STATES RTMENT OF k/IMERCE

LIGATION

National Bureau of 9ta„„ MAR 2 4 197|

NBS SPECIAL PUBLICATION 260-26

NATION/

CORPORATH

U.S. PARTMENT

OF

COMMERCE

National Bureau

of Standards

( DO

m

Co

Standard Reference Materials:

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

Standard Reference Materials:

National Bureau of Standards-U. S. Steel

Corporation Joint Program for Determining

Oxygen and Nitrogen in Steel

J. F. Martin

Analytical Chemistry Division

Institute for Materials Research

National Bureau of Standards Washington, D.C. 20234

ij \ S i National Bureau of Standards Special Publication 260-26

Nat. Bur. Stand. (U.S.), Spec. Publ. 260-26, 40 pages, (February 1971) CODEN: XNBSA

Issued February 1971

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.10:260-26), Price 50 cents.
Library of Congress Catalog Card Number: 79-610184

ii
PREFACE

Standard Reference Materials (SRMs) as defined by the

National Bureau of Standards are "well-characterized mate-

" rials, produced in quantity, that calibrate a measurement

j system to assure compatability of measurement in the nation."

| SRMs are widely used as primary standards in many diverse

j fields in science, industry, and technology, both within the

j United States and throughout the world. In many industries

! traceability of their quality control process to the national

measurement system is carried out through the mechanism and

use of SRMs. For many of the nation's scientists and technologists it is therefore of more than passing interest to know the details of the measurements made at NBS in arriving

at the certified values of the SRMs produced. An NBS series

! of papers, of which this publication is a member, called the

NBS Special Publication - 260 Series Is reserved for this

I purpose.

This 260 Series is dedicated to the dissemination of

j information on all phases of the preparation, measurement,

j and certification of NBS-SRMs . In general, much more detail

I will be found in these papers than is generally allowed, or desirable, in scientific journal articles. This enables the

' user to assess the validity and accuracy of the measurement

processes employed, to judge the statistical analysis, and

to learn details of techniques and methods utilized for work

entailing the greatest care and accuracy. It is also hoped

that these papers will provide sufficient additional information not found on the certificate so that new applications

in diverse fields not foreseen at the time the SRM was originally issued will be sought and found.

Inquiries concerning the technical content of this

paper should be directed to the author(s). Other questions

concerned with the availability, delivery, price, and so forth will receive prompt attention from:

Office of Standard Reference Materials

National Bureau of Standards

Washington, D. C. 20234

J. Paul Cali, Chief

Office of Standard Reference Materials

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vi
TABLE OF CONTENTS

PAGE

1. INTRODUCTION 2 2. MATERIALS AND EXPERIMENTAL WORK 3 A. Analyzers 3 1. Vacuum-Fusion Analyzer 3 2. Single-Crucible, Helium-Fusion Analyzer . . 4 3. Nitrogen Analyzer 5 4. Experimental IGF-GC Analyzer 5 B. Analytical Standards 9 3. RESULTS AND DISCUSSION 11

A. Analyzers 11

1. Vacuum-Fusion Analyzer 11

2. Single-Crucible, Helium-Fusion Analyzer . . 11

3. Nitrogen Analyzer 16

4. Inert Gas Fusion - Gas Chromatographic

Apparatus 16

B. Standard Reference Material Analysis 20

4. SUMMARY 2 5 5. ACKNOWLEDGMENT 26

6. REFERENCES 27 APPENDIX I 29 APPENDIX II 31

LIST OF TABLES

Table No.

1. Operating conditions for Inert gas fusion - gas chromatographic system 7

2. Nitrogen by single-crucible, helium- fusion

technique 10

3. Effect of sample weight on nitrogen recovery

(single-crucible, helium- fusion) 13

vii
LIST OF TABLES (con't)

Table No. PAGE

4. Effect of various bath materials on nitrogen

recovery (single-crucible, helium-fus ion) ... 14

5. Comparison of nitrogen values obtained with

Pt bath and Pt-Sn Bath (single crucible,

helium-fusion) 15

6. Nitrogen values obtained with the singlecrucible, helium- fusion technique 16

7. Nitrogen values on NBS samples obtained with

IGP-GC (no bath used) 19

8. Homogeneity analysis of NBS 1094 (maraging

steel) for Oxygen and Nitrogen (single-crucible,

helium-fusion) 22

9. Oxygen values on SRM 1094 obtained with

IGF-GC (no bath used) 23

LIST OP FIGURES

Figure No.

1. Schematic diagram of single-crucible,

helium fusion instrument 4 2. Flow diagram of inert gas fusion-gas

chromatographic apparatus 6 3. Six- way gas sampling valve 8 4. The new design of sample changer 18

5. Sampling scheme for homogeneity testing

of SRM 1094 21

viii
STANDARD REFERENCE MATERIALS:

NATIONAL BUREAU OF STANDARDS-U. S. STEEL CORPORATION

JOINT PROGRAM FOR DETERMINING OXYGEN AND NITROGEN IN STEEL

J. F. Martin*

Because of a need for a method for the rapid,

simultaneous determination of small concentrations

of oxygen and nitrogen in steel, a joint project

under the Industrial Fellowship Program of U. S. Steel and the Research Associate Program of the

National Bureau of Standards (NBS) was established.

After investigation of various techniques, and

analytical apparatus was constructed with excellent

sensitivity for both elements. This apparatus

consists of an inert gas fusion system coupled

to a gas chromat ograph . Recommended modifications

have been completed on this equipment, and simultaneous determinations of oxygen and nitrogen in

steel are presently being made. The detection

limit is <1 ppm for both oxygen and nitrogen, and

the analysis time is about 5 minutes

A direct result of this program has been

the issuance by NBS of a new Standard Reference

Material (SRM) , a maraging steel, for oxygen.

This SRM has the lowest oxygen concentration

(4.5 ppm) of any steel standard yet issued by NBS.

Key words: Inert gas fusion; one ppm of oxygen

and nitrogen in steel; Research

Associate Program; simultaneous

determination of oxygen and nitrogen;

Standard Reference Material for oxygen.

Industrial Fellow, 1968-69. Research Associate Program,

National Bureau of Standards. Present address: Applied

Research Laboratory, United States Steel Corporation,

Monroeville, Pennsylvania 14156.

1
1. INTRODUCTION

Under the Industrial Fellowship Program of U. S. Steel

and the Research Associate Program of the National Bureau of

Standards (NBS), a joint research project was conducted to develop a rapid instrumental technique for the simultaneous

determination of trace amounts (< 0.005 percent) of oxygen

and nitrogen in all grades of steel. This work was conducted

at the Gaithersburg , Maryland, laboratory of the NBS by

personnel from U. S. Steel and NBS. In this program the USS

Industrial Fellow worked at the laboratories of the NBS from

August 15, 1967, through September 30 3 1968, on a full-time

basis. NBS provided technical supervision and assistance,

research instruments, and general supplies. In addition,

two manufacturers of gas-analysis equipment provided their

analyzers for a limited time.

Although commercial equipment has long been advertised

as being applicable to the simultaneous determination of

oxygen and nitrogen in steel, non proved adequate. One

instrument, reportedly applicable to the simultaneous

determination of nitrogen and oxygen, an attachment for

another Instrument for nitrogen only, and another nitrogen

analyzer, however, have recently appeared on the market,

and these were evaluated as part of the joint project.

In addition to these three instruments, to obtain more versatility and better sensitivity than are available with

commercial systems, an inert carrier gas fusion apparatus

was designed and coupled to a gas chromatograph (IGF-GC)

for the determination of oxygen (as carbon monoxide) and

nitrogen. This apparatus was based in part on those of

Dallmann and Fassel [1] and one constructed earlier in the

U. S. Steel Applied Research Laboratory. Furnace designs

and the single crucible technique were investigated with this

equipment. The following is a report of this work.

2
2. MATERIALS AND EXPERIMENTAL WORK

A. Analyzers

Following are brief descriptions of the various analyzers

used.

1. Vacuum-Fusion Analyzer . Gerhardt, et al. [2]

developed a technique for the determination of nitrogen in

steel with a commercial vacuum- fusion unit. During the

analysis of successive steel samples by vacuum fusion,

graphite is eroded from the crucible walls. Some of this

graphite floats on the surface of the molten bath with the

result that the evolution of nitrogen is impeded. In this new technique, a crucible is employed that can be spun and

emptied between analyses without breaking the vacuum; thus

each sample is analyzed In a clean out-gassed crucible.

This method is referred to as the spinning-crucible technique.

To increase the fluidity of the melt, a small amount of

nickel-cerium alloy is added for each analysis so that the

eroded graphite that does form appears as nodules and not

as flakes.

The apparatus is equipped with a nondispersive infrared

system for measuring oxygen as carbon monoxide and a thermal

conductivity cell for measuring nitrogen plus carbon monoxide.

Nitrogen is determined by difference. Because cerium reacts

strongly with oxygen, only nitrogen can be determined conven- tionally by this technique. When oxygen is to be determined,

cerium, which forms a stable oxide, is not added to the

crucible. Successive analyses for nitrogen and then oxygen

are not practicable because once cerium is added to the

crucible, a small amount is retained after emptying the

crucible. An advantage of this instrument is that each

analysis may be simulated by use of a variable volume, gas dosing system.

3
2. Single-Crucible, Helium-Fusion Analyzer . An apparatus

developed by Lemm and Koch [3 S 4] employs fusion in helium

rather than in vacuum. The unique feature of this equipment

is the use of a new crucible for each determination (singlecrucible technique). The basic operation of this analyzer

provides for the measurement of oxygen as CO by infrared

absorption and nitrogen by gas chromatography, with the output integrated in a continuous flow. Figure 1. The Co is

measured first and then oxidized by Schutze reagent to CC^,

which is removed on a molecular sieve column; nitrogen and

hydrogen are separated on this column. The latter two gases

are determined by thermal conductivity. Results for H

2

, CO,

and N

2 are displayed on a strip-chart recorder and are presented in digital form. Analysis time for oxygen and

nitrogen determined simultaneously is about 3 minutes.

CALIBRATION VALVE

UNIT

EXTRACT ION FURNACE

SCHUTZE REAGENT

MOLECULAR

SIEVE

INFRARED

-ABSORPTION ANALYZER

THERMAL CONDUCTIVITY CELL

e

o®®o o®®o

D D

[^] © ® © © =

MEASURING

DEVICE

Figure 1. Schematic diagram of single-crucible, helium fusion

instrument

4
This apparatus has several unique features. A gas

dosing valve has been incorporated into the system so that

pure CO and N

2 can be admitted to the system both for calibration and for checking the response of the gas analyzers.

Further, the furnace section includes certain details not found in other commercial analyzers. These include:

1. An electric elevator for raising and lowering

the crucible to facilitate changing of crucibles and to exactly locate the crucible during heating,

2. An electroprogrammer to vary the temperature

while the gases are extracted from the sample,

3. A flow controller for directing the carrier

gas through the sample entry port, and through the furnace

section so that air does not enter the system when crucibles

are changed, and

4. Plug-in graphite heating elements.

3. Nitrogen Analyzer . Equipment based on the pulse

heating method described by Vasserman and Turovsteva [5]

and later used by Goldbeck, et al. [6] is now commercially

available. In this equipment a high current (between 300

and 400 amp) is pulsed through the graphite crucible so that

temperatures in excess of 2800 °C are attained. The and

CO extracted from the steel in a dynamic stream of helium

are oxidized to water and carbon dioxide, respectively, and

removed in absorption traps. The nitrogen is then measured

by thermal conductivity, and the concentration is presented

on a digital voltmeter. A new crucible is used for every

determination, and each determination requires 2-1/2 minutes

4. Experimental IGF-GC Analyzer . The fourth apparatus

evaluated was an inert gas (helium) fusion - gas chromatographic (IGF-GC) system designed and constructed at NBS

,

Figure 2. In this apparatus the sample is fused in a graphi
GC SUPPLY GAS

SHIELD GAS

SAMPLE

INLET

INDUCT ION FURNACE

COLLECTOR TRAP

VENT /

JLO_

SAMPLE

VALVE

SAMPLE

CARRIER GAS

INTEGRATOR

CARRIER GA VENTS

^-REFERENCE \ SAMPLE

GAS CHROMATOGRAPH

Figure 2. Flow diagram of inert gas fusion-gas chromatographic

apparatus

crucible insulated with graphite powder, both of which are contained in a quartz thimble. Purified helium is passed

over the crucible and through the system at a rate of 1.5

liters per minute. Table 1. The crucible assembly is heated

inductively, and an operating temperature of 1900 °C is

easily maintained. (Toward the end of the program a 10 kw,

450 kHz generator was installed to provide higher temperatures

because poor accuracy was obtained for nitrogen due presumably

to insufficient power to obtain the high temperatures necessary for the complete decomposition of metal nitrides.)

6
Table 1. Operating conditions for inert gas fusion - gas chromatographic system. Inert carrier gas Helium, 1.5 1/minute

Purification system 1/4-inch-diameter by 20-footlong copper tube packed with

molecular sieve 5A, column

temperature -I96 °C

Furnace heater

Outgassing temperature

Analysis temperature

Collecting loop

Chromato graph

Chromatographic carrier gas

Separating column

Detector

Recorder

Integrator

Water-cooled j induction 2.5 kw,

450 kHz

2100 °C

1850 to 1950 °C

Constant temperature, thermistor

detector

Helium, tank gage set at 60 psi

1/8-inch-diameter by 6-foot-long

copper packed with molecular

sieve 5A at 50 °C

Thermal conductivity

0 to 1 millivolt, variable chart

speed, and response time of 0.75

to 1 second full scale

Counting rate 6000/minute full

scale

Molecular sieve 5A at -196 °C

The helium sweeps out the CO, N

2

, and H

2 evolved from

the molten specimen during fusion. The gas mixture is

passed through a special valve (designed and built at NBS

and shown schematically in Figure 3) into a collecting loop

cooled to -196 °C with liquid nitrogen. The collecting

loop or trap is made of stainless steel and is filled with

about 5 grams of molecular sieve 5A . The CO and N

2 are adsorbed here during a 3-minute collection period, after

7
m w,

/ » mm % VAV/AV/AWAV/////////A

6 1 r

2 ^n- //X L _ __l — E

ANALYSIS POSITION

A - FURNACE GAS

B - CARRIER GAS He

C - TO CHROMATOGRAPH

D - VENT GAS

E - SHIELD GAS He

F - SAMPLE TRAP

COLLECTION

POSIT ION

Figure 3- Six-way gas sampling valve

which a valve is rotated to vent the furnace carrier gas (helium) to the atmosphere and to permit the chromatographic

carrier gas (helium) to pass through the collecting loop.

Simultaneously, this loop is electrically heated. The CO and N

2 are then desorbed and injected into the chromatograph

as a concentrated slug where they are separated on a molecular sieve (5A) column and detected by thermal conductivity.

The output from the cell is recorded on a 1-mv stripchart recorder. The latter is also connected to an integrator which measures and prints the area under the peaks

due to the CO and N

2

. The chromatographic apparatus is

calibrated by addition of known amounts of pure nitrogen

or carbon monoxide either through the furnace section or directly into the collecting loop. No significant difference

between the two methods of calibration was obtained.

8
B. Analytical Standards

To conduct this development work, National Bureau of

Standards Standard Reference Materials (SRMs) were used.

The oxygen standards, representing iron and two types of

steel, are listed in the Certificate of Analysis (Appendix

I); these were available in the form of rods. The SRMs for

nitrogen, representing a number of different types of iron

and steel, are listed in Table 2. Most of these were available only in milled form. For the milled standards, the

oxygen values were not recorded because surface cleaning

was not practicable. It will be noted that nitrogen values

are given in percent and parts per million (ppm) . The

former has been used for many years because nitrogen values

for low-alloy steels usually ranged between 0.005 and 0.02

percent with an estimated precision of 0.001 or 0.002 percent

and a similar detection limit. Recently, methods have been

developed with a detection limit of 0.0001 percent or 1 ppm

and with a precision considerably better than obtainable

earlier. For simplicity the ppm unit is now used for steel

almost exclusively. Oxygen in steel results are also now

expressed in ppm for the same reasons.

9
Table 2. Nitrogen by single-crucible, helium- fusion technique.

Nitrogen

NBS-SRM Analytical Average Recommended, Certified

value, ppm ppm ppm % 3a 84.4 79.4 82.9 82.2 86

3b 81.1 81.7 90.4 84.0 84

3b 83.3 83.5 84.3

3c 45.8 42.5 47.5 45.3 46

4i 70.7 69.1 71.2 70.3 70

4i 41.1 4l„3 41.3 41 0 2 37

^ i 79.4 80.1 78 3 78 5 5j 78.8 76.6 77.8 0.009

5k 111. 0 108.0 86.3 102 90

55.7 58.4 3D. O D<-> 5L 54.7 62.4 61.3

6f 38.4 32.8 32.1 34.4 50

8i 187.176.0 177.0 180

10g 146.0 142. 0 145.0 147 0.018

10g -loH . H 0.015

30f 76.1 78. 9 75ol 76.7 75 32e 85.0 88.2 87.2 86.8 0. 009

55e 42.5 42.1 43.3 42.6 0. 004 72f 102. 0 100.0 100. 0 101 0. 009

lOld 222. 0 224. 0 223.0 223 0.024

125a 42.6 38.2 36.8 38.7 0.002

125a 39.0 38.0 37.9

129b 132.0 126.0 128,0 129 0.014

163 93.9 77.1 73.1 81.4 91

343C 737.0 732.0 733.0 734.0 0.074

346° 4490 4530 4550 4520 4410

aRe commended values form NBS Technical Note 454, July 1968.

bThis has been shown to be incorrect. A recent recommended

value is 0.004 percent.

0 These samples were available in both chunk and milled form

All other samples were available in milled form only.

10
3. RESULTS AND DISCUSSION

A . Analyzers

1. Vacuum-Fusion Analyzer . With the vacuum- fusion

instrument no method could be developed for determining

nitrogen and oxygen simultaneously. Determination of one precluded determination of the other. The instrument also

has a rather poor detection limit for nitrogen (10 ppm)

,

and this element must be determined by difference. However,

certain analyses for nitrogen in steel were later performed

by using the spinning crucible on this instrument.

2. Single-Crucible, Helium-Fusion Analyzer . Although

the single-crucible, helium- fusion instrument Includes a number of novel features, it has several shortcomings. With

analysis for oxygen and nitrogen being performed in the flowing gas stream, the integrator is zeroed immediately be ore adding the sample to the crucible and the signal from the

blank is not recorded. Therefore, the magnitude of the blank

is not determined. If it becomes a significant fraction of

the gas from the sample, the accuracy of the analytical

results for both oxygen and nitrogen can be deleterious ly

affected. Further, results below 10 ppm may be in error because of drift in the digital integrator. In the analysis

of samples containing more than 4 ppm hydrogen, there is

poor resolution between hydrogen and nitrogen. This could

lead to erroneous results for the latter, but could be overcome by using a longer molecular sieve column to effect

adequate resolution. Finally, although the dosing valve

system facilitates calibration and checking of the apparatus,

it is not possible to reproduce each analysis because only

fixed calibration volumes are used.

Evaluation of the single-crucible, helium-fusion instrument was limited because NBS personnel also used it and

because the instrument was on loan from the manufacturer.

11
To analyze as many samples as possible, the equipment was

first evaluated with solid samples, NBS SRMs 1090, 1091,

and 1092. Nominal standard operating conditions were established and results for both oxygen and nitrogen were obtained.

After these parameters were fixed, many of the NBS milled

steel SRMs were analyzed for nitrogen. The SRMs ranged in

nitrogen content from 40 to 4550 ppm, and the results are listed in Table 2. After the above nitrogen values were obtained, certain

standard operating conditions were fixed. Temperature programming for the fusion of the sample was investigated because

this is recommended by the manufacturer and must be determined

for each type of material analyzed. With NBS SRM 5L, which

has a nominal nitrogen content of 58 ppm, samples were analyzed while the temperature at which the samples were introduced into the graphite crucible was varied from 1400 to 1850 °C. In addition, the normal heating cycle was programmed for

a temperature increase of about 100 °C during the last 10 to 20 seconds for the fusion and extraction period. This cycle

was varied with respect to both time and temperature to minimize saturation of the metal bath with carbon and to effect complete recovery of nitrogen. This study indicated

that some benefits were obtained with temperature cycling if

a final temperature of 1900 °C was attained and held for a minimum of 15 seconds. With an initial temperature of 1400 °C

it was not possible to reach 1900 °C in less than two minutes.

The best temperature program applicable to steels in general

seemed to be to introduce the sample at 1850 °C, fuse at this

temperature for 20 to 30 seconds, and then raise the temperature to 1950 °C for at least 15 seconds. However, optimum

conditions for the extraction of oxygen and nitrogen from

various metals and alloys should be determined for each type

of sample.

12
To ascertain the effect of sample weight on nitrogen

recovery, a series of samples containing about 150 ppm

nitrogen was analyzed by using samples weights from 0.05

to 1.0 gram. All but one of the results obtained (rounded

off to the nearest 10 ppm) , Table 3, agreed within 10 ppm

of the certified value obtained chemically.

Because complete nitrogen recovery from some steels

is difficult or presently impossible by conventional fusion,

the effect of bath material was also investigated. Copper,

nickel, and platinum were used as a bath in the ratio of 10 parts bath material to 1 part sample, and the results were compared with those obtained by the single crucible technique.

Table 3. Effect of sample weight on nitrogen recovery

(single-crucible, helium- fusion )

.

NBS-SRM lOg (Low alloy)

Nominal weight, g Nitrogen, ppm

0.05 145

0.06 173

0.07 163

0.08 150

0.09 147

0.1 149

0.2 154

0.3 158

0.4 159

0.5 143

0.5 145

0.5 141

0.5 147

1.0 143

1.0 141

1.0 144

a NBS certified value 0.015 percent nitrogen.

NBS certificate range 0.014 to 0.015 percent nitrogen.

13
NBS SRM lOg, from which nitrogen is easily recovered, was used for this part of the investigation. These results are shown in Table 4. Only the platinum bath technique yielded

results in agreement with the 140 to 150 ppm expected range. For reasons unknwon, values with the copper and nickel baths were low by 11 percent and 18 percent, respectively.

Table 4. Effect of various bath materials on nitrogen

recovery (single-crucible, helium- fusion)

NBS-SRM lOg (Low alloy) a Bath Nitrogen, ppm

None 146, 146

Copper 129, 128, 124, 129

Nickel 115, 120

Platinum 145, 144

aNBS certified value 0.015 percent nitrogen.

NBS certificate range 0.014 to 0.015 percent.

In addition to the straight platinum bath, a technique

proposed by Dallmann and Passel [1] using 18 percent platinum20 percent tin with a minimum ratio of 3(Pt*Sn):lFe was investigated. Results were obtained for both oxygen and

nigrogen on SRM 1091, and AISI Type 431 stainless steel,

with this technique and also with the platinum bath technique. Results are listed in Table 5. With the platinum

bath the oxygen values obtained agreed with the certified

result; the nitrogen results were low by comparison with

the NBS pressure bomb result which is preferred to that

obtained by vacuum fusion. The oxygen values obtained with

the platinum-tin bath were often high and low nitrogen values were again obtained.

14
Table 5. Comparison of nitrogen values obtained with Pt bath

and Pt-Sn bath (single-crucible, helium-fusion).

NBS-SRM 1091 (AISI Type 431)

Platinum Bath

NBS Certified

value

Oxygen, ppm Nitrogen, ppm

140 873' 137 850

133 858

l4l 869

133 860

132 851

0U/0 riaimum — dU/o lin nEiZn Oxygen, ppm Nitrogen, ppm

153 819

157 836

183 843

l4r0 833

147 815

139 827

I 131 945^

86lb

a Pressure bomb - not certified.

b Vacuum fusion - not certified.

Table 6 is a listing of nitrogen values obtained for

solid samples by using various metals as a flux in the ratio

of 1 part flux to 1 part sample. Experimental values for

SRM 1091 and 1092 were lower than the recommended values for

undetermined reasons, with the value of 1091 falling between

that obtained by the pressure bomb and Kjeldahl or vacuum- fusion methods. For SRM 1093 the average nitrogen value with

the platinum flux lies between the values obtained by the

Kjeldahl and pressure-bomb techniques. Use of platinum flux

material for SRM 1092 had no apparent effect on the nitrogen

recovery as compared with no flux at all.

15
Table 6. Nitrogen values obtained with the single-crucible,

helium- fusion technique.

Nitrogen, ppm

NBS-SRM Recommended

sample Flux Experimental Average or other

1091 a None 920 917 911 945

b (AISI 927 926 921 865^

Type 431) 903 922 906 86l d 913 911 901 915

1092 Platinum 1.4 2.3 1.5 3.6

b (Vacuum 1.2 1.2 1.2 10.0

Melted 2.1 1.9 1.6 4.0

Steel) None 1.1 1.5 2.6

3.6 2.7 2.0 2.3

1093 Cobalt 4540 4350 4420 44'40 4 9 40 a (Valve Nickel 4440 4540 4440 4470

Steel) Platinum 4510 4570 4580 4550 3640 d

See also Table 6.

13 Pressure bomb.

c Kjeldahl.

d Vacuum fusion.

3. Nitrogen Analyzer . The nitrogen analyzer based on the pulse-heating method was used for nitrogen analyses.

Because of the very good agreement obtained between this

analyzer and NBS certified values, a recommendation was made

to both the Laboratory and NBS that this apparatus be investigated further. Subsequently, equipment of this type was purchased and evaluated by the Applied Research Laboratory

and is presently being used routinely.

4 . Inert Gas Fusion - Gas Chromatographic Apparatus . The apparatus that was designed and built at NBS had a sufficiently good detection limit for both oxygen and

nitrogen, as noted earlier. As originally designed,

however, the apparatus did not have sufficient power to

16
attain the high temperature needed to extract the nitrogen,

but when a 10 kw power unit was installed, temperatures in excess of 2300 °C were attained. Various modification of

crucibles were tested to ascertain the practical number of

samples that could be analyzed before a crucible change.

This experimental apparatus was found to have a sensitivity of 120 counts/ul for carbon monoxide and 122 counts/yl

for nitrogen or 168 counts/ug oxygen and 98 counts/yg

nitrogen. (A count is an arbitrary unit of area printed by an electronic peak-area integrator.) The results are estimated to have a reproducibility of about 20 counts;

therefore, a detection limit of 0.2 yg for both oxygen and

nitrogen can be conservatively attained. If necessary, this

sensitivity can be increased by increasing the bridge current

on the thermal conductivity cell.

It was found that increasing the temperature and using

a large crucible and a platinum bath resulted in lower

nitrogen values than were obtained by the Kjeldahl or pressure-bomb dissolution techniques. This is evidenced by

values of 124 ppm that were obtained on samples of SRM lOg

certified at 0.015 percent nitrogen. Because of this low

nitrogen recovery with the multiple-sample technique, it was decided to modify the apparatus.

The recommended modification (put into effect after

the writer left NBS and returned to the Laboratory) involved

the use of a crucible for a single determination, rather

than using the same crucible for successive extractions.

(See Figure 4 and Ref. 9.) Thus, the influence of previously

analyzed samples is eliminated. In addition, the carbon

content of the molten bath arising from erosion of the

crucible is minimized and a more fluid bath results.

Finally, although it is possible to have a film of deposited

metals that volatilized from the bath and that may react with gases evolved from a succeeding sample, it is expected

17
Figure 4. The new design of sample changer.

that this film would be covered by graphite volatilized

during bake-out of a succeeding crucible.

The modified graphite crucible was prepared with the

following dimensions: 9/16 inch OD, 7/16 inch ID, and 3/8

inch high. A furnace tube, approximately 1 inch ID, was

18
fabricated of quartz. The crucible was supported from the

bottom by a graphite pedestal, the cross section of which

was reduced to minimize heat conduction. This pedestal was supported by a boron nitride rod which was held in place by

a slip joint fabricated in the ground-glass plug used to close the bottom of the furnace chamber. The closure designed

for the bottom of the furnace assembly employs O-ring seals

to permit rapid changing of the crucible, during which the

flow of helium through the furnace minimizes its contamination

with atmospheric gases.

With this equipment, it was possible, after outgassing

the crucible for 3 minutes at 2300 °C and by using a cyclic

heating technique, to obtain a nitrogen value of 147 ppm for

SRM lOg, which compares favorably with the certified value

of 0.015 percent. The crucible is changed for each sample.

The necessity for this was additionally demonstrated when

decreasing nitrogen values were obtained for second and third

samples of the same steel added to the crucible. The same experiment was repeated with SRM 1091. A nitrogen value of

950 ppm was obtained for the initial sample, as compared with

945 ppm by the pressure-bomb solution method, Table 7. However, when subsequent samples were added to the preceding

melt, the values dropped to 825 ppm. Nitrogen values for

Table 7. Nitrogen values on NBS samples obtained with

IGP-GC (no bath used)

.

Nitrogen, ppm

NBS-SRM This method Other methods

SRM 1090 60 61.

4

a

, 49.

5

b

, 60°

SRM 1091 950 945

SRM 1092 4 3.6 b

, 4.0 C

Kj eldahl-dist illation-photometry

.

Pressure bomb-distillation-photometry

.

Vacuum fusion.

19
SRMs 1090 and 1092 obtained with this equipment are also

listed in Table 7. Oxygen values for these samples obtained with this

system and used for certification appear in Appendix I. These values and the representative standard deviations

of a single determination agree favorably with the

corresponding data obtained by vacuum fusion and neutron

activation. Work performed at NBS after conclusion of the

USS Industrial Fellowship, but based on recommendations of

the Fellow, showed the apparatus to be applicable to the

simultaneous determination of nitrogen and oxygen in a number of different grades of steel by using the single

crucible technique. The detection limit for both oxygen

and nitrogen is <1 ppm, and the analysis time is about

5 minutes.

B . Standard Reference Material Analysis

During the period spent by the author at NBS , various

samples were analyzed as part of the NBS Standard Reference

Materials Program. Analyses of SRMs 1090, 1091 and 1092

by inert-gas fusion were performed, Appendix I, because

when these were first issued results for oxygen by vacuum

fusion only were reported and certified. However, the

certificate was recently replaced with one on which results

by three methods, vacuum fusion, IGF-GC, and activation

analyses, are reported. Nitrogen results on these samples

were also obtained as noted earlier, Table 7. In addition, NBS had been requested by various analysts

to provide a new SRM for oxygen in steel at a very low

level, and the Laboratory furnished a 12Ni-5Cr-3Mo maraging

steel for this purpose. This material (later designated

SRM 109^) was shipped in the form of a 1/2-inch-thick plate

approximately 3 feet by 6 feet and weighing 300 pounds.

Preliminary testing was done on this material by analyzing

samples from diametrically opposite corners, taking care

20
to obtain samples at least 1/2 inch from the edges which

had been flame cut during the processing of the sheet.

Analysis of this material by vacuum fusion indicated an oxygen content of <_5 ppm. Other samples of this material were then analyzed for

both oxygen and nitrogen with the IGF-GC apparatus. The

sampling scheme for the homogeneity testing is shown in

Figure 5- Strips of the maraging steel, approximately

1/2-inch wide, were cut by sawing from either side of the

plate. These strips were then hot-forged and swaged into

rods from the left and right sides of the plate, respectively,

were assigned the numbers 1 and 2 as the first part of their

identifying marks. Each rod was then cut after preparation

into specimens as noted.

ICI IC2

ORIGINAL PLATE

6ft x 3ft x 1/2 in.

2AI 2A2

2BI 2B2

2CI 2C2

SAMPLING POSITIONS, MARAGING STEEL Figure 5. Sampling scheme for homogeneity testing of SRM 1094.

For the homogeneity screening the single-crucible,

helium- fus ion equipment was used to determine both oxygen

and nitrogen, Table 8. As noted earlier, results (oxygen

in this case) below 10 ppm were suspect because of the drift

21
Table 8. Homogeneity analysis of NBS 1094 (maraging steel)

for Oxygen and Nitrogen (single-crucible,

helium- fusion)

Oxygen, ppm

Single- crucible

Sample helium-fusion

(no bath)

1A1 4.3

1A2 3-0

1B1 4.3

1B2 6.0

1C1 3-7

1C2 3.4

2A1 3.4

2A2 3-9

2B1 3-9

2B2 3-2

2 CI 4 .

2C2 3-2

1A1 3.7

1B2 3-0

Average 3.9

Nitrogen

Single cruciblehelium fusion

ppm

Spinning

crucible

(no bath)

_a

58.8

48.5

59.3

61.8

59.3

57.9

58.5

48.4

59.0

47.4

59.0

58.3

61.3

49 .0

(ni-Ce bath) NAb

64.0

NA

62.7

60 .7

58.6

NA

61.5

Sample lost. NA not analyzed.

in the digital readout. Because 71 ppm nitrogen was obtained

by the pressure-bomb method [7,8] as compared with 50 ppm

with the helium- fusion apparatus, analyses for nitrogen using

the vacuum- fusion, spinning-crucible method developed by

Gerhardt, et al. [2] were also performed. The results from

the latter compared more favorably with those obtained on the helium-fusion instrument than with those by the pressurebumb method.

22
Vacuum fusion was the method used to initally certify

SRM 1094 for oxygen (provisional certificate, Appendix II).

Analysis for oxygen, Table 9, was also performed with the

IGF-GC system. The sample numbers 1A1, 1A2 , etc., assigned

for homogeneity studies, refer to the position in the original

plate. The very low oxygen content of this material together

with optimum sample size of one gram made it necessary to measure microgram quantities of oxygen. Because it was possible to measure blanks at a level of 1 microgram per

minute of collection time with the IGF-GC apparatus, this

presented no problem.

Table 9- Oxygen values on SRM 1094 a obtained with IGF-GC

(no bath used )

.

Sample Oxygen, ppm Mean Oxygen, ppm

1A1 6. o> 4.• 0, 3. 8, 4 • 0, 3.• 9 4. 3 1A2 4,,0, 4,• 6, 4.,0, 3 5..6 4,.4 1C1 4,•1, 3..6, 3-•8, 3-9, 3.. 2 3..7 1C2 4,, 0, 5.,6, 4.•2, 3.•1 5 2 , • 5 3 .9 2A1 3..8, 5.•4, 4.•2, 3.,1, 2 ..7 4..0 2A2 4..0, 3.•2, 5..6, 5..4, 4..2 4,.5 2C1 3 .• 5, 4,.3, 4., 0, 6,. 0, 3..6 4,• 3 2C2 3.•1 3 3.,0, 5.,6, 2 , .6, 3..2 3.• 5 lx 5-,6, 4.•1, 6,2, 3.•2, 5.,1 4.• 5 2x 3- 0, 3.,0, 3.4, 2.,6, 2 .• 9 3.. 0 iy 4. 0, 3.2, 4.5, 2 .,6, 3..3 3..5 2y 4.8, 3..1, 4. 5, 3.•3, 4.,8 4,.1

All Value, I I

Certified oxygen content based on 109 determinations by

IGF-GC and vacuum fusion = 4.5 ppm.

23
For certification, samples taken from the original

material were analyzed, as were samples selected randomly

from the entire portion of material after forging, swaging,

and centerless grinding. The random samples are designated

x and y. Results based on 109 determinations gave a value

of 4.5 ppm with an uncertainty of ±2 ppm that includes the

variations due to inhomogeneity as well as to the apparatus

and operator biases. This final oxygen value is based on both inert-gas-fusion and vacuum- fusion analyses and will

be stated on a new certificate.

In addition to the above-mentioned steel SRM, two

platinum SRMs were certified for oxygen content. The first,

SRM 680, is a high-purity platinum metal that contains 4 ppm

oxygen; the second, SRM 681, platinum metal doped with

twelve elements, contains 7 ppm oxygen. Tentative values

were established on two high-purity gold SRMs, SRM 685W

at 2 ppm oxygen and SRM 685R and <2 ppm oxygen.
4 . SUMMARY

U. S. Steel and the National Bureau of Standards have

conducted a joint project in which various methods were evaluated for the determination of nitrogen and in particular

the simultaneous determination of oxygen and nitrogen in

steel. An inert gas fusion-gas chromatographic apparatus

with excellent sensitivity for both elements has been

constructed. Recommended modifications have been completed

on this equipment, and simultaneous determinations of oxygen

and nitrogen in steel are presently being made. The detection

limit is <1 ppm for both oxygen and nitrogen, and the analysis

time is about 5 minutes.

A direct result of this program has been the issuance

by NBS of a new Standard Reference Material, a maraging

steel, for oxygen. This SRM has the lowest oxygen concentration (4.5 ppm) of any steel yet issued by NBS.

25
5. ACKNOWLEDGMENT

Appreciation is expressed to the U. S. Steel Corporation

for my appointment as an Industrial Fellow at the National

Bureau of Standards under their Research Associate Program.

I would also like to express my sincere thanks and appreciation to Dr. W. Wayne Meinke, Chief of the Analytical

Chemistry Division, NBS , and to Dr. Oscar Menis, J. T. Sterling and other members of the Analytical Coordination

Chemistry Section of this NBS division. The work at NBS

was carried with assistance of J. T. Sterling. In addition

to the above named, I would like to acknowledge the continuing cooperation by Mr. R. E. Michaelis of the Office of

Standard Reference Materials and Dr. George Gordon and

P. R. deBruyn of the Office of Industrial Liaison, NBS.
6 . REFERENCES

Dallman, W. E. and Fassel, V. A., Anal. Chem. 39., 133R

(1967).

Gerhardt, A., Kraus, T . , and Frohberg, M. G .

,

"Development of a Method for the Rapid Determination

of the Nitrogen Content of Steel Based on the Vacuum

Fusion Method with Special Consideration of the

Degassing of Carbon Saturated Iron Melts", PhD.

Dissertation of A. Gerhardt for the Institute of

General Metallurgy of the Technische Universitat,

Berlin, 1965 3 Balzers High Vacuum Co., Liechtenstein.

Lemm, H., "A New Instrument for the Rapid Determination

of Nitrogen in Steel", presented at the 1967 Pittsburgh

Conference on Analytical Chemistry and Applied Spectroscopy . Lemm, H. and Kock, W., "New Apparatus for the Rapid

Determination of Oxygen and Nitrogen in Steel by the

Fusion Extraction Carrier Gas Method", presented at the 1968 Pittsburgh Conference on Analytical Chemistry

and Applied Spectroscopy.

Vasserman, A. M. and Turovtseva, Z. M. , Zh. Analit . Khim.,

20_, 1359 (1965).

Goldbeck, C. G., Turel, S. P., and Rodden, C. J.,

Anal. Chem., 40, 1393-4 (1968).

Menis, 0., Editor, Analytical Coordination Chemistry

Section: Summary of Activities July 1967 to .June 1968,

U. S. National Bureau of Standards Technical Note 454,

Superintendent of Documents, U. S. Government Printing

Office, Washington, D. C. 20402, 1968.

Sterling, J. T . , "Differential Thermal Analysis", 53-66,

Analytical Coordination Chemistry Section: Summary of

Activities July 1968 to June 1969, U. S. National

Bureau of Standards Technical Note 504, Superintendent

of Documents, U. S. Government Printing Office,

Washington, D. C. 20402, 1970.

Menis, 0. and Schultz, J. I., Editors, Analytical

Coordination Chemistry Section: Summary of Activities

July 1969 to June 1970, U. S. National Bureau of

Standards Technical Note 544, Superintendent of Documents, U. S. Government Printing Office, Washington,

D. C. 20402, 1970.

27
I

!


of Commerce

Stans

$f Standards

irectar Certificate of &nalps:ts;

Standard Reference Materials 1090, 1091, 1092

Oxygen in Ferrous Materials

The oxygen content of these three standards has now been determined by three independent methods of analysis, namely, vacuum fusion, inert gas fusion and fast neutron

activation analysis. The results of the vacuum fusion determinations were reported on a Certificate of Analysis dated March 1, 1966. At this time recommended or certified values

are not being reported because of small but undetermined systematic biases which exist in one or more of the methods used. For this reason, all the pertinent analytical data are presented in tabular form below. Oxygen in Ferrous Metals (ppm by weight)

SRM

No. Material Vacuum

Fusion Neutron

Activation

Inert Gas Fusion X s n X s n X s n 1090 Ingot Iron 484 14 216 492 28 6 497 13 12 1091 Stainless Steel (AISI 431)

131 8 286 132 7 6 129 8 11

1092 Vacuum Melted

Steel 28 2 105 28 4 5 29 5 20 X = mean oxygen value; s = standard deviation of a single determination; n = number

of independent determinations. (Note: the standard deviation includes error due both to the imprecision of the

analytical method and to possible heterogeneity of the materials analyzed).

Size: SRM 1090 and 1092 are in the form of rods 1/4 in in diameter and 4 in long. SRM 1091 is 5/16 in in diameter and 4 in long. The analyses reported herein were performed by staff members of the Analytical Chemistry Division, Institute of Materials Research, National Bureau of Standards. Additional technical information on this series of standards, including the names of cooperators who participated in the original analyses is to be found in NBS Miscellaneous

Publication 260-14, Standard Reference Materials: Determination of Oxygen in Ferrous

Materials; SRM 1090, 1091 and 1092, (1966).

Caution: Oxygen determinations should be made on thoroughly and freshly cleaned samples that represent the full cross-section of the rods. Washington, D. C. 20234 W. Wayne Meinke, Chief March 1, 1966 Office of Standard Reference Materials

April 2, 1969 (Revision) (over) 29
SUPPLEMENTARY INFORMATION

Although not certified at this time, the nitrogen content of these three SRMs has been

determined by the analytical methods shown below: COM Nitrogen No. Method of Analysis (PPm by weight) X s n 1090 Acid digestion — distillation — 61.5 3.5 14 indophenol — photometry

Pressure bomb — distillation — 59.4 2.0 4

indophenol — photometry Vacuum Fusion 60 1 15 1091 Pressure bomb — distillation — 945 20 12 indophenol — photometry

and titrimetry Vacuum Fusion 861 3 48 1092 Pressure bomb — distillation — 3.6 0.7 8

indophenol — photometry

and titrimetry Vacuum Fusion 4.0 1 6

The symbols X, s, and n have the same meaning as given for the oxygen

determinations and the note appended thereto applies here as well.

30
U. S. Depa

Mauric

of Commerce

Stans

National Bfc^gatf^rf Standards

A. V. Asas^Director Gterttftrat? nf Analgata

STANDARD REFERENCE MATERIAL 1094

Oxygen in Maraging Steel

This standard is intended primarily for application by vacuum and inert gas fusion methods for the determination of oxygen. The low value of oxygen at 4 ppm in the maraging steel material makes the determination by neutron activation analysis difficult. SRM No. 1094

Description

Maraging Steel

Oxygen, ppm

(by wt)

4. 5 a

Results determined by vacuum fusion techniques on 1-gram samples. The value given is the grand mean based on 109 deter- minations on 32 samples. The values found ranged from 2.5 to 7.5 ppm. Determinations made over a period of several months

indicate the existence of systemic errors of the order of 2 ppm. Examination of the data indicates that the material is homogeneous relative to the magnitude of the systematic errors in the method. This standard is supplied in rods y± in (0.6 mm) in diameter a^d 4 in (8.2 cm) long. The material for this standard was furnished to NB$ b^ihes^^plied Research Laboratory of the U. S. Steel Corp., Pittsburgh, Penn^yai^a. \f^v>

'

Analyses were performed by J. T. Sterling, J. FV\M&rtin, and 0. Menis. The overall direction and coord

tification were under the chairman The technical and sjt issuance of this Si Standard Refe

technical measurements leading to the cer- D. LaFleur.

Washington, D. C. 20234 March 4, 1969

(Revised June 12, 1969)

cts concerning the preparation, certification and

rence Material were coordinated through the Office of by R. E. Michaelis.

W. Wayne Meinke, Chief

Office of Standard Reference Materials CAUTION : Oxygen determinations should be made on thoroughly and freshly cleaned samples that represent the full cross section of the rods.

(over)
SUPPLEMENTAL INFORMATION

OTHER ELEMENTS: Nitrogen, at 71 ppm, was determined by a pressure bomb-distilla- tion-indophenol-photometric method. The nitrogen value obtained by vacuum fusion was

61 ppm. Work is now underway at NBS to discover and resolve systematic biases exist- ing in either or both of these methods. PREPARATION FOR THE DETERMINATION OF OXYGEN:

1. Samples should be cut from the original rod in such a manner as to minimize

heating of the sample ; i.e., by a hand hacksaw.

2. All surfaces of the cut sample should be thoroughly cleaned with a fine file. 3. Samples should be washed with C. P. ether, acetone, or other suitable solvent, dried in a stream of warm clean air and then handled only with clean forceps.

4. Analyses should be made as soon as possible after cleaning the sample. CONDITIONS FOR ANALYSIS AT NBS:

Method

Furnace temperature Furnace pressure

Collection time Bath material Carbon monoxide determination

Vacuum fusion 1675 °C

<10-5 torr 4 min

High-purity nickel Infrared absorption

32
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