NBSIR 73-160

Examination of Failed Six Inch Cast

Iron Pipe Natural Gas Main, Atlanta

Gas Light Company, Atlanta, Georgia

T. Robert Shives

Mechanical Properties Section

Metaiiurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

April 20, 1973

Failure Analysis Report

Prepared for Bureau of Surface Transportation

National Transportation Safety Board

Department of Transportation

Washington, D. C. 20591

NBSIR 73-160

EXAMINATION OF FAILED SIX INCH CAST

IRON PIPE NATURAL GAS MAIN, ATLANTA

GAS LIGHT COMPANY, ATLANTA, GEORGIA

J. Robert Shives

Mechanical Properties Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

April 20, 1973

Failure Analysis Report

"This document has been prepared for the use of the Bureau of Surface Transportation, National Transportation

Safety Board, 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."

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

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director
T
TABLE OF CONTENTS

Page

S UNITARY iii

1. INTRODUCTION 1

1.1 Reference 1

1.2 Part Submitted 1

1.3 Background Information 1

2. PURPOSE 2

3. RESULTS OF EXAMINATIONS AND ANALYSES 2

3.1 Visual Examination 2

3.2 Macroscopic Examination 3

3.3 Metallographic Examination 4

3.4 Chemical Analysis 5

3.5 Hardness Measurements 6

4. DISCUSSION OF RESULTS 6

5. CONCLUSIONS 7

6 . ACKNOWLEDGEMENT 8

REFERENCE 8

FIGURES

1. Length of pipe containing fracture as received 9

2. View of the two fractured pieces of pipe with the

coupling removed 10

3. View of the inside of one of the fractured pieces

of pipe showing the sand deposits 11

4. View of the inside of the same fractured piece of

pipe as shown in figure 3 showing the sand

deposits 12

i

Page

FIGURES (Continued)

5. One of the fracture faces 13

6. Fracture surface at top of pipe 14

7. Part of one of the fracture surfaces 15

8. Entire fracture surface after cleaning 16

9. Photomacrographs of longitudinal cross sections

intersecting the fracture surface 17

10. Polished transverse slice about one inch from

the fracture surface 18

11. Polished transverse slice about eight inches

from the fracture surface 19

12. Unetched fields showing microstructure of

cross section B 20

13. Unetched field from cross section D showing

graphitization gradient 21

14. Unetched field from cross section B 22

15. Etched field from cross section D near inside

wall surface . 23

16. Etched field from cross section D about midway

between inside and outside wall surfaces 24

17. Etched field from cross section D in an area where

graphitization is beginning 25

18. Etched field from cross section D in an area where

graphitization is nearly complete 26

ii

SUMMARY

A five foot length of six inch grey cast iron pipe from

a natural gas main which had failed in service was submitted

for examination. The failure was from a pipeline which had

been installed in Atlanta, Georgia in 1951.

The pipe had fractured transversely at the approximate

center of the submitted length. The opposing fracture faces

were covered with soil and patches of a waxy appearing

substance except for about a four inch circumferential length

at the bottom of the pipe which appeared to be fresh fracture.

The fracture apparently initiated at the top of the pipe and

occurred in at least two stages, with the fresh fracture

region being the last to fail.

There was sand deposited in one of the fractured pieces

in a pattern that indicates that the piece had been rotated

about its longitudinal axis.

Graphitization (corrosion) , penetrating up to 3 0 percent

of the wall thickness in one area, was found in all cross

sections examined, although most of the material was free of

graphitization. Where graphitization existed, it had started

at the outside wall surface of the pipe. The most severely

graphitized region found was adjacent to the fracture.

Nothing abnormal for grey cast iron was noted in the

chemical composition, hardness, or microstructure of the

material in the pipe.

The likely cause of failure was the application of an

external load to produce a bending stress in an area of the

pipe weakened by graphitization.

•Examination of Failed Six Inch Cast Iron Pipe Natural

Gas Main, Atlanta Gas Light Company, Atlanta, Georgia

1. INTRODUCTION

1 . 1 Reference

Bureau of Surface Transportation, National Transportation

Safety Board, Department of Transportation, Washington, D. C.

This investigation was conducted under NTSB order no, AB3006

accepted by NBS on January 10, 1973.

1 . 2 Part Submitted

A fractured five foot length of six inch diameter cast

iron pipe natural gas main was submitted to NBS for examination

on September 21, 1972. The pipe had fractured completely in

the approximate center of the submitted length. A coupling

and seal which connected the two fractured parts of the pipeline was still in place at the time the pipe section was

received at NBS.

1 . 3 Background Information

The information in this section was furnished by the

Bureau of Surface Transportation via telephone conversations

with T. R. Shives of the National Bureau of Standards.

The pipeline, which had been installed in 1951, was

found to contain a fracture on August 31, 1972. This pipeline is composed of 16 foot lengths of six inch diameter

cast iron pipe connected by mechanical joints. The

section that fractured had been installed over a large

concrete sewer line with about three inches clearance

between the sewer and the gas main. Gas pressure in the

line had been 10 psig.

- 1 -

2 . PURPOSE

The submitting agency requested an analysis of the

failure.

3. RESULTS OF EXAMINATIONS AND ANALYSES

3.1 Visual Examination

According to information obtained from the National

Transportation Safety Board, the plug in the coupling connecting the two fractured pieces was at the top of the pipe

as it was positioned in the ground. Top and bottom views

of the pipe length as received are shown in figures la and

lb, respectively.

Much of the outer surface of the pipe was encrusted

with a deposit which appeared, for the most part, to be

soil. This deposit was generally yellowish-orange in

color, but some areas were grey. Some of this material

could be crumbled away with the fingers, but most of it

adhered firmly to the pipe surface. The material did not

appear to be deposited on the pipe in any particular

pattern. Some of the deposit can be seen in figure 1.

The outside surface areas of the pipe not covered with

the deposit were covered with a fairly thin scale.

Removal of the coupling revealed a relatively clean

pipe surface where the coupling had been. This can be

seen in figure 2 where the two fractured pieces are

shown separated. At both edges of this clean length

(where the edges of the coupling had been) , there v/as a black ring (arrows, figure 2) of sealing material. The

white chalk lines indicate the top of the pipe. The

deposited material on the outer surface of the pipe

which had not been covered by the coupling can also be

seen in this figure.

- 2 ~

The inside surfaces of the two fractured pieces were

covered with a light scale. One piece had sand adhering to

the inside wall surface in a pattern which indicated that it

had been rotated approximately a full turn about its longitudinal axis with the pipe essentially in a horizontal

position while the sand was wet. As viewed from the fracture

surface, and with reference to a clockwise rotation direction,

most of the sand adhered to the wall surface in the 6 o'clock

to 12 o'clock region. There was a small amount of sand

between 12 o'clock and 6 o'clock which appeared to have

deposited after that at the first location, and after the sand

had dried somewhat. Two views of the pipe exhibiting the sand

pattern are shown in figures 3 and 4. A one inch transverse

slice containing the fracture surface had been removed before

the photographs shown in figures 3 and 4 were taken. The sand

pattern was not disturbed by the dry cutting of the slice.

The second fractured piece did not exhibit this sand pattern,

and no deposit of sand was observed in the gap between the

fracture surfaces when the coupling was removed.

Both fracture faces were covered with what appeared to

be soil (yellow clay in color) and a fev7 patches of a grey,

waxy appearing substance except for about a four inch

circumferential length at the bottom which was bright and

had the appearance of a fresh fracture . One of the fracture

surfaces is shown in figure 5. The region appearing as

fresh fracture is bracketed.

3 . 2 Macroscopic Examination

There were some areas of mechanical damage on the fracture

surfaces which may have been caused by the opposing faces

contacting each other. The opposing surfaces had similar or

mating features essentially everywhere except at the top of

the pipe. There was a small chip(s) missing from the top

outside surface (figure 6) at the fracture, and a small amount

of material missing across the fracture between the opposing

.lalves

One of the fracture surfaces and the adjacent inner and

outer surfaces of the pipe were cleaned ultrasonically with

detergent and water at NBS. After cleaning, the fracture had

the brittle appearance to be expected from a grey cast iron

fracture. The surface of the pipe under the missing chip

was fairly bright with little apparent corrosion after

cleaning. Figures 7a and 7b show one part of the fracture

surface before and after cleaning, respectively. The entire

fracture surface after cleaning is shown in figure 8

- 3 -

Four longitudinal cross sections about one inch long

each were cut to intersect the cleaned fracture surface.

The locations of these are indicated in figure 8 . Various

amounts of graphitization* were found in each of the cross

sections, and in each case, there was graphitization

adjacent to the fracture. Photomacrographs of parts of

cross sections A, C, and D are shown in figures 9a, 9b, and

9c, respectively. The degree of graphitization in cross

section B is intermediate to that in cross sections A and D.

There is graphitization evident under the missing chip

(figure 9b) . Where there was graphitization, it began at

the outside wall surface and proceeded inward. It appeared

as a gradient, the degree of graphitization decreasing with

distance from the outside wall surface.

The maximum depth of penetration in cross section D was

about 1/10 inch, or about 25 percent of the v/all thickness.

The maximum depth of penetration in cross section C,

including the depth of the missing chip, occurred adjacent

to the fracture and was greater than 1/10 inch (about 30

percent of the wall thickness) . This was the greatest

penetration of graphitization found in any of the cross

sections examined.

Two transverse slices, at one inch and eight inches from

the fracture, respectively, were examined for graphitization.

There was some graphitization in both slices, but not at the

top of the pipe in either case (figures 10 and 11) . The

deepest penetration (about 1/16 inch, or 16 percent of the

3/8 inch wall thickness) was found in the slice one inch from

the fracture. This area is shown in the insert in figure 10.

3 . 3 Metallographic Examination

The four longitudinal cross sections that were examined

macroscopically were also examined metallographically

Unetched fields intersecting the outside and inside surfaces

of the pipe are shown in figures 12a and 12b, respectively.

The microstructures shown in figure 12 are representative of

the four cross sections examined. Both of these fields are

from cross section B, and the field shown in figure 12a is in

an area where there was very little graphitization. The

graphite flakes (dark phase) near the outside surface are of

the A, B, and C types (figure 12a) . In the center of the wall

thickness and near the inner wall surface, the graphite flakes

are primarily of the A and C types (figure 12b)

The term "graphitization" as used in this report indicates

a corrosion process.

- 4 -

An unetched field from cross section d exhibiting

graphitization at and adjacent to the outside pipe surface

is shown in figure 13. The graphitization gradient is quite

evident. Graphitization is essentially complete near the

outside surface (at the left in the figure) and is just

beginning about one-fourth of the distance in from the right.

At the far right in the figure, there is no evidence of

graphitization. Figure 14 (at a higher magnification than

figure 13) shows an area where graphitization is beginning

in cross section B. There is considerable corrosion product

surrounding the graphite flakes.

Etched fields from cross section D which are representative of the four cross sections examined are shown in

figures 15 through 18 (at higher magnifications than figures

12 and 13) . The microstructure near the inside wall surface

of the pipe (figure 15) consists of graphite flakes partially

surrounded by ferrite (light color) in a matrix of pearlite

(grey) and iron-iron phosphide eutectic (light) . Midway

through the wall thickness, the microstructure consists of

graphite flakes in a matrix composed primarily of pearlite

and iron-iron phosphide eutectic (figure 16) . An etched

field (at a lovzer magnification than figures 15 and 16) where

graphitization is beginning shows graphite flakes surrounded

by corrosion product (figure 17) . The matrix is again

composed of pearlite and iron-iron phosphide eutectic. An

area where graphitization is nearly complete is shown in

figure 18. There is still a small amount of the iron-iron

phosphide eutectic component of the matrix remaining.

3 . 4 Chemical Analysis

A sample from the pipe was analyzed for chemical

composition by a commercial laboratory using spectrographic

and other techniques. The results of that analysis are as

follows

Element Percent

Carbon, total 3,26

Carbon, combined 0.80

Manganese 0.34

Phosphorus 0.64

Sulfur 0. 085

Silicon 1.30

Nickel 0.06

Chromium < 0.05

Molybdenum < 0.05

Copper 0,10

The results of the chemical analysis indicate that the material

is an unalloyed grey cast iron.

- 5 -

3.5 Hardness Measurements

Rockwell B (Rb) hardness measurements were made on a cross section through the pipe wall. The hardness ranged

from about Rg 92.5 near the inside wall surface to about Rg

94.5 in the center of the wall thickness to about Rb 98

near the outside wall surface.

4. DISCUSSION OF RESULTS

The most deleterious condition found in the material

examined from the submitted length of pipe was graphitization

All four longitudinal cross sections examined revealed

graphitization adjacent to the fracture. Graphitization was

also found in two transverse slices taken parallel to, and

one and eight inches from the fracture.

The deepest penetrations of the graphitization were

about 1/10 inch in cross section D and slightly greater than

1/10 inch in cross section C (including the depth of the

missing chip) . The penetration in cross section C represents

about 3 0 percent of the 3/8 inch wall thickness. The deepest

penetration in either of the two transverse slices was about

1/16 inch found in the slice taken one inch from the fracture.

Most areas of the cross sections adjacent to the outside

wall surface, but away from the fracture surface, exhibited

no graphitization. Where graphitization was found, it started

at the outside surface of the pipe and proceeded inward. It

occurred as a gradient, the severity being essentially

inversely proportional to the distance from the outside wall

surface of the pipe.

The pattern of the sand adhering to the inside wall

surface of one of the fractured pieces indicates that the

piece was apparently rotated approximately a full turn about

its longitudinal axis while in a horizontal position. There

was no sand in the second piece, nor was sand observed in the

fracture gap at the time of disassembly. This sand pattern

appears to have no bearing on the failure of the pipe.

Chemical analysis indicated that the pipe material was

an unalloyed grey cast iron. The chemical composition does

not appear to fit the chemical composition limits for any of

the ASTM class 20 through 60 grey cast irons.

- 6 -

The results of the hardness measurements indicate a hardness gradient from about Rb 98 near the outside wall

surface to about Rb 92.5 near the inside wall surface.

This difference does not appear to be of any great

significance.

The microstructure of the material from the pipe in the

areas which had not become graphitized appeared to be normal

for grey cast iron.

This gas main pipe apparently failed from the application

of an external load which produced a bending stress in an

area weakened by graphitization . The source of the external

load might have been, among other things, settling of the

soil around the pipe or movement of heavy equipment over the

pipe.

The deposits of soil and a waxy appearing substance on

the upper three-fourths of the fracture surfaces indicate

that the fracture probably initiated at the top of the pipe.

The fresh fracture appearance at the bottom indicates that

this was probably the last part to fail. The elapsed time

between the fracture initiation and final failure is not known.

5. CONCLUSIONS

1. Although most of the material adjacent to the outside

wall surface of the pipe exhibited no graphitization,

the four longitudinal cross sections examined v/hich

intersected the fracture showed graphitization adjacent

to the fracture.

2. Some graphitization was found in transverse slices

through the pipe one inch and eight inches from the

fracture

3. The deepest graphitization penetration found was

slightly greater than 1/10 inch (30 percent of the wall

thickness). This was adjacent to the fracture surface.

4. lAfhere graphitization existed, it started at the outside

wall surface and proceeded inward, forming a gradient.

5. The presence and pattern of sand deposits in one piece

of the fractured pipe length indicated that that piece

had been rotated about its longitudinal axis. The

presence of the sand does not appear to be related to the

failure

- 7 -

6. Chemical analysis indicated that the material was an

unalloyed grey cast iron.

7. The microstructure and hardness of the material

appeared to be normal for grey cast iron.

8. The fracture appears to have initiated at the top of

the pipe.

9. The apparent cause of failure was the application of

an external load which produced a bending stress in

an area weakened by graphitization

' 6. ACKNOWLEDGEMENT

Messrs. C. H. Brady and L. C. Smith of the Mechanical

Properties Section performed the metallographic specimen

preparation, and Mr. Smith performed the hardness

measurements

REFERENCE

1. Metals Handbook, American Society for Metals, Eighth

Edition, Volume 1, 1961.

- 8 -

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of pipe showing the sand deposits primarily between

six o'clock and twelve o'clock. The marker is at

the twelve o'clock (top) position of the pipe as

it was located in service. X 1

Figure 4. View of the inside of the same fractured piece

of pipe as shown in figure 3 showing the sand

deposits primarily between twelve o'clock and

six o'clock. The marker is at the twelve o'clock

(top) position of the pipe as it was located in

service. X 1

Figure 5. One of the fracture faces. Bracketed region

had the appearance of fresh fracture. Unbracketed region was covered with what appeared

to be yellow clay soil. Smallest division of

scale = 1/16 inch.

Figure 6. Fracture surface at top of pipe. Arrow indicates

location of missing chip(s) . X 4

b

Figure 7. Part of one of the fracture surfaces. X 4

a. Fracture surface before cleaning. Arrow

indicates area of waxy appearing substance.

b. Same area of fracture surface after

cleaning

Figure 8. Entire fracture surface after cleaning. Arrows A, B, C,

and D indicate locations of longitudinal cross sections

cut from the pipe for macroscopic and metallographic

examinations. Top of pipe is at top of photograph.

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to the outside surface of the pipe. These areas

are darker than the adjacent material where there

is no graphitization. The bracketed area contains

the deepest penetration of graphitization in this

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Figure 11. Polished transverse slice about eight inches from

the fracture surface. Top of pipe is at top of

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graphitization gradient. Graphitization is

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Figure 14. Unetched field from cross section B.

Graphitization is just beginning here.

There is a considerable amount of corrosion

product surrounding the graphite flakes.

X 500

Figure 15. Etched field from cross section D near inside

wall surface. Microstructure consists of

graphite flakes partially surrounded by ferrite

in a matrix of pearlite and iron-iron phosphide

eutectic. Etch: 3% nital. X 500

Figure 16. Etched field from cross section D about midway

between inside and outside wall surfaces.

Microstructure consists of graphite flakes in

a matrix of pearlite and iron-iron phosphide

eutectic. Etch: 3% nital. X 500

Figure 17. Etched field from cross section D in an area

where graphitization is beginning. The graphite

flakes are surrounded by corrosion product.

The matrix is primarily pearlite and iron-iron

phosphide. Etch: 3% nital. X 200

Figure 18. Etched field from cross section D in an area

where graphitization is nearly complete.

Small patches of the matrix remain.

Etch: 3% nital. X 200

jroRM NBS-IHA <I-7I) U.S. DEPT. OF COMM. 1. 1 MJ 1^ LIC AT ION OR IU-:PORr NO. 2. Gov't Acct-ssion BIBLIOGRAPHIC DATA mSlR 73-160

3. Recipient's Accession No.

4. 11 ri.ii AND SLiiii i ri.i-: EXAMINATION OF FAILED SIX INCH CAST IRON PIPE

NATURAL GAS MA.IN, ATLANTA GAS LIGHT COMPANY,

ATLANTA, GEORGIA

5. Publication Date April, 1973

6. F-'crfonninf. Organization Code

[ /. AU 1 HOlU S; T. Robert Shives

fi l*i»rfi'>rniinp f)r£*aniZ^tltJIl NBSIR 73-160

9. FLKFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OE STANDARDS

DEPARTMENT OE COMMERCE

\^AorillN0 1V.'l\, U.V^.

10. Pro)ect/Task/'Work Unit No. 3120415

11. Contract/Grant No.

12. Sponsoring Organ izarion Name and Address

Bureau of Surface Transportation

National Transportation Safety Board

Department of Transportation

Washington, D. C. 20591

13. Type of Report & Period

Covered p-^ilvirG

Analysis Report

14. Sponsoring Agency CoJe

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.)

A five foot length of six inch grey cast iron pipe from a natural

gas main in Atlanta, Georgia, was submitted for examination. The

pipe had fractured at the approximate center of the submitted length.

The fracture appeared to have initiated at the top of the pipe and

occurred in at least two stages, with the region at the bottom of

the pipe being the last part to fail. Graphitization (corrosion),

penetrating up to 30 percent of the wall thickness in one area

adjacent to the fracture, was found in all cross sections examined,

although most of the material examined was free of graphitization.

The likely cause of failure was the application of an external load

to produce a bending stress in an area weakened by graphitization.

17. KEY UORDS (Alphabetical order, separated by semicolons)

Cast iron; corrosion; gas pipe; graphitization

18. AVAILABILITY STATEMENl 19. SECURITY CLASS

(THIS REPORT)

21. NO. Ol- PAGES

] UNLIMITED.

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