DK143916B - PROCEDURE FOR THERMAL DETERMINATION OF THE CARBON EQUIVALENT OF OVERUTECTIC CASTLE - Google Patents

Description

1439 16

The invention relates to a method for thermally determining the carbon equivalent of hypereutectic cast iron by cooling a sample of the molten material and determining the liguidus discontinuity on the cooling curve.

10 The principle of carbon equivalent determination by pyrometry is based on the exact measurement of the initial or melting point's thermal discontinuity temperature, which occurs in a sample of molten cast iron as it begins to solidify. The carbon equivalent (CE) can be defined as the total 15 percent carbon content plus one third of the total percent silicon content plus one third of the total percent phosphorus content in a cast iron sample, based on the total weight of the sample. Under carefully controlled and regulated conditions, the melting point temperature 20 (liquidus temperature) of hypoeutectic cast iron, i.e., cast iron with a carbon equivalent of less than 4.35%, is readily observed. .

The temperature at which the melting point's thermal discontinuity appears depends directly on the carbon equivalent of the metal and is not appreciably affected by normal amounts of manganese, chromium, nickel or other commonly found contaminants. With accurate detection, the measurement of the thermal discontinuity of the melting point curve is completely reproducible, and this method is far faster and easier to perform than chemical analysis in connection with casting in practice. In practice, the carbon and silicon content are the two main variables in cast iron, since the phosphorus content is generally so low that it is relatively ineffective for any given carbon equivalent. The silicon content can be determined on the basis of cure for search, leaving only the carbon content to determine. By determining the carbon equivalent from a cooling curve of the cast iron sample, the carbon content 5 can be calculated on the basis of its relationship to the carbon equivalent. With this knowledge of the carbon content, a cast before pouring will know whether the cast-iron mixture meets the required conditions.

A suitable expandable phase change detector device for use in determining the carbon equivalent of molten cast iron is disclosed in U.S. Pat.

3267732. The carbon equivalent technique comprising the cooling curve test is also described in an article entitled "Carbon Equivalent in Sixty Seconds", which is found in the March issue of I5 1962 by Modern Castings on pages 37-39. This article emphasizes on page 38 that the melting point curve crack for hyper-eutectic iron, i.e. iron having a carbon equivalent equal to or greater than ca. 4.35%, the eutectic point, is not clear and definite enough to record with this sample, and therefore the sample is limited to hypoeutectic iron, ie. iron with a carbon equivalent of less than 4.3%.

It has been proposed to extend the cooling curve sample for carbon equivalent determination of hypereutectic cast iron using a technique which comprises diluting the p c hypereutectic iron sample with a known amount of low carbon steel and then determining the carbon equivalent, as it usually does, with cooling supercool. After determining the carbon equivalent of the steel-cast iron mixture, a correction for determining the carbon equivalent of the cast iron sample is introduced. However, since the steel additive does not always melt and mix with the sample, the carbon equivalent is not fully reduced and the correction is inaccurate. Although the use of low carbon steel wire in the form of coil springs spatially distributed in the carrier sample has improved the reliability of the preceding dilution technique, it makes this method less attractive that it is necessary to place the fine wire at precise intervals in the container for to obtain reliable results and to provide the correct correction factor.

The present invention makes it possible to apply the carbon equivalent technique also to hypereutectic iron up to the kish point, i.e. the temperature at which free graphite is formed and secreted by molten hypereutectic cast is allowed to cool, without resorting to a dilution technique which alters the carbon equivalent of the cast iron sample from that of the melt.

Hypereutectic cast iron tends to form stable graphite as it cools from the melt to the solidification temperature of the cementite, through the formation of unstable iron ore which is immediately decomposed into iron and carbon (graphite). This produces complex heat effects which result in a poorly defined discontinuity on the cooling curve. This is eliminated by the process of the present invention, characterized in that a carbide stabilizer is added to the molten sample which retards the primary graphite formation during cooling of sample 20 to its solidification temperature and promotes a discontinuity at the liquidus temperature.

All substances capable of retarding graphite formation upon cooling of the hypereutectic cast iron sample to its solidification temperature, and which do not affect the carbon equivalent of the hypereutectic cast iron melt with which it is mixed (so far as it does not change the carbon equivalent of the sample from melt) can be used as a stabilizer in connection with the present invention. Stabilizers which are characterized by a strong carbide stabilizing effect are readily soluble in and dispersible in the molten iron sample and will produce a uniformly reliable thermal discontinuity temperature when the iron carbide conversion takes place.

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Stabilizers having the aforementioned properties are bismuth, boron, cerium, lead, magnium and tellurium. Such stabilizers need not necessarily be combined with the molten hyper-eutectic cast iron sample in the form of an element, but may be added in the form of a compound or in mixtures with other substances which do not change the carbon equivalent of the cast iron sample from the melt. Eg. boron can be added in the form of ferro-robor (FeB). Cerium can be added in the form of misch metal (a mixture of rare earth compounds, atomic numbers 57-71, in 10 metal form). Magnesium can be added in the form of copper magnesium (Cu-Mg), with magnesium being approx. 15% by weight. Various combinations or mixtures of the foregoing additives have also been found to be suitable for generating initial thermal discontinuity temperatures by cooling samples of hyper-eutectic cast iron. The following mixtures have e.g. all provided favorable initial thermal discontinuity temperatures: a mixture of tellurium, boron and misch metal; a mixture of boron and misch metal; a mixture of lead and misch-metal and a mixture of bismuth and boron.

2Q Graphite-inducing substances, ie substances which promote graphite formation during the cooling of the hypereutectic cast iron sample should be avoided in connection with the invention. Eg. is such a substance as ferrosilicon (FeSi) unsuitable as it not only causes graphite formation but also changes the carbon equivalent of the cast iron sample from that of the melt.

The amount of stabilizer used in the present invention may vary within wide limits depending on the stabilizer used, the carbon content of the sample and the amount and nature of other constituents of the molten sample.

Satisfactory curves can be obtained by cooling samples of cast containing as little as 0.05% by weight (based on sample weight) of the stabilizer. The amount of stabilizer is preferably used in the minimum amount needed to achieve the desired retardation of primary graphite formation and accompanying discontinuity in the cooling curve at the carbide formation temperature. Although successfully stabilized amounts of as much as 0.4% have been used, smaller amounts are generally preferred.

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The addition of the stabilizers to molten cast iron samples according to the invention can be carried out in any convenient way, as long as the temperature of the molten sample, at the time the stabilizer is added, is sufficiently high to allow the desired cooling curve to emerge. The stabilizer can e.g. added in pill or particle form to the molten sample immediately after the sample has been poured. Alternatively, the stabilizer may be added to the container before the molten sample is applied.

Other methods of combining the stabilizer with the sample will be apparent to those skilled in the art.

The present invention is further illustrated by the following drawings, in which: 1 is a cooling curve for a hypereutectic cast iron sample produced in accordance with the present invention; FIG. 2 is a photomicrograph of the microstructure x500 of the picrolet sample using the one shown in FIG. 1, FIG. 3 is a cooling curve for a hypereutectic cast iron sample of the same composition as the sample of FIG. 1, but poured in the usual manner without an additive; 4 is a photomicrograph of the microstructure x500 of the picrolet sample of FIG. 3, and FIG. Figure 5 is a graph showing the relationship between percent carbon equivalent of hypereutectic cast iron and initial thermal discontinuity temperature using carbide stabilizing additives of the present invention.

Referring to FIG. 1, a cooling curve of a hypereutectic cast iron sample obtained according to the present invention is illustrated by combining the sample in liquid state with a stabilizer capable of retarding primary graphite formation during cooling of the sample to the set temperature. The hypereutectic cast iron test of FIG. 1 had a composition of 4.12% by weight carbon, 1.62% by weight silicon and 0.076% by weight phosphorus. This composition gave a carbon equivalent (CE) of 4.69% [CE =% C + 1/3 (% Si +% P) I.

The sample was poured into a phase change detector apparatus of the type described in the aforementioned U.S. Patent No. 3,267,732. Such a detector apparatus consists of a small bowl with a volume of approx. 10 cubic inches or less. A chromel-alumel wire thermocouple 10 is passed out through the wall of the bowl and is designed so that the element's heat point is completely enclosed by the sample poured into the bowl and underneath any cavities created by cooling. connected to an appropriate curve recording apparatus which records the cooling curve of the sample as the temperature drops. The electrical connection between the detector thermocouple and the temperature measuring circuit in the recording apparatus is made by inserting the detector contacts into corresponding contacts in a rack arranged to carry the detector in a horizontal position for recording the sample.

In obtaining the cooling curve of FIG. 1, cerium in the form of a 1 g misch metal tablet was placed in the bowl before the molten sample was poured therein. The sample had a weight of approx.

As indicated above, the amount of stabilizer may vary within wide limits. There are e.g. have been produced satisfactory curves by the addition of as little as 1/4 g of misch metal and as much as 2 g to a 500 g molten sample of hypereutectic cast iron containing 4.2% C and 1.5% Si.

30 Referring to FIG. 1, it can be seen that when the molten sample of hypereutectic cast iron was poured into the bowl, the pen of the recorder moved upward from point A to point B until the actual temperature at the center of the sample was reached. At this point, the recorder started with recording the cooling pattern. The liquid curve or initial thermal discontinuity temperature appeared as a vertical segment showing that such a temperature at C of the curve was 2200 ° F. The initial thermal discontinuity temperature is usually reached at approx. 20-40 seconds depending on superheat and this temperature can then be converted to carbon equivalent using predetermined carbon equivalent percent versus temperature data. A graph based on such predetermined data is shown in FIG. 5. From this curve, it can be seen that a sample of hypereutectic cast iron with a 10 initial thermal discontinuity temperature of 2200 ° F has a carbon equivalent of 4.7%. This corresponds exactly to the carbon equivalent of 4.69% which results from a chemical analysis of the sample.

FIG. 2 is a photographer! of the microstructure of the picrol-15 etched sample of hypereutectic cast iron with that of FIG. 1 shows the cooling curve. The FIG. 2 microstructure is enlarged 500 times.

Referring to FIG. 3, it is illustrated here that a cooling curve is illustrated here for a hypereutectic cast iron sample of the same composition and poured from the same portion as the sample of FIG. 1, but poured into a bowl with a carbon equivalent detector without the addition of a stabilizer. This sample was poured in parallel with the sample of FIG. 1, but the obtained cooling curve for the sample, which did not contain the stabilizer, 25, as shown in FIG. 3, not a useful thermal discontinuity over the eutectic temperature discontinuity.

Thus, it was not possible to determine the carbon equivalent on the basis of the cooling curve of FIG. 3, although this sample consisted of the same iron and therefore had the same carbon equivalent and the same composition as the sample used to produce the cooling curve of FIG. First

In FIG. 4 shows a photomicrograph of the microstructure of the picrolyzed sample of hypereutectic cast iron which produced the cooling curve of FIG. 3. The microstructure shown therein is enlarged 500 times. The worm-like regions are graphite flakes, and they are long and solid in FIG. 4 in contrast to the short and fine graphite grains of FIG. 2, showing the effect of the misch metal additive on inhibiting or retarding the formation and / or growth of graphite during cooling 5 of the sample to the solidification temperature.

The initial thermal discontinuity in the cooling curve for hypereutectic cast iron shown at C in FIG. 1, can be produced with various stabilizing additives which are capable of retarding primary graphite formation during cooling of the sample to the set temperature. A number of stabilizers have been used to produce thermal discontinuity temperatures in samples of hypereutectic cast iron with a percentage carbon equivalent varying from the eutectic point at 4.35% to about 4.95%. The percent 15 carbon equivalents for these different samples were determined by chemical analysis. By plotting the thermal discontinuity temperatures against the percent carbon equivalent of the samples, the curve of FIG. 5th

The data set forth in the following table shows the carbon equivalent 20 for hypereutectic cast iron determined from the associated data of FIG. 5 and using carbide stabilizing additives for comparison with the percent carbon equivalent determined by chemical analysis.

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Thermal dis-% Carbon equivalent continuity Based on Chemical temp. ° F of mutual analysis

Stabilizer addition _ coherence _ 5 Tellurium + Boron + mischmetal 2315 4.84 4.79

Boron + misch-metal 2323 4.85 4.85

Lead + misch-metal 2280 4.80 4.80

Boron 2210 4.71 4.70 10 Misch-metal 2210 4.71 4.72

Misch-metal 2200 4.70 4.68

Boron + misch-metal 2217 4.72 4.65

Misch-metal 2192 4.69 4.51

Boron + misch-metal 2123 4.58 4.51 15 Bismuth 2360 4.89 4.77

Misch-metal 2340 4.86 4.80

Bor 2323 4.85 4.68

Misch-metal 2341 4.87 4.68

Bismuth + boron 2310 4.83 4.68 20 by comparison, it turns out that the percent carbon equivalent determined from the initial thermal discontinuity temperature produced in the hypereutectic cast iron of the present invention is well within the applicable limits. In fact, it has been found that this method of determining the percent carbon equivalent is easier to realize than chemical analysis, as the results of chemical analyzes of the same cast iron sample will frequently vary from one laboratory to another.

While the specification of the present invention 30 specifically refers to the expandable phase change detector apparatus of the type disclosed in the aforementioned U.S. Patent No. 3,267,732, any suitable phase change detector apparatus can of course be used. Such detector apparatus will normally have a small bowl which is open so that the molten cast iron sample can be poured therein, and have a temperature sensitive means which is inserted into the bowl and placed below the surface of the sample so that it can sense the temperature change in the sample. , as