I

United States Department of Commerce

I 1^31 National Institute of Standards and Technology

NISTIR 3947

IGNITION CHARACTERISTICS OF THE

NICKEL-BASED ALLOY UNS N07001

IN PRESSURIZED OXYGEN

James W. Bransford

Phillip A. Billiard

NISTIR 3947

IGNITION CHARACTERISTICS OF THE

NICKEL-BASED ALLOY UNS N07001

IN PRESSURIZED OXYGEN

Chemical Engineering Science Division

Center for Chemical Technology

National Measurement Laboratory

National Institute of Standards and Technology

Boulder, Colorado 80303-3328

"•Boeing Company, Seattle, Washington

September 1 990

Sponsored by

NASA/George C. Marshall Space Flight Center

George C. Marshall Space Flight Center, Alabama 35812

U.S. DEPARTMENT OF COMMERCE, Robert A. Mosbacher, Secretary

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, John W. Lyons, Director

James W. Bransford

Phillip A. Billiard^

CONTENTS

Page

ABSTRACT 1

INTRODUCTION 2

toERIMENTAL PROCEDURE 4

METAL IGNITION PROCESS 10

RESULTS 20

1. Linear Temperature Scan Tests 20

2. Differential Thermal Analysis Studies 25

3. Incremental Temperature Scan Tests 29

4. Data Accuracy 37

CONCLUSIONS 44

FUTURE EFFORT 45

ACKNOWLEDGMENT 45

REFERENCES 46

iii
List of Figures

Page

1. Test specimen configuration 5

2. Experimental setup 7

3. Test chamber configuration. 9

4. General variation of heat balance parameters as a function of

surface temperature, 15

5. Transition from spontaneous ignition to combustion for a typical

ITS test 16

6. Abrupt ignition and combustion event during an LTS test 19

7. Typical surface and interior temperature waveforms for an LTS test... 21

8. Typical surface temperature waveform features from an LTS test 23

9. LTS test series ignition temperatures vs oxygen pressure 27

10. LTS test series combustion temperatures vs oxygen pressure 28

11. DTA thermograph of N07001 alloy heated in oxygen 30

12. Typical surface and interior temperature waveforms for an ITS test... 32

13. Development of ignition in 3.60 MPa oxygen during an ITS test 34

14. Development of ignition in 14.41 MPa oxygen during an ITS test....... 36

15. ITS test series spontaneous ignition temperatures vs oxygen

pressure., 39

16. ITS test series enhanced oxidation temperatures vs oxygen pressure.. 40

17. ITS test series ignition temperatures vs oxygen pressure 41

18. ITS test series start-of-combus tion temperature vs oxygen

pressure 42
List of Tables

Page

1. Materials selected for the determination of ignition and combustion

characteristics 3

2. LTS test series ignition and combustion temperatures 26

3. ITS test series spontaneous ignition, enhanced oxidation, ignition,

and combustion temperatures 38

4. Linear least- squares curve fit coefficients for the ITS ignition

parameter data shown in Figures 15, 16, and 17 43

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r
Ignition Characteristics of the Nickel-Based Alloy

UNS N07001 in Pressurized Oxygen

James W. Bransford and Phillip A. Billiard

The development of ignition and combustion in pressurized oxygen

atmospheres was studied for the nickel-based alloy UNS N07001. Ignition of

the alloy was achieved by heating the top surface of a cylindrical specimen

with a continuous -wave CO2 laser. Two heating procedures were used. In the

first, laser power was adjusted to maintain an approximately linear increase

in surface temperature. In the second, laser power was periodically increased

until autoheating (self -heating) was established. It was found that the alloy

would autoheat to combustion from temperatures below the solidus temperature.

In addition, the alloy had a tendency to develop combustion zones (hot spots)

at high oxygen pressures when the incremental (step) heating test mode was

used. Unique points on the temperature- time curves that describe certain

events are defined and the temperatures at which these events occur are given

for the oxygen pressure range of 1.72 to 13.8 MPa (250 to 2000 psia)

.

Key words: alloys; combustion; ignition; ignition temperature; metal; nickel

alloy; oxygen; oxygen compatibility

1
INTRODUCTION

A number of serious accidents and catastrophic fires involving the

ignition and subsequent combustion of metals have occurred in oxygen systems.

As the operating pressure in oxygen systems has increased, the potential for

disastrous ignition and combustion has also increased due, in part, to the

increased reaction rate of materials with oxygen. To reduce the potential for

ignition and combustion failure in oxygen systems, a knowledge of the ignition

and combustion characteristics of metals is needed. Recognizing this need,

the George C. Marshall Space Flight Center (MSFC) initiated a program at the

National Institute of Standards and Technology (NIST) to study the ignition

and combustion characteristics of selected alloys in pressurized oxygen. A

list of the alloys being studied is given in table 1.

This report, the third in a series [1,2], details the results for the

nickel-based alloy UNS N07001 (WaspaloyTM*) . The studies involved heating the

top surface of cylindrical specimens of the alloy by a continuous -wave (cw)

CO2 laser in pressurized oxygen. The surface and interior temperatures were

recorded as the specimens were heated. High speed cinematographic records of

the ignition and combustion events were also obtained. From this information

the spontaneous ignition (critical) temperature, enhanced oxidation

temperature (our terminology), ignition temperature, combustion temperature,

ignition and combustion morphology, and other information were obtained for

the oxygen pressure range of 1.72 to 13.80 MPa (250 to 2000 psia)

.

*Trade names are furnished to identify the material adequately. Such

identification does not imply recommendation or endorsements by NIST, nor does

it imply that the materials identified are necessarily the best available for

the purpose

.

2
Table

1:

Materials

selected

for

the

determination

of

ignition

and

combustion

characteristics.

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in 1 in M

in

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1 o

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1 in d

9

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d

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CO o00 • • z 1 00o 2

z 1 o

—1 2 -c O ^ o

d (O o

d

o 2 o CD o

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Q> CD CD

$

1 o in

1 o o

CO od cd

K

mCNJ

1 o o — CM O 1 in in

CM

1 m in

C£)

1 in in

CM o*

inA

1 o in

05 00 V d d V CM 00 d — CM CM —' CM

(O 6.50- 7.50

7.60- 8.600.50

<0.75 <0.35 <0.50 <0.05 p

V p

V o

V p

V pV pV pV pV p

V

0.20- 0.50

in o in o o o o o o o O o o o 00 oo 00 00 00 00 00 00 00 00 CO CO o o o o o o o o o o o o o o

d d d d d d d d d d d d d d V V V V V V V V V V V V V V

in O in O in in in in in in in in o 00 00 CL o o o o o o o o o o o o o d d d d d d d d d d d d d V V V V V V V V V V V V V

in

CM 15

1 o o o 15

1 O O1 o o1 O LO1 o o

1 o o

1 o O1 o Ot o o

1 o o 15 Z d £i jQ d LO d CO od od o od o 00 CM d oi CM cri CO d “M* V CO ID A CM CM CM CM

1 1 1 1 in 1 o P o in o 00 00 o o o o o in

00 o 00 in CM CO in CNJ od dV

C

ID

00 OCM oin o

in

00 o o o o o p p p p p p

in

CM 2 dV dV dV V dV V V CMV

CMVCMV

CNJ V

CMVCMVCNJ V VCNJ V V O o 1 1 o CO oo o O (0 o o o o CO cO CO cd cO CO <0 CO CO od u. d d ih CNJ in d n n n jO n JO D iO V

V V V V

m O O o o 3 CSJ CM in 00 in o d d d d d V V V V V

w oCM

» o o1 o o CO o1 o o

1 in m1 o o1 o o <i o1 o o

1 o o1 o o1 o o1 o o

1 in o1 o o

o d d oo 00 ^ CO CO od fsj CD CO <3 od d <b od r^* 0> o CO od od cd d CM CM CM CM CM

o

1 o o CZ) in o CM d CM .o V

o

o 1 00 o

o

00

<3

00p

CMp

in in 00o

00o

00o

00O00o

1 in o

CD CM

00o in m

O dV d d dV dV dV dV dV dV dV dV dV dV d dV d d

O 1 o o

1 o o1 O o in < JO CM CD CM 00 Tf q 00 d d d dV

< o o in 00 o CO o o o 00 o o O CO 00 (0

CO 00 CM o in o o o o o o o o 00 00

LO in CO o r>- CM V CM CO V— o CM •«— .2 2 00 00 CO o o O o o CM CO o QC o o o o o o 00 CO CO 00 00 OO 00 CO 00 <

< < z z z z z CO CO CO CO CO CO CO CO CO tr z

3

Note:

Composition

in

percent.
EXPERIMENTAL PROCEDURE

Cylindrical test specimens (fig. 1) were machined from commercially

available UNS N07001 alloy rod that had a hardness of Rockwell C 38. The rod

had a chemical composition of 1.36% Al, 0.005% B, 0.048% C, 13.96% Co, 19.40%

Cr, 0.03% Cu, 1.24% Fe, 0.02% Mn, 4.12% Mo, 0.006% P, 0.002% S, 0.06% Si,

3.10% Ti, 0.066% Zr with the balance Ni. Specimen dimensions were 6.35 mm

(0.25 in) in diameter by 6.35 mm (0.25 in) in height for the linear

temperature increase tests and 8.00 mm (0.31 in) in diameter by 6.35 mm (0.25

in) height for the incremental temperature increase tests. A thermocouple

hole was drilled axially to within 0.25 ± 0.05 mm (0.010 ± 0.002 in) of the

top surface. The top surface of the specimen was maintained in the machined

finish, spiral groove approximately 0.08 mm (0.003 in) in depth, to improve

the absorption of the laser beam at the beginning of an experiment and also to

give a diffuse laser beam reflection which would minimize potential heat

damage to the chamber liner or other chamber components.

The experiments were carried out in commercially pure oxygen. Chamber

pressure was varied from 1.72 to 13.8 MPa (250 to 2000 psia) in 1.72 MPa (250

psia) steps. Oxygen pressures lower than 1.72 MPa could not be used because

gaseous oxides produced during the high temperature combustion phase of the

experiments produced splatter which destroyed the laser beam window after

several experiments. The chamber pressure was allowed to increase, due to the

heating of the oxygen, during the experiments and was recorded at the

beginning of specimen combustion. The chamber pressure increase at combustion

rarely exceeded the initial chamber pressure by 10 percent.

To assemble an experiment, a specimen was mounted in a machined graphite

block but was insulated from the block by approximately 3 mm of aluminum oxide

4
NOTES:

1 . Apex to top surface center.

2. Surface finish as specified

3. 6.35 mm or 8.00 mm dia. as req'd.

Figure 1. Test specimen configuration.

5
powder packed to a density of approximately 1 gm/cc. This assembly was placed

upon a ceramic foam pad which was then placed on a pedestal (fig. 2) . The

heat loss from this configuration was significantly less than would normally

be experienced by metallic materials in most oxygen systems.

The top surface of the specimen was heated by a cw CO2 laser beam. The

laser provided a controllable noncontaminating heat source. The laser was

detuned from the pure TEMqo mode so as to contain higher order modes . This

procedure distributed the laser power more evenly across the laser beam and

thus heated the specimen more uniformly. The laser beam was focused to cover

approximately 80 percent of the top surface of the specimen. This allowed the

beam to remain predominantly on the specimen surface despite random refraction

of the beam by convection currents of heated gas rising from the heated

specimen surface.

Two heating procedures were used in this study. The first procedure

established and maintained an approximately linear increase in specimen

surface temperature and was called the Linear Temperature Scan (LTS) heating

procedure. Surface heating rates of 100 K/min were most often used, however,

rates of up to 250 K/min were used occasionally. The LTS heating procedure

was used primarily to determine whether rapidly developing exothermic events,

including ignition, were present, but the tendency of a material to autoheat

(self-heat) , which is usually a much slower exothermic process, was also

detectable

.

The second heating procedure incrementally increased the laser beam

power. The surface was allowed to reach short-term equilibrium before another

power increment was applied. At some power level during the procedure, the

surface temperature would not reach equilibrium and the material would

6
Aluminum Oxide

Insulation

Graphite Crucible

Platform

Specimen

Thermocouple

Ceramic Foam

Heat Barrier

Pressure Chamber

Bottom Closure

Figure 2. Experimental setup.

7
autoheat to combustion. This procedure was called the Incremental Temperature

Scan (ITS) heating procedure.

The surface and interior temperatures of the specimen were the principal

dependent variables measured in this study. The surface temperature was

measured using two-color ratio pyrometry. Two pyrometers were ultimately

used. One pyrometer covered the temperature range of 973 to 1773 K (1292 to

2732* F) and had an output range which gave a temperature sensitivity of 8 K/mV

(14*F/mV) . The second pyrometer covered the temperature range of 1173 to 2673

K (1652 to 4352 *F) and had an output range which gave a temperature

sensitivity of 15 K/mV. These signals were recorded by a 4-channel digital

oscilloscope which used 15 bit analog to digital (A/D) converters. The A/D

converters have a maximum voltage resolution of 6.25 /xV. Thus, maximum

temperature resolution was 0.05 K (0.09*F) for the 973 to 1773 K range

pyrometer and 0.09 K (0.16*F) for the 1173 to 2673 K range pyrometer.

The pyrometers viewed the specimen through the pressure chamber

observation windows which were oriented at 45° from the vertical (fig. 3).

The specimen surface observed within the circular measurement field of view of

the pyrometers was elliptical and covered top surface dimensions that varied

from approximately 5 to 7 mm (0.2 to 0.3 in). The pyrometers did not measure

the maximum surface temperature, but responded to the overall temperature of

the viewed area. The temperature was not an average but was biased toward the

highest temperature within the measurement area due to the non-linear

dependence of radiated power on temperature

.

The interior temperature was measured with a platinum versus platinum10% rhodium (Type S) thermocouple. The thermocouple was insulated except for

the bead, which was in contact with the specimen at the top of the

8
Laser Beam

Polycrystalline

NaCI Window

Windows or Access Ports

at 45^ (4)

Sapphire

Window

Quartz

Splatter

Shield

Windows or Access Ports

at 90° (4)

Fire Wall

Experiment Platform

Figure 3. Test chamber configuration.

9
thermocouple well. The output of the thermocouple was referenced to a 273 K

cold junction and then digitized directly at the maximum sensitivity of the

digitizers, 6.25 /*V. This gave a temperature sensitivity of approximately 0.6

K (1*F) over the temperature range of interest, 973 to 1773 K.

Special tests in which the thermocouple was spot-welded to the top

surface were also run. These tests were used to obtain estimates of the

temperatures of the alloy/oxide interface.

METAL IGNITION PROCESS

A detailed mathematical description of the metal ignition process

requires a quantitative knowledge of a significant number of variables.

However, a general nonmathematical understanding may be gained by considering

a generalized temperature -dependent heat balance for a surface undergoing

oxidation and external heating (fig. 4)

.

The heat balance is composed of the

following parameters

:

1. Q(ioss) rate at which the oxidizing surface loses heat by

conduction, convection, and radiation;

2- Q(ext) -the rate at which external sources transfer heat to the

oxidizing surface by conduction, convection, and radiation.

3. Q(chem) rate at which oxidation of the alloy generates heat at

the oxidizing surface;

4. -the sum of and Q(chem) •

o To further simplify the description, Q(ext) be a constant, but

« different, value for each of the illustrations in figure 4. Both Q(ioss) and

•Q(chem) are dependent functions of the surface temperature as well as other

variables which will remain constant for this discussion.

10
# The initial application of Q(ext)» is less than that required to

initiate ignition, will increase the magnitude of Q(i„) to some value greater

than Q(ioss)* surface temperature, Tg, will increase and reach an

equilibrium temperature, T^q, where Q(ioss) equal Q(in> (fig- 4a). Since

Q(ext) is less than that required for ignition, the slope of the Q(i„) curve

o will be less than the slope of the Q(ioss) curve at T^q. The oxidizing surface

temperature cannot increase beyond T^q irregardless of the oxidation mode that

is occurring, protective or unprotective , because an increase in heating rate

will be less than the increase in cooling rate at temperatures greater than

Tgq. Therefore, T^q will be a stable surface temperature in the sense that it

cannot increase. However, the long-term stability of Tgq over time will

depend upon whether the surface is undergoing protective (time -dependent) or

nonprotective (time- independent) oxidation. If protective oxidation is

o occurring, Q(chem) decrease with time as the oxidation rate decreases with

increasing oxide layer thickness. Since Q(chem) decreasing with time, so

o is Q(in>; so will the oxidizing surface temperature decrease with time. If

nonprotective oxidation is occurring, Q(chem) will remain constant, assuming

all other variables remain constant, and the oxidizing surface temperature

will remain constant.

o As Q(ext) i® systematically increased, an equilibrium surface temperature

will ultimately be reached at which the slope of the Q(in) curve equals the

slope of the Q(ioss) curve (fig. 4b). This equilibrium temperature is defined

as the spontaneous ignition (or critical) temperature, Tgp [3]. Since Q(chem)

^ • and Q(ioss) both increase with increasing temperature, the slopes of the Q(i„)

o and Q(ioss) curves may be approximately equal over a fairly wide temperature

range, relative to most run-away oxidation reactions. We have previously

11
described this range as a quasi -stable temperature region [4]

.

The behavior

of an alloy within this region is of interest. Small shifts in the value of

various factors which change the heating or cooling rate will cause the

oxidizing surface to cool or to autoheat (self -heat). At Tgp, the

o • relationship between the Q(i„) and Q(ioss) curves may be used to define several

unique events (or ignition parameters). These are:

1. Spontaneous ignition temperature, Tgp - the temperature at which

^Q(in)/^’^S " <iQ(ioss)/^Ts>

2. Ignition temperature, T^g^ - the temperature at which [Q(in> ' Q(ioss)]

maximum.

These parameters are shown in figure 4b. It should be clear that Tgp and T^g^

are not fundamental material properties, but are functions of the relationship

between the Q(in) Q(ioss) curves. Therefore, the values of Tgp and T^gj^ will

9 O change as the shape of the heat balance parameters curves, Q(i„) and Q(ioss)»

changes

.

If autoheating is sufficiently slow, as may be the case with bulk

quantities of material because the bulk mass acts as a heat -sink, an

equilibrium surface temperature greater than Tgp may be established by

decreasing Q(ext) o^ice the surface temperature has exceeded Tgp. However, such

an equilibrium surface temperature must be considered unstable because the

• o slope of the Q(i„) curve is greater than the slope of the Q(ioss) curve and the

surface will tend to autoheat.

It is useful to consider the effect of physical state on the ignition

parameters that have been introduced. In general, the oxidation rate of the

solid alloy is less than that of the liquid alloy. Therefore, if Tgp occurs

while the alloy is solid, a phase -dependent ignition temperature, Tj^g^, may be

12
defined. However, defining a liquid phase ignition temperature would be

difficult because of the effects of fluid flow. As a liquid oxidizing fluid

begins to flow, the rate of material oxidation will be significantly increased

by the exposure of unoxidized material. This, in turn, significantly

increases the rate of heat generation and thus the surface temperature for the

flow interval. The net result of fluid flow is to disrupt the smoothness of

the nonflow Q(chem) curve. This makes it difficult to determine flow induced

oxidation effects from quiescent oxidation effects. For this reason, liquid

phase ignition parameters and their temperatures were not determined for the

ITS heating procedure (with the exception of the temperature at the beginning

of the flow interval, which is defined as combustion, ; only solid phase

ignition parameters were determined.

If Q(ext) sufficiently large, the Q(in) curve will not intersect the

Q(ioss) curve (fig. 4c). In this case, Tgp is not obtainable from the

experiment, however, T^g^^ is. T^g^^ does not differ in this case from the case

shown in figure 4b. This is due to the definition of T^g^j as being the

o o temperature at the maximum deviation between the Q(in) curve and the Q(ioss)

curve. This means that our experiments which use the ITS heating procedure

can be thermally overdriven without seriously affecting the value of T^gjj.

However, the excess external heat input cannot be orders of magnitude more.

e o The exact shape and magnitude of the Q(in> and Q(ioss) curves are not

known. Therefore, the determination of the ignition parameters must be based

upon other measurable quantities. In our studies the surface and interior

temperatures were used. However, the surface temperature does have several

problems associated with its use.

13
As the surface temperature increases , the absorptivity (which is equal

to the emissivity) of the oxide layer changes , apparently increasing at the

CO2 laser wavelength more than at other shorter wave lengths. In our test, in

which a laser was used as the external heat source, this changing absorptivity

appeared to result in an increased heating rate over time at a constant laser

power input without a corresponding increase in the radiative cooling rate.

If a positive heating rate accrues, it will appear to be autoheating on the

temperature- time waveform and indicate a false spontaneous ignition

temperature

.

To determine the spontaneous ignition temperature, the apparent changing

absorptivity effect on specimen heating had to be separated from the oxidation

contribution. This was accomplished by running a number of tests using

heavily preoxidized specimens. These tests, when compared to the standard

test, showed that the contribution to the heating rate from the apparent

changing absorptivity, for constant laser power, was linear or nearly linear.

Since the oxidation rate of metals and alloys is nonlinear with temperature,

spontaneous ignition was assumed to have occurred at the temperature where the

temperature- time waveform became nonlinear under constant laser power input

(fig. 5). For N07001, the interior temperature- time waveform must be used for

analysis because the pyrometer measurement was adversely affected by the

apparent nonlinear emissivity of the surface oxide. If no measurement problem

existed, the surface and interior temperature waveforms would be parallel.

This was not the case for N07001, as is seen in figure 5.

We have defined an additional ignition parameter that may be unique to

our experiments. This parameter defines the beginning of a region on the ITS

temperature- time waveform where an abrupt change in curvature occurs after Tgp

14
o3

CO

IbOaEo«««o«co0c3

CDCO

CDCO1

CD

cia•oc

CO -ia«oco

CO

CO>oeO3a

iZ

15
-- (S

(UE

>{

* a j j aduja_L

16

Figure

5.

Transition

from

spontaneous

Ignition

to

combustion

for a

typical

ITS

test.
and prior to (fig. 5). The region generally contains several abrupt

changes in temperature which, for many alloys, usually appear in conjunction

with clearly observable endothermic features in the surface temperature

waveform. For N07001, these endothermic features are not clearly observable;

however, the abrupt changes in curvature usually were discernible. Using all

ITS tests, sufficient evidence was available to indicate that endothermic

events were occurring in conjunction with the abrupt changes in curvature.

Thus, we have postulated, as we have with other alloys, that the source of

these events increased or enhanced the oxidation rate of the alloy. The

temperature at the beginning of this region we have called the enhanced

oxidation temperature, T^q, and appears to be associated with the beginning of

the rapid buildup of the oxide layer. We speculate, but have not

demonstrated, that Tg^ lies in a temperature region where the oxidation

mechanism is changing from protective to nonprotective oxidation. This

• • mechanism change has not been incorporated into the Q(chem) Q(in) curves of

figure 4 nor discussed extensively. The oxidation mechanism change, where it

exists, probably causes a rapid change in the Q(chem) curve slope.

The ignition temperature was the most straightforward parameter to

determine, because the definition of this event requires the existence of a

maximum heating rate of the specimen. Ignition was assumed to have developed

as the maximum rate of change in the surface, or interior, temperature

occurred (fig. 5)

.

A number of factors can influence the shape and magnitude of the Q(in)

o o Q(ioss) curves. The Q(ioss) curve alone can be easily modified by improving or

impeding heat transfer from the oxidizing interface. The Q(in) curve may be

modified by the existence of fluid oxide phases (which may exist above certain

17
temperatures) at the oxidizing interface. These phases may increase the

oxidation rate of many alloys [4,5]. The Q(chem) component of the Q(in) curve

is also modified when the oxidation rate changes from protective (timedependent) to nonprotective ( time- independent) . When either the Q(i„) or

Q(ioss) curve is changed, the values of the ignition events will also change.

In general, the lower the heat transfer rate from the oxidizing surface, the

lower the temperatures at which T^p, Tg^, and T^gj, will occur; conversely, the

greater the heat transfer rate, the higher the temperature at which T^p, Tg^,

and Tign will occur. The temperatures at which ignition parameters occur are

not fundamental material properties; instead, their values are controlled by a

number of material properties and external inputs

.

Under certain experimental conditions and/or at elevated oxygen

pressures, combustion may develop abruptly before the above defined T^g^

occurs, as shown in figure 6. These abrupt combustions may be due to cracking

or spalling of the surface oxides which expose minimally oxidized material.

If the surface oxides are impeding the transfer of oxygen to the oxidizing

surface the resulting rapid increase in oxidation rate may produce sufficient

heat, in a region of restricted heat loss, to melt a small volume of the

alloy. A feedback effect may result which produces either immediate

combustion or a small combustion region, hot-spot, which may expand rapidly.

This type of event has also been called ignition, thus creating an unavoidable

dual definition for T^g^.

The terms "combustion" or "combustion region" are used in this report.

This terminology usually implies evaporation and vapor phase oxidation of the

fuel material. For the N07001 alloy, vapor phase combustion does not occur.

For this alloy, and most others listed in table 1 with the exception of the

18
0) E

(S (S (S C3

(S in (S in

(S in OJ

OJ —• —

>{ ' 3 j j 3ciujaj_

19

Figure

6.

Abrupt

ignition

and

combustion

event

during

an

LTS

test.
aluminum alloys, combustion is the fluid flow enhanced oxidation of the liquid

alloy. Flow enhanced fluid oxidation generates very high temperatures, 2500 K

and higher, and develops in milliseconds as the specimen melts and collapses.

The beginning of the development of these high temperatures is called the

combustion temperature, T^n^, and is shown in figure 6.

RESULTS

1. Linear Temperature Scan Tests

Analysis of a number of combustion events in oxygen systems constructed

of aluminum, iron, or nickel based alloys, or a combination of those alloys,

indicated that combustion events appeared to develop rapidly, possibly in

milliseconds. The environmental history leading to these combustion events

were usually unknown. Thus, the immediate thermal history of the oxygen

system materials involved in the combustion event and the presence or absence

of an oxidation reaction induction period was also unknown. We therefore

decided that the initial course of investigation should assume an abrupt solid

phase ignition- to-combustion transition, similar to the liquid phase

combustion event shown in figure 6, and evaluate the alloy for this type of

event. To perform the evaluation, the LTS heating procedure, a variant of a

standard thermal analysis temperature scan technique, was adapted and modified

for laser heating. Surface heating rates of approximately 100 to 200 K/min

were used. Experimental data were obtained for the oxygen pressure range of

1.72 to 6.89 MPa (250 to 1000 psia) . The results for a typical experiment are

shown in figure 7. There are three major features in the waveform of figure

7. The first is the extreme nonparallelism between the surface (pyrometer)

waveform and the interior (thermocouple) waveform. The second is the change

20
UN5

N07001

‘ s j nq.E j aduuaj_

21

Figure

7.

Typical

surface

and

interior

temperature

waveforms

for

an

LTS

test.
from a linear interior temperature waveform to a nonlinear waveform beginning

at approximately -51 s. The third is the abrupt ignition with immediate

combustion that occurred above the liquidus temperature. Negative time in our

data indicates time prior to combustion. Not as obvious, due to compression

of the data, are endothermic (constant temperature) features in the waveform.

Several examples are shown in figure 8

.

The extreme nonparallelism between the surface and interior temperature

waveforms indicated that the pyrometer measurement was being severely affected

by the optical properties of the oxide surface. Since the pyrometer was a

two-color ratio instrument, the property that would introduce errors of the

magnitude observed was non-linearity with wavelength of the surface

emissivity. The shape of the surface temperature waveform indicates that the

emissivity of the oxides would be a function of wavelength, temperature, and

possibly oxide thickness, which was time -dependent. Because of the time

dependency of the waveforms, the pyrometer could not be calibrated to

eliminate the measurement error. Thus, the surface temperature waveforms

cannot be used to obtain accurate values for T^g^ However, estimates

of the range of the surface temperature values relative to the interior

temperature can be obtained from several tests and from other materials which

have been tested and have thermal properties similar to N07001. One such test

which appears to give accurate surface temperature values for T^g^ and Tj.„j3 is

shown in figure 7. From the sources noted we estimate that the surface

temperature at T^g^ and T^nj^ would be 75 to 100 K higher than the interior

temperature values

.

At interior temperatures between 1250 to 1300 K, the specimen began to

rapidly autoheat, as shown in figure 7. Laser power was reduced in order to

22
S)

in

r>j OJ

in

OJ

I

in

OJ

i

(S

m

I U3 in

(/)

(UE

I-

>1

‘ a j j aduja_L

23

Figure

8.

Typical

surface

temperature

waveform

features

from

an

LTS

test.
maintain the linear temperature ramp. However, the power reduction rate could

not offset the accelerating temperature change until the interior temperature

exceeded the liquidus temperature. At the solidus temperature, the oxidation

rate increased significantly and a thick oxide layer began to form. The thick

oxide layer, which impeded heat transfer from the surface to the interior

oxidation zone, along with reduced external power, allowed the heating ramp to

be re-established. The temperature continued to increase until an abrupt

ignition- to -combustion event occurred.

Abrupt ignition- to -combustion events are thought to occur when the oxide

layer cracks or spalls and exposes minimally oxidized material. The

subsequent rapid oxidation in a geometry that has reduced radiant and

convective heat loss may initiate a rapid melting and oxidation feedback

mechanism that is observed as an abrupt ignition- to-combustion event. There

are two potential sources for generating cracks and spallation in the oxide

layer: stress induced by differences in the density between the alloy and

product oxides, and stress from expansion of the liquefying alloy. Both

mechanisms are apparently involved, but the latter was probably the most

frequent mechanism, since combustion for the LTS heating procedure always

occurred at temperatures greater than the liquidus temperature.

Small endothermic features were present in both the interior and surface

temperature waveforms. Figure 8 shows examples of the endothermic features

from a surface temperature waveform. Not all features are noted. Several of

the features occur over a temperature range of ± 0.09 K (± 1 digitizing

level) . This indicates that the effect covers the field of view of the

pyrometer and dominates the temperature profile of that field of view. The

sources of most of the endothermic features have not been determined; however,

24
several are melting transitions of known oxides or of the alloy. There were

indications that the source of some endothermic events may be associated with

an acceleration of the rate of increase in specimen temperature, which was

interpreted as an increase in oxidation rate of the material.

The ignition and combustion data for the LTS test are listed in table 2

and plotted in figures 9 and 10. Figure 9 shows the ignition temperatures as

a function of oxygen pressure. Ignition always occurred at temperatures

greater than the alloy liquidus temperature. The ignition- to -combustion

transition occurred rapidly. Combustion was often concurrent with ignition,

as shown in table 2. There was no tendency for abrupt ignition and combustion

to develop at temperatures below the solidus temperature. Because there was

no trend toward solid state ignition and combustion in the data, testing was

halted at an oxygen pressure of 6.9 MPa (1000 psi) in favor of thermal

analysis (DTA) and ITS tests. The DTA tests were run to study the effect of

the surface oxides on autoheating while the ITS tests were run to study the

effect of autoheating on bulk specimens.

2. Differential Thermal Analysis Studies

The appearance of features in the temperature- time waveforms from the

LTS tests, interpreted as endothermic and exothermic events, led to efforts to

verify this interpretation. The approach was to utilize standard Differential

Thermal Analysis (DTA) techniques and equipment and search for endothermic and

exothermic events. Testing was carried out in flowing oxygen or helium at

atmospheric pressure using 15 to 50 mg specimens heated to temperatures

greater than the liquidus temperature of the alloy.

25
Table 2. LTS test series ignition and combustion temperatures.

Pressure,

MPa

Interior Temperature, K

^ign ^cmb

1.779 1826 1840

1.786 1805 1809

1.806 1814 1821

1.813 1828 1828

3.523 1675 1675

3.551 1741 1741

3.565 1819 1819

3.585 1734 1734

3.634 1728 1730

5.316 1766 1766

5.357 1679 1679

5.378 1769 1769

5.399 1655 1655

7.102 1675 1675

7.184 1725 1725

7.191 1775 1799

7.295 1723 1725

7.308 1727 1730

26
2000

CD

oS

I

—1

(S

r 1 ! - -T’"— f r- "7

S)

f ! -

(S

(S (S s

in cs in

3 j j adujsx

27

Figure

9.

LTS

test

series

ignition

temperatures

vs

oxygen

pressure.
2000

OD

-1—

(S (S G3

cs (S G)

in (S in

^"4

>^

‘ 3 j rv^.B j 3dujs_L

28

Figure

10.

LTS

test

series

combustion

temperatures

vs

oxygen

pressure.
The tests in helium provided a baseline for comparison to the oxidation

tests. These tests produced featureless thermographs except for the melting

transition. However, the tests in oxygen demonstrated that the oxides

produced during heating exhibited thermal activity, as shown in figure 11.

Endothermic and exothermic transitions were present, but the temperatures of

the individual transitions could not be firmly established because of their

number and weakness. The weakness of the transitions was due to the small

mass of responding material - a thin layer on the surface of a 15 to 50 mg

disk of alloy.

The presence of liquid and probably gaseous oxides was evident from the

discoloration and eventual destruction of the aluminum oxide sample holders.

Several investigators have postulated or observed the presence of liquid

oxides during the high- temperature oxidation of alloys [5,6]. These

investigators have also observed increased oxidation rates when liquid oxides

were present. This tended to corroborate our observations of apparently

increased oxidation rates at temperatures where some endothermic transitions

occurred.

These studies also corroborate the existence of low-energy exotherms and

the existence of at least one exothermic event as shown in figure 11. These

studies also provided an independent verification that the alloy does autoheat

at temperatures below the solidus temperature of the alloy.

3. Incremental Temperature Scan Tests

The experiments to investigate the tendency of bulk alloy specimens to

autoheat to destruction were performed using the ITS heating procedure. This

procedure applied constant laser power to the top surface of the specimen.

29
^ ‘3aniva3di/M3i iviiN3a3ddia

30

Figure

11.

DTA

thermograph

of

NO7001

alloy

heated

in

oxygen.
The power was increased at intervals after surface temperature equilibrium was

established, until the specimen began to autoheat. The external heating rate

was then held constant until autoheating ceased or combustion occurred.

Figure 12 shows the surface and interior temperature- time waveforms for a

typical experiment.

Plotting of the interior temperature waveform in figure 12 was

terminated where the next data point exceeded the valid temperature range of

the thermocouple, 2040 K. The surface temperature was monitored through

combustion, which exceeded the temperature limits on the plot, and cool-down.

Extreme non-parallelism existed between the surface and interior temperature

waveforms. This indicated that problems were being encountered in measuring

surface temperature in the ITS test mode as were encountered in the LTS test

mode. Consequently, the surface temperature waveform could not be used to

obtain accurate values of Tgp, Tg^, and T^g^-

Endothermic features were present in both the interior and surface

temperature waveforms. However, these features were significantly smaller

than those encountered for many other materials, see [1,2] for examples. The

source of these small features , several of which are shown in figure 13 , did

not appear to affect the rate of increase in temperature as appeared to be the

case with a number of other alloys [1,2].

Due to the unsuitability of the surface temperature waveform, the

interior temperature waveform was used to determine the onset of spontaneous

ignition and enhanced oxidation. However, both the surface and interior

temperature- time waveforms were used to determine the onset of ignition and

combustion.

31

Figure 13, an expansion of a section of figure 12, shows the development

of combustion in greater detail. The linear heating rate from -80 s to Tgp

shown on the interior temperature waveform of figure 13 was not due to

oxidation heating. Instead, it was due to the changing absorptivity of the

oxide surface. The absorptivity heating effect was differentiated from

oxidation heating by tests using preoxidized specimens. These tests

demonstrated that autoheating by oxidation generated a curvilinear

temperature- time waveform rather than a linear temperature- time waveform as

observed from -80 s to Tgp. At Tgp the shape of the interior temperature- time

waveform changed from linear to curvilinear indicating that oxidation heating

was beginning to influence the heating rate. The rate of increase in

temperature immediately after Tgp was approximately 1.9 K/s and increased to

approximately 2.6 K/s at Tg^. The curvature of the waveform section from Tgp

to Tqo is somewhat difficult to observe due to the graph scale and the effects

of several endothermic regions, which are noted on the plot. Small curvature

within a waveform section was detected by orthogonal polynomial curve fits

with the best fit selected by analysis of variance techniques. Tgp was chosen

as the beginning of a clearly established curvature of the interior

temperature- time waveform.

Tgo was chosen as the beginning of a clearly established abrupt change in

curvature of the interior temperature- time waveform. This occurred, as noted

on the plot, in a region where several endo therms have occurred. Curve fits

were used to establish that the slope at Tg^ changed from approximately 2.6

K/s to approximately 3 . 7 K/s . Though speculative , it is though that Tg^ may

lie in a temperature region where the oxidation kinetics change from

protective to nonprotective oxidation.

33
34

Figure

13.

Development

of

ignition

in

3.60

MPa

oxygen

during

an

ITS

test.
After Tgo occurs, the slope of the temperature- time waveform increases

rapidly to a limiting value at Tig^. It is desirable to use the surface

temperature- time waveform to determine since the response time of the

pyrometer was much faster than the thermocouple. However, due to the surface

temperature measurement problems, both the interior and surface temperature

time waveforms were used. As a result, the compromise that had to be reached

between the two waveforms leads to a T^g^ value at a point on the interior

temperature waveform where a small curvature could remain (fig. 13). However,

the curvature was slight or non-existent for higher oxygen pressures (fig.

14) . Because of the effect of oxide surface emissivity on the shape of the

surface temperature waveform and the small remaining curvature that could

result in the interior temperature waveform at the selected T^g^^ value, this

parameter may not always exactly fit our definition of T^g^. However, it was

considered that the value of T^g^ is probably within several percent of the

definition value.

Figure 14 shows the development of ignition at high oxygen pressure,

10.47 MPa (1519 psia) . The higher oxygen pressure has increased the oxidation

rate of the alloy and created a thicker oxide layer. The thicker oxide layer

reduced the measurement problems at the higher oxygen pressure, however, some

measurement uncertainty remained. The example shown was among the best

surface measurement results obtained for this alloy. The nominal 80 K

difference between the surface and interior temperature- time waveforms was

also close to the difference values obtained for other nickel-based alloys.

Figure 14 also shows that combustion, in the form of hot spots, could develop

while the surface temperature was below the solidus temperature.

35
C c

0) o eS

Q-c oo

> w

0) 3

O

O

O

S)

s

OJ

I

>

X

o

(S ta Q Q_

rv 51 Q

Z rv.

C/l Z Q

3 ^

^ 1 h-L-4

a

2>

<x>» e‘"

-fl) i.

- O

0) 1

1

Q S) G3 in C3 in (S

in OJ G9

(SU3

I

C9

CD

I

0)E

>^

* a j n'^.e j admax

36

Figure

14.

Development

of

ignition

in

14.41

MPa

oxygen

during

an

ITS

test.
Table 3 lists the values of the ignition parameters. These parameters

are plotted in figures 15 through 18. The data have been fitted to linear

least- squares curves so that the trends in data with respect to oxygen

pressure could be established. The coefficients of these curves are listed in

table 4. The values of Tgp, Tg^, and T^g^ decrease with increasing oxygen

pressure over the full range of the data. The high pressure tests have been

used to establish an estimate of the surface temperatures. It was estimated

that the surface temperature was probably 80 K greater than the interior

temperature

.

4, Data Accuracy

The surface temperature values of Tgp, Tg^, and T^gj^ are not presented in

this report due to the apparent large inaccuracy of the surface measurement.

The interior temperature data were obtained from thermocouples made from

material certified to conform to IPTS-68. The thermocouple thermoelectric

voltage, E(T)

,

was converted to temperature from a polynomial derived from the

temperature -E(T) table from NBS Monograph 125 [7]. The maximum deviation of

the polynomial value from the table value, in the range of the data, was 0.5

K. The largest uncertainty in the thermocouple data was generated by the

digitizers. This equipment has a stated accuracy of 0.15% full scale, which

was 0.15 mV in the E(T) value. At 1000 K, the error due to the digitizer

would be approximately 14 K. However, the thermocouple/digitizer combination

consistently detected the onset of alloy melting at 1603 K to within ± 6 K or

better with no deviations greater than ± 10 K.

37
Table 3. ITS test series spontaneous ignition, enhanced oxidation, ignition

and combustion temperatures.

Pressure, Tsp.K o

1- 7jgn ' ^ T K ' cmb’ ^ MPa

Interior Interior Interior Surface

1.793 1109 1160 1288 1933

1.813 1157 1188 1303 1921

1.820 1107 1176 1339 1772

1.827 1102 1122 1248 1849

3.544 1119 1150 1271 1909

3.565 1140 1207 1268 1758

3.571 1115 1176 1292 1792

3.599 1086 1181 1283 1715

3.599 1135 1191 1296 1762

5.240 1111 1133 1295 1771

5.323 1139 1190 1353 1765

5.337 1099 1157 1292 1695

5.337 1119 1182 1384 1779

5.357 1102 1144 1335 1674

5.743 1118 1184 1289 1683

7.171 1140 1155 1324 1756

7.191 1122 1182 1316 1739

7.302 1157 1187 1274 1597

7.315 1115 1168 1258 1691

8.639 1130 1158 1233 1549

9.060 1138 1170 1266 1426

10.473 1132 1177 1308 1472

11.018 1084 1126 1204 1509

1 1 .025 1158 1173 1308 1472

12.211 1137 1185 1243 1376

12.218 1176 1183 1257 1422

12.349 1101 1140 1255 1596

12.369 1135 1216 1349 1687

12.411 1138 1213 1322 1429

13.831 1066 1085 1150 1429

13.907 1096 1131 1221 1463

13.934 1106 1159 1210 1468

38
2000

<-o

0)w32oaE

.0)

^o

Oo£o

__ fVJ o

<3

0p

^X>

(S

(S

IN. csz

LO

-- CD

ro QI)

L.3

V)0

i)

CL

Q.

O0

—1

(S (S (S Q o (S

in IS in

>i s j j 3duj3j_

39

Figure

15.

ITS

test

series

spontaneous

ignition

temperatures

vs

oxygen

pressure.
2000

__ CVJ

-- 00

n) CL

2:

(U

1.3

M

M

U

L. CL

5jcs

G)

in

a

G)

G)

in

>j‘sjn-4.EJS duj s j_

40

Figure

16.

ITS

test

series

enhanced

oxidation

temperatures

vs

oxygen

pressure.
2000

-r- ^ ^ o <3> O O

0)3«

0>aEg

a>c

3£>

0)^

?oCO

CO (0 cc ;r O) . •£co

|S

<>

(S

(S

rs. QZ

LHZD

C

D1

0^0

H

H

OJ

-- CD

ro CLZV1.D

V>MV

L. CL

O"

(S

(S (S (S

(S (s (S

in (s in

>^ ‘ a j j sdujsj_

41

Figure

17.

ITS

test

series

ignition

temperatures

vs

oxygen

pressure.
2000

I-

<3e>

<3> ^ oe>

0^0

o o

— LD

%

oo

<>

0»^

0>r^

CO

CO (0 GC ^

O) , •ieo ®PCO

0)t_3

0>a

E

0)u

3

CO o

OJ

-- 03

(0 CLH

V

L.3

V

L. Q_

(S

(S

Q

Z

LO

JOE

u

-I

(S

r

^ .p.— T .If...

1 y

1 y

1.

cs

! “

(S

G) G) G)

ID G) ID

>1' s j j 3dujsj[

42

Figure

18.

ITS

test

series

start-of-combustion

temperatures

vs

oxygen

pressure.
Table 4. Linear least-squares curve fit coefficients for the ITS

Ignition parameter data shown In Figures 15, 16,

and 17.

Ignition

Parameters

Waveform Ai A2

^sp Interior 1.121E+03 -2.519E-02

^eo Interior 1.175E+03 -1.031E+00

"^ign Interior 1 .321 E+03 -5.185E+00

43
CONCLUSIONS

The LTS tests and data indicated that millisecond ignition to combustion

transitions would not occur at oxygen pressures up to 6.9 MPa. In addition,

the trend of the ignition and combustion temperature data versus oxygen

pressure was flat from 5.2 to 6.9 MPa. That trend would indicate that there

would be no tendency for this alloy to have abrupt solid phase ignition to

combustion transitions for oxygen pressures reasonably greater than 6.9 MPa.

The LTS tests and data also indicated that the solid phase oxidation rate of

the alloy was sufficient for the alloy to autoheat to destruction in our

reduced heat loss experimental setup. The autoheating usually began at an

interior temperature that ranged from approximately 1250 to 1290 K. Since the

interior temperature was on the order of 80 K less than the surface

temperature, the 1250 to 1290 K range matches well with the exotherm at

approximately 1340 K from the DTA runs

.

The ITS interior temperature data established that under the appropriate

low heat loss conditions, the oxidation rate of the alloy was sufficient to

cause autoheating to combustion to occur from temperatures below the alloy

solidus temperature. Combustion zones (hot spots) began to develop at oxygen

pressures of 8.62 MPa and greater. General combustion did not begin until the

surface temperatures were greater than the alloy liquidus temperature at all

oxygen pressures. However, this alloy may have a "sudden failure"

characteristic (millisecond transition from ignition to general combustion) at

oxygen pressures greater than our maximum test pressure of 13.9 MPa.

The rate at which general combustion developed after ignition increased

three-fold from an oxygen pressure of 1.7 to 13.9 MPa clearly indicating that

the alloy oxidation rate was oxygen pressure sensitive. The temperature at

44
which Tgp, Tqo, and occurred decreased with increasing oxygen pressure.

That trend indicates that these parameters are a function of the oxidation

rate of the material and that the oxidation rate increased more rapidly with

increasing oxygen pressure than the cooling rate.

FUTURE EFFORT

The program of which this study is a part has recently been expanded to

include the direct determination of oxidation rates of selected alloys. To

achieve this, a thermogravimetric apparatus (TGA) is being constructed. The

apparatus will use a microgram balance housed in a pressure chamber to

directly measure the oxidation rate. The system has been designed to operate

to pressures of 34.5 MPa (5000 psia) and to temperatures greater than 1700 K.

This equipment will be used to generate oxidation rate data as a function of

time, temperature, and oxygen pressure. From these data, the Q(chem) curve can

be determined. Once the Q(chem) curve is known, computer modeling of system

designs and potential abnormal operating conditions can be accomplished and

the most appropriate materials chosen for a given design.

ACKNOWLEDGMENT

The authors express their appreciation to John G. Austin, Jr.

,

retired,

and Richard T. Congo, technical representatives, George C. Marshall Space

Flight Center, for their many valuable contributions to this study and the

overall program and to the George C. Marshall Space Flight Center for funding.

45
REFERENCES

[1] Bransford, James W. , Billiard, Phillip A., Hurley, James A., McDermott,

Kathleen M. , and Vazquez, Isaura, Ignition Characteristics of the Iron

Based Alloy UNS S66289 in Pressurized Oxygen, NISTIR 88-3904, Nov. 1988

[2] Bransford, James W. , Billiard, Phillip A. , Hurley, James A. , McDermott,

Kathleen M. , and Vazquez, Isaura, Ignition Characteristics of the

Nickel-Based Alloy N07718 in Pressurized Oxygen, NISTIR 89-3911, April

1989.

[3] Mellor, A.M.

,

Heterogeneous Ignition of Metals: Models and Experiment,

Ph.D. Thesis, Princeton University, Princeton, N.J., 1968.

[4] Bransford, James W. , Billiard, Phillip A., Hurley, James A., McDermott,

Kathleen M. , and Vazquez, Isaura, The Development of Combustion from

Quasi-Stable Temperatures for the Iron Based Alloy UNS S66286. ASTM SP

986, pp. 146-157 (1988).

[5] Brenner, S.S., Catastrophic Oxidation of Some Molybdenum- Containing

Alloys, J. Electrochem. Soc., Vol. 102, No. 1, p. 16 (1954).

[ 6 ] Rathenau, F.W. and Meijering, J.L., Rapid Oxidation of Metals and Alloy

in the Presence of M0O3 , Metallurgia, Vol. 42, p. 157 (1950).

[7] Thermocouple Reference Tables Based on the IPTS- 68 , Nat. Bur. Stand.

Monograph 125

.

46 U.S. GOVERNMENT PRINTING OFFICE: 1990 - 573-577/43019
BL-114A

(5-90)

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

BIBLIOGRAPHIC DATA SHEET

1. PUBUCATION OR REPORT NUMBER

NISTIR 3947

2. PERFORMINQ ORGANIZATION REPORT NUMBEI

3. PUBUCATION DATE

September 1990

4.

TITLE AND SUBTITLE

IGNITION CHARACTERISTICS OF THE NICKEL-BASED ALLOY UNS N07001 IN PRESSURIZED OXYGEN

5.

AUTHOR(S) James W. Bransford, Phillip A. Billiard

6.

PERFORMINQ ORGANIZATION (IF JOINT OR OTHER THAN NIST. SEE INSTRUCTIONS)

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

BOULDER, COLORADO 60303-3328

7. CONTRACT/GRANT NUMBER

8. TYPE OF REPORT AND PERIOD COVERED

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (STREET. CITY, STATE, ZIP)

NASA/ George C. Marshall Space Flight Center

George C. Marshall Space Flight Center, AL 35812

10.

SUPPLEMENTARY NOTES

11.

ABSTRACT (A 200-WORD OR LESS FACTUAL SUMMARY OF MOST SIGNIFICANT INFORMATION. IF DOCUMENT INCLUDES A SIGNIFICANT BIBUOGRAPHY OR

UTERATURE SURVEY, MENTION IT HERE.)

The development of ignition and combustion in pressurized oxygen

atmospheres was studied for the nickel-based alloy UNS N07001. Ignition of

the alloy was achieved by heating the top surface of a cylindrical specimen

with a continuous -wave CO2 laser. Two heating procedures were used. In the

first, laser power was adjusted to maintain an approximately linear increase

in surface temperature. In the second, laser power was periodically increased

until autoheating (self -heating) was established. It was found that the alloy

would autoheat to combustion from temperatures below the solidus temperature.

In addition, the alloy had a tendency to develop combustion zones (hot spots)

at high oxygen pressures when the incremental (step) heating test mode was

used. Unique points on the temperature - time curves that describe certain

events are defined and the temperatures at which these events occur are given

for the oxygen pressure range of 1.72 to 13.8 MPa (250 to 2000 psia)

.

12. KEY WORDS (6 TO 12 ENTRIES; ALPHABETICAL ORDER; CAPITAUZE ONLY PROPER NAMES; AND SEPARATE KEY WORDS BY SEMICOLONS)

alloys; combustion; ignition; ignition temperature; metal; nickel alloy; oxygen;

oxygen compatibility

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