NBSIR 75-725/^)

Report No. 8

Ambient- and Elevated-Temperature

Mechanical Properties of AAR M128-69-B

Steel Plate Samples Taken From Fire Tested

Insulated Tank Car RAX 202

J. G. Early

Mechanical Properl

Metallurgy Divisio

Institute for Mater

National Bureau of Washington, D. C.

Early., J- G -i Mechanical properties of AAR

Ml 28 -69 -B steel plate samples taken from

ihsulated fire tested tank car RAX 202, DOT

Report No. FRA-ORScd/^^, 57 pages (Available

as PB/255-907 from the National Technical

Information Service, Springfield, VA, 22l6l,

June 1976). « S I 7^' 7

^

May 1975

Final Report

31:

Prepared for Federal Railroad Administration

Department of Transportation

Washington, D. C. 20591

NBSIR 75-725

REPORT NO. 8

AMBIENT- AND ELEVATED-TEMPERATURE

MECHANICAL PROPERTIES OF AAR M128-69-B

STEEL PLATE SAMPLES TAKEN FROM FIRE TESTED

INSULATED TANK CAR RAX 202

J. G. Early

Mechanical Properties Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

May 1975

Final Report

"This document has been prepared for the use of the

Federal Railroad Administration, Department of Transportation.

Responsibility for its further use rests with that agency. NBS requests that if release to the general

public is contemplated, such action be taken only

after consultation with the Office of Public Affairs

at the National Bureau of Standards."

Prepared for Federal Railroad Administration

Department of Transportation

Washington, D. C. 20591

/ V \

U.S. DEPARTMENT OF COMMERCE, Rogers C.B. Morton. Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

TABLE OF CONTENTS

Page

ABSTRACT

1. INTRODUCTION 1

2. PURPOSE 2

3. EXPERIMENTAL PROCEDURES 2

3.1 Ambient-Temperature Tests 3

3.2 Elevated-Temperature Tests 3

3.2.1 Resistance Heated Tube Furnace

Test Procedures 3

3.2.2 Internal-Resistance-Heating Test Procedures 4

3.2.3 Hot-Tensile Tests 4

3.2.4 Stress-Rupture Tests 5

3.3 Macroscopic and Metallographic Observations 5

4. RESULTS AND DISCUSSION 5

4.1 Ambient-Temperature Tensile Properties 5

4.1.1 Strain Ageing Effects 6

4.2 Hot-Tensile Properties

4.2.1 Comparison of Hot-Tensile Data for AAR

M128 Steel 9

4.3 Stress-Rupture Properties 11

4.3.1 Comparison of Stress-Rupture Data for Tank

Car Steels 11

4.4 Macroscopic and Metallographic Analyses 13

4.5 General Discussion 14

4.5.1 Ambient- Temperature Properties 15

4.5.2 Hot-Tensile Properties 15

i
TABLE OF CONTENTS (continued)

4.5.3 Stress-Rupture Properties

4.5.4 Metal lographic Analysis..

5. SUMMARY

6. CONCLUSIONS. . .

.

7. ACKNOWLEDGEMENT

REFERENCES

TABLES

I. Ambient- Temperature Tensile Properties of Specimens Taken From

Car RAX 202

II. Hot-Tensile Properties of Specimens Taken From Plate Sample

TC2-(11B)

III. Stress-Rupture Properties of Specimens Taker. From Tank Car RAX 202

FIGURES

1. Schematic Diagram Showing the location of NBS Samples in Tank Car

RAX 202 as Viewed from the Outside of the Tank Car.

2. Plate Sample TC2-(1), Shell Course 3.

3. Plate Sample TC2-(3), Shell Course 3.

4. Plate Sample TC2-(11), Shell Course 1 and Shell Course 2.

5. Elevated -Temperature Strength Properties of Plate Sample TC2-(11B),

AAR M128-B Steel, Taken from Tank Car RAX 202.

6. Elevated -Temperature Tensile Ductility Properties of Plate Sample

TC2- ( 1 1 B ) , AAR M128-B Steel, Taken from Tank Car RAX 202.

7. Elevated -Temperature Ultimate Tensile Strength Properties of

AAR M128-B Steel

.

8. Elevated -Temperature Yield Strength Properties of AAR M128-B Steel.

9. Stress-Rupture Data for Three Plate Samples from Tank Car RAX 202,

AAR M128-B Steel.

Page

17

19

20

22

22

24

i i
FIGURES (continued)

10. Stress-Rupture Data for Three Plate Samples from Tank Car RAX 202,

AAR M128-B Steel

.

11. Stress-Ruptures Data for Three Plate Samples from Tank Car RAX 202,

AAR M128-B Steel

.

12. Stress-Rupture uata of Several Pressure Vessel Steels.

13. Hot-Tension Test Specimen No. 21, Plate Sample TC2-(11B).

14. Hot-Tension Test Specimen No. 38, Plate Sample TC2-(11B).

15. RTF Stress-Rupture Specimen No. 25, Plate Sample TC2-(11B).

16. RTF Stress-Rupture Specimen No. 32, Plate Sample TC2-(11B).

17. IRH Stress-Rupture Specimens.

18. Photomicrographs of Fracture Profile Sections Showing the Fracture

Edge.

19. Photomicrographs of Fracture Profile Sections Showing the Fracture

Edge.

20. Photomicrographs of the Region Immediately Behind the Fracture

Surface.

21. Photomicrographs of the Region Immediately Behind the Fracture

Surface.

22. Photomicrographs of the Region Immediately Behind the Fracture

Surface.

23. SEM Fractographs of Selected Fracture Surfaces, Plate Sample

TC2-(11B).

24. SEM Fractographs of Selected Fracture Surfaces, Plate Sample

TC2-01B).

25. SEM Fractographs of Selected Fracture Surfaces.

i i i

ABSTRACT

Studies were undertaken at the National Bureau of Standards to measure the elevated-temperature mechanical properties and to determine

the elevated-temperature fracture behavior of selected AAR M128-B steel

plates taken from insulated tank car RAX 202. This tank car had been

previously tested to failure in a fire environment. Further, these

measured elevated-temperature mechanical properties were compared with

published data of other plate samples of AAR M128-B steel. A secondary

effort was to measure the ambient-temperature mechanical properties of

plate samples taken from tank car RAX 202 to determine if the requirements

of specification AAR M128-69-B were satisfied.

Three plate samples, designated TC2-(1), TC2-(3) and TC2-(11B),

representing two shell courses, were selected for this study. The NBS

results of check chemical analyses, hardness surveys, thickness measurements, macroscopic observations, and metal lographic analyses of these

three plate samples had been previously reported.

The results of ambient-temperature tensile tests showed that all

three plate samples met the ultimate tensile strength, yield strength,

and percent elongation requirements of specification AAR M128-69-B.

The results of hot-tensile tests showed a continuous decrease in

ultimate tensile strength and yield strength, and an increase in tensile

ductility, as measured by percent elongation, as the test temperature was

increased from 1100 F to 1250 F. These results indicated that dynamic

strain ageing was not significant in the temperature range investigated

for the testing speeds used.

Analysis of stress-rupture data for specimens from all three plate

samples in the temperature range of 1100 F to 1250 F indicated that a single straight line in a log-log plot of initial stress versus rupture

life reasonably represented the data at each test temperature. This

result appears to be independent of the plate sample or the specimen

orientation. Further, in the temperature and stress range investigated,

a decrease in the initial stress of approximately 20 to 30 percent

resulted in a twelvefold increase in the rupture life from 15 minutes

to three hours.

A comparison of the results of the metallographic analysis of

hot-tensile and representative stress-rupture specimens in this study

with the previously reported metallographic results on the initial

rupture site in the failed shell course, all from tank car RAX 202,

indicate the presence of the identical fracture mode. This mode is

characterized by many intergranular voids which originate primarily

at the proeutectoid ferrite-pearlite boundaries. These results

confirm the previously reported finding that the initial rupture of

tank car RAX 202 was a stress-rupture crack.

A comparison of hot-tensile and stress-rupture test results

between specimens from tank car RAX 201 and specimens from tank car

RAX 202 showed substantial disagreement. The steel plates used to
fabricate both tank cars were reportedly produced from the same heat

of AAR M128-69-B steel, and thus the origin of the discrepancy in the

elevated-temperature mechanical properties between specimens from these

two tank cars is not apparent.
1. INTRODUCTION

In recent years, a number of catastrophic failures of railroad

tank cars carrying hazardous materials have occurred. These catastropic

accidents have prompted the Federal Railroad Administration (FRA) to

sponsor research activities with the object of developing knowledge

concerning the various factors responsible for causing rupture and

failure of tank cars involved in accidents, and to use this knowledge to

prevent or minimize the effects of tank car failures.

In one phase of the FRA-sponsored research program, fire engulfment

tests were conducted on two full-scale rail tank cars filled with

liquified petroleum gas (LPG); uninsulated tank car RAX 201 and insulated

tank car RAX 202. The steel plates used in the fabrication of both tank cars were reported to be 5/8 inch-thick, fine-grained steel plate in the

as-rolled condition, produced to specification AAR Ml 28-69, Grade B,

Flange Quality, by Lukens Steel Company as part of Melt Number CO 485.(1) a

The fire tests on both full-scale tank cars were conducted at White

Sands Missile Range, New Mexico, under the direction of personnel from

the United States Army Ballistic Research Laboratory (BRL). The details

of the test procedures and instrumentation used in these fire tests have

been reported previously. (2,3) A metallurgical analysis of the failure

of the uninsulated tank car, RAX 201, indicated that the initial rupture

was the result of a stress-rupture crack which initiated near the top

of the tank car. (4)

The FRA requested that the National Bureau of Standards (NBS)

conduct a metallurgical evaluation of the failed insulated tank car,

RAX 202. The analysis of the failure led to the conclusion that the

initial rupture was a stress-rupture crack which formed as a result of

prolonged exposure to elevated temperatures. (5)

The results of the two full-scale fire tests and data from other

tank car failures (6) suggest that knowledge of the elevated-temperature

mechanical properties of tank car steels is essential to any understanding

of the failure behavior of tank cars carrying compressed gases subjected

to fire environments. At present, however, elevated -temperature data

available in the literature for AAR M128-B steel appears limited. Typical

of the available data are results from hot-tensile tests on specimens from

a failed tank car (7) and from an as-rolled plate (8), and hot-tensile

tests and short-time stress-rupture tests on specimens from the fire

tested uninsulated tank car, RAX 201 (4).

Accordingly, a program was initiated at NBS to measure the

elevated -temperature mechanical properties of AAR M128-B steel plates

used in the insulated tank car, RAX 202. Three plate samples, previously

The isolated numbers in parentheses refer to references listed at

the end of the report.

- 1 -
designated TC2-(1), TC2-(3) and TC2-(11B) D

, representing two shell

courses, were selected for this investigation. The NBS results of check

chemical analyses, hardness surveys, thickness measurements, macroscopic

observations, and metal logra phi c analyses of these three plate samples

have been previously reported. (5)

Plate sample TC2-(1), taken from the top of the failed shell course

in the region experiencing the highest temperature during the fire test,

contained a portion of the stress-rupture crack believed to have been the

site of the initial rupture of the tank car. Plate sample TC2-(3) was removed from the bottom of the failed shell course in the region heated

the least during the fire test. Plate sample TC2-(11B), taken from the

bottom of the tank car in an unfailed shell course, was selected because

it was located in an undeformed and relatively unheated region of the

tank car. After the fire test, the bottom of the tank car still contained

a portion of the original thermal protective coating which indicated that

this region was heated the least during the test. A schematic of a portion of the tank car, Figure 1, shows a representation of the plate

samples in their approximate locations, as viewed from the outside of

the tank car.

2. PURPOSE

The principal purpose of this investigation was to determine the hottensile and short-time stress-rupture properties of AAR M128-B steel plates

used in the fabrication of insulated tank car RAX 202 which had previously

failed in a fire engulfment test. Another purpose was to determine the

ambient temperature mechanical properties of these plates and to compare

the properties with the requirements of specification AAR M128-69-B.

3. EXPERIMENTAL PROCEDURES

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

TC2-(11B) for ambient-temperature and hot-tensile tests, and for stressrupture tests, were prepared in accordance with ASTM E8-69, Tension

b Plate sample TC2-(11) contained portions of the shell courses 1 and 2.

Tests and observations on this piate sample are reported for the particular

shell course involved; TC2-(11A) from shell course 2 and TC2-(11B) from shell

course 1

.
Testing of Metallic Materials. The specimens, 0.250 inches in diameter

in the reduced section with a one-inch-gage length, were taken as closely

as possible from the quarterthickness position of the plate. Longitudinal

and transverse specimens d were taken from plate sample TC2-(11B) while

only longitudinal specimens were taken from plate samples TC2-(1) and

TC2-(3). The location in the respective plate samples from which the

test specimens were prepared are shown in Figures 2, 3 and 4. Macroscopic

and metal lographic observation were made on selected hot-tensile and

stress-rupture specimens from plate samples TC2-(1), TC2-(3) and TC2-(11B).

3.1 Ambient-Temperature Tests

Eight specimens for ambient-temperature tensile tests were taken from

plate samples TC2-(1), TC2-(3) and TC2-(11B) and tested in accordance with

ASTM A370-73, Mechanical Testing of Steel Products. Two longitudinal and

two transverse specimens were tested from plate sample TC2-(11B), and two

longitudinal specimens were tested from each of the plate samples TC2-(1)

and TC2-(3).

3.2 Elevated-Temperature Tests

Three test procedures were followed during the evaluation of the

elevated-temperature mechanical properties.

3.2.1 Resistance Heated Tube Furnace (RTF) Test Procedures

Hot-tensile tests and a limited number of stress-rupture

tests were conducted using a three-zone resistance heated tube furnace

with separate temperature control for each zone. The temperature of

each zone was adjusted in order to minimize the temperature gradient

along the specimen gage length. The maximum temperature gradient

measured in the one-inch gage length was less than 6 F per inch. Three

thermocouples were attached to each specimen, one on each shoulder and

Four ASTM Recommended Practices are applicable to these three tests.

Ambient temperature tension testing is covered by ASTM E8, Tension

Testing of Metallic Materials. Hot-tensile testing is covered by

ASTM E21 , Elevated Temperature Tension Tests of Metallic Materials,

and stress-rupture testing is covered by either ASTM El 39, Conducting

Creep, Creep-Rupture, and Stress-Rupture Tests of Metallic Materials

or ASTM El 50 , Conducting Creep and Creep-Rupture Tension Tests of

Metallic materials under Conditions of Rapid Heating and Short Times.

The dimensions applicable to the standard 0.250 inch diameter, cylindrical

test specimen are the same for each Recommended Practice with two

exceptions. ASTM E21 and El 39 require tighter tolerances on the

gage length and diameter than the +/-0.005 inches required by ASTM E8 or E150. All specimens were prepared to a tolerance of +0.002 inches.

Longitudinal and transverse refer to the longitudinal and transverse

specimen axis orientation with respect to the principal plate rolling

direction.

- 3

-
one at the center of the gage length. The specimen was placed in the

center of the furnace, which had been preheated to the appropriate test

temperature. The temperature of the specimen was monitored until the

specimen reached thermal equilibrium at the test temperature. e The

test was then initiated by the application of the load, and the specimen

temperature was periodically monitored throughout the test. At the • conclusion of each test, the specimen fragments were quickly removed

from the furnace to avoid additional oxidation.

3.2.2 Internal-Resistance-Heatinn ( IRH) Test Procedures

Stress-rupture tests were conducted using an alternative

heating method based on the electrical resistance of the test specimen.

A thermocouple attached to the specimen at the center of the gage length

was used to maintain the appropriate test temperature by automatically

controlling an electric current passing through the specimen. The ends

of the specimen were clamped in water-cooled copper grips which resulted

in a temperature gradient along the specimen gage length. The temperature

gradient was believed to be substantially greater than that measured

in the RTF method. The test terminated upon the fracture of the

specimen which resulted in the breaking of the heating current circuit

and the rapid cooling of the specimen fragments.'

3.2.3 Hot-Tensile Tests

Six longitudinal specimens prepared from plate sample

TC2-(11B) were tested at temperatures of 1100 F, 1150 F, 1200 F, and

1250 F in accordance with ASTM E21-70, Elevated Temperature Tension

Tests of Metallic Materials. 9 Four specimens, one at each test

temperature, were tested at an initial crosshead speed of 0.005 inches

per minute until yielding was complete. After yielding, the crosshead

speed was gradually increased to 0.05 inches per minute for the

remainder of the test. The other two specimens were tested, one at

1100 F and one at 1200 F, with the crosshead speed maintained at 0.05

inches per minute for the entire test.

i he time to reach thermal equilibrium varied between approximately

60 minutes to 75 minutes depending on the test temperature.

When specimen necking occurs using the i nternal -resistance-heati ng

method, the temperature of the specimen may increase above the

nominal test temperature if the thermocouple is not located at the

point of necking. Therefore, in this investigation, a meaningful

test result was obtained if the fracture site and the thermocouple

location are coincident.

Exceptions to the procedure of E21 were as follows: (i) single

specimens were tested at each temperature, (ii) the strain was calculated from the crosshead displacement.

- 4 -
3.2.4 Stress-Rupture Tests

Thirty-one specimens for stress-rupture tests were taken

from plate samples TC2-(1)» TC2-(3) and TC2- ( 1 1 B) : Fifteen longitudinal

and six transverse specimens from plate sample TC2-(11B), four longitudinal

specimens from plate sample TC2-(1), and six longitudinal specimens from

plate sample TC2-(3). Tests were conducted at temperatures of 1100 F,

1150 F, 1200 F and 1250 F. Twenty-nine of the 31 specimens were tested

by the internal -resistance heating technique (IRH) in accordance with

E150-64, Conducting Creep and Creep-Rupture Tension Tests of Metallic

Materials under Conditions of Rapid Heating and Short Times. The other

two specimens were tested in an electric resistance tube furnace (RTF)

in accordance with El 39-70, Conducting Creep, Creep-Rupture, and StressRupture Tests of Metallic Materials.

3.3 Macroscopic and Metallographic Observations

The exterior surfaces of five hot tensile specimens and three

stress-rupture specimens were macroscopically examined after testing in

both the optical microscope and the scanning electron microscope (SEM).

Metallographic observations were made on fracture profile sections,

taken parallel to the specimen axis, from one of the specimen halves

from each of the six hot-tensile specimen and from eight of the stressrupture specimens. SEM fractographs were taken of the fracture surfaces

of the other half of the fourteen specimens used for the metallographic

analyses

.

4. RESULTS AND DISCUSSION

4.1 Ambient-Temperature Tensile Properties

The results of ambient-temperature tensile tests of specimens

taken from plate samples TC2-(1), TC2-(3) and TC2-(11B) are given in

Table I, along with the tensile property requirements for AAR M128-B-69

steel plates. These results show that, for all plates tested, the

ultimate tensile strength values were within the specification limits

limits of 81 to 101 ksi, and the yield strength and percent elongation

values exceeded the minimum specification requirements of 50 ksi and

19 percent, respectively.

For plate sample TC2-(11B), the average values of longitudinal and

transverse ultimate tensile strength were 85.9 and 86.3 ksi, respectively,

and the average value of yield strength was 55.1 ksi for both orientations.

These values indicate that there was little anisotropy in the strength of

the plate in the plane of the rolling direction. However, the average

tensile ductility values, measured by percent elongation, were 30.2 percent

and 27.2 percent, respectively, for longitudinal and transverse specimens.

Thus, the longitudinal elongation values were approximately ten percent

higher than the transverse values. This finding was consistent with the

general observation that the tensile ductility of rolled steel plates

tends to be greater in the longitudinal direction than in the transverse

direction.

- 5

-
A review of the ambient- temperature mechanical properties of three

plate samples of AAR M128-B-65 steel and one plate sample of AAR M128-A-69

steel (9) shows: (i) Little difference between the average ultimate

tensile strength and yield strength values of longitudinal and transverse

specimens, (ii) The average longitudinal ductility, measured by percent

elongation, was approximately ten percent higher than the transverse

ductility. The anisotropy of tensile ductility observed in this study,

as well as that observed in earlier work on plate samples taken from tank cars which failed under a variety of conditions, is attributed to the

hot-rolling operation used to fabricate AAR Ml 28 steel plates.

As shown in Table I, the average ultimate tensile strength value

obtained from plate sample TC2-(3) was 87.3 ksi, as compared with 84.6 ksi

for plate sample TC2-(1). The tensile ductility values for plate sample

TC2-(3) were 26.6 percent elongation and 63.0 percent reduction-i n-area,

as compared with 29.6 percent elongation and 65.2 percent reduction-in-area

for plate sample TC2-(1). Thus, specimens from plate sample TC2-(3) were

found to exhibit a higher average ultimate tensile strength and lower

average tensile ductility than specimens from plate sample TC2-(1). These

results are consistent with observations and measurements previously

reported (5) for plate samples TC2-(3) and TC2-(1). In the earlier work,

it was found that plate sample TC2-(1), taken from the top of the tank car in the region experiencing the highest temperatures, was softer and

contained a greater degree of iron carbide spheriodization than plate

sample TC2-(3), taken from the bottom of the tank car in the least

heated region.

The yield strengths for both specimens tested from plate sample

TC2-(3) were unusually high, averaging 72.0 ksi, which was approximately

30 percent higher than the average values of 55.9 ksi and 55.1 ksi,

respectively, for plate samples TC2-(1) and TC2-(11B). This observation,

characteristic of the yield strength data, but not the tensile strength

data, is possibly related to the plastic deformation which occurred in

the region containing plate sample TC2-(3) as a result of the partial

flattening of the fragment of shell course 3 immediately after the

failure of tank car RAX 202. It is known that such plastic

deformation may increase the yield strength, but not the tensile strength

of ductile metals.

4.1.1 Strain Ageing Effects

Strain ageing manifests itself in susceptible steels by the

presence of a sharp yield point and by increases in flow stress, ultimate

tensile strength, and hardness relative to the unstrain aged material.

Peterson (10) studied the strengthening effects of strain ageing on low-(0.09 weight percent carbon) and medium-(0.46 weight percent carbon)

carbon steels. The ambient-temperature tensile properties were measured

on specimens that had been plastically deformed at temperatures up to

1000 F. Measured values of the 0.2-percent yield strength were up to

- 6 -
120 percent above tne values for undefomed specimens and were accompanied

by increases in ultimate tensile strength of up to 50 percent above the

undefomed values for botn the low- and medium-carbon steels.

Specimens tested from plate sample TC2 - ( 3 ) did not exhibit

sharp yield points n although small y^eld points were observed for all

specimens tested from plate samples TC2-0) and TC2- ( 1 1 B ) , as shown in

Table I. These observations are consistent with what is believed to be

the previous thermal -mecnan i cal history of these three plate samples.

Plate sample TC2-(11B) was selected for this study because of its location

in an undeformed ana relatively unheated region of the tank car. This

plate sample is believed to reasonably represent an aluminum deoxidized,

hot-rollea plate of kAR M128-B steel in the stress-relieved condition.

The observed small yield point in specimens from TC2-(1 IB) was not unexpected,

since stress-relieving, an aluminum deoxidized, hot-rolled carbon steel

does not necessarily eliminate strain ageing effects (11). Further, the

presence of yield points in specimens from plate sample TC2-(11B),

representative of tne tank car snell plates in the fabricated and stressrelieved condition, suggests that yield points should be observed in the

other plate samples studied in this investigation. All of the plates useo

in tank car RAX 202 were rolled from the same heat of steel and would be

expected to have the same strain ageing susceptibility. The presence of a yield point in specimens fror plate sample TC2-(1) indicated that the

effect of tne fire exposure was insufficient to eliminate the yield point.

Plate sample TC 2

- ( 3 ) was plastically deformed as a result of the

catastrophic failure of tne tank car and thus would be expected to

exhibit a yield point if ageing occurred subsequent to the deformation or

if the amount of plastic deformation was insufficient to eliminate the

yield point. The absence of the yield point in specimens from TC2-(3)

suggests that the deformation was sufficient to eliminate the yield point

and that subsequent ageing effects were too small to produce a measurable

yield point.

The high yield strength values measured on specimens from TC2-(3)

are not believed to be tne result of strain ageing for the following

reasons. First, the absence of a measurable yield point indicates that

the strain ageing effect would be too small to explain the magnitude of

the increase in yield strength for specimens from TC2-(3) compared to

specimens from TC2-(11B). Second, the large increase in yield strength

was not accompanied by either a significant increase in ultimate tensile

strength, as shown in Table I, nor an increase in hardness (5) compared

to specimens from TC2-(11B).

4.2 Hot-Tensile Properties

The results of the elevated-temperature tensile tests of specimens

from plate sample TC 2

-

( 1 IB) are given in Table II and Figures 5 and 6.

The tensile machine used for these tests was sufficiently stiff to

allow observation of snarp yield points.

- 7

-
The results show, that for a given test procedure, there was a continuous

decrease in ultimate tensile strength and yield strength and a continuous

increase in percent elongation as the test temperature was increased

from 1100 F to 1250 F. The percent reduction-in-area values, although

less temperature sensitive than the strength or elongation values, showed

a gradual increase as the test temperature was increased.

The ultimate tensile strength, measured at the slower crosshead

speed of 0.005/0.05 inches per minute, 1 decreased from 29.9 ksi at

1100 F to 16.8 ksi at 1250 F, whereas the yield strength decreased from

20.7 ksi to 9.5 ksi over the same temperature interval. The tensile

ductility values for these specimens were 39.2 percent and 46.5 percent,

respectively, for percent elongation and percent reduction-in-area at

1100 F and 57.4 percent elongation and 53.7 percent reduction-in-area at

1250 F.

At the higher crosshead speed of 0.05 inches per minute, the

ultimate tensile strength decreased from 34.1 ksi at 1100 F to 24.1 ksi

at 1200 F, whereas the yield strength was 32.4 ksi at 1100 F and 21.1 ksi

at 1200 F. The corresponding values for percent elongation and percent

reduction-in-area were, respectively, 27.0 and 37.2 at 1100 F and 44.9

and 41.3 at 1200 F. The yield strength-to-ultimate tensile strength ratio

at this higher crosshead speed was much greater than that at the lower

crosshead speed (0.95 versus 0.69 at 1100 F and 0.87 versus 0.57 at

1200 F), due to the fact that yield strength is more rate sensitive than

is tensile strength.

The observed tensile ductility behavior is consistent with

conclusions reported by Smith (12) in a review of the elevatedtemperature static properties of carbon steels. The ductility of the

steels referred to by Smith, as measured by percent elongation and

percent reduction-in-area, generally increased as the test temperature

was raised above the maximum strain ageing temperature of 600 F.

The data show that with increasing test temperature, the ultimate

tensile strength and yield strength values decrease monotonical ly, thus

indicating that dynamic strain ageing probably did not affect these

strength properties over the temperature range investigated. This

conclusion is in agreement with work of Peterson (10) and Miller (11),

who found that dynamic strain ageing effects on a large number of

carbon steels were not observed in hot-tensile tests conducted above 600 F.

The initial crosshead speed of 0.005 inches per minute was not

held constant for the entire test. After loading past the yield,

the crosshead speed was gradually increased to 0.05 inches per

minute as allowed by ASTM E21 . Thus a quantitative comparison of

crosshead speed effects on the mechanical properties cannot be made

as the effective crosshead speed of these tests is somewhere between

0.005 inches per minute and 0.05 inches per minute.

- 8 -
The data further show that the effect of strain rate or crosshead

speed on tensile strength is somewhat greater as the temperature of

deformation increases. At 1100 F, a specimen tested at a crosshead

speed of 0.05 inches per minute had a tensile strength 14 percent

greater than that of a specimen tested at a crosshead speed of 0.005/0.05

inches per minute. At 1200 F, the increase in tensile strength was 19

percent for these two crosshead speeds. These results are in agreement

with the results of Miller, Smith and Kehl (13) who investigated the

influence of strain rate on the strength of pearl itic carbon-molybdenum

steel and found that crosshead speed had a larger effect on tensile

strength at 1100 F than at 850 F.

4.2.1 Comparison of Hot-Tensile Data for AAR Ml 28 Steel

It is of interest to compare the hot-tensile data from this

study with data from other investigators in order to assess the degree

of similarity in behavior between the various sets of data. Hot-tensile

data are available from other measurements of the elevated-temperature

mechanical properties of aAR M128-B steel. However, these data appear

to be limited to the results of hot-tensile tests from only three other

investigations: Three specimens from a fragment of the uninsulated tank

car, RAX 201, were tested at 900 F, 1050 F and 1200 F (4), ten specimens

from a fragment of tank car GATX 89971 from the Laurel, Mississippi

accident were tested from room temperature to 1200 F (7), and three pairs

of specimens from an as-rolled and stress-relived plate were tested at

930 F, 1110 F and 1290 F (8). J The temperature dependence of the ultimate

tensile strength and yield strength data from these three studies (only

tensile strength data available for tank car RAX 201), together with the

results of this investigation, are shown in Figures 7 and 8. Large

variations in tensile and yield strengths for identical temperatures are apparent: Comparing the data of plates taken from failed tank cars, the

values from tank cars RAX 201 and RAX 202 are the highest and lowest,

respectively, and the values from tank car GATX 89971 are intermediate.

The strong effect of strain rate (or crosshead speed) on elevated- temperature tensile properties necessitates that details of

the test procedure must be stated before a precise comparison of data

can be made. The strain rate used in two of the other three investigations

was not reported; however, the tests on specimens from tank car GATX 89971 were reportedly conducted in accordance with established procedures and by

appropriate aSTM specifications. The tests on the normalized and stressrelieved specimens, which were prepared from as-rolled plate, were

Six additional specimens from another as-rolled plate, identified

as AAR TC128-B steel, were tested at the same three temperatures in

the normalized condition. However, the reported check chemical

analysis for this plate revealed sufficient vanadium to classify

this plate as a TC128-A steel and thus were not included for comparison

in this report.

- 9

-
reportedly conducted at strain rates of 0.5 percent per minute and five

percent per minute for the determination of the 0.2_percent yield strength

and ultimate tensile strength, respectively. These testing rates are

identical to those used in the current study, if it is assumed (as an approximation) that crosshead speed is a suitable basis for calculating

strain rate.

A comparison of ultimate tensile strength data from the

normalized and stress-relieved specimens with the results of this

investigation on specimens in the as-rolled and stress-relieved condition

shows excellent agreement over the temperature range studied. The

agreement between the yield strength data is not as good as that of the

tensile strength data, possibly as a result of using crosshead displacement

measurements in this study to compute the 0.2-percent strain. The overall

agreement between the strength data of the normalized and stress-relieved

specimens compared to the as-rolled and stress-relieved specimens is

good and is consistent with observations by other investigators. Miller (11)

reported that for the same nominal composition and ferrite grain size,

the tensile strengths of the carbon steels that he investigated were comparable, whether made to coarse- or fine-grain practice or whether

in the as-rolled or stress-relieved condition.

In addition to the strain-rate effects on elevated-temperature

strength properties, variations in ambient-temperature strength properties

may also result in similar variations of elevated-temperature strength

properties. Smith (12) concluded that, in general, elevated-temperature

tensile and yield strength vary as the ambient values and, to a first

approximation, the former can be predicted from the latter in some cases.

The reported ambient -temperature ultimate tensile and yield strength

values of specimens from GATX 89971 were approximately 95.0 ksi and

63.5 ksi, respectively, compared to 85.9 ksi and 55.1 ksi, respectively, for

plate sample TC2-(11B), which was tested in this investigation. Therefore,

the elevated- temperature tensile and yield strength values for specimens

from GATX 89971 would be expected to be higher than those for specimens

from TC2-(11B) and this is consistent with the values actually measured.

Surprisingly, the largest difference in the tensile strength

data, which were available for comparison, occurs between results from the

uninsulated tank car, RAX 201, and the insulated tank car, RAX 202, both

of which were reportedly fabricated from the same heat of AAR M128-B steel.

At 1200 F, the tensile strength of the plate sample from RAX 201 was about 49 ksi, which is more than twice that of plate sample TC2-(11B)

from RAX 202. Although the ambient-temperature strength properties for

the plate sample from RAX 201 were not reported, these properties are

not expected to be significantly different from those of RAX 202.

Differences in the rate of testing could account for the large difference

in ultimate tensile strength values. However, the rate of testing for the

specimens from tank car RAX 201 would have to have been exceptionally high

and well outside of ASTM recommended practices. Thus, large differences

in the measured values of the elevated-temperature tensile strength of

specimens taken from plates fabricated from the same heat of steel and

tested at two different laboratories are not readily explainable.

- 10 -
4.3 Stress-Rupture Properties

The results of stress-rupture tests conducted on specimens prepared

from plate samples TC2-(1), TC2-(3) and TC2-(11B) are given in Table III.

The data are grouped by test temperature and are plotted in the traditional

form of the logarithm of the initial stress as a function of the logarithm

of the time-to-rupture, as shown in Figures 9a, 9b, 10a and 11. Each

temperature group contains the results of specimens from each plate sample

and orientation tested by either of the two stress-rupture test

techniques, described earlier in Sections 3.2.1 and 3.2.2. The data at

each test temperature were analysed by a least-snuares technique, and

the resulting calculated line was plotted with the individual data points

as shown in the graphs.

Inspection of each graph reveals that, at each test temperature, a single straight line adequately represents all of the data, and the

linear relationships are independent of plate sample location and

specimen orientation. Further, there is no evidence of a change in slope

in the stress versus time-to-rupture curves for the data presented.

k

The single straight lines, obtained in the current study at each test

temperature, indicate that specimen oxidation was not significant, and

that a single fracture mode was probably operative.

The results obtained using the IRH technique were verified by two

stress-rupture tests conducted with a conventional resistance tube

furnace (RTF). These tests were conducted at 1200 F at two stress

levels on longitudinal specimens from pJate sample TC2-(11B). These

results are included in Table III. A log-log plot of the data obtained

with the IRH method on longitudinal specimens from plate sample

TC2-(11B) is shown in Figure 10b. Superimposed on the plot are the two

RTF test results showing that the agreement between the two heating

techniques is good and that for the stress levels and temperatures used

in this study, the IRH technique will produce reliable time-to-rupture

data.

4.3.1 Comparison of Stress-Rupture Data for Tank Car Steels

The amount of stress-rupture data on tank car steels such as ASTM A212 or AAR M128 found in the literature is rather limited (4, 16).

A change-in-slope or a break in the linear behavior for many alloys

can sometimes be correlated with a change in the fracture mode, and

such behavior has been reported for carbon steels and low-alloy

steels. (14) However, a change-in-slope does not always indicate a change in fracture mode. The results of White, Clark, and Wilson (15)

for a carbon steel demonstrated that the observed break in the stress

versus rupture time curves was a consequence of oxidation of the test

specimens. Protection of the specimens against the oxidizing

atmosphere (air) resulted in a linear relationship for the test

conditions studied. Tne stress levels used in the present study were sufficiently high so that the rupture times were much shorter than

those reported by White, et al.

- 11 -
One investigation (16) of the stress-rupture properties of a plate of

ASTM A212 steel was carried out at 1000 F and at stresses such that the

minimum rupture time was about four hours. The only data reported for

AAR M128-B steel (4) consists of seven stress-rupture tests conducted on specimens taken from the uninsulated tank car, RAX 201. In the analysis

of the data from tank car RAX 201, the investigators assigned a rupture

life of 0.03 hours (1.8 minutes) to the three hot-tensile tests and

combined these three data points with the seven stress-rupture data points

to make a total of ten data points.

In the current study, the data from tank car RAX 201 were

analysed by a least-squares technique and plotted together with the results

of this study on tank car RAX 202, as shown in Figure 12. A comparison of

the data obtained from specimens taken from plates fabricated from the

same heat of steel and tested at two different laboratories shows large

differences in measured values of stress-rupture strength: The stressrupture strength values at 1200 F reported in earlier work (4) on a plate

sample from tank car RAX 201 are much greater than those for plate samples

from tank car RAX 202 tested in the present study. These differences are

as striking as the differences in the elevated-temperature tensile strength

values which were discussed in Section 4.2.1 of this report.

Smith (12), in his review of carbon steels, concluded that the

scatter of elevated-temperature rupture strength values obtained from

nominally identical carbon steel plates was substantial and that these

differences in rupture strength values are partially associated with

differences in composition and manufacturing practice allowable under

current specifications. However, the results of previously reported

check chemical analyses for the plate samples from tank car RAX 202 (5)

indicated a high degree of chemical homogeneity not only between different

areas within a plate sample and between different plate samples, but also

between these check chemical analyses and the producer's ladle analysis.

Although check chemical analyses were not reported for any plate sample

from tank car RAX 201, significant differences in chemical composition

between the two tank cars would not be expected. Thus, it is highly

probable that the differences in stress-rupture strength values between

specimens from tank car RAX 201 and specimens from tank car RAX 202 are

not a result of compositional variations.

The absence of ambient-temperature mechanical property data

from a relatively unheated and undeformed region of tank car RAX 201

precludes a determination as to whether there may have been differences

in the fabrication procedure or stress-relieving practice between the

two tank cars. However, since these two tank cars were fabricated

specifically for the Tank Car Safety Program, it is unlikely that

substantial variations in the fabrication and/or heat treatment procedures

Occurred

.

Finally, there remains the possibility that the significant

variations in these elevated- temperature mechanical properties are the

- 12 -
result of differences in the rupture environment seen by the plate

samples tested. Apart from this last speculation, however, there

appears to be no explanation for the substantial differences in

elevated-temperature mechanical properties observed between specimens from

tank car RAX 201 and specimens from tank car RAX 202.

4.4 Macroscopic and Metallographic Analyses

Macroscopic examination of the hot-tensile test specimens and

selected stress-rupture test specimens revealed that the specimen surface,

within the reduced cross-section, contained irregular, secondary cracks

adjacent to and parallel to the fracture surface. These cracks were,

in general, perpendicular to the applied stress. The hot-tensile specimens

and the RTF stress-rupture specimens contained many secondary surface

cracks in a region up to approximately 0.5 inches on either side of the

fracture surface on a given specimen. The IRH stress-rupture specimens,

however, contained comparatively fewer secondary surface cracks and the

cracks were limited to a region approximately 0.1 inches on either side

of the fracture surface. These differences are believed to be a result

of the higher temperature gradient existing in the IRH specimens.

Optical photographs, Figures 13a, 14a, 15a and 16a, and SEM

photographs, Figures 13b, 14b and 15b, show that the secondary cracks

are distributed around the specimen circumference and become larger and more numerous as the fracture surface is approached.

Representative photomicrographs of fracture profile sections are shown in the unetched condition in Figures 13c, 14c, 15c, 16b and 17.

The extensive development of internal voids or cracks is apparent in

the vicinity of the fracture surface. The voids in the IRF stressrupture specimens are limited to a smaller specimen volume than the

voids found in the hot-tensile specimens or the RTF stress-rupture

specimens; this behavior is analogous to the secondary surface crack

observations discussed earlier.

Photomicrographs taken on fracture profiles are shown in the

etched condition in Figures 18 and 19a. Large numbers of voids or

cracks can be seen, as well as a general elongation of proeutectoid

ferrite grains in the direction of the applied stress. Higher

magnification photomicrographs, Figures 20, 21 and 22a, were taken of

the region directly behind the fracture surface. These photomicrographs

show that the voids are intergranular or interphase in character since

they are located primarily at the boundaries between the proeutectoid

ferrite grains and the pearl ite. Often, these voids appear to form as a result of a pulling apart (or separation) of the ferrite grains from

the pearl ite, as seen in Figure 20b.

SEM fractographs were taken of representative fracture surfaces

of hot-tensile and stress-rupture specimens; cracks or voids in the

fracture surface are shown in Figure 23. At higher magnifications,

- 13 -
Figures 24 and 25a, the generally dimpled character of the fracture

surface is revealed, indicative of ductile fracture.

Macroscopic observations of the region believed to be the site

of the initial rupture in the insulated tank car, RAX 202, (5) revealed

that, on both plate surfaces near this portion of the fracture, numerous

small secondary cracks were present and they were aligned parallel with

the fracture surface and perpendicular to the principal tensile or hoop stress direction in the tank car. The majority of these surface

cracks were confined to a region of within one-half- to one-inch of the

fracture surface, where most of the decrease in plate thickness (or

thinning) occurred. Metallographic examination indicated that the

rupture was initiated by cracks or voids which formed primarily at

interfaces between proeutectoid ferrite and pearl ite, as shown in

Figures 19b and 22b. An SEM fractograph, Figure 25b, showed the

dimpled character of the fracture surfaces as well as the presence of

voids.

A comparison of the results from the analysis of representative

test specimens from this study with the results of a similar analysis

of the region believed to be the site of the initial rupture in

insulated tank car RAX 202, shows a high degree of similarity between

the features observed on and near the fracture surfaces. Both

investigations revealed the presence of secondary surface cracks,

interphase voids, and a generally dimpled fracture surface indicating

ductile fracture.

4.5 General Discussion

The purposes of this investigation were several fold. The primary

effort was to measure the elevated-temperature mechanical properties

and to determine the elevated-temperature fracture behavior of selected

AAR M128-B steel plates taken from the insulated tank car, RAX 202.

Further, these elevated-temperature properties were to be compared with

the published properties of other plate samples of AAR TC128-B steel. A

secondary effort was to measure the ambient-temperature mechanical

properties of plate samples taken from tank car RAX 202 to determine if

the requirements of specification AAR M128-69-B were satisfied.

For these studies, three plate samples representing two shell courses were selected: a) plate sample TC2-(1), taken from the top of

the tank car in the failed shell course, shell course 3, b) plate sample

TC2-(3), taken from the bottom of the tank car in the failed shell course,

and c) plate sample TC2-(11B), taken from the bottom of the tank car in

a shell course that did not fail, shell course 1. All of the steel

plates used to fabricate tank car RAX 202 were reportedly produced from

a single heat of steel

.

- 14 -
4.5.1 Ambient-Temperature Properties

Ambient-temperature mechanical properties were determined

from longitudinal specimens taken from plate samples TC2-(1), TC2- ( 3

)

and TC2-(11B). The measured values of ultimate tensile strength,

0.2-percent yield strength, and percent elongation from all specimens

tested met the requirements of specification AAR Ml 28-69 . In addition,

check chemical analyses of the three plate samples, previously reported

(5), also met the requirements of AAR Ml 28-69 . The average values of

tensile strength, yield strength, and percent elongation, 86.0 ksi,

55.5 ksi and 28.8 percent, respectively, compare favorably with the

average values of 87. 0 ksi, 56.7 ksi, and 30.6 percent, respectively,

reported for longitudinal specimens from a shell course sample of

AAR Ml 28-B-65 steel taken from another failed tank car. (17)

4.5.2 Hot-tensile Properties

The results of the hot-tensile tests indicated a behavior

consistent with that observed in a large number of carbon steels.

The decreasing ultimate tensile and yield strengths with increasing

test temperature indicated that dynamic strain ageing was not a significant effect for tne specimens tested in the temperature range

of 1100 F to 1250 F. However, strain rate or crosshead speed effects were observed in this temperature range; the measured values of

ultimate tensile strength and yield strength increased and the

measured values of tensile ductility decreased as a result of increasing

the strain rate. These strain rate effects reflect the fact that with

increasing strain rate (or crosshead speed), the strengthening effects

of not-strain hardening are relatively greater than the thermal

softening effects.

A comparison of the elevated-temperature tensile strength

values for plate samples reportedly produced to specification AAR M128-B

from two of three failed tank cars, tank car GATX d9971 (7) and tank car RAX 202 in the present study, and an as-rolled plate (8), shows a maximum variation in ultimate tensile strength in the temperature range

1100 F to 1 250 F of 26 to 40 percent, respectively; the maximum

variation at 1100 F was 29 ksi to 36.5 ksi and at 1250 F, the variation

was 20 ksi to 28 ksi. However, specification AAR M128-B allows a variation in the ambient-temperature ultimate tensile strength of up to

25 percent above the minimum specified value, from 81 ksi to 101 ksi,

and the typical variation in testing speeds used in the temperature range

of the present study can result in differences in tensile strength of

another 10 to 20 percent; 29.9 ksi to 34.1 ksi at 1100 F and 20.3 ksi

to 24.1 ksi at 1200 F. Thus, the observed variation in elevatedtemperature tensile strength values for these three plate samples appears

to be consistent with the variation in ambient-temperature tensile

strength allowed by specification AAR M128-B, and with the effect of

testing speed on the elevated-temperature tensile strength.

- 15 -
The one exception in the agreement between hot-tensile

strength data from various sources is the data representing a plate

sample from the uninsulated tank car, RAX 201 (4). The large difference

in ultimate tensile strength between the sample from RAX 201 and the

other plate samples from RAX 202 is difficult to understand. The

discrepancy between the data from tank cars RAX 201 and RAX 202 is all

the more puzzling because both tank cars were reportedly fabricated

from plates produced from a single heat of steel. The plate samples

from RAX 202 met both the chemical requirements and ambient- temperature

strength and tensile ductility requirements of specification AAR M128-69-B,

and there is no reason to believe that the steel plates in RAX 201 failed

to meet these specification requirements.

The elevated-temperature tensile strength values reported

for tank cars RAX 201 (4), RAX 202 (this study) , and GATX 89971 (7) were determined for plate samples taken from tank cars which had failed

in fire environments. The evidence reported in these studies suggests

that plate sample TC2-(11B) from RAX 202 and the plate sample from

GATX 89971 were taken from portions of the respective tank cars which were heated the least (if at all) by the fire exposure. The plate

sample taken from RAX 201, however, was located near the top of the

tank car and contained what is believed to be the site of the initial

rupture. The available evidence indicates that the top of the tank car experienced the highest temperatures during the fire test (2), and thus

it is possible that this plate sample could have been affected by both

the elevated temperature and the deformation associated with the subsequent

rupture and failure of the tank car.

A comparison of ambient-temperature tensile properties and

elevated- temperature rupture properties can be made, however, between

heated and relatively unheated plate samples from tank car RAX 202.

The ambient-temperature tensile properties and elevated-temperature

rupture properties measured on specimens from plate sample TC2-(1),

taken from the top of the failed shell course and containing a portion

of the initial rupture, were not significantly different from the

values representative of plate sample TC2-(11B), taken from the bottom

of the tank car in an unfailed shell course. This evidence suggests

that the properties of the plate sample from RAX 201 were probably not

affected by the elevated temperature and subsequent rupture and failure

of the tank car. The lack of information on the ambient- temperature

mechanical properties and the hot-tension-test procedures followed in

conducting tests of the specimens from RAX 201 makes it difficult to

comment on the reason or reasons why these tensile strength data depart

widely from the consistent data from plate samples from the other three

tank cars.

The design and specifications for a variety of pressure

vessels involve a burst pressure parameter which can be calculated from

the design equations of the vessel. These equations are used to

compute the burst pressure from the geometry of the vessel and from

materials properties such as the ultimate tensile strength. The results

- 16 -
of the comparison made in this report between a limited number of sets

of hot-tensile data suggest that significant variations in the hotultimate-tensile strength exist for steel plates produced to the

AAR Ml 28-69 -B specification. This variation is sufficiently large so that hot-tensile-strength data from a single plate sample are probably

inadequate as a basis for caluclating tank car burst pressures at

elevated-temperatures. Other investigators (4), using one set of

data (7) previously reported, have computed a tensile design curve of

tank car burst pressure as a function of temperature''. However,

the data represented only the hot-tensile strength data from a single

plate sample from a tank car, GATX 89971, which failed as a result of

a derailment and subsequent fire. Thus, the calculated burst pressuretemperature curve would not necessarily describe the expected behavior

of tank cars fabricated from different AAR M128-B steel plates.

The knowledge of the mechanical properties of a specific

sample of a structural material is not an adequate basis upon which to

design a structure using this material. The usefulness of any data

in the prediction of the anticipated behavior of a material depends on whether or not the data is representative of different samples of a material produced to the same specification. The comparison made in

this of the limited amount of hot-tensile data available suggests

that a tensile design curve of tank car burst pressure versus temperature

cannot be presently calculated in the temperature range of 1100 F to

1250 F.

4.5.3 Stress-Rupture Properties

In the present study, the results of the short-time stressrupture tests were in good agreement with the results of the hot-tension

tests. The ultimate tensile strength values of specimens taken from

plate sample TC2-(11B), measured from tests using controlled crosshead

speeds, were equivalent to rjpture lifetimes, uncorrected for strain

rate effects, of approximately 0.16 hours or less in the temperature

range of 1100 F to 1250 F. Other investigators have correlated ultimate

tensile strength values with rupture lifetimes. White, et al , (15)

found that for a carbon steel, the tensile strength values in the

temperature range of 1 200 F to 1 300 F corresponded to rupture lifetimes

of 0.15 hours or less. Thus, in the current study, the equivalent

rupture lifetimes of the el evated- temperature tensile strength values

of specimens from plate sample TC2-(11B) are consistent with the

equivalent rupture lifetimes reported for a carbon steel.

The results of the least-squares analyses of the stressrupture data indicate that a single straight line reasonably represents

all of the data for each test temperature irrespective of the specimen

The question as to what equations are appropriate for calculating the

elevated-temperature burst pressure are not discussed in this report.

- 17 -
orientation or the plate sample from which the specimens were taken.

Neither the effect of the fire environment on plate sample TC2-(1) nor the deformation received by plate sample TC2-(3) was apparently

sufficient to cause a measurable difference in their short-time

stress-rupture properties compared to the average stress-rupture

properties of all specimens tested at each test temperature. In

addition, the stress-rupture strength values of the transverse specimens

from plate sample TC2- ( 1 1 B ) were not measurably different from the

stress-rupture strength values of the longitudinal specimens from the

three plate samples. This latter observation is consistent with the

finding that there is little difference in the ambient- temperature

strength properties of the longitudinal and transverse specimens from

TC2-(11B).

The absence in the literature of stress-rupture data for

AAR M128-B steel plates makes it difficult to establish whether or not the data measured in this study are representative of this

grade of steel. A comparison of the stress-rupture data from the plate

sample from tank car RAX 201 with the data from tank car RAX 202,

measured in this study, revealed the same large variance in rupture

strength values as the previously discussed variance between the

respective hot-tensile strength data from these two tank cars. The

stress-rupture data and hot-tensile data of each tank car are self-consistent, and thus the origin of this discrepancy in elevatedtemperature mechanical properties between nominally identically steel

plates is not apparent.

Stress-rupture data for another widely used tank car steel,

ASTM A212 steel, was found to be very limited. The time-to-rupture

curve for a sample of ASTM A212 steel tested at 1000 F, shown in

Figure 12, was drawn by graphically extrapolating the data reported by

Smith (16) from lower stress levels to higher stress levels. The

rupture strength values for AAR M128-B steel in the temperature range

investigated in the present study are consistent with the data taken

from a single plate of a similar tank car steel, ASTM A212.

Stress-rupture data taken at 1100 F for a low-carbon boilerplate steel (18) were also included in Figure 12. The boiler-plate

steel contained 0.13, 0.27 and 0.03 weight percent carbon, molybdenum,

and columbium, respectively, as compared to the specification requirement

of 0.25 and 0.08 maximum weight percent carbon and molybdenum, respectively,

for AAR M128-B steel plates. The increased stress-rupture strength of the

boiler-plate steel as a result of the molybdenum addition is apparent,

even though the reported ambient- temperature tensile strength value of

this boiler-plate steel is approximately 70 ksi as compared to a minimum

of 81 ksi for AAR M128-B steel.

In the current study, the rupture lifetime was observed to

be strongly dependent on the initial applied stress level, in the temperature

and stress ranges investigated. By extrapolation of the stress-rupture

data from this study, shown in Figure 12, to longer rupture times, with

the longest extrapolation being less than a factor of ten in lifetime, it can be shown that a 20 percent decrease in the 1 5-mi nute-1 ifetime stress

- 18 -
at 1100 F would cause a twelvefold increase in the rupture life from

15 minutes to three hours. Similarly, a 30 percent decrease in the

15-minute-lifetime-stress at 1150 F, 1200 F and 1250 F would result in the same twelvefold increase in the rupture lifetime. The insulated tank

car, RAX 202, remained intact for approximately one and one-half hours

before failure, although the time during which the top of the tank car experienced temperatures in excess of 1000 F is estimated to be less

than 30 minutesm . Although changes in the metallurgy of tank car steels through selected alloy additions can substantially enhance the

stress-rupture strength by increasing the rupture lifetimes at higher

stress levels than can be attained with AAR Ml 28 -B steel, a significant

increase in rupture life for AAR M128-B steel can be achieved by reducing

the time during which the tank car experiences the maximum internal

pressure or reducing the maximum internal pressure". Reducing the internal

pressure by using additional relief valves, larger flow capacity relief

valves, or lower opening pressure relief valves appear to be possible

approachs to increasing the elevated-temperature rupture lifetime

requirement of tank cars.

4.5.4 Metal 1 ographic Analysis

The results of the metal! ographic analyses in this study show

the presence of the same fracture mode in both the hot-tension specimens

and the stress-rupture specimens. This fracture mode is characterized

by the presence of irregular, secondary surface cracks lying perpendicular

to the applied stress direction and by large numbers of interphase cracks

or voids in the region immediately behind the fracture surface. These

voids originate primarily at the boundaries between the proeutectoid

ferrite and the pearl ite. SEM fractographs of the fracture surface reveal

the presence of these voids and confirm the ductile character of the

fracture surface by the presence of dimples. The primary cause of

failure is the formation of these numerous cavities or cracks, although

at the instant of catastrophic failure, transgranular fracture of some proeutectoid ferrite grains may occur.

The observation that single-slope lines can reasonably be

fitted to the stress-rupture data for each test temperature is

consistent with the metal lographic findings of a single fracture mode

in the stress-rupture specimens. The observation of the same fracture

Approximately 60 minutes after the beginning of the fire test on RAX 202, the instrumentation system recording temperature and

pressure data failed. The maximum temperature recorded at the time

of the instrumentation loss was 850 F. If the rate of temperature

increase remained constant for the remainder of the test, from 60 to

94 minutes, then an extrapolation of the temperature-time data would

show that the time during which the temperature exceeded 1000 F would

be close to 15 minutes.

n A typical relief-valve-opening pressure of 250 psig is equivalent to

a maximum wall stress, calculated using the Bach equation (19), of

about 20 ksi.

- 19 -
mode in both hot-tensile and stress-rupture specimens is not unexpected,

since most of the stress-rupture tests were conducted at initial stress

levels that were close to or above the yield strength of the steel at

the test temperature.

The characteristics of the fracture mode observed in both

the hot-tensile tests and the stress-rupture tests are the same as those observed in specimens taken from what is believed to be the

initial rupture site in the insulated tank car, RAX 202. These results

further support the earlier findings (5) that the features of the

initial rupture region in the insulated tank car are characteristic

of failure by a stress-rupture mechanism.

5. SUMMARY

1. For this investigation, three steel plate samples, designated

TC2-(1), TC2-(3) and TC2-(11B), were taken from an insulated tank car,

RAX 202, which failed while subjected to a fire environment. Test

specimens were taken from these plates for ambient-temperature tensile

tests, hot-tensile tests, and stress-rupture tests. Metal lographic

observations were made on selected specimens after testing.

2. These plate samples were removed from two shell courses. Two

samples were taken from shell course 3, the only shell course that

fractured: TC2-(1), which contained what is believed to be the site of

the original rupture near the top of the tank car; and TC2-(3), located

at the bottom of the tank car, which was relatively unaffected by the

fire exposure. The third sample, TC2-(11B), was taken from a relatively

undeformed and unheated region at the bottom of the tank car in a shell

course that did not fail, shell course 1. Plate sample TC2-(11B)

was believed to be representative of the as-fabricated steel plates

used in tank cars RAX 201 and RAX 202, which were reportedly produced

from the same heat of steel

.

3. Ambient- temperature tensile tests conducted on longitudinal

specimens from plate samples TC2-(1), TC2-(3) and TC2-(11B) showed that

all three plate samples met the ultimate tensile strength, yield strength,

and percent elongation requirements of specification AAR M128-69-B.

Yield strength values for specimens from TC2-(3) were approximately

30 percent higher than the levels for plate samples TC2-(1) and

TC2-(11B). The increase is believed to be the result of a small amount

of plastic deformation which occurred in this plate sample as a result

of the failure of the tank car and the expulsion of shell course 3 out

of the test pit.

4. Ambient-temperature tensile tests conducted on longitudinal and

transverse specimens from plate sample TC2-(11B) showed little anisotropy

in the strength properties in the plane of the rolling direction. Tensile

ductility values, as measured by percent elongation, showed that the

longitudinal elongation values were approximately ten percent higher than

the transverse elongation values.

- 20 -
5. Hot-tensile tests conducted at 1100 F, 1150 F, 1200 F and 1250 F on longitudinal specimens from plate sample TC2-(11B) showed a continuous decrease in ultimate tensile strength and yield strength,

and an increase in percent elongation as the test temperature was

increased. The percent reduction-in-area values were less temperature

dependent than the percent elongation values, showing a pronounced

increase only above 1200 F.

6. Results of tests at a testing rate of 0.05 inches per minute

showed increases in ultimate tensile strength and yield strengths

and decreases in percent elongation and percent reduction-in-area

values, when compared with the values found for tests at an initial

testing speed of 0.005 inches per minute.

7. If the effects of testing speed and differences in ambienttemperature strengths are taken into account, the results of the

hot-tensile tests from this study are in good agreement, with one exception, with the results from other studies of AAR M128-B

steel plates.

8. Stress-rupture data measured at 1100 F, 1150 F, 1200 F and 1250 F on longitudinal specimens from plate samples TC2-(1) and TC2-(3) and

on longitudinal and transverse specimens from plate sample TC2- ( 1 1 B) were analysed by at least-squares technique. A single-slope line in

a log-log plot of initial stress versus rupture time represented the

data at each test temperature. This result appears to be independent

of the plate sample or specimen orientation.

9. The results of the stress-rupture tests indicate that in the

temperature and stress range investigated, decreases in the initial

stress level of approximately 20 to 30 percent result in a twelvefold

increase in the rupture lifetime from 15 minutes to three hours.

10. Stress-rupture tests conducted with a resistance tube furnace

(RTF) to heat the specimens indicated that tests conducted with an internal -resistance-heating (IRH) technique gave reliable time-to-rupture

data at the stress levels and temperatures used in this study.

11. Little stress-rupture data on AAR M128-B steel were found in the

1 iterature for comparison with the results of this investigation. In

the one instance where comparable data were found, reasonable agreement

was obtained in a comparison of the present results with data from a related tank car steel, ASTM A212, which was tested at 1000 F.

12. A comparison of results from hot-tensile and stress-rupture tests

on a plate sample taken from the uninsulated tank car, RAX 201, with

those taken from the insulated tank car, RAX 202, showed substantial

disagreement. Tne steel plates used to fabricate both tank cars reportedly were produced from the same heat of steel, and thus the

origin of the wide variance in elevated-temperature properties between

these samples is not explainable.

- 21 -
13. The results of the metal lographic analysis of hot-tensile and

representative stress-rupture specimens and the previously reported

results on the initial rupture in the failed shell course, all from

tank car RAX 202, indicate the presence of the identical fracture

mode in all three types of metal lographic samples. This mode is

characterized by the presence of many interphase ( intergranular) cracks

or voids in the region immediately behind the fracture surface. The

voids originate primarily at the proeutectoid ferri te-pearl ite

boundaries. SEM fractographs revealed the generally ductile character

of the fracture surface by the presence of dimples which resulted from

void nucleation and coalescence. These results confirm the previously

reported findings that the initial rupture of tank car RAX 202 was a stress-rupture crack.

6. CONCLUSIONS

1. The variation in the ambient-temperature ultimate tensile strength

allowed by specification AAR M128-B and the effect of the rate of testing

can result in a significant variation in the elevated-temperature strength

properties. The lack of substantial elevated-temperature mechanical

property data in the literature appears to preclude the development, at

the present time, of a design or trend curve for the variation of burst

pressure with temperature for AAR M128-B steel. Knowledge of the lower

bound to the burst pressure- temperature curve for AAR M128-B would be

useful in the evaluation of existing relief-valve design.

2. The rupture life of AAR M128-B steel, as measured by uniaxial

stress-rupture tests in the temperature range of 1100 F to 1250 F, is

a strong function of both temperature and applied stress. Therefore,

modifications of tank car technology which would either reduce the

temperature dependence of the properties of the steel or reduce the

maximum stresses and/or time at maximum stress experienced by the

pressurized tank cars, could be important in efforts to reduce the

possibility of a tank car failing catastrophical ly when subjected to

a fire environment.

3. The results of the hot-tensile tests indicate that dynamic strain

ageing in AAR M128-B steel is not significant in the temperature range

and at the testing speeds used in this investigation.

4. In general, there were few differences in the measured ambient- and

elevated-temperature mechanical properties between plate samples regardless

of whether or not the plate sample was from the most or least heated

regions of tank car RAX 202.

7. ACKNOWLEDGEMENT

The author wishes to express thanks to Drs. C. G. Interrante and

B. W. Christ for their helpful comments and suggestions, and to

Mr. D. E. Harne for his excellent contributions to the metal lographic

- 22 -
and photographic portions of this report. In addition, thanks to

Mr. C. H. Brady for his skillful and timely assistance with the

scanning electron microscopy portion of this report. Special thanks

are extended to Dr. W. F. Savage and Mr. John Lippold, Materials

Engineering Department, Rensselaer Polytechnic Institute, for conducting

the stress-rupture tests using the IRH technique.

- 23 -
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. 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.

3. 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.

4. C. Anderson and E. B. Morris, "FRAGMENTATION AND METALLURGICAL

ANALYSIS OF TANK CAR RAX 201," B.R.L. Interim Memorandum Report

No. 213, USA Ballistic Research Laboratories, Aberdeen Proving

Ground, Maryland, April 1974.

5. J. G. Early and C. G. Interrante, "A METALLURGICAL INVESTIGATION i OF A FULL-SCALE INSULATED RAIL TANK CAR FILLED WITH LPG SUBJECTED

TO A FIRE ENVIRONMENT," Report No. 7, NBSIR 75-657, January 1975.

6. "PHASE 01 REPORT ON SUMMARY OF RUPTURED TANK CARS INVOLVED IN PAST

ACCIDENTS," Report RA-01-2-7, Railroad Tank Car Safety Research

and Test Project, Association of American Railroads, Chicago,

Illinois, July 1 , 1972.

7. P. Gordon and D. L. Albright, "METALLURGICAL EVALUATION OF SAMPLES

FROM TANK CARS DERAILED AT LAUREL, MISSISSIPPI," Appendix A,

AAR Report No. MR-453 AAR Research Center, Chicago, Illinois,

July 1969.

8. A. P. Coldren, Report ISJ-669, Climax Molybdenum Company,

June 9, 1972.

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

OF FOUR TANK-CAR ACCIDENT REPORTS," Report No. 5, NBSIR 75-655,

January 1975.

10. J. L. Peterson, "STRAIN HARDENING IN HYPOEUTECTOID STEELS," Trans.

ASM, 56, 304, 1963.

11. R. F. Miller, "THE STRENGTH OF CARBON STEELS FOR ELEVATED- TEMPERATURE

APPLICATIONS," ASTM Proc, 54, 964, 1954.

12. G. V. Smith, "ELEVATED TEMPERATURE STATIC PROPERTIES OF WROUGHT

CARBON STEELS," ASTM, STP 503, 1972.

- 24 -
13. R. F. Miller, G. V. Smith, and G. L. Kehl , "INFLUENCE OF STRAIN RATE

ON STRENGTH AND TYPE OF FAILURE OF CARBON-MOLYBDENUM STEEL AT 850,

1000 AND 1100 DEGREES FAHR.," Trans. ASM, 31_, 817, 1943.

14. T. Prnka and V. Foldyna, "THE CREEP PROPERTIES OF LOW-ALLOY

Cr-Mo-V STEELS WITH LOW CARBON CONTENT," High-Temperature

Properties of Steels, Proceedings of a BISRA-ISI Joint Conference,

ISI Publication 97, 1967.

15. A. E. White, C. L. Clark and R. L. Wilson, "THE FRACTURE OF

CARBON STEELS AT ELEVATED TEMPERATURES," Trans. ASM, 25, 863,

1937.

16. G. V. Smith, "AN EVALUATION OF THE ELEVATED TEMPERATURE TENSILE

AND CREEP-RUPTURE PROPERTIES OF WROUGHT CARBON STEEL," ASTM

Data Series, DS11S1 , 1970.

17. C. G. Interrante, G. E. Hicho, and D. E. Harne, "A METALLURGICAL

ANALYSIS OF ELEVEN STEEL PLATES TAKEN FROM A TANK CAR ACCIDENT

NEAR CALLAO, MISSOURI," Report No. 4, NBS Report 312.01/51,

September 19, 1972.

18. T. Wada, "CREEP RUPTURE PROPERTIES, RESISTANCE TO TEMPER

EMBRITTLEMENT AND TO HYDROGEN ATTACK OF A Mn - 0.25% Mo-Cb

STEEL," Report RP-32-71-02, Climax Molybdenum Company,

January 31 , 1973.

19. J. M. Barsom and S. T. Rolfe, "FATIGUE AND BURST ANALYSIS OF

HY-140(T) STEEL PRESSURE VESSELS," Jour. Eng. Ind. (Trans. ASME,

Series B), 92, 1 1 , 1970.

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TABLE III Stress-Rupture Properties of Specimens

Taken From Tank Car RAX 202

Specimen

Shell t*?** Stress Time To

OrieTitationU) Identi fic* tion Course No.

TemP^ture,

K$i Rupture , Hours

11-L TC2- (3) 3 nod 22.3 2.16

37-L TC2- (11B) 1 1100 26.1 0.05

40-L TC2- ;hb) 1 1100 21 .5 2.18

31 -L TC2- C 1 IB) 1 1 1 00 5 21 .5 1 .79

44-L TC2- (1TB) 1 1100 20.0 1 .92

4-L TC2- (1) 3 1150 20.1 1 .21

20-L TC2- (3) 3 1150 22.5 0.40

16-L TC2- (3) 3 1150 20.0 0.76

42-L TC2- [11B) 1 1150 22.5 0.26

29-L TC2- [1 IB) 1 1150 20.0 0.26

39-L TC2- ;iib) 1 1150 20.0 0.43

46-T TC2- [11B) 1 1150 20.0 1.11

52-T TC2- [llB.) 1 1150 17.0 2.88

6-L TC2- 3 1200 18.9 0.27

3-L TC2- 3 1200 17.5 0.73

13-L TC2- ,3) 3 1200 17.6 0.84

14-L TC2-I:3) 3 1200 14.8 1.52

33-L TC2-IM 1 B")

1 1200 19.9 0.20

30-L TC2-I;iib) 1 1200 17.4 0.49

43-L TC2- [TIB) 1 1200 15.6 1 .04

32-L(b) TC2- ;iib) 1 1200 17.5 0.36

25-L(b) TC2-I;iib) 1 1200 19.5 0.19-0. 23(c)

57-T TC2-I: i ib) 1 1200 18.5 0.41

2-L TC2-< 1) 3 1250 —c n 17.5 0.09

19-L TC2-I!3) 3 1250 15.9 0.16

45-L TC2-I 11B) 1 1250 20.0 0.05

26-L TC2-I 1 IB) 1 1250 17.4 0.19

41-L TC2-( 11B) 1 1250 14.4 0.37

55-T TC2-I 116) 1 1250 17.4 0.10

56-T TC2-( 11B) 1 1250 15.9 0.15

54-T TC2-( 11B) 1 1250 12.2 0.95

(a) L and T refer to the longitudinal and transverse specimen axis orientation

with respect to the principal rolling direction of the plate.

(b) Stress-rupture test conducted in a resistance - tube furnace (RTF).

(c) Load accidently reduced for brief time just before the end of the test

so that the true time to failure is not known. True time to failure must

lie between actual time to rupture of 0.23 hours and time at which the full

load was operative, 0.19 hours.

- 28 -
CIRCUMFERENTIAL WELDSSHELL COURSE 1

TC2-01B)

LONGITUDINAL WELD

TC2-0O)

SHELL COURSE 2

TC2-(11A!

SHELL COURSE 4

1 Foot

Figure 1. Schematic Diagram Showing the location of NBS

Samples in Tank Car RAX 2 02 as Viewed from the

Outside of the Tank Car.

The specimens used in this study were taken

from the plate samples marked as V//////A .

Figure 2. Plate Sample TC2-(1), Shell Course 3

Photograph shows the inside plate surface at the top

of the tank car. The inset photograph shows the

location from which the test specimens were removed.

The white rectangles identify the test specimens and

show the orientation of the specimens. Mag. X 1/12

Figure 3. Plate Sample TC2-(3), Shell Course 3 Photograph shows the inside plate surface with the

bottom center line of the tank car marked BOT. The

inset photograph shows the location from which the

test specimens were removed. The white rectangles

identify the test specimens and show the orientation

of the specimens. Mag. X 1/12

Figure 4. Plate Sample TC2-(11), Shell Course 1 and Shell Course 2.

Photograph shows the outside plate surface at the bottom

of the tank car. Plate sample TC2-(11A) , shell course 2,

is marked A and plate sample TC2-(11B), shell course 1,

is marked B. The white rectangles identify the area

from which the test specimens were removed and show the

orientation of the specimens. Mag. X 1/12

Figure 5. Elevated Temperature Strength Properties of Plate

Sample TC2-(113) , AAR M128-B Steel, Taken from

Tank Car RAX 2 02.

<

Ld 60

<

z

z 50

o

H

o

Q 40

UJ cr

H

Z

UJ 30

O

OSUJ CL

20

Crosshead

Speed, C.S.

A 0.005-0.05 in/min • 0.05 in/min

100 1150 1200

Test Temperature -F

1250

Figure 6. Elevated Temperature Tensile Ductility Properties of

Plate Sample TC2-(11B), AAR M128-B Steel, Taken from

Tank Car RAX 2 02.

75

70 — \

\

\

65

60

I 55

X

I-o

z

UJ

QZ 50

co

UJ=! 45

(0

z

UJ huj 40

<

I- 3 35

\

30

25

20

Tank Car RAX 201 , (4)

Tank Car GATX 89971 , (7)

Tank Car RAX 202 , C.S. = 0.005 -0.05 in/min

Tank Car RAX 202, C.S. = 0.05 in /mm

As-Rolled Plate, Normalized and Stress - Relieved , S.R. = 0.005-0.05 per min. , (8)

850 900 950 1000 1050 1100 1150

Test Temperature - F

1200 1250

Figure 7. Elevated Temperature Ultimate Tensile Strength Properties

of AAR M128-B Steel.

20

10

\ \

Tank Car GATX 89971 , (7) TanK Car RAX 202 , C.S. = 0.0C5 - 0.05 in/mm

Tank Car RAX 202, C.S. = 0.05 in /min

As - Rolled Plate , Normalized and Stress-Relieved , S.R. = 0.005-

0.05 per mm., (8 )

This Study

850

Figure 8.

900 950 1000 1050 1100

Test Temperature - F

1150 1200 1250

Elevated Temperature Yield Strength Properties of

AAR M128-B Steel

.
1
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Figure 9. Stress-Rupture Data for Three Plate Samples from Tank

Car RAX 202, AAR M128-B Steel.

a. Test Temperature = 1100 F

b. Test Temperature = 1150 F

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Figure 10. Stress-Rupture Data for Three Plate Samples from Tank

Car RAX 202, AAR M128-B Steel.

a. Test Temperature = 1200 F, all specimens included.

b. Test Temperature = 1200 F, longitudinal specimens

from TC2-(11B) tested by both IRH and RTF methods.

0.05 0.07 0.1 0.2 0.3 0.5 0.7 1.0 1.5 Time to Rupture -Hours

Figure 11. Stress-Ruptures Data for Three Plate Samples from

Tank Car RAX 202, AAR M128-B Steel.

Test Temperature = 1250 F

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Tested at 1100 F at a crosshead speed of 0.005/0.05 inches

per minute.

a. Photograph of exterior of specimen surface at the

fracture surface. Mag. X 15

b. SEM photograph of exterior of specimen surface at

the fracture surface. Mag. X 18

c. Photograph of fracture profile section, arrows

indicate some of the secondary surface cracks. Mag. X 20

c

Figure 14. Hot Tension Test Specimen No. 38, Plate Sample TC2-(11B)

.

Tested at 1200 F at a crosshead speed of 0.005/0.05 inches

per minute.

a. Photograph of exterior of specimen surface at the

fracture surface. Mag. X 15

b. SEM photograph of exterior of specimen surface at

the fracture surface. Mag. X 16

c. Photograph of fracture profile section, arrows indicate

some of the secondary surface cracks. Mag. X 20

c

Figure 15. RTF Stress-Rupture Specimen No. 25, Plate Sample TC2-(11B).

Tested at 1200 F, initial stress = 19.5 ksi, time-torupture = 0.19/0.23 hours.

a. Photograph of exterior of specimen surface at the

fracture surface. Mag. X 15

b. SEM photograph of exterior of specimen surface at the

fracture surface. Mag. X 17

c. Photograph of fracture profile section, arrows

indicate some of the secondary surface cracks. Mag. X 20

b

Figure 16. RTF Stress-Rupture Specimen No. 32, Plate Sample TC2-(11B).

Tested at 1200 F, initial stress = 17.5 ksi, time-torupture = 0.3 6 hours.

a. Photograph of exterior of specimen surface at the

fracture surface. Mag. X 15

b. Photograph of fracture profile section, arrows

indicate some of the secondary surface cracks. Mag. X 20

b

Figure 17. IRH Stress-Rupture Specimens.

Fracture profile sections, arrows indicate some of

the secondary surface cracks.

a. Specimen No. 42, plate sample TC2-(11B), tested at

1150F, initial stress = 22.5 ksi, time-to-rupture = 0.26 hours. Mag. X 20

b. Specimen No. 6, plate sample TC2-(1) ( tested at

1200 F, initial stress = 18.9 ksi, time-to-rupture = 0.27 hours. Mag. X 20

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Sample TC2- (11B)

.

a. Hot tension test specimen No. 21, test temperature = 1100 F. Mag. X 200

b. RTF stress-rupture specimen No. 25, test temperature

1200 F. Mag. X 170

c. IRH stress-rupture specimen No. 33, test temperature

1200 F. Mag. X 180

b

Figure 24. SEM Fractographs of Selected Fracture Surfaces, Plate

Sample TC2- (11B)

.

a. Hot tension test specimen No. 27, test temperature = 1150 F. Mag. X 470

b. RTF stress -rupture specimen No. 32, test temperature = 1200 F. Mag. X 430

Figure 25. SEM Fractographs of Selected Fracture Surfaces

a. IRH stress-rupture specimen No. 31, plate sample

TC2-(11B), test temperature = 1100- F. Mag. X 430

b. Region of initial rupture of tank car RAX 2 02,

plate sample TC2-(1). Mag. X 525

NBS-lUA (REV. 7-73)

U.S. DEPT. OF COMM. 1, PUBLICATION OR REPORT NO. 2. Gov't Accession

BIBLIOGRAPHIC DATA No. sheet NBSIR 75-725

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Report No, 8

AMBIENT- AND ELEVATED-TEMPERATURE MECHANICAL PROPERTIES OF

AAR M128-69-B STEEL PLATE SAMPLES TAKEN FROM FIRE TESTED

INSULATED TANK CAR RAX 202

5. Publication Date May 1975

6. Performing Organization Code

7. AUTHOR(S)

J. G. Earlv

8. Performing Organ. Report No. NBSIR 75-725

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 3120413

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Fori rcucio Y*A 1 Q A l 1 v»naH AHmi ni c v»a + 1 r\ n a i r\a i i r uau nuin I ill b Lid L 1 un Department of Transportation

Washington, D. C. 20590

13. Type of Report & Period Covered

Failure Analysis

14. Jiponsoring Agency Code

15. SUPPLEMENTARY NOTES

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

bibliography or literature survey, mention it here.) Studies were undertaken at NBS to measure the

elevated- temperature mechanical properties and to determine the elevated-temperature

fracture behavior of selected AAR M128-B steel plates taken from tank car RAX 202. A

octuuuaijr ciiur u wao uj iiicaour c tile ainu 1 ell tTM tempera LUrc mcCHaniCal propGrtlGS ZO QGlGTmine if the requirements of specification AAR M128-B-69 were satisfied. Three plate

samples were selected for this study. The NBS results of check chemical analyses, hardness surveys, thickness measurements, macroscopic observations, and metal lographic analyses of these plate samples had been previously reported. Results of ambient- temperatun

tensile tests showed that all three plate samples met the strength and ductility requirements of specification AAR M128-B-69. Results of hot-tensile tests showed a continuous

decrease in strength properties and an increase in tensile ductility in the temperature

range from 1100 F to 1250 F. Analysis of stress- rupture data for all three plate

samples indicated that a single straight line represented the data at each test temperature. In the temperature and stress range investigated, a decrease in the initial stres<

of approximately 20 to 30 percent resulted in a twelvefold increase in rupture life

A comparison of the results of the metal lographic analyses of hot-tensile and representative stress-rupture specimens in this study with the previously reported results on the

initial rupture site in the failed shell course indicated the presence of an identical

fracture mode. These results confirm the previously reported finding that the initial

rupture of tank car RAX 202 was a stress-rupture crack. A comparison of hot-tensile and

stress-rupture results between specimens from tank car RAX 201 and specimens from tank c< W^B^^oSflQW^ f<sw&5£a&£rt§sl; J^^rc&w^ capiTh&eQShQihB toft /the- <cf y&c n&pancyo ^"not ^apparent.

name; separated by semicolons)

Ambient- temperature; elevated-temperature; insulated rail tank car; stress-rupture

properties; tensile properties.

18. AVAILABILITY Unlimited

IXX1 For Official Distribution. Do Not Release to NTIS

I I Order From Sup. of Doc, U.S. Government Printing Office Washington. D.C. 20402. SD Cat. No. C13

1 1 Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

21. NO. OF PAGES

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

22. Price

USCOMM-DC 29042-P74