HighTemperature Vacuum Furnace for Tensile TestingE. A. Smith and R. W. Guard Citation: Review of Scientific Instruments 27, 386 (1956); doi: 10.1063/1.1715580 View online: http://dx.doi.org/10.1063/1.1715580 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/27/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Quenching Furnace for Elevated Temperature Tensile Test Rev. Sci. Instrum. 37, 893 (1966); 10.1063/1.1720357 Vacuum Furnace for HighTemperature Diffusion Anneals Rev. Sci. Instrum. 34, 193 (1963); 10.1063/1.1718309 Compact HighTemperature Vacuum Furnace Rev. Sci. Instrum. 32, 924 (1961); 10.1063/1.1717563 HighTemperature PressureVacuum Furnace Rev. Sci. Instrum. 30, 885 (1959); 10.1063/1.1716372 HighTemperature Laboratory Vacuum Furnace Rev. Sci. Instrum. 30, 290 (1959); 10.1063/1.1716602

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THE REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 27, NUMBER 6 JUNE, 1956

High-Temperature Vacuum Furnace for Tensile Testing

E. A. SMITH AND R. W. GUARD General Electric Research Laboratory, Schenectady, New York

(Received December 8, 1955; revised version received April 9, 1956)

A resistance furnace using a tantalum tube heater has been constructed for tensile testing wire specimens of metals that require protection from the atmosphere, at temperatures up to 1600°C. Specimens of molybde­num and tungsten have been successfully tested in the range from 1000° to 1600°C.

INTRODUCTION

W ITH the demand increasing for alloys that must function at extremely high temperature, it be­

comes necessary to understand more about their behavior in the range above 1000°C. The furnace to be described was designed for tensile testing wire specimens of metals that require protection from the atmosphere at temperatures up to 1600°C. Although it was built specifically for use with an Instron testing machine, it could be adapted for use with other machines.

Tests on molybdenum and tungsten wires in the range from 1000° to 1600°C have been successfully carried out.1,2

FURNACE ASSEMBLY (FIG. 1)

The tantalum heater (A) is 0.010 in. thick, 10 in. long, and! in. in diameter. The copper supports (B) on which it is mounted carry the current. The heater is mounted rigidly on the bottom cross-member of the supporting assembly (G), along with a ring of alumina (D) that supports a molybdenum radiation shield (C).

The top of the heater is flexibly connected through heavy copper jaws (B) and flat phosphor bronze springs (E) to the top cross-member (G' ). This flexibility allows the heater tube to expand and contract without buckling. To cut off radiation from the top of the heater, a cap of alumina (F) fits down over the opening. It has a hole at the center to accomodate the specimen. The main supports, two !-in. diameter copper rods, are insulated from and fastened to a !-in. thick brass base. Insulators are so arranged that the supports conduct current to the heater through the cross-members (G and G').

VACUUM CHAMBER

The vacuum chamber consists of a is-in. length of nine-inch-diameter Pyrex tubing with a i-in. wall thick­ness and a /6-in. Duralumin cap with a gasket recessed into its under side. The cap is fastened to the gasketed bottom plate by four rods with wing nuts.

The bottom plate has a vacuum connection on the under side and the lower anchor rod of the load string is fastened to the upper side. Power and thermocouple leads are brought through it using glass-to-metal seals. Grooves in the bottom of the plate ride two rails that are

1 R. P. Carreker, Jr., and R. W. Guard, J. Metals 8, No.2, 178 (February, 1956).

2 J. W. Pugh (personal communication).

mounted on a steel base. The base IS bolted to the moving head of the tensile machine.

POWER SUPPLY

A 5-kva stepdown transformer with a large variable transformer in the primary circuit was used to supply power. 160 amperes at 12 volts was required to heat the furnace to 1600°C.

LOAD CELL CONNECTIONS

An assembly of stainless steel sylphons, with gasketed flanges at either end, is bolted at the top to a standard 50-pound Instron load cell (K), and at the bottom to the cap of the vacuum chamber (Figs. 2 and 3).

Leaks through the load cell are sealed by a small sylphon (L), welded to the upper part of the load string (M) and fitted to a flange with rubber gasket inserted between the large sylph on assembly and the bottom rim of the load cell.

FIG. 1. Sketch of heater assembly showing position of thermocouple.

386

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VACUU1VI FURNACE 387

Load variation due to extension of the small sylphon is negligible for the movement is exactly the amount the weigh bar deflects.

OPERATION

To insert a specimen for testing, the furnace is slid forward to clear the cap and the outer glass cylinder is removed. The specimen is placed in a small tube, which is guided down through the holes in the ceramics, and the end of the specimen is placed in the lower clamp. The tube is removed and the clamp tightened, securing the specimen at the bottom. The top clamp with its T bar is fastened to the top of the specimen.

The glass cylinder is replaced, the enclosure pushed back into position and bolted down, and the upper end of the load string hooked to the T bar on the specimen. Wing nuts are drawn down until the enclosure is tightly sealed.

The complete assembly is shown in Fig. 3.

INSTRON LOAD CELL (Kl

t,"" ...... I>"-----SMALL SYLPHON ASSEMBLY (I)

SMALL SYLPHON (L)

LARGE SYLPHON ASSEMBLY (N)

~-~-=l-_ UPPER MEMBER OF LOAD STRING (M)

FIG. 2. Sketch of sylphon assembly on upper pull rod.

A vacuum system is connected and the chamber evacuated. It is possible to pump down to 0.5 micron; however, it is sometimes necessary to heat the furnace a little to drive off water vapor from the ceramics. About four pounds on the load scale must be balanced out during evacuation.

Since the specimen clamps are outside the hot zone, it is necessary to provide a means of establishing a gauge length. To do this, a reduced section is ground near the center of the specimen. A gauge length having t the cross-section area has been found to be satisfactory.

A platinum-platinum-lO% rhodium thermocouple mounted near the center of the heater is used to measure temperature. A temperature variation of two degrees Centigrade is maintained over the gauge length.

FIG. 3. Furnace assembly showing connection to load cell, glass envelope, and the position in the testing machine.

The Instron testing machine is designed to give a constant rate of head travel during a test. By using a reasonable gauge length (2 to 5 in.) and by making the components of the load string sufficiently heavier than the gauge section, it is possible to consider the head travel as being the extension of the gauge length.

OPERATING PROBLEMS

There are three main difficulties that limit the tem­perature of operation to 1600°C. The glass cylinder becomes overheated by radiation from the heater as­sembly. A water-cooled metal enclosure could be substi­tuted. The rubber gaskets soften as they get hot. They could be made of silicone rubber. The ceramics have a tendency to liberate adsorbed gases or water vapor at high temperature. The use of a getter in the chamber might remedy this condition. These improvements might raise the temperature at which the furnace can be operated.

ACKNOWLEDGMENTS

Roland P. Carreker, Jr. drew up the original design of the apparatus. J. D. Lubahn made several helpful sug­gestions connected with the design. During construction of the equipment, J. P. Horaczek made many helpful contributions to the final design.

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