REPORT NO. DOT/MT^B/OHMR-78-4

Analysis of the Pneumatic Burst of a

Large Seamless Steel Pressure Vessel

in Natural Gas Service

by

Bruce W. Christ

Fracture and Deformation Division

Center for Materials Science

National Bureau of Standards

U.S. Department of Commerce

Washington, D.C. 20234

.< oftr*.

December, 1978

Final Report

Issued March 1 979

Document is available to the U.S. public through

the National Technical Information Service,

Springfield, Virginia 22161

Prepared for

U.S. DEPARTMENT OF TRANSPORTATION

MATERIALS TRANSPORTATION BUREAU

Office of Hazardous Materials Regulations

Washington, D.C. 20590
NOTICE

The contents of this report reflect the views of the author who is

responsible for the facts and the accuracy of the data presented herein.

The contents do not necessarily reflect the official views or policy of

the Department of Transportation. This report does not constitute a standard, specification or regulation.

This document is disseminated under the sponsorship of the Department of

Transportation in the interest of information exchange. The United States

Government assumes no liability for its contents or use thereof.
NBS1R 78-1589

ANALYSIS OF THE PNEUMATIC

BURST OF A LARGE SEAMLESS

STEEL PRESSURE VESSEL IN NATURAL

GAS SERVICE

Bruce W. Christ

Fracture and Deformation Division

Center for Materials Science

National Measurements Laboratory

National Bureau of Standards

U.S. Department of Commerce

Washington, D.C. 20234

Failure Analysis Report

December, 1978

Issued March 1 979

Prepared for Office of Hazardous Materials Regulations

Materials Transportation Bureau

Department of Transportation

Washington, D.C. 20590

Contract No. DOT AS-40034
NOTICE

The contents of this report reflect the views of the author who is

responsible for the facts and the accuracy of the data presented herein.

The contents do not necessarily reflect the official views or policy of

the Department of Transportation. This report does not constitute a standard, specification or regulation.

This document is disseminated under the sponsorship of the Department of

Transportation in the interest of information exchange. The United States

Government assumes no liability for its contents or use thereof.
Technical Report Documentation Page

1 . Report No.

DOT/RSPA/MTB- 78/4

2. Government Accession No. 3. Recipient’s Catalog No.

4. Title and Subtitle Analysis of the Pneumatic Burst of a Large Seamless

Steel Pressure Vessel in Natural Gas Service

5. Report Date March 1979

6. Performing Organization Code

8. Performing Organization Report No.

NBSIR 78-1589

7. Author^ s) Bruce W. Christ

9. Performing Organization Name and Address Fracture and Deformation Division

National Bureau of Standards

Washington, DC 20234

10. Work Unit No. (TRAIS)

11. Contract or Grant No. DOT AS 40034

13. Type of Report and Period Covered

12. Sponsoring Agency Name and Address Office of Hazardous Materials Regulation

Materials Transportation Bureau

U.S. Department of Transportation

Washington, DC 20590

14. Sponsoring Agency Code D0T/MTB/0HMR

15. Supplementary Notes

16. Abstract /\£ the request of the Office of Hazardous Materials Regulation (OHMR) - U.S. Department of Transportation (DOT), the NBS Fracture and Deformation Division

carried out a metallurgical evaluation and fracture analysis of a pneumaticallyburst seamless steel compressed gas cylinder which was used for over- the- highway

transport of natural gas. The relevant specification for this pressure vessel is

Specification DOT 3T. The cylinder was 34-feet long by 22-inches diameter, with a wall thickness of about 1/2-inch. It was new and had been in service for about a week. The cylinder burst during filling. Reported burst pressure was believed to be

2,200 to 2,500 psi. Expected burst pressure was 6,800 psi. Tensile strength of this

quenched and tempered low alloy steel pressure vessel (0.44 carbon - 0.92 manganese - 0.98 chromium - 0.20 molybdenum) ranged between 136,200 and 170,500 psi. The cylinder

ruptured due to environmentally-assisted cracking. The fracture originated at a part-through crack in a region of high tensile strength. At the time of rupture, the

fracture origin was about 1.3-inches long and penetrated half the wall thickness from

the inside. Four other part-through cracks were found in the vicinity of the fracture

origin. They were linked to tiny circumferential cracks (blister cracks) in rather

hard microzones (HRC 40 to 45) within about 0.01-inches of the inside wall. Up to 50C

ppm by volume of hydrogen sulfide was detected in gas samples taken from the natural

gas wells after the incident. Evidence of sulfur was found on the fracture origin anc on the inside wall. It was concluded that the rupture was due to the internal gas

pressure acting on the region of the fracture origin. It is probable that the fracture origin and the other part- through cracks developed from service stresses acting

on , bl ister__c_racks. The blister cracks prqbably formed as diffusihle hydrogen (r.nnt. d)

17. Key Words Pressure vessel. Pneumatic rupture.

Natural gas. Steel, Hydrogen sulfide, DOT

Specification 3T, Stress corrosion

18. Distribution Statement This document is available to the U.S.

public through the National Technical

Information Service, Springfield, Virginia

22161

19. Security Classif. (of this report) Unclassified

20. Security Classif. (of this page)

Unclassified

21. No. of P ages 120

22. Price $6.50

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized
Technical Report Documentation Page

DOT/RSPA/MTB-78/4 Analysis of the Pneumatic Burst of a Large Seamless

Steel Pressure Vessel in Natural Gas Service

By: Bruce W. Christ

Fracture and Deformation Division

National Bureau of Standards

Washington, DC 20234

16. Abstract (cont'd)

from the decomposition of hydrogen sulfide interacted with the hard microzones close

to the inside wall. It is likely that the rupture occurred in such a short time

due to the combination of high hydrogen sulfide concentration in the natural gas

and rather high-hardness microzones in the steel near the inside wall.

IV
ANALYSIS OF THE PNEUMATIC BURST

OF A LARGE SEAMLESS STEEL PRESSURE

VESSEL IN NATURAL GAS SERVICE

by

Bruce W. Christ

Fracture and Deformation Division

National Bureau of Standards

Washington, DC 20234

V

TABLE OF CONTENTS

Page

1. GENERAL INFORMATION 1

1.1 Reference 1

1.2 Background 1

1.3 Identification and Description 5

2. PURPOSE 7

3. COMPARISON OF TEST RESULTS WITH REQUIREMENTS OF SPECIFICATION 8

DOT 3T

3.1 Chemical Analysis 8

3.2 Tensile Properties 9

3.3 Charpy Tests and Results 12

3.4 Wall Thickness 13

3.5 Surface Hardness of the Cylindrical -Section 14

4. FRACTURE DESCRIPTION 15

4.1 Location and Description of Fracture Origin 15

4.2 Fracture Path and Its Relationship to the Site 17

Configuration After the Incident

4.3 Other Cracks Near the Fracture Origin 20

4.3.1 Two Types of Cracks 20

4.3.2 Etching to Reveal Banding 21

4.3.3 Microhardness in Banded Regions 22

4.3.4 Explanation of Type C Crack Formation 24

4.3.5 Explanation of Type L Crack Formation 25

4.3.6 Summary of Cracking in Cylinder 9959 26

4.4 Examination of Inside Wall 27

4.4.1 Surface Topography 27

4.4.2 Evidence of Sulfur 30

4.5 Examination of Fracture Origin 34
TABLE OF CONTENTS (continued)

Page

j. MATERIAL DESCRIPTION 38

5.1 Grade of Steel and Heat Treatment 38

5.2 Microstructure from Heat Treatment 39

5.3 Other Features of Microstructure - Decarburization 39

and Banding

5.4 Characterization 40

5.4.1 Background 40

5.4.2 Kj

S(

-. c Measurement in the Laboratory 42

6. GAS ANALYSIS 45

6.1 Objective of Gas Analysis 45

6.2 Hydrogen Sulfide Analysis 45

6.3 Moisture Analysis 48

6.4 Other Chemical Species 48

7. HARDNESS SURVEY 50

8. STRESS ANALYSIS VIA FRACTURE MECHANICS 52

8.1 Review of the Fracture Origin 52

8.2 Tiffany and Masters Equation 52

8.3 Battelle Equation 53

9. DISCUSSION 57

10. CONCLUSIONS 60

11. ACKNOWLEDGEMENTS 62

12. BIBLIOGRAPHY 64

viii
ANALYSIS OF THE PNEUMATIC RUPTURE OF A

LARGE PRESSURE VESSEL IN NATURAL GAS SERVICE

ABSTRACT

At the request of the Office of Hazardous Materials Regulation (OHMR) - U.S. Department of Transportation (DOT), the NBS Fracture and Deformation

Division carried out a metallurgical evaluation and fracture analysis of

a pneumatically-burst seamless steel compressed gas cylinder which was used for over-the-highway transport of natural gas. The relevant

specification for this pressure vessel is Specification DOT 3T. The

cylinder was 34-feet long by 22-inches diameter, with a wall thickness

of about 1/2-inch. It was new and had been in service for about a week.

The cylinder burst during filling. Reported burst pressure was believed

to be 2,200 to 2,500 psi. Expected burst pressure was 6,800 psi. Tensile

strength of this quenched and tempered low alloy steel pressure vessel

(0.44 carbon - 0.92 manganese - 0.98 chromium - 0.20 molybdenum) ranged

between 136,200 and 170,500 psi. The cylinder ruptured due to environmentallyassisted cracking. The fracture originated at a part-through crack in a region of high tensile strength. At the time of rupture, the fracture

origin was about 1.3-inches long and penetrated half the wall thickness

from the inside. Four other part-through cracks were found in the vicinity

of the fracture origin. They were linked to tiny circumferential cracks

(blister cracks) in rather hard microzones (HRC 40 to 45) within about 0.01-

inches of the inside wall. Up to 500 ppm by volume of hydrogen sulfide

was detected in gas samples taken from the natural gas wells after the

incident. Evidence of sulfur was found on the fracture origin and on the inside wall. It was concluded that the rupture was due to the internal

gas pressure acting on the region of the fracture origin. It is probable

that the fracture origin and the other part-through cracks developed from

service stresses acting on blister cracks. The blister cracks probably

formed as diffusible hydrogen from the decomposition of hydrogen sulfide

interacted with the hard microzones close to the inside wall. It is likely

that the rupture occurred in such a short time due to the combination of

high hydrogen sulfide concentration in the natural gas and rather highhardness microzones in the steel near the inside wall.

IX
; v,

-

!

.
1. GENERAL INFORMATION

1 . 1 Reference

U.S. Department of Transportation, Materials Transportation Bureau,

Office of Hazardous Materials Regulation, Engineering Branch, 2100 -

2 nd Street, S.W., Washington, DC 20590. This report was prepared at

the request of Mr. Arthur J. Mallen, Engineering Branch Chief, in a

letter dated January 5, 1979. Contract DOT AS-40034 supported this

work.

1 .2 Background

The information in the following four paragraphs was provided by

Mr. A. J. Mallen.

On October 27, 1977 at about 6:30 p.m., a seamless steel compressed

gas cylinder burst while it was being filled with natural gas at the

wellhead. The incident occurred near Litchfield, Kentucky in an isolated

rural setting.

The DOT Serial Number stamped on the burst cylinder was 9959. The

burst cylinder was new. It had been manufactured in July, 1977. The

history of the cylinder between July and October was not recorded.

The cylinder was one of twelve cylinders mounted in fixed positions

on a "tube trailer" used for transporting compressed gases over the

public highways. The trailer permit was issued by the Kentucky State

Fire Marshal's Office on October 6, 1977. The relevant specification for

manufacture of the cylinder was Department of Transportation Specification

3T, appearing in Part 178, Subpart C, Section 178.45 of the Code of

Federal Regulations, Title 49.^ Mr. Arthur Mallen supplied NBS with

l
2 copies of the test reports prepared by the independent inspector approved

by the Department of Transportation to carry out inspections and testing

during manufacture. The revised test report was dated July 25, 1977.

The initial report was dated July 6, 1977.

Following the incident, DOT-OHMR issued rule making to amend the

regulations to incorporate a definition of methane, because of evidence

that the regulations were not being applied as intended. This amendment

emphasized that whereas the regulations (Code of Federal Regulations

- Title 49 - Subparagraph 173. 302- (a)(3)) authorize shipment of methane

in DOT specification cylinders 3AX, 3AAX and 3T, the regulations do not

authorize shipment of natural gas.

The further information reported in this section was gathered

by B. W. Christ from a number of individuals who were involved with the

incident. Some were on the scene shortly after the incident occurred.

Others participated in the clean-up during the following weeks. These

individuals are listed below.

Mr. Gilbert Ellis

Assistant State Fire Marshal

Frankfort, Kentucky

Mr. Rodney Raby

Deputy State Fire Marshal

Frankfort, Kentucky

Mr. Gobel Mattingly

Compressed Gas Corporation

Litchfield, Kentucky

Mr. William Piehl

Western Sales and Testing

Amarillo, Texas

Mr. Maurice Bolinger, Jr.

Gladstone Laboratories

Cincinnati, Ohio

Mr. Roger McCombs

Gladstone Laboratories

Cincinnati, Ohio

The trailer containing the cylinder which ruptured was identified

as trailer number T-256. It was standing on a gravel pad and was

being filled via a pipe connection to a large compressor which was

pumping natural gas from two wells located about two miles away. Well

pressure was about 400 psi, which was the pressure carrying gas to the

compressor through the gathering lines. All twelve cylinders on the

2
trailer were connected via a common manifold. A second trailer, identified

as trailer number T-254, was standing on the gravel pad alongside the

first. It was awaiting filling.

The charging pressure when the burst occurred was believed to be

2,200 to 2,500 psi. Minimum expected burst pressure was 6,800 psi.* The large

difference in pressures suggests the presence of a discontinuity somewhere

in the cylinder.

Eleven large pieces formed as a result of the burst. Pieces were

flung from 200 to 2,000 feet. Besides the burst cylinder, six others

came off trailer T-256. Each remained intact, except for the manifolding.

One cylinder landed beneath trailer T-254. Ingition of the natural gas

occurred at some time during the incident. Flow of the natural gas was

shut off at the compressor shortly after the burst. However, the eleven

cylinders from trailer T-256 continued jetting natural gas, which was

burning in some cases.

1 *Specification DOT 3T does not contain an explicit statement about design

burst pressure. However, a minimum burst pressure for a DOT specification

3T cylinder can be estimated by substituting the minimum allowed tensile

strength, 135,000 psi according to Section 178.45-7, into the BachClavarino formula for wall stress found in Subparagraph 1 78. 45-7 ( b)

,

C - P(1.3D

2 + 0.4d2

) D

2

- d

2

where S = wall stress (hoop stress) in psi = 135,000 psi

P . =

9 as pressure in psi at burst

D = outside diameter in inches (assumed to be 22 inches)

d = inside diameter in inches (assumed to be 21.06 inches)

t = wall thickness in inches = = -2-

~ 2—Q- = 0.470 inches

This estimate is only approximate because the Bach-Clavarino formula is derived

from elastic, not plastic, analysis.

> . , s(p

2

-d

2

) min

1.3D

2 + 0.4d

2 135.000 (22

2 -21.06 2

) . 6>800 , 1.3 (22 T + 0.4 (21.06)*

The value of P .. will vary slightly from cylinder to cylinder, depending on exact diameter and wall thickness.

3
Each cylinder was equipped with rupture disks, in accord with the

regulations. A disk was installed in each end of each cylinder and was

set to operate at a pressure of about 3,800 psi. Most of the rupture

disks were recovered after the incident. None of those recovered had

3

ruptured. Testing of the recovered disks verified that their operating

pressure was about 3,800 psi.

A diagram showing the incident site afterwards appears in Figure 1.

The layout of the site itself was traced from a surveyor's diagram

provided to NBS by the Kentucky State Fire Marshal's Office. The

positions of the pieces of the burst cylinder were noted by Mr. Rodney

Raby* of that office. The location of two pieces, RR-18 and RR-19 was

not established.

Clean up of the incident site was not immediate. Pieces of the

burst cylinder lay where they landed in cornfields for about a week.

Eventually, all pieces were dragged to the gravel pad, then loaded on a

truck and taken to a bonded warehouse in Louisville, Kentucky.

The weight of the eleven pieces trucked to Louisville on November

2 4, 1977 was 3694 pounds, as compared to the test-report tare weight for

the as-manufactured cylinder of 3642 pounds. This tare weight appears

in the revised test report dated July 25, 1977. The difference may be

attributed in part to slight inaccuracy in the weighing and/or adhering

soil, which was conspicuously evident.

On December 28, 1977, the eleven pieces were trucked to the Gladstone

Laboratories in Cincinnati, Ohio for inspection and analysis. Following

a request by Mr. Arthur Mallen, Dr. Bruce W. Christ of NBS visited

Gladstone Laboratories on January 5, 1978 to provide direction and

*Mr. Rodney Raby participated in the recovery of the eleven pieces. His

initials are the source of the "RR" designation.

4
assistance in their investigation. Eventually a cooperative investigative

effort developed between NBS and the Gladstone Laboratories.

By March, 1978, material from cylinder 9959 and from two other

cylinders was being transmitted to NBS for metal! ographic and fractographic

examination, mechanical properties measurements and chemical analysis.

Seven preliminary NBS memo reports related to the burst of cylinder

9959 were submitted to the Office of Hazardous Materials Regulation

between January 5, 1978 and July 19, 1 978.

Also in March, 1978, Battel le Memorial Institute of Columbus, Ohio

began looking into the feasibility of shipping natural gas from the

wellhead in DOT 3T specification trailer tubes for Goodyear Corporation,

Akron, Ohio. Mr. William Maxey surveyed the eleven pieces of the burst

tube 9959 at the Gladstone Laboratories and obtained some material for

metal 1 ographic and fractographic examination and for mechanical properties

testing. Information was exchanged freely between NBS and Battel le. A

5 report on the Battel! e findings was issued.

1.3 Identification and Description

The seamless steel gas cylinder which burst is identified as follows.

according to information supplied by Mr. A. J. Mallen. Some of the

information was obtained from the revised July 25, 1977 test report:

Specification

Test Date

Date of Test Report

Serial Number

Identifying Symbol

Minimum Sidewall

Marked Service Pressure

Heat Code

Type of Steel

Water Capacity

Size

DOT 3T

June 19, 1977

July 6, 1977 (Original Date)

July 25, 1977 (Revision Date)

9959

U.S.

J.B.K.

0.470 inches

2725 psi

A29

Quenched and tempered - ASTM A-372, Class 5, Type E 600 Gallons

22-inches Diameter, 34-feet Length
A photograph of the eleven pieces of cylinder 9959 laid out on

the floor of the Gladstone Laboratories appears in Figure 2a. A diagram

of these pieces appears in Figure 2b. The remainder of trailer T-256

after the incident appears in Figure 3. For comparison, an intact

tube trailer is shown in Figure 4.

Review of color photographs of the pieces of cylinder 9959 and

visual inspection of the pieces laid out as in Figure 2a did not reveal

any signs of heating. Evidently, all pieces escaped significant heating

from the fire which occurred shortly after the cylinder ruptured.

6
2. PURPOSE

1) Evaluate the compliance of cylinder 9959 with those requirements

of specification DOT 3T which relate to:

• steel chemistry

• tensile properties

• Charpy properties

• wall thickness

• surface hardness of the cylindrical section

2) To identify the cause of the burst of cylinder 9959. As will become

evident, this has involved a wide range of investigative activity,

including:

• natural gas sampling and analysis

• fractographic and metal! ographic examination

• electron microprobe analysis

• hardness testing and microhardness testing

• Kj SCC testing (a fracture mechanics stress corrosion test)

• stress analysis via fracture mechanics

7
3. COMPARISON OF TEST RESULTS WITH REQUIREMENTS OF SPECIFICATION DOT 3T

3.1 Chemical Analysis

Chemical analysis was carried out on a specimen cut from piece

RR-12. Drillings were obtained from the piece labeled "Chem" in Figure 5.

One analysis was handled by the Gladstone Laboratories and another was

handled by NBS. In both instances, material was sent to analytical

laboratories which rely on NBS Standard Reference Materials for calibration.

Results appear in Table I. The scatter between the two sets of chemical

composition determinations is not unreasonable for routine chemical

analyses of the same material. Also appearing in Table I is a tabulation

of the specification requirements. It is clear that these results comply

with the chemistry requirements in Section 178.45-5 of Specification DOT 3T.

These results are also consistent with results appearing in the independent

2 inspector's report.

The relatively low level of sulfur reflects favorably on the steelmaking

process. Low levels of sulfur favor good toughness.

The carbon level suggests strong responsiveness to hardening by

quenching and tempering.

Residual elements are cited in Table II. The copper and nickel

contents are well within the guidelines suggested in the American Iron

and Steel Institute's Steel Products Manual , ^ a widely-recognized specification

for the chemistry of steels.

The aluminum level in Table II suggests that deoxidation was

accomplished with aluminum. A benefit from deoxidation with aluminum

is the formation of fine austenite grains (probably ASTM 7-8) which are

8
desirable from the standpoint of heat treatment and mechanical properties.

The low titanium level suggests that titanium was not used for deoxidation.

The nitrogen and oxygen levels are typical for good steelmaking

practice.

Arsenic, antimony, tin and phosphorus are well known for their

7 tendency to promote temper embrittlement. It is thought that interactions

occur between these elements and alloying elements in low alloy steels to

promote embrittlement of grain boundaries. Only trace concentrations

(a few parts per million) are needed. Temper embrittlement of this

particular steel specimen from arsenic, antimony and tin is not

ascertainable, because the concentrations of these elements fall below

the limits of detectability. However, the phosphorous level is close

to the range thought to be critical for promoting temper embrittlement

7 in alloy steels containing chromium and manganese. This point is

mentioned again in Section 4.3.5.

3.2 Tensile Properties

3.2.1 Test Results

Several tensile specimens were prepared and tested in accord with

Section 178.45-13 of Specification DOT 3T. Both longitudinal and

transverse specimens were tested. Gage length was two-inches. Nominal

width of the gage section was either 0.5 or 1.5-inches. Nominal gage

thickness was the full wall thickness. The inside and outside wall

surfaces were not machined.

Pieces of the burst cylinder were distorted, as shown in Figure 2.

Thus, when necessary, material from which test specimens were machined

was flattened at room temperature in a press before machining the tensile

9
specimens. Cold work introduced during catastrophic failure or flattening

would have the tendency to raise the strength of the material. Yield

strength is usually more sensitive to cold work than is tensile strength.

Tensile test specimens were taken from two pieces of the burst cylinder --

piece RR-12 and piece RR- 10. Piece RR-12 was chosen because it contained

one side of the fracture origin. Piece RR- 1 0 was selected because it

was remote from the fracture origin. There was no evidence of heating from

fire or any other source on these pieces.

Tensile test results appear in Table III. There is a large variation

in tensile properties. The range in properties of longitudinal specimens

is listed below. Tensile test results from the Battelle reports are also

1 isted.

Range

Property

NBS Gladstone Battelle

Rectangular Rectangular Rectangular+ Rounds*

Yield Strength (psi

)

Tensile Strength (psi)

% Elong. in two-inches

(t x 1/2)

7o Elong. in two-inches

(t x 1 1/2)

114.6 - 138.0

150.3 - 165.0

13.6 - 16.4

16.0

- 19.8

114.0

- 148.0

136.5 - 170.5

13.0

- 17.5

148.7 105.6 - 143 170.5 139.4 - 162 13.0 12.8 - 18

*Specimens were 0.25-inches diameter by 1-inches long.

+0nly one rectangular specimen was tested.

Results from all three laboratories are consistent with one another in

that they cover about the same range. The range for these results is

broader than is customary in production tensile testing of uniform material,

and it is concluded that these results indicate significant material

variability in cylinder 9959.

3.2.2 Specification Requirements

In Subparagraph 178. 45- 15-(a)-(l ) of Specification DOT 3T, it is

stated that, "The tensile strength may not exceed 155,000 psi." Four of

10
the nine longitudinal tensile strengths listed in Table III exceed

this limit.

Many of the tensile strengths listed in Table III are higher than

2 the tensile strengths cited in the independent inspector's report.

The data in the inspector's report are usually obtained from specimens

taken from a test ring made from the same heat of steel from which the

cylinder is manufactured. The test ring is heat treated along with as

many as five cylinders. Properties measured on specimens from the test

ring presumably are representative of all cylinders made from the same

heat of steel. The observed differences in tensile strength may be

accounted for in part by material variability.

In Subparagraph 178. 45-1 5(a)(2) of Specification DOT 3T, it is

stated that, "the elongation must be at least 16 percent for a two-inch

gage length." This requirement was met for the test specimens in Table

III which had a width of 1 1/2 inches. This width is within the range

specified for tensile specimens and favors high elongation.

All of the elongations listed in Table III are lower than the.

elongations cited in the independent inspector's report. As in the case

of the tensile strength differences, material variability may account

in part for the observed differences in elongation.

For purposes of comparison, two longitudinal tensile test specimens

were obtained from another cylinder, 10041, which was also on trailer T-256.

The specimens were prepared and tested in accord with Section 178.45-13

of Specification DOT 3T. The cross-section was 1.5-inches x 0.480-inches,

and the gage length was two-inches. These specimens were tested along

with the specimens which gave the results in rows 3, 4 and 5 of Table III.

Results appear below:

li
Specimen

Identification

Yield Tensile % Elongation

Strength (psi) Strength (psi ) in Two Inches

1-1

1-2

123.000

122.000

143.000

142.000

22.6

24.6

It is clear that these results conform with the tensile strength and

elongation requirements of Specification DOT 3T.

3.3 Charpy Properties

Full-size Charpy specimens were prepared and tested in accord with

Section 178.45-13 of Specification DOT 3T. Test temperature 0 °F. Both

longitudinal* and transverse* specimens were tested (There is no specification

requirement on transverse Charpy specimens). Specimens were taken from

two pieces of the burst cylinder — piece RR-12 and piece RR-10E. These

pieces were selected to provide data from material close to the fracture

origin and remote from it. There was no evidence of heating on these

pieces. By judiciously selecting reasonable flat pieces, it was not necessary

to flatten the material from which test specimens were machined. The

effect on the Charpy data of the flattening during the rupture is unknown.

Results appear in Table IV. The data indicate that the material

from piece RR-10E is somewhat tougher than the material from piece RR-12.

As expected, the transverse specimens show less toughness than the longitudinal

specimens. This result arises because the crack in transverse specimens

propagates parallel to elongated sulfide inclusions and regions of banding,

both of which are zones of low fracture resistance. These features of

the microstructure develop parallel to the drawing direction during

fabrication of the seamless tube and are not easily eliminated.

*Longitudinal Specimen: Axis of specimen parallel to axis of cylinder and

crack propagating circumferentially.

Transverse Specimen: Axis of specimen in circumferential direction and

crack propagating parallel to axis of cylinder.

12
Requirements of specification DOT 3T for full size, longitudinal

Charpy specimens appear in Table IV. Absorbed energies for the longitudinal

Charpy specimens from piece RR-12 do not meet the specification requirements,

whereas results on the longitudinal specimens from piece RR-10E do

meet the specification requirements.

Energies for longitudinal Charpy specimens from piece RR-12 are lower

than energies cited in the independent inspector's report. On the other

hand, energies for longitudinal specimens from piece RR-10E are comparable

with results in the inspector's report.

These Charpy test results are comparable to results in the Battel le

Report.

5

3.4 Wall Thickness

In the revised test report, July 25, 1977, the minimum wall thickness

was cited as 0.470-inches. This is the required minimum wall for marked

service pressure of 2725 psi, as determined under Section 178.45-7 of

Specification DOT 3T. Wall thickness measurements were made with a

hand-held micrometer at various positions along the fracture surface

evident in Figure 2a. Typical measurements ranged between 0.458 and

0.472-inches. Slight thinning may occur close to the slant fracture

surface due to stretching during ductile fracture. Hence, these measurements

may not give an accurate indication of the as-manufactured wall thickness.

It is noted that these measurements are generally below the required

minimum wall, 0.470-inches.

Wall thickness measurements also show up in Table III, where each

first number in the third column represents the full wall thickness. These

thicknesses are representative of the as-manufactured thickness, since

they are from regions remote from the slant fracture surface. These

thicknesses are generally below the required minimum wall, 0.470-inches.

13
3.5 Surface Hardness of the Cylindrical Section

Surface hardness was measured in accord with Subparagraph 178.45-15

(a)(4) of Specification DOT 3T. Results appear below.

Location

Rockwell C-Scale Hardness

Below Decarburized Layer

(0.033-inches below surface)

Piece RR-12 (near

fracture origin)

34.5

Piece RR-17 33

These results fall below the specification allowable maximum of Rockwell

C-36, indicating that this requirement of the specification was met.

- 14 -
4. FRACTURE DESCRIPTION

4. 1 Location and Description of Fracture Origin

The cylinder which ruptured was in the top row of a four-wide by

three-high array of cylinders on the tube trailer. It was on the outside

at the right, as viewed from the rear of the trailer. Figure 6. The

serial number stamped in the cylinder is 9959. For convenient reference

to positions of cylinders on the tube trailer, the cylinders are

numbered 1 through 12, as shown in Figure 6. Thus, the cylinder

which ruptured is also designated as cylinder 4.

The fracture origin was located in the sidewall of the cylindrical

section. It was first identified by R. McCombs of the Gladstone Laboratories,

and subsequently verified by B. W. Christ of the National Bureau of

Standards and W. A. Maxey and T. Groeneveld of Battel le. It was about

12.5 feet from the front head, and between the 6 and 7 o'clock positions*,

as shown in Figure 6. THe fracture origin was between pieces RR-12

and RR-16.

The edge of piece RR-12 containing the fracture origin appears in

Figure 5. It is located along the fracture edge where the words "saw cut"

appear upside down. One side of the fracture origin was removed from

piece RR-12 by sawing along the "saw cut" line. The other side has

been preserved intact on piece RR-16. Care was exercised in the sawing

so that coolant from the band saw did not flow over the fracture surface.

A heavy deposit of various products coated the entire fracture surface.

*12 o'clock is located at the top of the cylinder, when the tube trailer is

viewed from the rear.

15
The fracture origin was a thumbnail-shaped part-through crack about

1.3-inches long and 0.25-inches deep. It reached from the inside wall

to the mid-wall position. The long axis of the crack was approximately

parallel to the longitudinal axis of the cylinder.

It is possible that the fracture origin was not a single crack,

but rather, three cracks that linked up, as suggested in Figure 7.

Figure 7a is an overall view of the 17-inch long segment sawed from

RR-12. This segment was about 1/2-inch wide at one end and 3/4-inches

wide at the other. The outside wall is facing the viewer in Figure 7a.

A view directly down on the fracture origin appears in Figure 7b.

Figure 7c is an artist's sketch clarifying Figure 7b and showing the position

of the fracture origin on the 17-inch long segment. The step in the

fracture origin between labels "1" and "2" is suggestive of 2 cracks.

Yet a third crack is suggested by the asymmetry of the fracture origin

in the region labeled "3".

Tiny shear lips were noted along the edge of the fracture origin which

intersected the inside wall. These show up in Figures 8a and 8b. The

presence of a shear lip suggests internal crack growth, with the shear

lip forming at the time of catastrophic separation. The shear lip is

probably in the decarburized layer (see Section 5.3), which was detected

along the inside wall of the cylinder.

Other notable features of the fracture origin include: 1) "flat

fracture" radially through the wall thickness at the position of the

thumbnail-shaped part-through crack, and 2) a transition from flat

fracture to full slant fracture within a few inches longitudinally on

either side of the fracture origin. Slant fracture indicates ductile

separation, whereas flat fracture indicates separation with little or no

ductility.

16
4.2 Fracture Path and its Relationship to the Site Configuration After

the Incident

The primary fracture path is marked by the heavy black lines in

Figure 2b. Secondary fracture paths are indicated by the dashed lines.

They probably developed in response to bending loads on an alreadyfractured section of the cylinder.

Numerous flat spots were found along the entire fracture surface of

cylinder 9959. The flat spots were usually 1-2 inches long. Their

presence shows an intermittent transition from ductile (slant) to brittle

(flat) fracture and may indicate numerous small brittle zones throughout

cylinder 9959. These observations are consistent with a finding in

5 the Battel le report, namely that drop weight tear test data "... indicate

a variable material with no consistent transition temperature. 11

A fast-running shear fracture runs at a velocity of about 1000 to

22

3000 feet per second. Taking the cylinder length as 34 feet and positioning

the fracture origin at 12.5 feet from the front, it is estimated that the

primary crack running at 1000 feet/second reached the front of the cylinder

in about 12.5 milliseconds and the rear in about 21.5 milliseconds.

Heavy flanges threaded onto the outside of the necks at each end

of the cylinder to hold it to the trailer bulkhead acted as crack arrestors.

When the primary crack reached each head, it followed a circumferential

path along internal threads machined in the neck for manifolding and

turned back into the cylinder. There was no evidence that the fracture

originated at the internal threads in either head.

Near each head, the fracture path followed forging seams which

formed from gathered metal that accumulated when the heads were forged.

Figure 9a. Such forging seams were found at the heads of other

17
cylinders on trailers T-256 and T-254, as determined by visual examination

through the 3 1/2-inch diameter openings at each head. Some forging

seams were metal lographically examined in cross sections taken from the

front head of cylinder 10041. These seams were about 1/32-inches deep

and were partially filled with a black substance which was not identified.

A typical as-polished cross-section is shown in Figure 9b. Etched sections

showed that the decarburized surface layer (see Section 5.3) followed the

contour of the forging seams, verifying that the contour developed during

forging and not afterward. There was no evidence that the fracture of

cylinder 9959 originated in the forging seams.

Reference to the incident site diagram in Figure 1 shows that the

front head was launched frontward across the gravel road in a line almost

parallel to the longitudinal axis of the tube trailer as it sat on the

gravel pad. The rear head was propelled rearward at an angle to the tube

trailer axis. The reaction force from the rear head's launch may have

swung about the rear of the tube trailer and helped to tip it on its

side. It is probable that the thrust of the decompression wave propelled

the front and rear heads. Piece RR-16 flew along a line almost perpendicular

to the longitudinal axis of the cylinder and piece RR-12 landed somewhat

rearward to the trailer. These observations are consistent with the position

of the fracture origin.

Cylinders 1, 2 and 3, Figure 6, were flung from the trailer in the

direction of the compressor. The trailer bulkhead is designed with a

separate piece at the top to accomodate installation of each cylinder in

the top row. Figure 3. Each separate piece is bolted in place. The

sidewards and upwards thrusting forces applied to cylinder 3 from the

18
longitudinal opening flap of cylinder 4 are probably the primary forces

which propelled cylinders 1, 2 and 3 to their final positions. There

were numerous circumferential scrape marks on the front half of cylinder 3

between the 10 o'clock and 6 o'clock positions. The bolting of the

separate pieces in the front bulkhead. Figure 3, was apparently easily

overcome by the rupturing forces, allowing the front ends of cylinders

3, 2 and 1 to readily separate and lift from the trailer. A sidewards

impact from the opening flap of cylinder 4 along the front half of

cylinders 1 and 2 would spin them end for end, and result in their

orientations as shown in Figure 1. The positions in which cylinders 1,

2 and 3 landed are consistent with the position of the fracture origin.

It is likely that the impacts from the frontward opening flap of

cylinder 4 along cylinder 3 led to the flexing and bending which developed

more secondary crack paths at the front than at the rear of cylinder 4.

The line labeled ABC in Figure 2b shows up clearly as a "hit mark" on

cylinder 8, Figure 10a. As expected from the relative positions of cylinders

on the trailer shown in Figure 6, this marking is in the quadrant from

9 to 12 o'clock. Furthermore, cylinder 8 is bowed significantly in

response to the blow represented by the hit mark, Figure 10b. The

position of the hit mark and the bowing suggest that cylinder 4 and 8

were in their normal relative positions at the time of the hit and that

8 was bolted firmly in place at the time of the hit. The blow to cylinder

8 flung it beneath trailer T-264, as shown in Figure 1. These observations

are consistent with the position of the fracture origin.

19
4.3 Other Cracks Near the Fracture Origin

4.3.1 Two Types of Cracks

In addition to the fracture origin, some other cracks were detected

in piece RR-12. These other cracks were of two distinct types.

Type C - These are subsurface cracks close to the inside wall. The

plane of the crack is roughly parallel to the inside wall.

The depth below the inside wall varied from from 0.01 to

0.10-inches. Several Type C cracks were found. As explained

in Section 4.3.4, Type C cracks result from diffusible

hydrogen entering the steel. Often, Type C cracks are referred to as "Blister Cracks".

Type C cracks were detected in transverse and longitudinal

sections. They have their long axis in the longitudinal

direction.

Type L - The plane of these cracks is roughly perpendicular to the

inside wall. The fracture origin was a Type L crack. As

explained in Section 4.3.5, Type L cracks probably result

from the hoop stress and other service stresses acting on radial components of Type C cracks. The Type C cracks can

be thought of as sources of the Type L cracks. Six Type L cracks were found which had intersected the inside surface.

Type L cracks were detected in transverse sections. Their

long axis is in the longitudinal direction.

Some of these other cracks were found at NBS in transverse sections

of the 17-inch segment containing the fracture origin. Figure 7. Examples

of Type C and Type L cracks appear in Figure 11. This photo shows the

right hand face of piece TR-1, as indicated in Figure 7c. Four Type L

cracks which intersected the inside wall were discovered by workers at

the Gladstone Laboratories. Two of these cracks were examined at Battelle

5 and are described in the Battelle Report. One of these cracks was

examined at NBS. The results of these examinations are consistent with

one another. Each of these three Type L cracks was similar to the fracture

origin, which is described in Section 4.5. The fourth crack remained

intact at the Gladstone Laboratories. A summary of the cracks found in piece

20
RR-12 appears in Table V. Visual examination of all the other pieces

of cylinder 9959 did not reveal additional cracks intersecting the inside

wall

.

Visual examination and random metallographic sectioning did not

reveal either Type C or Type L cracks in other pieces of cylinder 9959.

Nor were such cracks found in the few randomly selected sections of

cylinder 10041 which were examined. Nevertheless, there might be other

cracks in the remains of cylinder 9959. For the sake of providing a

complete picture of the distribution of cracks, it would be informative

to search the inside wall of cylinder 9959 for additional cracking using

non-destructive inspection techniques. Ultrasonic testing and magnetic

particle testing would be appropriate. Searching other cylinders on

trailers T-256 and T-254 would also be appropriate.

4.3.2 Etching to Reveal Banding

Etching of transverse sections revealed a feature of the metallurgical

structure called banding. The Type C cracks often were detected in whiteetching banded regions which were near the inside wall. An example of a Type

C crack in a banded region appears in Figure 12. Etching with 2 % nital*

or nital with an ammonium molybdate additive** revealed the banding.

Etching with the additive gave a sharper definition of the microstructural

features.

Banding is a well known feature of the metallurgical structure of

wrought commercial steels. Banding refers to microzones which have

variable chemistry. The origin of banding is chemical segregation

*2% nital consists of 2 ml. of nitric acid and 98 ml. of ethyl alcohol.

**Nital with the ammonium molybdate additive is prepared as follows: 100 ml.

ethyl alcohol mixed with 2 ml. of the following solution - 15 gm. ammonium

molybdate dissolved in 100 ml. water and mixed for four days with 100 ml.

nitric acid.

21
n

which occurs during solidification of the ingot. Such segregation occurs

in the last regions of the ingot to solidify, which are along the

centerline and near the top. The segregation is explainable from

phase diagram considerations, which show that the last liquid to solidify

O is enriched in alloying elements. When a tube round* is rolled from an

ingot, the enriched regions are stretched out longitudinally along the

centerline. Elongated banded regions result. The geometry is as if

stacks of tiny, wavey playing cards were embedded in the cylinder wall,

with the long axis of the cards parallel to the long axis of the cylinder.

Piercing and roll forging the tube round to make the seamless cylinder tends

to elongate banded regions longitudinally still more. Because the tube

round is pierced along its centerline, the banded regions with the most

severe chemical segregation remain close to the inside wall. Banding

cannot be eliminated by conventional heat treatments.

Electron microprobe analysis was carried out to characterize the

microchemistry of the white-etching microzones. Specimen L in Figure 7c

was selected for this examination. The microzone examined appears in

Figure 13. Results are listed in Table VI. No carbon segregation was

detected. Nor was sulfur segregation found. Phosphorus was the most

intensely-segregated element, followed by chromium and molybdenum, and

then manganese and silicon. Such microsegregation is probably typical

in all the white-etching microzones.

4.3.3 Microhardness in the Banded Regions

Microsegregation can contribute to high hardness through solid solution

strengthening, precipitation hardening and resistance to softening during

*"Tube round" is steel mill terminology for the semi-finished product resulting

from rolling a 15 to 20 ton ingot into starting material for the seamless tube

mill. Tube rounds are solid cylinders a few feet long with diameters up to 14-inches.

22
tempering. A survey of microhardness was carried out in order to

determine if the hardness of the white-etching microzones in which the

Type C cracks occurred was different from the hardness of the surrounding

material. Five transverse sections were examined. They are labeled TR1, TR-2, TR-3, TR-5, and R in Figure 7c. Results appear in Table VII.

The white-etching microzones were significantly harder than the surrounding

material, as illustrated in Figure 3. Often, the Knoop microhardness

measurements in the white-etching microzones converted to Rockwell -C

scale hardnesses of 40 or greater. For comparison, the surrounding

material was softer and exhibited equivalent hardnesses on the Rockwell

C scale of 35 to 40.

Material from other pieces than the piece in Figure 7 was examined

metal lographical ly and tested for microhardness. One transverse section

came from the left-hand end of the segment labeled "D" in Figure 5.

Three transverse sections came from a ring section cut from within a

foot of the front head of cylinder 10041. (See Figure 6) Results from

these specimens appear in the last four rows of Table VII. One finding

from the metal lographic work was that the banded regions in these four

specimens did not etch as white as the banded regions in the other

specimens. Light-dark etching did occur in these last four specimens,

and it provided the contrast which revealed the banding. Figure 14. As

shown in Table VII, the Rockwell C-scale equivalent of the Knoop microhardness measurements for these four specimens was around 30 + 3.

Microhardness was somewhat higher in the light etching bands.

It is not clear why the banded regions in the last four specimens

listed in Table VII did not show the sharper white contrast or the

higher hardness found in the other specimens. The lower values of

microhardness for the last four specimens suggest that the amount of

chemical segregation is less.

23
4.3.4 Explanation of Type C Crack Formation

Hard microzones in banded steel are often susceptible to cracking

in the presence of diffusible hydrogen.* A widely-accepted rule of

g thumb is that susceptibility increases with increasing hardness.

Although researchers disagree among themselves about the details of

hydrogen-assisted cracking mechanisms, there is general agreement that

hydrogen is attracted to preferred sites in steel, e.g., hard microzones,

and causes internal cracking, sometimes called "blister cracking." The

internal cracking is often attributed to extremely high internal hydrogen

gas pressure which develops when diffusible hydrogen atoms, H°, recombine

1

7

to form molecular hydrogen, . Thus, a high level of externallyapplied stress is not required for such cracking. The key factors for

promoting hydrogen-assisted cracking are a susceptible microstructure

and a source of diffusible hydrogen.

The natural gas being pumped into cylinder 9959 at the time of

rupture contained about 500 ppm by volume of hydrogen sulfide at the inlet

to the compressor. (See Section 6.2) It is well known that hydrogen

sulfide gas can react with steel at ambient temperatures to produce

diffusible hydrogen.^ A simplified expression for the decomposition

reaction is^

H

9 S + Fe -> FeS + 2H°

C. A The presence of liquid water favors this reaction.^ 1 Iron sulfide

produced by this reaction, FeS , generally adheres to the steel surface

*Diffusible hydrogen means atomic hydrogen, H°. The hydrogen molecule, H^, is

tgo large to diffuse appreciable distances in short times in steel. However, the

H° species could saturate a 0.050- inch diameter steel cylinder in about a minute

at room temperature. (Ref. 11) Impediments to diffusion could increase the

saturation time to about 5 days at room temperature.

24
as a black substance, such as that appearing in Figure 21. Sulfur signals

were obtained via electron microprobe analysis from this region of the

fracture origin (See Section 4.5), suggesting that the black substance in

Figure 21 is iron sulfide. Hence, the evidence suggests that a source of

diffusible hydrogen was was present.

13 Guidelines issued by the National Association of Corrosion Engineers

suggest that 500 ppm hydrogen sulfide in natural gas is more than sufficient

to promote sulfide stress cracking in a steel with a Rockwell C-scale

hardness of 40. Such gas is termed "sour gas." A conclusion to be drawn

from the foregoing information is that the Type C cracks found in hard,

white-etching microzones in transverse sections of cylinder 9959 resulted

from hydrogen-assisted cracking.

Sulfide inclusions are another preferred site for internal cracking in

steels. An example of hydrogen-assisted cracking in a laboratory specimen

from cylinder 10041 (see Figure 6) appears in Figure 15. In this case,

the specimen was saturated with diffusible hydrogen from an aqueous

acidified solution saturated with hydrogen sulfide, 14 which is known as a

severely corrosive solution. The low pH of this solution favors the entry

of hydrogen. Some cracks in Figure 15 have formed along sulfide inclusions

in the absence of an externally-applied stress.

4.3.5 Explanation of Type L Crack Formation

It is likely that the Type L cracks developed as a result of the hoop

stress and other service stresses acting on radial jogs in the Type C cracks.

An example of an incipient Type L crack forming as suggested appears in

Figure 16. This mechanism of formation is consistent with the observations

that the Type L cracks often intersect Type C cracks. For example, as shown

25
in Section 4.5, the fracture origin (which is a Type L crack), has intersected

5 a Type C crack. Figure 22a. In the Battel le study, a Type C crack was

found at about the mid-point of a Type L crack, as shown in their Figure 15.

An interesting aspect of the growth of Type C cracks is shown in Figure

12b. The crack tip is shown at a magnification of 800X. The crack path

is intergranular. It may be that the prior austenite grain boundaries have

undergone temper embrittlement and that hydrogen-assisted intergranular

crack growth is thereby aided.

7 In any case, the jog formed in the Type C

crack can be acted on by the hoop stress and other service stresses, and

thereby serve as a starter for a Type L crack.

The presence of diffusible hydrogen could easily assist the growth

of Type L cracks when they form as subsurface cracks. Once a Type L crack

penetrates the inside wall, any harmful chemical species in the cylinder

could assist in promoting further growth. Hence, the Type L cracks may have

grown via two distinctly different processes:

1) Hydrogen-assisted crack growth while subsurface.

2) More general stress corrosion by any harmful chemical species, once

the inside wall is penetrated.

The Type L cracks examined in this study are not tight. The crack

openings are rather wide, typically 0.0004 to 0.004-inches. An extremely

wide Type L crack appears in Figure 11. It is possible that the Type L

cracks were tighter in the intact cylinder and that these cracks widened

during the catastrophic rupture.

4.3.6 Summary of Cracking in Cylinder 9959

Reference to Figure 7 and Table V shows that the cracking detected

in piece RR-12 was localized to the vicinity of the fracture origin.

26
Furthermore, cracking was not detected in other pieces of cylinder 9959.

The general impression from metallographic examination and microhardness

testing is that regions of piece 12 of cylinder 9959 exhibit exceptionally

high hardness and microchemical segregation. Such regions occasionally

occur during steelmaking and fabrication. They originate from chemical

segregation which occurs during the solidification of the ingot. (See

Section 4.3.2).

In the case of a cylinder containing a harmful chemical species,

there may be direct contact between the chemical species and banded

regions which are highly susceptible to environmentally-assisted cracking.

In the case of cylinder 9959, the diffusion path between the harmful chemical

species (diffusible hydrogen, H°, from the decomposition of hydrogen sulfide)

and the highly susceptible microstructures (hard, white-etching microzones

beneath the inside wall) was about 0.01 to 0.1-inches.

4.4 Examination of Inside Wall

4.4.1 Surface Topography

Adhering soil and other products coated the inside wall of the pieces

of cylinder 9959. To the eye, these inside surfaces appeared brown and

rusty. Some regions of the inside wall were cleaned by workers at the

Gladstone Laboratory with a soft bristle brush to facilitate visual

examination. Further cleaning was done on pieces examined at the Bureau

of Standards. Ultrasonic cleaning in petroleum ether, and solution cleaning

with ammonium citrate* or buffered hydrochloric acid** solutions were the

*Ammonium citrate cleaning solution: This is a saturated aqueous solution of

ammonium citrate.

**Buffered hydrochloric acid solution: 3 ml HC1 , 4 ml 2-butyne-l , 4 dial, 50 ml

deionized FLO. This cleaning solution tends to minimize the solution of

substrate metal.

27
methods used to prepare surfaces for microscope examination. Not all inside

wall surfaces of the pieces from cylinder 9959 were cleaned. The focus

for cleaning was on piece RR-12. Specific mention is made in the text

when the surface under discussion was solution cleaned. Otherwise, the

surface was examined after bristle brushing.

Many transverse sections were taken near the fracture origin (see

Figure 7c) to examine the integrity of the inside wall and the surface

pofile of the inside wall. Generally, the surface profile was rough and

uneven. Sometimes such roughness is due to surface laps. Occasionally,

surface laps form along the inside wall during manufacture. These laps

result when oxide scale is pressed into the wall during hot forming.

An example of a surface lap appears in Figure 17. It was found at NBS

during examination of a transverse section from cylinder 10041. No

surface laps were detected in the NBS study of cylinder 9959. However,

one was found at Battel le Memorial Institute during examination of a

specimen from piece RR-12. There was no indication that surface laps

caused the fracture.

An unexplained pattern of sets of parallel circumferential lines

was embossed on the inside wall of piece RR-12. These lines were also

seen on other pieces and in other cylinders on Trailers 256 and 254. About

7-8 lines were in each set. In Figure 18a, one set runs through the

upside-down "R's." The systematic nature of these lines suggests they are

related to the manufacturing process. There was no evidence that the

fracture was due to these lines.

Generally, the surface of piece RR-12 was uneven, pitted and pockmarked,

as shown in Figure 18. The region of piece RR-16 near the fracture origin

28
was similar. The inside wall of other pieces from cylinder 9959 were

not as uneven and pockmarked. The inside wall of a ring taken from

near the front end of cylinder 10041 was smooth in comparison to that

of RR- 12. The general impression from comparison between the inside walls

of RR-12 and RR-16, and other pieces, was that the topography of much of

pieces RR-12 and RR-16 near the fracture origin was a shade rougher and

more marked up by grinding scratches.

It may be more than coincidence that the fracture origin was in

the vicinity of this rough surface. For example, the surface depressions

could have acted as "potholes" and held liquid water.* There was a strong

possibility that liquid water was present, as noted in Section 6.3. In

Section 4.3, it was mentioned that liquid water facilitates the decomposition

of hydrogen sulfide to form diffusible hydrogen.

Some of the uneven surface topography of piece RR-12 appeared to

be grinding scratches. One example is above the marking "PT" in Figure

18a. Normal manufacturing practice involves reaching inside the 3 1/2-inch

hole in the cylinder head with a long-handled pneumatic grinder to remove

surface discontinuities detected during ultrasonic testing of the finished

cylinder. This grinding is intended to remove stress concentrations and

notch effects.**

*As noted in Section 4.1, the fracture origin was located between the 6 and 7

o'clock positions, as shown in Figure 6. The cylinder is fixed to the trailer,

so the fracture origin location was at the bottom where liquid water could

accumulate.

**"Notch effects" is terminology used in mechanical metallurgy to describe the

tendency of stress concentrations near sharp corners to promote brittle fracture.

Ferrous materials are often evaluated for their "notch sensitivity" via the

Charpy test.

29
Microscope examination of regions on the inside surface of piece

RR-12 revealed two more interesting features of topography, 1) craters at

low magnifications, and 2) an uneven appearance at high magnifications.

Observations are shown in Figures 19 and 20.

The region shown in Figure 19 at a magnification of 24 times is

the inside wall of specimen TR-1 (see Figure 7c for location) after

ultrasonic cleaning and cleaning with ammonium citrate. The valleys running

from top left to lower right are grinding scratches. The hemispherical

craters probably formed from a shot peening procedure used in manufacture

to remove oxide scale from the inside wall.

Figure 20 shows the general uneven appearance of some regions of

the inside wall of specimen TR-5 (see Figure 7c for location) after solution

cleaning. A strip of remaining corrosion product runs from top left to

lower right in Figure 20. Hemispherical craters like those in Figure 18

are also evident in Figure 20a. Figure 20b shows a close-up of one of the

uneven regions. Cross-sections through some of these regions revealed that

the penetration was limited to less than 1/32-inches. In general, the

uneven regions are suggestive of chemical reactions occurring at the inside

wall. The Type C cracking discussed in Section 4.3 of this report suggests

that one of the chemical reactants might be hydrogen sulfide.

4.4.2 Evidence of Sulfur

Evidence of sulfur-bearing chemical species on the inside wall of

cylinder 9959 was found by four independent methods. The sulfur-bearing

species probably resulted from the decomposition of hydrogen-sulfide, as

discussed in Section 4.3.4. Detection methods included: 1) sulfur printing,

2) sodium azide testing, 3) electron microprobe analysis, and 4) x-ray

diffractometry.

30
Workers at Gladstone Laboratories used the sulfur printing

method, which is often used to detect sulfide Inclusion distributions in

steel ingots. They reported to NBS that a survey of pieces from one end

of cylinder 9959 to the other via the sulfur printing technique resulted

in strong indications of sulfur. The surfaces examined were on pieces

RR-9 (front head), RR-12 (about 12 feet from front head), and RR-10 (rear

head). A sulfur print made on the front head of cylinder 10041 also gave

strong sulfur indications. The surfaces were cleaned by bristle brushing,

and also by rubbing with 180 grit abrasive paper and wiping with a solvent.

It was reported that in general, the abraded surfaces gave more intense

sulfur prints than the bristle-brushed surfaces.

Sodium azide testing is a chemical solution method used for quick

determinations of the presence of sulfur. The region to be examined is

exposed to a solution to promote dissolving and the scent of hydrogen

sulfide is taken as an indication that sulfur is present. Workers at

Battelle Memorial Institute used this method on a section of piece RR-12.

5 They concluded that sulfur was present.

Electron microprobe analysis was carried out at the National Bureau

of Standards to search for sulfur-containing compounds. In this method,

characteristic x-ray signals from atoms excited by x-ray bombardment

serve to identify the chemical species. Caution must be used in this

procedure to sort out ambiguities from x-ray lines at nearly- identical

energies which come from different kinds of atoms. For example, sulfur,

molybdenum and lead have nearly the same emission energies, as indicated

in the tabulation below:

31
Characteristic Emission Energy

Element X-ray in KeV

Sulfur K

1 a 2.308

K

2

2.306

aS

1 2.464

Molybdenum La

1 2.293

s

1 2.395

2.518

Lead M

1 a 2.346

M

2 e 2.443

Suitable analysis procedures coupled with the use of high-resolution

spectrometers can overcome ambiguities in the analysis of electron

microprobe data.

Electron microprobe analysis was carried out on the inside wall of

two specimens from piece RR-12. A transverse section of piece RR-8

(see Figure 7c) was examined. This specimen was placed in a metal lographic

mount, after inside wall surface cleaning involving light bristle brushing

and ultrasonic cleaning in petroleum ether. Four linear scans were made

along paths starting in the steel, progressing through the surface product

and ending in the mounting material. In the metal, the sulfur signal was

insignificant. Strong signals from sulfur were detected as the beam

entered the surface layer for all four scans. Potassium, silicon, and aluminum

signals were also detected at the location of the surface product. Presumably,

these signals were from adhering soil.

The other specimen examined in the electron microprobe for sulfurbearing species on the inside wall was the piece containing the fracture

32
origin. (See Figure 7c). A region of the inside wall was examined in

nine places. Bristle brushing was the only surface clean-up technique used

initially. The predominant signals detected were from iron and silicon,

with low intensity signals from sulfur, chromium, manganese, aluminum,

potassium and calcium. After cleaning the surface via replica stripping,

the signal intensity was variable. Strong sulfur, iron, silicon, chromium,

aluminum, potassium and calcium signals were detected in some places, but

not in all locations sampled. These results suggest that some soil

remained on the inside wall after the replica stripping. They also

suggest that the sulfur distribution was non-uniform.

It was thought that some of the sulfur signals may have come from

fertilizers in the soil. To examine this possibility, a light scratch

was made in a corner of the specimen with a jewelers screwdriver. The

scratch removed surface product and revealed bare metal. The intensity of

the sulfur line from the scratch increased substantially. This result

suggested the presence of a heavy, adherent layer of sulfur-bearing chemical

secies along the inside wall near the fracture. This result is consistent

with results from sulfur prints of abraded surfaces obtained by workers

at Gladstone Laboratories.

X-ray diffractometry was used to search for sulfur bearing species on

the piece containing the fracture origin (see Figure 7c). Chromium radiation

was used with a pyrolytic graphite monochromator. Initially, the surface

was cleaned via bristle brushing and ultrasonic cleaning. Patterns from

alpha quartz and the gamma-Fe

2 03 were detected. After vigorous cleaning the

inside wall in the ultrasonic cleaner, the alpha quartz pattern disappeared

but the gamma Fe

2 0

3 pattern remained. In addition, indications of metal

sulfide patterns showed up.

33
The alpha quartz pattern was probably from adherent soil particles.

These x-ray results indicate that soil particles obscured the pattern

from metal sulfides which were adhering to the inside wall along with

gamma-Fe 203 .

4.5 Examination of Fracture Origin

A description of the as-received fracture origin was presented in

Section 4.1. Views of the fracture origin appeared in Figure 7. As

noted in Section 4.1, the half of the fracture origin on piece RR-12

was examined. The other half remained on piece RR-16.

The fracture origin was examined using the techniques of optical

microscopy, scanning electron microscopy and electron microprobe analysis.

A view of the as-received fracture surface viewed from the inside wall

appears in Figure 8a. The fracture surface was heavily coated with soil

and other products. Initially, the fracture surface was cleaned by

stripping cellulose acetate replicas. The surface in Figure 21 developed

after stripping away 3 replicas. Further clean up efforts with replica

stripping were abandoned because the process was slow and tedious. The

fracture surface was then cleaned using a buffered HC1 solution.* A

portion of the cleaned fracture surface containing the step appears in

Figure 22a. Occasional regions with surface product remained, but in

general, the fracture surface exhibited a dull metallic luster at this

stage. Some interesting details were revealed.

The dark horizontal line in Figure 22a is a Type C crack. It is

marked in the figure. This Type C crack occurs at the same depth below

the inside wall as the Type C cracks discussed in Section 4.3, i.e., between

*See Footnote on p. 27 for composition.

34
about 0.01 and 0.10-inches. Finding a Type C crack at the fracture origin

is consistent with the interpretation of Type C and Type L cracks in

Section 4.3. To the left of the Type C crack at 0.026-inches below the

surface in Figure 22a is a saucer-like depression (evident in stereographic

views) which may be the site where a Type L crack developed from this Type

C crack.

The possibility of the fracture origin being three independent Type L

cracks which linked up was discussed in Section 4.3. The step between

regions 1 and 2 of the fracture origin (see Figure 7c) may represent the

link-up of two Type L cracks which emerged from radial jogs located at two

different circumferential positions on two different Type C cracks. Region

3 may differ from region 2 on the fracture origin because each Type L

crack represented by these regions originated at a different Type C crack.

The thumbnail-shaped fracture origin is macroscopically flat. Typical

surface topography in this region is shown in Figure 22. The topography

might be likened to numerous plateaus and valleys. Figure 22b at a

magnification of 250 times is a closeup view of the same structure.

Beyond the thumbnail region — longitudinally and radially towards

the outside wall — was a very different surface structure. Here the

surface was quite rough on a macroscopic scale. It exhibited a texture with

directionality in the longitudinal direction. Tiny shear lips were

evident along the outside wall. Presumably, this region beyond the

thumbnail region represents the remaining ligament which failed in a

short time, e.g., milliseconds to seconds, due to tensile overload.

The fine details of surface structure in Figure 22 are probably not

representative of what the fracture surface was like when the fracture

35
origin formed. It is likely that corrosion during the lifetime of this

fracture surface obscured most of the origin surface topography. The

buffered cleaning solution used to remove adherent surface product may

also have contributed slightly to the loss of original topography, and

also to a slight exaggeration of the opening of the Type C cracks. These

slight modifications have no effect on the major observations or conclusions.

Electron microprobe analysis was carried out in three locations on

the fracture surface after the surface was cleaned by stripping 3 replicas.

These locations are labeled A, B and C in Figure 21. Strong sulfur signals

were emitted from each region. The emission of sulfur signals from the

fracture surface suggest that the fracture origin penetrated the inside

wall over at least part of its length for some period of its lifetime.

After making the examinations of the as-received fracture origin as

described above, a transverse section through the fracture origin was

prepared. The objective was to look for hard, white-etching microzones and

Type C cracks such as those appearing in Figures 11, 15 and 16. The cut

was made as shown in Figure 7c. The piece containing the step was

examined on its transverse face via optical microscopy at six successivelydeeper levels as the face was ground away.

Hard, white-etching microzones were found at all six levels. Three

Type C cracks were found. An example showing two within 0.6-inches of the

fracture origin appears in Figure 23a. These type C cracks are within the

banded region close to the inside wall.

Figure 23b is a close-up of the region containing the Knoop hardness

indentations which appear at the upper right in Figure 23a. The white-etching

36
microzones are evident in Figure 23b. The hardness at each indentation

is listed below.

MICROHARDNESS AT KNOOP INDENTATIONS

APPEARING IN FIGURES 23a AND 23b

Rockwell C-scale

Location Knoop Equivalent

G-in white microzone 452 44

B-in white microzone 439 43

I-in white microzone 430 42.5

E 424 42

C 418 41

A 394 39

F 392 39

D 383 38

H 378 38

0.3 inches below inside wall 373 37

These results indicate that the region of the fracture origin had a banded

microstructure near the inside wall which was highly susceptible to Type

C cracking as discussed in Section 4.3.

37 L
5. MATERIAL DESCRIPTION

5.1 Grade of Steel and Heat Treatment

The chemistry of the steel used under Specification DOT 3T is

listed in Table I. This chemistry corresponds to grades known as ASTMA372, Class 5, Type E and AISI 4145. This low alloy steel is a high

strength pressure vessel steel which is usually used in the quenched and

tempered condition.

The heat treatment for 3T cylinders is described in Specification

DOT 3T. In Subparagraph 178. 45-8(a) (1 ) , it is noted that the steel

temperature on quenching must never exceed 1750°F and in Subparagraph

178.45-8(a)(2) , it is noted that after quenching each cylinder must be

reheated to a temperature not less than 1050°F.

All 3T cylinders are heat treated after the heads are forged.

Normal practice involves heating as many as five cylinders simultaneously

in a car-bottom furnace. The ends of the cylinders are plugged during

heat treatment. Temperature is monitored at about five positions in the

cylinder array during austenitizing and also during tempering. Cylinders

are quenched in a turbulent oil bath one at a time. Transfer time from

furnace to quench bath is about one-minute. Tempering involves heating

as many as five cylinders simultaneously in a car-bottom furnace,

followed by air cooling. During heat treatment, cylinders are supported

at several positions along their length to minimize shape distortion.

A "test ring" is heat treated with each batch of cylinders, in

accord with Specification DOT 3T. A test ring may be 18-inches long and

is the full diameter and wall thickness of the cylinders. Its ends are

38
sealed. Specimens are taken from the ring for tensile testing and

Charpy testing. Each test ring represents a different heat of steel

from the current stream of 3T cylinder production.

5.2 Microstructure from Heat Treatment

The microstructure which results from quenching and tempering a low

alloy steel is usually tempered martensite or tempered bainite. Martensite

forms throughout thin sections and as an exterior "skin" on thick sections.

Tempered bainite forms beneath the skin.

A typical example of the microstructure found in cylinder 9959 appears

in Figure 24. This fine scale, acicular structure shown at a magnification

of 1000X is tempered bainite. Similar microstructures were found in all

the microscope specimens from cylinder 9959 and cylinder 10041.

5.3 Other Features of Microstructure-Decarburization and Banding

Two other features of the microstructure showed up in transverse

sections — decarburization and banding. These features appear in

Figure 25.

The two specimens shown were cut from opposite ends of the segment

in Figure 7c. The inside wall of each specimen is towards the middle.

An appreciable decarburized layer exists in each piece along the inside

wall and the outside wall. Estimated depths are tabulated below and

compared with similar observations from the study at Battel le Memorial

Institute.

Depth of Decarburized Layer in Specimens From Cylinder 9959

Study Specimen Inside Wall Outside Wall

NBS L 0.027 inches 0.027 inches

NBS R 0.020 0.013

Battelle RR126A 0.026 0.033

Battel!

e

RR12GC 0.026 0.017

39
The depths of the decarburized layers exhibit a little variability, but

are comparable in the specimens examined. Moreover, these depths are

typical for hot worked steel which has been quenched and tempered. The

decarburized layer represents a surface region which has lost carbon via

diffusion of carbon to the surface and chemical reaction with air or a

furnace atmosphere. Decarburization occurs at elevated temperatures.

The microstructure of the decarburized layer is not bainite, but

ferrite. Ferrite is extremely ductile. The tiny shear lips along the

inside wall in Figure 8 are probably in the ferrite layer.

The banding evident in Figure 25 is more pronounced in specimen R

than in specimen L. Banding is evidenced by the vertical streakiness in

specimen R. The banding is more intense near the inside wall. These

two specimens came from about the same longitudinal line along the edge

of piece RR-12. The variation in banding shown by these two specimens

which were about 17-inches apart is typical. Banding was discussed in

Section 4.3.2.

5.4 Characterization

5.4.1 Background

"Kiscc" is notation used in the field of fracture mechanics. This

is a field of stress analysis which quantitatively evaluates the influence

of cracks on the stress which a structure can safely support. The

development of this field during the last thirty years resulted from the

widespread recognition that most engineering structures may contain

cracks which could lead to failure at average applied stresses well

40
below the yield stress. The utility of fracture mechanics lies in its

potential to predict critical crack sizes in terms of applied stress and

resistance to crack propagation. A structure which cracks below the

critical size would remain intact, whereas a structure with larger

cracks would fail.

Fracture mechanics measures the ability of a material to resist

crack propagation in terms of "fracture toughness," signified by " K

lc"

for plane strain conditions, and "K " for plane stress, and with units

of Ksi - /inches. Typical values of fracture toughness for steels may range

between 50 and 400 Ksi - /inches. Typical aluminum alloys may exhibit

fracture toughnesses in the range, 20 to 100 Ksi - /inches. Fracture

toughness can be measured on precracked test specimens in an ordinary

1 5a

tensile testing machine. ASTM Standard Method E 399 - 72 describes

plane strain fracture toughness testing of metallic materials.

The concept of "Kj<~qq m is like that of "Kjq" and "K ". The difference

is that "K

ISCC

" relates to crack growth under stress corrosion conditions.

K ISCC

the threshold stress- intensity factor for stress-corrosion

cracking under plane strain conditions. Low values of Kj SCC signify

high susceptibility to stress corrosion cracking. Often, in K ISCC

testing, it is not clear if plane strain conditions prevail. In such

cases, the K- value is called an "apparent K^qq-"

Kjscc for a given material is less than K^ for that material. For

15b

example, in the case of a well-known bridge steel, K

IC was about 50

ksi /in. , whereas K

ISC(

, was about 15 ksi /in. Another feature of K ISSC

is

that it decreases with increasing yield strength. To illustrate this

point, some data obtained in a laboratory study of AISI 4340 steel are

1

6

listed below. The variation in yield strength resulted from different

heat treatments.

41
Yield Strength (ksi) K

ISCC

(ksi ' ^

130 111

180 26

220 10.5

5.4.2 K

j 5 CC

Measurements in the Laboratory

An experiment was carried out to estimate K^qq for the type of

steel used under Specification DOT 3T for tube trailer cylinders. Test

specimens were obtained from a ring section cut from cylinder 10041 near

the front head. This steel has yield and tensile strengths of 122.5 and

142.5 Ksi, respectively.

A wedge-loaded double cantilever beam test specimen was used, Fig.

26. Specimen thickness was 0.375-inches, which fulfills the requirement

1 5a

in ASTM E399-75 for a valid plane strain test for stress intensities

up to 50 ksi /in. The specimens were cut from the ring section such that

the long axis of the specimen was parallel to the longitudinal axis of

the cylinder. The axis of the crack was perpendicular to the wall

thickness, so that the crack grew longitudinally. This orientation was

selected to approximate the growth of the type L cracks discussed in

Section 4.3.

Each specimen was wedge loaded to a predetermined stress intensity,

and immersed in an aqueous acidified solution (0.5 wt. pet. acetic acid

. 14

in distilled water) saturated with hydrogen sulfide. The idea behind

this type of testing is that the crack will grow until the stress intensity

falls to the threshold level, i.e., Kj SCC

- After five days, the specimen

was removed and examined for crack growth. When crack growth was detected,

a value of Kj SCC was estimated. These results may overestimate i.e. :

42
the true value of K ISCC might have been lower if the specimen had been

re-immersed and the crack had been given further opportunity to grow.

For these initial estimates of however, it was felt unnnecessary

to re-immerse the specimens.

Test results appear in Table VIII. K

ISC(

, for this steel appears to

be about 40 ksi - /in. The most valid estimate of was from

specimen number 5, which was coated with wax to inhibit crack growth

into the arms. The determinations from specimens 2 and 6 seem reliable

despite the arm cracking which occurred. Clearly, the 35 ksi - /in.

applied to specimens 1 and 3 was below the threshold stress intensity.

In a given batch of test specimens, it sometimes happens that crack

initiation does not occur where it would be expected. This seems to be

the case for specimen 4.

The utility of this estimate of Kj SCC

is that it facilitates an

evaluation of hydrogen-assisted crack growth in cylinder 9959. The

critical crack size can be roughly estimated from the following equation

for flat plate,

K

2 1.2 a

2 a

- 0.212(^)

2

( 1 )

where

K

= critical stress intensity

o = applied stress

a

- crack depth

0

=

2 crack geometry factor (0 varies from 1 for a long, shallow

crack to 2.45 for a semicircular crack)

°y

= yield strength

43
For a long, shallow type L crack and applied stress at the crack tip

equal to the yield stress, a is estimated to be 0.07-inches. For a

^ I semicircular type L crack and applied stress equal to the yield stress

at the crack tip, a cr

is estimated to be 0.165-inches. A critical

crack size of 0.07-inches or less would be consistent with the size of

the radial jogs on type C cracks which are described in Section 4.3

as probably giving rise to the type L cracks.

It is felt that these estimates of a cr approximate roughly the limits

on critical crack size. The approximation is rough due to simulating

the real case with assumptions, such as assuming a surface crack rather

than an embedded crack, and estimating from a piece of cylinder

10041 rather than from piece RR-12 of cylinder 9959. The higher yield

strength of piece 12, 137.5 ksi vs 122.5 ksi for the piece from cylinder

10041 , suggests that a somewhat lower K^qq would be measured for piece

RR-12. This in turn suggests a somewhat smaller critical crack size for

piece RR-12.

44
6. GAS ANALYSIS

6.1 Objective of Gas Analysis

The evidence suggesting that cylinder 9959 ruptured due to

environmentally-assisted cracking made it desirable to obtain an

analysis of the natural gas being pumped into the tube trailer at the

time of rupture. Gas samples were being taken at the time of rupture,

1 ^

and gas analysis had been carried out earlier in the life of the two

natural gas wells. However, these analyses were for the heat content

of the gas and did not focus on potentially harmful chemical species

which might be present at the parts per million level. Consequently,

gas samples were taken and analyzed at the request of QHMR-DOT as part

of this investigation. The focus was on hydrogen sulfide and moisture

concentration in the parts per million range.

6.2 Hydrogen Sulfide Analysis

Samples were taken at the two natural gas wells and at the inlet

to the compressor, which was about two miles from the wells. The date

was May 9, 1978. The sample containers were aluminum alloy compressed

gas cylinders especially designed for sample taking. Less than 24 hours

elapsed between the time the gas samples were obtained and the completion

of the analysis for hydrogen sulfide. The analysis was carried out in

18

a laboratory where such analyses are done routinely.

Results appear in Table IX. These results indicate that a substantial

amount of hydrogen sulfide was present in the natural gas — enough to

classify it as a "sour gas" and cause concern about hydrogen-assisted

cracking (also called sulfide stress cracking) of steel. For example.

45
13

guidelines issued by the National Association of Corrosion Engineers

suggest that 400 parts per million hydrogen sulfide in gas at 2000 psi

is sufficient to cause "sulfide stress cracking" in steel of hardness

greater than Rockwell C-22. Hardness enters this guideline because of

the well established rule of thumb that susceptibility of a steel to

g cracking increases with increasing hardness.

Another guideline to compare with the amounts of hydrogen sulfide in

Table IX is found in the code of Federal Regulations, Title 49, Subparagraph

192.475-(c). This subparagraph of the Federal Gas Pipeline Safety Standard

deals with the acceptable gas composition for "pipe-type or bottle-type

holders." The upper limit is set at 0.1 grains of hydrogen sulfide per

100 standard cubic feet of natural gas. This converts to about 1.5 parts

per million by volume of hydrogen sulfide in pure methane, which is well

below the values appearing in Table IX. Thus, comparison with this guideline

indicates that the levels of hydrogen sulfide listed in Table IX exceed

significantly that allowed for gas pipeline hardware.

The Bureau of Mines Circular 8749, "Analysis of Natural Gases, 1976,"

was reviewed for possible background information about the hydrogen

1

9

sulfide content of natural gas at the wellhead. However, this document

did not report hydrogen sulfide with sufficient accuracy to be helpful.

Table IX shows that there was a reduction of hydrogen sulfide content

between the wells and the compressor. This reduction probably represents

reaction of hydrogen sulfide with the gathering lines. There is no

reason to believe there would be any significant change in the hydrogen

sulfide concentration as it passed through the compressor and into the

cyl inders.

46
The gas wells were essentially inactive between November, 1977 and

May 9, 1978. It is not known what effect this would have on the hydrogen

sulfide concentration. It is possible that dry hydrogen sulfide also

reacted with the steel of cylinder 9959 to cause environmentally-assisted

20

cracking. That such cracking can occur has been reported. However,

environmentally-assisted cracking by hydrogen sulfide is usually favored by

the presence of liquid water.

It is informative to review results of an earlier analysis for hydrogen

sulfide. This analysis was done at the request of 0HMR-D0T as part of

this investigation. Some gas samples were obtained in stainless steel

compressed gas containers on March 7, 1978. Analysis was carried out

on March 30, 1978 in the same laboratory reporting the results appearing

1

8

in Table IX. Results of this earlier analysis appear below.

Hydrogen Sulfide

Location Concentration (ppm by volume)

Gas Well -Renfro #1 No H

2 S Detected

Gas Well -Renfro #3 315 + 25

It is believed that the long time between sample taking and analysis

allowed reaction involving hydrogen sulfide, other components of the

natural gas and the container itself. The result was a substantial

reduction in the hydrogen sulfide concentration. Such absorption of

hydrogen sulfide during gas sampling is recognized in U.S. Bureau of

Mines Circular 8749.^

6.3 Moisture Analysis

The moisture content of the natural gas was 800 ppm, as shown in

1

8

Table IX. This converts to about 40 pounds of moisture per million

cubic feet of natural gas. The saturation concentration for moisture

47
in natural gas at 72°F and various pressures is tabulated below, as

calculated from Reference 21.

Saturation Concentration for

Moisture in Natural Gas at 72°F

Pressure(psi

)

Concentration

(Pounds per Million Cu. Ft.)

400

2000

2200

2500

2600

54.1

17.1

16.3

15.3

15.0

Comparison with data in the table indicates that the moisture content

of the gas, 40 pounds per million cubic feet, exceeds the saturation

concentration at shipping pressure. In a cylinder holding 16,000 cubic

feet of natural gas, about 0.4 pounds of water would precipitate. This

result suggests that liquid water was present in cylinder 9959 at the

time of rupture. As noted in Section 4.3.4, the presence of liquid

water favors the decomposition of hydrogen sulfide, which results in

diffusible hydrogen.

As noted in Section 1.1, the pressure in the gas gathering lines

was about 400 psi. At this pressure, the moisture in the gas would exist

as vapor.

6.4 Other Chemical Species

1

8

Other components of the May 9th set of gas samples were analyzed.

Results appear in Table X. These results are presented only for the

sake of completeness. There is no reason to believe these components

of the gas were related to the environmentally-assisted cracking of

cylinder 9959.

Carbon dioxide in conjunction with moisture and carbon monoxide is known

22 23

to promote stress corrosion in compressed gas containers. ’ Carbon

48
dioxide was determined to be 0.01 mole percent at each gas well and at the

1

7

input to the compressor. It would be helpful to know the carbon monoxide

content of the gas, since the possibility exists that crack growth via

the C0

2 -C0-H

2 0 mechanism might have occurred when subsurface cracks

penetrated the inside wall. It is believed that in the absence of

22 23 carbon monoxide, the proposed mechanism cannot operate. ’

49
7. HARDNESS SURVEY

During the course of this investigation, it was convenient to make

hardness measurements on numerous specimens from along cylinder 9959

and from other cylinders as well. The objective in making these

measurements was to obtain an overview of hardness variations throughout

cylinder 9959 and to estimate the variability of hardness among cylinders.

These measurements also provide a basis for estimating the susceptibility

of the steel to environmentally assisted cracking. A widely-accepted

g rule of thumb is that susceptibility increases with increasing hardness.

Hardness measurements were made on transverse sections. Rockwell C-scale

measurements were made on unmounted specimens with parallel faces. Some

measurements were made on the milled shoulders of tensile specimens.

Measurements were made carefully to avoid edge effects. Occasionally, a

hardness measurement was made on a specimen in a metal lographic mount

to obtain an approximate value of hardness. Results are reported in

standard ASTM notation, e.g., "HRC 22."

There is no requirement in Specification DOT 3T for hardness on

transverse faces. Hence, it is not appropriate to compare the hardness

measurements reported herein with any part of it.

Results of the hardness survey appear in Table XI. These results

were obtained at the National Bureau of Standards and also at the Gladstone

Laboratories. These results indicate a variable hardness in piece RR-12.

The range is from HRC 31.6 to HRC 39. These results suggest that the

hardness of piece RR-12 which contained the fracture origin is somewhat

greater than the hardness of pieces RR-10 and RR-17A.

50
Comparison of all the results on cylinder 9959 with results from

the two other cylinders listed in Table XI indicates that cylinder 9959

is somewhat harder than the other two cylinders. It is expected from

the comparison of tensile data obtained on cylinders 9959 and 10041 cited

in Section 3.2 that cylinder 9959 would be harder.

Following the rule of thumb that susceptibility to environmentallyassisted cracking increases with increasing hardness, cylinder 9959 would

be expected to be the most susceptible of the three cylinders listed

in Table XI.

Results in Table XI suggested making a further comparison between

hardnesses measured on the three cylinders. Hardnesses were measured

on the cylindrical surface, as shown in Table XII. These results also

indicate that cylinder 9959 is somewhat harder than the other two

cylinders. These results show further that meaningful hardness results

can only be obtained by grinding away a substantial layer from the

outside surface to get below the decarburized layer. Some measurements

of the depth of the decarburized layer appear in Section 5.3.

51
8. STRESS ANALYSIS VIA FRACTURE MECHANICS

8.1 Review of the Fracture Origin

The fracture origin was described in Sections 4.1 and 4.5. It

was thumbnail shaped and was about 1.3-inches long and 0.25-inches deep.

It may have formed via the link-up of 3 closely-spaced cracks. A notable

feature of the fracture origin is that it was macroscopical ly flat,

Figure 23a. Yet, on a microscale, it consisted of plateaus and valleys.

Figure 22b. Macroscopical ly, the fracture origin looked like a brittle

fracture. Microscopically, the fracture origin appeared somewhat ductile.

Two analyses have been carried out using fracture mechanics analysis

methods to estimate the critical crack size which would result in

catastrophic rupture. The analysis in Section 8.2 is for brittle fracture

of a structural material with a surface flaw. The analysis in Section

8.3 is for ductile fracture of a cylinder with a through-wall flaw.

8.2 Tiffany and Masters Equation

24 Tiffany and Masters published a method of fracture mechanics

analysis for brittle fracture of structural materials with surface

flaws. Their equation is

(q) critical " (-^)

2 (2)

where

a = crack depth

Q

= crack shape parameter

Kj£ = plane strain fracture toughness

a = applied stress

52
Kjc for the grade of steel used in 3T cylinders has been estimated in

25 laboratory tests as follows:

Heat Treatment

Quench and temper at 9Q0°F 110 ksi - /in.

Quench and temper at 1100°F 128 ksi - /in.

Transverse tensile specimens from the vicinity of the fracture

origin indicated the following strengths, as noted in Table III.

Yield Strength = 128 ksi

Tensile Strength = 159 ksi

For a crack which is longer than it is deep, Q

= 1.2. Assuming the

stress level at the crack tip equals the yield strength, and substituting

Kj

C

- 128 ksi - /in. into equation (2), a cri

- tl- ca -| = 0.31. For Kj

C

= 110 ksi

/in., a cri-t i ca -| = 0.23-inches. Both of these estimates of the critical

crack depth compare favorably with the observed value of 0.25-inches.

Hence, analysis via the brittle fracture theory of Tiffany and Masters

suggests an initially brittle fracture. This view is consistent with

the macroscopically flat fracture origin.

8.3 Battelle Equation

The following fracture mechanics analysis of cylinder 9959 is based

on an empirical analysis formulated by Maxey and co-workers "at Battelle

Memorial Institute. This analysis was developed for ductile fracture of

pipe with yield strengths from 40 to 70 ksi. It is recognized at the

outset that this analysis was not formulated for higher strength

materials, but a brief exploration of its applicability and predictions

relative to cylinder 9959 with a yield strength of 128 ksi seems worthwhile.

53
Moreover, this analysis was not formulated for a part-through crack growing

from the inside wall, but for a part-through crack growing from the

oustide wall. For this reason, this brief analysis will be confined to

the through-wall crack in cylinder 9959 which extended as flat fracture

over a few inches longitudinally to either side of the fracture origin,

as shown in Figure 7c. Beyond this region, slant fracture (ductile

fracture) developed. It seems appropriate to apply this ductile fracture

analysis in this region.

As a starting point, it is noted that stress magnification will

occur at the ends of a crack. Consequently, plastic flow and fracture

can occur at nominal wall stresses lower than the yield strength. In the

case of a longitudinal sidewall crack in a cylinder, stress magnification

is due to redistribution of the hoop stress around the crack and to bending

moments caused by bulging near the crack.

The stress magnification factor due to bulging of tubular products

containing a through-wall, longitudinal defect, M^, was developed by

Folias and is given by

2 4

M

y

= (1 + 1.255 " 0.0135 1/2 (3)

In the above equation,

d = depth of a part-through crack

2c = length of a through-wall crack

R = cylinder radius

t = wall thickness

54
The stress magnification factor shows up in the equations used

to calculate the hoop stress at failure. Equations have been defined

I O by Maxey for two types of fractures, classified as "flow stress controlled"

(plastic behavior) and "toughness controlled" (elastic-plastic behavior).

Equations for both types of behavior appear below.

In the case of a through-wall, axial crack, the equation for

toughness-controlled behavior, ay, is as follows:

in sec \

(M

ToT

/a

f

) = K*ir/C8ccjf) = 1 2 Ett(

C

v /Ac

)/ (8ca^ ) (4)

For a through-wall defect, the equation for flow stress controlled

behavior is

Oj = Of/M-j- (5)

In the above equations,

*Oy = hoop stress at failure for flow stress controlled behavior

ay = the flow stress of the steel (assumed by Maxey and others

to be yield strength + 10,000 psi

)

c = plane stress fracture toughness of material

Cv /A

c

= impact energy in foot-lbs. per unit area of a Charpy test

specimen (half size or full size)

E = Young's modulus (30 x 10

6 psi)

2 c = the length of the crack

My = stress magnification factor given by Equation 3.

Suitable analysis of equations 4 and 5 leads to the family of

parametric curves plotted in Figure 27. Taking the energy for a full-size

transverse Charpy specimen from near the fracture origin as 10 foot-pounds

55
from Table IV, and the yield strength for a transverse tensile specimen

from near the fracture origin as 128 ksi from Table III gives a

value for the toughness parameter on the right hand side of Figure 27 of

12C E 12(10) (30 x 10

6

) \

=

o = 0.68

A o^/rT CO. 1 24) ( 1 28,000 + 10,000)^ /(ll )(l/2)

The hoop stress at the moment of burst, cjy, is estimated from Oy ~2"

( ?/ 2)

1

1

^ = 55 ’ 000 P si - Hence. ll = 55,000 =

’ ^ i -3Q nnn u.hu. 138,000

PR

t

From these two parameters, 0.68 and 0.40, a point can be located on

2c Figure 27 as shown, thus leading to an estimate of — = 1.4. This gives

/RT

a value for the critical crack length of 2c = 1.4 /(ll )(l/2) = 3.3-inches.

Reference to Figure 7a indicates that the transition from flat fracture

to slant fracture began when the flat region was about 3-inches long.

26“ 28

Hence, the empirical analysis formulated by Maxey and co-workers

predicts a result which is consistent with the observations. It may be

concluded that the sidewall opened up via a flat fracture which became

slant fracture after a short extension. The time frame for the extension

of the flat fracture is uncertain. It may have been a few milliseconds

or even a few seconds.

56
9. DISCUSSION

Results presented in this report lead to the conclusion that

cylinder 9959 ruptured due to environmentally-assisted cracking*,

as evidenced by a well-defined fracture origin which intersected the

inside wall, several other cracks near the fracture origin, indications

of hydrogen sulfide in the compressed gas, and evidence of sulfur near

and on the fracture origin. The combination of highly susceptible steel

and a high concentration of hydrogen sulfide is the probable cause of the

rupture of this new cylinder in about a week. Less susceptible steel

and/or a lower concentration of hydrogen sulfide would have prolonged

the service life of cylinder 9959. However, the combination of a high

concentration of a harmful chemical species in the product being shipped

and the high allowable tensile strength, 155,000 psi, of steel used under

Specification DOT 3T would probably have eventually led to rupture of a

cylinder on one of the tube trailers.

Ruptures of a similar nature in compressed gas containers were

30 reported in England in 1947. Low alloy steel cylinders (nickel-chromiummolybdenum, and manganese-molybdenum) in the hardened and tempered

condition had tensile strengths around 130,000 to 140,000 psi. These

steels exhibited environmentally-assisted cracking at applied stresses as

low as 20,000 psi in hydrogen sulfide.

*Environmentally-assisted cracking is mechanical metallurgy terminology for

crack growth via interaction between one or more harmful chemical species and

stressed steel at the crack tip. Some researchers distinguish between hydrogenassisted cracking and other stress corrosion processes involving chemical

species other than hydrogen.

57
The general impression from the results appearing in Section 3 is

that cylinder 9959 was a marginal cylinder, in terms of its tensile and

Charpy properties, and in terms of its wall thickness. The inside

wall near the fracture origin showed numerous grinding marks, indicating

that this region drew attention at the time of inspection. Severe

banding and high hardness microzones in the microstructure close to the

fracture origin were further evidence of an undesirable metallurgical conditio

which promoted susceptibility to environmentally-assisted cracking. A

comparison of mechanical properties test results among pieces from cylinder

9959 (tensile, Charpy, hardness) indicated noteworthy material variability,

with the highest strength and hardness, and lowest toughness occurring in

test specimens taken from piece RR-12. Material variability in cylinder

9959 is also suggested by the frequent transitions between slant and flat

fracture observed along the entire fracture surface, as noted in Section 4.2.

Results of drop weight tear tests carried out at Battelle Memorial

Institute also suggest significant material variability in cylinder

9959.

5

The concentration of hydrogen sulfide in the natural gas was more

than sufficient to promote environmentally-assisted cracking in a steel

with a tensile strength upwards of 150 ksi, according to guidelines

1

3

published by the National Association of Corrosion Engineers. The

primary uncertainty about the environmentally-assisted cracking process

is the time required for cracks to grow to the critical size for catastrophic

rupture, which was calculated to be about 0.2 to 0.3-inches deep. Estimates

of Kj<-qq indicated that the critical crack size for hydrogen assisted

growth was less than 0.1 -inches deep. Several blister cracks near the

fracture origin and close to the inside wall provided evidence of hydrogen

58
assisted cracking. Once cracks had penetrated the inside wall, any

other harmful chemical species in the natural gas may have assisted in

the crack growth process.

The fracture mechanics analyses in Sections 8.2 and 8.3 indicate

that cylinder 9959 probably ruptured initially under plane strain conditions.

with a Kj£ value between 110 and 128 ksi-/in. . A through-wall flat fracture

about 3-inches long resulted, then a fast-running shear fracture developed.

The transition to shear fracture indicated a change from plane strain to

plane stress conditions, which might have occurred due to lessened

constraint from the cylindrical structure when the through-wall defect

developed. A plane stress value of fracture toughness was estimated from

28

the Battel le correlation,

K

2 =_ 12C

v

E _ (12) (10) (30 x 10

6

) (0.394) (0.315)

giving K

c

= 170 ksi-/in. . The occurrence of the fast running shear

fracture indicates that 170 ksi-/Tn. was insufficient fracture toughness

to contain a 3-inch long, through-wall flaw. Therefore, the critical

defect size for catastrophic ductile fracture was less than 3-inches for

the prevailing stress conditions.

( 6 )

59
10. CONCLUSIONS

1. The evidence gathered in this investigation strongly suggests that

cylinder 9959 ruptured due to environmentally-assisted cracking.

2. Cylinder 9959 was new and ruptured after about a week in natural gas

service. Rupture in this short time was probably due to the combination

of highly susceptible steel and a high concentration of hydrogen sulfide.

3. Cylinder 9959 appeared to be a marginal cylinder in terms of its

tensile and Charpy properties, and in terms of its wall thickness.

Generally, the wall thicknesses are less than these appearing in

the inspector's report. Charpy and tensile properties were variable,

and in some locations in cylinder 9959, did not meet the specification

requirements. Limited tensile data obtained on steel from another 3T

cylinder did meet specification requirements.

4. Metallurgical banding was observed in steel from cylinder 9959 and

from cylinder 10041. In both cases, the banding was close to the

inside wall of the cylinder. Subsurface cracking occurred in banded

regions of piece 12 of cylinder 9959 and was detected in transverse

sections of metal lographic specimens. Subsurface cracking was not

detected elsewhere in cylinder 9959 or in pieces examined from cylinder

10041. It is likely that the higher hardness in the banded regions

in piece RR-12 led to a higher susceptibility to subsurface cracking

in these regions of piece RR-12 than in other regions of cylinders

9959 and 10041.

5. To deal with environmentally-assisted crack growth problems in the

future, it appears necessary to develop a data base from which to

60
make decisions about tolerable combinations of:

1) strength and hardness of steel, and

2) concentrations of various harmful chemical species.

61
11. ACKNOWLEDGEMENTS

Numerous NBS staff workers contributed to the contents of this

report. Outstanding metal lographic work was carried out by Mr. Charles

Brady. Mr. Dave Ballard assisted in scanning electron microscopy work

and contributed significantly to the interpretation of scanning electron

microscope fractographs. Mr. Len Smith assisted in mechanical testing,

photographic work and report assembly. Mr. George Hicho performed the

Kj sec tests » ancl Mr. Don Harne performed the Charpy tests. Mr. Dale

Newbury and Mr. Chuck Fiore carried out the electron microprobe work.

Dr. E. Passaglia provided valuable contributions to the review of this

report.

Mr. Tom Groeneveld and Mr. William Maxey of Battel le Memorial

Institute made significant contributions to developing findings about

cylinder 9959 and interpreting them.

Mr. A. Mai len, Mr. J. Jones and Mr. J. Osteen of OHMR-MTB-DOT made

significant contributions to the planning and implementation of this

investigation. Mr. Mai len and Mr. Osteen provided valuable contributions

to the review of this report.

Mr. Maurice Bolinger, Jr., Mr. Roger McCombs, and Mr. William Tholke

of Gladstone Laboratories provided valuable assistance in analysis of the

incident site, and in obtaining material from cylinder 9959 for analysis.

They also provided useful tensile and metallographic data and photographic

documentation.

Mr. Gobel Mattingly of Compressed Gas Corporation provided valuable

assistance in analysis of the incident site and in the matter of gas

sampling.

62
Messrs. A1 Weber, John Nasci , Robert Schell, Herb Phelps, Alex

Loginow, Ray McCartney, William Forgeng, Jr., and John Gilligan of U.S.

Steel Corporation provided valuable commentary and assistance throughout

the course of this investigation.

Messrs. Steve Wechter, Frank Kramer and Woody Sarrow of Airco

Industrial Gases were extremely cooperative, skilled and dedicated in

carrying out the gas analyses reported in Section 6.

63
12. BIBLIOGRAPHY

1. Code of Federal Regulations, Title 49, Transportation p. 657,1977.

U.S. Government Printing Office, Washington, D.C. 20402.

2. Report of Inspection of Gas Cylinders, July 6, 1977-Original Date/

July 25, 1 977--Revision Date. Robert W. Hunt Company, Engineers,

810 South Clinton Street, Chicago, Illinois 60607.

3. "Pressure-Testing Relief Valves," Correspondence Dated November 21,

1977 from C. A. Robinson, Robinson Engineering, 4920 Bellemeade

Avenue, Evansville, Indianna, 47715 to Mr. Gilbert M. Ellis, Assistant

State Fire Marshall, Department of Housing, Buildings and Construction, Frankfort, Kentucky 40601.

4. Progress Reports from Bruce W. Christ, National Bureau of Standards,

to Arthur J. Mallen, 0HMR-D0T.

a. January 5, 1978 "Pneumatic Burst of 3T Trailer Tube No. 9959.

b. January 10, 1978 "Visit to Gladstone Laboratory, Cincinnati,

Ohio, To Survey Pieces of Burst 3T-Tube 9959".

c. April 14, 1978 "Susceptibility of Steels Used for Compressed

Gas Containers to Sustained Load Environmental

Cracking.

d. April 13, 1978 "Gas Analysi s-Hydrogen Sulfide Samples Taken

from Two Natural Gas Wells in the Shrewsbury

Field Near Litchfield, Kentucky.

e. June 8, 1978 "Determining Effects of Harmful Chemical Species

in DOT Specification 3T Trailer Tubes (Seamless

Pressure Vessels for Over-The-Highway Use).

f. July 6, 1978 "Technical Feasibility of Shipping Natural Gas

in Steel DOT Specification Compressed Gas

Containers.

g. July 19, 1978 "Specimen for Electron Microprobe Analysis

from Ruptured 3T Trailer Tube 9959."

5.

W. A. Maxey and T. P. Groeneveld: "Evaluation of the Use of DOT Type

3T Cylinders for the Transportation of Natural Gas", Report Issued

to the Goodyear Tire and Rubber Company, Sept. 1, 1978, From BattelleColumbus Laboratories, Columbus, Ohio.

64
6. American Iron and Steel Institute (AISI) Steel Products Manual. "Alloy

Steel: Semifinished; Hot Rolled and Cold Finished Bars", AISI, Washington,

D.C. , August, 1970.

7. a. J. R. Low, D. F. Stein, A. M. Turkalo and R. P. Laforce: Transactions of the Metallurgical Society of AIME, 1968, vol . 242, p. 14.

b. C. J. McMahon, Jr.: Temper Embrittlement in Steel, ASTM STP 407,

pp. 127-67, ASTM, Phi la. , PA. 1968.

c. K. Yoshino and C. J. McMahon, Jr.: Transactions of the Metallurgical

Society of AIME, 1974, vol. 5, p. 363.

8. R. M. Brick and A. Phillips: Structure and Properties of Alloys , pp. 65-68.

McGraw-Hill Book Company, Inc., New York, 1949.

9. B. F. Brown, Stress Corrosion Cracking Control Measures , National Bureau

of Standards Monograph 156, U.S. Government Printing Office, Washington,

D.C., 1977.

10. C. A. Zapffe and C. E. Sims: "Hydrogen Embrittlement, Internal Stress

and Defects in Steel", paper reproduced on pp. 13-59 of Hydrogen Damage , edited by C. D. Beachem, Published by American Society for Metals,

Metals Park, Ohio 44073, (1977).

11. "Corrosion of Oil and Gas Well Equipment", Book 2, American Petroleum

Institute, Division of Production, Dallas, Texas.

12. L. S. Darken and R. W. Gurry: Physical Chemistry of Metals , pp. 445-450.

McGraw-Hill Book Company, Inc. New York, 1953.

13. NACE Standard MR-01 -75 (1978 Revision), "Material Requirement-Sulfide

Stress Cracking Resistant Metallic Materials for Oil Field Equipment",

National Association of Corrosion Engineers, P. 0. Box 986, Katy,

Texas 77450.

14. NACE Task Group T-IF-9, "Specification for A Standard Sulfide Corrosion

Cracking Test", Draft Document Being Prepared. National Association of

Corrosion Engineers, P. 0. Box 986, Katy, Texas 77450.

15. S. T. Rolfe and J. H. Barsom: Fracture and Fatigue Control in Structures , Applications of Fracture Mechanics , p. 300. Prentice-Hall, Inc., Englewood

Cliffs, New Jersey.

16. ASTM E 399-72, "Standard Method of Test for Plane Strain Fracture Toughness

of Metallic Materials", American Society for Testing and Materials, 1916

Race Street, Philadelphia, PA 19103.

16a. J. A. Bennett and H. Mindlin: "Metallurgical Aspects of the Failure of the

Point Pleasant Bridge", ASTM Journal of Testing and Evaluation, Vol. 1,

No. 2, p. 152, 1973.

- 65 -

'/
17 . Gas Quality Laboratory Test Reports. Suction of Compressor, RENFRO #1,

RENFRO #3, Robinson Engineering, 4920 Bellemeade Ave., Evansville,

Indianna, 10/27/77.

18. Mr. S. Wechter: Airco Industrial Gases, Riverton, New Jersey 08077.

19. B. J. Moore: U. S. Bureau of Mines Circular 8749, "Analysis of Natural

Gases, 1976", U.S. Government Printing Office, Washington, D.C. 20402.

20. W. G. Clark, Jr., "Stress Corrosion Crack Initiation in High Strength

Type 4340 Steel", Flaw Growth and Fracture , ASTM STP 631, pp. 121-138,

American Society for Testing and Materials, Philadelphia, PA, 1977.

21. ASTM Dll 43-63, "Standard Test Method for Water Vapor Content of Gaseous

Fuels by Measurementof Dew-Point Temperature", Part 5.3, American

Society for Testing and Materials, 1916 Race St., Philadelphia, PA 19103.

22. A. Brown, J. T. Harrison and R. Wilkins: "Electrochemical Investigations

of Stress Corrosion Cracking of Plain Carbon Steel in the Carbon DioxideCarbon Monoxide-Water System", p. 686, Stress Corrosion Cracking and

Hydrogen Embrittlement of Iron Base Alloys , National Association of

Corrosion Engineers, 1440 South Creek, Houston, Texas 77084, 1977.

23. M. Kowama and S. Nagata: "Stress Corrosion Cracking of Mild and Low

Alloy Steels in CO-CO 2 -H 2 O", p. 680, Stress Corrosion Cracking and

Hydrogen Embrittlement of Iron Base Alloys , National Association of

Corrosion Engineers, 1440 Southcreek, Houston, Texas 77084, 1977.

24. C. J. Tiffany and J. N. Masters: "Applied Fracture Mechanics", Fracture

Toughness Testing and its Applications , ASTM STP 381, pp. 249-278,

American Society for Testing and Materials, Philadelphia, PA 1965.

25. A. W. Loginow and E. H. Phelps: "Steels for Seamless Hydrogen Pressure

Vessels", Paper Presented at 1974 Petroleum Mechanical Engineering

Conference, Dallas, Texas, Sept., 1974, From Research Laboratory, U.S.

Steel Corporation, Monroeville, PA 15146.

26. W. A. Maxey, J. F. Kiefner, R. J. Eiber and A. R. Duffy: "Ductile

Fracture Initiation, Propagation, and Arrest in Cylindrical Vessels."

ASTM STP 514, Fracture Toughness . ASTM, Philadelphia, PA 1972.

27. J. F. Kiefner, W. A. Maxey, R. J. Eiber and A. R. Duffy: "Failure Stress

Levels of Flaws in Pressurized Cylinders." ASTM STP 536, Progress in

Flaw Growth and Fracture Toughness Testing . ASTM Philadelphia, PA, 1973.

28. W. A. Maxey: "Fracture Initiation, Propagation and Arrest," pp. J-l to

J- 31 in Fifth Symposium on Line Pipe Research, Houston, Nov., 1974.

Pipeline Research Committee of American Gas Association, 1515 Wilson

Boulevard, Arlington, VA 22209.

66
29 . E. S. Folias: "The Stresses in a Cylindrical Shell Containing an

Axial Crack," ARL 64-174, Aerospace Research Laboratories, 1964.

30. W. P. Rees: "Note on Stress Corrosion Cracking of 11021

Presence of Sulphur Compounds," pp. 333-335, Symposium

Stresses in Metals and Alloys , The Institute of Metals,

Gardens, London, S.W.l, 1948.

Steels in the

on Internal

4 Grosvenor

67
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Chemical

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X 43 !h c ID • cn 3 3 3 3 3 3 3 I—1 3 G 3 3 03 ft rH

id 3 P id 44 tp

U 3 3 5 3 S 3 £ a S 3 44B 3 cn rH

B 4J >i 44

O •H id 43 cn cn cn

O •H cn ’ ' 3 3 id O 44 CD ft a; 44 44 44 44 — 44 44 44 44 3 cs P 44 44

On 3 E 03O 44 cn Pi

O

O

O

O 3 O

O 44 3 rQ

O 44 43 Sh id ID ID •H r ft 3 • 3 • 3 • 3 3 • 3 3 ft E 03 • rd 4J US ft Sh P ID 44 0 Sh c Sh G Sh G Sh c Sh c Sh P cn P Sh C CQ 44 o CO 44 O 3 O 3 O 3 O 3 U 3 O rH CS 3 C U <D ID U ID cn B B B e E 3 o , E 44 •H id 43 ft Sh id 43 P U •H U •H U •H ft •H ft •H ft SH u hH -H 3

Sh 44 as • Sh EhG o o o o o 3 • 3 o o 3 s c a 3 3 as 3 P

Pi 3 3 3 3 3 3 3 3 s G 3 3 3 CS Pi CD ft ft ft ft ft ft ft Pi ft Pi 3 rH ft ft 3

44

p

u

R cn

B ft • 3 R cn

R cn

R cn

R cnR US

R cn E rH

R US

G 0 a) o 44 44 •H 44 43 44 44 44 44 44 44 44 •H 3 44 •H c o ft O o CD O 44 cn cn cn cn cn O o 44 CO E

03 3 03 a) ft 3 3 •H 3 3 •H 3 •H 3 •H 3 3 3 44 (L) *H rd

us & o C Sh ft >1 Sh 44

C 43

G 43

G 43

C 43C 43

C SH ft 3 G 43

X < cn 44 O o cn 44 O 3 O 44 O 44 O 44 O 44 a 44 o o US PQ O 44 3 1 us £

44 44 1 1 o id pH rH 3 US 0) rd ^ 3

S-l c S us

G rd

U 3 3 •H s

CQ o 43 CM rH in in

p P U •o o 00 CO CM rH rH p G 3

43 •H c o o f—1 o rH 3 44

OC

CD 44 US -H • • • • •H *r| •rH & a G o o o o o R CP 44

r§ £ oQ H rH G G O o3 X 1 O rH W 3 e CO •P 3 43 3 r» 3 in E 4-> 43 in CP 44 Sh I— ID 1

us •P ft

O CN o 1 0 P CO rH ro

44X 3 G • • z cn • • • o id Q -H o o 3 o o o

id S as uu S

ft T1 3 3 £ X! US a) U &

+J 3 Sh 3 B tP 43 P m ft

Eh •H

G

O cn | CN Sh LD CP cn

X a) c P0 44 id rH

p r-~ o 00 P

id i-3 -H • O as • cn • • • 1—1 S rH s S x-x o o o O ft O 44 O' < • 44 CP 3 H

44 ft x—

O CN CM CN (N CN o

id rH rH rH rH 3 rH t"

Sh i i i i 3 i U g 2 rH ft LD 2 cn g • 2 2 i 2 1 2 2 CP

MH ft ft •H 0 a> <D Eh 3 Eh x—s 3 lO 3 ft o U O o O o 1 O G a) in (D G r- 3 G t- 3 ft 3 3 0 H •H O •H O •H Eh •H 3

•H ft • ft •H • ftH • ft ft cn

+j CP 44 CP 44 CP

C

id ft •H ft id •H ft 3 •H ft 0 ft o 0 ft O O ft 0 O ft 0 •H 0 P 0 0 O 44 G

ft 3 3 CD rH ID 3 rH 3 3 3 3 3

CP ID CP a> CP 3 CP C4 CP P cn P 44 cn P 44 cn P O P 44 w W id W 3 W rH w 3 C0

•H

G 44

a) id B o

•rH -rH 3 rH CO O ft Sh rH 1—1

rH cs

a) -h P c i 1) i 1 ft -P 44 CN CN CN CN

cn c O CP rH rH rH rH

a) id •H ft i i i i P Sh )H < < £

ft

H

gggg ft O 1—1

in CO 2 2 2 2

Except

where

noted

otherwise,

all

cracks

were

examined

at

the

National

Bureau

of

Standards.
Table VI

* Results Of Electron Microprobe Analysis

Of A Banded Region in Specimen L

Element

Multiplying

Factor Above

Background

Background (wt. pet.)

(as determined

from Table I)

Estimated

Concentration

in Band (wt. pet.)

Phosphorus 1.5 0.014 0.021

Chromium 1.3 0.98 1.274

Molybdenum 1.3 0.20 0.26

Manganese 1.2 0. 92 1.104

Silicon 1.2 0. 27 0.324

Carbon 1.0 0.44 0.44

Sulfur 1.0 0.017 0.017

* These results were obtained at the U.S. Steel Applied Research Laboratory,

Monroeville, PA. through arrangements made with Dr. H. Phelps.
Microhardness Survey of Banded Regions

Hardness Inside

White-Etching

Microzones

Hardness Adjacent

to White-Etching

Microzones

1 Rockwell Rockwell

Specimen C-Scale C-Scale

Identification Location of Specimen Knoop* Equivalent Knoop* Equivalent Remarks

lj| Fracture

j Origin

1A Edge of piece RR-12 at 443 43 393 39 White-etching bands were

location TR-1 (see 440 43 412 40.5 easily detected.

Fig. 7c) 440 43 418 40.5

ji 427 42 402 40

I 425 42 391 39

1 422 41.5 393 39

i 453 44

3A Edge of piece RR-12 at 418 41 372 37 White-etching bands were location TR-3 (See 410 40.5 388 39 easily detected.

I Fig. 7c) 388 39

2A Edge of piece RR-12 at 413 41 White-etching bands were

1 location TR-2 (See 365 36.5 easily detected.

Fig. 7c) 416 41

| 390 39 390 39

5A Edge of piece RR-12 at 422 41.5 363 36 White-etching bands were

location TR- 5 (See 356 35.5 easily detected.

Fig. 7c) L Edge of piece RR-12 46 40 White-etching bands were

at end (See Fig. 7c.) 45 41 easily detected. (See 39 Fig. 25.) 42

R Edge of piece RR-12 at 462 44.5 352 35 White-etching bands were

end (See Fig. 7c) 410 41 401 40 easily detected. (See 410 41 376 37 Fig. 25)

D Transverse section of 381 38 No white-etching bands

left end of specimen 378 38 were detected.

labeled "D" in Fig. 5 363 36 of piece RR-12 366 36

381 38 3B Transverse section 257 21 No white-etching bands near head of cylinder 307 29 were detected. Indenta10041 on trailer - 347 30 tions were made in light T-256 (See Fig. 6) 334 33 and dark-etching bands.

5B Transverse section 310L 30 No white-etching bands

near head of cylinder 292D 27 were detected. Indenta10041 on trailer

“ “ 320L 31 tions were made in light T-256 (See Fiq. 6) and dark-etching bands.

6B Transverse section 284L 26 No white-etching bands

near head of cylinder 312D 30 were detected. Indenta10041 on trailer ~ 330L 32 tions were made in light T-256 (See Fig. 6) 322L 31 and dark-etching bands.

1 330L 32

Knoop microhardness measurements were made with a 500 gram load to facilitate conversion to the Rockwell C-scale equivalent using the standard tabulation of conversions issued by the Wilson

Instrument Division of American Chain & Cable Company, Inc.
(Yield

Strength =

122.5

ksi

Tensile

Strength =

142.5

ksi)

(Steel

Specimens

From

Cylinder

10041)

cop

TJ P as 03

as o 43 as as c as 4M 44 c

44 P 44 0 CO • p 0 3 o C 03 03 4-> 4M c 3 e 44 B as ^ X C 0 0 44 o 0 03 CO 03 as as •H

!m a:p

C P 43 > 43 44 CO a:

a o • CP 03 as +j as 44 0 P cs .

ccS c e s 43

p 0 as 03 c 0 P as 0 CO E o 4-1 CP 4m p p P as +j U B 44 >i •H P •H 0 g CS E H 03 Cn 0 as •H •H

ai o s 03 CO 4-S

p 43 as

O 0 43 as 0 o3 a: 03 P 44 0 43 as r—1

44 CP 1—1CD S o as 03 44 P CP

CO 03 -p c 44 CO 0 . 0 P -p p 03 0 G 03 P - 0 c 0) 0 O H U 0 •H 0) 0 C as +j 0 X 44 4M o tji C •H 0 43 •H P 0 as O P •H 44

CO (0 CO CO 03 • c >1

p 44 0 CP CO a: (CS c 4m CCS as 03 1—1 C p c Pm p s as 0 3= CO C1S rH as CP •H CO c as 0

oj +Jp 44 rHp •H as 44 £ 11 * X p C as O -H •H 4-> Cn CO e X P

as 1 c cup

p as +j P G o3 •H as

p

ec; l-H 03

g 44 CO c AS •H

O 03

CO i •H as as

U £ as CO i 01 0) •H 03 03 OS 0 Ai CP as as H 43 G (0 CO c CO P P O CO 43 G

CO o O AS 03 co a3 03 o CO 03u o

Ai c 05 S P COG

•H CO

m £ CO as s O •H •H CO in • 1 CO ro • G >1 G 43 0 as 43 1 CO co x CO 0 X 0) 43 03 as 4-1 p p 44 CM 0 p rH p p e -P 03 i 4m CP rH p

>1 s rH

u >1 •H S •H P G CM o

pH 0 • n3 f—C 0 o O cn • U

O Ai t as •H • 03 44 P O 44 p 0) p 03 as 4m a c 3 O c tjc G c Co CP Co P as CP 03 as Co G G

OS 03 oJ <V CO as -p CO 44 P e •H 0 TJ o3

!m Ai c P P a; AS 43u CP

O

H 44 P C P

03

u 0 OS o COO +J as CO as

U 03 > 0

CP CCS >1 03 CP oS •H 03O +j •H

a as as 4M as >i 03

CP P as

a CP P 43 P G as 43X 43 CP 43 as

C

<c o CQ rtf c o Em

O 03 TS Em as 44 CO < o m 03

03

as • 44

03B * u

l§

H cn 1 44 H *H

CO w

Ai co

K*

CM CM

rH CTs PO

u

css H + co a -h H COP 03

b 03 PM 43 as as as as o 44 B ——— 44 44 44 s E co U o o

44 0 h as as CD as as CM G p 43 44 rH 44 44 rH

as o CO

O as rH as as •—i

CM

44 >i

Ga • Q

a • • X AS 03 *H o o O w U a — as as as 03 c c G P as 0 0 o U > Z z z

•H

Pm

>i '—1 44 c

rtf CO

H

M •H CO CO w P as G 1 LD LO LD LD o

•Hp as •H 00 oo LO

a 44 44 CO H in cH '

’ C

(1) Sm

CP Z

co

cn

m or LD <o * + * *

This

tabulation

shows

crack

growth

beyond

the

base

of

the

chevron

(See

Fig.

26).

Calculated

via

the

method

reported

by

R.B.

Heady:

"Sulfide

Corrosive

Cracking

in

Gas

and

Oil

Wells

XIB,

Technical

Progress

Report

BRC-Corp

5-74-B,

Shell

Oil

Company,

Houston.

Smaller K

values

might

have

been

determined

if

the

crack

had

not

turned

into

the

specimen

arm.

-LO
Table IX

Results of Hydrogen Sulfide and Moisture Analysis

Hydrogen Sulfide Moisture

Location Concentration (ppm)Concentration (ppm)

Input to compressor 490 +25 800

Gas Well-Renfro #1 603 +25 800

Gas Well-Renfro #3 572 +25 800

* Concentration is given in parts per million (ppm) by volume.
Table X

Component

Ethane

Propane

ISO-Butane

n-3utane

n-Pentane

Other C c Hydrocarbon

D Nitrogen

Methane

* Results Of Gas Analysis

(Volume Percent)

Input To

Compressor Renfro #1 Renfro #3

4.23 4.2 4.22

1.89 1.87 1.90

0.17 0.17 0.17

0.37 0.37 0.40

0.10 0.10 0.10

0.08 0.07 0.08

17.0 17.6 17.6

Balance Balance Balance

Hydrogen sulfide and moisture appear in Table IX.
Results

of

Rockwell

C-Scale

Hardness

Survey

On

Transverse

Sections

op • • to to

<4-4 C G G G

CD 0 to 1 a) a> ID ID 1—

1

p <D g g g g n c a -P •H *H •H •H

<d o t •H o o o o to CO P -H g 43 4) cd a) ID 0) G G

id to 3 S 0) ft ft ft ft O O

ft p c co to to to to •H •H

CO g <d a) — •p -p 43 0 > to 43 0) <D ID 03 id id P O C to -P to rH rH rH rH P •P

id 0 CD 0) •H *r| •H •H c G g cd u c CD c to to to to a) ID

<D P X t 0 G C c G T3 t

ft <d tx p •rH N <D a) ID ID G G

X! It to o -p -p -P -P •H •H

to 43 p p

0) 13 0 P O r~x C G G C CO CO PGP 0 -H • 0 o 0 0 CM rH

r-H P C_ g H

id o 03 H T3 X5 T3 <c <4-1 <4H > <w g (D tP > ID 0) a) CD o O

C G p p p P

<1) 0) CL -H -H CD p p P P • • to to o flJ rC H to to to to CO CO coo -P O 43 id id id id > >

43 43 C 43 -P id id a> ID ID < <

& +> ft 0 <D Eh s s s £

in

ur in co in u co CO i CO CO o

ft i 1 <43 [- CO <4> ID 1 rH • • S3 co r—

1

m ro m CO • CN CO o o

co CO rH CO CO CO CO

43O - G o

•H

1 o

i" c- t3 m

£3 rH ID o • rH • •H g to ID CO C -P 0 -H 43 -H

id id p ft id ft g o <4-4 rH •H -H G G U <4-1 to -h a i£> rn 10 -H

(1) -H G 1 1 1 G

ft -P ID -P ft ft ft ID = CO £3 g G eh Eh Eh i-3 ft g Q U Q

0) -h a) •H • TS o g o H CD & ID T3

ft ID ft G

co to co id

£4 T3 43

CD id <4H u t CM CM ID O O

C G 43 rH o -H • <=c • P -P o

•H rH CN O Co t" to P id id -

4-) 1 1 rH *H rH *H G -P o

id u _ ft _ ft O C H

o ft ft ft P ID H CO 0 to ft 2 a) 2 a) <4-4 O Id

£3 0) CD 5 -P 0 CO CO P P ID P

rH id Id 03 O

<c ID d> 'H 43 Z Z to (d

g . O i£> in

'xT CN to i m p Eh CO CD CO T3 >H C <D •H i—

I

u p

G • G • O <4> O P1 X m in ^ • rH rH CN • CO IN •. i4> 2!o

• <4> 1 «

p Eh P Eh^1

• O a) ID Co rHo3 P to P p to

•H G a) -h C CD -H

ft •H rH Cm •H rH Cm rH 'H i

1 *H

ID >i id <d >i id id

ID U P CD U P ID to -P CO '— -P CO
Rockwell

C-Scale

Hardness

At

Outside

Surface

cd cd 0 M-l cd >-i X 3 1 m

Cfl < m in o oj

m

in co O'

h

h

in ni

n (Nm

O oo o noO

I—I 'CT CO <Ti CO

f\| (N

M (N CM

co no o o o m om oo o r~ no no no CN no

04

CD

<1)

CO

<d -Cuc

•HI noO

CO CM

O CO CO oo o in in o no cm o in

H

CO 00 00 CO O'

CM CM CM CM CM

co co oo co

CM CM

N CM CM CD CD r~ CD t-' CM CM CM CM CM

u

CD CD >0 CD

cd

XOc

xs

H

CD 1 N CO •H no u O p • XI O M

cd *H U M-l CD O Q

TO S a 0 -H

1—1 U

CD

u

CQ '—

LCO

O LTl f'- .H CN CN oo CD CD

H CM CM Mf M1 M1 M1 cf

no no no no no

no no no cn cm no no no no no

00 00 OO 00 OM

CN net CN CN CN

+Jaon

tnu

cda> 2

T)

cd

CD X

CD T>c

•H

rH

>i u

COLO

CN cn

cn S-i OO CD oo T)c

iH *H

>o cd u M — +j i—I *(H >1 Cd U Vm — +J
&HR-14 «20on'

«456‘

Rtar Head (ZZ2>

RR-12

aRR 16 W450'

Tubs 7

RR-1S

B/L

»600*

Figure 1. Incident site after the pneumatic burst. Number by each

piece indicates approximate distance from remains of trailer

T-256, for those pieces for which this information was

provided. The relative sizes of the pieces is indicated

roughly by the size in the figure. The position of each

piece was indicated by Mr. Rodney Raby of the Kentucky

State Fire Marshal's Office. The positions of pieces RR-18

and RR-19 were not identified.

Figure 2a. Eleven pieces of cylinder 9959 laid out on the floor at

Gladstone Laboratories, Inc. The rear head is in the

foreground. (Photo courtesy of M. Bolinger, Jr., Gladstone

Laboratories, Inc., Cincinnati, Ohio).
,
FRONT

X <1) p £

Eh «w * 0 • a> w a) X c p 3 0 3 s

•H •p P o • »d m £ o p 0 <JH 4h i—i

>1 0

cd

p >i

p Ifl CO

nJ E a) E •H p

•H P p X ft 3 0 0 p <u O a x a p E

(ti ItH <sp

cr> 0 Cn c m

•H

x •H £ p D 0 <0 -

x: ft

CO

<d • V .c CO 01 p x: m p . 01 co <a 01 5 ft • 0 p .c <u < D P v A

<N0) p&

•H

Cn

Figure 3. Remains of trailer T-256 after the incident,, as seen from front. (Photo courtesy of Mr. W. Piehl, Western

Sales and Testing, Inc., Amarillo, Texas.)

Figure 4. Intact tube trailer composed of specification DOT 3T

cylinders. The array of cylinders on the trailer is

3-high by 4-wide. (Photo courtesy of Mr. W. Piehl,

Western Sales and Testing, Inc., Amarillo, Texas.)

Figure

5.

Inside

wall

of

piece

RR-12

showing

location

of

specimen

taken

for

chemical

analysis.

The

arrows

point

in

the

longitudinal

direction.

This

piece

is

about

30-inches

wide

by

about

50-inches

long.

(Photo

courtesy

of

M.

Bolinger,

Jr.,

Gladstone

Laboratories,

Inc.,

Cincinnati,

Ohio.)

TUBE TRAILER T-256

( Viewed from rear)

Figure 6. Array of specification DOT-3T cylinders on tube trailer T-256

as viewed from the rear. The cylinder which ruptured is on

the top row at the right. The location of the fracture origin

is shown. It was 12.5 feet from the front head. The "RR- " designations assigned to the pieces near the fracture

origin are indicated. (Identification number for each tube

supplied by Jack B. Kelly Enterprises, Inc., Amarillo, Texas.)

T3

0)

CD 0 -P X rH G 3 3

rH

G

r—1 •H 0

CD CO C •H i—1 •H P i—1 rH o 3

4H

CD CD X P 0 P 3 -P P

CnO CD

G cd P 0 p 3 1—1 4-1 P

03

O

CD 3

CN

X P i—1 Eh IP

Dh -p K • (0 0 rH 0 P M-t o 0 0 P 6 0 0 a e P

•H 4H

4-1 -p 0 G

G 0 0 0 O *H

Cn •p t G •rH 0 *H 0G s 0 cd 0 *H P p -P «P 0i—1 0

-P 0 X 3 O Eh O 0

-P 0 . c X, 0 0 Eh i—1 e cd & o 0 • 0 0 CD 0

O' 0 £ G P -p 0 3 -P

-Gp

O

r 3 G G i

l o

•HH cn o 1 r 0 G - 0 -P 0 0 0 3 -P p

•P

O •P 3 • G 0 -P 0 0 3 0 o i

i > <0 a cd cd 0 0 CL, p o

CO : 0 ip 0

cd

t"'

0)p3

C3 •H

(P

side

of

the

fracture

origin,

between

20

and

39

on

the
Figure

7b.

View

of

fracture

origin

looking

directly

down

on

it.

The

scale

is

against

the

outside

wall.
I

Figure

7c.

Artist's

drawing

of

the

segment

containing

the

fracture

origin.

Transverse

sections

cut

from

this

segment

are

labeled

"TR-

.

Figure 8a. Fracture origin on piece RR-12 before cleaning, viewed

toward inside wall.

Inside Wall

Figure 8b. Close-up view of step on fracture surface. Shear lips show

up as intermittent white markings along inside wall. These

shear lips are probably in the decarburized layer (see

Section 5.3).

Figure 9a. Forging seams evident on inside wall at front head of

cylinder 10041. The scale is in centimeters.
Figure 9b. Cross-section of typical forging seams found in front

head of cylinder 10041.
Figure 10a. Hit mark on cylinder number 8 (see Fig. 6 for location)

made by opening flap of piece KR-16. The lines AB and

BC in this figure correspond to the lines AB and BC in

Figure 2b. The rear head of the cylinder is to the

right.

Figure 10b. Bowing of cylinder Number 8 due to the blow resulting

in the hit mark shown in Figure 10a.

Xo m \

0) > X inC

G fd O p H • Eh •H C O O

d) -H fd £X -P W rd "

-p XI P G O d) in ip x! c C 0 O -H *H

-P

(/) fd

d) in -P P d) G 3 C d) Cn X3 XI 1—1 •H P C

(0 IX fd *H H x

-p <u in G <U d) in

<u

m i i d>

p fd

G

<u

u xi

M-l

W

p

g • i fd P X o X U i Ph 1 1 0 P P

*H E”* rH

U

u 0)

H c S g O Pi

d) c

O, fd in in <p G •H 0

d)

X •H

X) <N d) in •H rH . XI i—1

a>

in rH X) d>O •P

+J -P P P 3 £ o *H <U O d) o in

0)

X (0

X

O

<d

Cn f—I X X u

fd <u P tP O P

td

U 1 I

in Xu

cu <u fO cr

OjX rHG

>i E-f

X -H P 5

>1 0

XI

—

GX

G • •h in

(d

G P 0 G

PI -H M-4 0

-P 0 -H

<u <d

V

IX o 2 O

Jx 0 0 (D

E-i •—l p in

Cl) XI iH •H i—I

in

m

g s

Q)pp

tp •H

IX

full

wall

thickness.

(b)

Close-up

view

of

the
a

C -H

03 X

E co H C O O

03 *H

O-i X

CO 03 rH

03 (1) c u o

N CncH

5

•H O • E .C C4

CO -H

D-> 4-1 c c

•H o X -H O 4-1 4-> O Cn

0) OJ C

CO -H

4-> 03

I03

4-1 H

X

3c

•H

a

03 o

C/3 »—4 >4 03

03 >

> 03

C/3 03 C

03 -G

34 4-> E-| 03a,

TOc

po

44 0) ^

O

03

34U

CJ *H (—

I

4-1 03 03 P

C

Oi U 03 >i O 34

4-1 <H tji

44 C 03 0 0) 4-3 E G a -H H

•H O

4-3 0304 X • -P CO 0) 03

4-1 34

03 •G O • 03 4-1 44 1 1 X 03 rH

>, a O 03

i-4 5

0s u 03 0) 03 34 03 E 34 P *H P O Cn CO •H •H c

C 34 t4 -r4 0 03 g

i

1 0) o

E P 0) 4-3 c CO \ 03 — X

V 34 o

c Cn 03 (3 34 . 34 ,G 03 CD O O 4-1 1 4-1 c 00 44 w H tH O

rsi

03 03 0) 34P

Cn

•H
Etchant: 100X

Ammonium Molybdate

Figure 13. White-etching microzones found near inside wall in

transverse specimen L. Rockwell C-scale hardness

equivalents are shown. (see Figure 7c for specimen

location.

)

Etchant ammonium molybdate. 80X

'Figure 14. Banding near inside wall detected in transverse section

taken from a ring cut near the front head of cylinder 10041.

Figure 15.

Etchant: 2% nital. 100X

Cracking parallel to banding and inclusions in an unstressed

transverse section of a test specimen saturated with

diffusible hydrogen in the laboratory. This specimen was taken from a ring cut near the front head of cylinder 10041.

Circumferential
<u x

0)

o

Oj

U) <c

Figure

16.

Incipient

type

L

cracks

which

appear

to

have

originated

at

radial

jogs

in

a

type

C

crack.

This

cracking

was

found

in

specimen

TR-5.

(a)

As

polished

section

showing

cracks,

(b)

Same

section

as

in

16a

after

etching

with

ammonium

molybdate.
Inside

Wall

As Polished X400

Figure 17. Surface lap detected near inside wall of cylinder 10041.

Figure 18a. A portion of piece RR-12 showing the general surface

topography of the inside wall near the fracture origin.

The fracture origin is along the bottom near the words

"saw cut." (Photo courtesy of M. Bolinger, Jr.,

Gladstone Laboratories, Inc., Cincinnati, Ohio.)
Figure 18b. A view of piece RR-12 showing sets of parallel lines

running in the circumferential direction. (Photo

courtesy of M. Bolinger, Jr., Gladstone Laboratories,

Inc., Cincinnati, Ohio.)
X24

Figure 19. Surface topography of the inside wall of specimen TR-1

near the fracture origin after solution cleaning, showing

craters and grinding scratches. (Photo obtained in

scanning electron microscope.)
O O'

3 <4-1

M — (fl

a

oo

0

(N

0)1

•H

Cn
Figure 21 . Close-up view of the fracture origin after stripping three

cellulose acetate replicas. The black product on the

fracture surface became evident as the orange-brown product

was stripped away. This view is towards the inside wall.

Longitudinal

Figure

22.

Scanning

electron

microscope

views

of

fracture

origin

after

cleaning

with

buffered

HCe,

solution.

(a)

Region

of

step,

showing

a

type

C

crack,

(b)

Region

away

from

step,

showing

"plateaus

and

valleys"

structure.

Fracture

xo

LDpd3XOPwX

CM\

<u p3t

X)

t>i Pd

(0 xopw

p

res 1 0) to x p

-p tn

d 3 0) -H O TJ IP UP d

•p

Sp i—! < tn

0) tn £ 0) 0) • d p <—i t P i— p X 3 3

0) 3 Xo

CP 0)p

C

r0 o H •H •H

tn E 0 d x •H QJ

tn

X P P

>. fO

QJ ft

0)d o

•p> d

d 0 0

tn •H •H

in CP CP

qj 0) 0) d p p Xu T3 QJ •H QJ X X TJ P p d

tO ft 1—1 X 0 1—3 d s

ft

H QJ •H

tn > 3 x -— u ft

to 3 p l • CJ QJ d tn •H u 0

CP (—1

•H QJ

u P ft 0 ft • P X 3

0)

—

m

p 0 CM 3 3

-P P • QJ u 1—1 p 3 T3 1—1 3 P d 3 CP

<p to 5 H

ft

Xd QJ CP -p t d 3 CP •P -H 0 -H tn P P P P X 0 3 X P 0 CP p -rH d to QJ P o X

*H QJ P ft P O 0 o tO P p

0) ip 3

tn P 01 3 tn X

OJ tnpp

tn 3 p 3 QJ

p

QJ

p ft 3 > 3 ft

tn P 3 tn d O d

to 3 ft 0 p P •H •H

E-* ft i—1 p n

CM0) p3

cp H

ft
Figure 24. Typical microstructure of tempered bainite found in

cylinder 9959.

Etchant: Picral 1000X

Inside

Wall

1

1

L R 3.7X

Figure 25. Transverse sections from ends of the segment from piece

RR-12 shown in Figure 7c.
DCB SPECIMEN

NOTE

Grooves to be centered tv within +.002'' $ must ben opposite to within +.002"

A

Figure 26. Diagram of the double cantilever beam test specimen used

for K „ testing.

I SCC

o

CM

Figure

27.

Through

wall

flaw

size-toughness

relationship

for

cylindrical

pressure

vessels.

(Diagram

courtesy

of

W.

Maxey

of

Battelle

Memorial

Institute,

Columbus,

Ohio.)

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