NATIONAL BUREAU OF STANDARDB REPORT

7052

THERMAL CONDUCTIVITY OF A SPECIMEN OF

CARPENTER 20 STAINLESS STEEL

by

Thomas W. Watson and Henry E., Robinson

Report to the

National Aeronautics and Space Administration

Goddard Space Flight Center

Greenbelt, Maryland

U. $.. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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NATSONAS. BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1006-30-106^2 January 5, 1961 7052

THERMAL CONDUCTIVITY OF A SPECIMEN OF

CARPENTER 20 STAINLESS STEEL

by

Thomas W. Watson and Henry E. Robinson

Report to the

National Aeronautics and 'Space Administration

Goddard Space Flight Center

Greenbelt, Maryland

IMPORTANT. NOTICE

NATIONAL BUREAU OF ST/

Intended for use within the ( to additional evaluation and n

listing of this Report, either I the Office of the Director, Nal however, by the Government

to reproduce additional cople

ogress accounting documents Approved for public release by the subjected

j eproductlon, or open-literature

Director of the National Institute of on Is obtained In writing from Standards and Technology (NIST) permission Is not needed,

/-w 1 n irepared If that agency wishes on October 9, 2015.

<^BS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The^nial ' Conductivity of a S.pecinien of

Carpenter 20 Stain,ies.s Steel

by

k . . 1 Thomas W. Watson and Henry E. Robinson

1.

INTRODUCTION

A specimen of Carpenter 20 stainless steel was submitted

by the National Aeronautics and Space Administration, Goddard

Space Flight Center, Greenbelt, Maryland, for measurement of

its thermal conductivity in the temperature range 200° to

1400°F.

2

.

SAMPLE

The test sample was a bar 2.5^ cm in diameter and U6o0

cm long, A 9»0-cm length was cut from the submitted bar for

use in chemical analysis.. The letter of request (Ref,

S“9*+82-G) stated that the bar had been X-rayed and had undergone ultrasonic inspection.

The chemical composition as determined by an analysis

made at the National Bureau of Standards is given in Table 1,

3.

TEST APPARATUS AND METHOD

The thermal conductivity of the sample was determined by

means of a steady-state flow of heat longitudinally in a bar

specimen, with measurements of the temperatures existing at

the ends of six consecutive, approximately 3*5l-cm, spans

along the central length of the bar. Each determination

required a' pair of tests at moderately different temperature

conditions, and ‘yielded values of thermal conductivity at six

different mean temperatures.

The teat apparatus is shown schematically in Figure 1,

The specimen,’ a bar approximately 37 cm long ahd of uniform ext ernal dimensions , is supported at the top (coolant)

end concentrically within a stainless steel guard tube of

0,8-cm wall thickness, which in turn is held concentrically

within a cylindrical outer container. The specimen is drilled

at each end with a 1,35-cm hole cm deep. An electrical

heater is inserted and secured in ’the hole at the bottom (hot)
i
- 2 -

end, and the supporting fixture at the top end provides a liquid-tight connection for circulating a coolant through

the top drill-hole.

Temperatures along the specimen were indicated by seven

thermocouples located symmetrically about'the longitudinal

center of the specimen, spaced approximately 3»5l cm apart,

with one additional thermocouple near the bottom end of the

specimen. Thermocouples wese similarly located in almost

exactly corresponding longitudinal positions on the guard

tube.

The guard tube was equipped near its lower end with two

external circumferential electric heaters, as shown= The

guard tube was cooled at the top by means of a copper-tube

coil soldered cir cumf er entiallylat a position corresponding

in effect to that of the spe;cimen coolant well. Coolant was

pumped through the guard coil and specimen well in series

connection, as shown.

The electric heater for the specimen consisted of a porcelain cylinder 1.27 cm in diameter and 5=2 cm long, threaded

longitudinally with 26-gage nichrome heater wire. Its

resistance was approximately 22 ohms. Cur'rent was brought to

the heater through relatively large heater leads, to which

separate potential leads were connected at the,^,polnt where

they entered the porcelain core. The heater was energized by

an adjustable cons tant-voltage d-c source. Heater current

and voltage drop. ^measurements were made using standard resistors and the high precision manual potentiometer used for

thermocouple observations. The guard was heated with alternating current governed by a sensitive temperature controller

actuated by the guard temperature at- a selected positiono

-/:'A The thermocouples were made from 'calibrated chromel and

alumel 26-gage wires, electrically welded to form a spherical » junction about 0.10 cm in diameter. Junctions in the specimen were inserted into radially-drilled holes 0,11 cm i.n

diameter and 0,17 cm deep in the side of the bar, and tightly

secured by punch p'ficking the metal around the holet, The

thermocouple wires 'were individually insulated electrically

with fiberglass sleeving, and were wrapped around the bar

(one in each direction) and tied at the back' to ’Secure them’

in the transverse plane of the junction. The wires were

brought out through the powder insulation near the guard

tube. The thermocouples in the guard tube were similarly

attached to its exterior surface. The longitudinal positions

of the thermocouple junctions were taken as those of the

centers of the drilled holes, measured to the nearest 0.01 cm

wit.h a laboratory cathetometer

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After installation of the specimen, the space between it

and the guard tube was filled with diatomaceous earth powder

insulation, which also was used to insulate the space surrounding the guard tube.

In principle, if there were no heat exchange between the

specimen and its surroundings, the conductivity could be

determined from the measured power input to the specimen and

the average temperature gradient for each of the six spans

along the specimen, all of uniform known cross-sectional

area. In practice, a perfect balance of temperatures between

the bar and guard all along their lengths is not possible

because of differences in their temperature coefficients of

conductivity, and the effect of the outward heat losses of

the guard. In addition to heat exchanges between the bar

and guard from this cause, a not-insignificant longitudinal

flow of heat occurs in the powder insulation surrounding the

specimen, and the contribution of the specimen to this heat

flow must depend somewhat on the bar-to-guard temperature

unbalance

,

In order to evaluate the heat flow in the bar at the

center points of each of the six spans, a partly empirical

procedure was used. Two steady-state test-runs were made

with slightly different bar and guard temperatures and power

inputs. In the two tests, the heat flow and the observed

temperature drop from end to end of a given span differed,

as did also the approximate integral with respect to length

of the observed temperature differences between bar and

guard, summed from the hot end of the bar to the span center

point. It is thus possible to write for each span two equations (one for each test-run) of the form

4^^ + fS = Q Ax ^

where A is the cross-sectional area of the specimen,

k is the specimen conductivity at the mean temperature of the span.

At is the temperature drop from end to end of the

span.

Ax is the length of the span,

fS represents the total net heat loss from the bar

from its bottom end at the heater to the midpoint

of the given span, expressed as the product of S,

which is a close approximation of the integral
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Q is the measured power input to the specimen heater

»

By simultaneous solution of the two equations, the value

of k for each span, corresponding to the mean span mean temperature, is obtained. All of the computation of results is

effected by use of an IBM-704 digital computer, suitably programmed to compute the thermal conductivity and corresponding

mean temperature for each of the six spans. The computer was

used to fit a third-power equation to the thermal conductivity

and temperature results, by the method of least squares.

4. RESULTS

The r§su!lts of the thermal conductivity determinations

are shown in Figure 2 and Table 2» The 30 individual values

of thermal conductivity plotted represent five sets of tests,

each with values for the six spans o The solid line represents the trinomial fitting the data, as determined by a

digital computer, from which the values tabulated in. Table 2

were take.no

5. DISCUSSION OF RESULTS

The individual values of thermal conductivity plotted in

Figure 2 show moderate scattering from the smooth curve., The

extreme departure of an individual value from t.he smooth curve

is less than 3.0 percent.

- 5

-

TABLE 1

Chemical Composition — Percent

Mn Si Ni Cr Ko Cu Nb

0.0b2 0.75 0*7o 28.^ 19.8 2.2 3-3 Approx. 1=0

TABLE 2

Thermal Conduct ivity of a Sample of

Carpenter 20 Stainless Steel

op Btu/hr ft^(F/ino) op Btu/hr ft^(!F/in.)

200

- •9b. 6 900 137.7

300 100 0 9 1000 lb 3 o 8

boo 107.1 1100 150.0

500 113 ob 1200 156.2

600 119.6 1300 162.5

700 125.7 ibOO 168.8

800 131.7

USCOMM-NBS-DC

37.0

COOLANT

APPARATUS FOR MEASURING THE THERMAL CONDUCTIVITY OF METALS

USCOMM-NBS-DC

Fig. 1


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RMAL CbNiDiiCTIVlfry

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TEMPER

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FIG. 2

U.S. DEPARTMENT OF COMMERCE

Frederick H. Mueller, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

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