The effect of grain size on room temperature mechan-- ical properties, while generally accepted, is illustrated by With respect to inferior grain boundary corrosion resistance, the consensus of rnetallurgists is in line with that of Pierce and Grossman, The Book of Stainless- Steels, Thum, published by American Society for Metals, 1 page 164: 7

Grain size is a potential factor in intercrystalline corrosion. When material is fine grained, the grain boundary extent being this much greater, the severity of attackin any individual grain boundary is less than in coarse grained material.

Machinability, and especially the ability to take on a fine finish without special techniques, is also favored by fine grain. This statement refers to 300 series auste'nitic stainless steels which are soft and .draggy by nature, rather than to carbon and low alloy hardenable steels.

Orange peel or pebbly surfaces after deep drawing or severe cold forming are to be expected from coarse grain materials. Slip-or deformation take place along crystallographic planes so the coarser the grain, the rougher will be the surface after cold forming. Fine grained material deforms in exactly the same m'annemr but with smaller crystals the slip planes are too short to produce surface roughening. It will be apparentthat freedom from orange pee is highly desirable on tub'ular products which are finish machined and then subjected to one of several types of' cold forming operationsby the ultimate user.

I have chosen to describe the limits of this invention-v in terms of percentage increase in outside diameter necessary to effect a given grain size and, consequently, a given physical property. The relationship between grain size," and physical properties and-percentage increase in diam-'- eter is illustrated in Table II which gives the properties' obtained by expanding the casting shown in Figures- 4 Table II 1 Yield Tensile Percent- Reduc- Treatment Prior to Strength, Strength, age Elontlon of Testing psi. 7 p.s.i. gation Area,

. Percent AS Cost 'Structure 26, 500 72, 200 G5. 3 64. 8 Expanded 15.5% Anhealed 34, 000 82, 650 67. l 73. 2

The-results in Table II show generally superior properties for the fine grained structure, both higher strength and better ductility, whereas in normal steels, which undergo phase change with temperature, increased strength isobtained. at the expense of elongation, reduction of area, and impact resistance.

Utilizing the percentage of expansion of outside diarn-' Table III Yield Ultimate Percent Reduc- Percent Expansion Strength, Strength, Elen tion of psi. p.s.i. gatlon .Area,

Percent 32, 200 G9, 900 54. 3 72. 33, 800 70, 500 56. 5 68. 0 34, 400 72, 100 57. 9 69. 2 35, 500 72, 100 48. 6 69. 5 37, 000 75, 500 53. 6 66. 0 200 79, 800 (ll. 4 68. 3 36, 900 81, 400 65. 0 70. l 36, 200 82, 200 64. 3 72.0 36, 200 S2, 200 62. 8 72. 2 36, 600 83,000 62. 8 68. 9 36, 600 83, 500 64. 3 71. 6 '37, 200 83, 200 G2. 1 73. 1 36, 500 3, 600 62. 8 72. 2 37, 500 S4, 300 62. 1 70. 9 37, 900 84, 600 63. 6 72. 8 37, 200 500 65. 0 70. 4 37, 600 84, 800 63. 6 70. 7 37, 200 8'1, 900 61. 4 68. 5 37, 500 85, 200 60. 7 70. 7 37, 000 85, 800 62. l 68. 5 37, 300 800 66. 4 7l. 7 37, 600 84, 800 G9. 3 69. 4 36, 400 85, 200 67. l 71. 7 37, 300 85, 800 67. l 71. 4 37, 400 85, 800 66. 4 69. 7 36, 790 83, 400 65. 8 74. 0 37, 200 85, 800 64. 3 70. 5

These data. show that only a little recrystallization is.

effectedby expansion up to about 8%. From 8% to about 16% the most rapid rate of change in recrystallization occurs. Expansion beyond about 18 or 20% results in small increases in strength by virtue of a slight decrease in the recrystallized grain size, but the phenomenal change associated with the disappearance of the coarse cast grain size occurs between about 8 and 16% expansion. These data indicate, therefore, that a Type .316 stainless steel cylinder, expanded at least about 16% and annealed,.will exhibit a refined or recrystallized grain size with all the virtues pertaining to small grain products.

Similar data obtained with stainless steels of, other compositions indicate that the same order of expansion is applicable as with Type 316. Since a survey such as that described for Type 316 is laborious and time-consuming, a simpler and quicker means is helpful in determining the minimum degree ofexpansion required for recrystallization upon subsequently annealing. In going from one type of 300 series austenitic stainless steel to another, for exampie,or in taking up a new alloy, some simple means for determining the minimum amount is machined to predetermined dimensions. The specimen is then pulled apart in a tensile machine, annealed, split longitudinally, and macroetched as shown in Figures 8 and 9. Knowing the breaking load and the original cross-sectional area at the point where the recrystallization and unaifected structures meet, it is then possible to establish rather closely the load required to produce recrystallization. Using the data thus obtained as a guide, tensile rods are then stressed at three levels: (a) at the load corresponding to the transition from coarse to fine grains, (b) at a load somewhat higher, and (c) at a load somewhat lower. Elongation values are listed for these stressed but unbroken tensile rods. The rods are then annealed, remachined into standard tensile specimen and tested. The test results are used to determine what minimum expansion in diameter should be reached upon expanding the corresponding tube.

It will be apparent to those skilled in the art that expression of the degree of cold working, to be effected by the method of this invention, in terms of percentage increase in outside diameter necessarily limits the method to relatively large cylinders and to thinner walls in the smaller size range. This is for the reason that in smaller cylinders, with relatively thick walls, the percentage increase in inside diameter will be considerably greater than for the outside diameter. Inasmuch as the maximum degree of cold deformation effective to give recrystallization is limited only by the point of rupture of the cylinder, the method can be applied to cylinders of any size so long as minimum expansion of the outside diameter does not subject the inside diameter to suflicient expansion to cause rupture. Smaller diameter and thicker wall cylinders can be treated according to this invention by stepwise expansion, however. That is, the cylinder can be expanded to an intermediate degree, annealed, and then subjected to additional expansion and annealing steps until the desired degree of grain refinement is achieved.

Another novel feature of this invention concerns the fact that cold working is accomplished without either elongating or shortening the cast cylinders or having any contact with unyielding dies or hammers on either the outside or the inside diameter surfaces as is the case in cold drawing or swaging. The coarse crystals adjust themselves to accommodate the increase in diameter.

through block movements in an outward rather than in a longitudinal direction.

degree. Since the effects of this crystal elongation are not completely removed even upon a subsequent anneal, anyone familiar with these well known metallurgical principles will see the advantages of avoiding the preferred orientation inherent in conventional cold drawn cylinders. In our cold expanded casting, involving no significant change in length, the individual crystals are not elongated to any substantial degree in a longitudinal direction.

Another important feature of this invention is that cast cylinders are converted into cylinders having the desirable mechanical properties of wrought products, but

without imparting directional properties. Wrought prod- This results in a characteristic; coarsely crinkled or heavy orange peel elfect on bothncts commonly originate from large ingots by great reductions in cross-sectional area and corresponding elongai tions either through hot forging or rolling and/or cold finishing passes. Selective freezing in the large ingot results in primary dendrites consisting of large proportions of the highest melting element or phase, iron for example, with lower melting constituents, such as compounds of phosphorus, sulphur, and carbon, being concentrated to some degree in the interdenritic or interstitial areas. When the ingots are rolled out, non-metallic inclusions and the comparatively impure interstitial metal areas are elongated, thinned and flattened in the same proportion. When the non-metallic inclusions are flattened out, they occupy a greater percentage of the crosssectional area, so ductility in specimens taken at right angles to the direction of rolling is decreased. Figure 10 is an illustration of the effect on non-metallic inclusions of forging operations. The elongated non-metallic inclusions are clearly evident at several parts of the photograph (x 1000) which is an unetched section from hot rolled seamless tubing used in the recoil mechanism of U.S. Army tanks. The marked cross-sectional reduction and elongation from the ingot stage to a relatively thin walled cylinder has flattened and elongated the inclusion.

Steels differ as to this banding or fiber, varying with the cleanliness and purity of the steel as well as the percent reduction from the ingot to the final product. However, virtually all commercial wrought products do show different properties when tested in directions parallel to and at right angles to the direction of working, i.e.

they are not isotropic. Non-metallic inclusions are commonly pointed to as the major cause for directional properties, but in at least some instances a two-phase microstructure, if present at the time of rolling, results in distinct banding or layering of the two microscopical constituents. In certain instances, these structural constituents exhibit markedly different mechanical properties, so that directional characteristics are present to an extreme degree.

In contrast to wrought materials, castings do not show directional properties,i.e. they are isotropic, since they are not subjected to mechanical working. The cold expanded and annealed cylinders of this invention are changed in shape so little that they are also isotropic. For example, a Type 304 stainless steel casting 6.59" CD. by 5.21" I.D., 6 in length, was cold expandedto an CD. of 7.75" (17.6% increase), annealed by heating to 1950 F. and water quenching, and tested in both the transverse and longitudinal directions with the following results:

Table IV Yield Tensile Elonga- Reduction Test Direction Strength, Strength, tion, Perof Area, p.s.l. p.s.i. cent Percent Figure 11 is a photograph (x1000) of an unetched section of the cold worked cylinder the properties of which are shown in Table IV. The dramatic difference in effect on non-metallic inclusions will be seen by comparison with Figure 10.

The cold expansion of cast cylinders in accordance with this invention possesses another important advantage, distinct from the grain refinement and attendant improvement in mechanical properties. That is, the cold expansion helps overcome long-standing prejudices against the use of castings for critical services. Even with the best of non-destructive testing and inspection, there is a fear that some under-surface defect, e.g. a hot tear or blowhole, may be present and result in sudden failure of a hazardous nature. Since metal tubing is never intentionally placed in service under stresses above its yield strength, these cold-expanded cylinders will already have been subjected to stresses far in excess of service expectancy. Any gross defect will lead to failure during the cold expansion in manufacture. The method 0E this invention therefore constitutes a more rigorous proof test than any used heretofore for castings;

The process of this invention also provides a means for forming large diameter seamless tubing or cylinders from smaller castings. By cold expanding, annealing, andre-expanding and annealing several times, a very large increase in diameter is obtainable. Repeated expansions with intermediate annealings will result in somewhat smaller grain sizes than is the case with a single expansion. The cold deformation is not additive, however, since annealing removes cold work introduced by the preceding expansion, and restores ductility. The second and successive expansions, however, will be applied to cylinders having comparatively small grain sizes and when an equal degree of cold expansion is added, it is equivalent to a somewhat greater cold reduction on the coarse grain casting. While no further appreciable decrease in grain size will result from annealing and repeating the expansion, the increase in cylinder diameter is accomplished.

We claim:

1. The method for producing tubular metal articles of a work-hardenable ductile metal the grain size of which cannot be reduced by heat treatment alone, having an average grain size no larger than ASTM No. 1 and having isotropic physical properties which comprises, expanding a centrifugally cast tube of said metal at least about 16 percent in outside diameter by the application of uniform radial pressure simultaneously throughout the interior of said cast tube while the outside of said cast tube is unconfined, heating the so expanded cast tube to a temperature to effect recrystallization of grain crystals and then quenching said recrystallized tube.

2. The method as defined in claim 1 in which the ductile metal is selected from the group consisting of austenitic steels, brasses, bronzes, and nickel base alloys.

3. The method as defined in claim 1 in which the ductile metal is an austenitic stainless steel.

4. The method as defined in claim 1 in which the duetile metal is AISI Type 316 stainless steel.

5. The method as defined in claim 1 in which the duetile metal is AISI Type 304 stainless steel.

6. A tubular article at least about 8 inches in outside diameter and at least about one-half inch in wall thickness, consisting of a work-hardenable metal the grain size of which cannot be reduced by heat treatment alone, said article resulting from cold expansion of a centrifugally cast tube of said metal followed by heat treatment to effect recrystallization, the average grain size of said article being no larger than ASTM No. 1, non-metallic inclusions in said metal having the shape and characteristics common to castings of said metal, and the physical properties of said article being substantially isotropic.

References Cited in the file of this patent UNITED STATES PATENTS 1,552,848

1,692,521 Sturcke Nov. 20, 1928 OTHER REFERENCES Langenberg Sept. 8, 1925,

Claims (1)

1. THE METHOD FOR PRODUCING TUBULAR METAL ARTICLES OF A WORK-HARDENABLE DUCTILE METAL THE GRAIN SIZE OF WHICH CANNOT BE REDUCED BY HEAT TREATMENT ALONE, HAVING AN AVERAGE GRAIN SIZE NO LARGER THAN ASTM NO. 1 AND HAVING ISOTROPIC PHYSICAL PROPERTIES WHICH COMPRISES, EXPANDING A CENTRIFUGALLY CAST TUBE OF SAID METAL AT LEAST ABOUT 16 PERCENT IN OUTSIDE DIAMETER BY THE APPLICATION OF UNIFORM RADIAL PRESSURE SIMULTANEOUSLY THROUGHOUT THE INTERIOR OF SAID CAST TUBE WHILE THE OUTSIDE OF SAID CAST TUBE IS UNCONFINED, HEATING THE SO EXPANDED CAST TUBE

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