/

NBSIR 78-1470

Examination of Failed Four

Inch Diameter Cast Iron Pipe

Natural Gas Main, Hamden,

Connecticut

Fracture and Deformation Division

Center for Materials Science

National Measurement Laboratory

National Bureau of Standards

Washington, DC 20234

Failure Analysis Report

April 1978

Prepared for Bureau of Surface Transportation

National Transportation Safety Beard

Washington, OC 20594

T. Robert Shives

jfIBE COPY

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NBSIR 78-1470

EXAMINATION OF FAILED FOUR

INCH DIAMETER CAST IRON PIPE

NATURAL GAS MAIN, HAMDEN,

CONNECTICUT

T. Robert Shives

Fracture and Deformation Division

Center for Materials Science

National Measurement Laboratory

National Bureau of Standards

Washington, DC 20234

Failure Analysis Report

April 1978

Prepared for Bureau of Surface Transportation

National Transportation Safety Board

Washington, DC 20594

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director

TABLE OF CONTENTS

Page

SUMMARY ....... iii

1. INTRODUCTION 1

1.1 Reference 1

1.2 Parts Submitted 1

1.3 Background Information ........ 1

2. PURPOSE 1

3. RESULTS OF EXAMINATIONS, ANALYSES, AND TESTS 2

3.1 Visual Examination 2

3.2 Fractographic Examination 3

3.3 Metallographic Examination 3

3.4 Chemical Analysis ... 4

3.5 Hardness Measurements 5

3.6 Talbot Tests 5

3.7 Ring Tests 6

4. DISCUSSION 6

5. CONCLUSIONS 7

6. REFERENCES 8

7 . ACKNOWLEDGEMENT 8

FIGURES

1. The two pieces of four inch cast iron pipe as received at MBS.

2. Fracture surface of the piece of pipe designated E as received at NBS

before cleaning.

3. Fracture surface of the piece of pipe designated W as received at NBS

4. Fracture surface of the piece of pipe designated E after ultrasonic

cleaning

i
FIGURES (continued)

5. The region designated A in figures 2, 3, and 4 at a higher

magnification

.

6. Polished transverse section through the piece of pipe designated

E about 2 cm from the fracture.

7. SEM fractograph in the vicinity of C, figure 4.

8. SEM fractograph in the vicinity of D, figure 4.

9. Longitudinal section C near the top of the pipe.

10. Longitudinal section D in about the 10:30 o'clock position when

facing the fracture designated E.

11. Longitudinal section F2 at about the 7:30 o'clock position when

facing the fracture designated E.

12. Longitudinal section F2 showing graphitization at the outside of

the pipe and along the fracture.

13. Longitudinal section C showing the profile of the unusually flat

region of the fracture near the outside of the pipe.

14. Longitudinal section D showing the representative unetched microstructure near the outside of the pipe.

15. Longitudinal section D showing the representative unetched microstructure at the center of the pipe wall thickness.

16. Longitudinal section D showing the representative unetched microstructure adjacent to the inside of the pipe.

17. Longitudinal section D showing the representative etched microstructure near the center of the pipe wall.

18. Longitudinal section F2 showing the etched microstructure adjacent

to the fracture and the graphitization at the fracture.

19. Longitudinal section F2 showing etched microstructure adjacent to

the inside of the pipe.

li
SUMMARY

The submitted four inch diameter grey cast iron natural gas main

pipe had failed due to a transverse fracture. Failure was probably

caused by stresses from an external source. Loading was likely applied

in bending. Although there were no fracture features to indicate the

fracture origin, it appears likely that the top of the pipe was the last

to fail.

Most of the corrosion product on the fracture surface appeared to

be superficial. There was one region of significant graphitization at

the fracture near the bottom of the pipe. At its deepest, the

graphitization had penetrated about one half the pipe wall thickness.

The microstructure, hardness, and chemical composition of the pipe

material appeared to be satisfactory. The material satisfied the

Talbot test and ring test requirements of American National Standard

A21.9, 1970.

iii
£ -
Examination of Failed Four Inch Diameter Cast Iron Pipe Natural Gas Main,

Hamden, Connecticut

I . INTRODUCTION

1.1

Reference

National Transportation Safety Board, Washington, DC 20594. This

investigation was conducted at the request of Mr. Richard G. Marini,

Pipeline Safety Specialist, National Transportation Safety Board (NTSB)

The requesting letter was dated October 31, 1977.

1.2

Parts Submitted

Two pieces of the failed four inch diameter cast iron pipe were

submitted to the National Bureau of Standards for examination. The

nominal wall thickness of the pipe was 3/8 inch. Each piece of pipe

contained one of the two mating fracture faces. The two pieces of

pipe, designated E and W before being received at NBS , measured

approximately 30 and 30.5 cm, respectively, in length. The fracture

surface from the piece labeled E was used for most of this examination.

The fracture surface from the piece labeled W was not cleaned or sectioned, although the piece of pipe itself was cut in order to machine

specimens for Talbot tests. Both pieces were marked before being submitted

to NBS to indicate the top of the pipe as it had been oriented in service.

The parts are shown in figure 1 as they were received at NBS.

1 .Background Information

The information in this section was supplied by the National Transportation Safety Board. Failure of the pipeline occurred in a grassy area

adjacent to an apartment complex in Hamden, Connecticut. The grassy area

was subject to pedestrian traffic only. The break was detected during an investigation after an explosion in a building. There were about 41 5/8

inches of ground cover. At the point of the break, the pipeline was running parallel to and about two feet from the outside wall of the

apartment complex. The break was well removed from the paved street.

The pipeline was installed in 1953. Normal operating pressure was

0.25 psig.

2 . PURPOSE

The National Transportation Safety Board requested that the National

Bureau of Standards examine the submitted failed pipe and perform a visual

examination, chemical analysis, metallographic examination, determination

of graphitization , determination of fracture origin, Talbot tests, ring

tests, hardness and microstructure characterization.
3. RESULTS OF EXAMINATIONS, ANALYSES, AND TESTS

3.1 Visual Examination

The submitted pipe had fractured transversely. The outside surface

of the pipe had been cleaned for a length of about 10 cm on either side

of the fracture at what was reported to be the top of the pipe as it

had been oriented in service. Beyond the cleaned regions, the pipe was covered with fairly tightly adhering soil and surface scale (corrosion

product) . The surfaces of the pipe pieces near the ends opposite those

containing the fractures had been partially cleaned. In these regions, one of the pipe pieces had been marked E and the other had been marked

W. The top of the pipe is shown in figure 1 as it was received at NBS.

The two mating fracture surfaces are shown as received at NBS in

figures 2 and 3. The top of the pipe as it lay in the ground is at the

top of the photographs in both figures. Both fracture surfaces were

covered with corrosion product. There were two regions of special

interest in mating locations on the two fracture surfaces. One of

these regions is near the top of the pipe and is indicated by bracket A

in figures 2, 3, and 4. This region is adjacent to the outside surface

of the pipe and, at its thickest point, penetrates about 2/3 of the pipe

wall thickness. The fracture was smoother in this region than elsewhere,

and the corrosion product had a duller appearance than elsewhere. There

is a small apparent shear lip at the outside of the fracture face within

bracket A in figure 3. This feature appears as a depression on the

fracture face shown in figures 2 and 4.

The second region of interest is near the bottom of the pipe as the

pipe was oriented in service. This region is indicated by bracket B in

figures 2, 3 and 4. As in region A, the area of interest is adjacent to

the outside surface of the pipe. On the as-received (uncleaned) fracture

surfaces, the region appeared to have been graphitized.

The fracture surface from the piece of pipe marked E was cleaned

with detergent in an ultrasonic cleaner. Most of the corrosion product

was removed during the cleaning process in a relatively short time

indicating that most of the corrosion product was essentially superficial.

The cleaned fracture surface is shown in figure 4. The region designated

A in figures 2, 3 and 4 revealed a number of small shiny areas indicating

possible mechanical damage. Region A is shown in figure 5 at a higher

magnification than in previous figures. The mechanical damage could have

occurred from the mating fractures rubbing together.

After the fracture surface was cleaned, it was determined that there was indeed significant graphitization in region B. No other regions of

significant graphitization were detected on the fracture surface.

2
A transverse section was taken through the piece of pipe designated

E about 2 cm from the fracture. This section is shown in figure 6 after

it had been polished. There was one region of significant graphitization

near the bottom of the pipe (arrow, figure 6) . This region corresponds

to the location of the graphitization at the fracture surface. There was

some minor graphitization in a few other locations around the section. In

every case, the graphitization was adjacent to the outside of the pipe.

3 . 2 Fractographic Examination with the SEM

Three regions of the fracture surface were examined with the scanning

electron microscope (SEM) after ultrasonic cleaning. These regions are in

the vicinity of C, D, and F indicated in figure 4. There was still some corrosion product on the surface in all three areas, but where the corrosion

product did not mask the fracture features, cleavage was the dominant

fracture mode. Cleavage indicates a very low ductility or brittle fracture.

Brittle fracture is expected in cast iron.

More corrosion product remained on the fracture surface in region A

(figures 2, 3 and 4) than in the other regions examined. An SEM fractograph from region A is shown in figure 7. A considerable amount of

corrosion product is evident, but there is also evidence of cleavage.

An SEM fractograph from a region less affected by corrosion product

(near D, figure 4) is shown in figure 8. Cleavage is again the dominant

mode.

In the vicinity of the graphitized region (F, figure 4) , cleavage is

the dominant fracture mode where the fracture surface is not covered with

graphitized product.

3 . 3 Metallographic Examination

Longitudinal sections intersecting the fracture were taken at the

locations indicated in figure 4. One each was taken at locations C,

D, Fl, F2, and F3. The fracture profile at locations C, D, and F2 is

shown in figures 9, 10, and 11, respectively. A small amount of

graphitization and some other corrosion product (rust) can be seen on

the outer surface of section C, but there is no evidence of graphitization

on the fracture surface itself. In section D, somewhat more graphitization

can be seen on the outside surface of the pipe in the vicinity of the

fracture. The deepest penetration of the graphitization in this section

is about 15% of the wall thickness. No graphitization is evident on the

fracture itself.

In section F2, figures 11 and 12, more graphitization was evident than

in either sections C or D. An inward growing "spike" or graphitization

near the left of the figure had penetrated about 40% of the pipe wall

thickness. Graphitization was found on the fracture surface for about the

3
outer half of the pipe wall thickness. Part of the fracture profile of

section F2 showing some of the graphitization on the outer wall surface

and on the fracture is shown in figure 12. The graphitization in

sections FI and F3 was very similar to that in section F2.

The fracture profile of section C adjacent to the outside of the pipe

is shown in figure 13 at a higher magnification than in figure 9. The

fracture is quite flat and relatively smooth in this region, which is

about 15% of the wall thickness at this section.

The microstructure of the cast iron varied across the wall thickness

of the pipe. Unetched-as polished-fields from section D considered to be

representative of the microstructure adjacent to the outside, in the

center, and adjacent to the inside of the pipe are shown in figures 14,

15, and 16, respectively. The graphite flakes in the three regions can

be classified as follows

Outside mostly type A

Center mostly type B with some type E

Inside mixture of types A and B

An etched field from section D at the center of the wall thickness is

shown in figure 17. (Note: This figure is at a higher magnification

than the previous figures.) In addition to the graphite flakes, the

microstructure consists of pearlite (grey-fingerprint pattern) and

ferrite (light colored phase).

An etched field from section F2 showing graphitization along the

fracture appears in figure 18. The graphitization is at the right. The

microstructure of the material that has not been graphitized consists

again of graphite flakes, pearlite, and ferrite; but there is much

less pearlite evident here than in figure 17.

There was a layer adjacent to both the inside and outside surfaces

of the pipe that had a higher concentration of pearlite than the rest

of the material. Figure 19 shows this layer at the inside of the pipe.

3 . 4 Chemical Analysis

A sample of the cast iron pipe material was submitted to a commercial

laboratory for chemical analysis. The results of that analysis are as follows

:

Element Weight Percent

Total carbon

Combined carbon

Manganese

Phosphorus

Sulfur

Silicon

Nickel

Chromium

Molybdenum

Copper

3.66

0.38

0.37

0.60

0.083

1.70

0.02

0.05

< 0.01

0.18

4
The chemical composition of the material meets the requirements set forth

in American National Standard A21.9, Cast-Iron Pipe Centrifugally Cast in

Sand-lined Molds for Gas, 1970 edition. These requirements are as follows:

Element Percent, maximum

Phosphorus

Sulfur

0.90

0.12

3 . 5 Hardness Measurements

Hardness measurements were made on the transverse section shown in

figure 6 (after the photograph was taken) . Two attempts were made to

make Brinell hardness measurements, but in each case, the indentor

cracked the cast iron. Therefore, Rockwell B (HRB) hardness measurements

were made. The average for five measurements was HRB 79 and the range was HRB 76-81. A hardness of HRB 79 is approximately equivalent to a Brinell hardness of 1^7. The Brinell hardness of grey cast iron may range

from about 140 to 200 .

3 .Talbot Tests

Talbot tests were performed on three specimens machined from the piece

of pipe designated W after a transverse section containing the fracture had

been removed so that the fracture could be preserved. The tests were

conducted in accordance with American National Standard A21.9.

From the results of the Talbot tests, the modulus of rupture and the

secant modulus of elasticity were calculated according to the following

formulae as given in American National Standard A21.9.

where R = modulus of rupture, psi

W = breaking load, lbs

t = width of Talbot strip (pipe wall thickness) , inches

d = depth of strip, in (intended to be 0.50)

E 3 = secant modulus of elasticity, psi

y

= deflection of the strip at the center at the breaking load, inches

The calculated modulus of rupture and the secant modulus of elasticity for

the three specimens are as follows:

Specimen R, psi E^, psi

( 1 )

21. 3R

E

3 dy ( 2 )

1

2

3

51.900

49,000

50.900

10,800,000

11.400.000

10.500.000

5
The standard calls for a minimum of 40,000 psi for the modulus of rupture.

The secant modulus of elasticity must not exceed 250 times the actual

value of the modulus of rupture for pipe cast in sand-lined molds. All

three specimens met the requirements for both the modulus of rupture and

the secant modulus of elasticity.

3 . 7 Ring Tests

Ring tests were performed on three specimens machined from the piece

of pipe designated E in accordance with American National Standard A21.9.

Because of the limited amount of material available for testing, the

length of the ring test specimens was nominally three inches.

The modulus of rupture was calculated from the results of the ring

tests according to the following formula given in American National

Standard A21.9:

where R = modulus of rupture, psi

W = breaking load, lbs

d = average inside diameter of the ring, inches

t = average thickness of metal along the line of fracture, inches

b = length of the ring, inches

The modulus of rupture as calculated from the ring test data is as follows:

Specimen R, psi

The standard calls for a minimum of 40,000 psi for the modulus of rupture

as calculated from the results of the ring test. All three specimens met

this requirement.

This four inch diameter grey cast iron natural gas main pipe failed

due to the initiation and propagation of a transverse crack. At the

fracture, there was one region of significant graphitization near the

bottom of the pipe as it had been oriented in service. The graphitization

extended through about one half the pipe wall thickness in this region.

As would be expected, the graphitization initiated at the outside of the

pipe. The fracture exhibited very little if any ductility as is

characteristic of cast iron fractures.

1

2

3

61,600

59,900

59,900

4. DISCUSSION

6
The fracture origin could not be definitely established from the

features on the fracture surface. It does, however, appear that the

last part of the fracture to form may have been at the top of the

pipe as suggested by the slight lip on the outside of the fracture at

the top. It is not clear why the fracture appearance at the top is

different from the appearance of the rest of the fracture. If, indeed,

the top was the last to fail, then the origin of the fracture should be

well removed from the top and may have been at the region of graphitization

near the bottom of the pipe. Failure appears to have been caused by an external force probably applied in bending.

The chemical composition, modulus of rupture as calculated from both

the ring tests and the Talbot tests, and the secant modulus of

elasticity as calculated from the Talbot tests all met the requirements

set forth in ANSI 21.9.

The hardness is within the acceptable range for grey cast iron.

There was considerable variation in the microstructure of the material

across the wall thickness of the pipe. This variation was probably due

to the casting procedure employed and differences in the cooling rate

across the wall thickness. The microstructure appears to be satisfactory.

5.

CONCLUSIONS

1. This failed grey cast iron natural gas main pipe fractured

transversely apparently due to loading from an external source

probably applied in bending.

2. Most of the corrosion product on the fracture surface appeared to

be superficial.

3. The location of the fracture origin was not established.

4. There was one region of significant graphitization at the fracture

near the bottom of 'the pipe as it was oriented in service. The

graphitization penetrated about one half the pipe wall thickness

at the point of deepest penetration.

5. The fracture mode was primarily cleavage.

6. The chemical composition of the material appeared to be satisfactory

within the limits of ANSI Standard A21.9.

7. The microstructure of the material appeared to be satisfactory.

8. The hardness of the material appeared to be satisfactory.

9 . The calculated modulus of rupture and secant modulus of elasticity

met the requirements of ANSI Standard A21.9.
6. REFERENCES

1. Metals Handbook, 8th Edition, American Society for Metals, Vol. 7,

1972, p. 82.

%• 2. Charles F. Walton, Editor, Gray and Ductile Iron Castings

Handbook, Gray and Ductile Iron Founders' Society, Inc., 1971.

7. ACKNOWLEDGEMENT

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

assisted in this investigation. He performed the hardness measurements,

the metallographic specimen preparation, the photographic work, and he

assisted with the Talbot and ring tests.

8
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Figure 2. Fracture surface of the piece of pipe designated E ao received at NBS before cleaning. Regions designated

A and B are of special interest and are discussed in

the text of the report. The top of the pipe as it lay

in the ground is at che top in the figure. E 1

Figure 3. Fracture surface of the piece of pipe designated W as received at NBS. Regions designated A and B are in

the mating locations to the similarly designated regions

in figure 2. The top of the pipe as it lay in the

ground is at the top in the figure. X 1

Figure 4. Fracture surface of the piece of pipe designated E after

ultrasonic cleaning. The region labeled A has suffered

apparent mechanical damage. The region labeled B exhibits

graphitization. Arrows C, D, Fl, F2, and F3 indicate the

locations where sections were taken for metallographic

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Figure 6. Polished transverse section through the piece of pipe

designated E about 2 cm from the fracture. One region

of significant graphitization is indicated by the arrow

Minor graphitization is evident in other regions of the

section.

As polished X 1

Figure 7. SEM fractograph in the vicinity of C, figure 4.

A considerable amount of apparent corrosion

product is evident. The predominant mode of

fracture is cleavage. X 525

SEM fractograph in the vicinity of D, figure 4,

exhibiting cleavage as the predominant mode of

fracture. X 525

Figure 8.

Figure 9. Longitudinal section C near the top of the pipe. The outside

of the pipe is horizontal at the top, the inside is horizontal

at the bottom. The fracture profile is vertical at the right.

A small amount of graphitization is evident at the outside

surface

As polished X 7 1/2

Figure 10. Longitudinal section D in about the 10:30 o'clock position

when facing the fracture designated E. The outside of the

pipe is horizontal at the top, the inside is horizontal at

the bottom. The fracture profile is vertical at the right.

Somewhat more graphitization is evident in this section than

in the one shewn in figure 7.

As polished X 7 1/2

Figure 11. Longitudinal section F2 at about the 7:30 o'clock position

when facing the fracture designated E. The outside of the

pipe is horizontal at the top, the inside is horizontal at

the bottom. The fracture profile is vertical at the right.

A considerable amount of graphitization is evident.

As polished X 7 1/2

Figure 12. Longitudinal secrion F2 showing graphitization at the outside

of the pips (horizontal at the top) and along the fracture

(vertical at the right)

As polished x 50

Figure 13. Longitudinal section C showing the profile (vertical at the

right) of the unusually flat region of the fracture near the

outside of the pipe (horizontal at the top)

As polished X 50

Figure 14. Longitudinal section D showing the representative unetched

microstructure near the outside of the pipe. The graphite

flakes are mostly type A.

As polished X 100

Figure 15. Longitudinal section D showing the representative unetched

microstructure at the center of the pipe wall thickness.

The graphite flakes are mostly type B with some type E.

As polished X 100

Figure 16. Longitudinal section D shewing the representative unetched

microstructure adjacent to the inside of the pipe. The

graphite flakes are a mixture of types A and B.

As polished X 100

Figure 17. Longitudinal section D showing the representative etched

microstructure near the center of the pipe wall. The microstructure consists primarily of graphite flakes, pearlite

(fingerprint pattern), and ferrite (light colored phase).

Etchant: 1% nital X 500

Figure 18. Longitudinal section F2 showing the etched microstructure

adjacent to the fracture and the graphitization at the

fracture

Etchant: 1% nital X 200

Figure 19. Longitudinal section F2 showing etched

microstructure adjacent to the inside of

the pipe (horizontal at the bottom) . The

layer of high pearlite concentration adjacent to the inside of the pipe is evident.

Etchant: 1% nital X 100

3S-W4A (REV. 7-73)

U.S. DEPT. OF COMM.

bibliographic data

SHEET

1. PUBLICATION OK REPORT NO.

NBSIR 78-1470

2. Gov’t Accession

No.

TITLE AND SUBTITLE

(EXAMINATION OF FAILED FOUR INCH DIAMETER CAST IRON PIPE

NATURAL GAS MAIN, HAMDEN, CONNECTICUT

AUTHOR(S)

T. Robert Shives

PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

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

National Transportation Safety Board

Washington, D.C. 20594

SUPPLEMENTARY NOTES

3. Recipient’s Accession No.

5. Publication Date April 1978

6. Performing Organization Code

8. Performing Organ. Report No. NBSIR 78-1470

10. Project/Task/Work Unit No. 5620412

11. Contract/Grant No.

13. Type of Report & Period

Covered

Failure Analysis

14. Sponsoring Agency Code

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

bibliography or literature survey, mention it here.)

The submitted four inch diameter grey cast iron natural gas main pipe had

failed due to a transverse fracture. Failure was probably caused by stresses

from an external source. Loading was likely applied in bending. Although there

were no fracture features to indicate the fracture origin, it appears likely that

the top of the pipe was the last to fail.

Most of the corrosion product on the fracture surface appeared to be

superficial. There was one region of significant graphitization at the fracture

near the bottom of the pipe. At its deepest, the graphitization had penetrated

about one half the pipe wall thickness.

The microstructure, hardness, and chemical composition of the pipe material

appeared to be satisfactory. The material satisfied the Talbot test and ring

test requirements of American National Standard A21.9, 1970.

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; graphitization.

AVAILABILITY Unlimited

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