Applied Mechanics and Materials Vols. 148-149 (2012) pp 1473-1477 Online available since 2011/Dec/22 at www.scientific.net © (2012) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMM.148-149.1473
Emissivity Calibration and Temperature Measurement of High Strength Steel Sheet in Hot Stamping Process Hongqing Li1, a, Yisheng Zhang1, b,*, Liang Wang1, c, Xiaowei Tian1, d Chao Wang1, e and Bin Zhu1, f 1
State Key Laboratory of Material Processing and Die and Mould Technology, Huazhong University of Science & Technology, Luoyu Road, Wuhan, Hubei, 430074,China a
[email protected],
[email protected],
[email protected]
d
[email protected],
[email protected],
[email protected]
Keywords: Emissivity, Calibration, Temperature measurement ， High strength steel ， Hot stamping, Infrared thermography
Abstract. In order to meet the requirements of improving hot stamping process, the temperature distribution of high strength steel (HSS) sheets should be observed. However, the accurate measurement of temperature field in transfer where the temperature falls rapidly proves to be difficult due to the limitation of thermocouple. The infrared thermography could be used for accurate measurement of temperature field but the emissivity which is crucial to the accuracy and reliability of measurement results is not available. In this paper, a method to calibrate emissivity of HSS blank was developed for temperature measurement in hot stamping using infrared thermography. Experiments were performed on the emissivity calibration of the HSS Advanced 1500 in hot stamping. Emissivity dependence on temperature was investigated, so was the impact of little variation in emissivity on the temperature measurement. The validation tests were made and the results reveal that this calibration method of emissivity is accurate and feasible. This method is also appropriate for the calibration of emissivity in other temperature measurement situations. Introduction In the recent decades, high strength steel (HSS) has found its wide application in automotive industry, due to its contribution to reducing vehicle weight, improving safety, and crashworthiness qualities [1]. There are several production technologies for HSS components. The hot stamping technology (press hardening) is one of the most successful in producing complex components with superior mechanical properties [2]. Hot stamping, in which temperature measurement and control is extremely important, can be described by the following steps; punching of blanks, heating to 900◦C in a furnace to austenite followed by simultaneous forming and quenching in forming tools [3]. However, the accurate measurement of temperature, for which several techniques have been developed over time in various manufacturing processes, is not simple and straightforward in hot stamping process. This is mainly due to the uneven distribution of temperature and high cooling rate of steel sheet which could be as much as 250K/s. Thermocouple was used for temperature measurement of tool in hot stamping of HSS by Naderi [4]. As for the steel sheet, there are hardly any direct measurement methods investigated. Thus, although the embedded thermocouple is widely used in other cases, it is not appropriate for accurate temperature measurement of the sheet in hot stamping for its long response time and lack of convenience in operation [5]. Besides, thermocouple could only be used for spot temperature measurement. It is impossible to obtain the temperature field which is important in not only the actual production but also the finite element method of hot stamping [6]. Infrared thermography can be used for accurate measurement of temperature due to its three basic advantages：non-contact temperature measurement which has no impact on physical properties of objects under measurement and equipment; fast response and wide range of measurement. Emissivity which is crucial to the accuracy and reliability of measurement results, however, proves to be difficult to get. Many factors such as composition, temperature, surface state and measuring All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 218.199.85.98-17/01/12,11:33:21)
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direction, could affect the value of emissivity [7]. A little variation in emissivity may cause big changes in the measurement results, especially for the long wave thermography (8~13um). So it is very important of the appropriate setup of emissivity in hot stamping [8]. As a result, calibration of emissivity according to the measurement environment becomes prerequisite. However, few investigations were made on the emissivity of HSS in hot stamping. This paper develops a method to calibrate emissivity of HSS in hot stamping which can be used in the temperature measurement using infrared thermography. Emissivity dependence on the sample temperature while the other parameters have been set has been investigated. In addition, the parameters were set based on calibrated emissivity changing along with temperature in the FLIR Research IR (affiliated software of FLIR) to measure the surface temperature of HSS in hot stamping. Experimental Theoretical background. An infrared camera measures and images the emitted infrared radiation from an object. The captured energy is composed of three contributions: • Energy emitted by the surface which mainly depends on its temperature and emissivity. • Energy reflected on the surface. • Energy emitted by the column of air between the surface measured and the sensor system [9]. We can derive a formula for the calculation of the object temperature from the calibrated camera output according to that description： = + 1 − + 1 − .
(1)
Where ε is the emissivity of the object which indicates (1 – ε) is the reflectance of the object, is the transmittance of the atmosphere, , , and stand for the total radiation power, the radiation power of object, the radiation power of ambient reflection sources and the radiation power of atmosphere respectively. The camera output signal which is proportional to the power input, is generated from the received radiation power . We multiply each term of Eq. 1 by the constant C, replace the CW by the corresponding U according to the same equation, then solve for Uobj, and we can get: U =
U!! +
"
U#$%& +
"
U'!( .
2
The calculation could be made if the following parameter values are supplied: the object emissivity ε, object distance Dobj, the relative humidity, the temperature of the object surroundings Trefl and the temperature of the atmosphere Tatm. The first among these parameters is the most important because little variation could cause big change in temperature while the influences of the others could be omitted provided the surroundings do not contain large and intense radiation sources. Ideally the emissivity should be measured under the conditions of the required application if accurate measurement of temperature is required. Material and experiment methods. The steel sheet (100mm×30mm×1.5mm) used in this experiment was a type of high strength steel graded as Advanced 1500. The chemical components of the steel in weight percentage measured from the electro-spectrometer are shown in Table 1. Table 1 The chemical composition in [wt %] of the workpiece material (Advanced 1500) Material C Mn Si B P S Ti Mo Advanced 1500 0.2 1.64 0.85 0.001 0.005 0.001 0.022 0.01 The schematic diagram of experiment apparatus, shown in Fig. 1, mainly comprises of an infrared thermography，a furnace with gas protecting device and temperature controller, an ABB robot for delivery and data acquisition system. The Trefl (reflected apparent temperature) should be determined according to the direction of FLIR before the measurement. The specific parameters set in FLIR Research IR (affiliated software of FLIR) are shown in Table 2.
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Type FLIR A320
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Table 2 Specific parameters set in FLIR Research IR Relative humidity ε (default) Dobj [m] Tatm [℃] Trefl [℃] 0.7 2.5 27 22 0.5
Fig. 1 Temperature measurement facility A K-type thermocouple had been firmly welded on the surface of the steel sheet (Advanced 1500) before it was delivered to the furnace. Besides, ceramic tubes were put on the thermocouple to avoid contact of the two wires at high temperature. An ABB robot was used to deliver the steel sheet between the furnace and the press mold as it moves faster and this could be similar to the actual processing. After being heated in the NC furnace for 3minutes at constant 950℃ in nitrogen atmosphere to get full austenization, the steel sheet was delivered to the press mould. The temperature of the steel sheet surface is measured by bare K-type spot welded thermocouples, which was widely recognized. And meanwhile the temperature of the steel sheet surface was also measured by the infrared thermography (FLIR A320), where all the parameters including the emissivity (default) had been set. The sensitivity of the imager (spectral range 7.5 ~ 13µm) is below 50mK, with error rate at 2%. The measured data from thermocouples were collected with the precise NI USB6008 DAQ card (12bit ADC, 10kc/s). Later, the emissivity was adjusted in the FLIR Research IR so that the temperature of the welding position of thermocouple could be the same as it is measured in LabVIEW. The collected data were recorded and processed in the same computer so that time difference of two monitoring systems could be avoided. Ultimately, as an application, the calibrated emissivity of Advanced 1500 was used in the measurement of temperature field in hot stamping. Results and Discussion In Fig. 2, the calibrated emissivity of Advanced 1500 is shown as a function of the temperature. It is obviously noticed that the emissivity of the sample decreases as the temperature falls. However, while the temperature of the sample changes from 230℃to630℃, variation of emissivity from 0.611to0.647 is not large as expected. So, we can obtain the average emissivity value in this temperature range is 0.632. The phenomenon that the decline of emissivity with falling temperature slows down after 550℃ could be attributed to the formation of oxide in surface. As seen, the temperature measured from the infrared thermography with calibrated emissivity shows good agreement with the temperature measured with the K type thermocouple in Fig. 3 which also includes the influence of little deviation（0.1）in emissivity on the temperature measurement. However, if the emissivity is set 0.1higher (shown as green line), the mean
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temperature value measured is about 50 degrees lower than the temperature value measured with thermocouple at the same time, and vice versa. This indicates that the emissivity should be calibrated before using infrared temperature measurement.
Fig. 2 Relationship between emissivity and temperature Fig. 3 Calibration results of the emissivity Fig. 4 shows the temperature field of the steel sheet on the bottom die and the time temperature curve at a given time. The curve was drawn by the affiliated software according to the temperature of the zone in Fig. 4 marked as E1. The infrared signal of the bottom die was firstly captured by the thermography, then the hot steel sheet delivered from the furnace, and at last came the temperature of the bottom die when the steel sheet was moved away. The three stages above could account for the changes of temperature in the curve. It is noticed that the cooling rate of the air-cooled steel sheet was about 2.9℃/s. The measured temperature field could be used as reference for the finite element method. Reproducibility tests were made and the results showed a good repeatability with a mean dispersion less than 5%.
Fig. 4 Temperature field in FLIR Research IR Conclusions In this study, experiments were performed on the emissivity calibration of Advanced 1500 in hot stamping. The experiment results indicate that the calibration of emissivity is crucial for the temperature measurement using infrared facilities. A little variation in emissivity (0.1) could cause big changes in the measurement results (more than 50℃). So the emissivity should be measured under the conditions of the required application if accurate measurement of temperature is required.
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The temperature measured from the infrared thermography with calibrated emissivity shows good agreement with the temperature measured with the K type thermocouple, indicating that the method developed in this paper is accurate and feasible. This method is also appropriate for the calibration of emissivity in other temperature measurement situations. The calibration experiment showed that the emissivity of the Advanced 1500 decreases as the temperature falls. The future work will focus on the influence on emissivity of other factors. The average emissivity value of Advanced 1500 with temperature range from200℃to 650℃is 0.632, which could be used as reference emissivity value in infrared sensor as the emissivity of that could only be set as a constant. Acknowledgement This work was supported by National Basic Research Program of China (973 Program) under the contract No. 2010CB630803. References [1] Karbasian, H., Tekkaya, A. E, A review on hot stamping, Journal of Materials Processing Technology. 15(2010) 2103-2118. [2] Chao Wang, B. Z., Yisheng Zhang, Jie Shi,Han Dong, Hot-Stamping Process Simulation and Optimize Research for Collision Beam of Automobile Door, Advanced Materials Research. 3(2011) 201 - 203. [3] Komanduri, R., Hou, Z. B, A review of the experimental techniques for the measurement of heat and temperatures generated in some manufacturing processes and tribology, Tribology International. 34(2001) 653-682. [4] Naderi, M., Ketabchi, M., Abbasi, M., Bleck, W. Analysis of microstructure and mechanical properties of different high strength carbon steels after hot stamping, Journal of Materials Processing Technology. 211(2011) 1117-1125. [5] Chang-Da, W. Investigation of steel emissivity behaviors: Examination of Multispectral Radiation Thermometry (MRT) emissivity models, International Journal of Heat and Mass Transfer. 53(2010) 2035-2043. [6] Zhu, B., Zhang, Y., Li, J., Wang, H., Ye, Z, Simulation research of hot stamping and phase transition of automotive high strength steel, Materials Research Innovations. 15(2011) 426-430. [7] Del Campo, L., Pérez-Sáez, R. B., González-Fernández, L., Esquisabel, X., et al. Emissivity measurements on aeronautical alloys, Journal of Alloys and Compounds. 489(2010) 482-487. [8] Tadeusz, W. Emissivity measurements on electronic microcircuits, Measurement. 41(2008) 503-515. [9] Meca Meca, F. J., Rodrıǵ uez Sanchez, F. J., Sanchez, P. M. n, Calculation and optimisation of the maximum uncertainty in infrared temperature measurements taken in conditions of high uncertainty in the emissivity and environment radiation values, Infrared Physics & Technology. 43(2002) 367-375.
Mechanical Engineering, Materials and Energy 10.4028/www.scientific.net/AMM.148-149
Emissivity Calibration and Temperature Measurement of High Strength Steel Sheet in Hot Stamping Process 10.4028/www.scientific.net/AMM.148-149.1473
