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NBSIR 80-1 985i^ 00 HOT REMOVE

IP 19 1 Examination of Failed Eight

Inch Diameter Cast Iron Pipe

Natural Gas Main, Philadelphia,

Pennsylvania

T. Robert Shives

Fracture and Deformation Division

Center for Materials Science

National Measurement Laboratory

National Bureau of Standards

Washington, D.C. 20234

September 1979

Issued February 1 980

Failure Analysis Report

Prepared for National Transportation Safety Board

Washington, D.C. 20594

NBSIR 80-1985

EXAMINATION OF FAILED EIGHT

INCH DIAMETER CAST IRON PIPE

NATURAL GAS MAIN, PHILADELPHIA,

PENNSYLVANIA

T. Robert Shives

Fracture and Deformation Division

Center for Materials Science

National Measurement Laboratory

National Bureau of Standards

Washington, D.C. 20234

September 1979

Issued February 1 980

Failure Analysis Report

Prepared for National Transportation Safety Board

Washington, D.C. 20594

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
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TABLE OF CONTENTS

Page

SUMMARY 1

1. INTRODUCTION 2

1.1 Reference

1.2 Backgound Information

1.3 Parts Submitted

2. PLAN OF THE EXAMINATION

3. RESULTS OF THE EXAMINATION

3.1 Documentary Photographs

3.2 Cleaning the Fracture Surface

3.3 Fractographic Examination

3.4 Chemical Analysis

3.5 Examination of Graphitization and Metal lographic

Examination

3.6 Hardness Measurements

3.7 Ring Tests and Talbot Tests

4. DISCUSSION 7

5. CONCLUSIONS 8

6. ACKNOWLEDGEMENT 8

REFERENCES 8

FIGURES

1. Piece of eight inch diameter gray cast iron gas pipe containing

one face of the fracture from the 16 foot, 9 inch length of pipe.

2. Two-foot length of pipe identified as being from west end of

16 foot, 9 inch length as received at NBS.

3. Two- foot length of pipe identified as being from the 11 foot

length as received at NBS.

4. Two-foot length of pipe identified as being from east end of

16 foot, 9 inch length as received at NBS.

5. Fracture surface after cleaning.

6. SEM fractograph showing representative features of the fracture

surface.

7. SEM fractograph at higher magnification than in figure 6.

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TABLE OF CONTENTS (continued)

8. Polished transverse section through the piece of the pipe shown in

figure 1 ranging from 1/4 to 2 inches from the fracture.

9. Etched longitudinal section intersecting the fracture at the location

indicated at the lower right in figure 5.

10. Unetched longitudinal section showing essentially complete graphitization

at the left and partial graphitization at the right.

11. Unetched longitudinal section showing the fracture profile at the top.

12. Etched longitudinal section showing the fracture profile horizontally

at the top.

13. Etched longitudinal section showing representative microstructure of

the pipe material except for the region adjacent to the inside wall.

14. Etched longitudinal section showing the fracture profile horizontally

at the top.

15. Etched longitudinal section showing the representative microstructure

adjacent to the inner wall surface.
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SUMMARY

•At the request of the National Transportation Safety Board, the National

Bureau of Standards performed an examination of a fractured eight inch

diameter cast iron pipe natural gas main. An explosion and fire at the

location of the failure had occurred on May 13, 1979 at Tacony and

Margaret Streets in Philadelphia, Pennsylvania. The supporting soil

beneath the pipe was missing at the location of the failure. The pipeline had completely fractured circumferentially in three places, and

after fracturing, fell into the cavity created by the missing soil.

Failure appeared to be due to the application of a bending load to the

unsupported pipe.

The non-uniform wall thickness indicated that the pipe was pit cast and

therefore was probably rather old. Today, such pipe is normally centrifugal ly cast which results in a uniform wall thickness.

Only one of the fracture surfaces was selected by the National Transportation Safety Board for examination by the National Bureau of Standards.

Corrosion product on that fracture surface was rather easily removed,

indicating that the fracture was likely recent. Cleavage was the predominant fracture mode. The amount of graphitization at the fracture

was not considered excessive.

The hardness of the material was typical for gray cast iron. The chemical

composition was typical for this material, but the phosphorus content was higher than desirable according to today's practices. The microstructure

appeared to be satisfactory with the possible exception of more steadite

(iron-iron phosphide eutectic) than desirable according to today's practices.

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Examination of Failed Eight Inch Diameter Cast Iron Pipe Natural

Gas Main, Philadelphia, Pennsylvania

1 . INTRODUCTION

1 .1 Reference

National Transportation Safety Board, Washington, D.C. 20594. This investigation was conducted at the request of Mr. Jerry A. Houck, Metallurgist,

National Transportation Safety Board. The requesting letter was dated

June 25, 1979.

1 .2 Background Information

Information in this section was furnished by the National Transportation

Safety Board and by representatives of the Philadelphia Gas Works. On

May 13, 1979, there was an explosion and fire at Tacony and Margaret Streets

in Philadelphia, Pennsylvania. The explosion and fire were attributed to

the ignition of an accumulation of natural gas at the site.

After the explosion and fire, the eight inch diameter natural gas main pipe

at this location was found to be completely fractured circumferential ly in

three places. Two of these fractures were in an 11 foot length of pipe,

and the other was in a 16 foot, 9 inch length of pipe. There was another

16 foot, 9 inch length of pipe between the two lengths containing the

fractures.

The pipeline was under the paving of Margaret Street and had been buried

to a depth of 3 feet, 3 inches (measured from the top of the pipe).

When the pipeline in the region of the fracture was uncovered, the soil

beneath the pipeline was missing to a depth ranging from 3 inches to about

3 feet, 9 inches below the level where the pipeline had originally been

installed. This missing soil created a cavity and left the pipeline un- supported. When the pipe fractured, it fell into this cavity.

A fractured six inch diameter cast iron water main was in close proximity

to the fractured gas pipe.

1 .3 Parts Submitted

A small piece of the eight inch diameter gas pipe was submitted to NBS for

examination. This piece varied in length from about 3 inches to 4 1/4 inches

around the circumference and contained one of the fracture surfaces from the

fracture in the 16 foot, 9 inch length. This piece is shown as received at

NBS in figure 1. The other fracture surfaces were not submitted by the

National Transportation Safety Board.

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In addition, three two-foot pieces of pipe, one from each of the two pipe

lengths that had fractured and one from the 16 foot, 9 inch length between

the two lengths containing the fractures, were submitted. These pieces

did not contain a fracture surface. The pieces are shown as received at

NBS in figures 2, 3, and 4 and were identified respectively as from west

end of 16 foot, 9 inch section, from 11 foot section, and from east end of

16 foot, 9 inch section. The submitted fracture was from the same length

of pipe from which the two-foot piece identified as being from the east

end of 16 foot, 9 inch section was taken.

2. PLAN OF THE EXAMINATION

A meeting to discuss the examination of the pipe samples was held at the

National Bureau of Standards on May 30, 1979. Those attending that meeting

included R. D. Stout and LeRoy C. Schlagel representing the Philadelphia

Gas Works, Jerry Houck of the National Transportation Safety Board, and

T. R. Shives of the NBS Fracture and Deformation Division. At that time

it was agreed that the following tests and examination would be performed

on the submitted pipe samples.

1) Documentary photographs of the piece of pipe containing the fracture.

2) Cleaning of the fracture surface.

3) Fractographic examination.

4) Chemical analysis of material from the piece containing the fracture.

5) Sectioning for metal lographic examination and for examination for

graphitization.

6) Hardness

7) Talbot and ring tests on sections of. pipe furnished from three different

lengths of pipe, two of which had fractured. The third length was between the two fractured lengths.

3. RESULTS OF THE EXAMINATION

3.1 Documentary Photographs

Documentary photographs of the piece of pipe containing the fracture are shown in figures 1 and 5. Documentation of the other submitted pieces of

pipe are shown in figures 2, 3, and 4. Documentation of other aspects of

the examination is found in other figures throughout the report.

3.2 Cleaning the Fracture Surface

The fracture surface was covered with corrosion product when received at

NBS. The fracture was cleaned ul trasonical ly for ten minutes with a commercial detergent. Some, but not all, of the corrosion product was removed during this process. Further ultrasonic cleaning was carried

out with buffered hydrochloric acid. Most of the corrosion product was

removed. The fracture surface is shown after cleaning in figure 5.

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3.3 Fractographic Examination

The fracture was examined macroscopically and one area was examined with the

scanning electron microscope (SEM). The location of the fracture origin was

not evident from the fracture features. Two SEM fractographs representative

of the fracture surface are shown in figures 6 and 7. Both fractographs are

at relatively low magnification since the fracture features are rather coarse.

The predominant fracture mode is cleavage, indicating a low ductility or brittle fracture. This type of fracture is expected for gray cast iron.

Although there had been some corrosive attack, the fracture features had not

been obliterated by the corrosion process.

3.4 Chemical Analysis

Samples of material from each of the three pipe lengths examined were submitted to a commercial laboratory for chemical analysis. The results of

these analyses are given below as percent by weight:

West end, 16 ft. 11 ft. length East end, 16 ft.

-9 in. length 9 in. length

Total carbon 3.68 4.18 3.60

Combined carbon 1.04 1.65 1.29

Manganese Q.44 0.50 0.48

Phosphorus 1.34 1.45 1.35

Sulfur 0.069 0.086 0.104

Silicon 1.61 1.60 1.61

Nickel 0.03 0.03 0.03

Chromium <0.01 <0.01 <0.01

Molybdenum <0.01 <0.01 <0.01

Copper 0.09 0.08 0.08

The carbon equivalent^ (% total carbon +1/3 {% silicon + % phosphorus) for

the material from all three pipe lengths is greater than 4.3^ indicating that

the material is hypereutectic and would be expected to be relatively low in

strength. The phosphorus content in all three cases is rather high compared

to today's practice. Excessive phosphorus leads to the formation of steadite

(iron- iron phosphide eutectic). Steadite is hard and brittle, and if present

in sufficient quantities, may have adverse effects on the mechanical properties of the cast iron. As a basis for comparison only, proposed American

National Standard A21. 9-1970 for cast iron pipe centrifugally cast in sand

lined molds for gas service limits the phosphorus content to 0.90% maximum

and the sulfur content to 0.12% maximum. It should be noted that the pipe

being examined was pit cast and was produced before the quoted standard was

written.

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3.5 Examination for Graphitization and Metallographic Examination

Little if any graphitization was evident during a macroscopic examination

of the fracture surface. A transverse section was taken through the pipe

ranging from about 1/4 to 2 inches from the fracture. This section is

shown in figure 8 after polishing. No significant graphitization was

evident.

Two longitudinal sections intersecting the fracture were taken at the

locations indicated in figure 5. A small amount of graphitization was detected adjacent to the outside pipe wall surface in these sections. A

photomicrograph showing the entire wall thickness adjacent to the fracture

appears in figure 9. The graphitization extends about 0.12 inch in from

the outside wall surface. The wall thickness of the pipe varied around

the circumference between about 0.57 inch maximum and 0.42 inch minimum.

Graphitization is essentially complete right at and near the surface.

As distance from the surface increases, a region of partial graphitization

is reached. An area showing the change from essentially complete graphitization to partial graphitization appears in figure 10.

The unetched microstructure of the pipe material from the fractured piece

is shown in figure 11. The fracture profile is horizontal at the top. The

field shown in figure 11 is representative of the unetched microstructure

of the material except for the region adjacent to the inside wall surface

of the pipe. (The region adjacent to the inside wall surface is discussed

later in this report.) The thin, dark particles are graphite flakes with

a type A distribution?.

An etched field from one of the longitudinal sections from the fractured

piece of pipe is shown in figure 12. This field is representative of the

material in the pipe except for the region adjacent to the inside wall

surface. A field similar to that shown in figure 12 is shown at higher

magnification in figure 13. The microstructure consists of graphite flakes

(long, thin, dark particles), pearl ite (fingerprint pattern), ferrite (light

gray patches), and what appears to be steadite (small, rounded particles

within the ferrite patches).

The etched microstructure adjacent to the inner wall surface is shown in

figures 14 and 15. Again, the microstructure consists of graphite flakes,

pearl ite, ferrite and what appears to be steadite, but there is much more

ferrite and much less pearlite in this region than avyay from the inner wall.

The graphite flakes are smaller and there are more of them per unit volume,

than in the remainder of the material.

3.6 Hardness Measurements

An attempt was made to take Brinell hardness measurements on a transverse

section through one of the two-foot pieces of pipe, but the material cracked

under the load. Therefore, Rockwell K (HRK) hardness measurements were taken.

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The load for hardness measurements with this hardness scale is significantly less than the load for the Brinell hardness. Ten measurements

were made on one transverse section from each of the three submitted

two-foot long pipe pieces. The results of these measurements and the

approximate equivalent Brinell hardness number (HB) are as follows:

Hardness , HRK Approximate Equivalent

Pipe Identification Range Average Brinell Hardness

West end 16 foot, 9 inch length 89-94 91.7 155

11 foot length 91-93 1/2 92.2 157

East end 16 foot, 9 inch length 91 1/2 - 93 1/2

92.6 155

The typical range of Brinell hardness for ordinary gray cast iron is 140-200^.

3.7 Ring Tests and Talbot Tests

Two ring tests and two Talbot tests were performed on specimens machined from

each of the three submitted two-foot pieces of pipe in accordance with American

National Standard A21.9-70. The modulus of rupture was calculated for each

specimen from the results of these tests using the formulae given in ANSI

21.9-70. The results are as follows:

Pipe Identification Modulus of Rupture, psi

• Ring Test Talbot Test

West end of 16 foot, 9 inch length

Gl-1 37,300 36,400

Gl-2 39,800 35,700

11 foot length

G2-1 37,000 35,100

G2-2 40,000 37,400

East end of 16 foot, 9 inch length

G3-1 40,000 28,300

G3-2 37,900 29,400

In addition to the modulus of rupture. the secant modulus of elasticity was calculated from the results of the Talbot tests. These results are as follows:

Pipe Identification Secant Modulus of Elasticity, psi

West end of 16 foot, 9 inch length

Gl-1 7,110,000

Gl-2 7,615,000

11 foot length

G2-1 7,638,000

G2-2 7,350,000

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American National Standard A21.9-70 for cast iron pipe centrifugal ly cast

in sand lined molds for gas requires that the modulus of rupture as calculated from the results of both ring tests and Talbot tests be at least

40,000 psi, and that the secant modulus of elasticity must not exceed

250 times the actual modulus of rupture for any given pipe sample as determined from the results of the Talbot tests.

The pipe being examined appears to have been pit cast, and not centrifugal ly

cast, and the pipe was produced many years before the quoted standard was written. It is not clear what criteria were in effect at the time this

pipe was manufactured. Therefore, although ANSI 21.9-70 is not directly

applicable, it does provide a basis for comparison.

4. DISCUSSION

This eight inch diameter gray cast iron pipe natural gas main fractured

transversely in three places in a region where the soil under the pipe

was missing leaving the gas pipe in an unsupported condition. There was

a fractured water main pipe in close proximity to the failed gas main pipe.

There was some corrosion product on the one fracture surface submitted for

examination, but the corrosion product was reasonably easily removed indicating that the fracture was probably rather recent. The pipe failed in a brittle manner with cleavage being the predominant fracture mode. The

location of the fracture origin was not established.

A small amount of graphitization was found at the fracture surface adjacent

to the outside wall surface of the pipe. In a transverse section slightly

removed from the fracture, no significant graphitization was detected. The

graphitization was not considered excessive in the sections examined.

The chemical composition of the pipe material appeared to be satisfactory

with the possible exception of phosphorus. The phosphorus content was higher

than desirable according to today's practice. A high phosphorus content may

lead to the formation of excessive steadite, a hard, brittle constituent. It

may have also contributed to the high secant modulus of elasticity (relative

to the modulus of rupture) for the piece of pipe from the East end of 16 foot,

9 inch length as calculated from the results of the Talbot tests. The phosphorus content, modulus of rupture from most tests, and two of the secant

modulus of elasticity values would not meet today's ANSI standards for centrifugal ly cast iron pipe. However, it should be noted that the pipe examined

was pit cast and was produced before the standard cited was written.

The hardness of the material was acceptable for gray cast iron. The microstructure of the pipe material appeared to be satisfactory, although some

steadite was present. There was a region adjacent to the inside wall surface

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of the pipe where the carbon content appeared to be lower than that of the

rest of the material. The microstructure is also finer in this region than

in the rest of the material.

5.

CONCLUSIONS

1. This failed gray cast iron gas main pipe had fractured transversely in

three places in a region where the soil beneath the pipe was missing.

2. Failure appeared to be due to a bending load applied to the unsupported

pipe.

3. Most of the corrosion product on the fracture surface was rather easily

removed, indicating that the fracture was probably recent.

4. The fracture mode was predominantly cleavage, which is normal for cast

iron fractures.

5. There was some graphitization at the fracture, but not an excessive

amount. Graphitization did not appear to make a significant contribution

to the failure.

6. The fracture origin was not established.

7. The chemical composition of the material appeared to be typical with

the possible exception of a higher phosphorus content than is usual

in today's grades of cast iron.

8. The hardness of the material was typical for gray cast iron.

9. The microstructure of the material appeared to be satisfactory, with

the possible exception of more steadite than desirable.

6.

ACKNOWLEDGEMENT

Leonard C. Smith of the NBS Fracture and Deformation Division performed the

metal lographic work, prepared the SEM specimen, made the hardness measurements, and assisted in performing the ring and Talbot tests. Joel C. Sauter,’

also of the NBS Fracture and Deformation Division, and Mr. Smith performed

the photographic work.

REFERENCES

1. Gray and Ductile Iron Castings Handbook, Gray and Ductile Iron Founder's

Society, Inc., 1971.

2. Metals Handbook, 8th Edition, Volume 7, American Society for Metals, 1972.

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Figure 1. Piece of eight inch diameter gray cast iron gas pipe

containing one face of the fracture from the 16 foot,

9 inch length of pipe. The piece of pipe is shown as received at NBS. What was reported to be the top of

the pipe as it was oriented in service is at the bottom

in the figure. x 1/2

4J0 m

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05

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Figure

3.

Two-foot

length

of

pipe

identified

as

being

from

the

11

foot

length

as

received

at

NBS.

This

length

did

not

contain

a

fracture.

X

1/3

Figure

4.

Two-foot

length

of

pipe

identified

as

being

from

east

end

of

16

9

inch

length

as

received

at

NBS.

This

length

did

not

contain

a

fracture.

X

1/3

Figure 5. Fracture surface after cleaning. What was reported to be

the top of the pipe as it was oriented in service is at

the top in the figure. X 2/3

Figure 6. SEM fractograph showing representative features of

the fracture surface. X 19

Figure 7. SEM fractograph at higher magnification than in figure 6.

Cleavage is the predominant fracture mode. There is

some evidence of rather mild corrosive attack. X 107

Figure 8. Polished transverse section through the piece of

pipe shown in figure 1 ranging from 1/4 to 2 inches

from the fracture. No significant graphitization is

evident at this section. X 0.8

Figure 9. Etched longitudinal section intersecting the fracture at the

location indicated at the lower right in figure 5. The

fracture profile runs at about 45*^ from the upper left to

the lower right. The outside pipe wall is vertical at the

left and the inside pipe wall is vertical at the right. Some

graphitization can be seen adjacent to the outside wall surface.

Etchant: 1% nital X 11

•Figure 10. Unetched longitudinal section showing essentially complete

graphitization at the left and partial graphitization at

the right.

As polished X 100

Figure 11. Unetched longitudinal section showing the fracture profile

horizontally at the top. The field shown is not adjacent

to either the inside or the outside wall of the pipe. The

graphite flakes are present in a type A distribution.

As polished X 100

Figure 12. Etched longitudinal section showing the fracture profile

horizontally at the top. This microstructure is representative of the material except for a region adjacent to

the inside wall.

Etchant: 1% nital X 100

Figure 13. Etched longitudinal section showing representative microstructure

of the pipe material except for the region adjacent to the inside

wall. This figure is at a much higher magnification than figure 12.

The microstructure consists of graphite flakes, pearl ite (fingerprint pattern), ferrite (light gray patches), and what is probably

steadite (small rounded region within the ferrite patches).

Etchant: Mo nital X 1000

Figure 14. Etched longitudinal section showing the fracture profile

horizontally at the top. The field shown is representative

of the microstructure adjacent to the inner wall of the pipe.

Etchant: 1% nital X 100

Figure 15. Etched longitudinal section showing the representative microstructure adjacent to the inner pipe wall surface. This figure

is at a much higher magnification than figure 14. The microstructure consists of graphite flakes, pearl ite, ferrite and

what appears to be steadite.

Etchant: 1% nital X 1000

HBS-114A (REV. 9.78)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR - 1985

No;

4. TITLE AND SUBTITLE

Examination of Failed Eight Inch Diameter Cast Iron Pipe

Natural Gas Main, Philadelphia, Pennsylvania

7. AUTHOR(S)

T. Robert Shives

AcfiOSSItJli No*

5. Publication Date Feb. 1980

S* PeifOT»»Bg;Ofgamzation Code

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

iSi Pfeied/Ta«k/W<xk ifetit No.

11. Contract/Grant No. 5620-42

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS cSfreef, City, sr^fe, ZIP)

National Transportation Safety Board

Washington, D.C. 20594

13. Type of Report & Period Covered Failure Analysis

Ag«icy Cod?

\ V \X ' X ' ;-!E

15. SUPPLEMENTARY NOTES

Contract/Grant No. 5620-42

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

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

literature survey, mention it here.) At the request of the National Transportation Safety Board, the

National Bureau of Standards performed an examination of a fractured eight inch diameter cast iron pipe natural gas main. An explosion and fire at the location of the

failure had occurred on May 13, 1979 at Tacony and Margaret Streets in Philadelphia,

Pennsylvania. The supporting soil beneath the pipe was missing at the location of the

failure. The pipeline had completely fractured circumferential ly in three places, and

after fracturing, fell into the cavity created by the missing soil. Failure appeared to

be due to the application of a bending load to the unsupported pipe.

The non-uniform wall thickness indicated that the pipe was pit cast and therefore

was probably rather old. Today, such pipe is normally centrifugal ly cast which results

in a uniform wall thickness.

Only one of the fracture surfaces was selected by the National Transportation Safe

ty Board for examination by the National Bureau of Standards. Corrosion product on

that fracture surface was rather easily removed, indicating that the fracture was likel

recent. Cleavage was the predominant fracture mode. The amount of graphitization at

the fracture was not considered excessive.

The hardness of the material was typical for gray cast iron. The chemical composition was typical for this material, but the phosphorus content was higher than desirable according to today's practices. The microstructure appeared to be satisfactory

with the possible exception of more steadite (iron-iron phosphide eutectic) than

.sirahlp arrnrding

tn tnHay'<; practices ^ —

de17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separsted by aemicotona)

Brittle fracture; Cast iron. Cast

Graphitization

iron pipe, cast iron gas pipe, Cleavage;

18. AVAILABILITY I \ Unlimited

nCi For Official Distribution. Do Not Release to NTIS

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20402, SD Stock No. SN003-003-

I i Order From National Technical Information Service (NTIS), Springfield,

VA. 22161

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

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22. Price

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