NBSIR 74-594

Examination of Failed Four Inch

Cast Iron Pipe Natural Gas Main,

Philadelphia Gas Works,

Philadelphia, Pennsylvania

T. R. Shives

Mechanical Properties Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

October 1974

Failure Analysis Report

Prepared for Office of Pipeline Safety

Department of Transportation

Washington, D. C. 20590

NBSIR 74-594

EXAMINATION OF FAILED FOUR INCH

CAST IRON PIPE NATURAL GAS MAIN,

PHILADELPHIA GAS WORKS,

PHILADELPHIA, PENNSYLVANIA

T. R. Shives

Mechanical Properties Section

Metallurgy Division

Institute for Materials Researcti

National Bureau of Standards

Washington, D. C. 20234

October 1974

Failure Analysis Report

"This document has been prepared for the use of the Office of Pipeline Safety, Department of Transportation,

Washington, D.C. 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 Office of Pipeline Safety

Department of Transportation

Washington, D. C. 20590

U. S. DEPARTMENT OF COMMERCE. Frederick B. Oem. Secretary

NATIONAL BUREAU OF STANDARDS. Richard W. Roberts. Director

TABLE OF CONTENTS

Page

SUMMARY iii

1. INTRODUCTION 1

1.1 Reference 1

1.2 Background Information 1

1.3 Parts Submitted 2

2. PURPOSE 2

3. PLAN OF THE EXAMINATION 2

4. RESULTS OF TESTS, EXAMINATIONS, AND ANALYSES 2

4.1 Visual and Macroscopic Examinations 2

4.2 Fractographic Examination with the

Scanning Electron Microscope 4

4.3 Chemical Analysis 4

4.4 Metallographic Examination 5

4.5 Hardness Tests 6

5. DISCUSSION 6

6. CONCLUSIONS 7

7. ACKNOWLEDGEMENT 8

REFERENCES 8

FIGURES

1. Lengths of the gas main pipe and service taps as

received at NBS

2. Opposing fracture surfaces as received

3. North fracture surface after ultrasonic cleaning ...

4. Area A of the fracture surface shown in figure 3 ...

i
FIGURES (Continued)

5. Transverse cross section through the pipe two

inches north of the fracture

6. Transverse cross section through the pipe nine

inches south of the fracture

7. SEM fractograph showing the region of apparent

gas porosity

8. SEM fractograph exhibiting features typical of

the pipe fracture surface .

9. SEM fractograph exhibiting features typical of

the pipe fracture surface

10. Longitudinal section through the top of the pipe

showing the fracture profile

11. Area of figure 10 where the outside wall surface

of the pipe intersects the fracture

12. Typical field showing the microstructure of

the pipe material

13. Longitudinal section through the top of the pipe ...

14. Three fields from the longitudinal section

shown in figure 13
SUMMARY

The Office of Pipeline Safety submitted several pieces of four

inch diameter cast iron natural gas main pipe to the National

Bureau of Standards Mechanical Properties Section for examination. An accumulation of gas that had escaped from a

fracture in the pipeline resulted in an explosion in the 12 00

block of South Markoe Street in Philadelphia, Pennsylvania on

May 3, 1974. The fracture had occurred in a transverse plane

that passed through a service tap hole in the top of the pipe.

The fracture was brittle in nature and there was no evidence

to indicate the existence of a crack prior to the failure.

There was extensive graphitic corrosion in some areas of the

pipe, although this does not appear to be related to the

failure. A chemical analysis indicated that the phosphorus

content of the pipe material was higher than desirable.

The microstructure contained a considerable amount of ironiron phosphide eutectic. Failure apparently occurred from a single stressing event caused by bending loads from an

external source.

Examination of Failed Four Inch Cast Iron Pipe Natural Gas

Main, Philadelphia Gas Works, Philadelphia, Pennsylvania

1. INTRODUCTION

1 . 1 Reference

Office of Pipeline Safety, Department of Transportation,

Washington, D. C. This investigation was conducted at the

request of Mr. Lance F. Heverly of the Office of Pipeline

Safety under order number DOT-AS-10041 . The request was made

on May 21, 1974.

1 . 2 Background Information

The information in this section was furnished by

Mj:. Lance F. Heverly of the Office of Pipeline Safety,

Mr. John F. Flaherty of the Pennsylvania Public Utility

Commission, and Messrs. Joseph M. Devereaux and Leonard Orlando

of the Philadelphia Gas Works.

On May 3, 1974, there was an explosion at 1213 South Markoe

Street in Philadelphia, Pennsylvania, that severely damaged

several houses, killed two people, and injured nine others.

The explosion was attributed to an accumulation of natural gas

that had escaped from a complete fracture in the four inch

diameter cast iron gas main pipe located in South Markoe Street.

The gas main pipeline was under 36 inches of cover consisting of soil and the roadway pavement. A fourteen foot

length of the pipe had been excavated to a depth of several

feet below that of the gas main some weeks before the failure

in order to service another utility pipeline. The excavation

had been only partially refilled at the time of the explosion.

The pipe failed about two feet north of the excavation. Just

after the part of the pipe containing the fracture had been

exposed in the ditch, the top of the pipe at the south fracture

face was observed to be approximately at the level of the

bottom of the pipe at the north fracture face. The excavation

was under water for about 18 hours after the explosion, the

water having been used by fire fighting equipment.

The gas main had been installed in about 1897. The

service lines in the vicinity of the failure had been installed

in the 1920 's. Pressure in the main at the time of the

accident was 0.25 psig. The soil temperature at the point of

the fracture was estimated to be about 50° F.

- 1

-
1.3 Parts Submitted

Three lengths of pipe from the cast iron gas main, one

of which contained a service tap, and one separate service

tap were submitted for examination. The separate service tap

had been located at the point of fracture. The parts were

delivered to T. R. Shives of the NBS Mechanical Properties

Section by Captain James J. Leonard, Assistant Fire Marshal,

City of Philadelphia, and Mr. Harry J. Forr of the Philadelphia

Gas Works on May 21, 1974. The parts as received are shown in

figure 1 after they had been removed from the shipping crates.

The locations from which the pipe pieces had been removed from

the ditch are indicated in the figure legend. The total length

of gas main pipe submitted was about 4 3/4 feet.

2 . PURPOSE

The Office of Pipeline Safety requested the NBS Mechanical

Properties Section to examine the fractured pipe and, in so far

as possible, to determine the cause of failure.

3. PLAN OF THE EXAMINATION

At a meeting held at the National Bureau of Standards on

June 7, 1974, the general plan of the examination was discussed.

This meeting was attended by Mr. Lance F. Heverly of the Office

of Pipeline Safety, Mr. John F. Flaherty of the Pennsylvania

Public Utility Commission, Messrs. Joseph M. Devereaux and

Leonard Orlando of the Philadelphia Gas Works, Mr. Kumar

Kishinchand of the Philadelphia Water Department, and T. R.

Shives of the NBS Mechanical Properties Section. The failure

analysis was to include a f ractographic examination, a metallographic examination of the pipe material, an examination of

the surface condition of the pipe, hardness measurements, and

other examinations or tests deemed advisable as the analysis

progressed.

4 . RESULTS OF TESTS, EXAMINATIONS, AND ANALYSES

4 . 1 Visual and Macroscopic Examinations

There was a transverse fracture in the pipe in a plane

that passed through the approximate center of the tap hole

for the gas service line to the residence at 1213 South Markoe

Street. The location of the fracture is indicated by the arrow

in figure 1. The service line, nominally 1 1/4 inches in

diameter, was reported to be of galvanized steel and was

attached to the top of the gas main pipe by a threaded joint.

The separate service tap shown in figure 1 had been attached

to the gas main at the point of the fracture.

- 2 -
When received at NBS , the fracture surfaces were covered

with loosely adhering soil. There was a small amount of

loosely adhering soil and a considerable amount of a rather

tightly adhering material which appeared for the most part to

be soil on the outside wall surfaces of all the submitted

pieces of the gas main. There was a considerable amount of

soil inside the pipe pieces, especially near the fracture, as

can be seen in figure 2 in which the opposing fracture surfaces

are shown. It is likely that running water carried the soil

into the pipe at the point of fracture, because the failure

occurred about two feet north of the partially excavated ditch,

and the ditch had water running into it for about 18 hours

after the explosion. The soil was removed from the piece of

pipe containing the north fracture by ultrasonic cleaning.

There was corrosion product (rust) on both the inside and

outside surfaces of the pipe, but the corrosion did not

appear by visual examination to be severe enough to significantly affect the integrity of the pipe.

There was also some corrosion product on the fracture

surfaces. This corrosion product was removed from the north

fracture surface by ultrasonic cleaning. The fracture surface

is shown after cleaning in figure 3. Since the product could

be easily removed, the corrosion of the fracture surface

appeared to be superficial. The corrosion was likely due to

exposure to the water after the explosion. The absence of

any tightly adhering rust on any portion of the fracture

surface indicates that there was no pre-existing crack.

There was some evidence of what appeared to be gas porosity

(figure 4) on the fracture surface near the bottom of the

pipe. The fracture surface had also suffered some mechanical

damage, especially adjacent to the inside wall surface.

Examples of mechanical damage are indicated by arrows B in

figure 3. This damage could have occurred during the removal

of the pieces of pipe from the ditch or in subsequent handling.

There was no visual evidence of graphitic corrosion at the

fracture surface, but transverse sections through the pipe at

distances of 2 and 14 1/2 inches north of the fracture and 9

and 17 inches south of the fracture did exhibit this type of

corrosion. Examples are shown in figures 5 and 6. In one

area near the top of the pipe in the section shown in figure

5, the graphitic corrosion had penetrated nearly the entire

wall thickness of the pipe. There was no evidence of

graphitic corrosion in a transverse section 1/2 inch north

of the fracture.

- 3 -
The wall thickness of the pipe varied around the

circumference as can be seen in figures 3, 5, and 6. In the

plane of the fracture, the wall thickness ranged from about

0.45 inch at the bottom of the pipe to about 0.33 inch near v the top of the pipe. This pipe was installed in the latter

part of the 19th century and it is likely that it was

produced by the pit casting method. 1 It was not uncommon to

have such variations in wall thickness of pipe made by this

method.

Referring to figure 3, it is clear that the pipe wall

is thinner near the threaded hole than remote from it. The

wall thickness is about 0.33 inch at about one-half inch away

from the edge of the threaded hole, and about 0.2 0 inch

adjacent to, but not in, the threads. The thinning had

occurred on the inside of the pipe.

4.2 Fractographic Examination with the Scanning Electron

Microscope

Areas of the north fracture surface both near the top and

near the bottom of the pipe were examined with the scanning

electron microscope. There were no significant differences

found in the general fracture appearance among the areas

examined except for the area containing the apparent gas

porosity (shown in figure 4 by light microscopy) . A scanning

electron photomicrograph showing the area of gas porosity

appears in figure 7. Two scanning electron photomicrographs

showing areas typical of the fracture appearance are shown at

different magnifications in figures 8 and 9. The graphite

flakes in the cast iron are evident. Very little if any

dimpled rupture was detected on the fracture surface

indicating that the fracture v/as of a brittle nature, which

would be expected for this material.

4 . 3 Chemical Analysis

A chemical analysis was performed on a sample of the pipe

material by a competent commercial laboratory with the following

results

Element Per Cent (weight)

Total carbon

Graphitic carbon

Manganese

Phosphorus

Sulfur

Silicon

Nickel

Chromium

Molybdenum

Copper

< 0. 01

< 0. 01

< 0. 01

< 0. 01

3 . 54

2.43

0. 54

0.867

0.112

2.49

- 4
The phosphorus content is higher than is generally desirable.

Above about 0.50%, phosphorus can have adverse effects on the

mechanical properties of cast iron.

4 . 4 Metallographic Examination

An unetched longitudinal section through the top of the

pipe showing the fracture profile is shown in figure 10. The

fracture profile is at the right, the outside of the pipe wall

is at the top, and the inside of the pipe wall is at the bottom.

The large, dark areas in the section are regions of porosity.

The part of figure 10 showing the intersection of the fracture

with the outside surface of the pipe is shown at higher magnification in figure 11. The graphite flakes, which are

primarily types A and C, are the thin, dark particles. The

lack of apparent deformation adjacent to the fracture is

characteristic of brittle fracture. There is no apparent

graphitic corrosion in this section. An etched field exhibiting

the microstructure typical of the cast iron pipe material is

shown in figure 12. The microstructure consists primarily of

graphite flakes (long, thin, dark gray particles) and pearlite

(fingerprint pattern) with small amounts of ferrite (white)

and steadite (small rounded phase) . This microstructure

appears normal for gray cast iron except possibly for the

amount of steadite. If present in large enough quantities,

steadite, the iron-iron phosphide eutectic, may be detrimental to mechanical properties. 2 The amount of steadite

present depends on the phosphorus content, and since the

phosphorus content was higher than desirable (Section 4.3),

it follows that the amount of steadite is probably higher

than desirable.

A longitudinal section whose right side is about two

inches north of the fracture is shown in figure 13. Extensive

graphitic corrosion (dark area) has taken place over the right

half of the field shown. At the far right, the graphitic

corrosive attack has penetrated nearly the entire wall

thickness of the pipe.

Examination revealed that the graphitic corrosion started

at the outside surface and proceeded inward. This is illustrated in figure 13. Figures 14a, 14b, and 14c show, at

higher magnification, areas of figure 13 which illustrate

essentially no graphitic corrosion, partial graphitic corrosion, and nearly complete graphitic corrosion, respectively.

- 5 -
4.5 Hardness Tests

Rockwell hardness measurements were made on a transverse

section through the pipe two inches north of the fracture.

Fifteen measurements made at various points around the circumference ranged from Rg 81 to Rb 91 with an average of Rb 88.

This average is approximately equivalent to a Brinell hardness

of 17 6 which is within the range expected for ordinary gray

cast iron.

5. DISCUSSION

This four inch diameter cast iron natural gas main pipe

failed transversely through the center of a service tap hole.

This hole, which was at the top of the pipe could act as a

stress concentrator. The region of the pipe wall surrounding

the tap hole was thinner than the pipe wall away from the tap

hole. Both the tap hole and the thinned wall reduced the

cross section area of the pipe in the plane of the fracture

which, in turn, reduced the load carrying capability of the

pipe. Areas of significant graphitic corrosion were found in

several transverse planes through the pipe, but there appeared

to be no graphitic corrosion at the fracture; hence, whereas

the corrosion might be considered to be detrimental, it does

not appear to have contributed to the failure of the pipe.

The general corrosion of the inside and outside wall

surfaces of the pipe did not appear to be severe enough to

have any significant effect on the integrity of the pipe.

The fracture appeared to be brittle in nature, which

would normally be expected in cast iron. This is borne out

by the fracture features and the lack of apparent deformation

adjacent to the fracture. The gas porosity found at the

fracture surface and in metallographic cross sections, while

not desirable, is not considered to be excessive.

A chemical analysis of the pipe material indicates that

there is a higher than desirable phosphorus content. This is

borne out by the significant amount of iron-iron phosphide

eutectic (steadite) that was present in the microstructure of

the pipe material. Steadite is a brittle phase and can have

detrimental effects on the mechanical properties of cast iron

if present in large enough amounts. Except for the steadite

and the regions that had suffered graphitic corrosive attack,

the microstructure appeared to be satisfactory.

The hardness of the pipe material as measured on a transverse section v/as normal for ordinary cast iron.

- 6 -
The soil in the pipe apparently accumulated subsequent

to the failure and had no influence on the cause of the

failure.

Since the corrosion product on the fracture surfaces

appeared to be superficial, it is concluded that there was

no crack prior to the time of the failure of the pipe.

Failure v;as probably the result of a single stressing event

caused by loading from an external source such as soil

subsidence. The stressing condition may have been aggravated

by inadequate support beneath the pipe at the site of the 14

foot long excavation. Evidence of inadequate support and/or

a bending stress is the fact that, while the failed pipe v/as

still in place in the ditch, the south fracture face was

about a pipe diameter below the north fracture face.

The pipe probably failed where it did due to a combination of circumstances such as 1) the hole at the top of the

pipe for the service tap acting as a stress concentrator, 2)

reduced cross section in the plane of the fracture because of

the tap hole and because of the reduced pipe wall thickness

adjacent to the tap hole, and 3) possible constraint applied

to the pipe north of the fracture by three sources. These

sources are (a) the service line to 1213 South I^arkoe Street

which was attached to the gas main at the point of fracture,

(b) the service line to 1211 South Markoe Street which was

attached to the gas main about six inches north of the fracture,

and (c) the soil north of the fracture that had not been

disturbed by the excavation. Failure was apparently caused

by a single event bending stress from an external source.

6. CONCLUSIONS

1. This four inch cast iron natural gas main pipeline failed

in a brittle manner in a transverse plane that passed

through the center of a service tap hole.

2. There was no evidence to indicate the existence of a crack

prior to the failure.

3. Failure was apparently caused by a single event bending

stress due to external loading.

4. The location of the failure is probably related to the

tap hole acting as a stress concentrator, the reduced

cross section area of the pipe at the plane of the

fracture, and possible constraints imposed on the pipeline

by the service lines and by the undisturbed soil north

of the fracture.

- 7 -
5. There may have been inadequate support under the pipe

over the length of the excavation.

6. The pipe had suffered severe graphitic corrosive attack

in some areas, although there was no visual evidence of

this corrosion at the fracture.

7. The chemical composition of the pipe material appeared

to be satisfactory except for a somewhat higher than

desirable phosphorus content.

8. Except for what may be a greater than desirable amount of

iron-iron phosphide eutectic content and graphitic

corrosion, the microstructure appeared to be satisfactory.

9. The hardness of the pipe appeared to be satisfactory.

7 . ACKNOWLEDGEMENT

Mr. Leonard C. Smith of the NBS Mechanical Properties

Section performed the photographic work, prepared the metallographic specimens, and assisted in other aspects of this

investigation

REFERENCES

1. Development of the Metal Castings Industry , Bruce L.

Simpson , American Foundrymen ' s Association, 1948.

2. Metal Castings , Harry L. Campbell, John Wiley and Sons,

Inc. , New York, 1936.

3. Gray and Ductile Iron Castings Handbook , Charles F.

Walton, Editor, Gray and Ductile Iron Founders' Society,

1971.

- 8 -
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Figure 2. Opposing fracture surfaces as received. The north

fracture surface is at the left and the south fracture

surface is at the right. Large deposits of soil can

be seen inside the pieces of pipe, along with some

soil on the fracture surfaces and what appears to be

soil adhering to the outside of the pipe. The top

of the pipe as installed in the soil is at the top in

the photograph. X 1/2

Figure 3. North fracture surface after ultrasonic cleaning. The top of the pipe as installed in the

ditch is at the top of the photograph. The

threaded region is in the tap hole for the

service line to 1213 South Markoe Street.

Arrow A indicates an area of the fracture

surface shown in figure 4. Arrows B indicate

examples of mechanical damage. X 1
I
Figure 4. Area A of the fracture surface shown in

figure 3. This is "a region of apparent

gas porosity. X 4

Figure 5. Transverse cross section through the pipe

two inches north of the fracture. The dark

areas at the top and at about 4 o'clock are

regions of graphitic corrosion. Top of pipe

as installed in the soil is at top of

photograph. X 1

Figure 6. Transverse cross section through the pipenine inches south of the fracture. Regions

that have suffered graphitic corrosion can

be seen around much of the circumference.

The top of the pipe as installed in the soil

is at the top of the photograph. X 1

Figure 7 . SEM f ractograph showing the region of

apparent gas porosity (arrow A, figure 3)

The patches of very light colored matter

are foreign material. X 20

Figure 8, SEM f ractograph exhibiting features

typical of the pipe fracture surface. In

most cases, the narrow dark phase is

graphite. X 89

Figure 9. SEM fractograph exhibiting features

typical of the pipe fracture surface.

This figure is at a higher magnification

than that of figure 8. The dark phase

is graphite. X 4 60

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Figure 10. Longitudinal section through the top of

the pipe showing the fracture profile at

the right. The outside wall surface of

the pipe is at the top and the inside wall

surface of the pipe is at the bottom. The

dark patches are regions of apparent gas

porosity. Unetched. X 7.5

Figure 11. Area of figure 10 where the outside wall

surface of the pipe intersects the fracture

(vertically at the right) . Some general

surface corrosion can be seen on the outside wall surface. Most of the dark areas

are graphite flakes. Unetched. X 50

Figure 12. Typical field showing the microstructure of

the pipe material. The microstructure consists of graphite flakes (long, thin, dark

particles) , pearlite (fingerprint pattern)

ferrite (white) , and steadite (small rounded

particles)

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U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. inim.lf A 1 ION OK HI' I'OKT NO. 2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND .SUB I IT!. I- EXAMINATION OF FAILED FOUR INCH CAST IRON PIPE

NATURAL GAS MAIN, PHILADELPHIA GAS WORKS,

PHILADELPHIA, PENNSYLVANIA

5. Publ i< at ion Date October, 1974

6. Performing Organization Gode

7. AUTliOR(S) t T. R. Shives

8. Pi rforming t'rg.iii. Report No. NBSIR 74-594

9. PERFORMING ORGANIZATION NAMI- ANH ADDRl-SS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Pro)rci/Task/Work Unit No. 3120418

11. Contract/Grant No.

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

Office of Pipeline Safety

Department of Transportation

Washington, D. C. 20590

13. I ype of Report &: Period

( overed pallure

Analysis Report

14. Sponsoring Agency Code

15. SUPFl.KMKNTARY NOTKS

16. Af^STRAf 1 (A 2 00- word or less factual summary of most significant information. If document includes a sifinificant bibliographv or literature survey, mention it here.)

The Office of Pipeline Safety submitted several pieces of four inch

diameter cast iron natural gas main pipe to the National Bureau of

Standards Mechanical Properties Section for examination. An accumulation

of gas that had escaped from a fracture in the pipeline resulted in an

explosion in the 12 00 block of South Markoe Street in Philadelphia,

Pennsylvania on May 3, 1974. The fracture had occurred in a transverse

plane that passed through a service tap hole in the top of the pipe.

The fracture was brittle in nature and there was no evidence to indicate

the existence of a crack prior to the failure. There was extensive

graphitic corrosion in some areas of the pipe, although this does not

appear to be related to the failure. A chemical analysis indicated that

the phosphorus content of the pipe material was higher than desirable.

The microstructure contained a considerable amount of iron-iron

phosphide eutectic . Failure apparently occurred from a single stressing

event caused by bending loads from an external source.

17. KKY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semic olons

)

Brittle fracture; cast iron gas pipe; graphitic corrosion;

steadite.

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