MTED STATES •ARTMENT OF IMMERCE NBS SPECIAL PUBLICATION 260-35

U.S. •ARTMENT

OF

OMMERCE

National Oil

•US'?

to- Si'

Standard Reference Materials:

THERMAL CONDUCTIVITY OF

AUSTENITIC STAINLESS STEEL,

SRM 735, FROM 5 to 280 K
NATIONAL BUREAU OF STANDARDS

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onal Bureau of Standards

/^^ Standard Reference Materials:

1

'^

. 3^ Thermal Conductivity of Austenitic Stainless Steel,

SRM 735, from 5 to 280 K

J. G. Hust and L. L. Sparks

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

U.S. DEPARTMENT OF COMMERCE, Peter G. Peterson, Secretary NATIONAL BUREAU OF STANDARDS, Lewis M. Branscomb, Director, Issued April 1972
Library of Congress Catalog Number: 72-600028

National Bureau of Standards Special Publication 260-35

Nat. Bur. Stand. (U.S.), Spec. Publ. 260-35, 22 pages (Apr. 1972) CODEN: XNBSAV

Issued April 1972

For sale by the Superintendent of Documents, U.S. Government Printing Office Washintgon, D.C. 20402 (Order by SD Caulog No. C 13.10:260-35). Price 35 cents.
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I vii

TABLE OF CONTENTS

PAGE

1. Introduction 2

2. Apparatus and Data Analysis 3

3. Specimen Characterization 4

4. Results 5

5. Summary 6

6. Footnotes and References 7 Appendix I 8 LIST OF TABLES

PAGE

Table No.

1. Characterization data of SRM 735

stainless steel 10

2. Parameters, a^ of equation (4) 11

3. Thermal conductivity of austenitic

stainless steel (SPll 735) 12

ix
LIST OF FIGURES

PAGE

Figure No.

la. Experimental thermal conductivity

deviations from mean values of three

specimens from 5 to 40 K 13

lb. Experimental thermal conductivity

deviations from mean values of three

specimens from 40 to 300 K 14

2. Thermal conductivity of SRM 735 15

X
THERMAL CONDUCTIVITY OF AUSTENITIC STAINLESS

STEEL, SRM 735, FROM 5 TO 280 K*

J. G. Hust and L. L. Sparks

Cryogenics Division, NBS Institute for

Basic Standards, Boulder^ Colorado 80302

Thermal conductivity data are presented for a wellcharacterized austenitic stainless steel. Thermal conductivity

and electrical resistivity measurements were conducted on two lots of this steel. Electrical resistivity measurements were performed on the second lot both before and after the

material was hot-swaged and reannealed to a size 1/10 the

original diameter. These measurements indicate that this

steel can be swaged and reannealed without an appreciable

change in thermal conductivity. Electrical resistivity

measurements as well as direct thermal conductivity measurements on several specimens from both lots indicate a material

variability in these lots of less than 1% in thermal con- ductivity .

Key words: Cryogenics; electrical resistivity; stainless

steel; Standard Reference Material; thermal conductivity-

*This work was carried out at the National Bureau of

Standards, Boulder, Colorado, under the sponsorship of NASASpace Nuclear Systems Office, Cleveland, Ohio, and the

National Bureau of Standards Office of Standard Reference

Materials (NBS-OSRM) , Washington, D. C.

1
1 . INTRODUCTION

This report is a result of a program to establish

several thermal conductivity Standard Reference Materials

(SRM's). Measurements are planned for SRM's of high, medium

and low conductivity. An earlier report [1] describes the

estciblishment of SRM 734 material as a medium-high thermal

conductivity SRM. The material reported on here, stainless

steel SRM 735* [2] , is in the low conductivity range.

Design and development engineers in the aerospace

industry continue to have urgent need for thermal and

mechanical property data for new materials. For most

materials, especially new or uncommon alloys, measured values

of thermal conductivity are not available and predictions

cannot be made with adequate confidence. To help satisfy

these needs, we have constructed an apparatus for the

simultaneous measurement of thermal conductivity, electrical

resistivity, and thermopower. We intend to measure several

specimens of materials that appear to be useful as standards.

SRM data are useful for intercomparison of existing thermal conductivity apparatus , for debugging new apparatus , and

for calibration of comparative apparatus. The apparent large

differences (50% is not uncommon) among the results of various

investigators for a given material is evidence of the need

for reliable thermal conductivity SRM's.

* This steel was originally prepared and distributed for use

as a Standard Reference Material for high- temperature thermophysical properties by the Advisory Group for Aerospace

Research and Development (AGARD) under the direction of NATO.

It was measured at this laboratory to establish a standard

at low temperatures as well. Temperatures from 5 to 280 K are included in this work. Later, as data from other laboratorys are reported, it will be possible to extend the range

to higher temperatures.
The availability of SRM's will result in more accurate and more permanent transport property data for technically

important solids.

The basic characteristics of a thermal conductivity

SEM are that it be: (a) stable and reproducible under the

conditions of use, (b) uniform throughout a single specimen

and from specimen-to-specimen, (c) similar in thermal con- ductivity to that of the materials to which it will be applied,

(d) readily machined and fabricated to appropriate size

and shape, (e) chemically inert with its environment, and

(f) usable over a wide temperature range. Stainless steel

SUM 735 satisfies these criteria reasonably well.

2. APPARATUS AND DATA ANALYSIS

The apparatus is based on the axial one-dimensional

heat flow method. The specimen is a cylindrical rod 11.3 mm in diameter and 23 cm long with an electric heater at one end and a temperature controlled heat sink at the other.

The specimen is surrounded by glass fiber and a temperature

controlled shield. Eight thermocouples are mounted at equally spaced points along the length of the specimen to determine temperature gradients in the range 4 to 300 K. The experimental data are represented by arbitrary

functions over the entire range and smooth tables are generated from these functions . The number of terms used

to represent each of the data sets is optimized, through the use of orthonormal functions, so that none of the precision

of the data is lost by underf itting , nor are any unnecessary

oscillations introduced by overf itting. A detailed description of this apparatus and the methods of data anlaysis are given by Hust, et al. [3]

.

3
3. SPECIMEN CHARACTERIZATION

Initially, AGARD supplied four steel rods for characterization and measurement. Two rods were 6 mm diameter and

the other two were 10 mm diameter. The smaller rods were used for electrical resistivity measurements while the larger

two were machined for therroal conductivity measurements

.

These rods are referred to as lot 1 and measurements on them

suggested that this steel would be a useful standard of thermal

conductivity. A second lot of this material was purchased

for further measurements and for stocking. The 12 rods

from lot 2 were each 35 mm in diameter and 1 meter in length.

Six five-cm-long pieces were cut from the ends of six of

these rods for electrical resistivity characterization. These

measurements showed that lot 2 is indistinguishable, within

1%, from lot 1. Preparation of specimens by NBS-OSRM was then begun.

Because of high specimen preparation costs and the excessive

waste that would result from machining the 35-mm rods to smaller size, we decided to hot swage the rods to the desired

diameters. However, this raised the question of whether the

specimens would remain unchanged after hot swaging and

reannealing. We performed additional characterization

measurements on six specimens prepared by this method.

The results, listed in table 1, show that specimens prepared

by this procedure are within 1% in resistivity of the

specimens from lots 1 and 2 as received. Based on a total

of 21 elctrical resistivity deteinninations , it is concluded

that the theinnal conductivity variability of specimens

from these lots is less than 1%. A discussion of the

connection between electrical resistivity and thermal con- ductivity variability is presented in Appendix I.

4
The nominal composition of stainless steel SRM 735 is as follows : (The composition of lot 1 as determined by the

supplier is given in parentheses

Element Percent, by weight

Ni 20.0-20.5 (19 .90)

Cr 16.0-16.5 (16.41)

Mn 1.0- 1.2 ( 1.20)

Si 0.2- 0.3 ( 0.27)

Nb 8-12 X C ( 0.10)

Mo <0.2 ( 0.01)

C <0.02 ( 0.009) N <0.01 ( 0.009)

P <0.015 ( 0.005)

S <0.015 ( 0.006)

Fe bal (bal)

4 . RESULTS

The thermal conductivity of three specimens, two from

lot 1 and one from lot 2, was measured from 5 to 280 K. These data were functionally represented with the following

equation

n . £nX = a. 15, nT]^^-^

where X = thermal conductivity in Wm "'"K and T = temperature

in K. Temperatures are based on the IPTS-6 8 scale above 20 K and the NBS P2-20 (1965) scale below 20 K. The parameters,

, determined by least squares for the mean conductivity

of these three specimens, are presented in table 2. Further

details of the method of data analysis are given by Hust,

et al. [3] , The deviations of the experimental data from

this equation are given in figures la and lb. Calculated

values of thermal conductivity are presented in table 3 and

figure 2. ^
A detailed error analysis for this system has been

presented previously by Hust, et al . [3]. Based on this

analysis of systematic and random errors, the uncertainty

estimates (with 95% confidence) are as follows:

4 2.5% at 300 K, decreasing as T to 0.70% at 200 K,

0.70% from 200 K to 50 K, increasing inversely with

temperature to 1.5% at 4 K.

5 . SUMMARY

We have established low temperature thermal conductivity

standard reference data for stainless steel SRM 735. Thermal

conductivity measurements have been made on this stQel from

4 to 300 K. These data were fitted to an empirical equation

that was used to generate tabular values . Material variability is estimated to be less than + 1% in thermal conductivity, and measurement uncertainty is less than 2.5%.

We wish to thank R. E. Michaelis of NBS-OSRM, for

assistance in specimens procurement and helpful discussions.

This measurement program has been carried out under the

helpful guidance of R. L. Powell.

6
6. FOOTNOTES AND REFERENCES

[1] Hust, J. G., Sparks, L. L., Thermal Conductivity of

Electrolytic Iron, SRM 734, from 4 to 300 K, Nat. Bur.

of Stand. Spec. Pub. 260-31, 19 p. (Nov. 1971).

[2] This SRM is available in the form of rods of three

diameters and may be ordered from the Office of

Standard Reference Materials , National Bureau of

Standards, Washington, D. C. 20234. SRM 735-S is

0.65 cm in diameter and 30 cm long. SRM's 735-Ml

and M2 are 1.25 cm in diameter and 15 and 30 cm

long, respectively. SRM's 735-Ll and tl are 3.5 cm in diameter and 5 and 10 cm long, respectively.

[3] Hust, J. G. Powell, R. L., and Weitzel, D. H.

,

Thermal Conductivity Standard Reference Materials

from 4 to 300 K. I. Armco Iron: Including Apparatus

Description and Error Analysis, J. Res. Nat. Bur. Stand.

(U.S.), 74A (Phys. and Chem. ) No. 5, 673-690 (1970).

7
Appendix I. Discussion of electrical resistivity and

thermal conductivity variability.

The electrical resistivity, p, and thermal conductivity,

X, of metals are intimately related, especially for pure

metals, but also for alloys to a lesser extent. This

relationship exists because in a metal most of the heat

is transported by the electrons . Some heat is also

transported by the lattice vibrations . The total thermal

conductivity is the sum of the electronic, X^, and the lattice

Xg, (the German word for lattice is Gitter) components.

X = X^ + Xg. (1)

In most pure metals X^ is small compared to X^, but in

transition metals X^ may be as large as 20% of X^, and in some alloys X^ is much larger than X^. For pure metals

and dilute alloys, the relationship between p and X at both high and low temperatures is reasonably well described

by the Wiedemann-Franz-Lorenz (WFL) law:

^ = = 2.443 x 10"^ vV^, (2) where is the Sommerfeld value of pX/T and T is the temperature. At intermediate temperatures, large deviations from

the WFL law are observed. For our purposes the ice point

is a sufficiently high temperature and liquid helium is a sufficiently low temperature to satisfy the WFL law. In complex alloys such as this steel eq (2) does not hold, but

it has been observed that the value of the Lorenz function,

pX/T, is reasonably independent of material within a given

class of alloys such as austenitic stainless steel (Fe-18CrSNi alloys) . This indicates a close but unexplained relationship between p and X even X is small compared to X .

8
In metals there are two mechanisms that account for

most of the scattering of electrons : the interaction of

electrons with chemical impurities and physical imperfections,

and the interaction of electrons with thermal vibrations of

the atoms of the lattice. The fomer mechanism is usually

taken to be independent of temperature while the latter

is temperature dependent. If we assume that each of these

mechanisms is independent of the other, we may assign a separate resistivity to each. The resistivity arising from

impurity and imperfection scattering is usually referred to

as the residual resistivity, p^, while the resistivity due

to thermal scattering is called the intrinsic resistivity,

p^(T). The total resistivity, p (T) , may be written as the sum of these two terms.

p(T) = pQ + Pi(T) . (3) This separation of the total resistivity into a constant

term (p^) and a temperature dependent term (p^(T)) is known

as Matthiessen ' s rule. Although Matthiessen ' s rule is not

strictly valid, it is a sufficiently good approximation for our purposes. For steels, the residual resistivity is a significant part of the total resistivity, even at room

temperature; thus, values of either room temperature

resistivity or residual resistivity can be used as indicators

of material variability. This differs from pure metals in

that p^ is much smaller than p(293 K) for pure metals; therefore, p(293 K) is not an indicator of purity. The variability

in resistivity for various specimens in a given lot of

material is an indication of the variability in chemical

composition and physical imperfection concentration in the

lot. These material variations also cause thermal conductivity variations as indicated by the Lorenz function for a given class of alloys. Therfore, a determination of resistivity variability will usually approximate the thermal

9
conductivity variability in alloys. The determination of

electrical resistivity is considerably easier than the

determination of A

.

Table 1. Characterization data of SRM 735

stainless steel.

Specimen Lot 1 Lot 2 Lot 2* P273 k/''""'^"^ 788 786 788

P4 ^^(^^^)±2

RRR = p.273 j^/p^ j^±0.002 1.322 1.331 1.334

hardness, (Rockwell) ±2 B45 B48 B46

grain size, (mm)±0.01 0.04 0.04 0.07

density, (g/cm"^) ±0 . 004 8.004 8.009 8.006

*after hot-swaging and reannealing.

10
Table 2.

i

Parameters, a^, of equation

a

.

1

(4).

1 -4.85984600

2 6.59025067

3 -3.74701178

4 1.16265324

5 -2.05457295 X 10"

6 1.93981539 X 10"'

7 -7.59098428 X

10"'

11
Table 3. Thermal conductivity of austenitic

stainless steel (SRM 735)

.

Temperature

(K)

Thermal

Conductivity

(Wra"^ k"-"-) Temperature(K)

Thermal

Conductivity

5

6

7

8

9 10

0.466

0.565

0.676

0.796

0.921

1.05

75

80

85

9095 100

7.97

8.27

8.55

8.80

9.04

9.25

12

14

16

18

20

1.32

1.58

1.86

2.13

2.40

110

120

130

140

150

9.65

9.99

10.3

10.6

10.9

25

30

35

40

45

50

3.07

3.72

4.34

4.92

5.47

5.98

160

170

180

190

200

11.1

11.4

11.6

11.9

12.1

55

60

6570

6.45

6.88

7.28

7.64

220

240

260

280

12.6

13.0

13.4

13.8

12
e

0

....|

0

-'^1 . 1 I 1 . I

. 1 4 12 20 28 S& 44 T.K Figure la. Experimental thermal conductivity deviations

from mean values of three specimens from 5 to

40 K.

13
2.4 t t I 0
15

FORM NBS.n4A (1-71) U.S. DEPT. OF COMM. 1. PUBLICATION OR REPORT NO. 2. Gov't Accession BIBLIOGRAPHIC DATA NBS SP 260-35

3. Recipient's Accession No.

4. TITLE AND SUBTITLE Thermal Conductivity of Austenitic Stainless

Steel, SRM-735, from 5 to 280 K.

5. Publication Date April 1972

6. Performing Organization Code

7. AUTHOR(s) Q_ jjust and L. L. Sparks 8. Performing Organization

9. PERFORMING ORGANIZATION NAME AND ADDRESS NATIONAL BUREAU OF STANDARDS DEPARTMENT OF COMMERCE Boulder, Colorado 80302

10. Project/Task/Work Unit No. Project #2750431

11. Contract/Grant No. F33615-70-M-5002

12. Sponsoring Organization Name and Address NASA-Space Nuclear Systems Office

Cleveland, Ohio

13. Type of Report & Period Covered Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16> ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant bibliography or literature survey, mention it here.) Thermal conductivity data are presented for a well-characterized

austenitic stainless steel. Thermal conductivity and electrical

resistivity measurements were conducted on two lots of this steel. Electrical resistivity measurements were performed on the second

lot both before and after the material was hot-swaged and reannealed

to a size 1/10 the original diameter. These measurements indicate that

this steel can be swaged and reannealed without an appreciable change

in thermal conductivity. Electrical resistivity measurements as well

as direct thermal conductivity measurements on several specimens from both lots indicate a material variability in these lots of less than

1% in thermal conductivity.

17. KEY WORDS (Alphabetical order, separated by semicolons) cryogenics; electrical re s i s t i vi ty ; stainless steel; Standard Reference Material; thermal conductivity.

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