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Evaluating strength at ultra-high temperatures—Methods and results
Materials Science and Engineering A 483–484 (2008) 587–589
Evaluating strength at ultra-high temperatures—Methods and results Rainer V¨olkl a,∗ , Bernd Fischer b , Manuel Beschliesser c , Uwe Glatzel a a
Dept. Metals and Alloys, University of Bayreuth, 95447 Bayreuth, Germany Dept. SciTec, University of Applied Sciences Jena, 07745 Jena, Germany c PLANSEE Aktiengesellschaft, Technology Centre, 6600 Reutte, Austria
b
Received 6 June 2006; accepted 28 September 2006
Abstract Proprietary equipment for mechanical testing at ultra-high temperatures by ohmic heating is outlined. Strain is measured with a video extensometer with an accuracy of up to ¯ε ≈ ±0.00025%. Stability and accuracy of the test system are evaluated on Pt- and refractory alloys. These specially designed and built test facilities are compared to commercially available high-vacuum test chambers with tungsten heater. © 2007 Elsevier B.V. All rights reserved. Keywords: High temperature; Creep; Strength; Extensometer; Tensile
1. Introduction
2. General design of test facilities
Mechanical properties of engineering materials at ultrahigh temperatures are required for the development of optimized alloys and processes, specifications in design engineering, modelling of component performance and for quality assurance in production. However, high prices and limited availability of these materials along with extremely high testing-temperatures make the use of commercial test facilities difficult. In suitable setups for ultra-high temperature testing, several problems have to be solved. In order to minimize costs the samples should be small and the test facility should be simple in design and operation. At an early stage of an alloy development program, limited available material also often dictates small specimen. Continuous strain measurement at temperatures above 1000 ◦ C should be performed without affecting the specimen. Both the absolute specimen temperature and the temperature distribution over the gauge length have to be controlled throughout testing time. The grip ought to allow the application of a definite load even at highest temperatures. Other difficulties arise when tests have to be performed in certain atmospheres in order to prevent oxidation or corrosion of the sample.
Special test facilities [1–5] for evaluating strength of metallic materials at ultra-high temperatures were designed and built. Ohmic heating was chosen for easy access to the sample, fast heating and cooling cycles, and simplicity in design and operation. Low temperatures at the grips allow the use of inexpensive copper or stainless steel. Usually, specimens in the form of thin wires or strips are tested. The test facilities permit tests either in air, vacuum or under a protective gas atmosphere. Both constant tensile load creep tests and high temperature tensile tests can be executed. A schematic diagram of the test facilities is given in Fig. 1. Temperature is measured with a pyrometer. Neuer and Jaroma-Weiland [6] recommend Pt/Rh alloys as reference materials with known emissivity. Hence, simple calibration of the test system for materials with unknown emissivity was performed with a thin foil of a Pt/Rh alloy pasted on the specimen. Alternatively, dual color pyrometers avoid many problems caused by a not precisely known emissivity. Strain is measured with a CCD camera controlled by the software SuperCreep [3–5] which continuously determines the distances between corresponding markers in the central zone of the specimen where the temperature is uniform. Suitable markers may have the form of small shoulders (Fig. 2), or simply by winding thin wires around the specimen. SuperCreep determines the distance between markers on digital image with a sub-pixel accuracy of about 0.2 pixels. At a maximum measurable strain of ε ≈ 60%, a maximum error
∗
Corresponding author. Tel.: +49 921 555553; fax: +49 921 555561. E-mail address: [email protected] (R. V¨olkl).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.09.171
588
R. V¨olkl et al. / Materials Science and Engineering A 483–484 (2008) 587–589
Fig. 1. Scheme of ultra-high temperature test facility.
ε ≈ ±0.07% of the strain was determined [5]. The repeatability therefore is >0.25% of full-range output. The accuracy can be improved by increasing the initial distance between the markers. However, the maximum measurable strain then decreases. Alternatively, the error ¯ε of the mean can be decreased by mean filtering over n single measurements according to √ ¯ε = ε/ n. Actual hardware with dual core processor can
grab and process images with the live frame rate. At the University Bayreuth, 1024 × 768 pixels images are processed with a frame rate of 30 s−1 , leading to an error of ¯ε ≈ ±0.0025% at a sampling-rate of 1 min−1 . Future setups with high-speed cameras will allow frame rates of up to 10,000 s−1 . Hence, an error of ¯ε ≈ ±0.00025% is anticipated at a sampling-rate of 1 min−1 .
Fig. 2. Self-radiating specimen with markers for the video extensometer.
R. V¨olkl et al. / Materials Science and Engineering A 483–484 (2008) 587–589
Fig. 3. Measured thermal strain of pure Pt compared to literature data [7,8].
3. Performance tests The temperature distribution was measured by a second pyrometer on a 100 mm long strip of a Pt–10% Rh alloy [4]. The maximum temperature was 1500 ◦ C at the specimen centre. In a zone 30 mm around the specimen centre the temperature was 1500 ± 5 ◦ C. In a zone 10 mm between the markers the temperature was within 1500 ± 2 ◦ C. During a following creep test the temperature between the markers could be held to 1500 ± 3 ◦ C until necking occurred. Due to ohmic heating the temperature outside a necked region is always lower, whereas in the necked region the temperature is kept at the desired value. To verify the performance of the video extensometer the thermal expansion of pure Pt was studied [5]. The standard deviations σ ε = ±(0.04–0.05)%, given in Fig. 3, are in good agreement with the estimated error of ε ≈ ± 0.07%. Fig. 3 further shows that the measured mean thermal strains lay always in between the values reported in literature [7,8]. Another tension test series was performed on tungsten sheet material which was taken from the commercial production line of PLANSEE [9]. Tensile tests under reducing atmosphere (Ar–5% H2 ) with an engineering strain rate of 2 × 10−2 s−1 were executed at the University of Applied Sciences Jena. For a cross-check, additional tensile tests were performed on the same tungsten batch with a conventional test facility at the Technology Centre of PLANSEE, Reutte, Austria. The specimen together with tungsten grips is surrounded by a tungsten heater, altogether placed in a water-cooled vacuum chamber evacuated to 10−5 mbar. The chamber itself is mounted in a conventional load frame. Strain was recorded via cross-head movement. Test results are shown in Fig. 4. The data recorded in Jena fit into the data which have been recorded at PLANSEE. However, the evolution of the yield strength is different although the abso-
589
Fig. 4. Tensile properties of tungsten sheet material at an initial strain rate of 2 × 10−2 s−1 .
lute deviation is only about 10 MPa. In the overlap-region the data recorded in Jena appears to be higher than the one determined at PLANSEE. Since determination of 0.2% plastic yield strength involves strain measurement the origin of error is found in different types of strain measurements. 4. Conclusions The presented test equipment proved to be reliable, cheap and easy in operation and maintenance. The equipment is capable of generating creep and tensile testing data at ultra-high temperatures of 2500 ◦ C. The video extensometer SuperCreep reaches currently an accuracy of ¯ε ≈ ±0.0025% at a samplingrate of 1 min−1 . Future versions are anticipated to reach ¯ε ≈ ±0.00025%. The results of tensile tests on tungsten are in good agreement with data recorded by a conventional load frame setup. References [1] B. Fischer, R. Helmich, H. T¨opfer, DD Patent 245576 A3, 1982. [2] B. Fischer, H. T¨opfer, R. Helmich, Silikattechnik 35 (1984) 329– 331. [3] R. V¨olkl, D. Freund, B. Fischer, D. Gohlke, in: DVM Berichtsband, 638, Deutscher Verband f¨ur Materialforschung und -pr¨ufung e.V., Berlin, 1998, pp. 211–218. [4] R. V¨olkl, D. Freund, B. Fischer, J. Testing Eval. 31 (2003) 35–43. [5] R. V¨olkl, B. Fischer, Exp. Mech. 44 (2004) 121–128. [6] G. Neuer, G. Jaroma-Weiland, Int. J. Thermophys. 19 (1998) 917–929. [7] A.G. Degussa, Edelmetall-Taschenbuch, second ed., H¨uthig-Verlag, Heidelberg, 1995. [8] J.W. Arblaster, Plat. Met. Rev. 41 (1997) 12–21. [9] B. Fischer, S. Vorberg, R. V¨olkl, M. Beschliesser, A. Hoffmann, in: G. Kneringer, P. R¨odhammer, H. Wildner (Eds.), Proc. 16th International PLANSEE Seminar on Powder Metallurgical—High Performance Materials, 1, PLANSEE, Reutte, Austria, 2005, pp. 504–517.
Evaluating strength at ultra-high temperatures—Methods and results Rainer V¨olkl a,∗ , Bernd Fischer b , Manuel Beschliesser c , Uwe Glatzel a a
Dept. Metals and Alloys, University of Bayreuth, 95447 Bayreuth, Germany Dept. SciTec, University of Applied Sciences Jena, 07745 Jena, Germany c PLANSEE Aktiengesellschaft, Technology Centre, 6600 Reutte, Austria
b
Received 6 June 2006; accepted 28 September 2006
Abstract Proprietary equipment for mechanical testing at ultra-high temperatures by ohmic heating is outlined. Strain is measured with a video extensometer with an accuracy of up to ¯ε ≈ ±0.00025%. Stability and accuracy of the test system are evaluated on Pt- and refractory alloys. These specially designed and built test facilities are compared to commercially available high-vacuum test chambers with tungsten heater. © 2007 Elsevier B.V. All rights reserved. Keywords: High temperature; Creep; Strength; Extensometer; Tensile
1. Introduction
2. General design of test facilities
Mechanical properties of engineering materials at ultrahigh temperatures are required for the development of optimized alloys and processes, specifications in design engineering, modelling of component performance and for quality assurance in production. However, high prices and limited availability of these materials along with extremely high testing-temperatures make the use of commercial test facilities difficult. In suitable setups for ultra-high temperature testing, several problems have to be solved. In order to minimize costs the samples should be small and the test facility should be simple in design and operation. At an early stage of an alloy development program, limited available material also often dictates small specimen. Continuous strain measurement at temperatures above 1000 ◦ C should be performed without affecting the specimen. Both the absolute specimen temperature and the temperature distribution over the gauge length have to be controlled throughout testing time. The grip ought to allow the application of a definite load even at highest temperatures. Other difficulties arise when tests have to be performed in certain atmospheres in order to prevent oxidation or corrosion of the sample.
Special test facilities [1–5] for evaluating strength of metallic materials at ultra-high temperatures were designed and built. Ohmic heating was chosen for easy access to the sample, fast heating and cooling cycles, and simplicity in design and operation. Low temperatures at the grips allow the use of inexpensive copper or stainless steel. Usually, specimens in the form of thin wires or strips are tested. The test facilities permit tests either in air, vacuum or under a protective gas atmosphere. Both constant tensile load creep tests and high temperature tensile tests can be executed. A schematic diagram of the test facilities is given in Fig. 1. Temperature is measured with a pyrometer. Neuer and Jaroma-Weiland [6] recommend Pt/Rh alloys as reference materials with known emissivity. Hence, simple calibration of the test system for materials with unknown emissivity was performed with a thin foil of a Pt/Rh alloy pasted on the specimen. Alternatively, dual color pyrometers avoid many problems caused by a not precisely known emissivity. Strain is measured with a CCD camera controlled by the software SuperCreep [3–5] which continuously determines the distances between corresponding markers in the central zone of the specimen where the temperature is uniform. Suitable markers may have the form of small shoulders (Fig. 2), or simply by winding thin wires around the specimen. SuperCreep determines the distance between markers on digital image with a sub-pixel accuracy of about 0.2 pixels. At a maximum measurable strain of ε ≈ 60%, a maximum error
∗
Corresponding author. Tel.: +49 921 555553; fax: +49 921 555561. E-mail address: [email protected] (R. V¨olkl).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.09.171
588
R. V¨olkl et al. / Materials Science and Engineering A 483–484 (2008) 587–589
Fig. 1. Scheme of ultra-high temperature test facility.
ε ≈ ±0.07% of the strain was determined [5]. The repeatability therefore is >0.25% of full-range output. The accuracy can be improved by increasing the initial distance between the markers. However, the maximum measurable strain then decreases. Alternatively, the error ¯ε of the mean can be decreased by mean filtering over n single measurements according to √ ¯ε = ε/ n. Actual hardware with dual core processor can
grab and process images with the live frame rate. At the University Bayreuth, 1024 × 768 pixels images are processed with a frame rate of 30 s−1 , leading to an error of ¯ε ≈ ±0.0025% at a sampling-rate of 1 min−1 . Future setups with high-speed cameras will allow frame rates of up to 10,000 s−1 . Hence, an error of ¯ε ≈ ±0.00025% is anticipated at a sampling-rate of 1 min−1 .
Fig. 2. Self-radiating specimen with markers for the video extensometer.
R. V¨olkl et al. / Materials Science and Engineering A 483–484 (2008) 587–589
Fig. 3. Measured thermal strain of pure Pt compared to literature data [7,8].
3. Performance tests The temperature distribution was measured by a second pyrometer on a 100 mm long strip of a Pt–10% Rh alloy [4]. The maximum temperature was 1500 ◦ C at the specimen centre. In a zone 30 mm around the specimen centre the temperature was 1500 ± 5 ◦ C. In a zone 10 mm between the markers the temperature was within 1500 ± 2 ◦ C. During a following creep test the temperature between the markers could be held to 1500 ± 3 ◦ C until necking occurred. Due to ohmic heating the temperature outside a necked region is always lower, whereas in the necked region the temperature is kept at the desired value. To verify the performance of the video extensometer the thermal expansion of pure Pt was studied [5]. The standard deviations σ ε = ±(0.04–0.05)%, given in Fig. 3, are in good agreement with the estimated error of ε ≈ ± 0.07%. Fig. 3 further shows that the measured mean thermal strains lay always in between the values reported in literature [7,8]. Another tension test series was performed on tungsten sheet material which was taken from the commercial production line of PLANSEE [9]. Tensile tests under reducing atmosphere (Ar–5% H2 ) with an engineering strain rate of 2 × 10−2 s−1 were executed at the University of Applied Sciences Jena. For a cross-check, additional tensile tests were performed on the same tungsten batch with a conventional test facility at the Technology Centre of PLANSEE, Reutte, Austria. The specimen together with tungsten grips is surrounded by a tungsten heater, altogether placed in a water-cooled vacuum chamber evacuated to 10−5 mbar. The chamber itself is mounted in a conventional load frame. Strain was recorded via cross-head movement. Test results are shown in Fig. 4. The data recorded in Jena fit into the data which have been recorded at PLANSEE. However, the evolution of the yield strength is different although the abso-
589
Fig. 4. Tensile properties of tungsten sheet material at an initial strain rate of 2 × 10−2 s−1 .
lute deviation is only about 10 MPa. In the overlap-region the data recorded in Jena appears to be higher than the one determined at PLANSEE. Since determination of 0.2% plastic yield strength involves strain measurement the origin of error is found in different types of strain measurements. 4. Conclusions The presented test equipment proved to be reliable, cheap and easy in operation and maintenance. The equipment is capable of generating creep and tensile testing data at ultra-high temperatures of 2500 ◦ C. The video extensometer SuperCreep reaches currently an accuracy of ¯ε ≈ ±0.0025% at a samplingrate of 1 min−1 . Future versions are anticipated to reach ¯ε ≈ ±0.00025%. The results of tensile tests on tungsten are in good agreement with data recorded by a conventional load frame setup. References [1] B. Fischer, R. Helmich, H. T¨opfer, DD Patent 245576 A3, 1982. [2] B. Fischer, H. T¨opfer, R. Helmich, Silikattechnik 35 (1984) 329– 331. [3] R. V¨olkl, D. Freund, B. Fischer, D. Gohlke, in: DVM Berichtsband, 638, Deutscher Verband f¨ur Materialforschung und -pr¨ufung e.V., Berlin, 1998, pp. 211–218. [4] R. V¨olkl, D. Freund, B. Fischer, J. Testing Eval. 31 (2003) 35–43. [5] R. V¨olkl, B. Fischer, Exp. Mech. 44 (2004) 121–128. [6] G. Neuer, G. Jaroma-Weiland, Int. J. Thermophys. 19 (1998) 917–929. [7] A.G. Degussa, Edelmetall-Taschenbuch, second ed., H¨uthig-Verlag, Heidelberg, 1995. [8] J.W. Arblaster, Plat. Met. Rev. 41 (1997) 12–21. [9] B. Fischer, S. Vorberg, R. V¨olkl, M. Beschliesser, A. Hoffmann, in: G. Kneringer, P. R¨odhammer, H. Wildner (Eds.), Proc. 16th International PLANSEE Seminar on Powder Metallurgical—High Performance Materials, 1, PLANSEE, Reutte, Austria, 2005, pp. 504–517.