NATIONAL INSTITUTE OF STANDARDS &

TECHNOLOGY

Research Information Center

Gaithersburg, MD 20899

NEW NIST PUBLICATION

April 20, 1 98?

N I STIR 88-3904

IGNITION CHARACTERISTICS OF THE

IRON-BASED ALLOY UNS S66286

IN PRESSURIZED OXYGEN

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

November 1988

Stimulating America's Progress 1913-1988

NISTIR 88-3904

IGNITION CHARACTERISTICS OF THE

IRON-BASED ALLOY UNS S66286

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

November 1988

Sponsored by

George C. Marshall Space Flight Center

Marshall Space Flight Center, Alabama 35812

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secretary

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, Ernest Ambler, Director

CONTENTS

Page

List of Figures v

List of Tables v ^

Abs tract 1

Introduction 2

Experimental Procedure 2

The Metal Ignition Process 8

Results 18

1. Linear Temperature Scan Tests 18

2. Differential Thermal Analysis Studies 26

3. Incremental Temperature Scan Tests 27

4. Data Accuracy 38

Conclusions 38

Future Efforts 39

Acknowledgement 40

References 40

Appendix 41

iii

List of Figures

Page

1. Test specimen configuration 4

2. Experimental setup 6

3. Test chamber configuration 9

4. General variation of heat balance parameters as a function

of surface temperature 11

5. Development of ignition during an ITS test 15

6. Abrupt ignition and combustion event during an LTS test 17

7. Typical surface temperature waveform from an LTS test 19

8. Surface temperature waveform features from an LTS test 20

9. Ignition temperatures vs. oxygen pressure from the LTS test... 23

10. Combustion temperatures vs. oxygen pressure from

the LTS tests 25

11. DTA thermograph produced by heating a specimen of UNS S66286

alloy in oxygen 27

12. Typical surface and interior temperature waveforms from

an ITS test 29

13. Development of ignition in 3.57 MPa oxygen from an ITS test... 30

14. Development of ignition in 11.94 MPa oxygen from an ITS test.. 31

15. Spontaneous ignition temperature vs. oxygen pressure from

the ITS tests 35

16. Enhanced oxidation temperature vs. oxygen pressure from

the ITS test 36

17. Ignition temperature vs. oxygen pressure from the ITS test.... 37

Al

.

Development of ignition using a preoxidized specimen 42

v
List of Tables

Page

1. Materials selected for the determination of ignition and

combustion characteristics 3

2. LTS ignition and combustion temperatures 24

3. ITS ignition parameter temperatures 34

vi
Ignition Characteristics of the Iron-Based

Alloy UNS S66286 in Pressurized Oxygen

James W. Bransford, Phillip A. Billiard, James A. Hurley,

Kathleen M. McDermott, and Isaura Vazquez

National Institute of Standards and Technology

Boulder, Colorado 80303-3328

The development of ignition and combustion in pressurized oxygen

atmospheres was studied for the iron based alloy UNS S66286. 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 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 to rates greater than what would be expected

from increased temperature alone. It is suggested that the source of these

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 (25 to 2000 psia)

.

Key words: alloys; combustion; ignition; ignition temperature; iron alloy;

metals

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 oxygen system pressure has increased, the potential for disastrous ignition

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

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 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 that have been studied is given in table 1.

This report, the first in a series, gives the results for the iron-based

alloy UNS S66286. 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/combustion morphology and other

information were obtained for the oxygen pressure range of 1.72 to 13.8 MPa

(250 to 2000 psia)

.

Experimental Procedure

Cylindrical test specimens, figure 1, were machined from commercially

available age-hardened UNS S66286 alloy. THe ultimate tensile strength of the

alloy used was 1100 MPa (160 ksi) . The chemical composition of the alloy was

0.17% Al, 0.0065% B, 0.051% C, 14.02% Cr, 0.12% Mn, 1.11% Mo, 24.61% Ni

,

2
Table 1.

Materials

selected

for

the

determination

of

ignition

and

combustion

characteristics.

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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
0.011% P, 0.004% S, 0.15% Si, 2.15% Ti, 0.25% V, with the balance Fe. 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 (0.313 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 + 0.05 mm (0.010 + 0.002

in) of the top surface. The top surface of a specimen was 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 reduce 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. The pressure was allowed to increase during the experiments and

was recorded at the beginning of specimen combustion. This pressure 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 fixed to 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 TEM00 mode so as to contain higher order

modes. This procedure distributed the laser power more uniformly within the

laser beam and allowed for a more uniform heating of the specimen. The laser

beam was focused to cover approximately 80% of the top surface area of the

specimen. This allowed the beam to remain predominately on the specimen

surface as the beam was randomly refracted by gas convection currents rising

from the heated surface

.

5
Specimen

Figure 2. Experimental setup.

6
Two heating procedures were used in this study. The first procedure

established and maintained an approximately linear temperature increase in

specimen surface temperature and was called the Linear Temperature Scan (LT)

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 if rapidly developing exothermic

events were present; but the tendency of a material to autoheat (self-heat),

which is 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

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

surface temperature would not reach an 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 (1291 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 (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 voltage resolution of 6.25 /rv. Thus, the maximum temperature

resolution was 0.05 K (0.09 °F) for the 973 to 1173 K range pyrometer and 0.09

K (0.06 °F) for the 1173 to 2673 K range pyrometer. The accuracy of the

7
pyrometers was 1 percent of full scale when viewing black or gray body

surfaces

.

The pyrometers view the specimen through the pressure chamber windows,

figure 3, which are at angle 45 ° from the vertical. The specimen surface

observed within the pyrometers' circular measurement field of view was

elliptical and comprised 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 viewed area due to the Stefan-Boltzmann radiation law.

The interior temperature of the specimen was measured using a thermocouple

of plat inum-versus -platinum 10% rhodium (Type S) . 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 digitizer, 6.25 /iv. This gave a temperature sensitivity of approximately

0.6 K (1 °F) over the temperature range of interest, 773 to 1773 K. The

thermocouple junction temperature uncertainty did not exceed 0.5 K from 273 to

1373 K. From 1373 to 1723 K, the uncertainty increased but did not exceed 2 K

at 1723 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

temperature of the alloy/oxide interface.

The Metal Ignition Process

A detailed description of the ignition process of metals involves a

significant number of parameters. However, a general understanding of the

8
Laser Beam

O

Figure 3. Test chamber configuration.

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

oxidizing surface. The parameters that need to be incorporated are:

1 . Q( loss ) - the rate of heat loss from the oxidizing surface by

conduction, convection and radiation;

2 . Q(ext

)

the rate of heat transferred to the oxidizing surface

from external sources by conduction, convection and

radiation

;

3 . Q ( chem) - the rate of heat generation by oxidation of the

alloy

;

4. Q(in)

• • the sum of Q(ext) and Q(chem)

.

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

• • value for each of the illustrations in figure 4. Both Q(loss) and Q(chem) are

functions of surface temperature and other parameters which 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, T s , will then increase to some

equilibrium value,

• Te q, as shown in figure 4a. Since the slope of the Q(in)

curve is less than

• the slope of the Q(loss) curve at Te q, the oxidizing surface

temperature cannot increase on its own beyond Te q

because the increase in

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

greater than Te q. Thus, a stable surface temperature is reached. However, the

degree of 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 occurring, Q(chem) will remain constant and the oxidizing surface

temperature will remain constant if all other parameters remain constant.

10
•o

11
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, T S p . 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.

This range we have previously described as a quasi-stable temperature region

[1]

.

The behavior of an alloy within this region is of considerable interest.

Small shifts in other parameters which cause perturbations in any of the heat

gain or heat loss parameters, can cause the oxidizing surface to either cool or

autoheat. When Q(in) is sufficiently large for spontaneous ignition to occur,

several parameters may be defined. They are:

1. Spontaneous ignition temperature, T S p--the temperature at

which dQ(in)/dTs = dQ ( loss ) /dT s ;

2. Ignition temperature, T^gn --the temperature at which

9 • [Q( in) -Q( loss) ] = maximum;

3. Flame temperature, Tg--the maximum temperature reached by the

oxidizing surface.

These parameters are shown in figure 4b.

Equilibrium surface temperatures, greater than T S p , may be established by

9 decreasing Q(ext) once the surface temperature has exceeded TS p . However,

these equilibrium surface temperatures must be considered unstable since the

tendency is for the alloy to autoheat; i.e., the slope of the Q(in) curve is

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

At this point it is useful to consider the effect of physical state on the

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

12
solid alloy is less than that of the liquid alloy. Therefore, if T S p

occurs

while the alloy is solid, phase dependent parameters may be defined. However,

defining ignition parameters for the liquid phase in our experiments was

difficult because of fluid flow effects. When an oxidizing fluid moves, the

rate of oxygen transport to the oxidizing surface may be significantly

increased for a short time interval. This in turn increases the rate of

material consumption, which increases the surface temperature for the flow

interval. The net result of the fluid movement is to break up the overall

smoothness of the Q(chem) curve, which make it difficult to separate movement

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

liquid phase ignition parameters were not determined for the ITS heating

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

combustion, T cmb) ; 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 T S pis not

obtainable from the experiment; however, Tign an<^ Tf are. It is interesting

to note that T^gn does not differ in this case from the case shown in figure

4b. This is due to the definition of Tfgn

as being the temperature at the

• • maximum deviation between the Q(in) curve and the Q(loss) curve. This means

that our experiments, using the ITS heating procedure, can be thermally

overdriven without seriously affecting the value of Tfgn . This statement

assumes that the excess external heat input is moderately 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

now known, the determination of the ignition parameters must be based upon

13
other measurable parameters. In our case the surface and interior temperatures

were used.

To determine the spontaneous ignition temperature an apparent changing

emissivity contribution 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 apparent emissivity contribution to the specimen heating

rate, under constant laser power, was linear or nearly linear, as shown in the

Appendix. Since the oxidation rate of metals and alloys is nonlinear with

temperature, 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 data 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 temperaturetime waveform where an abrupt change in curvature occurs prior to ignition,

figure 5. The region generally contains a number of abrupt changes in

temperature which appear in conjunction with endothermic events. Since it

appears that many endothermic events are associated with the abrupt temperature

changes, it was 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 eo . T eo also 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

14
»'ajn;E j aduja_L

15

Figure

5.

Development

of

ignition

during

an S

TS

test.
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)] = maximum when dT s /dt = maximum. T^gn

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 [2,3]. The Q(in) curve is also

modified when the oxidation changes from protective (time dependent) to

• • nonprotec tive (time independent). When either the 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

temperature at which T S p, T eo , and T^gn will occur; conversely, the greater

the heat transfer rate, the higher the temperature at which T S p , T eo and T^gn

will occur.

Under certain experimental conditions or at elevated oxygen pressures,

combustion may develop abruptly before the above defined Tign 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.

(The surface oxides may also be impeding the transfer of oxygen to the

oxidizing surface)

.

The resulting rapid increase in oxidation rate that may

occur 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

16
2000

S

I

B

l

in

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W

II

fi

I—

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17

Figure

6.

Abrupt

ignition

and

combustion

event

during

an

LTS

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

T ign •

The terms combustion and combustion region are used in this report. This

terminology classically means vapor phase combustion (oxidation). For the

SS66286 alloy, vapor phase combustion does not occur. For this alloy, and

most other alloys listed in table 1, combustion is the fluid flow enhanced

oxidation of the liquid alloys. This type of fluid oxidation generates very

high temperatures, 2500 K and higher, and develops in milliseconds. The

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

combustion temperature, Tcmg , and is shown in figure 6.

Results

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

evaluation of the ignition characteristics of the S66286 alloy were carried out

using the LTS heating procedure. Heating rates of approximately 100 K/min were

used in order to determine if the alloy had any tendency to develop solid phase

ignition to combustion events that would occur in the millisecond to second

time period as occurs in some metals and alloys. Experimental data were

obtained for the oxygen pressure range of 1.7 MPa (250 psia) to 6.9 MPa (1000

psia) . The results for a typical experiment are shown in figure 7. Two

waveform features are immediately obvious. The first is the rapid increase in

the rate of change in temperature at approximately -30 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)

.

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.

18
UNS

S66286

*

E

S

in

OJ

s

s

E

m

I

s

ld

E

ri

M

OJE

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19

Figure

7.

Typical

surface

temperature

waveform

from

an

LTS

test.
MU

E

3jn;eja duu a j_

20

Figure

8.

Surface

temperature

waveform

features

from

an

LTS

test.
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, figure 7,

starting at approximately -30 s was considered to be due to a rapid increase in

oxidation rate of the alloy; this caused the specimen to autoheat faster than

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

the heating ramp was eventually re-established (although at a different rate)

.

In most tests, the autoheating appeared to develop in conjunction with

endothermic events

.

Endothermic events also appeared at other temperatures and often appeared

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

associated with the development of temperature spikes, rapid exothermic events,

ignition and the beginning of combustion.

The temperature spikes, such as shown in figure 8, were attributed to the

presence of foreign particles or by particles produced by the fracturing or

spalling of the oxide layer which would occur from mechanical stress such as

occurs from the normal oxidation of the alloy. A loose particle would have

poor thermal contact with the specimen surface and therefore would be heated to

high temperatures by the laser beam. A temperature spike would be generated

when the temperature of the particle began to affect the pyrometer signal. The

spike would disappear when the particle either vaporized or was melted, adhered

to the oxide layer, and cooled to the baseline surface temperature. The shapes

of the two spikes shown in figure 8 indicate that the particle heated, melted,

re-adhered to the substrate and then cooled to the baseline temperature.

However, the source of the endotherm that produced the first spike appears to

have increased or contributed to the increase in the oxidation rate of the

21
alloy. Temperature spikes did not appear to affect the- oxidation rate of the

alloy since permanent changes in the baseline temperature did not occur.

Rapid exothermic events, however, caused permanent changes in the baseline

temperature, figure 7. These events were usually abrupt, occurring within

miliseconds to several hundred milliseconds. They were often preceded by an

endotherm. Exothermic events are considered to be caused by rapid changes in

oxidation rate of the alloy.

At an oxygen pressure of 1.7 MPa (250 psia) the development of combustion

from ignition occurred over a period of seconds, usually between 5 and 10 s.

However, at an oxygen pressure of 3.4 MPa (500 psia) or greater, the

development of combustion from ignition occurred within milliseconds, the

second major feature of figure 7. In all cases ignition occurred when the

surface temperature was within or above the alloy melting range. The ignition

data for the LTS test are plotted in figure 9 and are listed in table 2.

Figure 10 shows the temperature at which the specimen began to collapse and

fluid- flow-enhanced oxidation (combustion) began. These data are also listed

in table 2

.

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

solid state; but they did not develop into ignition, at least to oxygen

pressures up to 6.9 MPa (1000 psia). These events appear to involve the

release of heat, since an increase in the baseline temperature or ramp rate

follows their occurrence. We concluded that the exotherms are due to a rapid

change in oxidation rate of the alloy. Since endotherms are often involved, it

seemed reasonable to assume that the oxidation rate of the alloy was affected

by the source of these events. However, the mechanism has not yet been

established. As the alloy specimen begin to melt, abrupt ignition to

22
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temperature

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23

Figure

9.

ignition

temperature

vs.

oxygen

pressure

from

the

LTS

tests.
Table 2. LTS ignition and combustion temperatures.

1

1

1

Pressure , MPa 1 ^ign ’

1

K 1 'I'cmb ’ K

1

1

I Interior Surface Interior Surface 1

1

1 1 . 793

1 1

1 1 1877

1

1

1

| 2009

1

1

1 1 . 793 1 1 1699 1 | 1992 1

1 1 . 793 | 1647 | 1706 | 1786 | 1864 1

1 1 . 800 | 1658 | 1685 | 1788 | 1985 1

1 1 . 806 | 1678 | 1747 | 1855 | 1893 !

1 1 . 827 | 1643 | 1722 1 | 1929 1

1 3.468 | 1674 | 1691 | 1771 | 1759 1

1 3.496 | 1610 | 1690 | 1629 | 1719 1

1 3 . 544 1 1 1679 1 | 1780 1

1 3.592 1 1 1715 1 | 1756 1

1 3 . 661 1 1 1754 1 | 1754 1

1 5.316 1 1 1692 1 | 1692 1

1 5.378 1 1 1696 1 | 1696 1

1 5 . 392 1 1 1696 1 | 1696 1

1 5.405 1 1 1706 1 | 1706 1

1 5.419 1 1 1695 1 | 1695 1

1 5.433 | 1688 | 1691 | 1688 | 1691 1

1 7 . 226 | 1646 | 1678 | 1646;1 16/8 1

1 7 .239 1 1631 | 1725 | 1631 | 1725 1

1 7 . 246 | 1663 | 1671 | 1663 | 1671 1

1 -L

7.474 1 1672 |

1

1760 | 1672

J

| 1760 1 _L_

T • 1 ign

^cmb

Ignition temperature

Temperature at the beginning of combustion

24
-Surface

temperature

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25

Figure

10.

Combustion

temperature

vs.

oxygen

pressure

from

the

LTS

tests.
combustion transitions occurred. These transitions were thought to occur as

the upper surface cracked or ruptured from 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 to 1450 K temperature

range. The rate of the temperature increase in this range suggested that the

alloy could autoheat to destruction. Because of this tendency, a series of

experiments was run to investigate this phenomena. 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 LTS

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

verify this interpretation. The approach was to use standard Differential

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

exothermic events. Testing was carried out using 15 to 50 mg specimens heated

to temperatures above the lower melting range of the alloy in flowing oxygen or

helium at atmospheric 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 considerable thermal activity as shown in figure 11.

In fact, so much thermal activity was present that the data were

not interpretable . It was clear that endothermic transitions were present but

the temperatures of the individual transitions could not be firmly established.

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

discoloration, and eventual destruction, of the aluminum oxide sample holders.

26
27
Several investigators have postulated or observed the presence of liquid

oxides during the high temperature oxidation of alloys [2,3], 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 release

exotherms and the existence of at least one high energy exothermic event as

shown in figure 11. It also provides an independent verification that the

alloy does tend to autoheat to destruction 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 used the ITS heating procedure. This procedure applied a constant

heating rate, increased at intervals after surface temperature equilibrium was

established, until the alloy 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 after the

last power increment, at -165 s, for a typical experiment. (In our figures,

the interior temperature waveform data is always terminated at the thermocouple

failure point. This point usually occurs at temperatures slightly above the

liquidus temperature.) These waveforms clearly show the endothermic regions

that had been suggested or very small in the waveforms from the LTS

experiments. The effect of the endothermic regions upon the rate of increase

in specimen temperature is also evident in the ITS waveforms. These features

and their effects are more clearly shown in figures 13 and 14, which are

28
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Figure

12.

Typical

surface

and

interior

temperature

waveforms

from

an

ITS

test.
>|'3jn;sj 3dLuaj_

30

Figure

13.

Development

of

ignition

in

3.57

MPa

oxygen

from

an

ITS

test
H

0) E

3jn}fcjadoiaj_

31

Figure

14.

Development

of

ignition

in

11.94

MPa

oxygen

from

an

STS

test.
expansions of waveform sections, chosen for their clarity, from several

experiments

.

Figure 13 shows the rate of increase in temperature, approximately 1 K/s

,

after the last power increment. This initial autoheating was due primarily to

the increasing absorptivity of the surface to the laser radiation; see

Appendix. An oxidation component to the heating rate was also present but very

small. However, as the specimen temperature increases, the oxidation component

to the heating rate steadily increases at a nonlinear rate. The oxidation

component ultimately becomes the principal heat source governing the rate of

increase in specimen temperature at the spontaneous ignition temperature. The

very slight curvature in the temperature - time waveforms between -100 s and T S p

was considered to be due to the increasing oxidation component to the heating

rate. T S p

was chosen as the beginning of the temperature- time waveform section

where it appeared clear that oxidation heat input to the system was the

dominate factor in increasing the specimen temperature. This was the beginning

of the clearly nonlinear section of the 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 T S p

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 T S pwas due primarily to increasing absorptivity of

the developing oxide layer to the laser radiation. The effect of the

endothermic regions was also clearer at the higher oxygen pressures. Also, the

rate of increase in temperature after Tpgn was greater than twice the values of

those shown in figures 12 and 13.

32
After spontaneous ignition was achieved, the specimen temperature began to

slowly increase. This increase was usually followed by a series of rapid step

increases in temperature which were associated with endothermic features. The

net result of these step increases 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 endothermic

features are associated with rapid changes in the oxidation rate of the alloy,

this temperature was called the enhanced oxidation temperature, T eo . After T eo

has occurred the rate of change in temperature continued to increase until a

maximum was reached. The temperature at which this maximum rate of change in

temperature occurred was considered to be the temperature at which the maximum

• • deviation between the Q(in) and Q(loss) curves occurred. Thus, this

temperature was taken as the ignition temperature, Tpgn . The values of the

ignition parameters - -T S p , T eo and T^gn

--are listed in table 3 and plotted as a

function of oxygen pressure in figures 15, 16, and 17. The data was fitted to

linear leas t- squares curves so that the trend of the data could be established.

The slope of the trend lines are noted on the plots.

An attempt was made to estimate the temperature of the oxidation zone by

spot welding a thermocouple of Pt vs. Pt 10% Rh to the top center of several

specimen. Typically, it was found that the thermocouple measurement was

approximately 40 K below the pyrometer at spontaneous ignition. This

difference decreased to approximately 30 K at ignition. After ignition, the

difference approached approximately 10 K as the alloy began to melt. These

estimates were made at oxygen pressures of 1.7 and 3.4 MPa (250 and 500 psia)

and are probably not valid for higher oxygen pressures.

33
Table 3. ITS ignition parameter temperatures

1

1

1 J_

Pressure

,

K

| Tsp ,

K

1

1

1

Te 0 , K T • ngn- K

1

| Interior

1 Surface | Interior Surface Interior Surface

1 1 . 724 | 1224 1335 | 1267 1383 1299 1421

1 1 .772 | 1181 1228 | 1267 1324 1308 1384

1 1 . 806 1 1229 | 1271 1319

1 1 . 806 | 1172 1213 | 1225 1261 1334 1370

1 1 . 848 | 1191 1258 | 1205 1274 1237 1321

1 3.420 | 1163 1215 | 1200 1252 1293 1367

1 3.447 1 1235 | 1253 1389

1 3.461 1 1256 | 1292 1433

1 3.468 | 1208 1248 | 1252 1292 1312 1358

1 3 . 503 | 1216 1269 | 1327 1369

1 3 . 571 | 1186 1228 | 1239 1288 1280 1324

1 4 . 109 | 1116 1223 | 1132 1238 1252 1397

1 4.440 | 1130 1226 | 1153 1249 1185 1293

1 5 . 288 | 1184 1242 | 1225 1282 1239 1302

1 5 . 295 1 1228 | 1260 1348

1 5 . 309 | 1172 1224 | 1218 1259 1284 1344

1 5.419 | 1158 1234 | 1208 1291 1266 1373

1 5.447 | 1173 1222 | 1199 1257 1257 1318

1 7 . 033 | 1187 1230 | 1208 1246 1279 1321

1 7 . 095 | 1143 1200 | 1184 1242 1255 1348

1 7.115 | 1151 1202 | 1253 1290 1314 1364

1 8 .839 | 1132 1181 | 1145 1193 1189 1284

1 8 . 867 1 1184 | 1250 1334

1 9 . 060 | 1120 1185 | 1149 1228 1176 1289

1 10.590 1 ini 1225 | 1135 1254 1179 1326

1 10.866 | 1106 1166 | 1130 1180 1306 1408

1 11.942 | 1117

_J

1211 |L

1144 1227 1227 1344

T S p

- Spontaneous ignition temperature.

T eo - Enhanced oxidation temperature.

Tjg n- Ignition temperature.

34
‘ 3jn;s jadiijaj_

35

Figure

15.

Spontaneous

ignition

temperature

vs.

oxygen

pressure

from

the

ITS

tests
-Interior

temperature

LD

CD

(\J

CD

CD

LD

LD O

Z

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36

Figure

16.

Enhanced

oxidation

temperature

vs.

oxygen

pressure

from

the

ITS

tes
Interior

temperature

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37

Figure

17.

Ignition

temperature

vs.

oxygen

pressure

from

the

ITS

tests.
4 . Data Accuracy :

The surface temperature values of T S p , Teo , and T^gn were determined with

a commercial two-color ratio pyrometer with an accuracy of 1 percent when

measuring black-or gray-body temperatures. This was verified by calibration

using a NBS 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 or

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 example, when the difference between the surface

and interior temperature curves in figure 12 was fitted to the leas t- squares

criteria, the maximum deviation from linearity was 3 K. The deviation occurred

during a period when the surface was undergoing color changes; thus, probably

changes in emissivity. Therefore, the pyrometer temperatures are considered to

be accurate to within the basic 1 percent error range of the instrument.

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

ignition parameters. However, it is considered that the data accurately

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

oxidation events occurred.

Cone lus ions

The LTS data indicated that milisecond ignition to combustion transitions

would not occur at temperatures below the solidus temperature for oxygen

pressures up to 6.9 MPa (1000 psia)

.

However, the ignition temperature data

38
have a definite downward trend with increasing oxygen pressure and there were

indications of exothermic events occurring at temperatures below the solidus

temperature. Therefore, millisecond ignition- to-combustion transitions at

temperatures below the solidus temperature and at oxygen pressures greater than

6.9 MPa (1000 psia) cannot be ruled out by rising the LTS data. The ITS data,

though, do provide this information

The ITS data established that the oxidation rate of the alloy was

sufficient for the alloy to autoheat to destruction, under the appropriate

experimental conditions, 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 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

decreases as oxygen pressure increases, but does not decrease to the

millisecond time frame. Therefore, this alloy did not exhibit an abrupt

ignition to combustion transition in the solid state. However, the development

of combustion after ignition can be rapid, occurring within several seconds at

the higher oxygen pressures. The temperatures at which the ignition parameters

occur, decrease with increasing oxygen pressure. This indicates that these

parameters are a function of the oxidation rate of the alloy and that the

oxidation rate increases more rapidly with increasing oxygen pressure than the

cooling rate, which is probably convection controlled.

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

39
achieve this, a thermogravimetr ic 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 (2600 °F). 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

e 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.

Acknowledgement

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

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

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

Marshall Space Flight Center for funding.

References

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

Isaura, The Development of Combustion from Quasi-Stable Temperature for

the Iron Based Alloy UNS S66286; ASTM SP 986.

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

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

[3] 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).

40
Appendix : Determination or Probable Emissivity Effects on Spontaneous

Ignition

Oxidizing surfaces are notorious for their nonuniform surface emittance.

This nonuniformity is in the magnitude of the emittance value at a constant

wavelength over time as well as in the variation in emittance value with

wavelength and over time.

In our experiments, the variation in emittance with wavelength will affect

the accuracy of the surface temperature measurement. However, inaccuracy in

this measurement was easily detected by comparing this measurement with that of

the interior thermocouple temperature measurement. If the difference between

these two measurements was constant or nearly so, then little or no error from

emissivity non-linearity was present. For S66286, there were no surface

temperature measurement problems, as shown by figure 5 in the text.

The variation of emittance with time at a constant wave length- - the laser

wavelength- -will affect the heating rate of the specimen. If the variation

occurs during a waiting period of an ITS test the effect may appear to be

oxidation heating. Therefore, it was necessary to separate the effect of

essentially pure oxidation heating on the temperature - time waveform data from

the effect of emittance driven heating.

This was accomplished by preoxidizing several specimens in air at

temperatures approaching the ignition temperature range. These specimens were

then tested using the ITS heating procedure.

A typical result is shown in figure Al . The oxidation heating effect was

evidenced by a steadily increasing slope once the effect began. This

contrasted with the initial linear or nearly linear increase in the

temperature - time waveforms after the beginning of autoheating for non41
r H

m

\-

H 1 1 j 1 1 f o ca © C3 in G3 in ©

in (\J

G3©

*r

1

>1 ' 0 jn;s jadujox

42

Figure

A1.

Development

of

ignition

using

a

pre-oxidized

specimen.
preoxidized test

waveform segment

linear or nearly

segment was due

radiation

.

This

increasing emis

probably has a s

spec imens -- see figure 5 in the text for an example of a linear

during autoheating prior to T S p . The conclusion was that the

linear waveform segment proceeding the clearly nonlinear

to an increasing absorptivity of the surface to the laser

increasing absorptivity was probably due almost entirely to

sivity at the laser wavelength in the early stages; but,

urface roughness component in the latter stages.

43

U.S. DEPT. OF COMM. 1. PU BLILA I ION OR i. Performing Organ. Report No. o. rublication U ate BIBLIOGRAPHIC DATA

SHEET (See in struction s)

REPORT NO.

NISTIR 88-3904 November 1988

4. TITLE AND SUBTITLE

Ignition Characteristics of the Iron-Based Alloy UNS S66286 in Pressurized Oxygen

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

Kathleen M. McDermott, and Isaura Vazquez

6. PERFORMING ORGANIZATION (If joint or other than NBS, see in struction s) National Institute of Standards and Technology

-W»f16l»*fPWWW-»F S T A ND A RD S DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

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

George C. Marshall Space Flight Center

Marshall Space Flight Center, AL 35812

10.

SUPPLEMENTARY NOTES

7. Contract/Grant No.

8. Type of Report & Period Covered

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

11.

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

bi bl iography or literature survey, mention it here)

The development of ignition and combustion in pressurized oxygen atmospheres was

studied for the iron based alloy UNS S66286. Ignition of the alloy was achieved

by heating the top surface of a cylindrical specimen with a continuous-wave CO

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—heat ing) 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 to rates greater than what would be expected from increased temperature alone. It is suggested that the source of these

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 (25 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; iron alloy; metals

13.

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Q For Official Distribution. Do Not Release to NTIS

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[Xj Order From National Technical Information Service (NTIS), Springfield, VA. 22161

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