national Bureau of Standards

Library, E-01 Admin. Bldg. NBS MISC. PUBL. 260 "1 2

o

o

Standard Reference Materials:

HOMOGENEITY CHARACTERIZATION OF NBS

SPECTROMETRIC STANDARDS III: WHITE CAST IRON AND

STAINLESS STEEL POWDER COMPACT

MS1

%f u $- Department of Commerce

National Bureau of Standards
THE NATIONAL BUREAU OF STANDARDS

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UNITED STATES DEPARTMENT OF COMMERCE • John T. Connor, Secretary NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

Standard Reference Materials:

Homogeneity Characterization of NBS

Spectrometric Standards III: White Cast Iron and

Stainless Steel Powder Compact

National Bureau of Standards Miscellaneous Publication 260-12

Issued September 19, 1966

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price 20 cents

H. Yakowitz, D. L. Vieth, and R. E. Michaelis

Institute for Materials Research

National Bureau of Standards Washington, D.C.

PREFACE

Within the framework of the NBS Institute for Materials

Research the area of standard reference materials is a broad

and important one, including the preparation, characterization and distribution of a wide variety of materials in such

diverse fields as metallurgy, polymers and inorganic

materials. In carrying out such a program there Is much

interaction with representatives of industry and science,

beginning with discussions as to which primary standard

materials will do most to advance technology, the furnishing

of materials and fabrication of samples, and the characterization and certification of the materials by cooperative

efforts. The many groups participating In a standards

program are very Interested in detailed information on specific aspects of the program -- but to date there has

been no publication outlet for such written discussions.

To meet this need, NBS Miscellaneous Publication 260

has been reserved for a series of papers in the general area

of "standard reference materials". This series will present

the results of studies and Investigations undertaken within

the Institute for Materials Research with emphasis on the

preparation and characterization of standard reference

materials. This subject-oriented series will provide a means for rapid dissemination of this detailed Information

and we hope will stimulate the use of standard reference

materials In science and industry.

W. Wayne Meinke, Chief

Office of Standard Reference Materials

iii
OTHER NBS PUBLICATIONS IN THIS SERIES

NBS Misc. Publ. 260, Standard Reference Materials: Catalog

and Price List of Standard Materials Issued by the National

Bureau of Standards., October 1965. 45 cents.* (Supersedes

NBS Misc. Publ. 241, March 1962.

)

NBS Misc. Publ. 260-1 , Standard Reference Materials:

Preparation of NBS White Cast Iron Spectrochemical Standards,

June 1964. 30 cents.*

NBS Misc. Publ. 260-2, Standard Reference Materials:

Preparation of NBS Coooer-Base Spectrochemical Standards,

October 1964. 35 cents.*

NBS Misc. Publ. 260-3, Standard Reference Materials: Metallographic Characterization of an NBS Spectrometric Low-Alloy

Steel Standard, October 1964. 20 cents.*

NBS Misc. Publ. 260-4, Standard Reference Materials: Sources

of Information on Standard Reference Materials, February

1965. 20 cents.*

NBS Misc. Publ. 260-5, Standard Reference Materials: Accuracy

of Solution X-ray Spectrometric Analysis of Copper-Base

Alloys, March 1965. 25 cents.*

NBS Misc. Publ. 260-6, Standard Reference Materials: Methods

for the Chemical Analysis of White Cast Iron Standards,

July 1965. 45 cents.*

NBS Misc. Publ. 260-7, Standard Reference Materials: Methods

for the Chemical Analysis of NBS Copper-Base Spectrochemical

Standards, October 1965. 60 cents.*

NBS Misc. Publ. 260-8, Standard Reference Materials: Analysis

of Uranium Concentrates at the National Bureau of Standards,

December 1965. 55 cents.*

NBS Misc. Publ. 260-9, Standard Reference Materials: Half

Lives of Materials Used in the Preparation of Standard

Reference Materials of Nineteen Radioactive Nuclides Issued

by the National Bureau of Standards, November 1965. 15 cents.*

iv
NBS Misc. Publ. 260-10, Standard Reference Materials:

Homogeneity Characterization of NBS Spectrometric Standards

II: Cartridge Brass and Low-Alloy Steel, December, 1965.

30 cents.

NBS Misc. Publ. 260-11, Standard Reference Materials:

Viscosity of a Standard Lead-Silica Glass. In press.

*Send orders with remittance to: Superintendent of Documents,

U. S. Government Printing Office, Washington, D. C. 20402.

Remittance from foreign countries should include an additional one-fourth of the purchase price for postage.

v
CONTENTS

PAGE

PART I: WHITE CAST IRON 2 REFERENCES 5 TABLE

1. Certified composition values for SRM 1175,

white cast iron (Pe by difference is

87.22$) 4

FIGURES

1. NBS-SRM 1175, white cast iron. (X500) .... 5 2. Ratemeter recordings for six elements in NBSSRM 1175 } white cast iron. All specimens were driven at 96 microns per minute with

respect to the stationary electron beam.

Chart represents 600 microns in length ... 6

PART II: STAINLESS STEEL POWDER COMPACT 7 INTRODUCTION 7 MATERIALS 7 PREPARATION 8 Weighing and Blending .. 8 Heat Treatment ..* 8 Mechanical Working and Heat Treatment 9 DISCUSSION 9 REFERENCE 10 TABLE

1. Twenty second count totals of points in

ferrite for Fe and Ni in stainless steel

powder compact. Ka line of Fe and Ni

monitored with a LiF crystal at 25 kV . . . 11

FIGURES

1. Final submitted stainless steel powder

compact. Unetched X750 12

2. X-ray scanning displays for Fe, Cr and Ni

at 500X. 2a. Fe-Ka at 25 kV. 2b. Cr-Ka

at 25 kV. 2c. Ni-Ka at 25 kV 15

vi
STANDARD REFERENCE MATERIALS:

HOMOGENEITY CHARACTERIZATION OF NBS SPECTROMETRIC

STANDARDS III: WHITE CAST IRON AND

STAINLESS STEEL POWDER COMPACT

H. Yakowitz, D. L. Vieth, R. E. Michaelis

Institute for Materials Research

National Bureau of Standards

ABSTRACT

This paper describes a continuation of the NBS

effort to characterize metal materials as to their

suitability for use in calibrating microanalytical

techniques such as the solids mass spectrometer and,

especially, the electron probe microanalyzer . An NBS

white cast iron sample (SRM 1175) and a specially prepared stainless steel powder metallurgy compact have been

investigated by means of electron probe microanalysis and

optical metallography. Results for six elements in the

cast iron and three in the stainless steel are given.

It is concluded that neither of these materials is suitable

for use for calibration in microanalytical techniques. It

is emphasized , however , that this in no way affects the

usefulness of the white cast iron material for macroanalytical techniques such as optical emission and x-ray

spectrochemical analysis.

Key words: microanalytical techniques , solids mass spectrometer, electron probe microanalyzer, NBS white cast

iron sample (SRM 1175) > stainless steel powder metallurgy

compact, optical metallography, macroanalytical techniques,

optical emission, x-ray spectrochemical -analysis

1
PART It WHITE CAST IRON

In the continuing program to characterize existing

NBS Standard Reference Materials as to their suitability

for microanalytieal techniques such as for the electron

probe microanalyzer and the solids mass spectrometer , the

white cast, iron designated SRM 1175 [1] was investigated*

The certified composition values for this material are given in table I [2], It was decided to investigate the homogeneity of

several elements by taking mechanical line scans with the

electron probe microanalyzer in precisely the same manner

as was done on NBS-SRM standards for brass and for

low-alloy steel [3]. A portion of the chill-cast face was prepared metallographically by grinding on SiC papers

through 600 grit and polishing to a final finish on 6- and

0.25-micron diamond respectively. No pits or inclusions

were observed; as expected, etching revealed no smeared

metal. The resultant structure is shown in figure 1. After removing the etch on 0.25 micron diamond, the sample was introduced into the electron probe microanalyzer.

Mechanical line scans were taken for six of the

eighteen certified elements l namely, Mn, Si, Ni, Cr, Mo

and Fe. Portions of each of these traces are shown in

figure 2; the total sample length examined for each element

ranged from 1.2 to 2.0 mm. The traces show that the

elements investigated are not homogeneously distributed at micron levels of spatial resolution. Therefore, SRM white

cast iron 1175 is not considered suitable for microanalytical techniques in that reproducible results do not appear

possible

It is emphasized, however, that this in no way affects

the usefulness of Standard Reference Material 1175 for

macroanalytical techniques such as optical emission and

2
x-ray spectrochemical analysis in which a large portion

of the chill-cast face is sampled. Further detailed

information on the macrohomogeneity of this material may

be obtained from reference

REFERENCES

[1] R. E. Michaelis and L» L. Wyman, Standard Reference

Materials: Preparation of NBS White Cast Iron

Spectrochemical Standards , NBS Misc. Publ. 260-1, 31

pp, (1964).

[2] J. I. Shultz, Standard Reference Materials: Methods

for the Chemical Analysis of White Cast Iron

Standards, NBS Misc. Publ. 260-6, 62 pp, (1965).

[3] H. Yakowitz, D, L. Vieth, K.F.J* Heinrich, and R. E. Michaelis, Standard Reference Materials: Homogeneity

Characterization of NBS Spectrometric Standards Hi

Cartridge Brass and Low-Alloy Steel, NBS Misc. Publ.

260-10, 28 pp, (1965).
Table 1. Certified composition values for SRM 1175 > white cast iron (Fe by difference is 87.22$).

Average {%) Average {%)

Carbon 1.97 Molybdenum 1.51

Magnesium 1.64 Cobalt 0 o ll

Phosphorus 0.652 Tin 0.025

Sulfur 0.017 Titanium 0.55

Silicon 5.48 Arsenic 0.22

Copper 1.50 Boron o.oo

5 Nickel 2.98 Antimony 0.020

Chromium 2.45 Tellurium 0.009

Vanadium 0.221 Lead 0.005

4
Figure 1. NBS-SRM 1175 j white cast iron. (X500).

5
I Figure 2. Ratemeter recordings for six elements in NBS-SRM

1175 j white cast iron. All specimens were driven at

96 microns per minute with respect to the stationary

electron beam. Chart represents 600 microns in

length.
PART II: STAINLESS STEEL POWDER COMPACT

1. INTRODUCTION

As part of the NBS Program to prepare and provide

standard reference materials of suitable homogeneity that

any micro- or macro-analytical technique can be carried

out without fear of inaccuracy due to inhomogeneity, it was decided to obtain an l8Ni-Cr8 stainless steel of

precise composition. It was the considered opinion of

the NBS scientists responsible that this stainless steel

best could be prepared by means of powder metallurgical

techniques. Specifically, it was felt that weighing Fe,

Ni and Cr powders of the finest size available to within

0.001$ of the desired values, followed by pressing and

sintering to yield a billet, was necessary; this billet

would then be worked and annealed to further improve

homogenization.

Due to the lack of direct NBS facilities to carry out such a program, it was necessary to arrange for this work

on a contract basis. The Powder Process Laboratory of the

General Electric Research and Development Center undertook

to prepare the desired material. Scientists in this

laboratory expressed apprehension at the NBS approach but

only because of the difficulties encountered in obtaining

fine (less than 10 micron), pure, oxygen-free metal

powders - especially chromium. What follows describes

the G. E. procedure for preparing the material which was supplied to NBS.

2 . MATERIALS

Powders were obtained on a "best available" basis.

The limited supply of iron powder dictated the size of the

billet made. Chromium was supplied as the finest size

available with minimum contamination. The analysis quoted

by the supplier was as follows:

7
Oxygen

Nitrogen

Hydrogen

Carbon

300 ppm

20

150

40

2 Sulfur

Iron 900

400

800

Nickel

Calcium

Chromium 99»74$ (by difference)

The powder was approximately 80 micron particle size, so that it was necessary to mill for 36 hours in toluene in

a nickel ball mill to attain a size comparable with the

iron and nickel used (viz., 10 microns or less).

The iron and nickel powders were quoted as being

99.99$ pure, but no further analysis was supplied. Microscope observation showed the powder to be in the low micron

range, about lOp. or less.

The three powders were weighed out at the Advanced

Technology Laboratories of the General Electric Company.

Mixing was effected by rolling the powders for three hours

in one -gallon glass jar. The mixture was then packed into

a rubber balloon and isostatically pressed at 30,000 psi.

A powder billet approximately 2-1/2" diameter x 3" long

was obtained.

hours. The sintered metal was cooled in the furnace „ Test

strips of clean stainless steel put in alongside the samples

came out of the furnace clean and shiny. A metallographic

sample taken at this time showed that the product was not

homogeneous. Areas believed to be agglormerated chromium

3. PREPARATION

3.1 Weighing and Blending

3.2 Heat Treatment

The green billet was sintered in pure dry hydrogen

(-80 °C dew point) followed by a soak at 1200 °C for four

8
were observed. These particles, however, were approximately twice as large as the particle size of the ingoing

chromium powder.

3.3 Mechanical Working and Heat Treatment

In order to promote homogenization the billet was hot extruded. To do this the billet was machined dry

into a right cylinder and fitted into a low-carbon steel

can. Machining was done under especially clean conditions.

The extrusion can was sealed by electron beam welding

after pumping down overnight. The canned billet was then

heated in dry hydrogen to 1075°C in 2.5 hours, and extruded

from 2-1/8" diameter down to 5/8" diameter (11.6/1 area

reduction)

.

The extruded product was annealed for 8 hours at 1300°C in dry hydrogen. This, however, failed to homogenize the sample as shown by metallographic examination.

Similarly, further annealing at 1350°C for 15 hours failed

to bring about the desired structure. The annealed

extrusion was swaged to 0.275" at 850-950°C. A small

piece was further swaged at room temperature from 0.275"

down to 0.249". Both samples were annealed in pure dry

hydrogen for (1) 72 hours at 1350°C and (2) 168 hours at 1400°C.

4. DISCUSSION

Metallographic examination at the various stages of

the processing has revealed that working and annealing

failed to break up the chromium aggregates. The reason

for the formation of these hard particles was not clear,

especially since they were larger than the powders used

in processing. It was noted that these materials appear

to be closely associated with an oxide constituent also

observed in the matrix. This is particularly evident in

the 750X micrograph of the -material in the extruded and

9
swaged condition where it can be seen that, even though

these particles had been severely deformed,, an enveloping

oxide film was still present (figure 3) . The contamination

is believed to be inherent in the commercial starting

materials, since the test strips of clean stainless steel

fired in the same furnace remained bright. It is suggested

that although the overall furnace atmosphere was reducing

to chromium, the oxide intimately associated with the

chromium was not reduced because the powder billet

densified readily leaving such areas cut off from dry

hydrogen.

Samples of this stainless steel sent to NBS for metallo—

graphic examination confirmed the G.E. observations

(figure 1). Nevertheless, it was decided to subject the

specimens to electron probe microanalysis. Figure 2 shows

scanning images for Fe, Cr and Ni. It is apparent that none of these three elements are homogeneously distributed.

Oxygen was not studied since the 0-K line lies between the

Cr-La and Cr-L-^ lines and cannot be separated from

either. Random point counts (avoiding Cr-rich areas insofar as possible) listed in table 1 show swings of as much

as 20$ of the Ni and Fe in ferrite depending on position.

Therefore, it must be concluded that this attempt to produce stainless steel of high homogeneity by powder

metallurgical techniques has failed. However, signal success has been achieved in preparing homogeneous W-20$

Mo by a similar powder process thus indicating the useful- ness of this approach^ It seems clear that the failure of

the stainless steel preparation rests solely on the

impossibility of obtaining suitable Cr starting powder.

5. REFERENCE

[1] P. Duncumb and D. A. Melford, Quantitative Applications

of Ultra-soft X-ray Microanalysis in Metallurgical

Problems, Tube Investments Res. Lab. Tech. Rept. No. 195, 27 pp (1965).

10
Table 1. Twenty second count totals of points in ferrite

for Fe and Ni in stainless steel powder compact,

Ka line of Fe and Ni monitored with a LiF

crystal at 25 kV.

Point Ni Fe

1 6l,4o6 526,653

2 54,940 520,080

3 48,840 405,734

4 53A78 459,058

5 43,334 552,068

6 72,332 517,W

7 50,706 544,124

8 56,592 488,173

9 61,882 535,193

10 54,840 537,676

11 57,860 513,749

12 58,282 527,730

In Cr rich area 4,230 13,314

11
Figure 1. Final submitted stainless steel

powder compact. Unetched X750.

12
2a 2b

2c

Figure 2. X-ray scanning displays for Fe, Cr and Ni at 500X. 2a. Fe-Ka at 25 kV. 2b. Cr-Ka at 25 kV. 2c. Ni-Ka at 25 kV.

15

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