US2464948A - Method for determining the solidification time of a casting - Google Patents

Description

March 22, 1949. g, 5 5 ETAL 2,464,948

METHOD FOR DETERMINING THE SOLIDIFIGATION TIME OF OAS'IINGS Filed Feb. 23, 1945 4 I v I I I I III/II m 8 3/11/1111,!!! [III/I 2 a 4 5 6789) 20 so 40 5060 BOIOO FREEZING POINT TEMPERATURE- D EG. F.

5 :0 so 40 so so 00 I00 TIME m MINUTES AFTER POURING I a 2 INVENTORS Clarence E; Sims, Samuel L. Case, and Charles G. MFCabe AGENTS -plicated mechanism for the purpose.

Patented Mar. 22, 1949 METHOD FOR DETERMINING THE SOLIDIFI- CATION TIME OF A CASTING Clarence E. Sims, Samuel L. Case, and Charles G. McCabe, Columbus, Ohio, assignors, by mesne assignments, to The Ferro Engineering Company, Cleveland, Ohio, a corporation of Ohio Application February 23, 1945, Serial No. 579,444

2 Claims. 1

This invention relates to methods for determining the solidification time of a casting. It

ls particularly concerned with a method of'determining the time required for complete solidification of an ingot from the characteristics of a cooling curve obtained from measurements made at a surface of the ingot or of the sinkhead.

In studies involving the freezing of cast metals, it is often desirable to determine the length of time required for complete solidification of the metal. For example, the ingot or casting should not be removed from the mold until it is completely, or at least essentially, solid. In addition, most metals shrink markedly on passing from the llquid'to the solid state, and, unless provision is made for feeding molten metal into the last portions of the casting to freeze, serious shrinkage cavities may result. Much research has been applied to the design of superior methods for the elimination of such internal cavities, and it is common practice to use hot tops on ingots and risers on castings, to supply a pool of molten metal suitable for feeding the ingot or other types of castings as they solidify. It will be readily apparent that such an arrangement can be effective only if the pool of metal remains molten vuntil the freezing of the main body of the casting is complete. Therefore, in order to determine the efiicacy of a sinkhead, it is desirable to determine the length of time required for complete solidification of the metal.

The usual practice in prior attempts to determine the solidification time of castings has been to insert a protected thermocouple in the center of the sinkhead and, by means of a normal cooling curve. to determine the time of complete solidification. Although this method has given useable results, the need for relatively heavy protection tubes to safeguard the couple from the action of the molten metal tends to retard the response of the couple, and, more important, the

presence of the couple tends to interfere with the normal solidification and freezing of the metal. In addition, it is often difiicult to provide a satisfactory method for suspending the couple in the desired location without developing a com- Furthermore, precious-metal couples are usually required if the casting freezes at a high temperature and, since some metal from the couple is always lost on each run, this results in considerable expense.

It is, therefore, an important object of this indetermining the solidification time of castings.

Another object is to provide methods for the determination of the solidification time of castings that do not interfere with the normal freezing of the casting.

A further important object of this invention is to provide a new and improved method of measuring the solidification time of a casting which eliminates the use of a precious-metal couple.

Other objects and advantages of this invention will become apparent from the following detailed description thereof when read in conjunction with the accompanying drawings in which Figure 1 is a schematic representation of one form of a thermocouple arrangement preferred for use in connection with this method, and

Figure 2 shows typical cooling curves obtained by the herein-disclosed process which enable an accurate determination of the solidification time of castings of various sizes.

In general, this invention relates to methods for determining the time required for complete solidification of a body of metal, which comprise graphing the temperature of the body of metal, as measured at or near the surface of the body, against the time elapsed subsequent to the casting of the metal, and ascertaining the time required for complete solidification as the period between the time of casting and the instant indicated by the point of tangency between the curved portion of the graph line, representing the interval during which the heat of fusion is released by the body, and the straight-line portion, representing the interval of normal cooling.

Before explalning the present invention in detail, it is to be understood that the invention is capable of being practiced or carried out in various ways, it being also understood that the phraseology or terminology employed herein is fg the purpose of description and not of limitat n.

For the purpose of illustration, this invention will be described primarily in relation to its use in connection with the freezing of steel ingots; however, it will be apparentthat the principles involved are applicable to the determination of the freezing characteristics of various other metals in various other types of castings.

It has been established empirically that when the surface temperature of a solid ingot, cooling normally from an elevated temperature, is plotted against the logarithm of the cooling time, the resulting time-temperature cooling curve has a relatively constant slope and approaches a straight line. On the other hand, if the rate of heat liberation from the interior is altered, for

example by change in internal conditions, this disturbance is reflected by a change in the slope of the cooling curve.

It is well known that as metals freeze they liberate considerable quantities of heat as the "latent heat of fusion." During solidification, the liberation of this latent heat of fusion acts as a disturbing factor and causes a gradual change in the slope of the cooling curve; however, once solidification is complete, the cooling curve of the ingot skin becomes essentially astraight line when plotted against the logarithm of the cooling time. The point of tangency of the curved and straight-line sections of the cooling curve may thus determine the instant of complete solidification of the ingot.

The temperature of the ingot surface may be measured in any suitable manner: however, the use of a thermocouple has been found most convenient. The combination of metals used in the thermocouple will depend upon the characteristics of the metal being cast. For example, for

metals melting at relatively low temperatures, a combination of Chromel and Alumel may be used; for steel, a thermocouple comprising the combination of iron and nickel, has been found to be particularly advantageous. This latter combination resists the mechanical stresses at the temperatures involved, has good sensitivity, and is relatively inexpensive.

In the actual practice of determining the freezing time of a steel ingot, the two thermocouple wires may be separately imbedded in the skin of the ingot; thus the ingot itself completes the circuit of the couple, and the hot Junction occurs at the surface or point of weld. The actual location of the couple is not extremely critical; however, it is advantageous to have it as near as possible to the region of final solidification. In normal ingot practice, it is desir-able to introduce the thermocouple through the hot top a short distance above its base.

Inasmuch as nickel wire has relatively little strength at temperatures approaching the melting point of steel, it is advantageous to provide some method of supporting the nickel wire and protecting it from stresses resulting from the contraction of the ingot. One method of achieving this protection is illustrated schematically in Figure 1, in which a nickel wire I is supported by an iron tube 2 which forms the other half of the couple. The wire I is insulated from the tube 2 by suitable insulation 3, such as Alundum cement. A lead wire I of the same composition as the tube 2 is attached thereto by a weld 5 and connects the tube 2 to the cold Junction (not shown) of the thermocouple. The nickel wire I may connect directly to the cold junction, although other suitable means such as extension lead wire 4 may be used. One end of the tube 2 and the wire I extends through a wall} of a hot top, and it is on this end of the couple that the hot junction is produced.

One adaptation of the design shown in Figure 1 may be conveniently prepared by drilling a hole through the long axis of a section of 9-gage iron wire with a Number 42 drill to produce the short length of iron tubing 2. A piece of lfi-gage iron wire was welded to this tube to provide the lead wire 4, the assembly serving as one leg of the thermocouple. The other leg of the couple comprised a piece of l i-gage nickel wire 'I, which was inserted through the iron tube 2 with Alundum cement serving as insulation 3 between slightly above the junction'of hot top and ingot.

The nickel wire I softens at the temperatures at which the steel of the ingot is molten, as does the iron tube 2, and the two fuse together and to the ingot surface to provide the hot Junction for the thermocouple.

Temperatures are recorded at frequent intervals and plotted against the logarithm of the time in minutes after pouring, as shown in Figure 2. The chart in Figure 2 shows the type of curves obtained on ingots of different sizes. Curves A, B, C, and D were obtained on ingots =weighing 376, 203, 93, and 20 pounds, respectively, which were cast in "Gathman-type big-endup molds.

As can readily be seen, the slope of the curve is constantly changing during the solidification of the ingot, but once solidification is complete, the curve approximates a straight-line untilfat relatively low temperatures, the thermal transformation of the steel again causes a deviation from the straight-line relationship. The freezing times for these ingots, as indicated by the arrows, were 19 minutes, 12 minutes, 7.5 minutes, and 3.2 minutes, respectively.

A cooling curve obtained by inserting a protected thermocouple in the liquid metal in the center of a sinkhead on an ingot cast under conditions similar to ingot A shown in Figure 2 gave a solidification time of approximately 17.5 minutes, which agrees quite well with that obtained by the surface measurement.

In order to determine whether the distance of the thermocouple from the last portion of the metal to solidify influences the shape of the surface temperature curve appreciably, cooling curves were obtained on a rectangular steel block weighing 500 pounds and cast in a sand mold. The casting was 9" x 18"in cross section and 12" high. Two iron-nickel thermocouples of the special design mentioned previously were projected through adjacent sidewalls of the mold along the center lines thereof at a point 6 inches from the base line. In addition, a shielded platihum-platinum rhodium thermocouple was located at the geometric center of the casting in order to check simultaneous readings taken by the aforementioned iron-nickel couples at points 4.5 inches and 9 inches, respectively, from the point of final solidification. The couple 9 inches from the center of the casting (which is the point of final solidification) indicated that approximately 67 minutes was required for complete solidification, while the couple located 4.5 inches from the center showed a solidification time of approximately 68 minutes. Since the couple at the center of the casting indicated that approximately 69 minutes was required for completion of the freezing, it is obvious that the distance of the couple from the point of final freezing has relatively little effect upon the time indicated for complete solidification.

the two metals. In this way. the nickel wire was In summarizing the above-described invention.

it is apparent that the method herein disclosed constitutes a new and advantageous method for determining the time required for the complete solidification of a casting. In applying this method, the surface temperature of .the casting is measured and the instant of complete solidification is indicated by the point at which the curve obtained by plotting the surface temperature against the logarithm of the time becomes essentially a straight line.

In determining the surface temperature, a selfwelding thermocouple may advantageously be used. The thermocouple wires are projected through the mold so that when the molten metal reaches the couple it fuses the two wires, forming a hot junction welded to the skin of the casting. In a preferred embodiment, an iron-nickel couple is used, with the nickel wire supported by an iron tube forming the second element of the couple.

The position of the couple with respect to the last portion of the metal to freeze is not extremely critical. The couple may be located in the surface of the hot top or riser, or in the surface of the casting. It is advantageous, however, to have the thermocouple located as close as possible to the point of final solidification.

By the use of the term periodic, as applied herein to the measurementsof the surface temperatures, it is to be understood that the temperature is measured at sufficiently frequent, although not necessarily uniform, intervals to yield data suitable for plotting a time-temperature cooling curve.

Variations and modifications of the abovedescribed invention will become apparent to those skilled in the art. For example, the temperature of the body of metal whose time of solidification is being determined may be taken within the body of the metal or within the mold as well as at the metal surface. Nor is it necessary that thetemperature measuring device used be a thermocouple, since other temperature measuring instruments, such as optical pyrometers, may be satisthe instant of pouring to beyond the time of complete solidification, plotting a curve of the surface temperatures against the logarithm of cooling time, and reading the time elapsed on the curve between the instant of pouring and the point at which the curve becomes essentially a straight line representing the point of complete solidification.

2. A method of determining the length of time for a casting to completely solidify from the instant of pouring, comprising measuring the surface temperature of the casting at intervals from the instant of pouring to beyond the time of complete solidification, plotting a curve of the surface temperatures against the cooling time, and reading the time elapsed on the curve between the instant of pouring and the point at which the curve becomes essentially a straight line representing the point of complete solidification.

The following references are of record in the file of this patent:

Pages 456, 457, and 459 of volume 42 of the Transactions of the American Foundrymen's Association, published in- 1935 by the American Foundrymens Association, Chicago, Illinois.

Pages 782, 783, 785, 786, and 787 of volume 48 of the Transactions of the American Foundry-

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