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A R C H I V E S

o f

F O U N D R Y E N G I N E E R I N G

Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences

ISSN (1897-3310)

Volume 8

Issue 1/2008

75 – 80

15/1

Continuous temperature measurements

on the pouring stand for casting moulds

W. Leśniewski

*

, A. Karwiński

Foundry Research Institute, Centre of Designing and Prototyping,

Zakopia

ń

ska 73, 31-418 Kraków, Poland

*

Corresponding author. E-mail address: wles@iod.krakow.pl

Received 16.07.2007; accepted in revised form 24.07.2007

Abstract

The results of temperature measurements of liquid iron alloys obtained by means of the pyrometer, PDR-1800 series, are presented in the paper. The measurements were performed in conditions determined by the kind of a pouring device. The results obtained for bottom-tap ladles were supplemented by laboratory measurements. These results allow explaining significant differences in the results of temperature measurements performed in pouring ladles by means of the pyrometric method and immersible thermocouple, which - in turn - improves assessment of metal thermal parameters in pouring devices.

Keywords: Pyrometer, Measuring temperature, Metal casting

1. Pyrometric methods of measuring

temperature

All bodies of a temperature higher than the absolute zero temperature are sources of thermal radiations. The radiation intensity increases with the increase of the body temperature and the maximum intensity range shifts in the direction of short waves. The intensity of monochromatic radiation L(T,λ), of a wave length λ, radiated by a black body of a temperature T, is determined by the Planck’s Function :

(

)

5 2 1

1

λ

1

T

c

exp

λ

c

λ

T,

L

=

(1)

where: C1 = 3,745 x 10 - 16

[Wm2 ] - constant C2 = 1,4388 x 10 - 2 [Km] - constant

The radiation intensity Lr(T,λ),emitted by the actual body surface of a temperature T, is always lower than the radiation intensity L emitted by the black body.

The ratio of both intensities is called the monochromatic emissivity:

ε(λ,T) = Lr/L ελ

E (0,1) (2)

The monochromatic radiation intensity L(T,λ) emitted by an arbitrary body of a thermodynamic temperature T and a monochromatic emissivity ε(λ,T) is given by the equation:

(

)

(

)

5 2 1

1

r 1

λT c exp

λ

c T

λ,

ε λ

T, L

⎠ ⎞ ⎜

= (3)

(2)

Correspondingly with the principle of measurements we distinguish three temperatures functionally related to the actual thermodynamic temperature and the body emissivity: energetic Te, black (luminance) T cz, and colour sensitive Tb .

The knowledge of relevant emissivities is necessary when the thermodynamic temperature is determined by the total radiation or monochromatic pyrometer, since it enables calculations of corrections for pyrometer indications.

The emissivity coefficient is nearly always the temperature function, and for liquid cast iron alloys a violent fluctuations in its value occurs. Quality of measurements performed by the total radiation or monochromatic pyrometers is often not sufficient for the technological needs. A better accuracy of the temperature measurement is provided when the spectral composition - not the energy - are compared. If ε(λ,T)=constant, then the determination of the thermodynamic temperature of the object is possible by means of measuring the ratio of radiation intensities emitted for two wave length λ1 i λ2 [1]:

(

)

(

)

1

2 1 2 2 2 1 2 1 1 2 2 2 1 λ λ 5ln λ λ c λ λ L L ln λ λ c λ λ T 1 − + −

= (4)

This equation can be simplified to the following formula:

B

L

L

Aln

F

1 2

+

=

(5)

where: A and B - pyrometer constants depending on its construction.

In this case, the ratio of radiation intensities is the explicit temperature function.

However, let’s assume that the condition ε(λ,T)=constant – is not met. In such case the colour sensitive temperature Tb indicated by the pyrometer differs from the actual temperature Tr of the tested body. In order to determine errors in the temperature measuring by bichromatic pyrometers the differences in measuring signals for the black body model and for the approximation of the known function of the wolfram radiation intensity were numerically calculated [2].

Calculations were done when taking into account the following conditions:

♦ Radiation of the tested object is in accordance with the Planck’s Function (1) and the approximate function of the wolfram radiation intensity.

ε (λ,T) = 0.5406 - 0.1507λ (6)

♦ Pyrometer detector is 100 % efficient - in the whole spectral range.

The obtained results point to the necessity of introducing the correction of indications in bichromatic pyrometers. However, advantages from the application of bichromatic pyrometers should be here clearly stated:

♦ Introduced correction is quite small,

♦ Pyrometer indications are not dependent on violent fluctuations of emissivity occurring in liquid iron alloys,

♦ Destruction (dirtying) of a significant part of optical system does not cause any essential measurement errors (radiation decrease by 50% will cause the change in monochromatic pyrometer indications on the level of 100K ).

Table 1.

Temperature differences calculated for two models of pyrometers utilising different wave length: the first model λ1=0.6;

λ2=0.7[μm]. The second model λ1=0.9; λ2=1.0[μm].

λ1=0.6; λ2=0.7[μm] λ1=0.9; λ2=1.0[μm] T[K]

ΔT=T(ε1)-T(ε2) ΔT=T(ε1)-T(ε2) 1000 1200 1400 1600 1800 11.0 15.6 21.0 27.1 34.1 26.1 37.0 49.9 64.6 81.3

2. Pyrometric temperature

measurement – general remarks

Pyrometric temperature measurement of metal poured into castings are very seldom used. There are several reasons of such situation, however in the first place a psychological factor should be emphasized. The measurement is considered useless since the result is obtained after the pouring of the casting mould. Another factor significantly limiting the application of pyrometry is the necessity of ensuring the correct temperature measurement of the metal stream undergoing dynamic disturbances. Pyrometers of PDR-1800 series were designed and tested in actual conditions of the casting house. It allowed to limit significantly the influence of the stream flow quality on the accuracy of temperature measurements.

Certain levels of the production repeatability take place in every casting house. Castings of similar sizes and from the same material are being produced. The temperature of metal poured into casting moulds should be practically the same. If during the metal pouring several measurements are performed (pyrometer enables taking measurements every second) the accuracy of the result will be up to 10K. Analysis of the obtained results allows to determine explicitly the temperature of the metal poured into the casting mould.

Therefore performing continuous temperature measurements and storing the results is very important. Such documentation significantly helps foundry engineers to determine reasons of casting defects and to introduce changes to the production process leading to quality improvements of the products.

3. Pyrometric temperature

measurement – current situation

(3)

a pouring ladle. Increasing requirements of clients as well as the necessity of lowering production costs force manufacturers to make systematic temperature measurements at various stages of the production process. These measurements should accurately determine the pouring temperature. In practice the measurement is done by means of expendable thermocouple tips fixed to an immersible probe of the stationary or mobile digital meter.

The measurement is done in the ladle before pouring the casting moulds and its result is generally considered the temperature of the poured metal. However, significant differences in temperatures indicated by pyrometers and thermocouple were noticed during an installation of the continuous measuring system for temperatures of cast iron poured into casting moulds on automatic casting lines (system of continuous temperature recording – RTO) as well as during training of employees for operating bichromatic manual pyrometers PGR-1800. Temperature differences often exceeded 80K. The user accustomed to applying thermocouple interpreted usually those differences as a low accuracy of pyrometric measurements. The reasons of those discrepancies seemed interesting and worth explaining, since it might allow the better control of the production quality.

Fig.1. Pyrometer PDR-1800 series equipped with memory of 512 measurements and the objective of a focal distance 135mm

4. Temperature measurements – in

practice

The performed measurements were aimed at assessment the thermal parameters of cast iron cooling in certain, selected pouring devices. In addition, the obtained results were to allow explaining the reasons of discrepancies between measurements done by means of the pyrometer and by immersible thermocouple.

Fig.2. Preparation of the transporting and pouring ladle for pouring the casting moulds in the moulding line

During the pouring process measurements were not made. Registered data allowed to present graphically the process of the metal temperature changes occurring in the ladle during its gradual emptying. Measurements were performed for two transporting ladles.

Pouring stands, in the vicinity of which it would be safe to perform pyrometric temperature measurements without the necessity of preparing the special stand, were selected. The casting house having both a bottom-tap ladle (of 5 ton capacity) and tilting ladles was chosen for performing measurements. It was possible to perform several dozen of pyrometric measurements during metal pouring from a ladle to a casting mould – at both stands. The pyrometer of PDR-1800 series with the objective of a focal distance 135mm was used. The pyrometer measuring angle was 2/135 radians and enabled relatively safe measuring from a distance of 1.5 to 2 meters. A metal stream was of a diameter minimum 4 cm, on both stands.

The measurement by means of the immersible thermocouple in the transporting ladle was practically possible only after its delivery. A characteristic phenomenon – possible to be registered only by means of the pyrometer – is a gradual increase of the metal temperature, when it is poured to the smaller ladle, followed by its gradual decrease by several dozen degrees. The difference of indicated temperatures was 20-30K for the first measurements done by the pyrometer and thermocouple.

Measuring data registered in the memory were sent to the computer and then presented in the graphic form. During each series of investigations also measurements with the expendable thermocouple tips were made.

4.2. Measurements in the bottom-tap ladle

Grey cast iron was obtained in an arc furnace. Metal temperature determined in the furnace by the thermocouple was equal 1350OC. Metal was poured into a bottom-tap ladle of a capacity of 5 ton. Pyrometric temperature measurements were done at the metal tapping into casting moulds. During the measurement the ladle was shifted above the successive moulds, which were poured by metal. The total duration time of the measurement during tapping was several dozen of minutes.

4.1 Measurements on the tilted ladle

(4)

Fig.3. Temperature changes of liquid cast iron registered by the bichromatic pyrometer on the pouring stand.

The obtained results indicate the significant temperature lowering during metal pouring from the furnace to the bottom-tap ladle. Temperature changes of liquid metal during its tapping from the ladle was so interesting, that it was decided to supplement those investigations by the laboratory tests.

5. Laboratory measurements

simulating industrial conditions

(5)

tube housing were placed in the metal bath. The first one was placed near the furnace lining, the second in the furnace centre. When the metal was heated to a temperature of 1500OC the

furnace supply was switched off. Temperature changes were recorded digitally, which enabled their graphical presentation.

Fig.4. Temperature changes of the cast iron poured to the casting moulds from the bottom-tap ladle of 5 ton capacity.

The obtained results indicate the significant temperature lowering during metal pouring from the furnace to the bottom-tap ladle. Temperature changes of liquid metal during its tapping

from the ladle was so interesting, that it was decided to supplement those investigations by the laboratory tests.

Fig.5. Final temperature changes of the metal cooling in the induction furnace.

Temperature changes recorded in the furnace centre are marked in blue colour, while those recorded near the furnace lining in black colour. Those temperatures were practically identical in the range from 1500 to 1295OC. This fact indicates the intensive heat exchange caused by convection movements. This phenomenon is taken into account in the software

(6)

6. Conclusions

Experiments performed both in the industry and in the laboratory indicate the possibility of significant differences in the temperature results obtained by the pyrometer and by immersible thermocouple. The required accuracy of the temperature of the liquid metal on the level of 10K is very problematic, when the expendable tips are used. In the majority of cases those results could not be interpreted as the temperature of the metal poured into casting moulds, because the condition of simultaneously performed processes of the temperature measuring and the metal pouring is not fulfilled. Temperature changes caused by cooling in the time elapsing between the measurement and the pouring can be so significant, that the results obtained by thermocouple should be used mainly for the control of the technological process.

The correct measurement of the temperature of metal poured into the casting moulds is only possible by means of the pyrometric method. When several moulds are poured from the

same ladle the metal temperature differences of several dozen degrees are to be expected. It is worth to compare the results obtained for the bottom-tip ladle with the results obtained in the Foundry Institute. In both cases we can notice temperature increase after reaching the freezing point curve. The effect recorded by chance - combined with the data obtained in the laboratory conditions - allowed to verify significantly the pyrometric measurements.

References

[1] Snopko W.N.: Spiektralnyje mietody opticzieskoj pirometrii nagrietoj powierchnosti, Nauka i Technika, Mińsk ZSRR, 1988.

Referências

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