NISTIR 89-3911

IGNITION CHARACTERISTICS OF THE

NICKEL-BASED ALLOY UNS N07718

IN PRESSURIZED OXYGEN N

James W. Bransford

Phillip A. Billiard

James A. Hurley

Kathleen M. McDermott

Isaura Vazquez

National Institute of Standards and Technology

(formerly National Bureau of Standards)

U.S. Department of Commerce

Boulder, Colorado 80303-3328

April 1989

NISTIR 89-3911

IGNITION CHARACTERISTICS OF THE

NICKEL-BASED ALLOY UNS N07718

IN PRESSURIZED OXYGEN

James W. Bransford

Phillip A. Billiard

James A. Hurley

Kathleen M. McDermott

Isaura Vazquez

Chemical Engineering Science Division

Center for Chemical Engineering

National Engineering Laboratory

National Institute of Standards and Technology

Boulder, Colorado 80303-3328

April 1989

Sponsored by

NASA

George C. Marshall Space Flight Center, AL 35812

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

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, Raymond G. Kammer, Acting Director

CONTENTS

Page

List of Figures v

List of Tables vi

Abstract 1

Introduction 2

Experimental Procedure 2

The Metal Ignition Process 8

Results 17

1. Linear Temperature Scan Tests 17

2. Differential Thermal Analysis Studies 27

3. Incremental Temperature Scan Tests 29

4. Data Accuracy 40

Conclusions 41

Future Effort 42

Acknowledgment 42

References 42

iii

List of Figures

1. Test specimen configuration

2. Experimental setup

3. Test chamber configuration

4. General variation of heat balance parameters

as a function of surface temperature

5. Development of ignition during an ITS test

6. Abrupt ignition and combustion event during an LTS test

7. Typical surface and interior temperature waveforms for an LTS

test

8. Surface temperature waveform features from an LTS test

9. Ignition temperatures vs. oxygen pressure from, the LTS tests....

10. Combustion temperatures vs. oxygen pressure from the LTS tests..

11. DTA thermograph of N07718 alloy heated in oxygen

12. Typical surface and interior temperature waveforms for

an ITS test

13. Development of ignition in 5.488 MPa oxygen during an ITS test..

14. Development of ignition in 14.410 MPa oxygen during an ITS test.

15. Spontaneous ignition temperatures vs. oxygen pressure

from the ITS tests

16. Enhanced oxidation temperatures vs. oxygen pressure

from the ITS tests

17. Ignition temperatures vs. oxygen pressure from the ITS test

Page

4

11

15

18

20

22

25

26

28

30

31

32

36

37

38

V
List of Tables

Page

1. Composition, in percent, of materials selected for the

determination of ignition and combustion characteristics 3

2. LTS ignition and combustion temperatures 24

3. ITS ignition parameter temperatures 35

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

ignition parameter data 39

VI
Ignition Characteristics of the Nickel" Based Alloy

UNS N07718 In Pressurized Oxygen

James W. Bransford, Phillip A. Billiard,

James A. Hurley, Kathleen M. McDermott,

and Isaura Vazquez

The development of ignition and combustion in pressurized oxyg n

atmospheres was studied for the nickel-based alloy UNS N07718. Ignition of

the alloy was achieved by heating the cop 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 destruction from temperatures below the

solidus temperature. In addition, endothermic events occurred as the alloy

was heated,. many at reproducible temperatures. Many endothermic events

occurred prior to abrupt increases in surface temperature and appeared to

accelerate the rate of increase in specimen temperature. It appeared that

the source of some endo therms may increase the oxidation rate of the alloy.

Ignition parameters are defined and the temperatures at which these

parameters 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; metals;

nickel alloys

.

1
Introduction

A number of serious accidents and catastrophic fires have occurred that

involved the ignition and subsequent combustion of metals in oxygen systems.

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

ignition and combustion has also increased. In order to reduce the potential

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

ignition and combustion characteristics of metals is needed. In 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 alloys that have been studied is given in table 1.

This report, the second in a series, gives the results for the nickelbased alloy UNS N07718. The studies involved heating the top surface of

specimens of the material by a continuous -wave (cw) CO 2 laser in pressurized

oxygen. The surface and interior temperatures were recorded as the specimens

were heated. High-speed cinematography of the ignition and combustion events

was 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)

.

Experimental Procedure

Cylindrical test specimens, figure 1, were machined from commercially

available UNS N07718 alloy rod that had a hardness of Rockwell C 24-27. The

rod had a chemical composition of 0.50% Al , 0.003% B, 0.036% C, 0.34% Co,

17.62% Cr, 0.09% Cu, 20.03% Fe

,

0.14% Mn, 2.90% Mo, 51.52% Ni , 0.007% P,

2
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1. Apex to top surface center.

2. Surface finish as specified.

3. 6.35 mm or 8.00 mm dia.

as required.

Figure 1. Test specimen configuration.

4
0,001% S, 0.06% Si, 0.92% Ti, 5,11% Cb + Ta. Specimen dimensions were 6.3 mm

(0.25 in) in diameter by 6.3 mm (0.25 in) in height for the linear

temperature increase tests and 8.0 mm (0.31 in) in diameter by 6.3 mm (0.25

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

hole was drilled axially to within 0,25 mm (0.010 in) + 0.05 mm (0.002 in) of

the top surface. The top surfaces of the specimen were maintained in the as

machined finish to improve the absorptivity of the surface to the laser beam

at the beginning of an experiment and to give a diffuse laser beam reflection

which would minimize potential heat damage to the chamber liner.

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, created splatter which destroyed the laser beam window after

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

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

beginning of specimen combustion. This pressure increase rarely exceeded the

initial chamber pressure by 10 percent.

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

block and was insulated from the block by packed aluminum oxide powder. This

assembly was placed upon a ceramic foam pad which was then placed on a

pedestal as shown in figure 2.

The top surface of the specimen was heated by a cw CO 2 laser beam. The

laser had been detuned from the pure TEMqq mode so as to contain higher order

modes. This procedure distributes the laser power more uniformly across the

laser beam and heats the specimen more uniformly. The laser beam was focused

5
Figure 2. Experimental setup.

6
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 gas convection currents rising from the

heated 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 a 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 autoheat to destruction. This procedure was called the

Incremental Temperature Scan (ITS) heating procedure.

The principal parameters in this study were the surface temperature and

interior temperature of the specimen. 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

7
of 15 K/mV (27°F/mV) . These signals were recorded by a digital oscilloscope

which used 15 bit analog to digital (A/D) converters. These converters have

a maximum signal voltage resolution of 6.25 //V. 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 windows

,

figure 3, at 45° from the vertical. The specimen surface observed within the

pyrometers' circular measurement field of view 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 using 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

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.

The Metal Ignition Process

A detailed description of the ignition process of metals requires a

significant number of parameters; however, a general understanding of the

8
Laser Beam

o

Polycrystalline

NaCI Window

Observation

Windows (4)

Sapphire

Window

Windows

(4)

Fire Wall

Experiment Platform

Figure 3. Test chamber configuration.

9
process can be obtained by considering a generalized heat balance for an

oxidizing surface, figure 4. The parameters that need to be incorporated

are

1. Q(loss) - the 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) - the rate at which oxidation of the alloy generates

heat

;

• • • 4. Q(in) - the sum of Q(ext) and Q(chem)

.

To further simplify the process, Q(ext) will be assumed a constant, but

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

•Q(chem) are functions of the surface temperature and other parameters wViich

are constant for this discussion.

An initial application of Q(ext)

,

which is less than that required to

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

than Q(loss)

.

The surface temperature, Tg , will then increase to some

equilibrium value Tgq, as shown in figure 4a. Since the slope of the Q(in)

curve is less than the slope of the Q(loss) curve at Tgq, the oxidizing

surface temperature cannot increase beyond Tgq because the increase in

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

greater than Tgq. Thus, a stable surface temperature is reached. However,

the stability over time will depend upon whether the surface is undergoing

time dependent or time independent oxidation. If time dependent oxidation is

occurring, Q(chem) will decrease with time and the oxidizing surface

temperature will also decrease with time. If time independent oxidation is

10
11
occurring, Q(chem) will remain constant and the oxidizing surface temperature

will remain constant if all other parameters remain constant.

As Q(ext) is 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(loss) curve, figure 4b. This equilibrium

temperature is defined as the spontaneous ignition (or critical) temperature,

• • Tgp [1]. Since Q(chem) and Q(loss) both increase with increasing

• • temperature, the slopes of the Q(in) and Q(loss) curves may be approximately

equal over a fairly wide temperature range. We have previously described

this range as a quasi-stable temperature region [2]. The behavior of an

alloy within this region is of considerable interest. Small shifts in other

parameters which change any of the heat gain or heat loss parameters, can

either cause the oxidizing surface to cool or to autoheat. Several

parameters may be defined for the case when Q(in) is sufficiently large for

spontaneous ignition to occur:

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

which dQ(in)/dTg = dQ(loss)/dTg

;

2. Ignition temperature, Tj^g^--the temperature at which

• • [Q(in) -Q(loss)

]

is maximum;

3. Flame temperature, Tf--the maximum temperature that can be reached

by the oxidizing surface.

These parameters are shown in figure 4b.

An equilibrium surface temperature greater than Tgp may be established

by decreasing Q(ext) once the surface temperature has exceeded Tgp. However,

such an equilibrium surface temperature must be considered unstable because

12
the tendency is for the alloy to autoheat since the slope of the Q(in) curve

is greater than the slope of the Q(loss) curve.

It is useful to consider the effect of physical state on the 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 may be definable.

However, defining an ignition temperature for the liquid phase in our

experiments was difficult because of the effects of fluid flow. When an

oxidizing fluid flows, the rate of oxygen transport to unoxidized material

may be significantly increased for a short time interval. This in turn

increases the rate at which material is consumed and significantly increases

the surface temperature for the flow interval. The net result of the fluid

movement is to disrupt the smoothness of the Q(chem) curve, which makes it

difficult to separate movement enhanced oxidation effects from quiescent

oxidation effects. For this reason, liquid phase ignition temperatures were

not determined for the ITS heating procedure (with the exception of the

temperature at the beginning of combustion, ; only solid phase ignition

parameters were determined.

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

Q(loss) curve until Tf is reached, figure 4c. In this case Tgp is not

obtainable from the experiment; however, and Tf are. "^ign not

differ in this case from the case shown in figure 4b, This is due to the

definition of as being the temperature at the maximum deviation between

• • the Q(in) curve and the Q(loss) curve. This means that our experiments,

which use the ITS heating procedure, can be thermally overdriven without

seriously affecting the value of Tfg^. This statement assumes that the

13
excess external heat input is slightly more than the minimum required to

achieve ignition and not orders of magnitude more.

• • Since the exact shape and magnitude of the Q(in) and Q(loss) curves are

not known., the determination of the ignition parameters must be based upon

other meas\irable parameters. In our case the surface and interior

temperatures were used.

As the surface temperature increases, the developing oxide layer

emissivity changes, usually increases. In our tests in which a laser was

used as the external heat source, this changing emissivity may result in

increased heating at a constant laser power input without a corresponding

increase in radiative cooling.

To determine the spontaneous ignition temperature, an apparent changing

emissivity effect to specimen heating had to be separated from the oxidation

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

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

showed that the contribution from the apparent changing emissivity to the

specimen heating rate, under constant laser power, was linear or nearly

linear [3]. Since the oxidation rate of metals and alloys is nonlinear with

tem.perature , spontaneous ignition was assumed to have occurred when the

temperature - time waveform became nonlinear under constant laser power input,

figure 5. The temperature at the beginning of the nonlinear waveform segment

was taken as the spontaneous ignition temperature.

We have defined an additional ignition parameter that is specific to our

data. This parameter defines the beginning of a region on the ITS

temperature- time waveform where an abrupt change in curvature occurs prior

to ignition, figure 5. The region generally contains a number of abrupt

14
UN5

N07718

>{

* a j rv^E j adujaj_

15

F

igure

5

Develop

n

»fc'r5l;

of

Irjiiition

during

an

ITS

tes
changes in temperature which appear in conjunction with endothermic features.

Since many endothermic events are associated with the abrupt temperature

changes, we postulated that the source of these events increased or enhanced

the oxidation rate of the alloy. Thus, the temperature at the beginning of

this region has been called the enhanced oxidation temperature, T^q, and

appears to be associated with the beginning of the rapid buildup of the oxide

layer

.

The ignition temperature was the most straightforward parameter to

determine since the definition of this parameter implies the existence of a

maximum heating rate of the specimen. This was assumed to have occurred when

the maximum rate of change in surface temperature occurred, i.e., [Q(in) -

•Q(loss)] is maximum when dTg/dt is maximum. was the beginning of the

region of maximum rate of change in surface temperature, figure 5.

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

• • and Q(loss) curves. The Q(loss) 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 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 (time dependent) to nonprotective (time independent) . When either

• • Q(in) or Q(loss) curve is changed, the values of the ignition parameters will

also change. In general, the lower the heat transfer rate from the oxidizing

surface, the lower the temperatures at which Tgp , TgQ , and will occur;

conversely, the greater the heat transfer rate, the higher the temperature at

which Tgp, TgQ, and Tj^gj-^ will occur.

16
Under certain experimental conditions and/or at elevated oxygen

pressures, combustion may develop abruptly before the above-defined

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

cracking or spalling of the surface oxides which expose unoxidized or

partially 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. This may result in immediate combustion or

in a small combustion region which rapidly expands. This type of event has

also been called ignition, thus creating an unavoidable dual definition for

TignThe terms "combustion" or "combustion region" are used throughout this

report. This terminology usually implies evaporation and vapor phase

oxidation of the fuel material. For the N07718 alloy, vapor phase combustion

does not occur. For this alloy, and most others listed in table 1,

combustion is the fluid- flow- enhanced oxidation of the liquid alloy. This

type of fluid oxidation generates very high temperatures, 2500 K and higher,

and develops in milliseconds as the specimen collapse. The beginning of the

development of these high temperatures is called the combustion temperature,

Tcmb > shown in figure 6.

Results

1. Linear Temperature Scan Tests : The initial experiments in the

evaluation of the ignition characteristics of the N07718 alloy were carried

out using the LTS heating procedure. Heating rates of approximately 100 to

200 K/min were used in order to determine whether the alloy had any tendency

to develop abrupt solid phase combustion events similar to the liquid phase

17
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18

Figure

6.

Abrupt

ignition

and

combustion

event

during

an

LTS

test.
combustion event shown in figure 6. Experimental data were obtained for the

oxygen pressure range of 0,7 to 13.8 MPa (100 to 2000 psia) . The results for

a typical experiment are shown in figure 7. The waveform shows two major

features. The first is the rapid increase in the rate of change in

temperature at approximately -52 s and the second is the abrupt ignition with

immediate combustion above the liquidus temperature. Negative time in our

data indicates time prior to the start of combustion or an arbitrary time

afterward. Not as obvious, due to the compression of the data, are

endothermic (constant temperature) features in the waveform, examples of

which are shown in figure 8. These features generally had a short duration

due to the temperature scan rate used in the experiments.

The rapid increase in the rate of change in temperature in figure 7

,

starting at approximately -52 s, appeared to be due to an increasingly rapid

rate of oxidation of the alloy which caused the specimen to autoheat faster

than control of the heating ramp could offset. However, as laser power was

continuously reduced and the oxide layer thickness increased (which reduces

heat transfer to the oxidation zone from the laser) , the heating ramp was

eventually reestablished (although at a different rate). In many tests, the

autoheating appeared to develop in conjunction with endothermic features in

the waveform. Endothermic events also appeared at other temperatures and

often appeared to increase the rate of change in specimen temperature. Also,

the endothermic features were often associated with the development of

temperature spikes, exothermic events, ignition, and the beginning of

combustion

.

The oxides produced during oxidation were significantly less dense than

the alloy. The density difference created considerable mechanical stress

19
UNS

N07718

QQ

in

ru

vt

Q)E

I

‘ a j j aduiaj^

20

Figure

7.

Typical

surface

and

interior

temperature

waveforms

for

an

LTS

test.
which was released by cracking and spalling of the oxide surface. This

cracking and spalling produced small particles and/or structures that could

be heated to very high temperatures before they would vaporize or melt,

adhere to the substrate oxide layer and cool to the baseline temperature.

The net effect of the particle and/or structure heating and cooling was to

produce a rapid increase, then decrease, in apparent surface temperature,

which we have called a temperature spike, as shown in figure 8. These spikes

did not appear to have any permanent effect upon the baseline temperature or

the rate of change in the baseline temperature. Therefore, the spikes are

not considered to have had an effect upon the oxidation rate of the alloy.

The series of spikes noted in figure 8 (others are also present) were

probably produced by the breakup of a fairly large piece of oxide.

Another surface temperature waveform feature had the appearance of a

fast exothermic event with limited energy as shown in figure 8. (In this

report we differentiate between exothermic features (events) , which occur in

milliseconds to several seconds and have an apparently small energy release,

and the much slower exothermic oxidation process or the exothermic combustion

process, which has a massive energy content.) These events appeared to

initiate permanent changes in the baseline temperature or increases in the

rate of change in surface temperature. These events were usually abrupt,

occurring within milliseconds to several hundred milliseconds and were often

preceded by an endothermic feature. Exothermic events are considered to be

caused by small rapid changes in oxidation rate of the alloy.

At an oxygen pressure of 0.7 MPa (100 psia) , the development of

combustion from ignition occurred only after significant melting of the

specimen. However, at an oxygen pressure of 1.7 MPa (250 psia) or greater,

21
in1

CD

r-OZ

inzz

O)O)c

03 cco_c

V-*O)L

XXo

n3 Q.Z

C^J X %ou LO c 00 LU ^ l 0)

132

C/3 CD

CDQ. q. E CO CD 1-I x: o

"Oc

LU

CO

CD JZoX

LU

CJ

in

Vi

(U in E

GJ

U3!

in

U3i G3 CD CO E3 ts C3

in o CO

Q in tS3

O'- CD in in OJ £3 » ‘ s j n':).^ j adiuaj^

22

Figure

8.

Surface

temperature

waveform

features

from

an

ITS

test.
combustion developed after ignition within milliseconds to several seconds

and with minimal melting of the specimen, the second major feature of

figure 7.

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

figure 9. Figure 10 shows the combustion temperatures, that is, the

temperatures at which the specimen began to rapidly melt and fluid flow

enhanced oxidation began; these data are also listed in table 2. As seen in

figure 9, the surface temperatures at ignition lie above the liquidus

temperature of the alloy for oxygen pressures up to 6.9 MPa (1000 psia) . At

oxygen pressures greater than 6.9 MPa the ignition temperatures began to fall

into the melting range, and at oxygen pressures greater than 9.3 MPa (1350

psia) they fall predominantly below the solidus temperature.

It was clear that exothermic events occurred while the alloy was in the

solid state and ignition began to develop from these events at oxygen

pressures greater than 9.3 MPa (1350 psia). These events released sufficient

heat to increase the baseline temperature or the rate of change in baseline

temperature following their occurrence. We concluded that the exotherms were

due to a rapid change in oxidation rate of the alloy. Since endotherms

,

probably melting transitions, are often involved, we assumed that the

oxidation rate of the alloy was affected by the source of these events.

However, the source has not yet been established. As the alloy specimen

began to melt, abrupt ignition to combustion transitions occurred. The

transitions were thought to occur as the upper surface cracked or ruptured

due to the expansion of the liquefying alloy. It also appeared that the rate

of oxidation of the alloy increased significantly as the surface temperature

entered the 1300-1450 K temperature range. The rate of the temperature

23
Table 2. LTS ignition and combustion temperatures

1

1 Pressure 1 "^ign, K 1

1 "^cmb > ^ 1

1

1

1 MPa

1

1 Surface Interior

1

1 Surface Interior

1

i

1 0.689 1 1718 1854 1 1752 1907 1

1 0.689 1 1642 1819 1 1644 1850 1

1 1.724 1 1670 1811 1 1692 1876 1

1 1.724 1 1696 1851 1 1733 1970 1

1 1.724 1 1734 1813 1 1780 1897 1

1 1.724 1 1685 1815 1 1685 1815 1

1 1.724 1 1606 1750 1 1647 1903 1

1 1.834 1 1667 1787 1 1676 1802 1

1 3.447 1 1620 1729 1 1620 1729 1

1 3.447 1 1652 1770 1 1652 1770 1

1 3.447 1 1637 1722 1 1678 1785 1

1 3.447 1 1575 1731 1 1575 1731 1

1 3.447 1 1603 1793 1 1603 1793 1

1 3.447 1 1542 1697 1 1542 1697 1

1 5.171 i 1594 1685 1 1594 1685 1

1 5.171 1 1539 1667 1 1539 1667 1

1 5.171 1 1541 1730 1 1541 1730 i

1 6.895 1 1494 1667 1 1494 1667 1

1 6.895 1 1599 1726 1 1598 1749 1

1 6.895 1 1633 1727 1 1633 1727 1

1 6.895 1 1549 1656 1 1554 1692 1

1 9.080 1 1524 1630 1 1524 1630 1

1 9.308 1 1539 1633 1 1539 1633 1

1 9.356 1 1526 1696 1 1526 1969 1

1 9.370 1 1487 1580 1 1509 1603 1

1 10.342 1 1431 1580 1 1431 1580 1

i 11.011 1 1412 1507 1 1500 1617 1

1 11.052 1 1340 1460 1 1430 1591 1

1 11.052 1 1213 1371 1 1357 1622 1

1 11.625 1 1291 1433 1 1397 1623 1

1 12.507 1 1380 1548 1 1383 1572 1

1 12.735 1 1206 1336 1 1322 1465 1

1 12.900 1 1339 1482 1 1387 1568 1

1 13.004 1 1254 1375 1 1314 1544 1

1 13.700 1 1321 1445 1 1321 1445 1

1 14.162 1 1412 1569 1 1461 1642 1

1 14.465 1 1492 1696 1 1492 1696 1

1 14.610 1 1368 1498 1 1403 1555 1 -L 14.644 1 1281 1420 1 1260 1564 -L

24
2000

C3 Q

Q CD Q in

LD

CD

TT

G)

a

CL2:

<u

L.J

M

M

U

L. CL

>1

‘ a J J adwaj^

25

Figure

9.

Ignition

temperature

vs.

oxygen

pressure

from

the

LTS

tests.
2000

U3

CD

m

CLz:

0)

L.3

V>M

0)L. OL

GJ

>1 ' a j rv;.E j sduus_L

26

Figure

10.

Combustion

temperatures

vs.

oxygen

pressure

from

the

LTS

tests.
increase in this range suggested that the alloy could autoheat to

destruction. Because of this tendency, a series of experiments were run to

investigate this phenomenon. Also, standard differential thermal analysis

(DTA) studies were made on oxidizing samples of the alloy.

2 . Differential Thermal Analysis Studies :

The appearance of features in the temperature- time waveform from the LT

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 15-50 mg specimens heated to

temperatures greater than the lower melting range of the alloy.

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 considerable thermal activity, as shown in

figure 11. This figure is a composite of two runs. It is occasionally

necessary to combine several runs to obtain an overall view of the events

which occur over an extended temperature range. 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

27

rn
ATMOSPHERE:

OXYGEN

{y{) dLuei |B!IU0J0U!G

c

<uO)

X

o

c

D

(1)

<0

d>

O)

o

£

o£

<

I-Q

o

3

O)

28

TEMPERATURE

(K)
liquid oxides during the high- temperature oxidation of alloys [4,5]. 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 many endothermic transitions

occurred.

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

the existence of at least one high-energy 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 the alloy to autoheat to

destruction were performed using the ITS heating procedure. This procedure

applied a constant laser power to the specimen which 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. These

waveforms show more clearly the endothermic and exothermic features that were

observed in the waveforms from the LTS experiments . Only a few features are

noted. The endothermic and exothermic features and their effects on the

baseline temperature and rate of change in the baseline temperature are more

clearly shown in figures 13 and 14; only the more prominent features are

noted. These figures are expansions of waveform sections, chosen for their

clarity, from several experiments. The converging slopes of the surface and

interior temperature waveforms of these figures also show that the surface

temperature measurement was adversely affected by the oxide surface, probably

29
>j

‘ a j j adujax

30

Figure

12.

Typical

surface

and

interior

temperature

waveforms

for

an

ITS

test.
inOJ

I

0

in

1

in

Q

CD

M

0) E

H

‘ a j n'^.E j adujaj_

31

Figure

13.

Development

of

ignition

in

5.488

MPa

oxygen

during

an

ITS

test.
Uf^S

M077

1B

Q)D)C

03 DCO)cL

XO

fO CLQ o

IDc

LU

CD. k. ' 2

CDCLE

CD

CDO

03 t:d

CO

w CO

CO

CD$o

Cl

LO

k_CD CD CO Cd i5

II c

03 :d|'o 00coo d

ro

k_CDO.E

CD H

0)c

in

-h oj

GJ

v>

03 E3 E

in

in

rv1 Q

CD

Q cn CO C3 G3

a

in o COa inQ

r- CD LO in CVl4 Q

)j ‘ a j n-i^B j adui3j_

32

Figure

14.

Development

of

ignition

in

14.410

MPa

oxygen

during

an

ITS

test.
due to the surface emissivity. However, the error introduced was minimal at

temperatures equal to or greater than Tgp . Because the error was minimal

(approximately 1 to 1.5 percent at Tgp and less at higher temperatures) the

surface temperature data for the ignition parameters was not adjusted.

Figure 13 shows a rate of increase in temperature of approximately 0.5

K/s (interior temperature waveform) after the last power increment. This

initial autoheating was due primarily to the increasing absorptivity of the

surface to the laser radiation [3]. An oxidation component to the heating

rate was also present but very small. However, as the specimen temperature

increased the oxidation component to the heating rate steadily increased at a

nonlinear rate. The oxidation component ultimately became the principal heat

source driving the increase in specimen temperature at the spontaneous

ignition temperature. The slight curvature in the interior temperature- time

waveform between -100 s and Tgp was thought to be due to the increasing

oxidation rate component to the heating rate. Tgp was chosen as the

beginning of the interior temperature- time waveform section where it appeared

clear that oxidation heat input to the system was the dominant heat source in

increasing the specimen temperature. This was the beginning of the clearly

nonlinear section of the interior temperature- time waveform.

Figure 14 shows the effect of increased oxygen pressure on the rate of

increase in temperature. As can be seen the rate of temperature increase

prior to Tgp was approximately the same as in the much lower oxygen pressure

experiment shown in figure 13. This was additional evidence that the heating

rate at temperatures below Tgp was due primarily to increasing absorptivity

of the developing oxide layer to the laser radiation. Also, the rate of

increase in temperature after was significantly greater than the values

33
shown in figures 12 and 13. This figure also shows that combustion may begin

while the alloy is in the solid state, or at least develop faster than the

data acquisition rate of 0.1 second per data point used in this experiment.

After spontaneous ignition was achieved, the rate of change in specimen

temperature began to slowly increase. This increase was usually followed by

several exothermic features (step increases in the baseline temperature)

which were associated with endothermic features or features which may be weak

endotherms . The result of these apparent exothermic events was an abrupt

change in curvature of the temperature - time waveform. An attempt was made to

quantify the onset of this abrupt change in curvature by relating it to the

temperature of features in the temperature- time waveform. Since it is our

premise that the exothermic features are associated with rapid changes in the

oxidation rate of the alloy, this temperature was called the enhanced

oxidation temperature, T^q. After T^q occurred the rate of change in

temperature continued to increase until a maximum rate of change was reached.

The temperature at which this occurred was considered to be the temperature

• • at which the maximum deviation between the Q(in) and Q(loss) curves occurred

for the solid phase oxidation. Thus, this temperature was taken as TignThe values of the ignition parameters -Tgp , T^q and Tign” listed in table

3 and plotted as a function of oxygen pressure in figures 15, 16, and 17.

The data were fitted to linear least- squares curves so that trends in the

data, with respect to oxygen pressure, could be established. The slopes of

the trends are noted on the plots and the linear fit parameters are given in

table 4.

The values of the ignition parameters decreased with increasing oxygen

pressure in a linear trend over the oxygen pressure range of the data.

34
Table 3. ITS ignition parameter temperatures

1

1

Pressure 1 T 1 ^sp >

1

K T^eo

»

K Tig, K 1

1

1 MPa 1 Interior Surface Interior Surface Interior Surface 1

1 1.834 1 1255 1309 1299 1359 1373 1450 1

1 1.834 1 1294 1337 1345 1384 1391 1437 _L

1 1.868 1 1262 1312 1317 1360 1368 1423 1

1 3.627 1 1324 1363 1441 1

1 3.640 1 1261 1316 1284 1341 1347 1428 1

1 3.654 1 1305 1362 1435 1

1 5.378 i 1265 1305 1294 1335 1353 1395 1

1 5.440 1 1245 1321 1263 1331 1321 1385 1

1 5.488 1 1236 1283 1288 1330 1324 1364 1

1 5.495 1 1299 1331 1362 1

1 7.226 1 1205 1298 1262 1330 1294 1360 1

1 7.253 1 1240 1294 1291 1330 1319 1354 1

1 7.260 1 1238 1291 1303 1337 1357 1399 1

1 8.991 1 1233 1308 1295 1361 1327 1394 1

1 9.060 1 1255 1315 1272 1330 1333 1392 1

1 9.101 1 1248 1291 1291 1340 1367 1414 1

1 10.832 1 1204 1299 1266 1330 1299 1372 1

1 12.169 1 1241 1326 1269 1330 1307 1353 1

1 12.493 1 1208 1318 1248 1353 1276 1372 1

1 12.686 1 1266 1313 1279 1324 1325 1374 1

1 14.348 1 1134 1251 1185 1311 1256 1387 1 -L 14.410 1 1222 1331 1248 1347 1294 1387 -L

35
Melting

Range

LD

03 Ql

<D

k_13ro

k_CD

C3- E

0) lCD

CD

03 TI3

C/5

I <*> <3

r\j

CD

k_ fX. 3

r^-

"co X—CD S

Q.

E

CD

z:

H m CL

o z CO

Q)C

z hfO CLH

OJD

(/) (U

i- D_

(

1 1 1 4 i—j 1 1 ^

1

1

1 1 1 1 \- Q

03 CO Q O COS S s

S CD IS s

(S s in

rvj —

>j

‘ 3jn:).EjadLU3_L

36

Figure

15.

Spontaneous

ignition

termperatures

vs.

oxygen

pressure

from

the

ITS

tests
Melting

Range

CD

O0 <J

03 D_ o

;><j<

C\J

<‘> \

03 a.

CD

CDLD

<1

<00 v:

yk4:^

(D

k_D

TO

k_

0)Q.E

CD

03O

03 t;

13

03

roi03Q.E

03 H

CO

— { I <J><]

n-sm

Z3 o

cu

t\i

-f- CD

(ti D_

2;

OJ

ti/i <u

i- CL S s CD COS s S Q o s S

(S CD 'flmQ in

rcj

—

^j'sjrv^isjs dm 3_L

37

Figure

16.

Enhanced

oxidation

temperatures

vs.

oxygen

pressure

from

the

ITS

tests.
Melting

Range

L CD

13

ro

k—(DQ.

E

0)

o>3

c5

0)a.E

CD

CD^ .2

-2

I

I 0 <]

fN- r-s

01

Z

Z

c

CD

ID

OJ

CD

rtJ 0_

(U

L.D

t/) M

(U

Q_

qS

CT> COS) S) S) S o S) S)

s CD S) LD

rvj — >—

‘ 3 J nq.E J 3C:luU3J_

38

Figure

17.

Ignition

termperatures

vs.

oxygen

pressure

from

the

ITS

tests.
Table 4. Linear least- squares curve fit coefficients for

the ITS ignition parameter data

1

1

1

Ignition

Parameter 1 Waveform

1

1 Ao

1

1

1

1 Al

1

1L_

Pressure |

1L

1

1 ^sp

1 Surface 1 1.313

I E+03 11 -7.321 E-01 1

1 1.83 < P < 14.41 1

1

1

1

1 Interior i 1.279

1 E+03 1

1 -5.170 E+00 1L_

tl

1

L

1

1 ^eo

1 Surface 1 1.359

1 E+03 11 -2.339 E+OO 1

1

tt

1

1

1

1

1 Interior 1 1.326

1 E+03 1

1 -5.913 E+00 11

IT

1

1

1

1 Tign

1 Surface 1 1.429

1 E+03 11 -4.595 E+00 11

tl

1

1

! i1 Interior 1 1.382

J

E+03 1

L

-6.742 E+00 1L_

Tt

1

L

39
However, this decrease was not large. Also, the surface temperature diverged

from the interior temperature for each ignition parameter. This was probably

due to an increasingly thicker oxide layer which developed as oxygen pressure

increased and impeded heat transfer to the interior.

4 . Data Accuracy :

The surface temperature values of Tgp , T^q, Tign^ '^cmb contained in

this report were determined with a commercial two-color ratio pyrometer with

an inherent accuracy of 1 percent when measuring black or gray body

temperatures. This V7as verified by calibration using a NIST standard lamp.

The assumption made in two-color ratio pyrometry is that the change in

emissivity over the measurement wavelength band is negligibly small.

However, oxidizing surfaces are notorious for the variation in the value of

emissivity with wavelength. To determine whether or not a pyrometer

measurement problem existed, the difference between the surface temperature

and interior (thermocouple) temperature was calculated. If this difference

was constant or changed negligibly, then the pyrometer measurement was taken

to be within its error limit of 1 percent. For N07718 a clear surface

temperature measurement error existed for the ITS data but not for the LTS

data. This error decreased as the surface temperature approached Tgp and was

minimal or disappeared at temperatures greater than Tgp. It was estimated

that the surface temperature values of Tgp were high with a maximum bias of 1

to 1.5 percent. The values of T^q and Tj|^g^ do not appear to contain any

significant bias and are assumed accurate to within the 1 percent accuracy of

the instrument.

An exact error value cannot be placed on each numeric value of the

ignition parameters. However, we consider that the data presented accurately

40
represent the temperature of the central region of the specimen where the

oxidation events occurred to within the limits discussed above.

Conclusions

The LTS data indicated that ignition could occur at temperatures below

the solidus temperature for oxygen pressures greater than 11 MPa (1600 psia)

.

Combustion occurred predominantly within or above the melting range.

However, several combustion events began at temperatures below the solidus

temperature at oxygen pressures exceeding 12 MPa (1750 psia) . Therefore,

this alloy has a "sudden failure" characteristic under this test mode if the

oxygen pressure is sufficiently high.

The ITS data established that (under the appropriate experimental

conditions) the oxidation rate of the alloy was sufficient for the alloy to

autoheat to destruction, from temperatures below the solidus temperature.

This was independently verified by the DTA data. These data also indicated

that certain oxides produced during heating undergo phase changes, probably

melting, at various temperatures. As some phase changes occur, apparent

increases in oxidation rate of the alloy also occur.

The time required for the development of combustion after ignition

decreased as oxygen pressure increased. Some combustion events developed

below the solidus temperature at oxygen pressures of 10.3 MPa (1500 psi) and

greater. The transition times from ignition to combustion at these pressures

were so short that the alloy may be considered to have a "sudden failure"

characteristic under this test mode. The temperatures at which the ignition

parameters occurred decreased with increasing oxygen pressure in a linear

manner. This indicates that these parameters are a function of the oxidation

41
rate of the alloy and that the oxidation rate increases more rapidly with

increasing oxygen pressure than the cooling rate for the ITS test procedure.

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) , which will use a

microgram balance housed in a pressure chamber to directly measure the

oxidation rate, is being constructed. 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.

Acknowledgment

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

representative, George C. Marshall Space Flight Center, for his many valuable

contributions to this study and the overall program and to the George C.

Marshall Space Flight Center for funding.

References

[1] Mellor, A.M.

,

"Heterogeneous Ignition of Metals: Model and Experiment."

Ph.D. thesis, Princeton University, Princeton, NJ , 1968.

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

Vazquez, Isaura, The Development of Combustion from Quasi -Stable

Temperatures for the Iron Based Alloy UNS S66286; ASTM SP 986, pages

146- 157 (1988).

42
[3] Bransford, James W. , et al . , Ignition Characteristics of the Iron Based

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

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

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

[5] Rathenau, G.W. and Meijering, J.L., Rapid Oxidation of Metals and

Alloys in the Presence of M0 O 3 , Metallurgia, Vol. 42, p. 167 (1950).

43
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NBS-n4A (REV. 2-80

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

4. TITLE AND SUBTITLE

1. PUBLICATION OR

REPORT NO.

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

NISTIR 89-3911 Ap r i 1 19 8 9

Ignition Characteristics of the Nickel-Based Alloy

UNS N07718 in Pressurized Oxygen

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

Kathleen M. McDermott, and Isaura Vazquez

6. PERFORMING ORGANIZATION (If joint or other than NBS, see in struction s) 7. Contract/Grant No.

National Institute of Standards and Technoloov

N-/V-T-tCM»hA-L-&W-REA-U-eF-S'FAWi)-AR&SDEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234

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

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 gnificant

bi bl iography or literature survey, mention it here)

The development of ignition and combustion in pressurized oxygen

atmospheres was studied for the nickel-based alloy UNS N07718.

Ignition of the alloy was achieved by heating the top surface of a

cylindrical specimen with a c ont inuous - wave CO 2 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 destruction from temperatures below the solidus

temp e r a t ur e

.

In addition, endothermic events occurred as the alloy was heated, many

at reproducible temperatures. Many endothermic events occurred prior

to abrupt increases in surface temperature and appeared to accelerate

the rate of increase in specimen temperature. It appeared that

the source of some endotherms may increase the oxidation rate of the

alloy. Ignition parameters are defined and the temperatures at which

these parameters occur are given for the oxygen pressure range of 1.72

to 13.8 MPa (250 to 2000 psia).

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) alloys; combustion; ignition; ignition temperature; metals; nickel alloys

13. AVAILABILITY 14. NO. OF

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