m s 19/3

NBSIR 73-111

1/

Report No. 10: Examination of a Failed

2-Inch Diameter Galvanized Steel Pipe:

Valley Gas Company, Woonsocket,

Rhode Island

C. G. Interrante, G. E. Hicho, and M. L. Picklesimer

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

February 23, 1973

Failure Analysis Report

Prepared for Office of Pipeline Safety

Department of Transportation

Washington. D. C. 20590

NBSIR 73-111

REPORT NO. 10: EXAMINATION OF A FAILED 2-INCH DIAMETER

GALVANIZED STEEL PIPE: VALLEY GAS COMPANY, WOONSOCKET,

RHODE ISLAND.

C. G. Interrante, G. E. Hicho, and M. L. Picklesimer

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

February 23, 1973

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."

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

iSIATIOIMAL BUREAU OF STANDARDS, Richard W. Roberts, Director

TABLE OF CONTENTS

Page

SUMMARY iv

1. INTRODUCTION 1

1.1 Parts Submitted 1

1.2 Objectives of the Examination 2

2. EXPERIMENTAL PROCEDURES 2

2.1 Macroscopic and Microscopic Examinations 2

2.2 Preparation of Specimens 2

2.3 Hydrogen Charging Procedures 3

2.4 Procedures for Impact and Tensile Testing 4

3. RESULTS 5

3.1 Macroscopic Examinations 5

3.2 Metallographic Observations 6

3.3 Fractography of the Failed Pipe 7

3.4 Impact Test Results 8

3.5 Fractography of Impact Specimens 9

3.6 Tensile Test Results 9

3.7 Interrupted Tensile Test Results 10

3.8 Macroscopic Appearances of Fractured

Tensile Specimens 11

3.9 Fractography of Tensile Specimens 11

4. DISCUSSION 12

4.1 Possible Causes of the Brittle Fracture 13

i

Page

.4.1.1 Geometry of the joint 13

4.1.2 Local corrosion 14

4.1.3 Galvanic cell action 14

4.1.4 Tensile and impact tests 14

5. CONCLUSIONS 15

ACKNOWLEDGEMENT 16

TABLES

1. Impact Test Results for Subsized Charpy V-Notch

Specimens 17

2. Tensile Test Results 18

FIGURES

1. Photographs of the Fracture Surface of the Short

Length of Pipe 19

2. Photographs of the Longer Length of Pipe AsReceived at NBS 20

3. Photographs of Typical Subsize Impact (A) and

Short Tensile (B) Specimens 21

4. Matching Corroded Fracture Surfaces at Locations

C and C 22

5. Photomicrographs of Longitudinal Cross Section

of Pipe 23

6. SEM Fractographs at Location A of Figure 1 24

7. SEM Fractographs at Location C of Figure 1 25

8. Macroscopic Appearance of Several Subsize Charpy

V-Notch Impact Specimens After Testing 26

9. SEM Stereo-Pair Fractograph of Impact Specimen

No. 5 27

10. SEM Fractographs Taken of Charged and Uncharged

Impact Specimens 28

ii

FIGURES

11. Fractures of Notched Tensile Specimens 29

12. Fractures of Notched Tensile Specimens 30

13. Fractures of Unnotched Tensile Specimens 31

14. SEM Fractographs of Uncharged Notched Tensile

Specimen No. 2 32

15. SEM Fractograph of Charged Unnotched Tensile

Specimen No. 11 33

16. SEM Fractographs of Notched and Unnotched

Tensile Specimens in the Charged Condition 34

17. Photomicrographs of Galvanized Coating Observed

on Second Thread from the Fracture Surface 3 5

Appendix A. Interrupted Tensile Test Results Using

IM Na-jSO^ as the Electrolyte 36

iii

SUMMARY

Two sections of a fractured 2-inch diameter galvanized

steel pipe used in a natural gas residential distribution

service were examined to determine the mode of fracture and

to analyze factors that could possibly have contributed to

the failure.

Notched impact and tensile specimens taken from the

failed section of pipe were tested in uncharged and hydrogen

charged conditions at temperatures of from 35 to 212° F for

the impact tests and of 35 and 70° F for the tensile tests.

The uncharged tensile specimens failed in an entirely ductile

manner by dimpled rupture with large elongations. The

hydrogen charged tensile specimens failed in a comparatively

brittle manner. The impact specimens failed in a similar

brittle manner and with low-energy absorption, when tested

at the lov7er temperatures whether charged or uncharged.

Impact specimens tested at 190 and 212° F did not fracture.

Scanning-Electron-Microscope (SEM) examinations of the

fracture surfaces of the failed pipe and the test specimens

shelved that the brittle fractures mentioned above were all

similar, being fractures of a predominantly transgranular

,

quasi-cleavage mode with some dimpled rupture being observable

at local sites

.

Chemical analysis and metallographic examination indicated

that the failed pipe was probably conventionally produced (welded

and annealed, hot-dip galvanized ) steel utility pipe. Surface

examination of the corrosion present on the pipe indicated

that the pipe had been in service, for a significant period of

time before the fracture.

The presence of a 10 degree bend in one section of the

pipe about 35 inches from the fracture, coupled with a dent

and surface scarring present at that point, indicates that the

pipe could have been hooked by ditch digging equipment before

the fracture occurred.

It was concluded that the fracture had been produced at the

root of the pipe thread at its junction with a pipe coupling as

the result of the application of an external load. The fracture

occurred at that particular point as a combined result of stress

concentration by thread and coupling geometry, reduced wall

i thickness at that thread root, and, possibly, a small previously

existing, short crack partially through the wall of the pipe at

that thread root. The fracture occurred in a brittle manner

with little or no plastic flow, and the brittleness could have

been caused by or worsened by the absorption of hydrogen discharged at the thread root, as a result of the galvanic current

produced by the galvanized coating used for corrosion

protection.

iv

Report No. 10

Examination of a Failed 2-Inch Diameter Galvanized Steel

Pipe: Valley Gas Company, Woonsocket, Rhode Island

1. INTRODUCTION

An examination of the fracture of a 2-inch diameter

galvanized steel pipe used in natural gas distribution service

in Woonsocket, Rhode Island, was requested on February 14, 1972

by Mr. Lance Heyerly for the Office of Pipeline Safety,

Department of Transportation. It was reported* that the pipe

had failed in service on February 8, 1972 and that the failure

was accompanied by an explosion which led to the deaths of 3

persons and to injuries to 6 others. It was suspected*

that the pipe may have been damaged earlier during the ditch

digging and back filling procedures used in a nearby sewer

installation. In addition**, a 2-foot boulder was found on

top of the pipe after the failure.

The fractured pipe was delivered on February 28, 1972 by

Mr. Gerald Harvey, New York Testing Laboratories, Inc. At

that time discussions were held between Mr. Harvey,

Mr. Lance Heverly (OPS) and Dr. M.L. Picklesimer (NBS) on

the examinations and tests to be conducted. In the course of

these discussions, Mr, Harvey obtained by telephone the

following additional information from a Mr. Bateman of the

Valley Gas Company, Woonsocket, Rhode Island: (1) the soil

around the filed pipe consisted primarily of loose gravel, and

(2) gas service to the house which exploded had been disconnected on June 1, 1971 by cutting out a 3-foot length of

pipe . ***

1. 1 Parts Submitted

The fractured pipe was delivered in two pieces, with each

piece containing one of the fracture faces of the failed pipe

section. One piece was short, about two feet in length, and

the other was relatively long, about seven feet in length,

with a coupling on one end. These are shown in Figures 1

and 2 , respectively

.

Conversations between Mr. Lance Heverly, Office of

Pipeline Safety, and Dr. M. L. Picklesimer, National

Bureau of Standards on February 14, 1972.

Conversation between Mr. Donald Whitcolm, Rhode Island

Public Utilities Commission and Dr. M. L. Picklesimer,

National Bureau of Standards on February 24, 1972.

No information was obtained about the acidity, or

electrical resistivity of the soil. The soil

temperature was estimated to be just above freezing.

- 1 -

1.2 Objectives of the Examination

The principal objectives of the examination to be

conducted on the failed pipe were (1) to characterize the

fracture surface to attempt to determine the fracture mode,

and (2) to attempt to determine the cause of the failure

and/or obtain information pertinent to this objective.

2. EXPERIMENTAL PROCEDURES

2 . 1 Macroscopic and Microscopic Examinations

Each of the two pieces of failed pipe was examined macroscopically and photographed. The fracture faces of both

pieces were also examined with a stereomicroscope at

magnifications of from 7X to about 20X.

The shorter of the two pieces of pipe was sectioned for

examination [in the Scanning Electron Microscope (SEM) ] of the

fracture surfaces at locations A, B, and C, as shov/n in

Figure 1, The sample from location C was also used to

determine the elements present in the surface coating on

the pipe. Each sample contained a fracture section about

3/8-inch wide by the full remaining wall thickness by about

1/4-inch long. In addition, a metallographic specimen was

taken as a longitudinal cross-section adjacent to the

location of sample C.

2 . 2 Preparation of Specimens

Longitudinal impact and tensile specimens were machined

from the lower portions of the shorter length of the failed

pipe. Typical impact and notched tensile 1-inch gage length

specimens are shown in Figure 3. All of the specimens

retained the nominal 2-inch diameter curvature of the pipe in

the gage sections.

The impact specimens were made as subsize Charpy V-notch*

type, 3/8-inch wide by 2 3/8-inch long by full v/all thickness,

with the notch located at the midpoint along tne length of

the specimen.

The tensile specimens were of three modifications of the

sheet-type geometry: (1) 1-inch gage length, notched, (2) 2-

inch gage length, notched, and (3) 2-inch gage length, unnotched.

The curvature of the pipe was retained in these specimens

and so they are non-standard subsize Charpy specimens.

- 2 -

The gage section of the tensile samples was approximately

3/8-inch wide by the full-wall thickness of about 1/8 inch.

The grip ends of these specimens were 1 1/8-inch wide by

1 1/4-inch long by 1/8-inch thick. Prior to testing, the

grip ends of the tensile specimens were flattened v;ith a hammer to make them compatible with the available tensiletesting grips. This caused no deformation of the gage

section. The unnotched specimens were tapered slightly along

the cut edges of the gage section, so as to promote failure

within the gage length. The notched-tensile specimens were

notched at the center of the gage length.

The notches on both the impact and the notched-tensile

specimens were located in the convex side of the specimen,

corresponding to the outer surface of the pipe. For the

notched tensile specimens, the notches were made with the

sharp edge of a jeweler's file, while being observed at 7X

under a stereomicroscope . The file produced a notch with a root radius of about 0.010 to 0.015-inch, and with a depth

of about 0.018 inch as gaged with an 0.018-inch diameter

copper wire. Filing was continued until the v/ire gage

could be placed flush with the surface of the specimen.

Before notching the impact specimens, corrosion products

and some of the parent metal were cleaned from the surface

over a band at the center of the specimen length. The notch

was then located in the center of this band, as shown at

location 2 in Figure 3 . These notches were machined to

provide a root radius of 0.010 inch at a notch depth of about

0.025 inch, in accordance with ASTM Standard E 23-66 for

Subsize Charpy V-notch specimens.

2 . 3 Hydrogen Charging Procedures

For most of the tensile and impact tests conducted,

specimens were prepared so that results for both hydrogencharged and uncharged conditions could be obtained. Hydrogen

charging for all of these tests, except for one tensile test,

was accomplished by placing the specimen in a 5 percent H2SO4

(sulfuric acid) aqueous solution, with the specimen as the

cathode and an annular stainless steel sheet as the anode in

an electrolytic cell established v/ith a 1.6V dry-cell battery.

The specimen was surrounded by the anode and v/as separated

from it by the electrolyte. Prior to charging with hydrogen,

the unnotched tensile specimens and the impact specimens were

coated with wax along their machined edges. The impact

specimen tested at 190° F was also waxed on both the convex

and concave surfaces except in the region of the notch.

- 3 -

The only exception to these charging practices was made

for the test referred to later in this report as the

"interrupted tensile test". For this test, the acid

electrolyte was replaced with a IM Na2S04 electrolyte to avoid

excessive corrosion from the long-time exposure to the acid

charging solution. The sulfuric acid electrolyte was found

to be too corrosive during the extended time tensile test of

Specimen No. 12, which broke near one of the grip ends of

the specimen after pronounced corrosive attack there.

2 . 4 Procedures for Impact and Tensile Testing

Impact specimens were tested at temperatures of 35, 70,

190, and 212" F. The test method used was in accordance with

ASTM E 23-66 for notched-bar impact testing of metallic

materials.

The tensile specimens were strained in tension at a crosshead rate such that a loading rate of 10 0 pounds per minute

was obtained in the range of loads near 1000 pounds. This

cross-head rate was established by successive approximations

during continuous loading in an electro-mechanical tensile

machine, and the testing machine was then allowed to run at

that cross-head rate until the specimen fractured.

One of the notched tensile specimens was tested with a

procedure referred to herein as the "interrupted tensile test".

This test was conducted by loading the specimen in the manner

given above until a load of 2 00 0 pounds was reached. This

load (2000 pounds) was near or only slightly less than the

maximum load observed in a companion specimen tested under

slightly different environmental conditions. After the

initial load application to 2000 pounds, the machine was

stopped so that the cross-head was fixed (i.e. "constant

strain" in the specimen) . After an initial load drop occurred

1 (completed in less than one minute) observations v/ere made

I over a period of several hours (1 to 17) to determine if the

load continued to drop. If it had, the specimen should

fracture eventually, indicating that fracture might have been

produced by the initial strain application in service. If no

i additional drop in load occurred during the observational

period, the load was reapplied in accordance with the schedule

given in Appendix A, until failure occurred during the last

application of load.

- 4 -

3 . RESULTS

3 . 1 Macroscopic Examinations

The fractured pipe was received in two pieces, each

containing one of the fracture faces of the failed pipe section

and both containing a longitudinal seam weld visible on the

interior. One piece was short, about two feet in length,

with one end cut with a saw and the other end being one face

of a fracture at the root of a pipe thread. The shorter

length of pipe had apparently been threaded into a coupling

on one end of the longer piece of pipe, where the face of a

fracture was still in place. The fracture had occurred at

the root of the thread just at the end of the coupling. The

fractured end of the shorter length of pipe is shown in

Figure 1, and several views of the longer length of pipe are

shown in Figure 2. Careful comparison of surface irregularities on the fracture faces of both sections of pipe showed

that the two faces mated, and the two had originally been one

piece of pipe. The mating fracture faces are shown in

Figures la and 2b.

The longer length of pipe was bent about 10 degrees at a

point located approximately 3 5 inches from the end of the

coupling containing the fracture. An opposite bend of

approximately 6 degrees was present at a distance of about 63

inches from the same end of the coupling. The coupling was

marked with a longitudinal stripe of "watery" orange paint

of some sort in the plane of both bends in the pipe. If

this stripe had been put on the top of the pipe while in

place in the ground before removal, then the pipe had been

pulled upward at the major bend. Both sections of pipe and

the coupling appear to be galvanized, and all had a wide band

of "rust" along the line of the paint stripe.

The point at which the pipe had received its major bend

is shown in Figure 2 as point G. The surface of the pipe at

point G was dented and scraped, as though it had been hooked

by a strong, pointed device and pulled to make the bend, dent,

and scrape. Point G is oriented about 180 degrees around the

pipe from the orange paint stripe and a local site at which

the fracture appeared to be ductile (progressing across a

thread from one thread root to another) . The apparent

ductility at this site and the brittle fracture appearance

of the balance of the fracture surface, indicated that this

local site was the last to fail.

- 5 -

Each of the two fracture faces contained some rust at

scattered locations, but the rusted areas matched clearly

at only one position on the fracture, location C on one and

location C on the other. These are shown in Figures 1 and

2f respectively, and the features are compared directly in

Figure 4. Since the rust was present on both faces in the

same pattern, and the rusted areas of both fracture surfaces

were depressed below the level of the "freshly fractured"

part of each, it appears probable that the rust formed in a crack that existed before the final fracture occurred.

Looking directly along the longitudinal axis of the 7-

foot long pipe from the end of the fracture, there is an angle

of roughly 45 degrees between location C and location G

(which lies in the plane of bending) where the pipe appeared

to have been externally loaded. It is probable that an

external force applied at location G could have produced high

tensile stresses at location C in a constrained piping system.

In addition, rust observed on other parts of the outer surface

of this pipe, as described below, was not generally present on

the surface at location G.

Corrosion products in the form of a reddish oxide,

presumed to be either iron oxide or oxides of a zinc-iron

intermetallic phase, were abundant on the exterior surface

of the shorter pipe. The coupling and much of the area of

the longer pipe were less prominantly covered with this

oxide. Other exterior parts of both pipes and the coupling

were also grey or white in appearance in parts and these

parts were presumed to be covered with zinc metal, zinc-iron

intermetallics and oxides of zinc. The interiors of both

pipes were grey and white in appearance and they were

presumed to be covered with zinc metal and zinc oxide.

3 . 2 Metallographic Observations

The longitudinal and transverse sections taken from the

shorter pipe showed nothing unusual in either the inclusion

content or the microstructure of the steel. Longitudinal

sections of the pipe in the unetched and the etched conditions

are shown in Figure 5. The inclusion content of the steel is

not high. It was roughly estimated to be below number 2 thin,

for inclusions of Types A and B using the ASTM E 45 Method

A rating.

- 6 -

The microstructure of the steel is a mixture of ferrite

and pearlite, which are present in proportions that would

correspond roughly to that of an SAE 1015 steel in the

annealed or the hot-rolled conditions, the ferrite grains

are irregular and elongated, and the pearlite colonies are

small in size. On both the inner and the outer surfaces of

the pipe, layers of intermetallic phases and zinc metal were

observed, with the thickness of this layer being generally

much greater on the inside of the pipe and lesser on the

outside in the locations examined. Most of the exterior

surfaces examined on the fractured pipe near the fracture

face appeared to have very little or no zinc or iron-zinc

intermetallics

.

Examination of the microstructure of a longitudinal

section taken at location C revealed that the microstructure

just beneath the corroded region was essentially the same as

that just beneath the fractured surface. In fact, these

microstructures are indistinguishable from those at the

other locations examined.

In concurrence with the findings from the macroscopic

examinations of the fracture surfaces , the stereomicroscopic

examinations of the fracture faces revealed that the fracture

had occurred with little evidence of ductility, except at the

local site that is about 180° from orientation G. This

examination also confirmed two presumptions made after the

macroscopic examination: (1) the corroded areas at location C

a'nd C were depressions on the mating fracture surfaces,

indicating that corrosion had occurred here on both faces,

and (2) the uncorroded parts of these same locations, C and C,

appeared to be mating fracture surfaces v/ith the "hills" on

one of the two faces matching in size and shape with the

"valleys" on the mating fracture face.

3. 3 Fractography of the Failed Pipe

In concurrence with the observations made v/ith the light

microscope, the fracture appearances were predominantly brittle

at locations A, B, and C, as viewed in the Scanning Electron

Microscope (SEM) . This is shown by fractographs taken on

specimens from locations A and C, respectively, in Figures 6

and 7 at magnifications of from 35x to 350X. The fracture

mode is a brittle mode of predominantly transgranular quasicleavage, with local sites (indicated by arrows within the

figures) where there is evidence of local ductility in

- 7 -

the form of dimpled rupture observable only at higher

magnifications. These dimpled-rupture sites are believed to

be the last sites to fail in the fracture process. It is

believed that these local sites could have been athermally

j heated by the preceding fracture processes. This would have

resulted in increased toughness and ability to flow

plastically at the sites that failed last. The resulting

fracture appearances would be a few local sites of ductile

fracture (dimpled rupture) in a sea of quasi-cleavage

fracture, as was actually observed on the samples examined.

There were occasional sites where intergranular cleavage

(as contrasted to transgranular cleavage) was observed on

I both the pipe fracture and the fractures of the impact and

I tensile specimens (discussed later) . This fact is not deemed

important enough to justify further comment.

Two of the fractographs taken at location C, given in

Figure 1 , show a deposit on the fracture surface which is

believed to be the corrosion product discussed previously,

j This deposit lies between the lines C-C and A-A in Figure 7a,

! and below line C-C in Figure 7b. Comparison of fractographs

made before and after ultrasonic cleaning of this specimen

j shows that only a small amount of the rust deposit was

removed by cleaning, as can be seen by comparing the same

area in Figure 7a (after cleaning) with that in Figure 7b

(before cleaning) . The surface features in this area

(marked U on the fractograph) were not as sharp or welldefined as those of nearby sites above the line C-C.

The principal elements present in the inner and outer

surface layers at location C on the pipe vrere determined by

x-ray emission analysis in the SEM. Both zinc and iron were

strongly detected on these pipe surfaces. These qualitative

analyses are consistent with the metallographic observations

previously discussed.

3 . 4 Impact Test Results

The results of the V-notch impact tests of longitudinal

specimens taken from the failed pipe are presented in Table 1.

The data in Table 1 show that over the range of temperatures

from 35° F to room temperature (about 7 0° F) and for both

hydrogen-charged and uncharged specimens, the steel was not

- 8 -

very tough, absorbing less than 3 ft-lbs of energy per

specimen. This indicates that, at service temperatures at

and below normal room temperature, the pipe would have had

very low resistance to fracturing by impact loads.

At temperatures of 190 and 212° F, the steel was much

tougher, absorbing 12 and 18 ft-lbs per specimen, respectively.

As shown in Figure 8, these specimens did not fail when

tested. They simply bent enough to be pushed through the

anvil by the striker of the test machine. In contrast with

this behavior, the specimens tested at (and below) room

temperature failed either completely or almost completely,

as also shown in Figure 8. In the latter cases a small ridge

of steel at the back of the specimen remained unfractured

after the test. Their fracture faces have many tiny crystallographic facets which are highly reflective to light,

indicating that the failure mode was relatively brittle.

3 . 5 Fractography of Impact Specimens

Fractographic examinations of the fractures of impact

specimens (numbered 1, 2, 4, and 5) tested at temperatures

at-and-below room temperature showed that all were similar

in appearance in the region of the specimen that had been

failed under the impact load. Hydrogen-charged specimens

could not be distinguished from uncharged specimens. These

appearances were not unlike those of the fracture of the

failed pipe discussed earlier.

The stereoscopic fractograph of specimen number 5, shown

as Figure 9, represents roughly the center 1/3 of the totalfracture surface. This stereo pair and the general

appearances of the other fractures, represented by fractographs of specimen Nos . 5 and 1 given in Figure 10, are not

unlike those shown earlier for the failed pipe given in

Figures 6 and 7. The fracture mode is brittle, being

predominantly transgranular quasi-cleavage , with local sites

(indicated by arrows in Figure 10) of dimpled rupture

observable at higher magnifications.

3.6 Tensile Test Results

The results of the tensile tests are given in Table 2.

The data indicate that (1) the steel is notch strengthened,

(2) hydrogen charging decreases the strength of notched

specimens, (3) hydrogen charging markedly decreases the

ductility of both the notched and the unnotched specimens,

and (4) the data for 1-inch gage length specimens is similar

to that for 2-inch gage length specimens.

- 9 -

The effect of notch strengthening is shown by comparing

results of uncharged, 2-inch gage length specimens in the

notched and unnotched conditions. The notched tensile

strength is about 24 percent greater than the unnotched

strength. The former is 68.0 ksi (average of 2 tests) and

the latter is 53.7 ksi (one test).

The decrease in notched strength produced by charging

with hydrogen is similar for the 1- and 2-inch gage length

specimens. When averaged together, the notched strength

data indicate that the average strength of uncharged

specimens (numbered 3, 2, 8, and 5) is 67.1 ksi, and that

of the charged specimens (numbered* 4, 1, 6, and 12) is 51.8

ksi, representing a decrease of about 23 percent in notchtensile strength due to charging with hydrogen.

Thus, the decrease in notch-tensile strength due to

hydrogen almost equals the increase in strength due to notch

strengthening. Consequently, the notched hydrogen-charged

specimens have about the same load carrying capacity per unit

area (of remaining metal) as unnotched, uncharged specimens.

The effect of hydrogen charging on the ductility of the

pipe is similar for the 1- and the 2-inch gage length specimens

and it is observable in both unnotched and notched specimens.

For the unnotched specimens, the reduction of area of the

uncharged specimen is 52.3 percent, whereas that of the charged

specimen is only 17.4 percent, representing a 67 percent

decrease (2/3 of the original value) in the reduction of area,

due to hydrogen. For notched specimens this value is a 70

percent decrease in reduction of area. The elongation values

of notched specimens show an even greater effect, with a

decrease of 85 percent of the original uncharged elongation

being observable in hydrogen-charged specimens. The average

elongation values for four specimens in each condition are

23.8 and 3.6 percent, respectively for the uncharged and

charged specimens.

3 . 7 Interrupted Tensile Test Results

The interrupted tensile test (see section 2.5) specimens

continued to sustain loads without noticeable slow-crack growth,

even at load levels near the ultimate load for the specimen.

Data for Specimen No. 14 is not included here because

it was tested differently.

- 10 -

This single test result indicates that slow-crack growth

could not be obtained in one of the notched tensile specimens

under the charging and loading conditions used. However, we

note that charging with the IM Na2S04 solution, most likely,

was not as severe as charging with the 5 percent H2SO4

solution used for the other test specimens. This conclusion

is partly confirmed by the slightly greater ductility and much

greater strength of the interrupted test specimen when

compared with specimen Nos . 6 and 12, which were continuously

strained to failure in the H2SO4 electrolyte.

3 . 8 Macroscopic Appearances of Fractured Tensile Specimens

The ductility losses of hydrogen-charged specimens are

observable on the photographs of the fractured tensile-test

specimens shown in Figures 11 and 12 for the notched specimens

and in Figure 13 for the unnotched specimens. Each photograph

shows two views of a test specimen; one view is along the

length of the specimen in the region near the notch and the

other view shows the fracture surface of the mating half of

the same specimen.

All of the notched and unnotched specimens show necking

(reduction of the cross-sectional area at the fracture) for

the uncharged specimens (those at the tops of the figures)

and little or no apparent necking for the charged specimens

(those at the bottoms of the figures). Furthermore, the

appearances of the fracture surfaces are markedly different

for charged and uncharged specim.ens. The uncharged specimens

show evidence of gross plastic flow in the form of large lips

of shear fracture at the edges of the fracture face, whereas

the charged specimens show virtually no shear lips. The

charged specimens are thus quite flat in comparison. On

these flat surfaces are seen numerous shiny facets which

reflect light. These fracture appearances indicate that the

fractures of the uncharged specimens are ductile while those

of the charged specimens are comparatively brittle.

3 . 9 Fractography of Tensile Specimens

The fracture surfaces of specimens numbered 1, 2, 4 to

8, 11, and 14, were examined in the SEM. The results of

the examinations are summarized in Figures 14 through 16.

In general, the fractography of these samples indicated that

all of the uncharged tensile samples examined (Nos. 2, 7, and

8) failed by a ductile mode (dimpled rupture) , whereas all

of the charged tensile samples examined (Nos. 1, 4, 11, and

14) failed by a comparatively brittle mode of predominantly

- 11 -
I
transgranular quasi-cleavage , with local sites of dimpled

j rupture observable only at higher magnifications.

' Furthermore, the fracture appearances of the charged tensile

specimens were not unlike those of the impact samples (Nos.

1, 2, 3, and 4) discussed earlier and not unlike those of

locations A, B, and C from the failed pipe.

Figure 14 shows a stereographic pair of fractographs of

an uncharged notched tensile sample (No. 2) taken at 13X

as well as a representative fractograph taken at 650X on this

fracture surface. The dimpled rupture shown here is

characteristic of the general appearance observed on the

fracture surfaces on all of the other uncharged tensile

samples.

Figure 15 shows a stereographic pair of fractographs

taken at 19X on the fracture of a charged tensile specimen

I (No. 11) . Fractographs taken at higher magnifications on

charged specimens (Nos. 4 and 11) are shown in Figure 16.

1 These fractographs show that charged specimens, whether

f notched or unnotched, failed with a fracture mode of

predominantly transgranular quasi-cleavage , with only

local sites of dimpled rupture.

j 4 . DISCUSSION

The features observed on the fracture surfaces of the

failed pipe indicate that the principal mode of fracture was

brittle, being predominantly transgranular quasi-cleavage

,

with ductile dimpled rupture being observed at localized

sites only at the higher magnifications. In addition, a rust

pattern was observed at one location on both fracture faces

v;hich indicated that a crack had probably existed there at

the root of the pipe thread for some time before the fracture

was finally completed.

From the macroscopic examination of the pipe and the

orientations of several features of the fracture surface

(the previously existing crack, the "tearing" and ductility

I shown by the movement of the fracture from one pipe thread

to a neighboring one, the orientation of the major bend

j and dent in the pipe, and the orientation of the scarred

I area in the major bend) , it appears likely that the pipe could

I have been hooked by ditch digging equipment. The "freshness"

of the scarred surface at the dent, in comparison to the

corrosion pattern on the pipe in the neighborhood of the dent,

, indicates that the dent and scar were relatively new, and

i certainly had been made an appreciable time after the pipe

' was originally placed in service. The dented and scarred

- 12 -

area was a fulcrum of the bend in the pipe, indicating that

the pipe was probably bent by the same force that made the

dent. It is believed probable that the process of denting

and bending the pipe produced high stresses in the threaded

part of the pipe at the coupling and that the failure

occurred because the metal at the root of the thread could

not accommodate (by plastic flow) the strain required.

4 . 1 Possible Causes of the Brittle Fracture

The failure occurred in a brittle manner despite the fact

that the low-carbon steels used for making galvanized pipe

normally have a high capacity for plastic strain, i.e., good

ductility, as was shown by the tests of the uncharged notched

and unnotched tensile specimens. The lack of ductility shown

by the fracture of this material in service warrants further

discussion

.

4.1,1 Geometry of the joint

The constraint of the coupling (a much more

massive section) and the shape of the threads of the pipe

tend to limit the maximum capacity of the pipe material

for plastic strain there. The large reduction in wall

thickness of the pipe at the root of the thread and the much

greater wall thickness of the coupling would tend to

concentrate the bending stress into the root of the thread.

The shape of the threads would also strongly limit the

"length" of the region over which plastic flow could occur.

This length is essentially that at the root of the thread.

The combined effects would then serve to concentrate both

stress and strain at the root of the pipe thread located

at the edge of the coupling.

Although believed to be improbable, it is possible that

the constraint provided by the coupling and the thread would

make the pipe fail at this point with little evidence of

ductility when slowly strained in bending at the estimated

temperature of the service failure. This concept could not

be adequately tested in the laboratory with the available

materials (the longer length of pipe was not the one that

failed in service) . While the notched tensile specimens

were tested under conditions of stress and geometry that

did not fully duplicate the constraint on the threaded pipe

wall in service before failure, the tests do show that

notched tensile specimens can be made to fracture very

ductilely, with large amounts of plastic flow even at the

root of the notch, in a manner very different from that

observed on the fracture produced in service.

- 13 -

4.1.2 Local corrosion

There is some evidence that a crack, about onethird of the remaining wall thickness deep, had existed in

part of the root of the thread at the junction with the

coupling for some period of time before the final fracture

occurred. If it had predated the actual failure it would

have augmented the effects of the local stress concentration

produced at the edge of the coupling, so that this region of

the pipe wall could have failed at a stress lower than that

required in neighboring regions on the same thread. Thus,

this location may have been the origin of the final fracture.

4«1'3 Galvanic cell action

The steel pipe was shown to have been galvanized

both x-ray emission analysis and microstructural examination)

on both the interior and exterior surfaces. Also, iron-zinc

intermetallic layers were observed on the crest of the second

thread from the fracture face, as can be seen in the photomicrographs shown in Figure 17. Since galvanized surface

layers produce their protective action against corrosion of

the steel pipe wall by galvanic currents, making the exposed

steel in the root of the threads cathodic, hydrogen could

have been first discharged on the steel surfaces in the root

of the threads and then absorbed by the steel. It is known

[1,2] ^ that absorbed hydrogen can severely limit the plastic

strain occurring before fracture in low-carbon steels.

Thus, it is believed probable that the "brittleness" of the

fracture in service was significantly increased by charging

with hydrogen discharged by cathodic currents produced by

the interaction of zinc, iron and the environment.

4.1.4 Tensile and impact tests

Tests of subsize notched impact specimens and of

hydrogen- charged tensile specimens (both notched and unnotched) taken from the failed pipe showed that the steel pip

could be made to fracture with little ductility at normal

service temperatures. Also, the fracture mode could not be

distinguished from that of the failed pipe by either light or

electron fractography (SEM) . These findings suggest that the

steel at the failed pipe thread would have had little ability

to accommodate strain in service if it had been stressed by

impact loads, or if it had been stressed slowly in a hydrogen

charging environment of sufficient severity.

Figures in brackets indicate the references at the end

of this report.

- 14 -

However, the charging conditions used for the specimens

used in the slowly stressed tensile tests may not nave

duplicated the conditions present in service at the time of

the failure. Factors which would have promoted high charging

fugacities (severe charging of hydrogen) in service, include

soil which contains poisons (such as hydrogen sulfide)

,

soils with high electrical conductivity and low pH , and large

galvanic cell potentials between the steel and its galvanic

protector. If all of these factors v^ere unfavorable in

service at the time of the failure, the charging conditions

of the experiments were probably less severe than the service

conditions. Much further work would be required to determine

if galvanized pipe can be made to fail in a brittle manner

under charging conditions that more accurately simulate the

actual service conditions.

5. CONCLUSIONS

The following conclusions were drawn from the examinations

and laboratory tests conducted:

(1) The pipe fractured in a brittle manner by a

predominantely transgranular quasi-cleavage fracture

mode, with local areas of dimpled rupture

observable only at high magnification.

(2) The failure was probably caused by the application

of an external load to the piping system and by the

concentration of the bending stress produced in the

root of a pipe thread where the pipe entered a coupling.

(3) The fracture faces observed on the two pieces of

pipe mated, showing that the two pieces had

originally been one.

(4) The microstructures and inclusion content of pipe

samples indicated that the pipe material was not

abnormal for welded and annealed, hot-dip

galvanized steel utility pipe.

(5) X-ray emission analyses detected zinc and iron on

both the inner and outer surfaces of the pipe

indicating that the pipe had been galvanized.

(6) Factors that may have contributed to the failure of

the pipe, by limiting the strain capacity of the

pipe metal, include thread and coupling geometry,

possible impact loading, hydrogen embrittlement

,

and corrosion at a thread root.

- 15 -

(7) A crack, about one-third of the remaining wall

thickness deep, may have existed in part of the

root of the fractured thread before the final

fracture occurred. If so, this region could

have started to fracture at a lower stress than

others in the neighborhood required, and could

have been the origin of the final fracture crack.

(8) The brittle fracture features of transgranular

quasi-cleavage observed on fractures of notched

impact specimens (both uncharged and hydrogencharged) and on fractures of hydrogen-charged

tensile specimens could not be distinguished from

the features observed on samples of the fracture

surface of the pipe that failed in service. On

the other hand, uncharged tensile specimens failed

in an entirely ductile mode, readily distinguished

even by eye from that of either the failed pipe

or the charged specimens

.

ACKNOWLEDGEMENT

The authors wish to acknowledge Mr. Donald E. Harne for

the extensive metallographic , photographic, and mechanical

testing work that he did in conjunction with this examination.

REFERENCES

1. "On some remarkable Changes produced in Iron and Steel

by the Action of Hydrogen and Acids" by W. H. Johnson,

Proc. Royal Soc . (London) 23_, p. 168 ff, Jan. 14 , 1875.

2. Metals Handbook, 8th Ed., Vol. 1, 1961, p. 1165

American Society for Metals.

- 16 -

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Figure 3. Photographs of Typical Subsize Impact

(A) and Short Tensile (B) Specimens.

Both specimens are notched at location

2. Measurements of reductions of area

and elongation were made on the tensile

specimens at locations 1, 2, and 3.

About IX.

Figure 4. Matching Corroded Fracture Surfaces at

Locations C and C. Root of thread is

indicated by the arrows D. The corroded

region extends about 1/3 down the fracture

surface from the root of the thread.

About 5X.

' 1 i

7^-

j.

(a)

(b)

Figure 5. Photomicrographs of Longitudinal Cross

Section of Pipe. Two iron-zinc intermetallic layers on the outer surface

of the pipe are shown just above line

D-D on each photomicrograph. About 10 OX.

(a) General microstructure . Etched

with 5% Nital.

(b) Inclusion content. Unetched.
I

1
Figure 6. SEM Fractographs at Location A of Figure 1.

(a) Fracture surface is above line A-A, side

of thread below. 35X

(b) Region of fracture surface at higher

magnification. Fracture is predominantly

by transgranular quasi-cleavage mode, but

some small amounts of dimpled rupture are

shown at arrows. 350X


Figure 7. SEM Fractographs at Location C of

Figure 1

.

(a) Junction of corroded and "fresh"

regions of fracture surface after

iiltrasonic cleaning. The face of

the pipe thread is located below

line A-A, a corroded region of

fracture surface between lines

A-A and C-C , and "fresh" fracture

above line C-C. Arrow U shows

. where the ultrasonic cleaning

removed some corrosion product.

60X

(b) Junction of corroded and "fresh"

regions of fracture surface before

ultrasonic cleaning. "White"

deposit below line C-C is a corrosion product that does not

conduct electricity readily and

consequently is charged by the

electron beam of the SEM. 33X

(c) Field near the center of the

"fresh" fracture region at higher

magnification, showing predominantly transgranular quasicleavage, with some evidence of

dimpled rupture (between arrows)

.

130X



Figure 8. Macroscopic Appearance of Several

Subsize Charpy V-Notch Impact

Specimens After Testing.

Magnification about 3X.

(a) Specimens 1, 4, 2, and 5 were

tested at or below room

temperature and show faceted

fracture surfaces.

(b) Specimens 3 and 6 were tested

at or above 190** F and did

not fracture.


Figure 9. SEM Stereo-Pair Fractograph of Impact

Specimen No. 5. Charged with hydrogen,

tested at 35° F. Fracture appearance

is representative of specimens 1 , 2

,

and 4 . Line A-A is the root of the

notch. 21X

Figure 10. SEM Fractographs Taken of Charged and

Uncharged Impact Specimens.

Both fractographs show the predominantelytransgranular quasi-cleavage fracture mode

observed on all impact specimens tested at

room temperature and below. Selected local

sites (at arrows) show dimpled rupture. lOOX

(a) Charged. Specimen No. 5.

(b) Uncharged. Specimen No. 1.


Figure 11. Fractures of Notched Tensile

Specimens. One inch gage length.

4X

(a) Specimen No. 3. Uncharged.

Tested at 75° F in air.

(b) Specimen No. 2. Uncharged.

Tested at 35** F in water.

(c) Specimen No. 4. Charged.

Tested at 75° F in 5% H2SO4

aqueous solution.

( (d) Specimen No. 1. Charged.

Tested at 35** F in aqueous

5% H HO4.



Figure 12. Fractures of Notched Tensile Specimens.

Two inch gage length. 4X

(a) Specimen No. 8. Uncharged.

Tested at 75° F in air.

(b) Specimen No. 5. Uncharged.

Tested at 35** F in water.

(c) Specimen No. 14. Charged.

Tested at 75*» F in IM, Na2S0^.

(d) Specimen No. 6. Charged.

Tested at 35° F in aqueous

5% H2SO4.


(a) (b)

Figure 13. Fractures of Unnotched Tensile Specimens.

Two inch gage length. 4X

(a) Specimen No. 7. Uncharged. Tested at

75° F in air.

(b) Specimen No. 11. Charged. Tested at

75° F in aqueous 5% H2SO4


Figure 14. SEM Fractographs of Uncharged NotchedTensile Specimen No. 2. Features are

typical of all uncharged tensile

specimens examined in the SEM (Nos. 2,

1 , and 8 )

.

(a) Stereo pair showing the ductile

fracture mode at low magnification.

The region between the lines A-A

and B-B is the notch deformed

during testing. 13X

(b) The ductile fracture mode (dimpled

rupture) shown at higher magnification

in region near center of fracture

surface. 650X


Figure 15. SEM Fractograph of Charged Unnotched Tensile

Specimen No. 11. Stereo pair shows the

comparatively brittle ( transgranular quasicleavage) mode which was dominant in all of

the charged tensile specimens examined in

the SEM (Nos. 1, A, 11, and 14). 19X

(a)

(b)

Figure 16. SEM Fractographs of Notched and Unnotched

Tensile Specimen in the Charged Condition.

The fracture surfaces and features of these

specimens are quite similar, being predominantely transgranular quasi-cleavage

.

Dimpled rupture occurring only at local

sites is shown by the arrows

.

(a) Notched Specimen No. 4. IIOOX

(b) Unnotched Specimen No. 11. 550X


Figure 17. Photomicrographs of Galvanized Coating

Observed on Second Thread from the

Fracture Surface. Specimen taken from

near location C of Figure 1.

(a) Location of galvanized coating on

tip of second thread from fracture

face is shown by arrow A. The

fracture face is shown by arrow B.

5% Nital etch. lOX

(b) Crest of second thread from fracture

face (shown in a) . Etched very

lightly with picral etch. 160X

(c) Crest of second thread from fracture

face at higher magnification. Two

layers of iron-zinc intermetallic

coating the outer surface of the

section can be detected and are

shown by arrows C. Etched very

lightly with picral. 4 00X


Appendix A: Interrupted Tensile Test Results Using 1¥.

isia2S0^ as the Electrolyte. Specimen No. 14.

Applied Load, Observed Load Upon

When Test Relaxation of

Date Time Interrupted Applied Load

11/30/72 9:15 A.M. 2080 1900

12:40 P.M. N.C. 1900

2:00 P.M. N.C. 1900

2:50 P.M. N.C. 1900

3:00 P.M. 2200 (a)

4:35 P.M. N.C. 2140

17/01/72 8:45 A.M. N.C. 2090

8:45 A.M. 2400 2090

9:45 A.M. 2450 (b) 2330

1:00 P.M. N.C. 2320

2:25 P.M. N.C. 2305

4:30 P.M. N.C. 2305

12/04/72 8:50 A.M. N.C. 2275

12:20 P.M. N.C. 2275

1:55 P.M. N.C. 2275

4:00 P.M. 2330 2310

12/05/72 8:50 A.M. N.C. 2310

12:45 P.M. N.C. 2310

12:45 P.M. 2360 2360

2:20 P.M. N.C. 2360

3:35 P.M. 2450 (a)

4:35 P.M. 2500 (a)

12/06/72 8:40 A.M. 2550 2480

10:00 A.M. 2600 2510

12:45 P.M. 2650 2500

3:30 P.M. 2700 2570

4:30 P.M. 2750 2520

12/07/72 8:40 A.M. 2800 2640

9:50 A.M. 2840 2700

11:00 A.M. 2900 2750

12:15 P.M. 2950 2830

1:15 P.M. 3000 2830

2:25 P.M.. 3020 (Specimen Broke)

N.C. - i.e. test held at constant cross-head displacement

(a) Not observed

(b) Holding for week-end.

roRM NBS-n4A (1-71)

BIBLIOGRAPHIC DATA No. SHEET NBSIR 73-111 i

3, Recipient's Accession No.

4. TITLE AND SUBTITLE

REPORT NO. 10: EXAMINATION OF A FAILED 2-INCH

DIAMETER GALVANIZED STEEL PIPE: VALLEY GAS COMPANY,

>700NS0CKET, RHODE ISLAND.

5. Publication Dace February, 1973

W. 1^ C L I L'li. ti 11 ^

^fcl il i jC .J L . l^. . V^./^t.

7. AUTHOR(S)

C.G. Interrante, G.E. Hicho, and M.L. Picklesimer

C. Performing Or^an iz^ NBSIR 73-111

9. PERFORMLN'G ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

tU. rtO)QC\./ laSiC nOiffc Uilll AO. 312041S

11. Contract/Grant No.

1 12. Sponsoring Organization Name and Address OFFICE OF PIPELINE SAFETY

Department of Transportation

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13. Type of Report &: Period Covered p^ii^^^e

Analysis Report

14. Sponsoring Agency Coco

15. SUPPLEMENTARY NOTES

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

j bibliography or literature survey, mention it here.) Two sections of a fractured 2-inch diameter galvanized steel pipe used | in natural gas residential distribution service in Woonsocket, Rhode j Island were examined to determine the mode of fracture. Notched tensilq

and impact specimens made from the pipe v/ere tested in the uncharged and

hydrogen-charged conditions. The fractures in the uncharged tensile j specimens were ductile, occurring by dimpled rupture. Those in the j impact specimens, the hydrogen-charged tensile specimens, and the j original pipe were "brittle" , occurring with very little to no I ductility by transgranular quasi-cleavage . Since the fracture 'l features of the original pipe fracture were duplicated by the hydrogen- i charged notched tensile specimens, it was concluded that the galvanized i pipe failed in service due to (1) an external force, (2) s-cress j concentration by the junction of the root of the pipe thread and the j face of the 9 0 degree pipe elbow, and (3) a low ductility at the root

of the pipe thread probably as a result of charging with hydrogen

produced by the galvanic current from the corrosion protection of the

galvanized coating.

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

brittle fracture; galvanized steel pipe; hydrogen charging; natural

gas service.

18. AVAILABILITY STATEMENT

I I UNLIMITED.

[rOK OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

46

22. Price
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