US2400495A - Centrifugal cast article and method of manufacture - Google Patents

Description

May 21, 1946. H. FORD ETAL 2,400,495

- CENTRIFUGAL CAST ARTICLE AND METHOD OF MANUFACTURE Filed Nov. 21, 1941 '20 v I 0 4' /Z l BY Qm/ f ATTORNEYS.

Patented Met e1; 1946 U V i 2,400 495 UNITED sures PATENT ,orrica 2,400,495 CENTRIFUGAL can narrow AND METHOD OF MANUFACTURE Henry Ford, Dear-born, Charles E. Soreiisen, De-

, troit, and Russell H. MoCarroll, Dearbornb Micln, assignors to Ford Motor Company, marborn MIc a corporation of Delaware Application November 21, 1941, Serial No. 419,934

icioim. (01. 309-2 Thisinvention relates to metal castings and to amethod of centrifugal casting therefor.

The class of articles to which this, invention has particular reference includes tubular or annular shapes, finished with thin with sections.

The casting method or this invention is per ticularly designed to replace forging in the manufacture of these articles. However, the same general types'of steels now used in forging are used, but a. much superiorarticle is obtained. In addition, the process utilizes relatively simple centrifugal casting machines rather thanthe large and costly installations required for forgmg.

Essential points of the inventionare the control of the cooling rate of the metal during casting and the balancing of the opposing force due to contraction and centrifugal action during the cooling period.

, At the present time, cylinder barrels are made by forging, which entails the production of a suitable steel, pouring it in ingots and subjecting it to all the rolling mill operations to form bar stock. This stock is then reheated for the forging process which consists of progressive steps of upsetting and drawing, forming a tubular blank. Thereafter, the 'forged' blank is annealed, the flash is trimmed off and it is brought 'down' to bull-am" size. machined, heat-treated, finish-machined, and

The blank is roughsubjected to pickling baths or other surface treatment. In each step the most rigorous care must be exercised to obtain a uniform article.

In contrast, by casting the blank, all the steps no q i t to rolling the bar stock and the forging dpe'ration are avoided. In this period of national a'ndginjdustrial emergency, when steel production andforg ng facilities-are taxed to their utmost, the release of the productive machinery now do: voted to the manufacture Ofthis one article alone is of considerable importance.

' The advantages are not limited to the economy of production. Even more important is the fact that the finished article is stronger when made by casting rather than by forging, and this is true even though the composition ofthe metal is identical. And it must be remembered that in aircraft work where every ounce counts and machine elements are stressed to the utmost, any increment in material strength is vital.

' 1 The defect inherent in the forged blank arises from the distribution of flow lines in the metal. Inasmuch as forging is essentially a process of extrusion, the dendritic formations and the-interstices therebetween are elongated parallel to the axis of extrusion. Even more serious, be-

' cause of their greater size, are the physical flaws or inclusions which are also extended. These elongated formations form lines along which cleavage may occur under stresses much lower than the metal should withstand normally. In a centrifugal casting, the dendritic formations are radially disposed. As there is no extrusion effect, neither the dendrite, nor the inclusions are extended or enlarged. Further, the dendritic formation, per sc, is finer in a centrifugal casting,- and while this does not seem to be due to faster cooling alone, if there is some centrifugal effect minimizing size it is not yet fully understood.

Whatever the reason, the structural difference between the forged and cast cylinders ls graphically shown under the bursting test. Tubularobjects so stressed normally tend to rupture more or less longitudinally since the transverse forces exceed those acting longitudinally. The forged cylinder accentuates this as it almost invariably ruptures on a straight longitudinal line which also corresponds to the zone. of weakness established by the extrusion process. In some cases, the rupture appears precisely like a rip or split. Thus, the inherent defect is such that it increases the probability of failure under actual conditions of use. The cast cylinder, however, merely. bursts as though punctured despite its tubular shape. These results clearly point the difference between the longitudinal weaknesses of the forging and the over-all uniformity of the casting. It need only be added that the cast cylinder withstands materially higher bursting pressures, as well as exhibiting tensile strengths at least equal to forged steels of the same chemical composition.

However, there have been pronounced limitations on centrifugal casting'arising from inability to cast certain steels or to obtain sound castings with the desired microstructure. ent invention, the same steel may,be used in either process and the identicalmicrostructural constituents obtained. Other limitations were the larger number of imperfect blanks obtained 45 and the inability to localize defects in the portion of the blank removed on machining. Each of these has been overcome.

It is, therefore, an object of this invention to obtain an article of 'the generally described 50 shape, structure and composition which isfree from flow lines within the metal. A further object is to evolve a method by which such articles I may be produced to a standardeconomically and quickly.

55 With these and other objects in view, the in- 60 accompanying drawing, in which:

With the pres-.

cylinder barrels.

Figure 1 is a quarter sectional view of the mold used in this invention enclosing a cast blank.

Figure 2 is a quarter sectional'view on an enlarged scale of a finished cylinder, the outline of the blank from which it was machined being indicated by dotted lines.

As stated, a cylinder barrel has been chosen as a specific example to illustrate this invention, particularly as it embodies the most difiicult circumstances of manufacture and use. It will be understood, however, that the method is equally applitial flange 13 on the blank H, which requires that the mold be separable. In instances where the blank is merely cylindrical, a one-piece mold may be used. The mold I is designed to be placed horizontally in a conventional centrifugal casting rotating machine and secured therein. After preheating the mold to a temperature of 450- 600 F., the molten metal at a temperature of approximately 2920 is introduced to the interior through the end 9 while the mold is in rapid rotation. The skirt I5 is provided to protect the mechanism from damage in case of'overfiow. The molten metal is projected toward the inner circumference of the mold by the centrifugal force and freezes in place in from 50-70 seconds to form a blank H, as indicated in Figure l. The mold I0 is removed from the spinning machine, separated in halves and the blank I removed therefrom for further processing. The mold, which ma be of cast iron or steel, is then sandblasted or washed with a mixture of sodium silicate and silica flour to clean its surface, reheated, and the operation repeated.

In Figure 2, the outline of the blank I4 is shown in dotted line and the finished cylinder barrel I6 is shown with relation thereto to indicate how it is formed from the blank. For the purpose of comparison, and not as a limitation, Figure 2 is shown substantially half the size of a cylinder barrel actually used. From. this it will be noted that the wall of thebarrel is very thin as compared to the diameter-scaling this drawing indicates this ratio to be about 1 to 40, and thicknesses of this order are representative of most It will be noted that it has a circumferential flange I! and rings Is by means of which the barrel is secured to the engine crankcase. It also has the threads l9 by means of whicha cylinder headis afiixed to the barrel. This form of cylinder is, of course, conventional and merely illustrative of one of the types which has been manufactured by this method.

the same steel. The following representative test data were obtained:

Heat treatment:

Normalize at 1700-1750 F.

It will be noted that the centrifugally cast cylinders have tensile stren ths and elastic limits which compare most favorably to those of the forged cylinders. More important in the present instance is that rigorous standards have been set, notably those of the United States Army, for coordination of tensile strength to hardness. The centrifugally cast cylinders excel in that the tensile strength and hardness are co-ordinated within the limits set. Furthermore, other tests on cylinders of corresponding steel to determine the bursting pressure under hydrostatic load show that the cast cylinders are approximately 20 per cent stronger than the forged.

In view of these obvious advantages, the continued use of forging as a method of construction at a. time when increased production is imperative and intricate forging machines are not readily available seems rather strange. The explanation, of course, is that heretofore the advantages which should flow from centrifugal casting have not been realized, principally because the casting process itself had not been perfected. To underj stand these difliculties and the means by which This barrel has heretofore been made by the forging process from bar stock of SAE 4140 steel having the following alloy composition:

Molybdenum .15- .25

For the purpose of exact comparison, the centrifugally cast cylinders have been made from they have been cured. it will be necessary to consider in some detail exactly what occurs during the making of centrifugal castings.

Centrifugal casting has been'used for many years in the production of cast-iron pipes and similar objects. It has been used more recently with various steels for producing pipes, gun barrels and other tubular objects, massive in form and relatively thick in section. It has also been used to cast annular articles such as rims, car .wheels, and the like. But none of the methods evolved for these types of articles has proved satisfactory when a thin-walled structure able to withstand high stresses is to be obtained.

The reason for this is that as molten metal is poured into the rotating mold. it is chilled on contact with the metal of the mold and tends to solidify. As it solidifies, it must contract. while the mold, being heated, tends to expand. At this stage, the casting is far from a solid or hard mass but, rather, may be thought of as being putty-like. The semifluid mass is subjectedto two opposing forces; the centrifugal force tends to drive it outwardly into contact with the mold, while the thermal stresses, on cooling, tend to contract it and draw it away from the mold. The result is obvious-the opposing stresses, when they do not cause a complete rupture and failure of the casting, cause a number of small cracksthroughout the structure which render the casting worthless,

This condition is a function. of the, rotating. speed'of the mold inasmuch as it determines the" centrifugal force; and of the cooling en'ectof the mold, since it determines the rate of contraction. It might be supposed that if the mold were so constructed that the heat of the metal could be removed practically instantaneously, a sound casting would result, by reason of the shortening of the period during which themetal is semifluid and subject to distortion. It might also be supposed that the speed of rotation could be decreased to lessen the centrifugal force during this period and obtain the same results.

Neither of these suppositions is correct. In the first place, a cold mold does not produce a. satisfactory casting and preheating of the mold to about 500 F.=is necessary troduced. Of course, this temperature level is considerably less than that ofthe molten metal and also lower than that customarily used; but it has been found that if the metal mass of the mold be increased to increase the speed of cooling, the surface of the casting will be chilled rather than cooled and will be minutely cracked or crazed. Similarly, if the speed is reduced, the molten metal is not subjected to enough force to must be segregated, after the casting has cooled, adjacent its inner surface. These defects must not be in the portion of the metal which is to form the finished article; and, unless the speed is adequate, these defects will be distributed throughout the body of the metal rather than localized in the inner surface.

Therefore, if a sound and clean casting having a uniform structure is to be obtained, the rotational speed and the cooling rate otthe mold must be balanced. This is the essence of this invention insofar as the method is concerned.

Ithas been discovered that the proper range of peripheral speed is between 1000 and 1750 feet per minute computed on the outer diameter of the casting. It has also been discovered that by the comparatively simple step of proportioning the effective metal mass of the mold to the amount of metal in the casting, a close control over the cooling rate is obtained. The metal of the mold is the primary cooling agent and its thermal capacity directly affects the cooling rate. The term effective mass has reference to the portion'of the mold readily available for heat. transferroughly, that included between the projected limits of the casting, as shown between dotted lines 20 in Figure l. The ratio between the weight of this effective mass and the weight of the casting should be within the range 3 to 4 to 1. In addition, the thickness of the mold wall should pref erably be not less than 1 /2 or 2 times'the average wall thickness of the casting.

Another consideration; perhaps as important as the heat capacity of the mold in regulating the cooling rate, is the characteristic of the mold surface as it affects heat transfer. It is essential that this heat transfer characteristic be the same throughout the mold surface. This is true whether the mold be coated, in which case the coating must be evenly applied, or if the mold is cleaned by sandblasting, it must present a uniform surface. There must be no condition on the mold surface which can cause a differential heat transfer at different parts.

When the speed of rotation and the ratio of mold weight to casting weight are maintained within the above ranges and surface conditions permit uniform heat transfer, the proper cooling before the metal is inrate will be obtained and castings will be free from harmful cracking or crazing and from imsound, uniform metal of the desired structure.

separate the lighter particles of impurities which blank will be obtained. Then,

Three advantages flow from the use of the outer metal. One complete machining operation required with forgings is avoided. The amount of metal inthe blank is greatly reduced. The structure of the metal in the outer portion of the blank is best suited to the intended use in that it is more dense, has the least injurious dendritic formation, and generally presents the optimum condition of grain form and arrangement.

Thus, by properly proportioning the mold weight to that of the casting at the ratio of from 3 to 4 t 1, by maintaining the peripheral speedof the mold from 1000-1750 feet per minute, and providing a uniform mold surface, a sound cast by utilizing substantially the outer metal in the blank for the finished article and applying the indicated heat treatment, a uniform sorbitic structure is obtained without included matter, cracks or perceptible flow lines.

Summarizing the advantages, an article of superior physical characteristics is obtained. The time for production and the machinery required therefor are greatly reduced. Unifomity of product is increased. The amount of metal required in the process is reduced at least 20 per cent and. in many cases, by asmuch as 50 per cent. The preheating level, held at approximately 500 F., simplifies production procedure and is also an important step in the cooling rate control. The entire process can be subjected to rigorous laboratory control and much less skill is demanded from the operator. Given metal of the proper'composition and temperature which is readily controlled, a casting machine set at an established speed and a, mold properly surfaced and preheated, all other variables are eliminated. Uniform production follows as a matter of course.

Some changes may be made in the arrangement, construction and combination of various parts of the improved device and in the steps of the approved method without departing from the spirit of the invention and it is the intention to cover by the claim such changes as may reasonably be included within the scope thereof.

We claim as our invention:

An article of manufacture, a cast steel cylinder sleeve, the wall thickness of which is very small in proportion to the diameter thereof and related thereto in the ratio of substantially 1 to 40, said sleeve being formed of steel'having a uniform sorbitic microstructure, the meta1 thereof being substantially free from extended grain distribution patterns and the dendrites therein being substantially radially disposed.

HENRY FORD. CHARLES E. SORENSEN. I RUSSELL H. McCARROLL.

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