National Bureau of Standard; Library, N.W. B"M=- JAN 1 3 1965 NBS MISC. PUBL. 260-3

Standard Reference Materials:

METALLOGRAPHIC CHARACTERIZATION OF AN NBS

SPECTROMETRIC LOW-ALLOY STEEL STANDARD

U.S. Department of Commerce

National Bureau of Standards
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UNITED STATES DEPARTMENT OF COMMERCE • Luther H. Hodges, Secretary NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

Standard Reference Materials:

Metallographic Characterization of an

NBS Spectrometric Low-Alloy

/ Steel Standard

R. E. Michaelis, H. Yakowitz, and G. A. Moore

Institute for Materials Research

National Bureau of Standards Washington, D.C.

Reprinted from the Journal of Research of the National Bureau of Standards—A. Physics and Chemistry

Vol. 68A No. 4, July-August 1964, with an appendix added

National Bureau of Standards Miscellaneous Publication 260-3

Issued October 22, 1964

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C, 20402 - Price 20 cents
Library of Congress Catalog Card Number : 64-60094
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.

Ill
OTHER NBS PUBLICATIONS OF INTEREST

NBS Miscellaneous Publication 260-1, Preparation of NBS White

Cast Iron Spectrochemical Standards , June 1964. 30 cents. NBS Miscellaneous Publication 260-2, Preparation of NBS CopperBase Spectrochemical Standards, October 1964. 35 cents. NBS Miscellaneous Publication 24l, Standard Materials Issued

by the National Bureau of Standards: A Descriptive List

with Prices, March 1962. 30 cents. Up-to-date supplementary insert sheets to Misc. Publ. 241 list new, renewal,

out of stock, and discontinued standards. This sheet is

available without charge directly from the National Bureau

of Standards, Washington, D.C., 20234.

NBS Miscellaneous Publication 250, Calibration and Test

Services of the National Bureau of Standards, November 1963.

70 cents. Send orders with remittance to: Superintendent of Documents,

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

IV
CONTENTS

PAGE

1. INTRODUCTION 1

2. SELECTION OF MATERIAL FOR STUDY 1

3. INFORMATION DESIRED AND EQUIPMENT USED 2

4. EXPERIMENTAL PROCEDURE 2 4.1. Electron Probe Microanalysis 2 4.2. Computer Metallography 3 4.3. Inclusion Counting 4

4.4. Studies on Etched Structures 6

5. RESULTS

.

6 5.1. Homogeneity 6

a. Nickel in the Cross Section .... 6

b. Iron in the Cross Section ..... 6

c. Nickel in the Longitudinal Section 6

d. Iron in the Longitudinal Section . 7 5.2. Inclusions in the Steel 8

a. Identification of the Inclusions . 8

b. Volume and Weight Percentages

of Nonmetallic Inclusions .... 8

c. Estimation of the Weight Percent

of Inclusions 8

d. Estimation of Number and Size

of Inclusions 8

e. Mean Free Path Between Inclusions 9 5.3. Analysis of the Etched Structure in

NBS-461 Steel 9

a. Ferrite and Pearlite Percentages . 9 V
CONTENTS - Continued

PAGE

b. Mean Free Paths for Ferrite and

Pearlite 10

c. Grain Size of the Steel 10

6. DISCUSSION 11

7. SUMMARY OF RESULTS ON NBS-461 STEEL 11

7.1. Homogeneity 11

7.2. Inclusions . . 12

7.3. Characteristics of the Steel 12

8. REFERENCES 12

APPENDIX 13

LIST OF TABLES

TABLE No. PAGE

1. NBS Spectrographic low-alloy steel

standard No. 46l 2

2. Nickel concentration in cross sections of

NBS-461 steel 6

3. Nickel concentration in longitudinal sections

of NBS-461 steel 8

LIST OF FIGURES

FIGURE No. PAGE

1. NBS-461 steel polished for inclusion counting

and microprobe examination 4

2. Typical inclusion in longitudinal section ... 5

3. Cross section of NBS-461 steel etched for 9 s in 3-1/2 percent HNO^ 7

4. Longitudinal section of NBS steel etched for

10 s in 3-1/2 percent HNO-^ . . 7

5. Histogram of ASTM grain size numbers in

NBS-461 steel 11

VI
Metallographic Characterization of an NBS

Spectrometric Low-Alloy Steel Standard

R. E. Michaelis, H. Yakowitz, and G. A. Moore

(January 2, 1964) The spectrometric standard steel designated NBS Low-Alloy Steel 461 was investigated by means of electron probe microanalysis and quantitative metallographic techniques employing a digital computer. Electron probe microanalysis showed the steel to be homogeneous in nickel and iron at two to four microns of spatial resolution. The average of

all determinations agreed with the certified values for these elements. Inclusions in the

steel were identified, classified as to size and shape, and counted. Mean free path data on

the inclusions were calculated. The ASTM ferrite grain size number was deduced as 13.5 for the steel in the unetched condition. From the mean free paths in ferrite and pearlite, it was found that the steel is structurally homogeneous at a five micron level. It is concluded

that the homogeneity level corresponds closely to the grain size of the material. It is further concluded that NBS-461 steel is sufficiently homogeneous that any present microanalytical technique can be carried out with little chance of inaccuracy due to inhomogeneity.

1. Introduction Most of the modern rapid instrumental methods

of analysis depend on the use of standard samples

of composition for calibration. The National Bureau

of Standards in cooperation with industrial and

government groups, plans, prepares, tests, and

certifies selected standards of composition to serve the urgent needs of science and technology in critical areas of calibration. Whereas generally suitable techniques have been

developed for the preparation of homogeneous

standard samples of composition to be applied in chemical analysis, or in the usual optical emission and x-ray spectrochemical analysis, [l]

1 some of the newer analytical methods are far more demanding

with respect to the requirement of homogeneity.

Included in the newer analytical techniques which

have stringent requirements for sample homogeneity

at the 1 to 50 m level are those involving the solids mass spectrometer, the laser excitation source in spectrochemical analysis, and the electron-probe microanalyzer. The latter instrument, for example,

can perform an analysis with a spot diameter of about 1 ju- An important objective of the spectrochemical standards program is to develop new or improved

procedures and methods for preparing, testing, analyzing, and applying the spectrochemical standards. In this, research is directed not only to techniques for providing standards with high meas- urement excellence, but also to procedures for increasing the applicability of existing and future

standards. The purpose of this report is to present the results

of a limited but critical evaluation of homogeneity

at the 2 to 4 ju level in one of the available NBS

1 Figures in brackets indicate the literature references at the end of this paper.

steel spectrochemical standards. The evaluation is intended (1) to provide a more complete characterization of the standard, and (2) to learn of its suitability for application to the newer analytical techniques. The investigation primarily involves a study by

the NBS electron probe microanalyzer and by new

quantitative metallographic techniques. 2. Selection of Material for Study

At present, the NBS does not have available specific standard samples of composition for some of the newer analytical techniques, such as the solids mass

spectrometer. However, a number of available NBS standard samples, designed primarily for appli- cation in spectrochemical analysis, was prepared and processed to yield material of high homogeneity. Based on the results of extensive testing several years ago by optical emission and x-ray spectrochemical

analysis, by chemical analysis, and by metallographic

studies, these standard samples were determined to be free from segregation, for the intended applications, for most contained elements. In particular, a

set of eight ingot iron and low-alloy steel standards, prepared in the years 1956 and 1957 to contain a graded composition range for about 25 elements, was

found to be homogeneous at a level of about 1 mm

(1000 ju) for most elements as based on the methods

of test then available. It was believed that one of these standards would serve the purpose of this investigation. The study of photomicrographs of these irons and

steels indicated structural inhomogeneity for all eight standards at the micron level ; however, at least two of the standards appeared to exhibit suitable structural homogeneity at the 10 to 20 ju level. The

latter level should be satisfactory for application to either the solids mass spectrometer or to the laser excitation source in optical emission analysis.
The material for each standard had been melted

in a one-ton induction furnace (high frequency) at the Naval Research Laboratory and cast into a single ingot. In an effort to reduce the number of inclusions, and to improve and standardize the recovery

values for the additions, each heat was given a "carbon boil" immediately after melt down. Also, as a possible aid to reducing the inclusions in the

final material and to obtaining a finer grain size, a

rare earth addition was made to the molten metal in the ladle prior to skimming and pouring into the mold. Each ingot was processed by forging to a slab having one dimension of the cross section four times that of the other dimension. After cropping top and bottom, 15 and 5 percent respectively, the slab was cut lengthwise and the central longitudinal sec- tion corresponding to one-fourth of the slab was

discarded. The remaining two slab portions were

hot rolled to oversize rods, annealed, straightened, and centerless ground to size. About 900 lb (408.6 kg) of finished rods were obtained for each standard

as follows: 100 lb (45.4 kg) of rods %2 in. (5.56 mm)

in diameter from the outer section near the bottom

of the original ingot; 400 lb (181.6 kg) of rods VA

in. (31.75 mm) in diameter from the outer sections near the middle of the original ingot that is currently

certified; and additionally, 400 lb of rods lK in. in diameter which will be issued as renewal material when the first 400-lb lots are exhausted. As would be expected, the rod material %2 in. in diameter received far more severe working than the

larger size of 1% in. in diameter, hence, the smaller rods exhibited a much finer grain size. The decision was made to confine the initial investigation by the electron probe microanalyzer and by the new quantitative metallographic tech- niques to NBS Spectrochemical Standard No. 461. The composition of this steel is shown in table 1. A random sample was chosen as typical of the

entire 100 lb lot. Previous metallographic examination of several selected samples revealed no significant

differences.

Table 1. NBS Spectrographic low-alloy steel standard No. 461

Provisional certified analysis (November 25, 1959) Element Wt Element Wt

Fe %*(96.47) 0.15

.36 .053

(.02) .047

.34 1.73

0.13

.024

.30 .022

(.01)

B %

0. 0002 .023

.012 «.005)

.011 .002

(. 005) .26

(. 003)

(. 001 s) (.001 5)

(. 020) (.006 )

C As Mn W

P Zr S Nb

Si Ta Cu _. Al Ni Co

Cr Pb V Ag —- Mo Ge

Sn O

Ti____ N

•Values in parenthesis are not certified, but are given for infor- mation on the composition. Iron percent is by difference.

3. Information Desired and Equipment Used

It was desired to determine whether the steel was

homogeneous in nickel and iron at micron levels of spatial resolution in the longitudinal, transverse, and

normal directions. Furthermore, the average of a

set of analyses for these two elements was to be checked against the certified value. For this purpose, the NBS electron probe microanalyzer was

employed. This instrument, which enabled analyses

to be made at the 2 to 4 m level, has been described

in detail elsewhere [2]. Since inclusions were found in the steel, they had

to be identified; here both the electron probe microanalyzer and chemical-etching techniques were employed. Additional desirable information concerning

the inclusions consisted of their volume percentage in the steel, the mean free path between them, and some

idea of their number and size distribution. Concerning the steel itself, the following in- formation was required: (a) a statement of the apparent ferrite and pearlite percentages in the worked structure of the steel, (b) the mean free path

for ferrite and pearlite, and finally (c) the grain size of the steel. It is apparent that in order to obtain the required

information, beyond the mere identification of the

inclusions, one must resort to accurate quantitative metallography.

4. Experimental Procedure

4.1. Electron Probe Microanalysis To check the chemical homogeneity of the steel, both nickel and iron were investigated for uniformity

of composition throughout the sample of 461. With

the NBS electron probe microanalyzer, 46 separate

spot determinations were made for nickel in the cross section and 62 determinations were made in the

longitudinal section'; For iron, the corresponding

determinations were 9 and 16, respectively. This

gives a total of 108 separate nickel determinations and 25 separate iron determinations. The electron probe microanalyzer was operated at

32.5 keV for all analyses. Specimen currents used were 3.5 X10-8 (±1%) A for the cross section and

1.0X10-7 (±1%) A for the longitudinal section. Observed line to background ratios were 20/1 for pure iron and 30/1 for pure nickel using the second

order unresolved Ka doublet for these elements in conjunction with an ADP crystal 2 and a flow proportional counter. Using the first order unresolved Ka doublet for nickel in conjunction with a LiF

crystal and a G-M counter, the observed line to background ratio was 210/1 for pure Ni. The probe

size was 2 to 4 m based on examination of the con- tamination spots formed.

2 ADP refers to an ammonium dihydrogen phosphate crystal, which has a 2d

spacing of 10.65 A.

2
The x-ray intensities from the sample were compared to those obtained for pure metal end- member standards. The corrections applied were

as follows: Absorption: Philibert [3]. Fluorescence: Birks [4] (enhancement of Fe-Ka

by 1.73% Ni). Atomic Number [5]: Not required. Effect of continuous radiation [6]: Neglected. An appendix showing the detailed calculations

for this paper is available on request [7]. 4.2. Computer Metallography

A process which makes accurate quantitative metallography economically practical is one which

employs a digital computer capable of accepting

suitable photomicrographs and of printing out the

desired information directly [8]. The computer

which has been used to date is the SEAC (Standards

Electronic Automatic Computer). Black and white

pictures may be directly introduced in binary machine code by a scanning device. This equipment

places a physical size limitation on the photomicrograph to be investigated since the scanning device- can accommodate only one picture, 44 mm2

, at a time. At present, 28 general operations may be per- formed on a suitable micrograph. Many of these have been described previously [9]. New operations used in this study are known as "SLICE," "BLOB

TRANSFER," "BLOB CUSTER," and "STATIS- TICS" respectively and these will be described

herein. The photomicrographs to be scanned must have

the highest possible contrast between black and white

areas because the accuracy of the machine is adversely

affected by gray components. The size and shape

of the areas to be shown as black must be truly representative of the sample at the magnification

chosen. Furthermore, all grain boundaries revealed by etching must be shown as completed networks

since the computer will find any openings and count

such paired grains as single grains. The computer scans a raster of square spots

0.25 mm on an edge. For example, if the photo- micrograph is taken at a magnification of 500 diam,

the computer unit will be 0.25/500 or 5X10_i mm

which corresponds to 0.50 n- Therefore, the magnifications used must be known accurately if data

such as mean free paths are to be meaningful. To obtain the metallographic information desired, the machine was required to state the area

of the black phase, to display the picture on an

oscilloscope screen, to "slice" the picture hori- zontally and vertically, and to perform lineal analyses

at right angles to each other. In certain cases, complete "blob" (any discrete black area) analyses were ordered from which complete data on each

blob in the picture were deduced and printed out. To accomplish all of these orders, a master routine

is fed into a portion of the computer memory. This

will cause the computer to demand a list of orders which are in the form of English words. Given this

list, the computer stores the commands and carries them out in the listed sequence. To perform a command, the computer searches the order tape

until it locates the matching title. It then reads

in the detailed orders and executes this set of orders. The operator, in effect, causes the machine to execute

literally thousands of orders in machine language by simply typing in a list of commands in English. To do the complete series of analyses required for obtaining all the information desired about one micrograph of the NBS-461 steel, the simplified English language command list to the computer is: SCAN. This causes the computer to be supplied with one picture 44 mm2

. A blank border 1 mm

wide is masked onto the picture. The net usable

area within the border is 28,224 bits. AREA. This counts the number of black bits in the image. Division by 28,224 gives the per- centage of the phase represented by the image. The area statement is checked by using a calibrated bar chart containing 14,112 black bits which is exactly half the usable picture area of 28,224 bits (704 SEAC words).

_ DISPLAY. The image which is stored in the computer memory is exhibited on a cathode ray tube. DUMP. The image in the machine is electrically recorded on wire or tape. SLICE HORIZONTALLY or SLICE VERTI- CALLY. The slice operation separately deter- mines the areas of 168 horizontal or vertical line

slices. The decimal print out classifies the 168

slices into a histogram of 27 classes. The upper

limit of each class and the number of slices found

therein is stated. The area is given in bits and in percent of the net image area (28,224 bits). A a value, based on the histogram of the line slice distribution, is given in percent and the tr value of the mean area is given in

bits and in percent. The <r values may be expected

to differ for horizontal and vertical slicings. The

larger of the two a values is used. LINE. This causes the computer to treat the

picture as an unbroken horizontal helical raster such

as would result from wrapping the picture around a

vertical cylinder and joining the right end of each

scan line to the left end of the next line. This

order produces a decimal table presenting a histogram

(on V2 classes) of line lengths in the black and white

portions of the picture. The mean free path is computed as well as the standard deviations of both

path length distributions and of the mean free path. ROTATE. The picture is turned 90 deg as may

be required for a directional interpretation. LINE. This second LINE order gives the lineal analysis at right angles to the first LINE order. BLOB TRANSFER AND RECORD. Each

black blob in the picture is analyzed separately. The computer prints out the serial number, area, addresses of the corners of the circumscribed rec- tangle, height, width, estimated length (L), estimated

thickness {T), and the shape factor (L/T). 5
BLOB CUSTER. This operation reads back the data from the BLOB TRANSFER AND RECORD

table together with an image of the single blob and

computes the perimeter (P) of each blob as well as the complexity factor, P2/axea. The table so constructed contains the input data for the statistical analysis. STATISTICS. The data from the BLOB CUS- TER table are classified into histogram tables of eleven blob parameters on logarithmetically increas- ing classes of base V2. Automatic corrections for blobs cut by the picture edges are made. END. Machine stops. 4.3. Inclusion Counting The longitudinal and cross sections were polished

for inclusion counting, i.e., to reveal the true number,

size, and distribution of the inclusions. This was

accomplished by performing the final polishing with

a quarter-micron diamond powder on micro-cloth for long times at low speeds. The sample was alter- nately etched and polished and was finished in the unetched condition. The criteria adopted for the inclusion polishing were as follows: (1) that there be no scratches in the area to be photographed when this area was subjected to rotation in polarized light, (2) that "open

circles" were not true inclusions, (3) that the inclu- sions have sharp boundaries at the magnification of

interest. When these criteria were satisfied, it was

believed that a true representation of the inclusion concentration had been obtained. After being photographed in this condition, the samples were

introduced into the NBS electron probe microanalyzer.

One criterion for obtaining true photographic

records of the surface was that two overlapping

areas taken at different exposure times and printed

for maximum contrast showed inclusions common

to both areas as the same size in both prints. The

micrographs used were below the limit where over- exposure could conceivably blot out smaller inclusions and reduce the size of larger ones. Furthermore, when sharp black inclusions were obtained against white backgrounds by contact printing, the print

truly represented the negative. Finally, the print was visually checked against the sample. When

these requirements had been satisfied, the micro- graphs were ready for computer analysis (figs, la and lb). Thirteen photomicrographs were processed. Six were of the longitudinal section and seven were of the cross section. Longitudinal section. Photomicrographs were pre- pared at 50 X, 100 X, and 200 X. The 50 X magnification encompassed the width of the rod in two 5X7 photomicrographs. The higher magnifications showed more of the detail of the inclusion shapes, however, the 200 X pictures were too high in magnification to give truly representative values. Accordingly, four 50 X and two 100 X pictures were

chosenfor examination. Since the computer analyzes

a central area on the picture of only 42X42 mm,

the pictures were chosen to give maximum, average, and minimum blackness wherever possible. The

six pictures chosen were: 50 X-maximum blackness 50 X-minimum blackness 50 X-average blackness 100 X-maximum blackness 50 X-average blackness 100 X-minimum blackness

Figure 1. NBS 461 steel polished for inclusion counting and microprobe examination.
Figure 2. Typical inclusion in longitudinal section.

a. Unetched, X1000. Note the definite duplexity, i.e., discrete bars with smaller alcoholic hydrochloric acid. Note attack by the sodium picrate on the small

crystallites between. crystallites between bars and slight ittnck (lighter color) on bars by the hydrochloric add, indicating presence of [•><) Mull largely in the crystallites. X1000.

b. Same inclusion after succesive etching for 10 s in 10 percent nital, 5 min in c. Same as (b) except followed by 10 min in 20 percent aqueous hydrofluoric

10 percent aqueous chromic acid (neither of which attacked the inclusions), acid. Note nearly complete removal of bars indicating presence of FeOSiOj.

followed by 5 min in boiling alkaline sodium picrate, and 5 min in 5 percent X1000. 5
The computer was ordered to furnish the area of the inclusions, to display them on the oscilloscope, to slice the pictures horizontally and vertically, and

to perform two lineal analyses of the pictures at

right angles to each other. Cross section. The area, display, slice, and lineal analyses were performed on seven pictures, six of which were at 100 X and one at 50 X- The micro- graphs were taken such that the cross section was

covered from one edge to nearly the other edge. The six 100 X pictures represented three from the

center and three from the edge. These were taken

for maximum, average, and minimum blackness from each position. The 50 X picture was randomly

selected near the middle.

4.4. Studies on Etched Structures Super picral. nital, and high-chlorine aqua regia

failed to cut cleanly through all of the grain boundaries; however, a Z% percent solution of concentrated HN03 in water successfully performed this task. Etching time was 9 to 10 s with the sample im- mersed in the solution. In addition to normal magnifications, the speci- mens were photographed, in some cases, at magnifications such that the statistics table printed out by the computer would classify the grain sizes directly into ASTM numbers. Since the computer program

establishes y2 size classes on the basis of maximum

blob area, and ASTM Method E-112 [10] defines grain size values in terms of average grain areas, an

adjustment by 72 on diameters or \2 on areas causes the two classifications to match. The adjust- ment factor was found to be nearly constant for each

of the 30 listed ASTM classes. This factor is 0.925,

i.e., if one were doing a grain size count at 1000 X by

conventional methods, a photomicrograph taken at 925 X and processed by the computer would give the same grain size. Based on these facts, photomicrographs were taken of both the longitudinal and cross sections at 462.5 X, 500 X, 925 X, and 1000 X. To

increase the contrast, the 8 X 10 photographs were

printed onto lithographic film and then printed onto

high contrast photographic paper. This revealed

the pearlite as white and the ferrite as black. Photo- micrographs at 500 X are shown in figures 3 and 4. Computer analyses were made on portions of each photomicrograph except those of the longitudinal

section taken at 925 X and 1000 X.

For these analyses, the computer was ordered to state the area of ferrite (black), display the picture,

slice horizontally and vertically, and to do lineal analyses at right angles to one another. On a photomicrograph of the cross section taken at 462.5 X, the computer was ordered to do a full blob

analysis and report the results in the "Statistics"

table. The information to be gained by these pro- cedures is the ferrite percentage, the mean free path

in microns for ferrite and pearlite, and the grain size. Furthermore, from the mean free path data and from

the manner in which the photomicrographs were

prepared, data for a grain size determination by the Heyn intercept method are available [10].

5. Results

5.1. Homogeneity

a. Nickel in the Cross Section Microprobe analyses were made on three separate

days. First, 15 random points on the sample were

analyzed, and then two traces across the sample were made at right angles to one another on successive days. These traces contained 1 9 and 12 points

respectively. Points taken just inside the edge

of the sample gave results which were very different from results taken from points a millimeter or more away from the edge. There were nine such points,

of which eight gave high results and one gave a low

result. The latter probably can be explained by

the fact that the beam overlapped into the mounting

material. The edge was not maintained completely

flat during polishing. This resulted in a higher x-ray

takeoff angle and caused the high results observed

for the remaining eight points. Due to the low nickel concentration and the low

x-ray emergence angle in the NBS microanalyzer

(5.5°), the 3<r counting error was about 0.45 wt

percent. Table 2 shows the expected concentration ranges and the observed ranges after the edge

affected points have been dropped.

Table 2. Nickel concentration in cross sections of NBS-461

steel*

Run number Number of points Expected

limits" Observedlimits Average

concentration

12 %1. 47-2. 35 1. 50-2. 38 %1.85

2 15 1.39-2.19 1. 33-2. 12 1.74 3 10 1.28-2.22 1.48-2.15 1.75

*See Appendix [7] for complete data. **Range shown is the certified value (1.73%) ±3 times the standard counting

error.

b. Iron in the Cross Section The observed range for the iron concentrations was 95.6 to 98.2 percent with an average of 97.0 percent. The iron concentration, by difference, based on the certificate is 96.5 percent. The largest deviation from the observed mean value is 1.44 percent. The appendix [7] contains the complete

data.

c. Nickel in the Longitudinal Section Two separate investigations were made; these

consisted of 48 and 14 analyses respectively. Of the

total of 62 points, 60 fell within the expected limits. The average concentration found for both investi- gations was 1.73 percent Ni, which agrees with the

certified value. The extreme edges were avoided. The complete data for the 62 random points in- vestigated are shown in the appendix [7], while

table 3 shows the expected and observed concentration ranges. 6
Figure 3. Cross section of NBS 461 steel etched for 9 s in 8-1/2 percent HN03 a. Ferrite white and pearlite black. X500

b. game as (a) except ferrite black and pearlite white as scanned into the computer. X600

Figure 4. Longitudinal section of NBS 461 steel etched for 10 s in 8-1/2 percent HN03 a. Ferrite white and pearlite black. X500

b. Same as (a) except ferrite black and pearlite white as scanned into the computer. X500

d. Iron in the Longitudinal Section with an average of 95.8 percent. Two points Sixteen analyses were made at random points deviated from the mean by 2.9 percent; the next with the extreme edges being avoided. The ob- largest deviation was 1.6 percent. The appendix

served concentration range was 93.0 to 98.6 percent [7] contains the complete data. 7
Table 3. Nickel concentration in longitudinal sections of NBS-461 steel

Number of Expected

limits Observed*

limits Average

concentration

48 %

1.22-2.23 1. 19^2. 05 1?73

14 1. 35-2. 12 1. 27-2. 03 1.73

•One point in each run was too low.

5.2. Inclusions in the Steel

a. Identification of the Inclusions The longitudinal section of the steel shows

stringers which appear to be composites under the microscope. The structure appears somewhat similar to that observed in eutectoid type structures,

i.e., barring occurs. These inclusions were in- vestigated by means of the electron probe with

the result that Mn and Cr were confirmed in them. There is no Ni present in the inclusions. There

appeared to be Fe, but this is indeterminate because

the matrix is so rich in Fe and the distribution in depth of the inclusions is unknown. An attempt was made to further identify the

inclusions by progressive etching [11]. This scheme

was followed until nearly all of the inclusion components had been removed or attacked, i.e., through

a 10 min etch in aqueous 20 percent HF. Photo- micrographs were taken of the same inclusion at each stage. Some of these are shown in figure 2. From the results it was concluded that: (1) The

bars are largely FeOSi02 , but they probably contain some MnOSi02 since the bars are slightly attacked by 5 min in 5 percent HC1 in C2H5OH. (2) The

other components are probably MnOFeO (note attack by 5 min in boiling alkaline sodium picrate) mixed with a carbide. It is surmised that the

carbide is Fe-Cr since the carbon bearing patches in the bulk of the steel behave in a like manner under

polarized light to the crystallites remaining after the FeOSi02 bars have been removed by 20 percent HF. It may be that these are only an iron carbide and that the Cr is combined as oxide or carbide.

b. Volume and Weight Percentages of Nonmetallic Inclusions Longitudinal Section. The area percent of a random cut has been shown to be equal to the volume percent of the material [12]. Considering

the previously established homogeneity of NBS-461

steel, it was believed that virtually any cut made on

the random sample would be truly representative

of the standard. Area results as given by the computer are then interpreted to be volume

percent of inclusions in the steel. The average of four 50 X pictures gave a volume percentage of inclusions equal to 0.40. A single 100 X picture chosen for average blackness (inclusion density) gave a volume percentage of 0.42. Cross Section. Two sets of photomicrographs were taken near the center and near the edge. These

were processed by the computer as separate entities to determine whether any large gradients of in- clusion density existed. For the set of three 100 X

pictures taken near the center of the sample, the volume percentage of inclusions was 0.38 percent. The three 100 X pictures taken near the edge gave an average volume percentage of 0.42 percent. Obviously, no appreciable gradient was found, and

the six pictures were then taken as representative

of the entire cross section. This gave an average volume percentage of 0.40 percent. Note that this

is the same result as that obtained for the longitudinal

section. Data for each picture processed will be found in the Appendix [7].

c. Estimation of the Weight Percent of Inclusions From the results, on both the longitudinal and

cross sections it is reasonable to say that the inclusion content is 0.40 volume percent. For an approxi- mate calculation of the weight percentage of the

inclusions, equal portions of FeO-Si02 and FeO-MnO

can be assumed in the inclusion. The density of FeO-Si02 is 3.5 while that of FeO-MnO is 5.6. The

average density is then about 4.5. The density of the metallic portion of the steel is about 7.9. Then

for the total density of the steel as issued: p= [(0.996) (7.9) + (0.004) (4.5)] p=7.8384+0.0180

p=7.8564

Weight percent of inclusions of 50 percent FeO-Si02 and 50 percent FeO-MnO is 0.0180/7.8564=0.23

percent. Thus, 0.23 weight percent of inclusion

is obtained. To obtain a better indication of the

absolute range for the weight percentage of the

inclusions, the value was computed by first assuming

the inclusions to be only FeOSi02 and then to be

only FeO-MnO. The weight percent in the first case is 0.18 and in the second, 0.28. It can thus be stated with confidence that the weight percentage

of the inclusions is 0.23 ±0.05.

d. Estimation of Number and Size of Inclusions The computer was ordered to do a full statistical analysis on the average 100 X picture taken of the

cross section near the center. From this analysis, some estimation of the number and size of the

inclusions can be made. The statistics table indicated that the average

height and width of the inclusions was equal and had a value of 5.25 /i. Furthermore, the mean

value of the major axis of elliptical shapes was 6.0 n. Based on this, it is possible to view the inclusions

in the cross section as sections of an elliptical cylinder having a mean diameter of approximately 5.5 m- Data on the area distribution of the inclusions from the statistical analyses, coupled with the fact that at 100 X the real area of metal represented by

the photomicrograph is 0.176 mm2

, enabled the cal- culation of the approximate number of inclusions per cubic millimeter of steel. This was first attempted
by using the method of Bergh andLindberg [13] which

assumes that the inclusions are spherical. However, the summation of the predicted spherical volume of the inclusions was over 100 percent

greater than the real volume as deduced by the computer. Therefore, the Bergh and Lindberg method was discarded. Visual observation indicates that the inclusions in NBS-461 steel are cylindrical in shape. On this basis, an attempt was made to find the average length

in terms of the diameter. Both mean free path data

in the inclusions, and the statistical analyses of longi- tudinal and cross-sectional photomicrographs were

used. The results from the former gave the length equal to 2.64 times the diameter, while the results from the latter gave the length as 2.73 times the

diameter. For purposes of calculation, the length was taken as 2.7 times the diameter. The conversion

of the number of inclusions in a given class per square

millimeter to inclusions of the same class per cubic

millimeter is then obtained by dividing by the particle length in millimeters. Since, for a 100 X photo- micrograph, the computer scans spots 2.5 li2

, the

smallest observable inclusion diameter is 2.5 n, as compared with 10 m minimum diameter frequently

reported. The total number of inclusions per cubic millimeter was found to be about 14,000 of which 76 percent had diameters of 5 n or less. These 76 percent repre- sent only about 20 percent of the total inclusion volume, however. The total inclusion volume per- cent was 0.44 from the photomicrograph in question; the calculated inclusion percentage was also 0.44. A table of probabilities for a 50 n spherical volume

to contain one inclusion of each size observed was

prepared. The 50 n spherical volume was chosen to be representative of the sample size for laser excita- tion sources and the solids mass spectrometer. Full data are shown in the Appendix [7].

e. Mean Free Path Between Inclusions The computer delivered data on the mean free paths in both the inclusions and in the steel, as obtained from lineal analyses of the photomicrographs. In the cross section, the average mean free path within an inclusion was about 4.5 n, while the average mean free path in the steel was about 1600 (i. From these figures the probability of a random 1 n

electron probe touching any inclusion is only about

one in 350. In the longitudinal section, the average mean free path in an inclusion was about 7 n while in the steel,

it was about 2700 m- Here the probability for a random 1 y. electron probe to touch an inclusion is only about one in 380. The fact that the mean free paths are smaller than the maximum dimensions of the inclusions is to be expected since the longest path

through the inclusion will not 'always be the path

chosen by the computer. Complete mean free path

data for each picture processed will be found in the Appendix [7].

5.3. Analysis of the Etched Structure in NBS-461

Steel

a. Ferrite and Peailite Percentages It is readily apparent from the photomicrographs

of the etched structure that this steel is extremely

fine grained. This presents an immediate dilemma

since a fairly large portion of the grain itself may be

lost in etching to clean cut grain boundaries. Thus,

the apparent ferrite percentage derived from a photo- micrograph of the etched structure can be very much

lower than the actual ferrite percentage. As an illus- tration for a steel with an ASTM grain size number

of 6.0, the average grain diameter is given as 45 n [10]. Etching a 1 m grain boundary into such a steel causes the loss of %s {}{ ix on each "side") or 2.2 percent of the grain diameter and 4.4 percent of the area. However, in a steel with ASTM grain size of say 13, the average grain diameter is about 4 /x and etching a 1 m boundary into such a steel causes the loss of 25

percent of the grain diameter. The resulting area

loss is about 44 percent. The problem, then was to obtain the ferrite and

pearlite percentages with and without the grain boundaries. Fortunately, this was possible with the computer display system. Each picture as scanned

into the computer was displayed on a cathode ray

tube and compared with the original. The sensi- tivity of the scanner was adjusted until all of the

grain boundaries were clearly cut through; the area deduced by the computer under these conditions was

the ferrite percentage in the etched condition. Then

the pictures were redisplayed and the scanner sensi- tivity adjusted until the grain boundaries were

apparently removed (this was when the calibrated bar chart gave an area value of nearly 14,112 bits). No particular encroachment on the pearlite areas could be discerned by visual comparison of the cathode ray tube and the actual photomicrograph. The area deduced by the computer under these conditions was considered to be the ferrite percentage in the "unetched" condition, i.e., the actual ferrite percentage in the steel. In the etched condition, the average volume per- cent of ferrite was found to be 32.8 percent, while in the unetched condition, the average volume percent

of ferrite was found to be 54.8 percent. From this, two things are apparent: etching this fine grained

steel has reduced the apparent ferrite surfaces by

more than 40 percent. Furthermore, the pearlite percentage seems very high for a steel containing

0.15 percent carbon. This high pearlite value is a re- sult of the history of the steel especially the fact that

it is not fully annealed. For an estimate of the pearlite percentage which would be apparent in a photo- micrograph of the annealed structure, NBS^461

steel may be compared to an 8615 steel which has roughly the same composition. In 8615 steel, the

eutectoid value is at approximately 0.65 percent

carbon. From this, an approximate value of 23 per- cent pearlite would be expected. Thus, as a result

of its history, the NBS-461 steel exhibits approxi9
mateiy twice as much pearlite as would be antici- pated in the fully annealed structure. Complete

data on ferrite percentages in all pictures processed

will be found in the Appendix [7].

b. Mean Free Paths for Ferrite and Pearlite The computer was ordered to do lineal analyses on

the steel in the etched condition. The average

ferrite mean free path was found to be 1.63 n while

the average pearlite mean free path was found to be

3.38 ju. From these data the probabilities for a 1 n

electron probe to be in a given constituent can be computed. The probabilities are about one in five that the spot is in pure ferrite, one in five that it is in a ferrite-pearlite mixture, and three in five that it

is in pure pearlite. With a 3.5 p. electron probe, one is always in a mixture of ferrite and pearlite. Complete mean free path data on all pictures processed will be found in the Appendix [7].

o. Grain Size of the Steel Despite the fact that the steel was in a worked

condition, it was felt that a meaningful grain size could be deduced. Furthermore, with the aid of the computer, a number of individual grains could be

analyzed separately and a histogram of ASTM grain

classes in the steel could be established.

It was desired to state the grain size in both the etched and unetched conditions; the grain size deduced for the unetched condition is believed to be

the more accurate statement for the real grain size. However, since one normally works with etched

structures when using the ASTM grain size chart, it was deemed necessary to present both values. With

both values available some idea of the effect of etch- ing on the apparent ASTM grain size in a very fine grained steel could be indicated. To obtain the grain size in the etched condition, the Heyn intercept procedure was adopted. The

average intercept lengths required are the same as the mean free paths given by the computer. From

these data, the number of grains per cubic millimeter may be calculated by the following relation [10]. ni«.= (Q.7) W(n«)W

where nUn is the number of grains per cubic millimeter.

iii is the average number of grains per millimeter

intercepted in the longitudinal direction. n t is the average number of grains per millimeter

intercepted in the transverse direction. nn is the average number of grains per millimeter

intercepted in the normal direction. Since photomicrographs had been prepared from

sections of the steel taken at right angles to one

another and since rotations had been performed in the computer, all of the necessary data were available. The values for nx and n t were taken from the

ferrite mean free path data in the longitudinal

section while the nn value was taken from the ferrite mean free path data in the cross section. Duplicate10

grain size analyses were made on photomicrographs

taken at 462.5 X and 500 X. The latter are shown

in figures 3 and 4. The calculation made on the

462.5 X picture will serve to illustrate. nltn =(0.7)(n1 )(nt)(nn) Since 1 mm equals 1000 y., the n value is in general equal to:

__ 1000 mean free path (in microns) for ferrite In the 462.5 X photomicrograph the mean free path

values were 2.03 n in the longitudinal direction, 1.48 n in the transverse direction, and 1.98 n in the normal direction. Thus ~ /1000\/1000\/1000\

nnn =117X106 grains/mm3. Since ASTM grain size 14 has 45.2 XiO6 grains/mm3 and grain size 15 has 128 XIO6 grains/mm3

, it can be

seen that the grain size in the etched condition is indicated to be 14.9. Precisely the same result is obtained when the mean free path data from the 500 X photomicrographs are used. To obtain the grain size in the unetched condition, mean free path data for the ferrite in the unetched

condition were required. These data were obtained by using the ferrite area values obtained in the unetched condition in combination with the total number of lines found in the ferrite by the lineal analysis performed with the computer on the etched structure. The effect is to add the grain boundary back onto each grain. In our example for ! the 462.5 X picture, the new mean free path values

j found were: 3.29 ix in the longitudinal direction, 2.55 m in the transverse direction, and 3.65 ju in the normal direction. These values led to an nUn j value of 22.9 XIO6 grains per cubic millimeter corresponding to an ASTM grain size of 13.3. When adjusted mean free path data were used in conjunction with the 500 X photomicrographs, the \ ASTM grain size for the unetched condition was ! found to be 13.5. Virtually the same effect, i.e., the addition of the | boundary area to the grain area, can be obtained by adding the perimeter of the etched grain to the

area of the etched grain. To obtain an indication

1 of the spread of grain sizes in this worked material, the computer was ordered to do a full statistical

j analysis, which includes the perimeter plus area 1 data, on a photomicrograph of the cross section taken at 462.5 X. In this case it was hoped that all the grain boundaries were cleanly cut through since any connectivity will be found by the computer and i treated as one unit. Of a total of 356 blobs analyzed by the computer, only about 30 exhibited this type

of complexity as evidenced by a second peak in the histogram of frequency versus ASTM grain size.
The histogram is presented in figure 5. The

peaks associated with the very small grain sizes are

ferrite particles in the pearlite while the larger grain

sizes are due to the previously mentioned complex

grains. The estimated Gaussian distribution curve

is also shown. From this figure, the mean ASTM

grain size is 13.2. Thus, in the etched condition, the apparent rounded ASTM grain size is 15 while in the unetched condition, the average grain size is 13.2 to 13.5. The

significant size range indicated by the Gaussian

distribution is about ASTM size 12.5 to ASTM

size 15.5. It is interesting to note that there are about two thousand grains for each inclusion. 6. Discussion

It has been found that NBS-461 steel is homogeneous in both nickel and iron at 2 to 4 n levels by means of electron probe microanalysis. These

results have been corroborated by Adler 3 who

made 10 determinations each for nickel and iron in the cross section of the steel using a 1 ju electron probe. The maximum volume of steel excited was

assumed to be about 7 ju3 (a hemisphere 3 m in di- ameter). This corresponds to about 5X10-11 g

of steel per determination. The average of a number of determinations for nickel and iron in both the

cross section and the longitudinal section was in close agreement with the certified value of 1.73 per- cent for nickel and with the predicted value for iron (by difference) of 96.5 percent. The operator of an electron probe microanaiyzer can easily avoid visible inclusions; even if he could

not, the probability for striking an inclusion with

a 1 ,u probe is only one in 350. Furthermore, if the x-ray path from the sample is assumed to pass through a 20 m

3 hemisphere of steel before the x rays emerge from the surface, the probability for an underlying inclusion to be anywhere in the x-ray path is less than 3 in 100. The steel has been shown to be structurally ho- mogeneous at a 5 n level since the mean free path in

ferrite is 1.63 ju while the mean free path in pearlite is 3.38 m- Thus, any 5 m probe will be in a structurally homogeneous region. Furthermore, other metallographic observations have shown that there

is no long range inhomogeneity in the structures. On this basis it is felt that at a 5 m level, the structural homogeneity predicts that the other elements

in NBS-461 steel, except those concentrated in the

inclusions, will be uniformly distributed.

It is reasonable to expect partition of some ele- ments between ferrite and carbide (or in pearlite). For example, manganese, phosphorus, sulfur, and

silicon are known to concentrate in ferrite areas while chromium, vanadium, niobium, tungsten, and tantalum are known to have an affinity for car- bide regions. Therefore, at less than the 5 n structural homogeneity level, these elements might well

give fluctuations in analysis.

3 1. Adler, private communication (1963).

£3 FINES IN PEARLITE NORMAL FERRITE GRAINS DOUBLETS 8 TRIPLETS

GRAIN SIZE NUMBER Figure 5. Histogram of ASTM grain size numbers in NBS

461 steel. The etched grain size is 14.9 and the mean unetched grain size is 13.5. For other microanalytical techniques, a 50 n sphere was assumed to be a representative sample. This corresponds to about 0.52 microgram of the steel or about ten thousand times the amount investi- gated by means of electron probe microanalysis. Since it takes about four inclusions of average size to make a 1 percent discrepancy in weight, such an occurrence is highly unlikely. A commonly found grain size in many steels cor- responds to ASTM number 5.5 or 6.0. In this case, the 50m spherical volume would sample only one or two grains. However, it has been shown

that the average grain size of NBS^-461 steel cor- responds to ASTM number 13.5. The resultant average number of grains in the 50m sphere is about

three thousand—certainly a fair sampling for an

analysis. It is believed that the homogeneity level of this

steel corresponds very closely to the grain size. In

this case, the fine grain size was achieved by the

addition of rare earths, aluminum, and other beneficial elements. Extensive cold-working was fol- lowed by a final process anneal just sufficient to produce recrystallization without grain growth.

It can be confidently stated that the homogeneity

level in NBS-461 steel is such that present day

microanalytical techniques can be carried out with

little fear of discrepancies due to inhomogeneity.

7. Summary of Results on NBS-461 Steel

7.1. Homogeneity

Nickel. 108 separate Ni determinations were made with the microprobe (2-4 y. spot). Of these only two fell outside of 3<r limits predicted by the number of counts. The longitudinal, transverse and normal directions were examined. The average

analysis was 1.73 percent Ni which agrees with the

certificate value.

Iron. 25 separate iron determinations were made.

Again all three directions were examined. The

largest deviation from the mean was 2.9 percent. The average analyses were 95.8 percent iron in the 11
longitudinal section and 97.0 percent iron in the

cross section. The certificate value (by difference)

is 96.5 percent Fe.

Other Elements. Probably homogeneous at 5 n

levels except for those contained in the inclusions.

7.2. Inclusions

Identification: Both the microprobe and chemical

etching tests were used. The inclusions were identified as consisting of FeOSi02 and FeO MnO. Some

Cr is also present in a combined form. Volume percentage: 0.40 Weight percentage: 0.23 ±0.05 (assumed equal

portions of FeOSi02 and FeO-MnO on a weight

basis). Mean free path in inclusion: 4.7m (cross section)

;

7m (longitudinal section) ; 5.75m average. Mean free path in steel: 1600m (cross section)

;

2700m (longitudinal section); 2100m average. Number of inclusions per cubic millimeter: 14,000

(approx.). Probability for random one micron probe to strike a surface inclusion: 1 in 350. Probability for x rays from a microprobe to strike an underlying inclusion: 1 in 34. Probability for an inclusion to affect an analysis

of a 50m sphere of steel by 1 percent: Negligible. Inclusion shape: Rod with average length equal

to 2.7 times the average diameter.

7.3. Characteristics of the Steel Ferrite percentage : 54.8 Apparent ferrite percentage after etching: 32.8 Mean free ferrite path: 1.63m Mean free pearlite path: 3.38m (thus a 3.5m spot must always be in a mixture; a lja spot has a one in

five probability of being in pure ferrite and a three

in five probability of being in pearlite)

.

Average ASTM grain size in etched condition: 14.9 (by Heyn method)

. Average ASTM grain size in unetched condition:

13.5 (Heyn and particle area methods). Average ferrite grain diameter (unetched): 3.5m. 8. References

[1] Standard materials issued by the National Bureau of Standards, NBS Misc. Publ. 241 (1962).

[2] J. R. Outhill, L. L. Wyman, and H. Yakowitz, Metallurgical microanalysis with the electron probe, J. Metals 15 (1963), in press.

[3] J. Philibert, The Castaing "Microsonde" in Metallurgical and mineralogical research, J. Inst. Metals 90, 241

(1962).

[4] L. S. Birks, Calculation of x-ray intensities from electron probe specimens, J. Appl. Phys. 32, 387 (1961).

[5] D. M. Poole and P. M. Thomas, Quantitative electron probe microanalysis, J. Inst. Metals 90, 228 (1962).

[6] M. J. Henoc, Contribution to electron probe microanalysis, French Postal and Telecommunications

Ministry, Study number 655 "P.C.M." 38 pp. (1962)

(a translation by H. Yakowitz (1963) is available for distribution).

[7] R. E. Michaelis, H. Yakowitz, and G. A. Moore, Appendix to Metallographic Characterization of an NBS

Spectrometric Low-Alloy Steel Standard (available on request).

[8] G. A. Moore, L. L. Wyman, and H. M. Joseph, Chapter

15 in Quantitative Metallography, F. N. Rhines, Ed. (McGraw-Hill Book Co., Inc., New York, N.Y.) in

press.

[9] G. A. Moore and L. L. Wyman, Quantitative metallog- raphy with a digital computer: Application to a Nb-Sn superconducting wire, J. Res. NBS 67A (Phys. and Chem.) No. 2, 127 (1963).

[10] Standard methods for estimating the average grain size of metals, ASTM Designation E-112 (revised 1963) 18 pp.

[11] Nonmetallic inclusions in steel, Metals Handbook,

American Society for Metals 1948, 445-452.

[12] A. E. Delesse, Recherches sur les Roches Globuleures,

Proc. Soc. de Geologie de France, Mem. 4, 67(1952).

[13] S. Bergh and O. Lindberg, On the microscopic determi- nation of the number and distribution of nonmetallic

inclusions, Jernkontorets Annaler 146, 862 (1962) [Henry Brutcher Translation No. 5772—7 pp. (1963)].

[14] A. Taylor, X-rav Metallography (J. Wiley and Sons,

Inc., New York, 1961) table 73, pp. 936-943.

[15] L. S. Birks, Fluorescent x-ray excitation efficiencies, Spectrochimica Acts 11, 148 (1961).

12

* U.S. GOVERNMENT PRINTING OFFICE : ISM 0—742-112
APPENDIX

to "Metallographic Characterization of an NBS Spectrometric Low -Alloy Steel Standard" by

R. E. Michaelis, H. Yakowitz, and G. A. Moore

Introduction ; This Appendix contains typical calculations and tabulated observational data re- ferred to in the above titled publication,, which appears in Volume 66A, page 343 of the National Bureau of Standards Journal

of Research. Explanations of the use made

of these data will be found in that publication. Definition of Symbols: Section A-l

M-/P9Xh

VoV

mass absorption coefficient X-ray take off angle

u./p Cosec p constant X A/Z , where: A is the atomic weight, and Z is the atomic number

intensity of line for specimen

intensity of line for standard, where the standard is pure A Lenard coefficient for electrons Concentration in weight percent

increment of radiation from ele- ment A due to fluorescence from

element B Excitation efficiency for fluor- escence

K excitation voltage (kV) operating voltage (kV) Section A-2

C = Concentration in weight percent

Sections A-3 and A-4

(X) alloy

5.5 ; Csc 9 = 10.43

722; ( X ) Fe = 701 h

alloy = hFe = °-°8?

a32. 5 keV = 1550

CFe

= 1.001 k

Fe for absorption

For nickel: WpYit = 56.2; (n/p) alloy = 354 Malloy = 37005 (X) N1 = 586 halloy = °- 08^ h

Ni = °-°9°

a = 1550

CNi = 2 - 86 k

Ni Fluorescence ' correction: (Birks

)

For iron being fluoresced by 1.73$ Ni

1 / \Ni (iVp) Pe

(V-VoT WpY. Hoy (V-Vo

Fe- E,„ „. is the excitation efficiency for

AZ.-1M1 Nj__ Ka pho tons to excite Fe-Ka

photons at A = I.65A0 (Ni-Ka)

.

The value is 21$ using data from

Birks. ***

Vo is the K series excitation voltage (kV)

a = Standard deviation of the asso- ciated quantity, as determined by

computer analysis of the internal variance of the image observed

during the determination APPENDIX

A-l Absorption Correction (Philibert) for Electron Probe Microanalyzer Data

(M-/p)pe = 67.3 Wp Fe 'Alloy 69.3

V is the operating volt; (kV) „ _ (362.5) (0.6) (0.0173) (0.21) [585.6]

f ~ 353. b o95.2

Journal of Research, National Bureau of Standards, 68A, 343 (1964), reference [3].

, reference [l4 ]

.

, reference [4 ]

.

, reference [15 ]

.

13
K

f = 0.002

1 + K

f = 1.002

n k

f 1.001) —» absorption

Fe Fe (1.002) —»> fluorescence

C Fe = °-999 k

Fe A-2 Actual Data Points From the Electron

Probe Microanalyzer

(1) Cross-section

Nickel Run No Point

I 1 0.559 1.60

I 2 0.653 1.80

I 3 0.772 2.21

I 4* 0.861 2.46

I 5 0.703 2.01

I 6 0.57^ 1.64

I 7 0.663 I.89

I 8 0.833 2.38

I 9 0.714 2.04

I 10 0.744 2.13

I 11* 0.507 1.45

I 12 0.635 1.81

I 13 0.524 1.50

I 14 0.655 1.87

I 15* 0.479 1.37

II 1 0.532 1.52

II 2 0.510 1.46

II 3 0.466 1.33

II 4 0.584 I.67

II 5 0.646 I.85

II 6 O.675 1.93

II 7 0.670 1.91

II 8 0.678 1.94

II 9 0.769 2.20

II 10* 0.823 2.35

II 11* 0.771 2.20

II 12* 0.848 2.42

II 13 0.742 2.12

II 14 0.648 I.85

II 15 0.557 1.59

jj 16 0 . 64l 1 83

II 17 o!562 l!6l

II 18 0.584 1.67

II 19 0.655 1.87

III 1 0.528 1.51

III 2 0.562 1.61

III 3 0.663 1.89

III 4 0.671 1.92

III 5* 0.718 2.05

III 6* 1.13 3.23

III 7 O.965 2.76

III 8 0.75^ 2.15

III 9 0.609 1.74

III 10 0.536 1.53

III 11 O.565 1.61

III 12 O.518 1.48

Point near edge

Point 0{fo) 1 O.956 95 • 5

2 O.966 96.5

0 . 978 97 .

7

4 0.956 95-5

5 0.981 98.0

6 0.982 98.1

7 0.980 97.9

8 0.978 97.7

9 0.959 95.8

(2) Longitudinal section Nickel

No

.

Point 0 \i°

)

I 1 0 . 629 i fin 1 . OO

I 2 0.600 1.72

I 0.534 1 52

I 4 0.539 1*54

I 5 0.577 1.64

I 6 0.601 1.72

7 0 4l5

I 8 0.534 1.52

I 9 O.683 1.96

I 10 0.719 2.05

I 11 O.582 1.67

I 12 0 .502 1.43

-L.2 0 613 1 • 75

I 14 o!520 1.49

I 15 0.629 1.80

I 16 O.638 1.82

I 17 0.668 1.91

I 18 0.620 1.78

0 720

I 20 o!6l2 1.75

I 21 0 .545 1.56

I 22 0 .554 1.58

I 23 o!6l3 1.75

I 24 0 .690 1.97

I 25 n ££fi 0 . 000 1 . 91

I 26 O.583 1.67

x 27 0 .665 1.90

I 28 0 .522 l!49

I 29 0.626 l .79

I 30 o!679 1.94

I 31 0 . 030 1 . 82

I 32 0.548 1.57

I 33 0.575 1.64

I 34 0.604 1.73

I 0.629 1.80

I 36 0.583 1.67

I 37 0.533 1.52

I 38 0.527 1.50

I 39 0.641 1.84

I 40 0.5^7 1.56

I 4l . 0.617 1.76

I 42 0.530 1.51

I 43 0.643 1.84

I 44 O.650 1.86

I 45 0.617 1.76

I 46 0.552 1.57

I 47 0.718 2.05

I 48 0.719 2.05

14
Run No

.

Point k(%) C(%)

II 1 0.712 2.03

II 2 0.635 1.81

II 3 O.566 1.62

II 0.549 1.57

II 5 0 . 004 1 . 90

II 6 0.702 2.01

II 7 0.665 1.90

II 8 0.624 1.78

II 9 0.548 1.57

II 10 0.647 I.85

II 11 0.444 I.27

II 12 O.588 1.68

II 13 0 . 594 1 . 70

II 14 0.565 1.61 Iron

Point 1, 1 af\ k(%) c(%)

1 0.945 94.4

2 0.954 95.3

3 0.953 95.2

4 0.931 93.0

5 0.950 95.0

6 O.961 96.0

7 0.945 94.4

QO 0 . 963 96 .

2

9 0.974 97.3

10 0.954 95.3 n 0.967 96.6

12 0.971 97 .0 13 O.958 95.7

14 0.964 96.3

15 0 . 987 yO . O 16 0.971 97.0

A -3 Volume Percentaeie of Inclusions and Mean Free Paths in the Longitudinal

Section as Deduced by Computer Average mean free Mean

Picture Vol(%) a 50X 0.33 0.09 average

blackness

50X 0 . 94 0 . 11 maximum

blackness

50X 0.19 0.05 average blackness 50X 0.16 0.04 minimum

blackness 100X 0.42 0.10

average blackness

path* in free path

Inclusions in steel

100X 0.09 minimum

blackness

0.02

8.48

10.7

7.38

6.90

4.99

3.28

2480

1392

4275

3435

1155

3264

Picture

5 OX average 100X average All

average

Vol(fo) 0.405

0.26

O.36

Average mean free path* in inclusions

8.36 4.14

6.96

Mean

free path

in steel 2896

2210

2667

As the 50X pictures may fail to resolve inclusions whose minimum dimension is much

less than 5 microns, the mean free path between inclusions can be expected to be somewhat greater in the 50X pictures than

in the 100X pictures. The 100X series does

not include an example of a maximum blackness area, hence the average Vol. % of the

two 100X pictures does not represent

average material. Mean free paths are in microns and are the averages of two lineal analyses done at right angles to one another. Volume Percentage of Inclusions and Mean Free Paths in the Cross-Section

as Deduced by the Computer

Vol(#)

0.60

Picture 100X middle maximum

100X middle

average 100X middle minimum

100X

edge maximum

100X

edge average 100X

edge minimum

50X Average

(3 middle) Average

(3 near

edge) Average

all 100X Average 0.37 all pictures

a0.08

0.44 0.06

0.02

0.52 0.07

Average mean free path* in inclusions

5.45

5.12

3.63

4.!

0.50 0.06 4.25

0.24 0.03

0.19 0.38

0.42

o.4o

0.03 6.75 4.07

4.73

4.40

4.74

Mean

free path

in steel

1132

3062

935

825

1172

3435

977 1694

1336 1636

15
Note the similarity of the 100X value to the value obtained for the longitudinal

section.

As the 50X pictures do not show the effects of inclusions less than 5 microns

in diameter, which amount to about h2% of the total count (Section A-l) , the mean

free path between inclusions may be expected to be abnormally long, and the apparent volume percent abnormally small. Mean free p'aths a re m microns

Cross-section

500X

34.1 0 .54 Cross-section

925X 34.95 0.73 58.5 Cross-section

1000X

34.2 0.62 54.4 Longitudinal

section 462. 5X 34.5 O.67 56.0 Longitudinal

section 500X

30.7 O.89 50.2 Average 32.8 0.27 54.8 A-4 Apparent Perrite Volume Percentage in

NBS 461 Steel in Both the Etched and Unetched Condition

Picture Cross-section

462. 5X (area I) Cross-section

462. 5X (area II)

Vol (56) ferrite

(etched) 29.5

Vol(%)

f errite

(unetched)

The unetched ferrite percentage is de- duced from the perimeter plus area data

in the statistics print out from the .com- puter for the 462. 5X cross-section photomicrographs

The average ferrite percentage in the unetched condition excludes the 462. 5X cross-section photomicrographs

0.55 59.7

32.0 0.55 59.7*

A-5 Mean Free Path Data for Ferrite and Pearlite in NBS 46l Steel Mean free Mean free Mean free Mean free path* in path in Average mean path in path in Average mean

ferrite ferrite free path in pearlite pearlite free path in Picture (normal) ( rotated 90° ) ferrite (normal) ( rotated 90° ) pearlite Cross-section

462. 5X (area I) 1.98 1 89 1.93 4.73 4.51 4.62 Cross-section

462. 5X (area II) 2.11 1 99 2.05 4.49 4.25 4.37 Cross-section

500X

2.12 2 06 2.10 4.10 3.99 4.05 Cross-section

925X

0.968 0 919 0.943 1.80 1.71 1.75 Cross-section

1000X

0.'913 0 880 0.895 1.76 1.70 1.72 Longitudinal

section 467. 5X 2.03 1 48 1.75 3.84 2.81 3.33

Longitudinal

section 500X

1.98 1 44 1.71 4.46 3.25 3.85 Average 1.73 1 52 I.63 3.6o 3.17 3.38

All mean free path data in microns

A-6 Number of Inclusions per Cubic Millimeter

This determination is based on cross-section photomicrographs of an area of average blackness. For a micrograph at 100X, the observed image field represents 0.1764 square milli- meters on the specimen. Thus the indicated number of inclusions in a 1 square milli- meter field is 1 = 5.668 times the number actually counted. 0.1764

16
The typical inclusion was determined to be a cylinder whose length perpendicular

to the field is 2.7 times its mean diameter. For this shape, the observed sections may be assumed to be true sections rather than the reduced sections which would be observed for spheroids.

If the cubic millimeter were subdivided into 1000 plain slices each 1 micron thick, each inclusion would appear in a number of slices, this number being on the average the

same as the length of the inclusions. Thus for size class 0, each inclusion would be seen on 6.75 slices, but only 0.675% of all of the inclusions in one cubic millimeter

are seen on one slice. The number of inclusions, of this class, observed on 1 square millimeter must therefore be multiplied by 1000 = 148 to give the number in the volume. For class 9, 3 .2h% of the inclusions will be observed, and. the multiplying factor is reduced to 31.Diameter Length Particles Particles per Particles per

Area class (microns ) ( microns ) in class square millimeter cubic millimeter

0 2.5 6.75 7 39.63 5873

2 3.5 9.45 6 33.96 3606

4 5.0 13.5 3 16.98 1260

5 6.0 16.2 2 11.34 700

6 7.0 18.9 6 33.96 1800

7 8.5 23.0 3 16.98 741

9 12.0 32.4 1 5.668 175 Total estimated particles of 2.5 micron diameter and larger, per cubic millimeter

is 14,155. Estimated probable error is 20$

A-7 Probability for One Inclusion to Occur in a Random Fifty Micron Diameter Sphere

of NBS 46l Steel

The volume of the fifty micron sphere is 65,450 cubic microns; there will be

15 j 279 such spheres per cubic millimeter. The volume of an inclusion is simply its length times the area of its base. The volume percent of such an inclusion is its volume divided by 65, 450, and its weight percent is approximately (0.23/0.40) times

the volume percent.

On the average, there will be 14,000/15,279 (0.93) inclusions, or about 1 inclusion

per 50u. sphere which is equivalent to 0.23 percent by weight. For the inclusions to have a 1 percent effect on the results by weight, about 2 percent by weight of the in- clusions would be required which corresponds to the presence of about 8 inclusions of average size in the 5 Op. sphere. Assuming these inclusions are randomly distributed in the steel, the chance of finding 8 in a 50[i sphere is negligible. Area Inclusions per Volume* of Total volume* of Probability for Volume Weight

class cubic millimeter one inclusion inclusion per mnK one inclusion percent percent

0 5873 35.5 208,000 0 384 0 054 0.031

2 3606 99.2 358,000 0 236 0 152 0.087

4 1260 284 358,000 0 082 0 434 0.250

5 700 477 334,000 0 046 0 729 0.419

6 1800 795 1,431,000 0 118 1 21 0.696

7 74l 1366 1,012,000 0 048 2 09 1.20

9 175 3842 672,000 0 Oil 5 87 3.38

tals 14,155 4,373,000 0 926

Volumes are in cubic microns; hence, 4,373,000 corresponds to 0.44 volume percent in one cubic millimeter (109 cubic microns)

17

Announcement of New Publications on Standard Reference Materials

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Dear Sir: Please add my name to the announcement list of new

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Announcement of New Publications on Standard Reference Materials

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Government Printing Office

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Dear Sir: Please add my name to the announcement list of new

publications to be issued in the series: NBS Miscellaneous

Publication 260 - Standard Reference Materials.

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