NBSIR 75-657 'R)

Report No. 7

A Metallurgical Investigation of a

Full-Scale Insulated Rail Tank Car

Filled With IPG Subjected to a

Fire Environment

J. G. Early and C. G. Interrante

Mechanical Proper,

Metallurgy Divisic

Institute for Mater

,

National Bureau of j Washington, D. C.^

January 1975

Final Report

Ef;rly, J. G., Interrante^ C. G.^ A me tallurf^ical investigation of a full-scale insulated

rail tank car filled with LPG subjected to a fire environment, DOT Report No. FRA-OmDjIj':-

52, 67 pages (Available as PE25O-587 from the

National Technical Information Service

,

Tan.

Springfield, VA, 22l6l, chrnr-—tg"?^ .

312

Prepared for Federal Railroad Administration

Department of Transportation

Washington, D. C. 20591

NBSIR 75-657

Report No. 7 A METALLURGICAL INVESTIGATION OF A

FULL-SCALE INSULATED RAIL TANK CAR

FILLED WITH LPG SUBJECTED TO A

FIRE ENVIRONMENT

J. G. Early and C. G. Interrante

Mechanical Properties Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington. D. C. 20234

January 1975

Final Report

Prepared for Federal Railroad Administration

Department of Transportation

Washington, D. C. 20594

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent. Secretary

NATIONAL BUREAU OF STANDARDS. Richard W. Roberts. Director

TABLE OF CONTENTS

Page

ABSTRACT V

1. INTRODUCTION ..... 1

2. PURPOSE . . . 2

3. EXPERIMENTAL PROCEDURES 3

3.1 Macroscopic Observations and Thickness

Measurements 3'

3.2 Chemical Analysis 3

3.3 Metallographic Observations and Hardness

Testing 4

4. RESULTS AND DISCUSSION 4

4.1 Chemical Composition . . . . ^ . 4

4.2 Macroscopic Observations and Thickness

Measurements , , . 5

4.3 Metallographic Analysis 7

4.3.1' Shell Course 3 7

4.3.2 Shell Course 1 ........... 9

4.4 Hardness Measurements 10

4.4.1 Shell Course 3 10

4.4.2 Shell Course 1 ............ 10

4.4.3 Stress-relieving Experiments .... 11

4.5 General Discussion 12

5. SUMMARY 16

6. CONCLUSIONS 18

7. ACKNOWLEDGEMENT 19

i
Table of Contents (Continued)

REFERENCES

TABLES

Page

20

I. Chemical Composition of Plate Samples from

Insulated Tank Car RAX 202 , . .

.II. Macroscopic Observations of Plate Samples

TC2-(1) and TC2-(3) from Shell Course 3 . . .

III. Hardness Data Summary , FIGURES

1. Insulated Tank Car RAX 202 positioned in the pit

prior to initiation of the fire test . . . . .

2. Photographs of fragments of Tank Car RAX 202

after failure-. . ,

3. Plate sample TC2-(1), shell course 3, taken

from fragment C ....... . .

4. Plate sample TC2-(3), shell course 3 , taken

from fragment C .

5. Plate sample TC2-(7)/ shell course 4, taken

from fragment B . .

6. Plate sample TC2-(10), taken from shell courses

1 and 2 of fragment A at the top of the tank

car .

7. Plate sample TC2(11) taken from shell courses

1 and 2 of fragment A at the bottom of the

tank car

8. Schematic diagram showing the location of NBS

samples in Tank Car RAX 202

9. Schematic showing the three mutually

perpendicular planes associated with the

rolling direction in a steel plate

10. Close-up of a portion of the inside surface of

plate sample TC2-(1)

11
FIGURES (continued)

11. Macrograph of fracture face of TC2-(1) between

locations 5 and 6

12. Macrograph of fracture face of TC2-(1) between

locations 6 and 7

13. Macrograph of inside plate surface of TC2-(1) at

the fracture surface

14. Profile view of fracture surface on plate sample

TC2-(7)

15. Profile views of fracture surface on plate

sample TC2-(1) , shell course 3 .......

16. Representative microstructure of plate sample

TC2'-(1) away from fracture face

17. Representative microstructure of plate sample

TC2-C3)

18. Photomicrographs of plate sample TC2-(1)

adjacent to hardness specimens

19. Photomicrographs of plate sample TC2-(3)

adjacent to hardness specimens

20. Photomicrographs of the plate surfaces of plate

sample TC2'-(1) near the fracture surface . .

21. Photomicrograph of an intergranular crack near

the fracture initiation site on plate sample

TC2-{1)

22. Photomicrographs of fracture surface of plate

sample TC2-(1)

23. Photomicrographs of the fracture surface and

region immediately behind

24. SEjM fractographs of the fracture surface of

plate sample TC2-(1) near outside plate

surface ,

25. SEM fractographs of the fracture surface of

plate sample TC2-(1) near midthickness

region
FIGURES (continued)

26. SEM fractographs of the fracture surface of plate

sample TC2-(1) near inside plate surface . . . .

27. Representative microstructures of plate sample

TC2-(10B) .

28. Representative microstructures of plate sample

TC2-(11B) . .

29. Photomicrographs of plate sample TC2-C10B)

adjacent to hardness specimens

30. Photomicrographs of plate sample TC2- (IIB)

adjacent to hardness specimens

31. Schematic of representative hardness profiles

from plate samples TC2- (3) and TC2-(11B) . . . .

iv
ABSTRACT

An analysis of the failure of an insulated rail tank car,

RAX 202, which had been tested to failure in a fire environment

at White Sands Missile Range, New Mexico, was requested by the

Federal Railroad Administration, Department of Transportation.

The tank car, filled with approximately 33,000 gallons of

liquified petroleum gas (LPG) , failed after approximately 94

minutes of exposure to a JP-4 jet fuel fire. The car fractured

into four fragments which were examined in the field. Five

plate samples from the four fragments were selected for laboratory

study at the National Bureau of Standards.

The results of laboratory check chemical analyses of five

specimens representing three shell courses indicated that all

of the samples tested met the chemical requirements of AAR TC128-B

steel. The chemical variability between individual plate samples

was low, and the levels of phosphorous and sulphur were notably

low in comparison with the levels found in six heats of TC128

steels previously analyzed at NBS

.

The results of metallurgical investigations suggest that a region approximately 30 inches in length near the top of the

tank car in shell course 3 was the site of the initial rupture

of the tank car. This rupture was attributed to the tensile

hoop stress due to internal gas pressure, as the crack was aligned

with the longitudinal axis of the tank car. The results of

stress-relieving experiments conducted on samples taken from

the top and bottom of the car indicated that the top of the tank

car experienced temperatures of 1200*'F to 1250*'F for times of

between 10 to 15 minutes.

The fracture features of the initial rupture were indicative

of failure by a stress-rupture mechanism. It was concluded that

this 30-inch stress-rupture crack led to tensile overload,

instability and to the onset of rapid crack propagation in a shear mode, with the initial shear fracture propagating as an

extension of the original stress-rupture crack. Within a short

distance, this shear fracture turned 90" and propagated in the

plate rolling direction, a result explained by the anisotropy

of the fracture resistance of this steel at the elevated

temperatures of the test. This reorientation occurred despite

the fact that the hoop stress which promotes fracture in the

longitudinal axis of the tank car is twice as large as the

stress that promotes fracture along the rolling direction of

the plate.

V

1. INTRODUCTION

A metallurgical evaluation of a full-scale railroad tank

car tested to failure was requested by the Federal Railroad

Administration, Department of Transportation. A fire test

(Fire Test 2) was conducted on tank car RAX 202 on December 6,

1973, at White Sands Missile Range, New Mexico, under the

direction of personnel from the United States Army Ballistic

Research Laboratory. The tank car tested was similar to a standard 33,000 gallon tank car with the exception of three

modifications. One modification was the addition of a second

manway to provide access to the interior of the tank car in

order to facilitate the installation of temperature and

pressure sensors at various locations inside the tank car.

Secondly, two ports were added to one side of the tank car

through which the instriimentation cables passed, and finally,

a white, protective, thermal coating was sprayed over the

entire tank car. The steel plates used in the fabrication of

tank car RAX 202 were reported to be 5/8 inch-thick, finegrained steel plate in the as-rolled condition and to have

been produced to Specification AAR M128-69, Grade B, Flange

Quality, by the Lukens Steel Company as part of Melt Number

CO 485 . (1)

The tank car, filled with approximately 33,000 gallons of

liquified petroleum gas (LPG) , was placed in a large pit. A

low earth dike that was continuously maintained at a level

of about 18 inches of JP-4 jet fuel surrounded the tank car.

When ignited, this pool of jet fuel, located beneath the tank

car, provided the thermal energy to heat the tank car. Figure

1 shows the tank car positioned in the pit surrounded by the

fuel dike prior to the fire test. (2)

The time to failure of the tank car was measured beginning

with the ignition of the JP-4. At about 60 minutes into the

test, the temperature and pressure recording systems failed.

This failure prevented the recording of any temperature or

pressure data for the remainder of the test. Approximately 94

minutes after ignition of the JP-4, the tank car failed and

fractured into four fragments. Two fragments, constituting

almost all of shell course 3, were propelled out of the pit.

The smaller of the two fragments remaining in the pit contained

the A head plate and shell courses 1 and 2 , and the larger

fragment remaining in the pit contained the B head plate and

shell courses 4, 5, 6, and 7, see Figure 2.

- 1 -
A total of five steel-plate samples from four shell

courses were selected, removed by torch cutting, and sent to

NBS for metallurgical investigation. The samples, designated

TC2-(1), TC2-(3), TC2-(7), TC2-(10)* and TC2-(11)*, were

photographed in the as-received condition and are shown in

Figures 3 through 7.

Plate sample TC2-(1), including the additional manway and

encompassing the top of shell course 3, was torch-cut along

the line BG (Figure 3) . The fracture surface follows the line

BCDEFG. Sample TC2-(3) was torch-cut from the bottom of shell

course 3 and contains a portion of the fracture surface along

HIJ (Figure 4), which is the continuation of the fracture BC

in TC2- (1)

.

Plate sample TC2-(7), containing a portion of the girth

weld which originally joined shell plates 3 and 4, was torchcut from shell course 4, located in the larger remaining

fragment in the pit, at the bottom of the tank car. A small

piece of shell course 3, seen at the lower left front of the

sample (Figure 5), contains part of the fracture surface, MNO,

which is also an extension of the fracture surface BC in TC2-(1).

The last two plate samples, TC2-(10) and TC2-(11), were

torch-cut from the smaller remaining fragment of the tank car

in the pit. Sample TC2-(10), taken from the top of the tank

car, and TC2-(11), taken from the bottom of the tank car

directly opposite from TC2-(10), were selected for removal

because they were from a relatively undeformed region of the

tank car. A schematic of a portion of the tank car. Figure 8,

shows a representation of the plate samples in their approximate

location

.

2 . PURPOSE

The principal purpose of this metallurgical investigation

was to determine the cause and location of the initial rupture

which led to the fracture of the tank car. Another purpose of

the investigation was to make observations of the fracture

characteristics and the elevated-temperature mechanical behavior

of this type of steel.

* Each of plate samples TC2-(10 and TC2-(11) contained

portions of shell courses 1 and 2. Tests and observations

of these two plate samples are reported here based on the

particular shell course involved; TC2-(10A) and TC2-(11A)

are from shell course 2 and TC2-(10B) and TC2-(11B) are

from shell course 1.

- 2 -
||| ^' EXPERIMENTAL PROCEDURES

Check chemical analyses of the plate samples were

; conducted for a comparison with the requirements of Specification AAR TC128-B-69. In addition, hardness surveys

j and thickness measurements were performed, and macroscopic

[and microscopic observations were made. Specimens for these

i tests and observations were taken from plate samples TC2-(1)

,

|tC2-(3), TC2-(10B) and TC2-(11B) with the following exceptions:

j (i) Chemical analysis was not performed on TC2-(10B) since

!TC2-(10B) and TC2-(11B) are from the same shell plate. (ii)

Thickness measurements were not made on TC2-(10B) or TC2-(11B)

because these two samples were taken from a shell plate that was

not noticeably affected by the failure of the tank car. A

chemical analysis was performed and macroscopic observations were

made on sample TC2-(7).

Mechanical property measurements, including ambientand hot-tensile tests, and stress-rupture tests, are currently

being conducted and will be reported on in a subsequent report.

3 . 1 Macroscopic Observations and Thickness Measurements

The fracture surfaces of samples TC2-(7) and TC2-(1) were

macroscopically examined and fracture profile sections were

taken, perpendicular to the fracture surfaces, at the locations

designated in Figures 4 and 10. Thickness measurements were

taken at various locations near the fracture surfaces and at

various distances perpendicular to the fracture face in samples

TC2-(3); these measurements were used to determine if plate

thinning occurred in regions near the fracture.

3 . 2 Chemical Analysis

Check chemical analyses (by combustion -conductometric

,

x-ray fluorescence, and emission spectroscopy methods) were

conducted by a commercial testing laboratory at a quarterthickness position of chemistry samples taken from plate

samples TC2-(1) and TC2-(3) (shell course 3), TC2-(7) (shell

course 4), and TC2-(11B) (shell course 1). The location of

the chemistry samples on each of these plates is shown in

Figures 3, 4, 5 and 7. Two chemistry samples, designated IB

and IC, were taken from sample TC2-(1) for analysis. Sample

IB was taken from an area near what is believed to be the

original rupture, and sample IC was taken from an area as

close as possible to the corner of the plate,* in accordance

The shell plates are fabricated by forming flat rectangular

plates into a cylindrical shape with the narrower plate ends

abuting each other, and are joined by welding the narrow sides

together. The plate corners are thus separated by this

longitudinal seam weld.

- 3 -
with Specification ASTM A20-67, "General Requirements for

Delivery of Steel Plates for Pressure Vessels", which specifies

that check analyses samples shall be taken from a corner of the

as-rolled plate.

3 . 3 Metallographic Observations and Hardness Testing

Representative photomicrographs, oriented as shown in

Figure 9, were obtained on the metallographic specimens taken

from plate samples TC2-(1), TC2-(3) (shell course 3), TC2-(10B)

and TC2-(11B) (shell course 1). Metallographic observations

were made on selected polished and etched areas of logitudinal

C planes, shown in Figures 4, 6, 7 and 10. The ferrite grain

size was measured in accordance with Specification ASTM E112-63,

"Estimating the Average Grain Size of Metals". Scanning electron

microscope (SEM) photographs were taken of the fracture face at

a location indicated in Figure 10.

Rockwell B hardness measurements were taken on specimens

adjacent to the metallographic specimens from plate samples

TC2-(1), TC2-(3), TC2-(10B) and TC2-(llB). Hardness measurements were taken on transverse B planes (see Figure 9) at

numerous locations on a 0.1-inch square grid in rows parallel

to the plate surfaces in order to determine the average hardness

of the specimens.

Additional groups of hardness specimens were prepared from

plate samples TC2-(3) and TC2-(11B).* Room-temperature hardness

measurements were taken on these specimens before and after

annealing at various temperatures. These qualitative tests were

conducted in order to estimate the temperature of the steel at

the time of failure by matching the hardness values of the

laboratory stress-relieved specimens taken from the bottom of

the tank car with those of specimens in the as-received condition

taken from locations near the top of the tank car.

'4. RESULTS AND DISCUSSION

4.1 Chemical Composition

The chemical compositions, as determined by the commercial

testing laboratory's check analyses of the five specimens from

three shell courses, are given in Table I along with the

producer's ladle analysis and the chemical requirements for

AAR TC128-69 Grade B steel. The results of the laboratory check

* The individual specimens in each group were cut adjacent to

each other and were located adjacent to the metallographic

specimens

,

- 4 -
analyses and the producer's ladle analysis are in good agreement,

and they indicate that the composition of the five samples met the

chemical requirements for AAR TC128-69 Grade B steel.

The compositional variability of samples taken from different locations within the same shell course, chemistry

samples IB, IC and 3 (shell course 3) , is quite low with the

possible exception of the manganese and sulfur levels in sample

3, which are below the mean value obtained from the other two

samples. The compositional variability between these samples

and those from other shell courses, chemistry sample 7 (shell

course 4) and chemistry sample llB (shell course 1) is also

low and is consistent with the fact that all of the plates for

tank car RAX 202 were produced from the same heat of steel.

The aluminum check analyses showed that the steel was

deoxidized by a combination of silicon and aluminum, indicating

that the steel was produced in accordance with fine-grain

practice. Vanadium was not detected by either the producer's

ladle analysis or by the check analysis. This result confirms

that the plates are Grade B steel, and it indicates that the

steel does not meet the minimum vanadium requirement of 0.02

percent-by-weight for Grade A steel.

The levels of sulfur and phosphorus are particularly low,

averaging respectively 0.013 and 0.006 percent-by-weight for

the five check analyses. These values compare with average

check analysis values of 0.026 and 0.018 percent-by -weight

for six steel samples taken from failed tank cars and tested

at NBS.(3) When compared with these six plate steels, the

relatively low level of phosphorus would be expected to give

lower impact transition temperatures, and the relatively low

level of sulfur would be expected to increase the upper-shelf

energy absorption values in impact tests of the steel. The uppershelf energy absorption values are important to the elevatedtemperature crack propagation behavior of the steel, and in

particular it is believed that they may relate to the critical

flaw size for initiation of ductile fracture.

4.2 Macroscopic Observations and Thickness Measurements

Macroscopic examination of plate samples TC2-(3), TC2-(7),

and TC2-(11) revealed that the outside surfaces of these plates

were either partially or completely covered with the charred

remnant of the protective thermal coating. Plate samples TC2-(3)

and TC2-(11) contained an inner layer of uncharred thermal

coating next to the outside plate surface and an outer charred

layer. These samples were taken from locations on the bottom

of the tank car where the fire exposure apparently was insufficient to burn off or completely char the thermal coating.

- 5 -
Approximately one-half of the outer plate surface of sample

TC2-(7), the half closest to the bottom of the tank car,

was covered with the charred coating residue, while the other

half of TC2-(7) was completely free of any coating residue.

These findings are consistent with the observations for TC2-(3)

and TC2-(11). Examination of samples TC2-(1) and TC2-(10)

revealed tiiat their outside surfaces were completely free of

the thermal coating. These two plate samples were removed

from the top of the tank car, where the fire exposure was

apparently sufficient to burn off the thermal coating.

The fracture face of sample TC2-(1) was examined before

specimens for other studies were cut from this sample. Macrographs of the fracture surface between locations 5 and 7 (Figure

10) are shown in Figures 11 and 12. The fracture appearance

shown in Figure 10 is typical of that observed along the region

from midway between locations 4 and 5 to midway between locations

- 7 and 8

.

The inside and outside plate surfaces of TC2-(1) near the

fracture surface were examined. Numerous secondary cracks

adjacent to and running parallel with the fracture face were

observed between locations 4 and 8 (Figure 10) on both the

inside and outside surfaces. Within a length of six inches on

either side of location 5, secondary cracks were found in an

area extending about one inch back from the fracture face. In

the other regions, the secondary cracks were confined to within

one-half inch of the fracture face. Figure 13, taken near

location 5, shows a portion of the fracture face and the

adjoining inside plate surface with a large number of the

secondary cracks being visible on the plate surface.

Two fracture modes were observed along the fracture face

of plate sample TC2-(1). One mode was characterized by a fracture surface nearly perpendicular to plate surfaces, and

the other fracture mode, a shear type, was characterized by a fracture surface inclined at an angle of about 45" to the plate

surfaces. Samples TC2-(3) and TC2-(7) contained only the shear

mode. Profile views illustrating the two fracture modes are

shown in Figures 14 and 15.

The fracture mode from B and C and from F to G (Figure 3)

in sample TC2-(1) is a shear type with the fracture plane at an

angle of about 45° with the surfaces of the plate and with one

edge of the fracture plane in a weld HAZ , as represented by Figure

14b. A shear mode was also observed from C to D and from E to F,

in both of which the fracture surface lies entirely in base metal

and makes an angle of about 45® with the plate surfaces, as

represented by Figure 14a. Profile views of the fracture surface

- 6 -
between D and E. near iocations 5 and 6, are shown m Fiau

15. This fracture surface, aithcugh rough m appear .nee

,

inacroscopica^iy nearly porpena icular to the plare surfaces

sn\ail prorrusicns , reser.bling shear-lips , being present at

the inside and outside plate surfaces.

Plate saT>ple TC2-(3) contains another portion of the

fracture face separating shell course 3 and shell course 4.

The fracture mode from H to I (Figure 4) is a mixed type,

represented by Figure 14c, with both perpendicular and shear

regions, and with one edge of the fracture plane anchored in a HAZ

.

From I to J, the fracture mode is a shear type with one edge of

the fracture plane in a HAZ.

Plate sample TC2-(7) contains a portion of the fracture

surface which intersects the circumferential weld joining shell

course 4 to shell course 3. The fracture mode from M to N

(Figure 5) is a shear type with tlie fracture plane at an angle

of 45° to the plate surfaces while from N to 0 the fracture mode

is a mixed type. The fracture surfaces from M to N and from N to

0 are illustrated by Figures 14a and 14c, respectively.

Thickness measurements taken at various locations on the

fracture surface of plate samples TC2-(1) and TC2-(3) indicate

that substantial plate thinning occurred only between locations

3 1/2 and 9 on plate sample TC2-(1), Figure 10. Plate thinning,

-as measured by percent reduction in plate thickness, reached a maximum of almost 4 0% near location 5 and dropped off to 20% at

locations 3 1/2 and 9. A tabulation of the fracture mode observations and plate thinning calculational results are given in

Table II.

4 . 3 Metallographic Analysis

4.3.1 Shell course 3

' Representative photomicrographs taken at low

magnification on logitudinal C planes of plate samples TC2-(1)

and TC2-(3) from shell plate 3 are shown in the etched condition

in Figures 16 and 17, respectively. Sample TC2-(3) was taken

from the bottom of the tank car, and sample TC2-(1) was taken

from near the top of the tank car. The microstructure of TC2-(1)

represents a region approximately two inches from the fracture

surface. The photomicrographs reveal evidence of decarbur ization

to a depth of about 0.005 inches on both surfaces. The presence

of decarburization on the inside plate surface indicates that the

observed decarburization on both surfaces probably occurred prior

to the fire test, perhaps during the plate-rolling operation or

the stress-relieving operation after the plates were welded

together. These surface layers are primarily ferrite with very

small amounts of pear lite. The tensile strength of these ferrite

7
surface layers is expected to be lower than that of the bulk

of the plate, and these surface layers are therefore expected

to have lower resistance to stress-rupture failure.

The micros tructures of these two specimens are typical

of a carbon-manganese steel in the hot-rolled condition. The

micros tructure consists of a mixture of proeutectoid ferrite

and pearlite with moderate banding, of alternate layers of

ferrite and pearlite, primarily in the midthickness region of

the plate. The ferrite grain size number for both plate samples

was determined to be 8 1/2, indicating that the proper finegrain deoxidation practice was followed, and more importantly,

indicating that either the finishing temperature of the plate

rolling operation was low enough to preserve the fine austenite

grain size which then transformed into a fine grain ferrite, or

the cooling rate through the transformation temperature was

sufficiently high to produce a high ferrite nucleation rate.

Photomicrographs taken of these two specimens at higher

magnification are shown in Figures 18 and 19. The pearlite

lamellae in plate sample TC2-(1) , shown at a magnification of

500X in Figure 18a, are not clearly resolvable. At a magnification of 1250X, Figure 18b, regions within some pearlite

colonies are shown to have retained their lamellar morphology

while substantial areas have not, because spheroidization of

the iron carbide phase has started. Figure 19a shows that in

sample TC2-(3), regions of lamellar pearlite are clearly resolved

at a magnification of 500X.* At a magnification of 1250X, Figure

19b, most pearlite colonies are seen to have regions of lamellar

morphology

.

A metallographic examination of the fracture surface and

adjacent regions near location 6 (Figure 10) of sample TC2-(1)

was conducted. The secondary cracks observed on the outside

and inside surfaces near the fracture face generally lie at an

angle of 45" to the surface as seen in Figure 20. Large numbers

of fissures or voids are present, some of which are aligned

ahead of advancing secondary cracks. The photomicrographs reveal

the elongation of the ferrite grains as the plate thickness

The ability to resolve at a given magnification the lamellar

morphology of pearlite depends not only on the inter lamellar

spacing but also on the extent to which the micros tructure

has been modified by thermal treatment; e.g., iron carbide

spheroidization. Since TC2-(1) and TC2-(3) are from the same

steel plate where the aus teni tizing temperature and cooling

rate through the transformation temperature are expected to be

similar, the average interlamellar spacing in both TC2-(1) and

TC2-(3) should be similar. The inability to resolve the pearlite

at a magnification of 500X in TC2-(1) suggests that additional

deterioration of the lamellae has occurred in plate sample TC2-(1)

compared to plate sample TC2-(3).

- 8 -
decreased under the applied stress. The character of these

secondary cracks can more easily be seen by looking at a small crack at high magnification. Figure 21, taken of a

region on the inside plate surface, shows the crack to be

intergranular and following interfaces between the ferrite

and pearlite. The micros tructure of the fracture profile.

Figure 22, shows extensive elongation of the ferrite combined

with the general degradation of the structure by the formation

of large numbers of extensive voids in the vicinity of the

fracture surface. Higher magnification micrographs of the

region shown in Figure 22b shows that the fissures and voids form

primarily at grain boundaries and particularly at the interface

between proeutectoid ferrite and pearlite (see Figure 23)

.

SEM fractographs were taken of the fracture surface near

location 6 of TC2-(1) (Figure 10) , as this region was believed

to have been part of the initial rupture. Evidence of fissures

or cavities (Figures 24, 25 and 26) can be seen throughout the

fracture interface from the outside plate surface to the inside

plate surface. Higher magnification fractographs, Figures 24b,

25b, 26b and 26c, reveal a general dimpled appearance throughout the fracture interface. The dimpled character of the

fracture interface indicates that ductile fracture occurred

with extensive microvoid initiation and coalescence. It is

often observed that overload fractures due to stress-rupture

are initiated by microvoids at grain boundary junctions .( 4

)

Additional active sites for microvoid nucleation are inclusions,

microcracks , and interfaces between the matrix and particles

such as carbides. The proeutectoid ferrite to pearlite interface

could provide excellent sites for microvoid initiation. The

observation that the small secondary cracks on the plate surfaces

are intergranular in character is additional evidence that this

fracture surface region was the origin of the initial rupture,

and that failure of this region occurred by stress rupture

through a combination of intergranular fracture and microvoid

coalescence.

4.3.2 Shell course 1

Representative photomicrographs taken at low

magnification on the logitudinal C planes of plate samples

TC2-(10B) and TC2-(11B) from shell course 1 are shown in

Figures 27 and 28. These microstructures indicate, as in the

case of the shell course 3 specimens, that decarburization

has occurred on both surfaces to a depth of about 0.005 inches.

The inside and outside surface layers are therefore primarily

ferrite compared to the bulk microstructure , and this is

additional evidence that this decarburization preceded the

fire test

.

- 9 -
The raicrostructures are typical of AAR TC128-B steel

in the as-rolled condition with a mixture of proeutectoid

ferrite and pearlite with a lesser degree of banding than was

observed in the shell course 3 specimens. The ferrite grain

size numbers were 8 and 8 1/2 for TC2-(10B) and TC2~(11B)

respectively, indicating that the grain-refining deoxidation

practice was preserved by proper finishing temperature during

the plate rolling operation.

Photomicrographs were taken of these ' two specimens at

higher magnification to determine what differences, if any,

existed between a specimen from the top region (hotter) of

the tank car and a. specimen from the bottom region (colder) of

the tank car. The micros tructures of TC2-(10B) and TC2-(11B)

are similar, contrary to the observations made on the specimens

from the fractured shell plate, shell course 3. Figures 29 and

30 reveal that, in both TC2-(10B) and TC2-(11B), the pearlite

colonies retained substantial regions of lamellar morphology

throughout the micros tructure indicating that the exposure to

elevated temperature during the fire test was insufficient to

cause significant additional spheroidization of the iron

carbide phase in the pearlite, even at the top of the tank

car in this region of this shell course.

4 . 4 Hardness Measurements

Representative hardness measurements were taken on

specimens from plate samples TC2-(1) and TC2-(3) taken from

near the top and the bottom, respectively, of shell course 3

and on specimens from- -plate samples TC2-(10B) and TC2-(11B)

taken from the top and bottom, respectively, of shell course 1.

The results are summarized in column 3 of Table III.

4.4.1 Shell course 3

The average hardness for TC2-(3) , taken from the

bottom of the tank car, is Rb 89. This hardness corresponds

to an approximate tensile strength of 87,000 psi.CS) The

average hardness for TC2-(1), taken from near the top of the

tank car, is R3 8 6 1/2, corresponding to an approximate

tensile strength of 82,000 psi.

4.4.2 Shell course 1

The average hardness for TC2-(11B) , taken from

the bottom of the tank car, is Rb 89, corresponding to an

approximate tensile strength of 87,000 psi. The average

hardness for TC2-(10B), taken from the top of the tank car,

is Rb 88, corresponding to an approximate tensile strength

of 86,000 psi.

- 10 -
These results show that the two plate samples, TC2-(3)

and TC2-(11B), which were heated the least during the fire

test, have the highest average hardness values. Plate

sample TC2-(1), taken from the failed shell course in the

region experiencing the highest temperature during the test,

the top of the tank car, has the lowest average hardness

value. Plate sample TC2-(10B) taken from the top of the

tank car in shell course 1, which did not fail, has only a slightly lower average hardness value than TC2-(11B), taken

from the bottom of the tank car. The difference in average

hardness between TC2-(l)"and TC2-(10B), samples from two

different shell courses, can arise not only from metallurgical

variables, such as the pearlite lamellae spacing prior to the

fire test or the chemical composition, but also as a result of

differing temperature-time environments experienced by different

regions of the tank car during the fire test. Possibly the

maximum temperature reached during the fire test or the

time at temperature may have varied along the length of the

litank car.

4.4.3 Stress-relieving Experiments

The hardness data used to determine the represen-

;tative hardness values of each of four plate samples were

llfound to vary depending on whether the measurements were near

jthe two plate surfaces or in the midthickness region. This

I observation is not surprising since the metallographic study

revealed moderate banding, principally in the midthickness

Jregion, in all of the plate samples. Thus, for a meaningful

comparison, the hardness data must be compared from the same

region of each test specimen. The grid network and the hardIness row identification are illustrated in a schematic of two

typical hardness specimens. Figure 31.

j A simple set of experiments was conducted in an effort

ijto duplicate the lower hardness values measured on specimens

'taken from TC2-(1) and TC2-(10B), from the top of the tank car,

ly stress-relieving specimens taken from TC2-(3) and TC2-(11B),

rom the bottom of the tank car, at various temperatures and

imes . * The results of these tests are summarized in columns

4, 5, and 6 of Table III.

Although both time and temperature are independent variables

which can affect the microstructure and thus the hardness

of a specimen, the particular set of time -temperature

parameters chosen were based on an estimate of the temperature and time-at-temperature of the top of the tank car.

It should be remembered, however, that a microstructural change

for a specific time-temperature combination will occur in a

shorter time at a higher temperature.

I

- 11 -
The results for the specimens from the failed shell

course, shell course 3, ir>digate that a 1221 F heat treatment for 15 minutes decreased the average hardness of TC2-{3)

specimens by 2 Rg points, whereas the difference between

TC2-(3) and TC2-(1) is 2 1/2 Rb points. The 1221 F treatment

was thus insufficient to match the decrease in hardness, and

either a longer time or slightly higher temperature would be

required for a decrement of 2 1/2 Rg points.

The results for the specimens from an unfailed shell

course, shell course 1, indicate that to reduce the hardness

of TC2-(11B) specimens to the level of TC2-(10B), it would

be necessary to heat into the temperature range of 1215 F to

1257 F for periods of 10 to 15 minutes: the difference in

average hardness values between TC2-(11B) and TC2-(10B) is

1 Rg, whereas with a 1215 F heat treatment for 10 minutes

the average hardness was lowered by 1/2 Rb* and with a 1257

F treatment of 15 minutes the average hardness was decreased

by 1 1/2 Rb- The 1215 F heat treatment for a somewhat longer

time would probably be sufficient to match the decrease in

hardness found as a result of the fire test.

The larger difference in hardness values between TC2-(1)

and TC2-(3) than between TC2-(10B) and TC2-(11B) is consistent

with the microstructural observation that TC2-(1) contained

greater iron carbide spheroidization than TC2-(3) while little

difference was observed between TC2-(10B) and TC2-C11B). Thus,

as expected, the plate tempered more severely had both a greater loss in hardness and a greater degree of spheroidization

These stress-relief test results further show that at any

temperature in excess of lOOO^F, the specimens from shell course

3 were softer and thus weaker then those from shell course 1.

Thus, of the two shell plates, shell course 3 would be expected

to fail by stress-rupture before shell course 1 at a given

temperature.

4,5 General Discussion

The purposes of this investigation were to determine the

probable cause and location of the initial rupture which led

to the initial fracture and eventually to complete failure of

tank car RAX 202, and to make observations on the fracture

characteristics and on elevated temperature behavior of AAR

TC128-B steel.

To these ends plate samples from four shell courses were

selected for laboratory investigation: a) the shell course

that fractured and failed, shell course 3, b) part of an

adjoining shell course, shell course 4, and c) two shell courses

that did not fail, shell course 1 and shell course 2.

- 12 -

t
I All of the steel plates in tank car RAX 202 were formed

jtrom a single heat of steel. The results of the check

[chemical analyses from the plate samples combined with the

Iproducer's ladle analysis of the melt indicated that all

jbf the samples tested met the respective chemical requiretients of AAR TC128-B steel. In addition, the variability

on an elemental basis between individual shell plates and

between the shell plates and the heat of steel was low. In

tihemical terms, the plate samples tested appear normal and

jbarely distinguishable from each other.

I Examination of the fracture edges of plate samples

TC2-(1), TC2-(3) and TC2-(7) indicated that only two fracture

modes were present, a shear mode with the fracture plane at

fen angle of 45° to the plate surfaces and a nearly perpendicular mode. The fracture mode in the region midway between

locations 4 and 5 to midway between locations 7 and 8 on

|rc2'-(l) (Figure 10), although exhibiting ductile behavior as

Evidenced by the substantial percent reductions in plate

thickness (20% to 40%) , has a rough irregular surface

riented approximately 90** to the plate surfaces.

f Metallographic examination established the interaranular character of small secondary surface cracks running

parallel to the fracture surface between locations 4 and 8

'(Figure 9). In addition, micrographs taken of regions just

ibehind the fracture face revealed substantial numbers of

Eissures or voids primarily originating at the interface

etween the proeutectoid ferrite and pearlite. SEM fractographs confirmed the ductile character of the fracture inter-

|face by the presence, throughout the fracture face of dimples

Iformed by microvoid coalescence. The ferrite grain size was

comparable to the size of the dimples, so that the SEM fractofraphs could not confirm the intergranular nature of the

racture face.

These combined results strongly suggest that the region .. ridway between locations 4 and 5 to midway between locations

and 8 on TC2-(1) (Figure 10), approximately 30 inches long,

was the site of the initial rupture of the tank car. The

features of this fracture region are characteristic of failure

y a stress-rupture mechanism. This type of failure occurs as

;a result of a high tensile stress being sustained for a sufficient time to reduce the net cross-sectional area of the

plate to such an extent that the remaining ligaments of metal

%re insufficient to carry the applied load, with the result

being failure by tensile overload. In this process, as

ILntergranular void formation and coalescence progresses, the

l|:emaining ligaments of metal are subjected to an ever increasing

I - 13 -
tensile stress. Concurrently " with void formation and

coalescence, cracks are formed and plate thinning occurs

as a consequence of these plastic processes resulting in

a reduction in the plate cross-sectional area and this

decreases the applied load necessary to cause tensile

overloading.

Many of the small stress-rupture cracks eventually

joined together to form a part-through crack at one''or more

locations. Subsequently, cracks completely penetrating the

cross-section formed and grew until the length of the stressrupture crack exceeded the critical value* required for the

onset of rapid crack propagation. For the steel of shell course

3, the 30 inch stress-rupture crack apparently exceeded this

critical length for the stress level present at the time of

the initial rupture. '

The ultimate catastrophic failure and complete separation

of shell course 3 from the tank car was probably the result

of three cooperative events. First, the stress-rupture crack

led to the initiation of rapid crack propagation. Secondly,

as the gas pressure in the tank car was being relieved by

the opening and further propagation of the cracks, the portions

of the shell course adjacent to the cracks were rapidly

forced apart by the force of the escaping gas, with the

momentum of these large masses of metal providing additional

energy to drive the crack propagation. Finally, as a result of

the rapid opening of the tank car by the progressing fracture,

a critical mixture of air and LPG at or near the crack opening

occurred and was. ignited by the fire, resulting in the

explosion which completed the fracture and caused the fragments

of shell course 3 to be propelled out of the pit.

The rapid crack propagation occurred by a shear mode.

This mode, observed on plate samples TC2-(3), TC2-(7), and

outside of the stress-rupture fracture on TC2-(1), was

generally a ductile shear type with the fracture plane at

45** to the plate surfaces and reductions in plate thickness

of between 10 and 20 percent. However, when the shear

fracture followed a weld HAZ , it contained some regions nearly

perpendicular to the plate surface and reductions in plate

thickness of less than 10 percent.

The shear fracture paths were in general related to the

circumferential or firth welds joining shell course 3 to

shell courses 2 and 4. The initial shear fractures, propagating from each end of the stress-rupture crack, occurred

with fracture planes transverse to the plate rolling direction.

However, within a very short distance, near locations 3 1/2

and 8, the shear fracture planes began to curve and align

themselves parallel to the plate rolling direction; i.e., the

- 14 -
longitudinal plate di}f;ection. During this reorientation

process from the transverse to the longitudinal direction,

one edge of the shear fracture plane reached the HAZ of

each of these two girth welds, and then propagated near and

parallel to these HAZ regions until the crack circumscribed

the tank car. The reorientation of these propagating cracks'

from the transverse to the longitudinal direction of the

plate is consistent with the findings of dynamic-tear (DT)^*^''^

tests at elevated temperatures on AAR TC128-B steel which

indicated that the resistance to crack propagation in the

direction transverse to the plate rolling direction is much

greater than in the direction parallel to the plate rolling

direction . (6) Over a broad range of elevated temperatures,

the impact energy absorption of longitudinal specimens

(transverse crack propagation) was as much as twice as large

as that for transverse specimens (longitudinal crack

propagation) . The shear cracks propagated in the circumferential

direction despite the fact that the tensile stress due to

the internal gas pressure in the tank car is twice as large

in the circumferential direction (hoop stress) as in the

axial direction. The large hoop stress should promote

crack propagation along the length of the tank car. However,

the anisotropy of fracture toughness of this steel can be

large at elevated temperatures and this anisotropy apparently

governed the crack .propagation behavior, even for a crack

that was initially propagating in a direction perpendicular • to the "weaker" direction.

The results of the stress-relieving experiments show

that the decrease in hardness found in specimens taken from

two different shell plates at the top of the tank car TC2-(1)

and TC2-(10B), can be explained if it is assumed that the top

of the tank car experienced temperatyr.es between 1200°F and

1250"? for times of between 10 to 15 minutes. The results

of the fire test (7) on an uninsulated tank car, indicate that

skin temperatures of the order of 12 00°F can be achieved from

the JP-4 fire and that flame temperature differences of over

SOO^F along the top of the tank car were measured. Since

other combinations of temperature and time-at-temperature can

lead to the observed decrease in hardness, the results of the

stress-relieving study cannot be used to determine the actual

shell plate temperature at the time of the initial rupture.
5. SUMMARY

1, An insulated tank car (RAX 2 02) , which was fabricated from

plates from a single heat of TC128-B steel, was tested to

failure in a JP-4 fire environment. Five steel samples

selected for this investigation were taken from the failed

tank car and were designated TC2-(1), TC2-(3), TC2-(7),

TC2-(10) , and TC2-(11)

.

2, The plate samples selected for investigation were taken

from four shell courses. Two samples were taken from shell

course 3, the only shell course that failed in the fire

test, TC2-(1), which contained what is believed to be the

site of the original rupture near the top of the car, and

TC2-(3), which was located at the bottom of the tank car in

this failed shell course. Sample TC2-(7), contained portions

of shell course 4 and the girth weld that joined shell courses

3 and 4. Portions of shell courses 1 and 2 were contained

in each of two samples, TC2-(10) and TC2-(1'1), taken from

relatively undeformed regions at the top and bottom of the

tank car, respectively.

3, The chemical compositions of the samples taken from shell

courses 1, 3, and 4 met the chemical requirements of AAR

TC128~69, Grade B steel. The chemical variability on an

elemental basis was low, both between individual plate

samples and between the check chemical analyses of the

plates and the ladle analysis of the heat.

4, Macroscopic examination of the outside plate surfaces of

the samples ' indicated that exposure to the JP-4 fire was

apparently insufficient to completely burn-off the . protective. thermal coating from samples located at the

bottom of the tank car' but the fire exposure was sufficient

to completely burn-off the thermal coating from samples

located at the top of the tank car.

5, Three of the five plate samples contained portions of the

fracture surfaces. Examination of these fracture surfaces

indicated that two distinct fracture modes are present.

The fracture mode in one region near the top of the tank

car in shell course 3, was rough and irregular with the

fracture surface nearly perpendicular to the plate surfaces.

This region is believed to be a 30-inch long stressrupture crack and the origin of the failure of the tank car.

Essentially all of the balance of the fracture occurred

predominately by a shear mode with the fracture plane at

an angle of about 45* to the plate surfaces.
Macroscopic observations revealed that, on both surfaces of

the plate near the fracture face of the stress-rupture

crack, numerous small cracks were present and they were

aligned parallel with the stress-rupture crack.

Thickness measurements indicated that substantial plate

thinning (2 0% to 4 0%) occurred only in and nearby the

region of the stress-rupture crack. Reductions in plate

thickness of between 10 and 2 0 percent were measured in

the shear fracture regions, except that reductions in

thickness of less than 10% were measured where the fracture

surface followed a weld HAZ

Results of metallographic examinations indicated that the

stress-rupture crack was initiated by intergranular voids

which formed primarily at interfaces between proeutectoid

ferrite and pearlite. Scanning electron microscope

fractographs indicated that this fracture surface was

formed by void coalescence which led to ductile dimpled

rupture throughout the cross section of the plate.

Metallographic examinations indicated that in the failed

shell course (no. 3) a greater degree of iron-carbide

spheriodization had occurred in the sample taken from the

top of the car when compared with that for the Scimple taken

from the bottom. In an unfailed shell course (no. 1), the

exposure to elevated temperature was . insufficient to cause

significant differences in the level of iron-carbide

spheroidization between samples taken from the top and

bottom of the tank car.

The results of hardness measurements showed that the plate

samples taken from the bottom of the tank car (in shell

courses 1 and 3) had higher hardness values than those taken

from the top of the tank car in these shell courses. The

plate sample from the top of the tank car in the failed

shell course (no. 3) had the lowest average hardness.

Further, the plate sample from the top of the tank car in

shell course 1, which did not fail, had only a slightly

lower average hardness than the sample taken from the

bottom of the tank car in this course. These hardness

results are consistent with results of the metallographic

observations of the relative levels of iron-carbide

spheroidization

The results of stress-relieving experiments on specimens

from the failed shell course, shell course 3, indicated

that with a heat treatment at 1221**F for 15 minutes, the

average hardness of specimens from the tank car bottom

- 17 -
could be decreased by 2 Rg points, which is slightly less

than the 2 1/2 hardness difference measured between

specimens taken from the top and bottom of this shell

course. Similar experiments on specimens from an unfailed

shell course, shell course 1, indicated that on specimens

taken from the bottom of the car, heat treatments of

1215°F for 10 minutes and 1257T for 15 minutes, respectively

decreased the average hardness by 1/2 and 1 1/2 Rg points,

whereas the average difference in hardness measured between

specimens from the tank car top and bottom was 1 Rg point.

Extension of the initial rupture occurred by shear

fracture which initially was aligned in the same direction

as the stress-rupture crack, transverse to the plate

rolling direction. Within a short distance from either

end of the stress-rupture crack, the ends of this propagating

shear crack turned 90" and propagated (still by shear

fracture) in the plate rolling direction. During this

reorientation process, from transverse to longitudinal, one

edge of each of the two shear fracture planes reached the

HAZ of a girth weld and propagation continued near to and

parallel to these girth welds until the fracture circumscribed

the tank car.

6 . CONCLUSIONS

The insulated rail tank car failed by rupture of the third

shell course after extended exposure to fire, v^^ith the

initial rupture being a stress-rupture crack of about 30

inches in total length that was oriented parallel to the

longitudinal axis of the tank car.

Growth of this stress-rupture crack occurred by a shear

mode initially in the same direction as the stress-rupture

crack, transverse to the plate rolling direction. After

propagating a short distance, the shear fracture plane

began to turn and the shear crack propagated in hoop

direction of the tank car, parallel to the plate rolling

direction.

This shear-fracture plane reorientation behavior is believed

to be due to the anisotropy of the fracture resistance of

this steel at elevated temperatures.

Hardness values of samples from the top and bottom of shell

course 3 differed by a wider amount than did hardness

values between specimens from the top and bottom of shell

course 1 and this observation is consistant with the
microstructural observation that a larger degree of ironcarbide spheroidization occurred in the specimens from

the top of shell course 3 than occurred in specimens

from the top of shell course 1

.

5. In both shell courses 1 and 3, the average hardness of

specimens taken from the top of the tank car was less then

that for specimens from the bottom of the car and this

can be explained by differences in temperature and timeat-temperature experienced by these regions as a result

of the fire exposure. The top of the tank car probably

experienced temperatures between 1200"? and 1250*^ for

times of between 10 and 15 minutes. The bottom of the tank

car is believed to have received insufficient high temperature exposure to cause a measureable decrease in the

strength of the steel.

6. Shell course 3 would probably fail by a stress-rupture

mechanism before shell course 1 at a given temperature of

exposure due to the relative strengths of these two plates.

The stress-relieving test results indicate that at

temperatures above lOOO'T shell course 3 specimens would

be softer and thus weaker than the shell course 1 specimens.

7 . ACKNOWLEDGEMENT

The author wishes to thank Dr. J. H. Smith for his comments

and suggestions, and Mr. D. E. Harne for his able assistance

in the metallographic and photographic portions of this report.

- 19 -
REFERENCES

1. Test Certificate, Lukens Steel Company, Mill Order Number

64931 1, Melt C0485, AAR-TC-128 GR. B, APPENDIX M FLG.

81000, February 29, 1972.

2. W. Townsend, C. Anderson, J. Zook, and G. Cowgill, "COMPARISON

OF THERMALLY COATED AND UNINSULATED RAIL TANK CARS FILLED WITH

LPG SUBJECTED TO A FIRE ENVIRONMENT", B.R.L. Interim

Memorandum Report No. 319, USA Ballistic Research Laboratories,

Aberdeen Proving Ground, Maryland, December 1974.

3. C. G. Interrante, J. G. Early, and G. E. Hicho, "ANALYSIS OF

FINDINGS OF FOUR TANK CAR ACCIDENT REPORTS", (Report No. 5)

,

To be published.

4. Metals Handbook Volume 9 , The American Society for Metals,

Metals Park, Ohio, 1974 , p . 7 5

.

5. Army-Navy Approximate Hardness Tensile Strength Relationship

of Carbon and Low Alloy Steels (AN-QQ-H-2 01 )

.

6. C. G. Interrante, G. E. Hicho, and D. E. Harne , "A ^TALLURGICAL ANALYSIS OF ELEVEN STEEL PLATES TAKEN FROM A TANK

CAR ACCIDNET NEAR CALLAO , MISSOURI", (Report No. 4), NBS

[ \r Report 312.01/51, September 19, 1972.

7. C. Anderson, W. Townsend, J. Zook, and G. Cowgill, "THE EFFECTS

OF A FIRE ENVIRONMENT ON A RAIL TANK CAR FILLED WITH LPG",

B.R.L. Interim Memorandum Report No. 288, USA BAllistic

Research Laboratories, Aberdeen Proving Ground, Maryland,

September 1974.

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a

Figure 2. Photographs of Fragments of Tank Car RAX

202 After Failure.

a. Fragment A contains the A-head plate

and shell courses 1 and 2. Fragment

B contains the B-head plate and shell

courses 4, 5, 6, and 7.

b. Largest fragment of shell course 3,

Fragment C, showing additional manway

.

c. Smallest fragment of shell course 3,

Fragment D.

Original color photographs courtesy U.S. Army.
c

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0)IH H

CIRCUMFERENTIAL WELDS

TANK CAR BOTTOM t

Figure 8. Schematic Diagram Showing the Location of NBS

Samples in Tank Car RAX 202, Viewed from the

Outside of the Tank Car. The direction of crack

propagation is indicated by the open arrows .

Figure 9. Schematic Showing the Three Mutually Perpendicular

Planes Associated with the Rolling Direction in a

Steel Plate.

Plane A is perpendicular to the short transverse

direction and is parallel to the plate surface.

Plane B is a transverse plane perpendicular to

the rolling direction, and plane C is a longitudinal

plane, perpendicular to the surface, parallel to the

rolling direction. The arrow shown on the C plane

indicates the rolling direction.

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'Figure 13. Macrograph of Inside Plate Surface of TC2-(1) at

the Fracture Surface.

Photograph was taken near location 5 and shows

numerous secondary surface cracks. A spot of

white marking paint is shown at A and a portion

of the fracture surface is shown at B. Mag. X 5


a

Figure 14. Profile Views of Fracture Surface on Plate Sample

TC2- (7) . A is shell course 4 , B is shell course 3, and

C is the weld HAZ

.

a. Location 7D-1, shear failure at 45° to the

plate surface.

b. Location 7D-2 , shear failure at 45° to the

place surface in weld HAZ.

c. Location 7C, mixed mode fracture in weld HAZ

with a nearly perpendicular region (N.P.)

bounded by two shear failure regions

.

Etch: 5% Nital. Mag. X 4


b

Figure 15. Profile Views of Fracture Surface on Plate Sample

TC2-(1), Shell Course 3.

Fracture surface is nearly perpendicular to plate

surfaces with evidence of very small shear lips at

the plate surfaces.

a . Location 6

b. Location 5

Etch: 5% Nital. Mag. X 4


Figure 16. Representative Microstructures of Plate

Sample TC2-(1) Away from Fracture Face.

AAR TC128-B-69 Steel.

Micrographs taken on the C plane of

shell course 3 near the top of the tank

car showing moderate banding at the

plate midthickness position. Ferrite

grain size number 8-1/2.

Etch: 5% Nital. Mag. X 100
Outside Surface

Inside Surface


Figure 17. Representative Microstructures of Plate

Sample TC2-(3). AAR TC128-B-69 steel.

Micrographs taken on the C plane of

shell course 3 at the bottom of the

tank car showing moderate banding at

the plate midthickness position.

Ferrite grain size number 8-1/2.

Etch: 5% Nital. Mag. X 100
Outside Surface

Inside Surface

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fd 0 H U n^

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fd

w

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Figure 20. Photomicrographs of the Plate Surface of Plate

Sample TC2-(1) near the Fracture Surface Showing

Extensive Deformation of the Ferrite Grains.

Arrows indicate the direction of the principal

or hoop stress.

a. C plane, outside plate surface.

b. C plane, inside plate surface.

Etch: 5% Nital. Mag. X 100
I

I

L&qioni-iq ©rii
b

Figure 21. Photomicrograph of an Intergranular Crack Near

the Fracture Initiation Site on Plate Sample

TC2- (1) . a. C plane. Mag. X 200

b. C plane. Mag. X 500

The gray layer on the inside plate surface,

crack surfaces , and voids near the surface is

believed to be iron oxide.

Etch: 4% Picral.

b

Figure 22. Photomicrographs of Fracture Surface of Plate

Sample TC2-(1) Showing the Profile of the Fracture

Edge , the Elongated Ferrite Grains and Cracks Near

the Fracture Edge.

a. C plane, near outside plate surface

b. C plane, near inside plate surface

Etch: 5% Nital. Mag. X 100
J.

I

J.

f - 1. 1

I
Figure 23. Photomicrographs of the Fracture Surface and

the Regional Immediately Behind the Fracture

Surface on Plate Sample TC2- (1) . a. C plane. Mag. X 200

b. C plane. Mag. X 5 00

The gray layer on the fracture surface and

crack surface is believed to be iron oxide.

Etch: 4% Picral.

b

Figure 24. SEM Fractographs of the Fracture Surface of

Plate Sample TC2-(1) in the Fracture

Inition Region Near the Outside Plate Surface.

a. Outside plate surface at the top of the

photograph, near location 6. Mag. X 14

b. Higher magnification photograph of the

region indicated by the arrows in (a)

.

Mag. X 190
'-^AS lo si
b

Figure 25. SEM Fractographs of Fracture Surface of Plate

Sample TC2-(1) in the Fracture Initiation

Region Near the Plate Midthickness Region.

a. Near location 6. Mag. X 13

b. Higher magnification photograph of the

region indicated by the arrows in (a)

.

Mag. X 190


a

Figure 26. SEM Fractographs of the Fracture Surface of

Plate Sample TC2-(1) in the Fracture Initiation

Region Near the Inside Plate Surface.

a. Inside plate surface at the bottom of the

photograph, near location 6. Mag. X 13

b. Higher magnification photograph of the

region indicated by arrows in (a) . Mag.

X 100

c. Higher magnification photograph of the

region indicated by arrows in (b)

.

Mag. X 190
c


Figure 27. Representative Microstructures of Plate

Sample TC2-(10B). AAR TC128-B-69 Steel.

'

, . Micrographs taken on the C plane of shell

; ^ course 1 at the top of the tank car showing

moderate banding at the plate midthickness

position. Ferrite grain size number 8.

Etch: 5% Nital. Mag. X 100
Outside Surface

Inside Surface


Figure 28. Representative Microstructures of Plate

Sample TC2-(11B). AAR TC128-B-69 Steel.

•: Micrographs taken on the C plane of

shell course 1 at the bottom of the tank

car showing moderate banding at the plate

midthickness position. Ferrite grain

size number 8-1/2.

Etch: 5% Nital. Mag. X 100
Outside Surface

Inside Surface

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ROW I

ROW 2

ROW 3

ROW 4

ROW 5

OUTSIDE PLATE SURFACE

0.

90 90 90 90 '/2 90 90 90 90 90 '/2

90 90!^2 90'/2 89'/2 90'/2

86'/2 87'/2 86'/2 87'/2 87 87 87 86'/2 86'/2

89 89 89 '/2 91 88'/2

90 90 90 89 '/2 90 90 90 90 89

>0.

>o.r

ROW I ROW 2

ROW 3

ROW 4

ROW 5

0.

INSIDE PLATE SURFACE

(a)

OUTSIDE PLATE SURFACE

0.

88'/2 89 89 89 89 88 '/2 89 88 '/2

88 89 88 88 88 '/2

90 89 87 87 '/2 87'/2 87'/2 88'/2 88'/2

88 89 89 89 '/2 89

89 89'/2 89'/2 91 89'/2 89'/2 89'/2 89

INSIDE PLATE SURFACE

(b)

"b" plane

"b" plane

Figure 31. Schematic of Representative Hardness Profiles

from Plate Samples TC2-(3) and TC2-(11B).

a. Specimen from TC2-(11B), B plane, as received

b. Specimen from TC2-(3), B plane, as-received.

NBS-lUA (BEv. 7-7-j)

U.S. DtPT. OF COMM. BIBLIOGRAPHIC DATA

SHEF.T

1. I'l ni l' A I |0\ OK Kl I'OK 1 .\0.

NBS IR 75-657

2. ( M IV t .Af < r s 1 Dii N.,.

4. I i I I.I- .\.M) ^1 IM I I I I Report No . 7

A ^E:TALLURGICAL INVESTIGATION OF A FULL-SCALE

INSULATED RAIL TANK CAR FILLED WITH LPG SUBJECTED

TO A FIRE ENVIRONMENT

3. I<< < ipitnt 's .Ac L 1 11)11 No.

5. I 'u SI i( at Ion I ).it 1 January, 1975

6. I'( rlotmin^ Or^.in i / ai mn ' odo

7. ALTHOR'^i

J. G. Early and C. G. I-nterrante

8. I'l rlormin^ OrLMii. Ki-purt No. NBS IR 75-657

9. HtHFORMI.\(. OK(.,A.\i/,A I I0.\ NAMl AND Al)^l<l..^s NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20J34

10. Fro|i cr ' l aslv ttiuk l.nit .No. 3120413

11. (oniraci (jrani Ni

12. Sponsoring Or^ .in 1/ at mn Name and ^omplcti- AJJrtss /Street, City, State, ZIP)

Federal Railroad Administration

Department of Transportation

Washington, D. C. 20591

13. Type of Ri-port & Pcrio.J

' "^"^'-^ Failure

Analysis Report

14. Sponsoring; .Age nt v (.

15. SLPPl.l-MI NTARV .\OTtS

16. AHSTR.AC r (A 200-word or less (actual summary ot most stSfxiticanl in tormation. If documfnl includes a signi ticant bibliography or literature survey, mention it here.)

An analysis of the failure of an insulated rail tank car, RAX 202, which had been tested to failure in a fire environment at White Sands

Missile Range, New Mexico, was requested by the Federal Railroad Administration, Department of Transportation. The tank car, filled with

liquified petroleum gas (LPG) , failed after approximately 94 minutes of

exposure to a JP-4 jet fuel fire. Five plate samples were selected for

laboratory study at the National Bureau of Standards.

The results of metallurgical investigations suggest that a region approximately 30 inches in length near the top of the tank car in shell

course 3 was the site of the initial rupture of the tank car. The results;

of stress-relieving experiments conducted on samples taken from the top ar.d

bottom of the car indicated that the top of the tank car experienced

temperatures of 1200°F to 1250°F for times of between 10 to 15 minutes.

The fracture features of the initial rupture were indicative of

failure by a stress -rupture mechanism. It was concluded that this 30-

inch stress-rupture crack led to tensile overload, instability and to the

onset of rapid crack propagation in a shear mode, with the initial shear

fracture propagating as an extension of the original stress-rupture crack. Within a short distance, this shear fracture turned 90** and propagated in

^^-^-'•i^"'^ direction, a result explained by the anisotropy of the

sLanctf uf .this .r.i VIl'K1)> (if\ lo r.ic'7vt' enlries, alpndoeTic name: separated by semic olon.<<

)

Anisotropy; ductile fracture; fire test; insulated rail tank car; stres$

rupture crack

18. A\ AIl.Ami.l 1 'I Unl imitcd 19. SIX IIKITV CLASS

' 1 HIS RLPORT)

21. NO. OF PAGES

XX For Offici.il distribution. Ho Not Release to NT IS L ASSil 11 0

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