Thermal Shock Testing of Refractory Materials Using anElectron Beam Materials Test Facility

Corinna Thomser,* Erik Skiera, Andreas Buerger, Jochen Linke,Thorsten Loewenhoff, Axel Schmidt, Lorenz Singheiser, and Rolf Steinbrech

Forschungszentrum Juelich, Institute of Energy and Climate Research (IEK-2), D-52425, Juelich,Germany

The thermal shock behavior of ceramic refractory materials is of particular interest. These materials are usually testedin downward thermal shocks by cooling hot specimens in water or air. In contrast to this method, the use of electron

beam material testing facilities offers an alternative approach to perform thermal shock tests on heating. The method ofthermal shock testing by an electron beam is introduced for ceramic refractory materials within this article. Finally, thematerial degradation of a carbon bonded MgO refractory material due to thermals shocks, applied with the electron beam,

is presented.

Introduction

For any field of steel production processes, ceramicrefractory materials are indispensable. The thermalshock behavior of these materials is a widely investi-gated research field for many years.1–22 However, dueto difficulties in the application of appropriate testingprocedures, conventional thermal shock tests are inmost cases performed in a downward mode by fastcooling of a hot specimen of the test material in a

medium like water, oil, or air.1–10 These test proce-dures have found acceptance in national as well asinternational test standards and testing procedures forrefractory materials.11–14

Basically, two facts of these conventional downwardthermal shock testing methods have to be criticized:

1. A downward mode for the thermal shock perfor-mance of refractory materials is not an applicationrelevant for steel production processes as “hot”liquid metal hits “cold” refractory material (differ-ence in thermal stress state).

2. The applied medium for the downward thermalshock tests can interact chemically with the refrac-

*c.thomser@fz-juelich.de

© 2012 The American Ceramic Society

Int. J. Appl. Ceram. Technol., 1–6 (2012)DOI:10.1111/j.1744-7402.2011.02714.x

tory material (the influence of the chemical reactionon the thermal shock performance cannot be deter-mined).In the past, a few experimental as well as numerical

approaches of performing thermal shocks on heatinghave already been made for ceramic refractory materialsby radiation, direct electrical heating, laser heating, liquidmetal, or even by application of gas welding torches.15–22

Nevertheless, a completely new and very fast test-ing method for ceramic refractory materials in vacuumby application of an electron beam testing facility isintroduced, mainly to prevent any chemical reactions oftested materials with a test medium and to apply a fastthermal shock on heating (application relevant thermalstress state). Electron beam material testing facilities arealready well known for tests of plasma facing compo-nents, which are used in nuclear fusion devices andconsists of materials like beryllium, tungsten, or car-bon-based materials for plasma facing components.23–34

As already mentioned before, these testing facilities alsooffer an innovative method for thermal shock of cera-mic refractories on heating.

The Electron Beam Test Facility JUDITH 2

The electron beam material testing facilityJUDITH 2 (Juelich Divertor Test Facility in the HotCells) is placed in a hot materials laboratory at Fors-chungszentrum Juelich (FZJ), and is equipped with acommercial electron beam generator (high power elec-tron beam gun EH 800 V; Von Ardenne Anlagentech-nik, Dresden, Germany).30–34 A schematic drawing ofthe test facility is shown in Fig. 1. This testing facilityis mainly used for thermal fatigue as well as for thermalshock experiments of carbon based beryllium and tung-sten materials for divertor and first wall components ofnuclear fusion devices.23,27,30–34

The maximum beam power of the JUDITH 2testing facility is 200 kW, and the acceleration voltagecan be adjusted between 40 and 60 kV. The vacuumchamber of JUDITH 2 was produced using TrinosVakuum-Systeme (Asslar, Germany) and is of cylindri-cal shape with 800 mm diameter and a length of1800 mm. Tested materials and components aremounted on a numerically controlled carrier system,which is fixed to the door of the vacuum chamber.31–33

Depending on the settings, a wide range of testparameters is available, like power densities up to

1 GW/m2 for a time scale between less than 1 ms anda few seconds. The maximum heat loaded area by anelectron beam is 400 mm 9 400 mm.30 As the pene-tration depth of the electrons into the material is onlya few micrometers, a nearly complete surface heat loadcan be assumed for ceramic refractory materials. Theelectron beam has a Gaussian-like profile with an elec-tron beam diameter between ~3 and ~17 mm (fullwidth at half maximum) depending on different param-eters, e.g., acceleration voltage, vacuum quality, andbeam power.31–33 Details of the electron beam profileare described by Loewenhoff.34

The Tested Material

Carbon bonded MgO-C fire bricks show uniquethermal, chemical, and mechanical properties. Theyhave a high thermal shock resistance, high structureflexibility, as well as an enormous spalling resistance.Therefore, they are well established in steelmaking pro-cesses, in particular, for applications in converters, elec-tric arc furnaces, and steel treatment ladles.5,35

The tested material is a resin-bonded magnesiumoxide refractory material with a graphite content of10% (MgO-C). It was carbonized at 1000°C, and hasa porosity of 12%. The bulk density is determined tobe 2.95 g/cm3, and the maximum grain size is 4 mm.Further details of the properties and production pro-cesses for this group of materials are described byAneziris and Hampel.5, 35, 36

Fig. 1. Schematic drawing of the electron beam facilityJUDITH 2.33

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Test Procedure

The test specimens have a cylindrical shape with adiameter of 50 mm and a height of 20 mm. The testassembly is presented in Fig. 2. Six refractory samplescan be installed in the vacuum chamber of theJUDITH 2 facility at the same time.

For a homogeneous distribution of heat within thisthermal shock experiment, a grid-like beam patternwith 61 points was chosen. This means that the beamwith an electron beam diameter of ~3 mm (full widthat half maximum) was guided in a grid over the samplewith a dwell time of 5 ls on each point, which is whyone pass of the electron beam takes 305 ls . As theloading period was chosen to be 100 ms followed by2 s for cooling (inter pulse time), the grid pattern wasrepeated for 328 times. The loading scenario is shownin Fig. 3. Due to the short dwell time, the grid-likebeam pattern and the electron beam diameter of~3 mm (full with at half maximum), a homogeneousheat loading over an area of 20 mm 9 20 mm, can beassumed.

The resulting average absorbed power density forthis area is ~42 MW/m2 using the assumption of 8%of electron reflection for the tested material. Details onpower density calculations were described by Schmidtand Loewenhoff.33,34 The vacuum in the test chamberis determined to be 3.5 9 10�4 mbar.

The loading scenario was repeated 100 times. Inthe manner described above, 100 thermal shock cyclesof 100 ms electron beam loading followed by 2 s inter

pulse time are applied in total. During this inter pulsetime, the electron beam was moved to the water cooledcopper beam dump, as the electron beam cannot beswitched on and off in such a short period of time.

Test Results

A macroscopic image of the test specimen aftertesting is presented in Fig. 4 (the in resin embeddedcross section is shown, which is cut in the middle ofthe sample). As expected, surface erosion is observed.The maximum measured scratch depth is ~3 mm, and

Water cooled copperbeam dumps

Test specimens

50 mm

Fig. 2. Test assembly for refractory materials in the electron beam test facility JUDITH 2.

Fig. 3. Schematic drawing for the chosen loading scenariowith the electron beam (beam diameter at full width at halfmaximum ~3 mm for 3.5 9 10�4mbar, absorbed power density~42 MW/m2, acceleration voltage 40 kV, pulse duration100 ms by repeating the grid-like beam pattern 328 times, interpulse time for cooling 2 s, 100 cycles applied).

www.ceramics.org/ACT Thermal Shock Testing of Refractory Materials 3

is always in the center of the sample. Due to theGaussian-like beam profile, small amounts of energy ofthe electron beam are also distributed outside the20 mm 9 20 mm test area, which leads to little mate-rial erosion on the outer side of the specimen. Thedepth of the erosion zone can be used for the quantita-tion of material erosion and compared with other typesof carbon bonded refractory materials.

A more detailed SEM picture of the materialwithin the erosion zone is shown in Fig. 5. A brittle

debonding of MgO grains in the eroded zone wasfound. Furthermore, it is clearly shown in the SEMimage that no decarburization took place during thetest due to the absence of any test medium in the vac-uum test chamber. The surface erosion was also docu-mented by an optical camera (Fig. 6). The straightbright lines in the picture correspond to the light emis-sion of the eroded particles. As their speed is quitehigh, they are documented as straight lines in the imageand not as spheres due to the exposure time of thecamera.

The maximum temperatures, measured with a fastsingle color pyrometer (KMGA 740, Kleiber Infrared,Saalfeld/Saale, Germany) in the heat loaded area, wereapproximately in the range of 2000°C. As evaporatedcarbon was deposited to the windows of the vacuumchamber during the tests, exact temperature measure-ments using thermographic methods were difficult toperform.

Conclusion & Outlook

A new approach of testing refractory materials inthermal shock tests by an electron beam materialtesting facility was presented. The test procedure

Fig. 4. Macroscopic image of cross section of the tested MgO-Cspecimen (surface erosion is documented).

Fig. 5. SEM image of the cross section of the test specimen ofthe erosion zone, MgO particles are disconnected and removed,and no additional decarburization in surface regions wasobserved.

Fig. 6. CCD image of particle erosion due to electron beamloading, brittle destruction.

4 International Journal of Applied Ceramic Technology—Thomser, et al. 2012

was described, and the resulting surface erosion forMgO-C refractory material containing 10% carbon wasshown.

The approach with an electron beam test facilityhas the following main advantages in comparison withconventional test methods of thermal shock testing forrefractory materials:

1. The chemical interaction of the tested material witha cooling or heating medium is excluded (tests per-formed under vacuum).

2. Thermal shock tests are done on heating (applica-tion relevant stress state).

3. Very fast heating of the sample surface available(ms range).

4. Wide range of parameters in terms of temperatureand heating time applicable.

5. No decarburization of refractory materials contain-ing carbon.

6. The surface erosion can be quantified easily.7. The number of thermal shock cycles is variable.8. The thermal shock is induced by a homogeneous

surface heat load.

For conventional thermal shock tests for MgO-Cmaterials, mechanical data are reported in literature,like the change of Young’s Modulus and deformationin three point bending tests due to the number ofdownward thermal shocks in compressed air from 950°C to room temperature.5 With the electron beam, theadherence of microstructure constituents can be directlytested as the material damage level is also higher due tomore severe thermal shocks.

Thus, the method of electron beam thermal shocktesting for ceramic refractory materials offers variouspossibilities for novel, comparative studies in the nearfuture between different carbon containing refractorymaterials.

Acknowledgments

We thank Michael Lowis and Karsten Dominiczakfor their assistance with the JUDITH 2 experiments,and Vasileios Roungos (TU Bergakademie Freiberg) forthe material making and Prof. Aneziris (TU Bergakade-mie Freiberg) for his helpful suggestions and comments.We gratefully acknowledge the German Scientific Foun-dation (DFG) for supporting the investigations in terms

of the Priority Program SPP 1418 “Refractory Initiativefor the Reduction of Emissions – FIRE.”

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