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NBSIR 80-2036^

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Examination of Failed Six-Inch

Diameter Cast-Iron Pipe Water

Main, Philadelphia, Pennsylvania

T. Robert Shives

Fracture and Deformation Division

Center for Materials Science

National Measurement Laboratory

National Bureau of Standards

U.S. Department of Commerce

Washington, D.C. 20234

January 1 980

Issued April 1980

Failure Analysis Report

Prepared for National Transportation Safety Board

Washington, D.C. 20594

NBSIR 80-2036

EXAMINATION OF FAILED SIX-INCH

DIAMETER CAST-IRON PIPE WATER

MAIN, PHILADELPHIA, PENNSYLVANIA

T. Robert Shives

Fracture and Deformation Division

Center for Materials Science

National Measurement Laboratory

National Bureau of Standards

U.S. Department of Commerce

Washington, D.C. 20234

January 1 980

Issued April 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 Productivity, Technology, and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

TABLE OF CONTENTS

SUMMARY

1. INTRODUCTION

1.1 Reference

1.2 Background Information

1.3 Parts Submitted

2. PURPOSE

3. RESULTS OF EXAMINATIONS, TESTS, AND ANALYSES

3.1 Visual Examination of the Pipe Wall Surface

3.2 Fractographic Examination

3.3 Examination for Graphitization

3.4 Chemical Analysis

3.5 Hardness Measurements

3.6 Metal lographic Examination

4. DISCUSSION

5. CONCLUSIONS

6. ACKNOWLEDGEMENT

REFERENCES

FIGURES

1. Piece of submitted pipe designated W1 as received at NBS showing the

fracture surface.

2. Piece of submitted pipe designated W2 as received at NBS showing the

fracture surface.

3. Pieces of submitted pipe designated W3 as received at NBS showing the

fracture surface.

4. Fracture surface W1 after cleaning.
5. Fracture surface W2 after cleaning.

6. Fracture surface W3 after cleaning.

7. SEM fractograph of W1

.

8. SEM fractograph of W1 at much higher magnification than figure 7.

9. Transverse section through W1 about 3/4 inch from the fracture

showing graphitization adjacent to the inside wall surface.

10. Transverse section through W2 about 3/4 inch from the fracture showing

graphitization adjacent to the inside wall surface.

11. Transverse section through W3 ranging from about 1/2 to 2 inches from

the fracture showing graphitization adjacent to both the inside

and outside wall surfaces.

12. Etched longitudinal section through W1 showing the fracture profile.

13. Unetched longitudinal section through W1 showing the fracture profile.

14. Etched longitudinal section showing the entire wall thickness of W3.

15. Etched longitudinal section through W1 in a region which has not been

graphitized.

16. Etched longitudinal section through W1 in a region that has been

essentially totally graphitized.
SUMMARY

At the request of the National Transportation Safety Board, the

National Bureau of Standards performed an examination of a fractured sixinch diameter cast-iron pipe water main from Tacony and Margaret Streets

in Philadelphia, Pennsylvania. An explosion and fire had occurred at

this location on May 13, 1979. At that time, both the six-inch diameter

water main pipe and the eight-inch diameter cast-iron natural gas main

pipe at the site of the explosion and fire were found to be completely

fractured transversely in three places. Failure of the water pipe

appeared to be due to an externally applied bending load.

The nonuniform wall thickness indicated that the pipe was pit cast

and therefore rather old. Today, such pipe is normally centrifugally

cast which results in a uniform wall thickness.

Each of the three fracture faces submitted for examination was covered with a rather tightly adhering layer of corrosion product. The

corrosion product was not easily removed, and the corrosion process had

obliterated some of the fracture features indicating that the fractures

were not fresh. Cleavage was the predominant fracture mode. There was significant graphitization close to the fractures.

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 practice. The

microstructure appeared to be satisfactory with the possible exception

of more steadite than desirable according to today's practice.
n

.
Examination of Failed Six-Inch Diameter Cast-Iron Pipe Water Main,

Philadelphia, Pennsylvania

1 . INTRODUCTION

1 .1 Reference

National Transportation Safety Board, Washington, DC 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

The information in this section was furnished by the National Transportation

Safety Board and by representatives of the Philadelphia Gas Works. On May 4,

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, both the eight-inch diameter cast-iron pipe

natural gas main and the six-inch diameter cast-iron pipe water main at the

site were found to be broken. The two pipelines were in close proximity

and both lines had fractured completely in a circumferential manner in

three places. (The results of an NBS examination of the failed gas pipe

can be found in NBS Report NBSIR 80-1985, issued February, 1980).

When the pipelines in the region of the fractures were uncovered, the soil

beneath the gas main 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 gas pipeline

unsupported. When the gas line fractured, it fell into this cavity.

1 .3 Parts Submitted

Pieces of the water main pipe containing one fracture face from each of

the three fractures were submitted to NBS for examination. The parts,

designated W1 , W2, and W3, are shown as received at NBS in figures 1, 2,

and 3, respectively. The pieces shown in figures 1 and 2 were complete,

intact transverse sections. The pieces shown in figure 3 (W3) were from

a transverse section that included the end of the pipe length. It was reported that this section was fractured longitudinally into the three

pieces shown while being removed from the adjoining pipe length. The

fracture in W1 passed through the approximate center of a service tap

hole in the pipe.

- 1 -
2. PURPOSE

The National Transportation Safety Board requested that NBS determine

the mode of failure for the broken water pipe.

3. RESULTS OF EXAMINATIONS, TESTS, AND ANALYSES

3.1 Visual Examination of the Pipe Wall Surface

Each piece of the submitted pipe was covered with a rather adherent

layer of soil and some corrosion product. The pieces were cleaned

ul trasonical ly, first with a conmercial detergent, and then with buffered

hydrochloric acTd. Essentially all of the soil was removed from the

outside of the pipe by the cleaning procedures. Some corrosion of both

the inside and outside walls of the pipe was evident.

There was a lack of uniformity in the pipe wall thickness indicating

that the pipe was pit cast, and therefore rather old. As an example of

the nonuniform thickness, the wall thickness at the fracture in W1

ranged from about 0.43 inch minimum to about 0.53 inch maximum.

3.2 Fractographic Examination

All three fracture surfaces were covered with corrosion product. The

fracture surfaces were cleaned ul trasonical ly at the same time the pipe

walls were cleaned. Some of the corrosion product adhered tightly to

the fracture surfaces, and much of it remained on the surfaces after

cleaning. The fact that the corrosion product was difficult to remove

indicates that the pipe had probably been broken for some time. The

cleaned surfaces of fractures W1 , W2, and W3 are shown respectively in

figures 4, 5, and 6.

Visually and macroscopically, all three fractures appeared similar.

Therefore, a detailed fractographic examination was performed on only

one of the fracture surfaces. After cleaning, part of the fracture

surface from the piece of pipe designated W1 was examined with the

scanning electron microscope (SEM). Many of the fracture features had

been obliterated by the corrosion process, but the fracture mode appeared

to be predominantly cleavage. Cleavage indicates a very low ductility

or brittle fracture, which would be expected for an overload fracture in

gray cast iron.

A low magnification SEM fractograph is shown in figure 7. Although some

of the detail has been lost due to the corrosion process, and some is

masked by the corrosion product that remained on the fracture surface,

evidence of cleavage can still be seen. A much higher magnification SEM

fractograph is shown in figure 8. Corrosion product remaining on the fracture

surface is quite evident.

- 2 -
3.3 Examination for Graphitization

Transverse sections were taken close to each of the three fractures.

These sections were polished metal lographical ly to determine the extent

of graphitization. The sections from W1 , W2, and W3 are shown in

figures 9, 10, and 11, respectively. Some graphitization adjacent to

the inside wall surface is evident in each section, and in some regions,

the depth of the graphitization is significant. In one region in the

section from W1 (figure 9), graphitization starting at the inside had

penetrated about 1/3 of the pipe wall thickness.

The transverse sections from W1 and W2 exhibited essentially no evidence

of graphitization adjacent to the outside wall surface, but there was significant graphitization adjacent to the outside wall surface of W3

over most of the circumference of the section. This section is very

close to the end of the pipe length.

3.4 Chemical Analysis

A sample from piece W2 was submitted to a commercial laboratory for

chemical analysis. The results of that analysis are as follows:

El ement Percent (by weight)

Total carbon 3.21

Combined carbon 1.01

Manganese 0.42

Phosphorus 1.97

Sulfur 0.084

Silicon 1.86

Nickel 0.04

Chromium 0.01

Molybdenum ^ 0.01

Copper 0.14

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

for the material is greater than 4.3%, indicating that the material is

hypereutectic and would therefore be expected to be relatively low in

strength. Current ANSI Standard A21. 6-1975 (American Water Works

Association Standard Cl 06-75) for cast iron pipe centrifugal ly cast in

metal molds, for water or other liquids, and ANSI Standard 21.8-1975

(AWWA Cl 08-75) for cast iron pipe centrifugally cast in sand-lined

molds, for water or other liquids, specify a maximum phosphorus content

of 0.50% and a maximum sulfur content of 0.12%. Since this pipe was pit

cast, and therefore probably rather old, the standards quoted would not

apply. They are cited for comparison only. Compared to today's practice,

however, the phosphorus content is quite high. Excessive phosphorus

leads to the formation of steadite (iron-iron phosphide eutectic).

Steadite is hard and brittle, and if present in sufficient quantities,

it may have adverse effects on the mechanical properties of the cast

iron.

- 3 -
3.5 Hardness Measurements

Rockwell K (HRK) hardness measurements were taken on the transverse

section from W2 that is shown in figure 10. Ten measurements were made

around the circumference. The range in hardness was HRK 85 1/2 to 97

and the average value was 92.8. The approximate equivalent Brinell (HB)

hardness values range from 136 to 176 and the approximate equivalent HB

average is 159. The variation in hardness is rather large, but the

average hardness is typical for ordinary gray cast iron^.

3.6 Metal lographic Examination

An etched longitudinal section through W1 showing the entire pipe wall

thickness with the fracture profile horizontal at the top appears in

figure 12. This section passes through a region of graphitization which

can be seen adjacent to the inside wall surface at the left. An as- polished longitudinal section through W1 showing part of the fracture

profile at higher magnification appears in figure 13. The thin, dark

particles are graphite flakes with an essentially type A distribution2 # A longitudinal section showing the entire wall thickness of W3 slightly

removed from the fracture appears in figure 14. This figure is at a lower magnification than figure 12. It should be noted that the region

shown is very near the end of the pipe length where it connected to

another pipe length while in service. There is significant graphitization adjacent to both the inside and outside wall surfaces.

An etched longitudinal section through W1 in a region which has not been

subject to graphitization is shown in figure 15 at much higher magnification than in figure 13. The microstructure consists of graphite flakes

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

(light gray patches), and probably steadite (small, rounded particles

within the ferrite patches). A region in a longitudinal section through

W1 exhibiting almost total graphitization is shown in figure 16.

4. DISCUSSION

This six-inch diameter cast-iron pipe water main fractured completely in

a circumferential manner in three places. There was corrosion product

on the fracture surfaces. This corrosion product adhered rather well to

the fractures, and some of the fracture features had been obliterated by

corrosive attack. These two factors suggest that the pipeline had been

fractured for some time. In contrast, the fracture examined from the

failed cast-iron pipe natural gas main from the same location appeared

to be relatively fresh (NBS report NBSIR 80-1985). The pipe failed in a brittle manner as expected for an overload fracture. Cleavage was the

predominant fracture mode. The locations of the fracture origins were

not established.

One of the fractures passed through the approximate center of a service

tap hole in the pipe. This tap hole would have reduced the cross section

of the pipe in the plane of the fracture which, in turn, would reduce the

load carrying capacity of the pipe somewhat at this location. A significant amount of graphitization was found adjacent to the fractures.

- 4 -
In a region near the end of the pipe length, there was graphi tization

adjacent to both the inside and outside wall surfaces of the pipe. Away

from the end of the pipe, graphitization was found only adjacent to the

inside wall surface. The graphitization would have also reduced the

effective cross section of the pipe somewhat.

It appears that the pipe failed due to the application of an external

bending load.

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 is unclear whether the high phosphorus content was a contributing factor to the failure.

The hardness of the material varied considerably, but the average hardness

was acceptable for gray cast-iron. The microstructure of the pipe material

appeared to be satisfactory although some steadite was present.

5.

CONCLUSIONS

1. This failed gray cast iron pipe water main had fractured completely

in three places in a transverse manner.

2. Failure appeared to be due to an external bending load applied to

the pipe.

3. Much of the corrosion product adhered rather tightly to the fracture

surfaces and the corrosion process had obliterated some of the fracture

features. These two factors suggest that the fractures had occurred

some time ago.

4. The fracture mode was predominantly cleavage indicating that the

fractures were brittle, which is normal for gray cast-iron.

5. There was significant graphitization in localized regions near the

fractures.

6. The fracture origins were not established.

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

for gray cast-iron 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, although exhibiting considerable

variation, was within the range typical for gray cast-iron.

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

the possible exception of more steadite than desirable.

- 5 -
5. ACKNOWLEDGEMENT

The metallography and hardness measurements were performed by Leonard C.

of the NBS Fracture and Deformation Division. The photographic work was performed by Leonard Smith and Joel C. Sauter, also of the NBS Fracture

and Deformation Division.

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.

Smith

- 6 -
Figure 1. Piece of submitted pipe designated W1 as received at NBS

showing the fracture surface. The fracture passed through

a service tap hole. X 2/3

Figure 2. Piece of submitted pipe designated W2 as received at NBS

showing the fracture surface. X 2/3

Figure 3. Pieces of submitted pipe designated W3 as received at NBS

showing the fracture surface. These pieces had formed a continuous transverse section which was fractured longitudinally

into three pieces in the field when it was removed from the

adjoining pipe length. X 2/3

Figure 4. Fracture surface W1 after cleaning. X 2/3

Figure 6. Fracture surface W3 after cleaning. X 2/3

Figure 7

Figure 8

SEM fractograph of W1 . The fracture mode is predominantly

cleavage. X 185

SEM fractograph of W1 at much higher magnification than

figure 7. Corrosion product is evident. X 1000

Figure 9. Transverse section through W1 about 3/4 inch from the fracture

showing graphitization adjacent to the inside wall surface. X 2/3

Figure 10. Transverse section through W2 about 3/4 inch from the fracture

showing some graphitization adjacent to the inside wall

surface. X 2/3

Figure 11. Transverse section through W3 ranging from about 1/2 to 2 inches

from the fracture showing graphitization adjacent to both the

inside and outside wall surfaces. X 2/3

Figure 12. Etched longitudinal section through W1 showing the fracture

profile (horizontal at the top). The entire wall thickness of

the pipe is shown. The inside wall surface is at the left and

the outside wall surface is at the right. Significant graphitization can be seen adjacent to the inside wall surface.

Etchant: 1% nital X 11

Figure 13. Unetched longitudinal section through W1 showing the fracture

profile (horizontal at the top). The thin, dark particles are

graphite flakes. The small gray particles appear to be non- metal! ic inclusions.

As polished X 100

Figure 14. Etched longitudinal section showing the entire

wall thickness of W3. Graphitization is evident

both adjacent to the inside wall surface (left)

and adjacent to the outside wall surface (right).

Etchant: 1% nital X 8

Figure 15. Etched longitudinal section through W1 in a region which has not been graphitized. The

microstructure consists of graphite flakes

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

and probably steadite (small rounded particles

within the ferrite patches).

Etchant: 1% nital X 1000

Figure 16. Etched longitudinal section through W1 in a region that has

been essentially totally graphitized.

Etchant: Mo nital X 1000

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR-2036

l.Gov't.' Accession No. 3. Recipient' a: Accession No.

4. TITLE AND SUBTITLE

Examination of Failed Six-Inch Diameter

Cast-Iron Pipe Water Main

Philadelphia, Pennsylvania

5. Publication Date March 1980

6. Performing Organization Code

7. AUTHOR(S)

T. Robert Shives

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS 10- Project/Task/Work Unit No.

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

11. Contract/Grant No. 5640-42

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (S freer. City, state, ZIP) 13. Type of Report & Period Covered

National Transportation Safety Board, Wash. DC 20594 Failure Analysis

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Contract/Grant No. 5620-42

|

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 bibliography 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 six-inch diameter cast-iron pipe

water main from Tacony and Margaret Streets in Philadelphia, Pennsylvania. An explosion and fire had occurred at this location on May 13, 1979. At that time, both the

six-inch diameter water main pipe and the eight-inch diameter cast-iron natural gas

main pipe at the site of the explosion and fire were found to be completely fractured

transversely in three places. Failure of the water pipe appeared to be due to an externally applied bending load.

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

rather old. Today, such pipe is normally centrifugally cast which results in a uniform

wall thickness.

Each of the three fracture faces submitted for examination was covered with a rather tightly adhering layer of corrosion product. The corrosion product was not

easily removed, and the corrosion process had obliterated some of the fracture features

indicating that the fractures were not fresh. Cleavage was the predominant fracture

mode. There was significant graphi tization close to the fractures.

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 practice. The microstructure appeared to be satisfactory

-with the possible exception of more steadite than desirable according to today's practty

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

separated by semicolons)

Brittle fracture; Cast-iron; Cast-iron pipe; Cast iron water pipe; Cleavage;

Graphitization

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