NBSIR 75-743 (^:)

Examination of Failed 3/4 Incli Steel

Pipe Natural Gas Service Line, Sierra

Pacific Power Company, Reno, Nevada

T. Robert Shives

Mechanical Properties Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

June 1975

Failure Analysis Report

Prepared for Office of Pipeline Safety

Department of Transportation

Washington, D. C. 20590

NBSIR 75-743

EXAMINATION OF FAILED 3/4 INCH STEEL

PIPE NATURAL GAS SERVICE LINE, SIERRA

PACIFIC POWER COMPANY, RENO, NEVADA

T. Robert Shi ves

Mechanical Properties Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

June 1975

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 COWMERCE, Rogers C.B. Morton. Secretary

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Acting Director

TABLE OF CONTENTS

Page

SUMMARY

1. INTRODUCTION 1

1 . 1 Reference . 1

1.2 Background Information 1

1.3 Parts Submitted 2

2. PURPOSE 2

3. RESULTS OF EXAMINATIONS, TESTS, AND MEASUREMENTS 2

3.1 Visual and Macroscopic Examination 2

3.2 Fractographic Examination 3

3.3 Tensile Tests 4

3.4 Metallographic Examination 6

3.5 Hardness Measurements 6

4. DISCUSSION 7

5. CONCLUSIONS 8

6. ACKNOWLEDGEMENT 9

FIGURES

1. The two pieces of pipe as received at NBS

.

2. The two opposing fracture surfaces of the pipe as received

at NBS.

3. The fracture surface from the part of piece "B" which was

not still attached to the coupling.

4. SEM fractograph from area 1, figure 3.

5. SEM fractograph from area 5, figure 3.

6. SEM fractograph from area 7, figure 3.

i
FIGURES (continued)

7. SEM fractograph from area 9, figure 3.

8. SEM fractograph from the tensile fracture produced in

the laboratory at a machine cross head rate of 0.02

inch per minute.

9. SEM fractograph from the tensile fracture produced in

the laboratory at a machine cross head rate of 0.02

inch per minute.

10. " SEM fractograph from the tensile fracture produced in

the laboratory at a machine cross head rate of 16 inches

per minute

.

11. SEM fractograph from the tensile fracture produced in

the laboratory at a machine cross head rate of 16 inches

per minute

.

12. SEM fractograph from the tensile fracture produced in the

laboratory at a machine cross head rate of 16 inches

per minute

.

13. As-polished longitudinal section that intersected the

fracture at location 5, figure 3.

•s 14. As-polished longitudinal section that intersected the

fracture at location 6, figure 3.

15. Etched longitudinal section intersecting the fracture at

location 6, figure 3.

16. Etched longitudinal section intersecting the fracture at

location 12, figure 3.
SUMMARY

At the request of the Office of Pipeline Safety/ the

NBS Mechanical Properties Section examined a fractured

3/4 inch diameter steel pipe gas service line from Reno,

Nevada. The pipe was reportedly fabricated according to

American Petroleum Institute specification 5L.

The gas line had failed about two feet from the house

it was serving, apparently by overload. The fracture

surfaces exhibited essentially no corrosion product and

there was no evidence to indicate a pre-existing crack.

Quasi-cleavage was the fracture mode over a significant

portion of the fracture. Comparison of the fracture with

laboratory produced uniaxial tensile fractures indicated

that failure was likely caused by the single application of

an essentially axial tensile load at a relatively rapid

loading rate.

The ultimate tensile strength of the material conformed

to that required by API specification 5L. There was some

banding in the micros tructure

.

Examination

Service Line,

of Failed 3/4 Inch Steel Pipe

Sierra Pacific Power Company,

Natural Gas

Reno , Nevada

1 . INTRODUCTION

1 . 1 Reference

Office of Pipeline Safety, Department of Transportation,

Washington, D.C. 20590. This investigation was conducted at

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

Safety under order number DOT-AS-1 0041 . The request was made

on October 1 , 1974

.

1 . 2 Background Information

The information in this section was furnished by

Messrs. Lance F. Heverly and Paul J. Cory of the Office of

Pipeline Safety, and by Mr. Charles Lefeber of the Public

Service Commission of Nevada.

On September 8, 1974, there was an explosion at the

residence at 701 Thoma Street in Reno, Nevada, which was

attributed to the ignition of escaping natural gas. A

break was found in the 3/4 inch diameter steel gas service

line to the residence at a point about two feet from the

house and 75 feet from the gas main at the street. Fracture

had occurred in a threaded portion near the end of a pipe

length where it was joined to another length with a coupling.

At the location of the failure, the pipe was under 26 to 3 0

inches of cover.

The pipe was reported to have been manufactured in

accordance with American Petroleum Institute specification

API 5L. The gas line had been installed in 1956. From 1956

to 1963, it had carried a mixture of propane and air. Since

1963, it had carried natural gas. At the time of failure,

pressure in the line was about 25 psig.

The gas main was cathodically protected, the nearest anode

to the failure being about 40 feet from the junction of the

main and the service line. There were no anodes attached to

the service line itself.

The sewer service line to the same residence was located

under the gas service line at the point of failure. The

sewer service line pipe was cast iron where it left the house;

from there to the main it was clay. The sewer service line

- 1 -
was under about eight feet of cover, and it was reported that

the backfill was "very poor." There had been some settling

of the soil. The sower service line was thought to have been

installed at about the same time as the gas service line..

The break in the gas line occurred under an area of the

yard where there were extensive plantings of shrubbery. The

shrubbery was frequently watered and fertilized. The ground

in the vicinity of the fracture was described as being

"water saturated."

The soil in this part of Reno was reported to be

"corrosive" in nature. An odor of "swamp gas" was detected

in the vicinity of the pipeline.

1.3 Parts Submitted

Two pieces of 3/4 inch diameter carbon steel gas service

pipe, each about 1 1/2 feet in length, were submitted to the

NBS Mechanical Properties Section for examination. The pipe

pieces were tagged "PSSC - S. P. P. Co., 701 Thoma Street,

Reno, Nev," "A" and "B." A coupling which had joined the

two pieces of pipe was attached to the piece tagged "A." The

piece tagged "B" had fractured in the coupling, and a small

part of the threaded end of this piece was still in the

coupling. The two pieces of pipe are shown as received at

NBS in figure 1.

2 . PURPOSE

The Office of Pipeline Safety requested that the NBS

Mechanical Properties Section examine the fractured pipe

in order to characterize the fracture and, insofar as

possible, to determine the cause of failure.

3 . RESULTS OF EXAMINATIONS, TESTS, AND MEASUREMENTS

3 . 1 Visual and Macroscopic Examination

Piece "B" of the pipe had fractured transversely in a threaded region about 7/16 inch from one end at a coupling

joining pieces "A" and "B." The 7/16 inch piece was still

in the coupling when the parts were received. This coupling

had "recessed" threads; i.e., the last 3/16 inch at either

end of the coupling was not threaded. The inside diameter

of the coupling in the unthreaded region was about 0.07 inch

greater than the greatest outside diameter of the pipe in a

threaded region. The pipe had fractured at the outermost

- 2 -
thread of the coupling.

The two opposing fracture surfaces are shown in figure 2

as received at NBS. There was essentially no corrosion

product on either of them, although some parts of the surface

of the fracture still attached to the coupling were covered

v/ith a tarry appearing substance. This substance, which

could be rather easily removed mechanically, was not found on

the other fracture surface. Both fracture surfaces had

suffered an insignificant amount of mechanical damage.

Except for about a 6 1/2 inch length surrounding (and

including) the coupling, the outside surfaces of the two

pieces of pipe were coated with a black, tarry substance.

The region around the coupling was wrapped with what appeared

to be black electrical tape. When the pipe was received at

NBS, this tape had been removed from piece "B" for a short

distance beyond the threads adjacent to the fracture. The

exposed area which had been covered with the tape was

essentially free of corrosion product.

The remainder of the tape and the heavier tarry coating

were removed at NBS from the piece of pipe labeled "B." There

was essentially no evidence of corrosive attack on the whole

length of pipe.

The tape was removed from the unfractured piece of pipe

(piece "A") where it entered the coupling. There was some

corrosion product on this piece of pipe, but the corrosive

attack appeared to be insignificant.

There was no evidence of any significant mechanical

damage to either piece of pipe or to the coupling.

Some deformation could be seen in the vicinity of the

step on the fracture surface. This region would have

probably been the last part of the fracture to form. The

step can be seen in figure 2 (arrow) on the part of the

fracture still attached to the coupling.

3 . 2 Frac tographic Examination

The fracture surface of pipe piece "B" (not attached to

the coupling) was examined with the scanning electron microscope after it was cleaned with petroleum ether. Some areas

of the fracture surface exhibited dimpled rupture as the

primary fracture mode, some areas exhibited quasi-cleavage

as the dominant mode, and some areas exhibited a mixed mode.

The fracture surface examined is shown in figure 3. The mode

- 3 -
exhibited by the fracture at each of the areas numbered in

figure 3 is as follows:

Area Fracture Mode

1 Predominantly dimpled rupture

2 Mixed - about 75% quasi-cleavage , 25% dimpled

rupture

3 Predominantly quasi-cleavage , some dimpled

rupture

4 Essentially all dimpled rupture

5 Mixed - mostly dimpled rupture near the inside

and outside walls, quasi-cleavage in the center

6 At outside, nearly all quasi-cleavage

7 At inside, mostly quasi-cleavage with some

dimpled rupture adjacent to the inside wall

8 Essentially all quasi-cleavage

9 Essentially all quasi-cleavage

10 Essentially all quasi-cleavage

11 About half quasi-cleavage and half dimpled rupture

12 Nearly all dimpled rupture

Representative fractographs from areas 1, 5, 7, and 9, figure 3,

are shown in figures 4, 5, 6, and 7, respectively.

3 . 3 Tensile Tests

It was decided to conduct uniaxial tensile tests on

specimens of the submitted pipe in order to produce fractures

under known conditions of loading rate for comparison with

that of the submitted failure. The decision to run the tests

was based on several criteria as follows. (1) There was a

considerable amount of quasi-cleavage on the fracture surface

of this reasonably ductile material. Quasi-cleavage is

normally associated with a lack of ductility. (2) There was

no evidence of a pre-existing crack which might suggest stress

corrosion cracking as a possible crack mechanism. (3) The

fracture did not appear to have been produced by the application

- 4 -
of a bending load.

Consequently, two specimens, each about three inches long,

were made from the piece of the submitted pipe labeled "B."

To simulate the configuration of the field fracture, each

specimen was threaded at one end. For the test, the threaded

end of the specimen was fitted into a fixture on the tensile

testing machine and the other end was secured with a pin

through a drilled hole.

One of the specimens was tested at a relatively low

loading rate and the other at a relatively high loading rate.

The specimen tested at the low loading rate was tested to

failure in tension at a cross head speed of 0.02 inch per

minute. As would be expected, failure occurred in the threads.

The fracture that was produced exhibited dimpled rupture as

the primary factor mode over the entire fracture surface

except for one small region which was close to the apparent

crack origin. In this region, the mode was mixed, consisting

partly of dimpled rupture and partly of quasi-cleavage . The

quasi-cleavage was near the center of the fracture surface;

i.e., not near the inside or outside wall surfaces of the pipe.

A representative fractograph from an area not close to the

apparent crack origin is shown in figure 8, Part of the area

showing a mixed mode is shown in figure 9.

The specimen that was tested at the higher loading rate

was tested to failure in the same way as the first specimen,

but the cross head speed was about 16 inches per minute. The

fracture surface was quite different from that produced at a cross head speed of 0.02 inch per minute. Approximately onethird of the area of the fracture surface exhibited quasicleavage as the primary fracture mode. A representative fractograph showing quasi-cleavage as the primary mode appears in

figure 10. About an additional third of the fracture surface

exhibited a mixed mode; i.e., both dimpled rupture and quasicleavage. An example of the mixed mode is shown in figure 11.

The remainder of the fracture surface exhibited dimpled rupture

as the primary fracture mode. This part of the fracture

included the apparent last part of the fracture to form. A

representative fractograph from this region is shown in

figure 12.

The increase in testing speed from 0.02 inch per minute

to 16 inches per minute significantly increased the percentage

of the fracture surface exhibiting quasi-cleavage as the

fracture mode, and correspondingly decreased the percentage

of dimpled rupture.

- 5 -
3 . 4 Metallographic Examination

Four longitudinal sections adjacent to the fracture

were examined metallographically . These sections intersected

the fracture at approximately locations 1, 5, 6, and 12,

figure 3. As-polished regions showing average and estimated

worst inclusion areas from these four sections are shown in

figures 13 and 14, respectively.

Etched fields from two of these sections are shown in

figures 15 and 16. The fracture profile in each case is

horizontal at the top in the figure. In figure 15, where the

section intersects the fracture at location 6 in figure 3,

the lack of distorted grains indicates that there is

essentially no mechanical deformation except possibly

adjacent to the fracture on the inside of the pipe (upper

right in the figure) . This lack of deformation is consistent

with the fracture mode in this region of the fracture which

is primarily quasi-cleavage . The microstructure consists

primarily of ferrite with small amounts of pearlite. There

is considerable evidence of banding (straight vertical

features). In the narrow bands, there appears to be only

ferrite and no pearlite. Sometimes manganese segregates to

the narrow ferrite bands.

Figure 16 shows part of an etched specimen intersecting

the fracture surface at location 12, figure 3. The fracture

exhibited dimpled rupture as the primary fracture mode in

this region. Consistent with this ductile mode of fracture,

there is considerable mechanical deformation adjacent to the

fracture as evidenced by the necking down and bending of the

material. In this particular section, no banding is evident.

The microstructure again consists primarily of ferrite with

small amounts of pearlite.

The two other sections exaunined showed some evidence of

banding, but to a lesser extent than that exhibited in

figure 15.

3 . 5 Hardness Measurements

A Knoop hardness traverse at a load of 500 grams (KHN^qq)

was made on the longitudinal section that intersected the

fracture at location 12, figure 3. The traverse was made on a plane perpendicular to the longitudinal axis of the pipe. The

results are as follows:

- 6 -
Distance from the outside

wall surface of the pipe Approx. equivalent

0.004

0. 008

0. 016

0.024

0. 032

0.040

0. 048

0.056

0. 064

0.072

0.080

0. 088

186

195

186

206

211

238

182

194

158

174

174

179

87

89

87

91

92

97

86

89

78

83

83

85

The hardness was highest (R^ 97) in one of the narrow banded

areas shown in figure 15. This is consistent with the way in

which the specimens responded to metallographic polishing.

It may be that the increased hardness is due to manganese

segregation to the narrow ferrite bands.

The ultimate tensile strength approximately equivalent

to the lowest of the above hardness values is well above the

60,000 psi minimum required by API specification 5L.

This API specification 5L steel pipe gas service line

failed after about 18 years use at a location about two

feet from the house it was serving. Failure occurred in a threaded region of the pipe where it was attached to a coupling. The threading operation had reduced the wall

thickness, and consequently the cross sectional area of the

pipe, especially in the thread roots, in the region of the

failure. The thread roots also acted as stress concentrators.

These are the likely reasons that the fracture occurred in

the threads

.

The fact that the fracture surfaces exhibited little or

no evidence of corrosive attack suggests that the fracture had

occurred only a short time before the pipe was removed from

the ground, and further, that there was no pre-existing crack.

Quasi-cleavage was the dominant fracture mode over a significant percentage of the fracture surface. This indicates

a lack of ductility. A relatively low-speed bending or tensile

overload failure at room temperature in a relatively low

strength steel such as API 5L would normally be expected to

4. DISCUSSION

- 7 -
result in a ductile fracture with dimpled rupture as the

primary fracture mode. The low strain rate tensile test

performed at NBS resulted in such a fracture. The high

strain rate tensile test performed at NBS resulted in a

fracture in which quasi-cleavage was the primary fracture

mode over about two-thirds of the fracture surface. The

fracture surface of the specimen tested at 16 inches per

minute was similar to that of the submitted failure.

Based on the appearance of the fracture and the results

of the NBS tensile tests, a reasonable premise is that the

fracture of the submitted pipe was caused by the single application of a predominantly axial tensile load at a relatively

rapid rate. This premise leads to the possibility that the

pipe failed as a result of the explosion at the house, rather

than that the break occurred before the explosion.

In the sections excimined, the inclusion content did not

appear to be abnormal. The microstructure , which consisted

of ferrite and a small cimount of pearlite, appeared to be

satisfactory. Some banding, which apparently had no

significant influence on the failure, appeared in places.

The results of hardness measurements which were converted

to tensile strengths indicated that the ultimate tensile

strength requirement of API specification 5L was satisfied.

The lack of corrosion product on the pipe suggests that

the heavy watering and fertilizing of the lawn and shrubbery

did not have any significant effect on the condition of the

pipe.

5. CONCLUSIONS

1. Failure occurred in this 3/4 inch diameter steel pipe

gas service line due to the apparent single application

of an essentially axial tensile load, resulting in an

overload fracture.

2. Quasi-cleavage was the primary fracture mode over a

significant portion of the fracture surface, indicating

less ductility than would normally be expected in a low

loading rate overload fracture in a steel of this type,

3. The fracture surfaces were essentially free of corrosion

product. There was no evidence of a pre-existing srack.

4. The outside wall surface of the pipe had suffered no

significant corrosive attack.

- 8 -
5. The ultimate tensile strength of the pipe material

(based on a conversion from hardness measurements)

met the requirement of API specification 5L.

'6. The microstructure of the pipe material appeared to

be satisfactory, although there was some banding.

6. ACKNOWLEDGEMENT

Mr. L. C. Smith of the NBS Mechanical Properties Section

carried out the metallographic specimen preparation, the

photographic work, and the hardness measurements.

Mr. W. A. Willard of the NBS Mechanical Properties Section

and Mr. Smith conducted the tensile tests.

- 9 -

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Figure 3. The fracture surface from the part of

piece "B" which was not attached to the

coupling. The step in the fracture

(where the fracture path jumped a thread) is at the bottom. The numbers 1

through 12 represent areas referred to

in the Section on the Fractographic

Examination. X 4

Figure 4. SEM fractograph from area 1, figure 3. The

primary fracture mode is dimpled rupture.

X 235

Figure 5. SEM fractograph from area 5, figure 3. The

fracture mode is mixed with dimpled rupture

adjacent to the inside and outside wall surfaces

at the left and all the way across at the right.

Quasi-cleavage appears in the center at the left.

The inside wall surface is at the bottom. X 78

Figure 6. SEM fractograph from area 7, figure 3. The

primary fracture mode is quasi-cleavage

.

There is some dimpled rupture near the inside

wall. X 420

Figure 7. SEM fractograph from area 9, figure 3. The

fracture mode is essentially all quasi-cleavage.

X 475

Figure 8. SEM fractograph from the tensile fracture produced

in the laboratory at a machine cross head rate of

0.02 inch per minute. The mode of fracture is

essentially all dimpled rupture. X 280

Figure 9. SEM fractograph from the tensile fracture produced

in the laboratory at a machine cross head rate of

0.02 inch per minute. The mode of fracture in the

center is quasi-cleavage . Around the periphery,

the mode is primarily dimpled rupture. X 215

Figure 10. SEM fractograph from the tensile fracture produced

in the laboratory at a machine cross head rate of

16 inches per minute. The mode of fracture in this

region is primarily quasi-cleavage . There is a

small amount of dimpled rupture. X 260

Figure 11. SEM fractograph from the tensile fracture produced

in the laboratory at a machine cross head rate of

16 inches per minute. The fracture mode is mixed;

there is about half dimpled rupture and half quasicleavage in this field. X 105
I
Figure 12. SEM fractograph from the tensile fracture produced

in the laboratory at a machine cross head rate of

16 inches per minute. The fracture mode is

primarily dimpled rupture. X 290

c

Figure 13. As-polished longitudinal section that intersected

the fracture at location 5, figure 3. This field

exhibits the average inclusion content for the

material examined. X 100

Figure 14. As-polished longitudinal section that intersected

the fracture at location 6, figure 3. This field

is estimated to have the greatest inclusion content

of any of the sections examined. X 100

Figure 15. Etched longitudinal section intersecting the fracture

at location 6, figure 3. The fracture profile is

horizontal at the top. The microstructure consists of

ferrite and pearlite except in the narrow bands which

consist of ferrite. An example of one of the narrow

bands is indicated by the arrows. The inside wall is

vertical at the right and the threads are at the left.

Etchant: 1% nital X 40

Figure 16. Etched longitudinal section intersecting the

fracture at location 12, figure 3. The fracture

profile is horizontal at the top. The microstructure consists of ferrite and pearlite.

Distorted grains are evidence of deformation

adjacent to the fracture. The inside wall is

vertical at the right and the threads are at

the left.

Etchant: 1% nital X 40

NBS-114A IREV 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUHI.IC^ATION OK RKFOR I NO. NBSIR 75-743

2. Cii,iv't Accession

N...

3, Rccipiffit's Accession No.

EXAMINATION OF FAILED 3/4 INCH STEEL PIPE NATURAL

GAS SERVICE LINE, SIERRA PACIFIC POWER COMPANY,

RENO, NEVADA

5. Publicanon Dai' June 1975

6» Performing Organ i zat ion C ode

7. AUTHOR(S)

T. Robert Shives

8. Performing Organ. Report No. NBSIR 75-743

9 PK H FOR ; OH( A \' 1/ A riO\ NJAMI- AWD ADDRKS's

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Taslc/Work Unu No. 3120418

11. Contract /Grant No.

12, '^pi.>nsorin^ Or>; tint/atK>n Nam*' .ind ( onipl<*K* AJdrrss trect , City , Stste, ZIP)

Office of Pipeline Safety

Department of Transportation

Washington, D.C. 20590

13. Type of Report & Period Covered _ . , Failure

Analysis Report

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOlhS

16. Af<STRA( r (A 200-word or less (actual summary of most significant intortnation. II document includes a significant

bibliography or literature survey, mention it here.)

At the request of the Office of Pipeline Safety, the NBS Mechanical

Properties Section examined a fractured 3/4 inch diameter steel pipe gas

service line from Reno, Nevada. The pipe was reportedly fabricated

according to American Petroleum Institute specification 5L.

The gas line had failed about two feet from the house it was serving,

apparently by overload. The fracture surfaces exhibited essentially no

corrosion product and there was no evidence to indicate a pre-existing

crack. Quasi-cleavage was the fracture mode over a significant portion

of the fracture. Comparison of the fracture with laboratory produced

uniaxial tensile fractures indicated that failure was likely caused by

the single application of an essentially axial tensile load at a

relatively rapid loading rate.

The ultimate tensile strength of the material conformed to that

required by API specification 5L. There was some banding in the

microstructure

.

17. KEY WORDS (nix to twelve entries; alphabetical order, capitalize only the first letter of tf»e firal key word unieas a proper name; separated try semicolons

)

Gas pipe; high strain rate fracture; overload fracture; pipe fracture

18. AVAlLAlilLlTY [_ Unlimited

rX For Official Distribution. Do Not Release to NTIS

[ Order From Sup. of Doc, U.S. dovernment Printing Office Washinxton, D.C:. 20402. SD ( at. No. CH

r Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

21. NO. OF PAGES

20. SE( URITV CLASS

(THIS PAGE)

UNCLASSIFIED

22. Price

USCOMM-OC 29042-P74