National Bureau of Standar

Library, E-01 Admin. Bldg. JUL 2 1971 A111Q3 0fiS70fi NBS SPECIAL PUBLICATION 260-25

Standard Reference Materials:

A STANDARD REFERENCE MATERIAL

CONTAINING NOMINALLY FOUR PERCENT

AUSTENITE

UNITED STATES DEPARTMENT OF COMMERCE • Maurice H. Stans, Secretary NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director

Standard Reference Materials:

A Standard Reference Material Containing

Nominally Four Percent Austenite

G. E. Hicho, H. Yakowitz,

S. D. Rasberry, and R. E. Michaelis

Institute for Materials Research

National Rureau of Standards Washington, D.C. 20234

(i'S, National Bureau of Standards Special Publication 260-25

Nat. Bur. Stand. (U.S.), Spec. PubL 260-25, 22 pages, (Feb. 1971) CODEN: XNBSA

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. 03.10:260-25), Price 30 cents.

Issued February 1971

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Standard Reference Materials (SRM) as defined by the

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vi
TABLE OF CONTENTS

PAGE

Introduction 2 General Description of the Standard 2 Preparation of the Compacts 3 Characterization of SRM-485 8 Conclusions 13 Acknowledgements 14 References Ik

LIST OF TABLES

PAGE

Table No.

1. Chemical Analysis of Starting Powder

Particles - Content in Weight Percent .... 3 2. Determinations for Establishing X-ray

Calibration Curve of Figure 6 10

3. Comparison of QTM and X-ray Fluorescence

Results 12

LIST OF FIGURES

PAGE

Figure, No

.

1. Scanning electron micrographs of the starting

powders 4 2. Fabrication steps for SRM-485 5 3. Structure of finished compacts - etched ... 6

vii
4. Cross-section of typical compact 7 5. Strip-chart recording of Ni-Ka signal vs. position in microprobe trace : a = Ni-signal

in austenite particle, b = Ni-signal in

ferritic matrix, c = Ni-signal at austeniteferrite matrix. Full scale about 10,000

counts /second , 9 6. Calibration curve for X-ray fluorescence

analysis characterization of SRM-485 .... 11

7. Number of samples in each composition range

from 3-0 to 5.3 percent austenite. The total

population of 134 standard samples is represented 13

viii
A STANDARD REFERENCE MATERIAL CONTAINING

NOMINALLY FOUR PERCENT AUSTENITE

G. E. Hicho, H. Yakowitz,

S. D. Rasberry and R. E. Michaelis

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

This standard was produced by powder metallurgical

techniques using known amounts of austenite. Using these

techniques, 134 specimens were prepared. Because these

standards are expected to be used primarily for the calibration of X-ray diffraction equipment, only one surface

of each standard is certified, and these surfaces range

from 3-1 percent to 5.2 percent in austenite content. To make the specimens, 310 stainless steel powder (austenitic)

was blended with 430 stainless steel powder (ferritic) to make a mixture of 5 percent austenite in ferrite. The

material was compacted, sintered, polished and etched so the

austenite appears white and the ferrite, a deep brown. Then

quantitative microscopy methods were used to determine the

percentage of austenite near the surface. Furthermore, the

310 powder contains 20 percent of nickel while the 430 powder

contains virtually no nickel. Therefore, after establishing

a meaningful calibration curve, X-ray fluorescence analysis

for the nickel content was also used as a direct measurement

of the amount of austenite on the surface of the compact.

Both procedures were carried out on fifteen specimens statistically selected from the total number of compacts produced. Agreement, within experimental error limits, was obtained between the X-ray fluorescence results and quantitative microscopy results. The X-ray fluorescence method

was used to characterize all additional compacts. X-ray

diffraction determinations of austenite content are in good

agreement with the X-ray fluorescence and quantitative microscopy results. The compacts may be used as X-ray diffraction

standards for austenite or in special cases as X-ray fluorescence standards for nickel content.

Key words: Austenite in ferrite; electron microprobe

;

powder metallurgy; quantitative microscopy;

SRM; X-ray fluorescence analysis.

1
INTRODUCTION

Standard reference materials for retained austenite

determination are required because the amount of retained

austenite in ferrous material critically affects metallurgical properties. The percentage of retained austenite in

ferrous materials is usually determined by X-ray diffraction

procedures requiring an accurate measurement of the integrated intensity under two or three selected diffraction

peaks. As the amount of retained austenite decreases, however, the accuracy of the intensity measurement deteriorates.

Therefore, the National Bureau of Standards has undertaken

to produce a series of standards containing from 1 to 25 percent austenite. The first of these standards is now ready for issue. This standard was prepared by means of

powder metallurgical methods.

The purpose of this report is to give a detailed description of the preparation and characterization of the

present group of 134 specimens. Because completely homogenous blending of the austenite and ferrite powders used

to make the compacts can not be achieved, despite considerable effort, each of the 134 specimens has been separately

characterized for austenite content. GENERAL DESCRIPTION OF THE STANDARD

This standard was prepared by mixing 5 percent by

weight of fully austenitic stainless steel powder (type 310)

with ferritic stainless steel powder (type 430). Large

amounts of chromium and nickel are contained in the 310

stainless steel. Hence, the diffusion of a portion of these

elements into the 430 stainless steel would not be expected

to alter the austenite structure of the 310 stainless steel.

The compact is 20.6 mm (13/16 in.) diameter by 2.54 mm

(0.100 in.) thick. One surface is polished and etched so that the austenite is white and the ferrite is a deep brown.

2
It is this surface which has been characterized with respect

to austenite content.

The standard is issued with one surface certified for

austenite content. No surface preparation is necessary. In fact, damage to the certified face renders the sample useless.

The lowest actual amount of austenite in the certified faces

of the samples is 3-1 percent, and the highest value is 5.2

percent

.

PREPARATION OP THE COMPACTS

The starting powders were irregularly shaped but uniformily sized. The entire lot of 310 powder passed through

a 53 ym sieve but none passed through a 44 ym sieve. All

the 430 powder passed a 44 ym sieve but none passed a 37 ym

sieve. Figure 1 shows the starting powder particles In detail. The chemical analysis of the powders is shown in

table 1.

Table 1. Chemical Analysis of Starting Powder

Particles - Content in Weight Percent.

Elements 310 Stainless Steel 430 Stainless Steel

Chromium 24.99 16.03

Nickel 20.41 0.09

Iron 53.5 83.5

Carbon 0.05 0.05

Manganese 0.20 0.10

Phosphorus 0.01 0.011

Sulfur 0.007 0.01

Silicon 0.75 0.40

Each step in the preparation of the compacts is shown

in the flow diagram given in figure 2. The structure of

the final compact was examined by optical and scanning electron microscopy. Figure 3 shows typical micros true tural details. The compacts contain several percent porosity and

3
b

Figure 1. Scanning electron micrographs of the starting powders

.

a. As received 310 stainless steel powderfully austenitic.

b. As received 430 stainless steel powderferritic

.

4
5
b

Figure 3- Structure of finished compacts - etched.

a. Optical micrographs showing austenite

(white) and ferrite (black).

b. Scanning electron micrograph showing

sharp delineation between large austenite particles and ferritic matrix.

6
the 310 particles are large islands in a matrix of ferritic

material. Scanning electron microscopy shows that there is

a sharp delineation between the austenitic particles and

the ferritic matrix as is evident in figure 3b. Modified

Groesbeck's etchant (sample immersed in boiling etchant of

30 g KMnO^, 30 g NaOH , and 100 ml distilled H

2 0) covers the

ferritic material with a friable layer of what is perhaps

a mixed oxide while the austenite is unaffected. The final

compact then appears as a dark brown ferritic matrix with

randomly placed austenite particles and some pores.

A section through one of the compacts is shown in figure

4. No important agglomeration of austenitic particles is seen throughout the section. However, it is clear that random cuts within the section may yield different percentages

of austenite when a single surface is characterized - as is

done in X-ray diffraction or optical microscopy. Consequently,

one face of each specimen was characterized and that face

only is certified with respect to its austenite content.

Figure k. Cross-section of typical compact.

7
CHARACTERIZATION OF SRM-485

There is virtually no nickel in the ferritic powder

while the austenitic powder contains 20.4 percent nickel

by weight. Therefore, if there is no diffusion of nickel

into the ferrite during the fabrication process, X-ray

fluorescence analysis for the nickel content of the com- pact can be used as a direct measure of the austenite content. For X-ray fluorescence analysis to be successful

however, a set of standards is required to calibrate nickel

(austenite) content versus X-ray intensity.

The electron probe microanaly zer was used to determine

whether or not any diffusion of nickel into the ferritic

matrix had occurred. The microprobe operating voltage was

20 kV and the Ni-Ka peak was monitored with a LiF crystalsealed proportional detector system. About fifteen separate

austenite-ferrite interfaces were examined. The nickel count rate in the austenite was approximately 4500 counts/second.

A typical trace through an austenite particle is shown in

figure 5. The nickel signal goes from near zero (ferritic)

to full signal (austenitic) in a space of about 3 urn. This

value is approximately the spatial resolution of the X-ray

signal from the microprobe. No measureable diffusion of

nickel into the ferritic matrix was observed. Also it was noted that the nickel content within a given austenite particle and from particle-to-particle was nearly constant.

To establish the X-ray intensity versus nickel

(austenite) content calibration curve, five special powder

compacts were prepared. These nominally contained 0, 10,

15 a 20, and 25 percent austenite. One of the compacts to be certified was also picked for this purpose. Except for

the completely ferritic specimen, these compacts were evaluated by quantitative microscopy to determine their austenite

content. The same austenite compacts also were examined by

X-ray diffraction (1).

8
a

Figure 5- Strip-chart recording of Ni-Ka signal vs. position in microprobe trace: a = Nisignal in austenite particle, b = Nisignal in ferritic matrix, c = Ni-signal

at austenite-ferrit e matrix. Pull scale

about 10,000 counts/second.

Quantitative microscopy could be used because the modified Groesbeck's etchant provides a complete color separation

of the austenite (white) and ferrite (brown). Thus, the percentage white phase (austenite) can be determined provided a porosity correction is made. Since the pores are "black" the

porosity correction readily can be made. The porosity of the

specimens averages about five percent.

Two quantitative microscopy devices were used to characterize the calibration specimens : a high-precision

NBS drum-scanner (2); a quantitative television microscope,

(QTM) . The NBS drum-scanner required 203 x 25^ mm (8 x 10 in.) high contrast micrographic prints as input. Final magnifications were at 125 diameters. Thus, a region 1.62 x 2.03 mm

9
on the specimen was examined. Five such areas were chosen

for each specimen which is only about five percent of the

total area of the specimen.

By means of the QTM, more than 95 percent of the surface could be sampled. In the QTM, an image of the actual

specimen is observed on a television monitor. The QTM then

reads percentage white, i.e., austenite in that field. The

working frame size on the monitor is 198 mm x 225 mm and the

specimen image is magnified 198 diameters. Under these con- ditions, 80 fields cover almost the entire surface. The

average of the 80 austenite determinations gives the austenite percentage on the surface. Care is taken so that suc- cessive areas do not overlap.

Extensive tests over a period of two months using three

different operators indicated that QTM drift was not a serious

problem. However, the absolute white-black threshold did seem to vary with time and operator. Exhaustive studies of

a previous set of four compacts having a nominal austenite

content of 5 percent indicated a standard deviation of 0.23

percent

.

The drum-scanner is far more precise than the QTM but

the scanner samples only 5 percent of the surface. Hence,

in establishing the calibration curve for X-ray fluorescence,

the drum-scanner and QTM values are given equal weight.

Table 2 gives the results.

Table 2. Determinations for Establishing X-ray

Calibration Curve of Figure 6. Nominal 5 10 15 20 25 Austenite Weight

Method\ Percentage

Drum scanner - 8.83 15-37 19.04 31.44

QTM 4.90 10.04 16.04 18.05 27-11

Calibration curve value 4.90 9.6 15-7 18.0 29.8

X-ray diffraction 4.8 9.4 16. 0 17.6 28.3

10
The calibration curve established with the results in

table 2 is shown in figure 6. The equation of the least

squares fitted curve is

:

C Ni (%) = -0. 0 332+2 .687xlO

_2| I+3.156xlO~ 8 I

2 -1.21xlO" 12

I

3 (1)

where I is the Ni-Ka signal in counts per second.

NICKEL %

0 5 10 15 20 25 30 AUSTENITE %

Figure 6. Calibration curve for X-ray fluorescence

analysis characterization of SRM-485.

At low nickel (austenite) values, a definite departure

from linearity exists. We believe that this is due to the

pronounced grain size effect caused by having discrete

nickel X-ray emitters scattered throughout the surface.

These emitters vary considerably in size and distribution

as the absolute nickel content increases. Claisse and

Samson (3) have discussed similar effects. Because of this

specific nickel distribution, more nickel intensity is observed than would be the case if the nickel were in solution

in an iron-base matrix. The X-ray self absorption is con- siderably reduced by concentrating the nickel into discrete

particles in the matrix.

11
Equation 1 was used to establish the nickel content and

hence the austenite content of all 134 specimens . The X-ray

fluorescence results were obtained on two different days, but

all samples were run at least once on each day. The X-ray

results are based on three determinations of Ni-Ka count rate. Standards were rerun approximately once every 90 minutes. No significant drift of the X-ray fluorescence unit was noted.

The operating conditions were as follows

X-ray tube: Tungsten

Crystal : LiP

Voltage : 44 kV

Current : 44 ma Detector : Scintillator g 1000 volts

Collimator: Fine

Specimens were placed in an air path. The primary X-ray

beam nearly irradiated the entire specimen face.

As a final test, fifteen specimens were run individually

on the QTM. The comparison of X-ray and QTM results is shown

in table 3. The agreement is considered satisfactory.

Table 3. Comparison of QTM and X-ray Fluorescence Results.

Specimen

Number

59 1921

42

496971

8081

86 117

119

128

141

QTM

Results

3-9

4.0

4.4

3.8

4.0

4.4

4.7

5.4

4.4

3.5

3.3

3.1

3.5

5.0

4.6

Std. Error

QTM

0.32

0.02

0.04

0*.24 0.28

0.34

0.37

0.40

0.27

0.17

0.22

0.36

0.30

0.41

0.51

X-ray a Results

4.2

3.7

4.0

3.7

4.0

3.9

Std. Error

X-ray

0 .01 0.03

0.01

0.02

0 .01 0.01

0.02

0.03

0 .04

0 .01 0.05

0.08

0.04

0.04

0.05

L Given in percent austenite

12
To show the overall distribution of the results, a histogram of number of samples versus austenite composition

was plotted for the entire population of 13^ specimens as shown in figure 7.

3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 AUSTENITE %

Figure 7. Number of samples in each composition range

from 3.0 to 5.3 percent austenite. The

total population of 134 standard samples

is represented.

CONCLUSIONS

SRM-485 is satisfactory for issuance as a standard for

X-ray diffraction determination of retained austenite at the

3 to 5 precent austenite range. In special cases, SRM-485

also may be used as an X-ray fluorescence standard for determining the nickel content in nickel-iron or nickelchromium-iron composites.

SRM-485 should not be used as a standard for quantitative microscopy. For single field determinations, too

few austenite particles are present for statistically meaningful results. Average values for many fields often give

unacceptably large standard errors as shown in table 3- In using SRM-485 care must be taken not to alter the

certified face.

13
ACKNOWLEDGEMENTS

The authors thank Dr. M. R. Meyerson for his guidance

in the initial stages of this program. His help in choosing

appropriate steel powders , blending times, sintering temperatures, and die pressures, is greatly appreciated.

The authors also thank Dr. G. A. Moore for assistance

with the NBS drum-scanner, and Dr. K. F. J. Heinrich for

the use of the microprobe. Special thanks are due to D. Harne for extensive metallographic aid, to C. J. Bechtoldt

for the X-ray diffraction determinations, and to D. B. Ballard for aid with the scanning electron microscope.

Thanks also are given to Dr. E. Passaglia, Dr. J. Filliben

and Dr. H. Ku for helpful discussions.

REFERENCES

[1] Ogilvie, R. E., Retained Austenite by X-rays, Norelco

Reporter, VI, 3, 60 (1959).

[2] Moore, G. A., Automatic Scanning and Computer Processes

for the Quantitative Analysis of Micrographs and Equivalent Subjects, Pictorial Pattern Recognition, p. 275

(Thompson Book Co., Washington, D. C, 1968).

[3] Claisse, F., and Samson, C, Heterogeneity Effects in

X-ray Analysis, Advances in X-ray Analysis, 5_, p. 335,

(1962).

14
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