NATIONAL BUREAU OF STANDARDS REPORT

403S

THERMAL CONDUCTIVITY OF A SPECIMEN OF INCONEL

Lloyd E„ Richards

Henry E„ Robinson

Report to

Argonne National Laboratory

Lemont, Illinois

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

by
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

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NATIONAL BUREAU OF STANDARDS REPORT

MBS PROJECT NBS REPORT

d

1003-30-l1ei5 April 11, 1955

THERMAL CONDUCTIVITY OF A SPECIMEN OF INCONEL

by

Lloyd E„ Richards

Henry E 0 Robinson

Heating and Air Conditioning Section

Building Technology Division

to

Argonne National Laboratory

Lemont, Illinois

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, re

unless permission ij

25, D, C, Such per

cal ly prepared if t

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 201 5.

in part, is prohibited

andards, Washington

port has been specifi-

»port for its own use.

4038

THERMAL CONDUCTIVITY OF A SPECIMEN OF INCONEL

by

Lloyd E„ Richards and Henry E 0 Robinson

I Introduction

A specimen, designated as Inconel, was submitted by the

Argonne National Laboratory of Lemont, Illinois for measurements of thermal conductivity.

II Preparation of the Specimen

The general arrangement of the test apparatus is shown

in Figure 1,

The lower end of the specimen was heated by an electrical

heating element inserted in the lower well of the specimen;

the upper end was cooled by water circulated through the upper well. The heating element, having about 56 ohms resistance, was made by passing nichrome wire through a multiple

hole porcelain cylinder l/2 inch in diameter, Chromel-alumel

thermocouples were attached at intervals of about 4 cm. along

the length of the bar by peening the junctions into one-millimeter radial holes about 2 mm deep in the side of the specimen,, An additional thermocouple was attached to the lower

end of the specimen. The specimen was supported on a thin

nichrome pin located in the bottom of a thick-walled stainless steel guard cylinder. The guard cylinder was also

equipped with a heater element and cooling coil and with thermocouples at appropriate positions.

The specimen-guard assembly was suspended in a large

sheet-metal container and the entire system insulated with a fine granular insulation of known thermal conductivity characteristics.

III lest Method

Electrical energy from a d.c, source was supplied to the

heater elements and adjusted so that a minimum temperature

difference between bar and guard existed at the thermocouples

just above the heaters. Cooling water at a constant temperature of about 40°C was pumped through the well in the speciment and through the coil on the guard. When steady temperature conditions had been attained, the emf’s of the thermocouples and the current through and voltage drop across the

2 =

bar heater were measured by means of standard resistors and

a precision potentiometer 0 Temperature conditions were considered satisfactorily steady when no thermocouple on the

bar changed temperature at a rate greater than 0.5 degree

C per hour; several readings taken over a period of three

or more hours after steady condtions were attained were averaged for each test.

To calculate the thermal conductivity, the observed temperatures of the bar and guard were plotted versus position

along the bar as abscissae and smooth curves were drawn

through the points . Corrections to the measured heat input

to the bar to account for heat interchange between the bar

and guard were made on the basis of the temperature differences

between them determined from the curves and using the thermal

conductance of the granular insulation at the appropriate

mean temperature. The corrections were made for the heat interchange (a) between the lower end of the bar and the guard,

(b) between the bar and guard at the heater region and (c) between bar and guard for each thermocouple span. The average

rate of heat flow between any two adjacent thermocouples on

the bar was thus computed and used, together with the measured

distance and temperature difference between them and the crosssectional area of the specimen, to calculate the average thermal conductivity for that span.

The maximum difference for any thermocouple span between

the computed heat flow in the span and the measured electrical

input to the heater ranged from 0.3 to 9»2 percent of the input, in the various ' tests . It is believed that since the calculated values of heat interchange could be evaluated with an

uncertainty of not more than 20 percent, the uncertainty in

the rates of heat flow used in computing the thermal conductivities were of the order of not more than 2 percent.

For each of the separate tests, the thermal conductivity

was computed for each of the six thermocouple spans on the

bar, corresponding to the mean temperature existing in each

span.

IV Test Results

The values of thermal conductivity obtained for each test

span were plotted against their corresponding mean temperature,

as shown in Figure 2, and are represented by a curved line

drawn through the plotted values. The line was determined by

fitting a quadratic equation to the data.
.
- 3 -

Table 1 lists thermal conductivity values for various

mean temperatures, as taken from the curve,

V Discussion of Results

The plotted points show some scatter from the curve.

Such scatter is an inverse measure of the precision of the

measurements, as affected by such factors as small random

inaccuracies in determining the thermocouple locations on the

bar, slight heat conduction along the thermocouple wires near

the hot junctions, and possible slight inhomogeneities in the

thermocouple wires. To minimize heat conduction effects, the

thermocouple wires were led away from the hot junctions

and wrapped around the bar for a few centimeters in the plane

of its cross section, in which the temperature should be fairly

uniform. However, since the temperature gradients along the

bar ranged from 11 to 33 degrees C per centimeter in the several

tests, some conduction effect on individual thermocouple readings probably could not be avoided.

Since the factors involved in inaccuracies in measurements

of thermocouple positions and temperatures were random in nature,

their effect was probably to decrease the precision of the values obtained for each span rather than to affect the overall

results in any one direction.

Test number one was found to give thermal conductivity

values materially higher than the other five tests so it was

not used in the calculation of the curve in Figure 2, This

shift of thermal conductivity with time seems to be an effect

of the thermal history of the specimen, and has been observed

in similar magnitude in several other measurements made on

Inconel, It may be of interest that the data for test number

one were taken less than 24 hours after first heating the specimen and that for test number two about 4$ hours after that for

test number one.

-4-

TABLE I

INCONEL

Mean Temperature

(C)

Thermal

(Watts

Conductivity

cnrlC- 1

) 100 o„ 136

200 o„ 149

300 0ol64

400 0 o l£0

500 0,19$

600 0„21$

700 0 0 240

370

CENTIMETERS

p.m'-r- r i

j. l—

0 5 10 15 20

APPARATUS FOR MEASURING THE THERMAL

CONDUCTIVITY OF METALS

F ig.l

rooj~ o^oof^'-^u') ^

* CVJcM-cvj — — — —; — ~

,.0,.lM0 Sll VM - AllAllOnaNOO 1VAIH3H1

IOO

200

300

400

500

600

700

TEMPERATUREC

FIGURE

2

THE NATIONAL BUREAU OF STANDARDS

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The functions of the National Bureau of Standards are set forth in the Act of Congress, March

3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and

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