NATIONAL BUREAU OF STANDARDS REPORT

3 $6£

THERMAL CONDUCTIVITY OF A SPECIMEN OF STAINLESS STEEL

TYPE 446

by

Lloyd E. Richards

Henry E. Robinson

Report To

Champion Spark Plug Company

Toledo, Ohio

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

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©Office of Basic Instrumentation ©Office of Weights and Measures.
NATIONAL BUREAU OF STANDARDS REPORT

MBS PROJECT NBS REPORT

1003-30-1015 January 5, 1955 3S6g

THERMAL CONDUCTIVITY OF A SPECIMEN OF STAINLESS STEEL

TYPE 446

by

Lloyd E. Richards

Henry E. Robinson

Heating and Air Conditioning Section

Building Technology Division

for

Champion Spark Plug Company

Toledo, Ohio




0. §. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, reprf

unless permission Isob 25,D,C. Such permli

cal ly prepared If thal

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

part, is prohibited

dards, Washington

rt has been speclfiirt for its own use,

THERMAL CONDUCTIVITY OF A SPECIMEN OF STAINLESS STEEL

TYPE 446

by

Lloyd E. Richards and Henry E. Robinson

I. Introduction

A specimen, designated as Stainless Steel Type 446,

was submitted for measurements of thermal conductivity

by the Champion Spark Plug Company of Toledo, Ohio*

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, and the upper end was cooled by water circulated

through the upper well* The heating element, having about

53aresistance, was made by passing nichrome wire through

multiple-hole porcelain tubing. 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. Test 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

- 2 -

the thermocouples just above the heaters. Cooling

water at constant temperature was pumped through the

well in the specimen 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 bar heater

weremeasured by means of standard resistors and a

precision potentiometer. Temperature conditions were

considered satisfactorily steady when no thermocouple

on the bar changed temperature at a rate greater than

0.3 degree C per hour; several readings taken over a

period of three or more hours after steady conditions

were attained were averaged for each test.

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

versts 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 thermocouples on the bar was thus computed and used,

together with the measured distance and temperature difference between them, and the cross-sectional 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

2 to 10 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.

- 3 -

For each of the separate tests, the thermal

conductivity was computed for each of the six 4 cm

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 temperatures.

The results of the measurements are shown in

figure 2, and are represented by a straight line drawn

through the plotted values* The line was determined

by the method o.f least squares.

Table 1 lists thermal conductivity values for

various mean temperatures, as taken from the straight

line.

V. Discussion of Results

The plotted points show some scatter from the

straight line. 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'5 wires. To minimize heat conduction effects, the thermocouple wires were led away from the hot junction 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 12 to 29 degrees

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

TABLE I

Stainless Steel Type 446

Mean Temperature

r°ci

Thermal Conductivity

(Watts cm-^-C -^-)

100 0*207

200 0 « 216

300 0*224

400 0*233

500 0.241

600 0.250

700 0.253

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THE NATIONAL BUREAU OF STANDARDS

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

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