CH651486A5 - Method for determining an area as an aid for adjusting or correcting the composition of iron/carbon melts before casting - Google Patents

Description

651 486

2nd

PATENT CLAIM Process for determining an area as a tool for adjusting or correcting the composition of iron-carbon melts before casting by thermal analysis of a small sample of the melt, which is poured into a standard test specimen and the differential calorimeter curve (D) is recorded , characterized in that the values of this curve (D) are stored in a computer and subtracted from the values of a specific total heat line (G) which is also stored there, resulting in an area of the differences which determine the time course of the release of the heat of solidification as well as the total solidification heat, whereby from the shape of this surface and the total heat of solidification indicated, statements about the microstructure formation and the cavity behavior of the melt can be read and the necessary measures for adjusting or correcting their composition can be taken.

The invention relates to a method for determining an area as an aid for adjusting or correcting the composition of iron-carbon melts prior to casting by thermal analysis of a small sample of the melt, which is poured into a standard test piece, the differential calorimeter curve of which is recorded and draws conclusions about the melting state. Thereafter, the germ state can be changed either by overheating the melt and / or the composition can be corrected by alloying.

Such a method is known from DE-PS 2 316 035, namely that the conclusions are drawn on the melting state based on the comparison of this curve with a differential calorimeter curve, which corresponds to a melt, which gives castings of the desired quality. This known method has proven itself in practice for several years, however, in order to make the comparison possible, one must either carry out a comparison test in each case, which provides the required comparison curve, or one must have a larger number of curves of pre-tested samples on hand in order to have a test curve suitable for the specific purpose. These types of procedures are somewhat cumbersome or complex.

The invention is based on the surprising finding that the time course of the release of the heat of solidification of a small test specimen in comparison with a calculated total heat flow and the total heat of solidification of the test specimen allows conclusions to be drawn about the microstructure formation of a much larger casting.

The invention is based on the object of improving a method of the type mentioned in the introduction such that it does not require the creation of test curves.

This object is achieved in a method of the type mentioned at the outset in that the values of the differential calorimeter curve of the standard test specimen are stored in a computer and are subtracted from the values of a specific total heat line which is also stored there, from which a surface of the differences, which shows the temporal course of the release of the heat of solidification and the total heat of solidification, whereby statements about the microstructure formation and the cavity behavior of the melt can be read from the shape of this area and the total heat of solidification indicated and the necessary measures for adjusting or correcting its composition can be taken.

The advantage of the new method compared to the known method described above is that curves of pre-tested samples that have been found to be good are no longer required as a basis for comparison. Instead, the values of the differential calorimeter curve of the sample to be examined, which are stored in a computer, are subtracted from the values of a total heat line stored in this computer, and the heat of solidification is thus ejected. With appropriate experience, the person skilled in the art can judge from the amount of calories released from the melt whether the melt is suitable for casting a specific object to be produced.

The total heat line is the curve which shows the time course of the heat emission of a comparison test sample 1S pers of the same size, shape and composition in the same molding material in the event that the solidification takes place homogeneously without deposition of different phases. According to D. Rabus and K. Wutzl, Material und Technik, vol. 5 (1977), No. 2, pp. 100-102, it can be calculated as follows: The so-called root law applies to the total heat emission:

25th

Q = kl / To,

Q = cp • T • G

(1)

(2)

wherein

Q - total heat quantity (kJ)

k = variable (kJ m ~ 2 s ~ v ') = constant of the root law

30 t = time seconds (s)

O = surface of the sample (m2)

cp = specific heat (kJ.kg-1 • K-1)

T = temperature (K)

G = mass (kg)

35

k is determined experimentally by recording a cooling curve in the liquid and solid range, i.e. in the range not disturbed by solidification processes, which supplies the k values according to the following equation depending on the time: With T = Tj - T2 and] / t = ] / t2 - tj becomes (1) and (2)

45

k =

IT, - Tjl-C ^ G

(3)

\ FTZ ~ T? - °

!

The k-values increase considerably between liquidus and solidus, which can only be explained by the better thermal conductivity of the already solidified parts of the melt and the constant increase in the temperature difference between the center and the edge of the test specimen as a result of the heat of solidification released later on can.

55

The also measured time course of the temperature difference between the center and the edge of the sample during the solidification of the melt serves as the basis for the determination of the k values between liquidus and solidus, in which these limit values are connected by the course curve. It is therefore assumed that the temperature conductivity between the center and edge of the sample increases or decreases in accordance with the increase or decrease in the temperature difference between the center and edge of the sample.

65

On the basis of the k values found in this way, the total amount of heat is dependent on the time, i. H. the total heat line, calculated according to the root law:

3rd

651 486

Q t -t = kt -t Z2 1 2 1

. ] Jt2 - • o [kJ]

(4)

Q'2-

tl kt2-ti is used

Heat emission of the test specimen in the time between ti and t2 the variable k in the time between ti and t2

t2 "t1 =

2_t1

(-]

lkg;

the total heat line can be converted into temperature values according to equations (3) and (4), where t2-ti = At, namely the difference time chosen for recording the differential calorimeter curve:

3a & 3b representations corresponding to FIGS. 2a and 2b for a melt which delivers good castings;

Fig. 4 is a diagram of the dependence of the strength of the casting on the temperature Tp at the start of pearlite-10 formation.

In the figures are

A = time-temperature curve (measured on sample) = Ab-

Cooling or solidification curve D = 1st derivative of curve A 15 G = total heat line = 1st derivative of the curve of heat emission from (3) and (4) calculates G '= axis on which the difference D-G is plotted. D '= D-G, related to the axis G'

M = position of the 1st minimum of the 1st derivative D of curve A.

V *,

Cp

= T

The k values depend on the structure, which depends on the carbon equivalent (CE). They are predetermined for different iron alloys with different CE. With these k values found, the total heat line can be calculated as a function of time in the specified manner. The total heat line of a number of the different alloys with different carbon equivalents (CE) expected to be required are stored in advance in the computer.

The total heat lines are also dependent on the casting temperature, which is therefore kept approximately the same for the selected total heat line and the differential calorimeter curve of the sample in terms of measurement technology or calculation. The appropriate total heat line is selected from the stored curves according to the CE of the melt, on which the microstructure depends. The computer itself determines the CE of the iron in the sample using known methods from the measured solidus and liquidus values of the same.

If you look at the total heat line as the zero line, the area between this and the differential calorimeter curve in the area between the liquidus and solidus gives the respective proportion of heat of crystallization, which can be distributed in a known manner to the structural components.

The invention is illustrated by the following embodiment, which refers to the accompanying figures. Show here:

Figure 1 is a schematic representation of a cooling curve A and the associated differential calorimeter curve D of a standard specimen and a calculated total heat line G.

2a shows the corresponding curves, as recorded by the computer for a standard sample and the selected total heat line, for a melt which gave castings with voids, and

2b shows a representation of the difference between the differential calorimeter curve and the total heat line of FIG. 2a, which the computer has created on the basis of these values and which, by its area, indicates the amount of heat released during solidification.

example

Ribbed cylinders of quality GGL 25 are to be cast. The following requirements are placed on this casting 25:

1. No microporosity

2. Tensile strength at least 250 N / mm2

A standard test specimen with 30 dimensions of 35 mm x 35 mm x 38 mm was cast from the melt, the differential calorimeter curve of which was recorded and stored in a computer. The total heat line for a GGL 25 melt had already been stored in the computer. When measuring the standard test specimen, the computer first printed curves G and D shown in FIG. 2a and then curve D 'of FIG. 2b. Finally, he also printed the values of the corresponding horizontal line in the following table.

The table shows that when the melt was inoculated with 40 0.2% FE Si 75 178 J / g, eutectic heat of reaction was released. This amount of heat is, as the expert knows from experience, too small and indicates that too little eutectic graphite has been excreted. In fact, the castings obtained from this melt had a 45 reject rate of 75% due to microporosity.

However, if this melt is treated with 0.2% Fe Si + cerium mixed metal, the eutectic heat of reaction increases to 191 J / g. This reduced the reject rate due to microporosity to zero. The kalori-50 see difference between the cases where good and bad castings are achieved is only about 13 J / g. This difference is clearly displayed by the device, automatically calculated and printed out in numbers.

55 The amount of heat of crystallization q corresponds to the area between the first derivative D of the time-temperature curve A and the total heat line G (FIG. 1). Curve D is the differential calorimeter curve which, strictly speaking, can be referred to as the difference quotient curve AT / At. The division of the total heat of crystallization between eutectic (dotted area) and dendrites (hatched area) is carried out automatically by the differential and analysis device according to the first minimum (point M in FIG. 1) of the differential calorimeter curve D.

Fig. 2a shows the time-temperature curve A, the differential calorimeter curve D and the total heat line G for the casting of rib cylinders with 0.2% FE-Si 75 inoculation, which, as mentioned, provides 75% rejects.

651 486

4th

The melt was kept in the pan for about 6 minutes after the inoculation and then poured with the casting temperature TG of 1280: C.

From the curves D and G of FIG. 2a, the device calculates the curves of FIG. 2b, which shows the temporal release of the heat of solidification. 2b, the base line G 'is the linearized total heat line G and the curve D' is the distance between D and G in the respective time intervals, based on G '. You can see the surface that reflects the successive release of the heat.

The hatched area W of 3 cal / g is calculated from the time t in seconds multiplied by the difference quotient AT / At and by the specific heat cp as follows:

q = ÉÏ • t (sec) • cp = 0.5 • 30 • 0.2 = 3 cal / g = 12.5 J / g At

Fig. 3 shows the analog curves for the casting with 0.2% FE-Si + Cer mixed metal treatment, which provides castings without rejects. This melt was also treated in the pan for about 6 minutes after the inoculation and then poured at Tg = 1262 ° C.

Another advantage of the method according to the invention over the known method, in which the strength of spheroidal cast iron was determined by comparing the curve shape in the area of austenite-pearlite transformation with test curves, is that when the first derivative of the time-temperature curve, that is the differential calorimeter curve D, through which the pearlite reaction changes direction, the temperature at which the conversion begins is ejected by the computer. As an example, FIG. 4 shows how the expected tensile strength for a spheroidal cast iron depends on the temperature Tp at which the pearlite reaction begins.

table

Casting: vaccination 0.2% vaccination 0.2%

Ribbed cylinder GGL 25 FeSi 75 FeSi + Cer-

Time: pan approx. 6 min. (Fig. 2) mixed metal

(Fig. 3)

analysis

Carbon equivalent CE 3.97 3.94

Degree of saturation Sc 0.933 0.927

Casting temperature Tg ° C 1280 1262 heat of crystallization cal / g or J / g

Total cal / g

59.4

60.9

Y / g

249

254

Dendrites cal / g

16.8

15.3

Y / g

70

64

Eutectic cal / g

42.6

45.6

Y / g

178

191

5

10th

15

20th

25th

30th

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40

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55

60

65

S

3 sheets of drawings