Continuous temperature measurements on the pouring stand for casting moulds

Transcription

1 ARCHIVES of FOUNDRY ENGINEERING Published quarterly as the organ of the Foundry Commission of the Polish Aademy of Sienes ISSN ( ) Volume 8 Issue / / Continuous temperature measurements on the pouring stand for asting moulds W. eśniewski *, A. Karwiński Foundry Researh Institute, Centre of Designing and Prototyping, Zakopiańska 73, 3-48 Kraków, Poland * Corresponding author. address: wles@iod.krakow.pl Reeived ; aepted in revised form Abstrat The results of temperature measurements of liquid iron alloys obtained by means of the pyrometer, PDR-800 series, are presented in the paper. The measurements were performed in onditions determined by the kind of a pouring devie. The results obtained for bottom-tap ladles were supplemented by laboratory measurements. These results allow explaining signifiant differenes in the results of temperature measurements performed in pouring ladles by means of the pyrometri method and immersible thermoouple, whih - in turn - improves assessment of metal thermal parameters in pouring devies. Keywords: Pyrometer, Measuring temperature, Metal asting. Pyrometri methods of measuring temperature All bodies of a temperature higher than the absolute zero temperature are soures of thermal radiations. The radiation intensity inreases with the inrease of the body temperature and the maximum intensity range shifts in the diretion of short waves. The intensity of monohromati radiation (T,λ), of a wave length λ, radiated by a blak body of a temperature T, is determined by the Plank s Funtion : ( T, λ ) = λt 5 λ exp where: C = 3,745 x 0-6 [Wm ] - onstant C =,4388 x 0 - [Km] - onstant The radiation intensity r (T,λ),emitted by the atual body surfae of a temperature T, is always lower than the radiation intensity emitted by the blak body. () The ratio of both intensities is alled the monohromati emissivity: ε(λ,t) = r / ε λ E (0,) () The monohromati radiation intensity (T,λ) emitted by an arbitrary body of a thermodynami temperature T and a monohromati emissivity ε(λ,t) is given by the equation: ( T, λ ) = ε( λ, T ) λt 5 r λ exp Radiation harateristis of the body under testing are ompared in the optial pyrometers with orresponding harateristis of the blak body. Those harateristis an be ompared aording to the total equality priniple or the spetral radiation intensity or by means of the spetral omposition identity gauge. Thus, for the temperature measuring we an use either: total radiation pyrometer, monohromati pyrometer or olour sensitive pyrometer. (3) ARCHIVES of FOUNDRY ENGINEERING Volume 8, Issue /008,

2 Correspondingly with the priniple of measurements we distinguish three temperatures funtionally related to the atual thermodynami temperature and the body emissivity: energeti T e, blak (luminane) T z, and olour sensitive T b. The knowledge of relevant emissivities is neessary when the thermodynami temperature is determined by the total radiation or monohromati pyrometer, sine it enables alulations of orretions for pyrometer indiations. The emissivity oeffiient is nearly always the temperature funtion, and for liquid ast iron alloys a violent flutuations in its value ours. Quality of measurements performed by the total radiation or monohromati pyrometers is often not suffiient for the tehnologial needs. A better auray of the temperature measurement is provided when the spetral omposition - not the energy - are ompared. If ε(λ,t)=onstant, then the determination of the thermodynami temperature of the objet is possible by means of measuring the ratio of radiation intensities emitted for two wave length λ i λ []: T = λ λ ln + λ λ ( λ λ) ( λ λ) λ λ 5ln This equation an be simplified to the following formula: F = Aln + B where: A and B - pyrometer onstants depending on its onstrution. In this ase, the ratio of radiation intensities is the expliit temperature funtion. However, let s assume that the ondition ε(λ,t)=onstant is not met. In suh ase the olour sensitive temperature T b indiated by the pyrometer differs from the atual temperature T r of the tested body. In order to determine errors in the temperature measuring by bihromati pyrometers the differenes in measuring signals for the blak body model and for the approximation of the known funtion of the wolfram radiation intensity were numerially alulated []. Calulations were done when taking into aount the following onditions: Radiation of the tested objet is in aordane with the Plank s Funtion () and the approximate funtion of the wolfram radiation intensity. (4) (5) ε (λ,t) = λ (6) Pyrometer detetor is 00 % effiient - in the whole spetral range. The obtained results point to the neessity of introduing the orretion of indiations in bihromati pyrometers. However, advantages from the appliation of bihromati pyrometers should be here learly stated: Introdued orretion is quite small, Pyrometer indiations are not dependent on violent flutuations of emissivity ourring in liquid iron alloys, Destrution (dirtying) of a signifiant part of optial system does not ause any essential measurement errors (radiation derease by 50% will ause the hange in monohromati pyrometer indiations on the level of 00K ). Table. Temperature differenes alulated for two models of pyrometers utilising different wave length: the first model λ =0.6; λ =0.7[μm]. The seond model λ =0.9; λ =.0[μm]. λ =0.6; λ =0.7[μm] λ =0.9; λ =.0[μm] T[K] ΔT=T(ε )-T(ε ) ΔT=T(ε )-T(ε ) Pyrometri temperature measurement general remarks Pyrometri temperature measurement of metal poured into astings are very seldom used. There are several reasons of suh situation, however in the first plae a psyhologial fator should be emphasized. The measurement is onsidered useless sine the result is obtained after the pouring of the asting mould. Another fator signifiantly limiting the appliation of pyrometry is the neessity of ensuring the orret temperature measurement of the metal stream undergoing dynami disturbanes. Pyrometers of PDR-800 series were designed and tested in atual onditions of the asting house. It allowed to limit signifiantly the influene of the stream flow quality on the auray of temperature measurements. Certain levels of the prodution repeatability take plae in every asting house. Castings of similar sizes and from the same material are being produed. The temperature of metal poured into asting moulds should be pratially the same. If during the metal pouring several measurements are performed (pyrometer enables taking measurements every seond) the auray of the result will be up to 0K. Analysis of the obtained results allows to determine expliitly the temperature of the metal poured into the asting mould. Therefore performing ontinuous temperature measurements and storing the results is very important. Suh doumentation signifiantly helps foundry engineers to determine reasons of asting defets and to introdue hanges to the prodution proess leading to quality improvements of the produts. 3. Pyrometri temperature measurement urrent situation iquid ast iron prepared in upolas or in indution furnaes is usually poured into a transporting ladle, whih often onstitutes 76 ARCHIVES of FOUNDRY ENGINEERING Volume 8, Issue /008, 75-80

3 a pouring ladle. Inreasing requirements of lients as well as the neessity of lowering prodution osts fore manufaturers to make systemati temperature measurements at various stages of the prodution proess. These measurements should aurately determine the pouring temperature. In pratie the measurement is done by means of expendable thermoouple tips fixed to an immersible probe of the stationary or mobile digital meter. The measurement is done in the ladle before pouring the asting moulds and its result is generally onsidered the temperature of the poured metal. However, signifiant differenes in temperatures indiated by pyrometers and thermoouple were notied during an installation of the ontinuous measuring system for temperatures of ast iron poured into asting moulds on automati asting lines (system of ontinuous temperature reording RTO) as well as during training of employees for operating bihromati manual pyrometers PGR-800. Temperature differenes often exeeded 80K. The user austomed to applying thermoouple interpreted usually those differenes as a low auray of pyrometri measurements. The reasons of those disrepanies seemed interesting and worth explaining, sine it might allow the better ontrol of the prodution quality. Fig.. Pyrometer PDR-800 series equipped with memory of 5 measurements and the objetive of a foal distane 35mm 4. Temperature measurements in pratie The performed measurements were aimed at assessment the thermal parameters of ast iron ooling in ertain, seleted pouring devies. In addition, the obtained results were to allow explaining the reasons of disrepanies between measurements done by means of the pyrometer and by immersible thermoouple. Pouring stands, in the viinity of whih it would be safe to perform pyrometri temperature measurements without the neessity of preparing the speial stand, were seleted. The asting house having both a bottom-tap ladle (of 5 ton apaity) and tilting ladles was hosen for performing measurements. It was possible to perform several dozen of pyrometri measurements during metal pouring from a ladle to a asting mould at both stands. The pyrometer of PDR-800 series with the objetive of a foal distane 35mm was used. The pyrometer measuring angle was /35 radians and enabled relatively safe measuring from a distane of.5 to meters. A metal stream was of a diameter minimum 4 m, on both stands. Measuring data registered in the memory were sent to the omputer and then presented in the graphi form. During eah series of investigations also measurements with the expendable thermoouple tips were made. 4. Measurements on the tilted ladle The first measurement series was performed on the Disamati prodution line. The temperature was measured when the ast iron was poured from the transporting ladle to the smaller pouring ladle. A apaity of the pouring ladle allowed to perform pouring for 8 to 0 minutes. Fig.. Preparation of the transporting and pouring ladle for pouring the asting moulds in the moulding line During the pouring proess measurements were not made. Registered data allowed to present graphially the proess of the metal temperature hanges ourring in the ladle during its gradual emptying. Measurements were performed for two transporting ladles. The measurement by means of the immersible thermoouple in the transporting ladle was pratially possible only after its delivery. A harateristi phenomenon possible to be registered only by means of the pyrometer is a gradual inrease of the metal temperature, when it is poured to the smaller ladle, followed by its gradual derease by several dozen degrees. The differene of indiated temperatures was 0-30K for the first measurements done by the pyrometer and thermoouple. 4.. Measurements in the bottom-tap ladle Grey ast iron was obtained in an ar furnae. Metal temperature determined in the furnae by the thermoouple was equal 350 O C. Metal was poured into a bottom-tap ladle of a apaity of 5 ton. Pyrometri temperature measurements were done at the metal tapping into asting moulds. During the measurement the ladle was shifted above the suessive moulds, whih were poured by metal. The total duration time of the measurement during tapping was several dozen of minutes. ARCHIVES of FOUNDRY ENGINEERING Volume 8, Issue /008,

4 Fig.3. Temperature hanges of liquid ast iron registered by the bihromati pyrometer on the pouring stand. The obtained results indiate the signifiant temperature lowering during metal pouring from the furnae to the bottom-tap ladle. Temperature hanges of liquid metal during its tapping from the ladle was so interesting, that it was deided to supplement those investigations by the laboratory tests. 5. aboratory measurements simulating industrial onditions Comparative measurements were performed in the indution furnae of a apaity 40kg. The applied ast iron was of the following omposition: C.8%, Si.88%, Mn 0.49%, P 0.035%, S 0.0%. Two platinum thermoouples in the quartz 78 ARCHIVES of FOUNDRY ENGINEERING Volume 8, Issue /008, 75-80

5 tube housing were plaed in the metal bath. The first one was plaed near the furnae lining, the seond in the furnae entre. When the metal was heated to a temperature of 500 O C the furnae supply was swithed off. Temperature hanges were reorded digitally, whih enabled their graphial presentation. Fig.4. Temperature hanges of the ast iron poured to the asting moulds from the bottom-tap ladle of 5 ton apaity. The obtained results indiate the signifiant temperature lowering during metal pouring from the furnae to the bottom-tap ladle. Temperature hanges of liquid metal during its tapping from the ladle was so interesting, that it was deided to supplement those investigations by the laboratory tests. Fig.5. Final temperature hanges of the metal ooling in the indution furnae. Temperature hanges reorded in the furnae entre are marked in blue olour, while those reorded near the furnae lining in blak olour. Those temperatures were pratially idential in the range from 500 to 95 O C. This fat indiates the intensive heat exhange aused by onvetion movements. This phenomenon is taken into aount in the software MAGMASOFT-Stil. However, the module intended for the simulation of the ast iron solidifiation does not take into aount this phenomenon. ARCHIVES of FOUNDRY ENGINEERING Volume 8, Issue /008,

6 6. Conlusions Experiments performed both in the industry and in the laboratory indiate the possibility of signifiant differenes in the temperature results obtained by the pyrometer and by immersible thermoouple. The required auray of the temperature of the liquid metal on the level of 0K is very problemati, when the expendable tips are used. In the majority of ases those results ould not be interpreted as the temperature of the metal poured into asting moulds, beause the ondition of simultaneously performed proesses of the temperature measuring and the metal pouring is not fulfilled. Temperature hanges aused by ooling in the time elapsing between the measurement and the pouring an be so signifiant, that the results obtained by thermoouple should be used mainly for the ontrol of the tehnologial proess. The orret measurement of the temperature of metal poured into the asting moulds is only possible by means of the pyrometri method. When several moulds are poured from the same ladle the metal temperature differenes of several dozen degrees are to be expeted. It is worth to ompare the results obtained for the bottom-tip ladle with the results obtained in the Foundry Institute. In both ases we an notie temperature inrease after reahing the freezing point urve. The effet reorded by hane - ombined with the data obtained in the laboratory onditions - allowed to verify signifiantly the pyrometri measurements. Referenes [] Snopko W.N.: Spiektralnyje mietody optizieskoj pirometrii nagrietoj powierhnosti, Nauka i Tehnika, Mińsk ZSRR, 988. [] Miller D.: Pirometry, Wydawnitwa Naukowo-Tehnizne, Warszawa, Polska ARCHIVES of FOUNDRY ENGINEERING Volume 8, Issue /008, 75-80