NBS

Reference publications

NBSIR 84-3013 AlllOb EEfiSHt,

LASER-INITIATED COMBUSTION

STUDIES ON METALLIC ALLOYS IN

PRESSURIZED OXYGEN

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303

August 1984

pQC

100

,1156

No , 34-3013

1934

NBSIR 84-3013

HATIONAL BUREAU OF STANDARDS * LIBRARY

LASER-INITIATED COMBUSTION

STUDIES ON METALLIC ALLOYS IN

PRESSURIZED OXYGEN

J.W. Bransford

Chemical Engineering Science Division

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303

August 1984

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

CONTENTS

Page

I. INTRODUCTION 1

II. EXPERIMENTAL 2

A. Facility Development

1. High Pressure Chamber 2

2. Surface Temperature Measurement 6

a. Broadband Pyrometer 8

b. Narrowband Pyrometer 8

3. Interior Temperature Measurement 8

4. Mass Measurement 9

B. Experiment 10

1. Materials 10

2. Specimen Geometry 10

3. Experimental Arrangement 12

III. RESULTS 14

A. Specimen Heating Rate 14

1. A1 uminum A1 1 oys 15

2. Iron, Nickel, and Cobalt Alloys 18

B. Ignition and Combustion Test in Atmospheric Flowing Oxygen . . 21

1. Aluminum 6061 21

2. Iron and Nickel Base Alloys

a. Stainless Steel: AISI 302, 304, 304L, 316, 321, and

347 27

b. 440C Stainless Steel 29

c. Nickel Based Alloys: UNS N06600, N06625, N07718 ... 29

d. Monel 400 and K-500 29

i i i
CONTENTS ( continued )

Page

C. Ignition and Combustion Test in Pressurized Oxygen 29

1. A1 uminum 6061 . 30

2. Cobalt Based Alloy, Haynes 188 34

3. Iron Based Alloys 37

a. Carbon Steel 1018 37

b. Stainless Steel: AISI 302, 304, 304L, 316, 321, and

347 37

4. Nickel Based Alloys: UNS N06600, N06625, N07718, and

N07750 40

IV. SUMMARY AND FUTURE EFFORT 55

V. ACKNOWLEDGMENTS 56

APPENDIX A. Theory of Two-Color Ratio Pyrometry A-l

APPENDIX B. Data Definitions B-l

iv
LIST OF FIGURES AND TABLES

Figure Number Page

1. High pressure combustion chamber 4

2. View of combustion chamber in position 5

3. Polycrystall ine sodium chloride window assembly 7

4. Experimental set-up in combustion chamber 13

5. Heating rate of an aluminum alloy specimen in atmospheric

nitrogen 16

6. Heating rate of an aluminum alloy specimen in atmospheric

air 17

7. Heating rate of a 300 series stainless steel specimen in

atmospheric air 19

8. Heating rate of a preoxidized 300 series stainless steel

specimen in atmospheric air 20

9. Typical signals for an aluminum 6061 combustion in atmospheric

flowing oxygen 23

10. Expanded results at ignition for aluminum 6061 24

11. Brightness and mass correlation for an AISI 300 series stainless steel 26

12. Typical results for an AISI 300 series stainless steel in

flowing oxygen 28

13. Average surface temperatures at ignition for aluminum 6061 . . 31

14. Temperature at ignition for a 0.5 mm diameter surface area

for aluminum 6061 32

15. Surface temperatures for an aluminum 6061 for ignition on cool-down 33

16. Average surface temperature at ignition for Haynes 188 .... 35

17. Average surface temperature at combustion for Haynes 188 ... 36

18. Interior temperature at ignition for carbon steel 1018 ....

19. Average surface temperature at ignition for carbon

steel 1018 39

20. Interior temperature at ignition for AISI 302 41

v
LIST OF FIGURES AND TABLES ( continued )

Figure Number Page

21. Average surface temperature at ignition for AISI 302 42

22. Interior temperature at combustion for AISI 302 43

23. Average surface temperature at combustion for AISI 302 .... 44

24. Interior temperature at ignition for AISI 304 45

25. Average surface temperature at ignition for AISI 304 46

26. Average surface temperature at combustion for AISI 304 .... 47

27. Interior temperature at ignition for AISI 316 48

28. Average surface temperature at ignition for AISI 316 49

29. Interior temperature at combustion for AISI 316 50

30. Average surface temperature at combustion for AISI 316 .... 51

31. Interior temperature at ignition for UNS N06625 52

32. Average surface temperature at ignition for UNS N06625 .... 53

33. Average surface temperature at combustion for UNS N06625 ... 54

B.l. Expanded view of an aluminum alloy ignition B-2

B.2. Average surface temperature history for a test of a 300 series

stainless steel B-3

B.3. Expanded view of the ignition and combustion sequence for a 300 series stainless steel B-5

Table Number

1. Material composition 11

vi
Laser-Initiated Combustion Studies on Metallic Alloys in Pressurized Oxygen*

James W. Bransford

National Bureau of Standards

Boulder, Colorado 80303

The interim results of ignition and combustion studies on aluminum, cobalt,

iron, and nickel based alloys are presented. It was found that aluminum alloys

could be ignited below the melting point of the product alloy oxides. It was

also found that the cobalt, iron, and nickel based alloys generally ignited

slightly below to slightly above the melting range of the respective alloy. Unsupported combustion could not be achieved until the alloys and oxides were in

the liquid state.

Key words: alloy; combustion; ignition; metal; oxygen compatibility

*Certain commercial equipment, instruments, and materials are identified in this

paper to adequately specify the experimental procedure and materials studied. ! no case does such identification imply recommendation or endorsement by the

National Bureau of Standards, nor does it imply that the material or equipment

identified is necessarily the best available for the purpose.

1. INTRODUCTION

This interim report presents the follow-on work on Government Order H43201B

- Combustion of Metals in Oxygen. The initial report 1 on this contract described the results of experiments carried out on small i rregul ar-shaped samples

of four aluminum alloys, three stainless steels, and two copper alloys. The experiments provided qualitative information on the i gnitabi 1 i ty, combustibility,

and brightness profile, and quantitative data on combustion temperatures and

oxygen consumption rate. From the oxygen consumption rate, brightness, and

photographic data, several conclusions about the ability to obtain an ignition

point (Appendix B), using the laser heating technique, were arrived at. These

were

1. For vapor burning materials - aluminum and aluminum alloys, titanium and

titanium alloys, for example - an ignition point could be readily determined since signal profiles were sharp and unambiguous.

2. For liquid surface burning materials - stainless steels, nickel alloys,

for example - the signal profiles had slow rise-times and irregular

shapes. Thus, defining an ignition point was expected to be difficult

but achievable.

With the establishment of the feasibility of obtaining an ignition point,

or at least a point above which ignition was highly probable, the actions required for the follow-on effort were defined. These actions were:

1. Construct a high pressure chamber and obtain support equipment.

2. Develop an adequate flameproof lining for the chamber.

3. Develop interior and exterior high temperature measurement capability.

4. Develop mass measurement capability under pressurized conditions.

5. Establish criteria for defining the ignition point.

iBransford, James W. and Clark, Alan F. , NBSIR 81-1647, April 1981.

1
The above actions have subsequently been carried out and are discussed in the

following sections along with the results of measurements made to 6.9 MPa.

II. EXPERIMENTAL

A. Facility Development

The original combustion facility was set-up primarily to study titanium combustion at atmospheric pressure with the sample blanketed by flowing oxygen. A

small chamber was available but had very limited viewing; the pressure capability

was from vacuum to less than 0.2 MPa. Temperature measuring capability was limited to a radiant intensity pyrometer and to a rapid scanning spectrometer, both

of which were inadequate for the fast, precise temperature measurements needed

for the high pressure experiments. The pyrometer was adversely affected by smoke

and window clouding; the rapid scanning spectrometer was too slow and could not

be used for continuous measurements during the experiment. Mass measurements

were made on an electronically modified laboratory balance, which could not be

adapted for chamber operation. Consequently, a high pressure combustion chamber

had to be built and several major items of equipment procured and/or developed.

The major items of equipment provided for the high pressure facility are described briefly below.

1. High Pressure Chamber:

The usual procedure used to minimize safety risk in high pressure oxygen experiments is to limit the experiment volume. This procedure was not feasible in

this case since the ultimate experiment size and configuration were unknown and

considerable viewing area was considered necessary. After several iterations,

the final chamber configuration and interior volume was established. The interior dimensions were 12 cm in diameter by 23.5 cm in height. These dimensions

would allow the manual placement and adjustment of specimens and support equipment, and pressure changes during the experiment would be small. Four equally

2
spaced ports were placed perpendicul ar to the chamber axis and four equally

spaced ports were placed at 45° to the axis. Each port allows a circular viewing

area of approximately 11.3 square cm at the sample. The ASME Unfired Pressure

Vessel Code was used to determine minimum shell and closure dimensions. Considerable material was then added for additional safety (in case of wall combusion)

and also to simplify casting of the item. Casting was used to eliminate welding.

A sectional view is shown in Fig. 1 with Fig. 2 showing the finished chamber in

position.

There were two critical chamber components. The first was the fire wall.

This component must be capable of stopping a burning metal drop without being

penetrated or cracking and exposing the chamber surfaces. Several configurations

and materials were considered and tested. The configuration chosen was composed

of several layers of heavy asbestos cloth heavily impregnated with a silicate refractory cement. The layers are applied wet and allowed to dry in place, providing a tight seal to the metal contours. The wall has been impacted a considerable number of times with molten burning material and impinged by the reflected

laser beam without failure.

The second critical chamber component was the window for the laser beam.

There are few materials which will pass 10.6 ym wavelength radiation. Of those

that do, most have a large index of refraction and therefore require some type of

antireflection coating. These coatings have two potential problems. The first

was the possible adverse effect on the coating due to pressure stress. The

second is radiation adsorption at particle adhesion points on the bottom surface.

The energy adsorbed at these adhesion points could be sufficient to cause thermal

cracking of the window and result in sudden window failure during an experiment.

Because of the potential trouble with the higher strength high index of refraction materials, it was decided to use a low index of refraction uncoated

3
Laser Beam

o

F i g u r e High pressure combust i on c h amb e r

4
Figure ?. View of combustion chamber in position.

5
material, as a first attempt, even though this meant using a low strength material. The material chosen was polycrystalline sodium chloride. This material

was readily available, relatively low cost, and potentially refurbi shable. A

window, using this material, was designed and several purchased. 2 The window

has been tested to pressures of 14 MPa with no sign of failure even though considerable interior surface damage has occurred. It is expected that this item

can be used to pressures significntly beyond 14 MPa. Figure 3 shows an exploded

view of the window and mounting arrangement.

2. Surface Temperature Measurement

The experiment is arranged to allow the sample to be heated from the top.

This top surface then will be the first to undergo ignition and combustion. To

obtain an accurate temperature of this surface a noncontact method must be used.

This prevents biasing of the temperature profile or changing the ignition temperature as would happen if a contact method was used. Infrared two-color ratio

pyrometry was chosen as the means to measure the surface temperature. This twocolor method differs from the method first chosen and reported in the first interim report. The method is discussed further in Appendix A.

Two pyrometers are used since energy spikes in the laser beam and convection

currents from the sample generates moving hot spots. One pyrometer, a modified

commercial unit, called a broadband pyrometer in this report, views the entire

sample top surface. The temperature generated by this unit was an integrated

average temperature and is called an average temperature in this report. The

second pyrometer views a small area which can vary from 0.5 mm to 3 mm in diameter and is called a narrowband pyrometer in this report. A brief description of

each pyrometer follows.

^Bransford, James W. , Review of Scientific Instruments, Vol . 55, No. 1,

pp. 125-126, January 1984.

6
Figure 3. Polycrystalline sodium chloride window assembly.

7
a. Broadband Pyrometer

The broadband pyrometer was a commercial unit which has been modified to extend the valid temperature range from 1173 K to 2673 K. The field of view was

approximately 6 mm in diameter at the sample, which covered the complete specimen

top surface. The instrument was capable of making valid measurements with up to

95% signal attenuation (Appendix A). The spectral response range extends from

700 nm to 1050 nm with one signal derived from the entire spectral range centered

at 950 nm and the second signal derived from a narrow wavelength band centered at

1050 nm. The accuracy of the instrument was typically 1% or better on smooth

surfaces with some decreased accuracy on rough oxidized surfaces and surfaces

that contain iron oxides.

b. Narrowband Pyrometer

The narrow band pyrometer was an in-house designed and constructed unit

which operates on the same principle as the broadband unit. The major differences are that this unit has a narrow field of view and each signal's optical

bandwidths are very narrow. The short wavelength signal was centered at 905.1 nm

and the long wavelength signal was centered at 1060 nm. The bandpass for each

wavelength was 10 nm FWHM.

3. Interior Temperature Measurement

It was considered highly desirable to have a specimen interior temperature

in addition to the surface temperature due to the physical nature of the oxides

produced during heat-up. The oxides of most of the alloys are tight protective

oxides during much of the heat-up period. However, at the higher temperatures

preceding ignition the oxide layer rapidly increases in thickness and develops a

scaly or granular nature. This produces an insulating effect which may introduce

a significant temperature differential between the upper oxide surface and the

oxide-metal interface.

8
There are two techniques that can be used to measure an interior temperature

in the ignition ranges expected. One technique, a blackbody radiant method, utilizes an iridium coated quartz fiber. The upper temperature limit is approximately 2000 K. The second technique utilizes a thermocouple. The measurement

method chosen was the thermocouple since this device is less costly and easier to

implement. Two types of thermocouples were used. They were tungsten 5% rhenium

vs. tungsten 26% rhenium, which has the highest temperature capability but has

oxidation problems, and platinum 10% rhodium vs. platinum which resists oxidation

but has lower temperature capability. EMF data are available for temperatures up

to 2593 K for the tungsten system and to 2041 K for the platinum system. Wire

diameter for each thermocouple system was 0.13 mm (0.005 in).

4. Mass Measurement

The metals and alloys of this study produce nonvolatile oxides which remain

mostly within the reaction zone. Thus, if a sufficiently sensitive mass measurement device can be found and a stable baseline maintained during the experiment,

the rate of the oxidation reaction can be followed. Two mass sensors have been

used, one exclusively for the open atmosphere experiments, and one exclusively

for pressure chamber experiments.

The mass sensor used for the open atmosphere experiments was a standard

laboratory electronic balance. The balance electronics were modified to increase

the response time and to stabilize and amplify the output signal. The sensitivity of this unit was 10 mg/mV.

The second mass sensing unit, which was used for experiments in the pressure

chamber, is based upon an LVDT force transducer. This device was modified to

withstand pressures up to 70 MPa. The unit has a full scale range of 100 grams.

In use, the output signal is offset and amplified to give a sensitivity of approximately 6 mg/mv. The unit has one drawback. The spring mounted platen

9
assembly is undamped and thus was very sensitive to vibration and impulse loads

produced during combustion. Never-the-1 ess, the precombustion oxidation rate can

be obtained with good accuracy.

B. Experiment

1. Materials

For this effort, the materials consisted of eighteen metallic alloys and

three nonmetallic materials. A list of these metallic alloys and their chemical

composition is given in Table 1. Several other materials will also be included

in this study. These will provide a link to previous experiments and also provide information on the behavior of several pure materials which form the basis

for the alloys being studied as well as several other alloys of interest. Several of the additional materials that will be studied are pure iron, pure nickel,

monel 400, monel K-500, and pure copper. As the results of previous work are

compiled, other materials may be added.

2. Specimen Geometry

The original study utilized small irregular-shaped specimems cut from sheet

stock. This type of specimen does not lend itself well to accurate interior or

exterior temperature measurements; thus a change in specimen geometry was necessary if these data were to be obtained. It was also desirable to change the

specimen geometry in order to increase the mass data signal -to-noise ratio.

In arriving at an applicable specimen geometry, several requirements had to

be kept in mind. These were:

1. Maximum laser power was between 75 to 85 watts.

2. Ultimate specimen temperature needed to approach 1800 K or higher

for the iron, nickel, and cobalt alloys and to exceed 2330 K for the

aluminum alloys in order to be assured of reaching an ignition temperature.

10
Other 1

iO6 01 Q

3©< ©

La 0.04

—

Ca <0.01

N

©

a*oo c

Nl oo

CMoV i * CMCO > g

CMo oo P 0

01o

CM01 o o•©

s.

CM

©

CO

©

CMo ©©o o©

CM

0) iveto 1 CD

©

K

®

O o o o o o o

CMa6 o ©o ©od

CMoo

CMooCMoo

N.5

CO o

<0oo o

COoo ocooo o

COoo o©oo o©oo o©oo ooo o©oo ©ooo ©ooo

CMoooCMooo ooo

Ol

©aoo

IOaoo ©aod

©aoo ©aoo ©aod

©aoo

CMoo oaoo 3oOo

CMooo ©ooo Kooo

n.ooo z

coi © 01 ©

©01 ©

CMi © *o

CMi 3 ©•

K.aa

CM

am

CM

CM

aa

CM©©

©

CM©

a*a©©©o

CMk c2 Oo o

CM o

CM o

CM o

CM o o

CM o« © o ©

CM oo oo ©oo ©6 o

Mo 1oo

CM CO

©

CM

©

N.o o©2

CM2fx. N.© 3

CM

a» 2 i ©©

CM* do 1 a©

do ooV

Fe o

04o aoV

a

CO

a

CD

a

CD

a

CD

a

CD

a

CD

a

CD

a

CO

a

CD

© o

CM©

©

h*o ?

CM

K.©

©3

n! oO o

04o

CMoV

©oo

CMoo o

CMo oo 6

CMoV

©i© 31 fx. o

CM© o

CM© «© 31 fx. 31 ©©

CO

©1 ©

CM

CM o3©

CM

ai o

CM

CM

©o©

©©©

Co

a©© 3r

CM

©od

CMoo

Cb

Ta

nO& O3

CO

CM

CM© 3

fx. o o

IOo ©o 3o

COoo «oo ©Oo ©oo

r*» CMOo ©o3CM o*- oo ©ooo ©oo aoo 3o ©

CMoo

CD

©ooo ©ooo

CMooo < a

CD

*5

CD

CM

CMd

CM

Ko 3ao

CM©o ©

CO1 < 0

to1 3•c®H

©

COo

CO

CMo

«0 8

CO

’llao

CO

"©©

CM©

he*©

CM©

CM1 <

*bo

•a

a- <

>» 5aZ

*©©a•c>aI

CM>3®a O Z

CM>x .2ma«a5

CM©

CM©•eoo£

CM©

*x» ®eoo£

CM• O c © o ^ o w £ *

cm •>

11

Table

1.

Material

composition.
3. Heat-up time to maximum temperature needed to be kept to less than

three minutes maximum, and preferably to about two minutes.

Several specimen configurations and dimensions were tried. A cylindrical

specimen with height equal to diameter (5 mm), to minimize heat loss by

radiation, was found to meet most of the above requirements adequately and give

consistent results. A 0.635 mm diameter thermocouple well was drilled axially to

within 0.25 mm to 0.40 mm of the top surface. The thin web of material at the

top of the specimen requires that the experiment be completed beyond the point of

ignition before normal nonprotect i ve oxidation completely destroys the web and

also the thermocouple. This destruction of the web by the nonprotecti ve

oxidation mechanism usually requires two to three minutes after reaching high

temperature. The thermocouple will sense an environment which is hottest

directly above the bead but cooler at the sides. The temperature measured by the

thermocouple was an "averaged" temperature biased slightly to the low side from

the true oxide-metal interface temperature.

3. Experimental Arrangement

The experimental set-up for the open atmosphere experiments and the pressure

chamber experiments was essentially the same. The major difference was that the

spray shield for the open atmosphere set-up had a smaller top opening for the

laser beam and oxygen was introduced through a small side opening with flow

across the top of the sample. Figure 4 shows a side view of the pressure chamber

set-up. The mass sensors were utilized as the experiment support and were protected from excessive heat by a foam fire brick. A graphite crucible was used to

contain the specimen and molten material produced during combustion. Heat transfer from the specimen to the crucible must be minimized in order to achieve the

desired heating rate and maximum temperature. To achieve this, the thermocouple

was bonded in place in the specimen well in the graphite block. Then the

12
Spray Shield

Graphite Crucible

Aluminum Oxide

Insulation

Heat Barrier

LVDT Mass Sensor

Pressure Chamber

Bottom Closure

Figure 4. Experimental set-up in combustion chamber.

13
specimen well was lined with packed aluminum oxide powder to a thickness of approximately 1 run on the side and 2 mm on the bottom. The specimen was then inserted firmly over the thermocouple and seated firmly on the aluminum oxide

powder. The thermocouple leads are then connected to the chamber feed-thru leads

and the mass sensor calibrated.

A typical experiment records at least five quantities which are:

1. Mass

2. Interior temperature

3. Overall top surface temperature

4. Spot top surface temperature

5. Brightness

High-speed movies and/or spectral measurements may also be made. All electronic

data were recorded on digital oscilloscopes with the possible exception of spectral data which can be recorded as digital, analog, or photographic data. All

digital data were passed through a desktop computer for processing and recording

onto permanent magnetic tape or disc files.

III. RESULTS

A. Specimen Heating Rate

A number of specimens of several alloys representative of the overall group

of materials were heated in air to confirm that the desired maximum temperature

range could be reached in the required time. (Air contains sufficient oxygen to

give an oxidized surface similar to that obtained in pure oxygen without supporting combustion). The thermocouple was positioned at the predetermined depth of

0.25 mm to 0.40 mm from the top surface as well as at other positions. The surface temperature was measured using the broadband pyrometer. The results of

these heating experiments were compared to computer generated temperature distributions and heating rates. Two comparisons were made, one for the high thermal

14
23

Typical

signals

for

an

aluminum

6061

com

bust

ion

in

atmospheric

flowing

oxygen.
24
ignition, and the ignition temperature determined from this trend curve. However, this procedure was used only if the data are considered to be extrapolatable. The values obtained for the temperatures at ignition for aluminum 6061

ranged from approximately 1700 K to approximately 2100 K with most data above

1825 K.

2. Iron and Nickel Base Alloys

The ignition and combustion behavior of the larger specimens, 750 mg and

heavier, used in this phase of the program, was essentially the same as for the

smaller specimens used in the initial study. A small hot spot would develop

within the laser beam impingement area and then develop into combustion and

spread across the specimen surface. All of the iron and nickel alloys studied

could be ignited and would burn to completion. Monel 400 and Monel K-500 could

also be ignited and would burn to completion. Nickel 200, essentially pure

nickel, was the only material that could not be reliably ignited. This was due

to the development of a heavy layer of nickel oxide which has a very high melting

point, 2363 K, and thus prevented oxygen transport to the molten metal. In fact,

if the experiment run time was lengthened at the higher temperatures, the heavy

build-up of oxides could prevent ignition of all of the materials studied.

The determination of the ignition temperature, for this phase of the study,

was a major problem since a surface temperature measurement was not available.

Previous investigators had used the most rapid rate of change in the brightness

signal (ds/dt = max.) as the time reference in determining the ignition temperature. To test this definition against oxidation rate data, a number of experiments were run in which the brightness signal was correlated with the mass

signal. Figure 11 is a typical result for a stainless steel. It was found that

generally ds/dt was a maximum during combustion, not at ignition, and therefore

could not be used to determine the time at which ignition occurred. To further

25
ld (S3

m

LD c

fd

i_

«+-C .

id 0 . — 0

CM +-> +-> cd iA 0 iA

i- (A

L. 0 o <

u C

iA

— - (A fd s OJ iA -p —

£ fd w

h— E

T5 0 C

fd 0

(A (A

iA LO 0 (S C (S

+->

m

_c

DIM

•— LD

Sw i—

m CE

LQ 0

13D1

6lu ‘

26
complicate the problem, there was an experiment-to-experiment and a material-tomaterial variation in the specific detailed shape of the brightness curve as well

as the relation of the curves general features to the oxidation rate curve.

The problem presented by the inconsistent brightness curve is partly resolved by using mass data to determine the beginning of combustion and then working backward to determine the most probable time for ignition to have occurred.

This procedure requires that the ignition reaction continue sufficiently long to

consume a detectable amount of material. For most of the materials studied this

criterion appeared to be met; however for several, the product oxides suppressed

adequate consumption of material so that ignition point could not be estimated

from the mass curve.

Figure 12 shows the relationship between interior temperature (upper curve),

weight (middle curve), and brightness (lower curve) obtained from the combustion

of a 650 mg specimen of 304L stainless steel. This data set was representati ve

of those obtained from all 300 series stainless steels. For this test, the oxidation rate began to accelerate at five seconds. Since there was an absence of

any indication of abrupt surface temperature transitions prior to this time, ignition was assumed to occur at that time.

The results of the atmospheric flowing oxygen experiments can be grouped according to alloy type, i.e., 300 series stainless steels, nickel based alloys,

etc. These results are summarized below.

a. Stainless Steel: AISI 302, 304, 304L, 316, 321, and 347

Based upon oxidation rate data, these alloys appeared to ignite within their

respective melting ranges, 1672-1728 K, with some data indicating ignition temperatures may be as low as 1600 K. Unsupported combustion was not concurrent

with ignition; the alloys must be in the liquid phase in order for unsupported

combustion to occur.

27
» ‘ 3 j j sduuaj_

co

QJ

L.

QJ • CO C

QJ (S3 CD

(S3X

m

x i—

LD CD

i—i c

CE -3

£ZO

rd .

c+-

i_

C

<-t- •—

CO +> QJ — QJ +>

CO CO

QJ

M

CO

— QJ

rd >

U C

Q_ rd I— CO

ru

QJ

13cn

l_L_

28
b. 440C Stainless Steel

This was the only 400 series stainless steel tested. The data indicate a

probable broad ignition range with a low end approaching that of pure iron, approximately 1400 K, but with definite ignitability in the liquid phase, 1644-

1783 K. Unsupported combustion can only occur if the alloy is liquid, greater

than 1600 K.

c. Nickel Based Alloys: UNS N06600, N06625, and N07718

These alloys typically ignited within their respective melting ranges, 1533-

1700 K. Some data indicated ignition at slightly less than the solidus temperature. Unsuported combustion can only occur if the alloy is fully liquid.

d. Monel 400 and K-500

The data for these two alloys are erratic, but ignition appears to occur at

temperatures greater than 1500 K, with combustion occurring in the liquid phase.

C. Ignition and Combustion Test in Pressurized Oxygen

The ignition and combustion characteristics for the vapor burning alloys did

not change with increasing oxygen pressure. However, for the liquid surface

burning alloys, there were two significant changes in behavior. The most noticeable change occurred in the amount and size of the droplets and spray which is

ejected during combustion. The increased pressure decreased the size of vapor

bubbles which drive the droplet and spray mechanism. This allowed the vapor bubbles to accumulate and build in size until a critical size was reached. This

critical size then overcomes the surface tension, which maintains a stable fluid

volume, expands and literally blows the molten sample from the crucible in large

drops. The most severe action occurred in the 0.34 MPa (50 psia) to 0.69 MPa

(100 psia) pressure range. For pressures greater than 1.38 MPa (200 psia) the

ejection of large droplets of material was essentially suppressed for the specimen size used.

29
The second change in behavior occurred in the ignition sequence. In general, the surface temperature at ignition experienced a sharp increase

(Figure B.3) and the time between the development of an ignition zone (hot-spot)

and the development of full combustion was considerably shorter than in the atmospheric tests. For some cases, the delay between ignition and combustion was

only a few data acquisition time periods and could be considered to be concurrent.

The results of the testing, to date, at elevated pressures is presented in

the following sections.

1. A1 uminum 6061

Aluminum 6061 is the only aluminum based alloy tested at elevated pressure

to date. It was possible to obtain data on this alloy to pressures of 2.41 MPa

(350 psia) but not beyond, due to sample cooling by the higher density gas. An

interesting and unexpected event occurred during this series of tests. Longer

than normal heating times had to be used to reach ignition temperature; several

of these tests failed to ignite and the tests were terminated; however, during

cool-down the specimen ignited and burned. These data points are marked by

circles in Figures 13 and 14. It is thought that a stress fracture developed in

the oxide shell exposing the molten metal. The energy of the resulting surface

oxidation reaction was trapped within the crack and raised the temperature of the

immediate material to the ignition point, causing ignition and combustion.

Figure 15 presents the data from both pyrometers for one such event. The top

curve shows the narrowband pyrometer data; the bottom curve, the broadband pyrometer data.

Figure 13 presents the average top surface temperature at ignition. As can

be seen there is a slight ignition temperature decrease, 9.12 K/MPa

(0.0629 K/psi), at pressures beyond 0.14 MPa (20 psia). Figure 14 represents

30
2500

rd Q_

s:

(DD

V\

V\

<U CL

C

-Pcm

-p

rd

(U

L.

+->rdi_

OJQ_E

OJ •p

OJu

rd <+-p3

(/)

OJm

rdP

<D>

CEm

E3 GJ C3 QJ in G) in P

OJ CS3 r^

—

OJ OJ * > m

>1'; 3jn;e j gduja^

Li_

31

fora1

um i

num

G06 1.
2500 djrvjcvj— S rv

y\

1 9 j J 9ciuJ9]_

32

Pressure

,

MPa

Figure

14.

Temperature

at

ignition

-For

a

0.5

mm

diameter

surface

area

for

aluminum

G061.
(S

LD

-- LD 0)E

LO

OJ

S

LD

OJ

OJ

csIS

OJ

lhO- s

in

y\ ‘ 3 j j aduuaj_

in

QJ

l.D

m

L_

33

Surface

temperatures

of

aluminum

G061

for

an

iqnition

on

cool-down.
the temperatures of a 0.5 mm diameter area on the top surface at ignition. This

data shows the variation that can exist in surface temperatures. Maximum surface

temperatures are considered to be approximately 150-175 K above the average surface temperature linear least squares curve. The temperature of the oxide-alloy

interface is considered to be within +20 K to the linear least squares curve.

2. Cobalt-Based Alloy, Haynes 188

Haynes 188 was the only cobalt-based material to be tested. Material in

suitable configuration could not be obtained. Specimens were therefore cut from

flat stock and stacked. This type of specimen only allowed surface temperatures

to be measured. From the existing data and examination of samples that did not

ignite, it appears that the material must be within the melting range, 1575 to

1630 K or above to ignite, and definitely must be fluid in order for unsupported

combustion to occur.

The material is extremely difficult to ignite due to the thick oxide layer

produced during heat-up. This oxide layer appears to have a melting range significantly above that of the alloy and thus restricts heat transfer to the base

alloy, preventing easy ignition and combustion. A number of attempts to ignite

the samples failed. These samples sustained severe surface damage, i.e., oxidation, melting and vaporization, without burning. Ignition temperatures (see Appendix B) were difficult to obtain because the nature of the oxide layer required

extended run times to obtain combustion. Those that were obtained are presented

in Figure 16 while temperatures at combustion are presented in Figure 17. These

temperatures are probably considerably higher than the oxide metal interface because of the oxide thickness.

34
2250

-- 00

-- LO

LD

r^- LD

C\J LD

rs.

rc) CL

z:

<DD

V)

CO

IU

c_ Q_

-- oj

GJ

Ccm

-p

id

OJ

+->rd

t_0

CLE0

+->0 • u oo

fd CD

H-

—

S^. 13

(/) 0

to0C

m (d

ra

X0> o x <+-

LD0

L.D

CD

>| ‘ j aduj3j_

35
w

h- 1—

oo o

-*

o o

--

03

03

T—

LD

OO O

LJ

2i

>-

cex

OO --

OO -•

1 (. —1 1 1 1 Jj H 1 1 1 1 1 1 1 Q (3 Q K> ID in in in

rvi r- OJ OJ —

—

00

CO

’SJrvj

<T3 CL

SI

(Uc

3

M

toW

c

CL

C

-P

00

=5 _Q

E

Q

U

-P

ftf 0

L.

•P

fO

t_0

CL

E

QJ

+->

QJ • U 03

rtf 03

<-t- —

£_D CO

00 QJC

0 X

m ixi

rtf I

£_0 L. > o

x hfN0

L.

13C30 X 3 J n^B J 9duJ9J_

36
3. Iron-Based Alloys

a. Carbon Steel 1018

A carbon steel was included since atmospheric flowing oxygen tests had indicated that the 300 series stainless steels may ignite below the solidus temperature. It was thought that ferrous oxide, carbon or iron-carbides were the most

probable source of the ignition. Since many of the materials to be studied contain significant quantities of iron, and some carbon, it was considered expedient

to determine the ignition and combustion characteristics of an iron-carbon system

for comparison to other iron containing alloys. Carbon steel 1018, which was

readily available in the laboratory, was used for this work.

The temperature data from the broadband pyrometer shows considerable regional surface temperature activity. Temperature spikes are apparent from the

activation of this instrument at approximately 1173 K until combustion begins.

Mass data, however, indicate that little or no detectable mass was consumed prior

to ignition. Consequently, these temperature spikes are not considered to be ignition zones but may be due to the rapid heating of loose oxide layer scales or

the ignition of microscopic phases on the specimen surface. Combustion develops

rapidly from a major ignition event. Figure 18 presents the interior temperature

data and Figure 19 the average surface temperature data at ignition. There is a

slight pressure dependence in ignition temperature with increasing oxygen pressure, but not large enough to be considered significant at this time. Rapid combustion occurs only in the fluid state. Ignition and combustion may be considered to be concurrent.

b. Stainless Steel: AISI 302, 304, 304L, 316, 321, and 347

The detectability of the ignition point of the AISI 300 series alloys tested

improved dramatically with increased oxygen pressure, Figure B.3. The ignition

temperatures also decreased to within the melting range or below, if the solidus

37
CD

CD

-- •^r

a ED ED ED

ED ED El E)

ED LTD E) LD OJ — —

(XI CL

QJ

i_3

CO

CO

IU

s_ CL

OJ

ED

y\ ‘ 3 j n^.E j adwa^

L.

<PdPc

CD P

fCJ

(DL.

15 P

rtJ C-

(DQ_E

OJ P

c.Od

QJ PC rtf I l u

CD

QJdD

CD

Li

38

rbon

steel

1018.
2000

rd CL

(DL.DM

V)

QJl_ CL

C

-P

V)

=5 _QEou

-p

rd

QJp

Z5 -P

rdp

QJCLE

QJ -P

QJU

rd

<-h ^r p Qd m

(/) i—

qj in

CD I—

rd CE p

QJ P >

cr hLD

c\j

<s GJ

Q QJ LD Q in P

r- in

—

•—4 •—

-JCD

>j ' Qjn^E j aduj3j_

47
— V)

0

o

oo e> o

H

cn

S

CD CD CD cd CD LD CD LD S fN- LD C\J OJ —

y\

1 a j j adiua^

U1

i—

cr

L.o

4-

C

- LD

4->c

cn

+->

fO rd Q_X- OJ

QJ L_

-

sti_—

\

15

(/) 4-> V

rd UD OJL. Q_

s_

QJ m Q_

E

i—

OJ LQ +->

i—

CL - DJ

L.

QJ • -P LD

C —

i—i m

rru

OJD

m

b_

48
G3 r- LD OJ

oj

—

—. —

CD

UD

OJs

rd Q_0)

(A

00OJi_ Q_

y\ ‘ a j j 3duuaj_ co

+->c

CD

+->rd0

i_

15

+->rd

QJQ_£

QJ

+->0

CJ

rd <-H LD

s_

—m

C/1 I—

0 in

CD l i rd CH

L.0 £- >

CL t+-

CD

fvj0

L.

13CD

Ll_

49
2000

-- CD

ro

-- sr

ro Q_

OJi_D

</) m

s_ Q_

njS

in OJ

y\ ‘ 3 j j sdiu9j_

<+-c

+->to13 _DEu

-4-> rd

QJ

=5

+->

rti

CDQ_E

CD

+->

L.H) +JcI—

cn

OJu

L.D

cn

u_

50

nisi

316.
0

-r LD

EJ S s ts S LD s LD G3 in rvj c\j <—

—

—

rtJ CL

<U

V\

QJ

i_ CL

nj

BD

y\ ‘ 9 j nq.2 j 9duj3j_ c4->

(/) D

_QEou

+->rd

OJ£_

13

+->rdL0

Q_E0

-+-> 0u

rd

<-t- UD L «—

Z3

m

M l—

0 in

cn^

rd QZ 10 L > O

cr <+- sm0LD

CD X

51
--

--

m

oj

CD S CD S S LO ts LO

cs rv LD OJ OJ —

—

—

(U CL

QJ

i_DM

V)

ajL.

Ills » ‘ 3 J J sdiua^

LDz

ZD

L.o

H-C

+->c

CD

4->rd

QJ

=5

-+-> rd

£_QJD_E

QJ

4->

L.QJ

+->Cm

QJ

L.

15cn

L_

52

N0B625.
--

m

-- OJ

GD (S (S (S <S LD s in S r- LD OJ OJ —

-—

—>

a) CL

QJ

i_d

V)

v\

QJ S C

+->C

CD

+J

rtf

QJ

L_15

+->

ftf L.QJQ_E

QJ

+->

QJU LD

ro ru

«+- LD

i_

13

(/)

LD

C3

QJ LD

cnZ

ra ZD

OJ>

>j ' a j j 3duuaj_

CZ hOJm

QJ

s_D

cn

L_

53
n

1— 1—

-

LH

-•

PJ -*

- LD

LD -- s

o o z -

LH

Z

ID --

O <•> O _ _ 0 <G> 0><2> - —1—»—1

<•> —i 1 1

1 1

—iH 1

1 1 1 H

CD CD S (D CD LD CD in

S r-- LH CVJ CVI —

—

^r

m

rvj

CD

ro

CLUJL.3

M

{/) (U

L. Q_

»

‘ 3 j n^E j adwaj^

c

0

+->01=5 _Q

E

o

u

+->rd

QJL.D

4->rd

L_

QJQ_

£

QJ

+->

QJ • U LD

HJ PJ

<+- CD

J- LDS

00 2

QJ LSI cnZ

rc) ID

d

QJ S- > O

CE <+- n

m

QJi_Z5Ol

Ll

54
IV. SUMMARY AND FUTURE EFFORT

Several experimental problems have developed during the testing at elevated

oxygen pressures. A summary of these difficulties and their resolution follow:

1. Insufficient laser power to reach ignition temperatures at high pressure. This difficulty has been resolved by acquiring a new 500 plus Watt

1 aser.

2. Mass sensor disturbance by building vibrations. This difficulty is to

be resolved by mounting the pressure chamber on a vibration damped table.

3. Mass sensor disturbance by convection currents. This difficulty has

been partially resolved by restricting the area above the sample. However,

additional work must be done.

4. Premature failure of the sample thermocouple. This difficulty was resolved by changing materials. However, the new material cannot be used beyond 2040 K. Therefore, interior temperature measurements on aluminum alloys cannot be made if ignition exceeds this temperature limit.

The acquisition of the new laser will ease the experimental problems. This unit

can be computer controlled; thus heating rates can be programmed and maintained.

Also, there is sufficient power to allow an increase in specimen size and to

allow the sample to be "bathed" in the laser beam, allowing a more uniform surface temperature.

It has been found that aluminum alloys can be ignited at temperatures below

the melting point of the surface oxide layer, in some cases, at temperatures significantly below. For these alloys, ignition and combustion are concurrent or

almost so.

For the cobalt, iron, and nickel alloys, it has been found that unsupported

combustion can only take place in the fluid state. Iron alloys can be ignited at

temperatures slightly below the lower bounds of the melting range while cobalt

55
and nickel alloys can be ignited only when a fluid phase is present. To date,

the ignition temperatures have no large oxygen pressure dependence.

Present plans call for bringing the new laser online under computer control.

Programmed heating rates will be implemented and data will be generated for the

present set of materials to the 13.8 MPa (2000 psia) pressure level, the maximum

pressure that can be attained without pumps.

V. ACKNOWLEDGMENTS

The author would like to express his gratitude to Mr. James D. Breuel

,

Mr. James A. Hurley, and Mr. Ke Nguyen for their assistance in this study, and to

the George C. Marshall Space Flight Center, Mr. John G. Austin, Jr., technical

representative, for supporting this program.

56
APPENDIX A: Theory of Two-Color Ratio Pyrometry

Two color ratio pyrometry is a straightforward procedure which has one very

strong feature: attenuation of the radiation, within reasonable limits, does not

affect the temperature measurement. The theory of the method is described as

follows for a unit wavelength interval.

Starting with the Planck equation for the spectral radiation, Wg(x), for

a blackbody surface

W

B (X) = 2 7T c

2 h x“5 (exp(hcAkT-l)" 1 [W/m3

], (A-l)

apply the Wien approximation

exp(hc/\kT) lx exp(hc/AkT)

for x << hc/kT

and obtain

W

B (X) = 2 tt c

2

h A -5exp(-hc/AkT) [W/m 3

] (A-2)

where c = velocity of light in vacuum, m/s

h = Planck's constant, J.s

X = wavelength of light, m

k = Boltzman's constant, J/k

T = absolute temperature, K.

The constants may be grouped and the blackbody spectral radiant emittance

converted to graybody by

W(x) = e(A) W

B (X) = C lE (x) x"

5 exp(-c

2

/xT) (A-3)

where

p c^ (first radiation constant) = 2 tt h c

c^ (second radiation constant) = hc/k

and e(x) is the spectral emissivity.

A-l
A ratio of the spectral radiant emittance at two wavelengths may now be defined:

(A-4)

Taking the logarithm of the ratio and grouping constants obtain _ n _ , r ,1

(A-5)

^2

Aq = 5/n — and A^ = C

The wavelengths chosen for the ratio computation must be such that e(A]_)

= £(*2) (° r nearly so); otherwise an emissivity correction would have to be

appl ied.

To expand the argument to a radiation detector signal it need only be observed that the detector signal is directly proportional to the radiation intensity

The argument presented herein is for a unit wavelength. To expand the

argument to a wavelength band complicates the development considerably and is

best approached by numerical methods. To implement the numerical development, a

detector, optical system, and wavelength band are defined and the detector signal

calculated from

thus

S * W(A) or S = K W( A)

R = W(A

1 )/W(A2

) = S

l /Sz (A-6

)

(A- 7

)

A

1

A-2
where:

Ln R = Ln

radiation

to 1/T.

G - geometric factor

t - optical transmission factor

R(x) - absolute responsivity of the detector

F(x) - filter transmission function

e( x) ,W( x) - are as previously defined.

(S^/S 2

) may then be calculated from the Planck (or Wien approximation)

equation. It can then be seen that this result is linear with respect

A-3

APPENDIX B: Data Definitions

The definition of terminology to accurately describe an event or events in

an experiment which has widely varying and interacting phenomena is usually difficult; this study is no exception. A theoretical definition of ignition can be

fairly easily defined. However, to extract this point from the experimental data

in which multiple events take place is far from easy and straightforward except

for the vapor burning materials. The definition of ignition and combustion

point, as determined from the experimental data, for vapor burning and liquid

burning materials is discussed below.

1. Vapor Burning Materials

The materials of the vapor burning type of interest to this program are

aluminum and its alloys. Figures B.l shows an expanded view of the ignition sequence of aluminum 6061 in 0.69 MPa (100 psia) oxygen. Data were taken at 20 ms

intervals. The upper curve was the average surface temperature of the specimen,

and the bottom curve was the mass. An ignition event is considered to be developing when a temperature increase breaks from the general average surface temperature trend. The ignition point is the last data point--or projected data

point--before this trend break. For the data in Figure B.l, the trend break occurs at 20.12 s, and the major temperature step was at 20.16 s. This short interval between the trend break and the major temperature step is typical of an

aluminum alloy ignition. Because of the typically short time between ignition

development and full combustion, ignition and combustion are considered to be

concurrent for this report and the ignition value is used. For the case shown,

the average surface temperature at the ignition time of 20.12 s is 2101 K.

2. Liquid Burning Materials

The cobalt, iron, and nickel based metal alloys burn in the liquid

phase. Figure B.2 shows an overall view of the average surface temperature of a

B-l
009

000E 0i_3*-> CO

k_®aE®®o

CO

u.3

CO0

O)0L-0>

00fr 002 o

CL

id - CL

a: z — CD

10 to Q

to CD

IS

inQ

.. cvj

is

is

is

r\j

ISin

CD

000 2 000 

y\ ‘ s j j sdws_L

M0E

IS

(S

cn

is

in• _ 00 0 -

B-2

Figure

B.l.

Expanded

view

of

an

aluminum

alloy

ignition.
cq s

j aduisx

B-3

Figure

B.3.

Average

surface

temperature

history

for

a

test

of

a

300

series

stainless

steel
test of a 300 series stainless steel. Figure B.3 is an expanded view of the ignition sequence of this same test. It must be pointed out that this example has

been chosen to clearly illustrate the technique in determining the ignition and

combustion points and is not typical of all test results.

The ignition sequence usually begins with the development of an ignition

zone (hot-spot). This development occurs within the temperature region just

prior to the beginning of the melting range to past the melting range. Thus, the

waveforms of the data are complicated by phase and geometry changes. In addition

to these changes, the oxide layer has built up to a significant thickness and may

have a scaly or irregular surface texture which can give momentary hot-spots.

These hot-spots appear to be an ignition zone on the surface temperature waveforms but are not. For a true ignition zone to be established, a small amount of

material must be consumed. This can usually be detected on the mass waveform as

well as the interior temperature waveform. The beginning development of the ignition zone is shown in Figure B.3 by the first set of markers at 15.28 s. The

interior temperature response is delayed because the thermocouple interior position is a short distance from the ignition zone. To obtain a numerical value for

the ignition temperature--interior, spot, average, etc. --a data point may be used

or an extrapolating or averaging technique may be employed. The combustion point

is found by determining the beginning time of an accelerating surface temperature

trend ending in an almost instantaneous temperature step. In many cases the temperature accelerating time occurs for only a few data acquisition time periods.

In other cases, the surface temperature acceleration occurs over several seconds

and occasionally longer. As can be seen from Figure B.3, the existence of

numerous hot-spots can mask the determination of the combustion point. However,

for the example in Figure B.3, the combustion point is straightforward and occurs

at 20.06 s. As in the case of the ignition temperature, the combustion

B-4
B-5

Figure

B.3.

Expanded

view

of

the

ignition

and

combustion

sequence

for

a

300

series

stainless

steel
temperature may be an individual data point or an averaged or extrapolated value.

Occasionally the ignition point and the combustion point may be considered to be

concurrent.

B-6
nbs- ] I4A I RE V. 2-8C I U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions,)

1. PUBLICATION OR

REPORT NO.

NBSIR 84-3013

2. Performing Organ. Report No. 3. Publication Date

August 1984

4. TITLE AND SUBTITLE

Laser-Initiated Combustion Studies on Metallic Alloys in Pressurized Oxygen

5. AUTHOR(S)

J. W. Bransford

6. PERFORMING ORGANIZATION (If joint or other than N BS, see instructions)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

George C. Marshall Space Flight Center

Marshall Space Flight Center, Alabama

10.

SUPPLEMENTARY NOTES

J Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a si gnifi cant

bibliography or literature survey, mention it here)

The interim results of ignition and combustion studies on aluminum, cobalt,

iron, and nickel based alloys are presented. It was found that aluminum alloys

could be ignited below the melting point of the product alloy oxides. It was

also found that the cobalt, iron, and nickel based alloys generally ignited

slightly below to slightly above the melting range of the respective alloy.

Unsupported combustion could not be achieved until the alloys and oxides were

in the liquid state.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon

alloy; combustion; ignition; metal; oxygen compatibility

13. AVAILABILITY

| X I Uni imited

| | For Official Distribution. Do Not Release to NTIS

I Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

I X| Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGE

76

15. Price

$ 10.00

•U.S. GOVERNMENT PRINTING OFTICt 1'iM ;«0 «> . • «»<