ML20129F611

Final Rept on Investigation of Failed Field Anchor Heads HV016 & HV038 from Jm Farley Nuclear Power Plant Unit 2 Containment

ML20129F611
Person / Time
Site:	Farley
Issue date:	04/09/1985
From:	Groeneveld T Battelle Memorial Institute, COLUMBUS LABORATORIES
To:
Shared Package
ML20129F600	List: ML20129F596 ML20129F611 ML20129F633
References
NUDOCS 8507170410
Download: ML20129F611 (42)

Text

I lO FINAL REPORT r

l i on s

i l

INVESTIGATION OF FAILED FIELD ANCHOR HEADS

HV016 AND HV038 FROM THE J. M. FARLEY

, NUCLEAR POWER PLANT UNIT 2 CONTAINMENT I to I

~O INRvCO. INCORe0RATEo

April 9,1985

i by T. P. Groeneveld BATTELLE Colutus Laboratories 505 King Avenue Colud us, Ohio 43201 Battelle is not engaged in research for advertising, O- saies promotion. or posiicity purposes aad this report ey not be reporduced in full or in part for such purposes.

8507170410 850710~

I PDR ADOCK 05000348 S PDR

i l

i

. TABLE OF CONTENTS Page l

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 SUMMA'RY ............................. 3 METALLURGICAL STUDIES ..................'.... .

5 Chemical Composition .................... 5 s-Tensile Properties ..................... 5

Impac t Prope rties . . . . . . . . . . . . . . . . . . . . . . 7 Hardness .......................... 10 i Visual Examination ....'................. 11 j Scanning-Electron-Microscope Examination .......... 15 Anchor Head HV016 ................... 15 I Anchor Head HV038 ................... 21 0 EDAX Analyses of Deposits on Tendon Hole Surfaces ... 24 t

Fracture Surfaces of the Tensile and Impact Specimens From the Anchor Heads ............ 27 l

Metallographic Examination ................. 28 Mi c ros t ruc tu re . . . . . . . . . . . . . . . . . . . . . 28 Cleanliness Ratings .................. 31 Examination of Failed Tendon Wires ............. 31 DISCUSSION OF RESULTS ...................... 32 l

i i

O l

I

l I

INVESTIGATION OF FAILED FIELD ANCHOR HEADS HV016 AND HV038 FROM THE J. M. FARLEY

. NUCLEAR POWER PLANT UNIT 2 CONTAINMENT by T. P, Groeneveld INTRODUCTION On January 27, 1985, at the Joseph M. Farley Nuclear Power Plant

, operated by Alabama Power Company in Dolton, Alabama, Field Anchor Head

. HV016 from Tendon V17 in the Unit 2 containment was discovered to havs t

failed. The failed anchor head was discovered during inspection of the containment posttensioning system while Unit 2 was shut down for refueling.

On January 30, 1985, Field Anchor Head HV038 (from Tendon V21) was found to have failed also. The exact time of failure of those two field anchor heads was not known. Both had been in service since 1977, and the post-O tensioning system was last inspected during June,1983. At that time, all of the anchor heads inspected were found to be intact.

The two failed field anchor heads were part of the posttensioning system for the Unit 2 reactor containment. Both were machined from 10-inch-diameter hot-rolled-and-annealed steel rounds produced in accordance with the requirements set forth in ASTM A322 for Grade 4140/4142 alloy steel.

Both anchor heads reportedly were fabricated from rounds produced from the

. same heat of steel (Republic Steel Company Heat No. 6061524), which was produced in 1973. In addition, both failed anchor heads were heat treated

!l in the same lot by Downey Steel Treating Company, Downey, California, in accordance with Military Specification MIL-H6875 to a hardness of 40 to 44 Rockwell C (Rc )*

The field anchor heads are 9.375 inches in diameter by 4 inches i high. One hundred and seventy 0.257-in .n .iiameter holes are drilled longitudinally through the central region of the anchor heads forming a l honeycomb region. Cold-drawn-steel tendon wires ( ASTM A421) pass through those holes and are cold headed during installation of the posttensioning I .-

e l

2

'O system. The field anchor heads are then pulled away from the bearing plate l by jacks so as to stress the tendons, and ASTM A36 steel shims are inserted between the anchor head and the bearing plate when the proper tendon stress level.is reached. After tensioning, grease covers are placed over the anchor heads, and corrosion-protection grease (Visco 0 2090P) is pumped into the system (the field anchor grease cans, tendon conduit, and the shop anchor grease cans). At the Farley Plant, the grease covers and the tendon conduit were made of galvanized steel.

When the grease covers of both of the failed anchor heads were removed and examined subsequently, water was found in the grease. Analyses of the grease and water were performed by Inland Steel Company and Suburban Laboratories, Incorporated. In addition, flakes of zine were found in the grease.

,j The failure of Field Anchor Head HV016 resulted in six major pie-shaped fractured pieces and numerous other smaller fragments. Two of the major pie-shaped pieces, identified as Pieces 4 and 5, were provided Q to Batte11e's Columbus Laboratories for this investigation. The failure of Field Anchor Head HV038 produced two major pieces, with the fracture occurring essentially along a diameter of the anchor head. The plane of the major fracture was at an angle of approximately 30 degrees to the shim-stack gap. In addition, another crack had occurred through a portion of

one of the broken halves, and a small fragment had broken out of the honey-comb region. Anchor Head HV038 was inspected at Inland Steel on l February 18, 1985, by Inland Steel Metallurgical Laboratory personnel, I

Mr. G. Schmidt of Bechtel Corporation, Mr. H. Presswalla of INRYCO, and l! Mr. T. Groeneveld of Battelle. Following that inspection, one of the I

sections that represented approximately one-half of the failed anchor and the fragment that broke away from the honeycomb region were provided to Battelle for this study. The other half of the failed anchor, which contained the second major crack, was kept by Inland Steel for evaluation.

This report describes the metallurgical studies conducted at Battelle on Field Anchor Heads HV016 and HV038 to establish the most j probable cause(s) of their failure.

O l

1 l

3

.O -

SUMMARY

t The infonnation obtained from the metallurgical evaluation of failed Anchor Heads HV016 and HV038 'has shown that the most probable cause of their failure was hydrogen-stress cracking. The primary reasons for that conclusion were as follows:

(1) The fracture mode at the suspected origins and some other regions on the fracture surfaces was inter-granular fracture.

(2) The anchor heads were heat treated to hardnesses in the range from 40 to 44 Rockwell C.

i (3) The anchor heads were subjected to high stresses with tensile components during service.

f (4) Water was present in the grease cans surrounding the anchor heads, and particles of zine were entrained in the grease. In addition, zine was O detected on the fracture surfaces and the surfaces of the tendon holrs.

(5) The failures occurred after the posttensioning system had been in service between 6 and 7 years.

(6) The absence of intergranular fracture on the fracture surfaces of the tensile and impact specimens tested in the laboratory indicated that the anchor heads were not embrittled by mechanisms that can result in inter-granular fracture under purely mechanical loading, such

, as temper enbrittlement or tempered-martensite enbrittlement.

(7) Corrosion products usually associated with stress-corrosion cracking were not present on the fracture surfaces of the failed anchor heads.

(.8) The absence of heat-treating scale on the fracture surfaces showed that quench cracks were not present in the suspected origin regions of the failures.

l i

O It is well established that low-alloy steels heat treated to j' hardnesses in a range that includes the 40 to 44 Rc range of the anchor

! heads are susceptible to hydrogen-stress cracking when they are subjected to app' lied tensile stresses of suitable magnitude and' eith'er' contain atomic j hydrogen absorbed during prior processing or are placed in environments from j which they can absorb atomic hydrogen during service. The high stresses in the anchor heads that result from the loading of the tendons would be suffi-cient to cause failure if the steels contain atomic hydrogen or absorb atomic hydrogen during service. The presence of water and zinc particles in the

immediate vicinity of the anchor heads most likely was the source of the atomic hydrogen. Since zinc is electrochemically anodic to steel, it Will preferentially corrode and thereby cathodically protect the steel if moisture is present. As a result of that cathodic protection, atomic hydrogen will be generated at the surface of the steel, and some of that

hydrogen will be absorbed by the stressed steel, thus resulting in hydrogen-l stress cracking after some period of time during which the cracks initiate and grow. When the hydrogen-stress cracks have grown to a critical size, which is a function of the toughness of the steel, its strength, and the applied stress level, the remaining section will fail rapidly by overstress failure.

( ,

The steel used to fabricate the anchor heads essentially met the chemical-composition requirements of ASTM A322, Grade 4140/4142, steel and the hardness requirements specified for the anchor heads. The minor

. deviation in average carbon content of the steel from Anchor Head HV038 was not msponsible for its failure. The steel from both anchor heads i was relatively dirty with respect to sulfide inclusions, a condition that

contributed to lower ductility and impact properties in the transverse i di rection. However, that condition was not the cause of the cracking in the anchor heads.

To prevent similar failures of anchor heads in the future, steps should be taken to prevent water from contacting the surfaces of the steel.

Those steps should include thuroughly coating the anchor-head surfaces

'O

(

I '

ao

, with corrosion-prevention grease and eliminating the source (s) of water i in the posttensioning system. In addi. tion, the design of the post-tensioning system should be reviewed, and consideratton should be given to modifications that would permit t.he anchor heads to be made of a lower strength steel that will be less susceptible to hydrogen-stress

$ cracking and other environs 7 tally induced failure mechanisms.

A METALLURGICAL STUDIES

?

Chemical Composition i

A section from Piece 4 of Anchor Head HV016 and two sections,

h one from near the outer cylindrical surface and one from near the tendon holes, from Anchor Head HV038 were used to detennine the chemical compo-sitions of the steels using spectrographic-analysis techniques. The Q results of those analyses are listed in Table 1, along with the product-4 check-analysis range for ASTM A322, Grade 4140/4142, steel.

The results of those analyses indicated that the chemical compositions, except for the carbon content of the steel from HV038, were within the allowable limits for product analyses for ASTM A322, Grade 4140/4142, steel. For the steel from Anchor Head HV038, the average carbon content was 0.48 percent, or 0.01 percent above the maximum allow- '

l able amount. The sulfur content of the steel from both anchor heads was somewhat high, but well within the specified range. The aluminum content l- indicates that the heat of steel was aluminum killed. The levels of the other residual elements were low.

i Tensile Properties

?

The tensile properties of the >tesi from both anchor heads wrse j determined with both longitudinal and transverse round tensile specimens.

The axes of the longitudinal specimens were parallel to the axes of the Q cyiindrical anchor heads and the axes of the transverse specimens were 90 degrees to axes of the anchor heads, or parallel to the circular faces of the '

1 _ -_. .- .

I 6

'O TABLE 1. RESULTS OF CHEMICAL ANALYSES OF SAMPLES FROM ANCHOR HEADS l

l HV016 AND HV038 Chemical Composition, weight percent

' ASTM A322, Grade 4140/4142, Element Sample HV016(a) Sample HV038(a) Product-Analysis Range C 0.43 0.48(b) 0.36/0.47 Mn 0.98 0.96 0.69/1.04 P 0.012 0.011 0.040 max S 0.026 0.023 0.045 max Si 0.22 0.23 0.13/0.37 Cr 1.02 1.09 0.75/1.15 Mo 0.21 0.21 0.13/0.27 Cu 0.094 0.12 0.35 max Ni 0.13 0.14 0.25 max A1 0.013 0.014 --

Sn <0.005 0.009 --

Sb <0.005 <0.001 --

As ND(c) <0.005 V <0.001 <0. 01 --

Co <0.05 <0.10 --

Cb <0.01 <0.01 --

Pb <0.001 0.0002 --

Zr <0. 01 <0.01 --

i Ti <0.01 <0.01 --

l <0.05 W <0.05 --

B <0.0005 <0.0005 --

0 (a) Spectrographic analyses, average of two determinations per element.

1 (b) The individual carbon contents for the two analyses were 0.46 and

, 0.50; the higher carbon content was measured in the sample near i the tendon-hole region.

, .(c) ND = not determined.

l l --- --

i i.i 7

O anchor heads. Because of the limited sizes of the two sections from Anchor Head HV016 received for this study, ths' tensile specimens from it were limited to a 0 250-inch-diameter reduced section and a 1-inch gage length. The specimens from Anchor Head HV038 had a 0.5-inch-diameter reduced section and a 2-inch gage length. Duplicate specimens of each orientation from each anchor head were tested. The tensile properties of the steels from the two anchor heads are listed in Table 2.

The results of the tensile tests showed that the average ultimate tensile stmngth of the steel from both anchor heads was about 200 ksi and that the average yield strength ranged from 181 to 186 ksi..

, There was relatively little difference in the strength properties among the specimens from the two anchor heads or for the specimens from the two orientations. Similarly, there were no substantial differences in the ductilities of the steels from the two anchor heads, although the ductility

. of the steel from Anchor Head HV038 was somewhat lower than that of the O steel from Anchor Head HV016. However, as would be expected, the ductilities measured with the transverse specimens from each anchor head were substan-tially lower than those obtained with the longitudinal specimens.

l Impact Properties The impact properties of the steel from both anchor heads were

measured at room temperature and boiling-water temperate.e using full-size Charpy V-notch specimens machined from blanks removed from each anchor head i

in both the longitudinal and transverse directions. The impact properties

!' obtained are listed in Table 3.

l The results listed in Table 3 show that the impact properties of the specimens from both anchor heads were similar and that the energy absorbed by the transverse specimens was approximately one-half of that of longitudinal specimens. However, the values measured are considered

.; reasonable for this grade of steel heat treated to the 200-ksi ultimate-tensile-strength level. All of the impact specimens tested at room 1O

.I I -

O O O TABLE 2. TENSILE PROPERTIES OF ASTM A322, GRADE 4140/4142, STEEL FROM ANCHOR HEADS HV016 AND HV038

• Specimen Ultimate Yield Strength Elongation, Reduction Specimen Tensile Strength, (0.2% Offset), YS percent in in Area, Number Orientation (8) ksi ksi UTS 4D percent AnchorHeadHV016(b) 5-1 L 203.3 186.1 0.92 13 47.4 5-2 L 199.0 181.1 0.91 12 47.0-Average 201.2 183.6 M T2.T TT 2 4-1 T 197.4 177.6 0.90 8.5 23.6 4-2 T 201.4 184.4 0.92 7.5 16.2 Average 199.4 181.0 UT BU TFY Anchor Head HV038(c) 1 L 198.8 183.0 0.92 11 44.4 2 L 201.4 185.1 0.92 12 46.5 Average 200.1 184.1 W TTT 45.5 3 T 198.0 179.1 0.90 6 17.6 4 T 207.8 193.3 0.93 6.5 12.3 Average 202.9 186.2 0.92 6.5 15.0 (a) L-specimen axis parallel to the axis of the cylindrical anchor head.

T-specimen axis parallel to the circular faces of the anchor head and approximately normal to a diameter.

(b) Determined with round tensile specimens with 0.250-inch-diamete,r reduced sections.

(c) Determined with round tensile specimens with 0.500-inch-diameter reduced sections.

.I l

e 9

O TABLE 3.' CHARPY-V-NOTCH-SPECIMEN IMPACT-PROPERTIES OF THE e

STEELS FROM ANCHOR HEADS HV016 AND HV038 i

i Energy Specimen Absorbed

• Specimen ft-lb Number. Orientation I3) 72 F 212 F i Anchor Head HV016 5-1 L 11 -

, 5-2 L 12 -

5-3 L -

18

,, 5-4 L -

19 4-1 T 5 -

4-2 T 5.5 -

O- 4-3 1 -

20 4-4 T -

8'. 5 Anchor Head HV038 L-1 L 15 -

L-2 L 13 -

L-3 L -

21 L-4 L -

21

!!- T-1 T 7 -

T-2 T 7 -

] '

T-3 T -

9.5 T-4 T -

9.$

(a) L specimens were parallel to the axis of the cylindrical anchor head. T specimens

] were parallel to the circular surfaces of the anchor and essentially normal to a diameter.

O I

r.

F l

10 h temperature exhibited 90 to 95 percent brittle fracture. In addition, the

transverse specimens tested at 212 F exhibited between 80 and 90 percent brittle fracture; however, the longit 0'inal d specimens _ tested at 212 F exhib;ited considerably less brittle fracture, typically only 40 to 50 percent.

Hardness i Rockwell C hardness measurements were made in essentially radial paths traversing approximately 1-1/2 inches of the 2-inch-wide rim section 4

of Piece 4 from Anchor Head HV016. Those traverses were made on the shim-

, side surface and on a plane approximately 1 inch from the shim-side surface.

Similar Rockwell C hardness traverses were made on Anchor Head HV038 on the shim-side surface, the button-head surface, and on planes approximately 3/4 inch and 1-1/2 inches from the button-head surface. The results of those hardness traverses are listed in the following tabulation:

C Traverse Location Rockwell C Hardness Range Anchor Head HV016 Shim-side surface 41.5-44 1 inch from shim-side 41-43 surface Anchor Head HV038 Button-head surface 43-43.5 t 3/4 inch from button- 41-45 ii head surface 1-1/2 inch from button- 39-43

'I head surface i Shim-side surface 42-44 .

g The results of those hardness measurements indicate that the hardnesses of both anchor heads were within the specified range' of 40 to j 44 Rockwell C. As is noted in the tabulation, for Anchor Head HV038 there was one hardness value (45 Rc ) that was 1 point above the maximum and one

, hardnest value (39 Rc ) that was 1 point belcw the minimum value specified.

A Those two deviations from the specified range out of 27 measurements are not considered significant with regard to either the failure of these anchor heads or compliance with the specified hardness range.

I I

t 11 O

Visual Examination 1

The fracture surfaces on the' two pieces (4 and 5) of failed Anchor Head HV016 are shown in Figures 1 and 2, respectively. Those surfaces first were examined visually and with a low-power. stereomicro-scope. Because of the time constraints to develop the mechanical-property data, that initial examination was perfonned quickly prior to sectioning the pieces for tensile and impact specimens. That examination indicated that the direction of crack propagation in these sections was from the shim side to the button-head side of the anchor. The fracture surfaces exhibited a primarily brittle appearance. However, a small shear lip:was present along the edge of the fractures on the shim side of the anchor.

The presence of that shear lip and the absence of other distinguishable markings led to the conclusion that the origin of the failure was not present on the pieces provided for this study. As is shown in Figures i and 2, pieces of the steel had fractured from the button-head surfaces; O probably in the latter stages of the failure of this anchor head.

The fracture surface on the portion of Anchor Head HV038 pro-vided for study is shown in Figure 3. A distinct fracture origin was not obvious on that surface. However, the general appearance of that surface indicated that the fracture most likely initiated in one or more of the ligaments or webs between the tendon holes in the central region of the anchor head near the shim face. Several of those regions contained no shear

! lips adjacent to their surfaces and, thus, they were considered to be potential origin sites. The markings on the 2-inch-wide rim portions of f the anchor head indicated that those surfaces most likely were formed during the latter stages of the failure of the anchor head. Those fracture

! surfaces were quite flat and exhibited a woody texture that most likely was related to the nonmetallic inclusions and banding (alloying element segre-j gation) in the steel. Small shear lips were present on the rim fracture surfaces adjacent to the shim-side surface. The presence of those shear j lips and the absence of other distinct features of the surfaces of the frac-ture through the rim regions indicated that the failure did not initiate O in those regions.

l 12 G -

~

?

, \ u

'? p% ,.;s.

'.'. f: l I

a As Received 7317-1 l a. Button-Head Surface g-.

0 ,

, ]-

N [; y_. l l'#

.b ,

4 ,

$h , .,

c' 1

• 'x.',**

.b 1

,. p y ..[f.g.

1. ' k',; [.>m,t.n .

.9 - i n n

( .

e ,' [f.. ... Ik'-

_ f; k * , h*- l

( 4 *.* ' -

4 ,f$, f - : . ; .;Q ' , .. . .

4 v,.

,. r i

x. ,

l i

i ..

As Received 7317-4 As Received 7317-6 l

l b. Fracture Surface on Left Side c. Fracture Surface on Right Side t

l FIGURE 1. PHOTOGRAPHS OF PIECE 4 FROM ANCHOR HEAD HV016 l0 l

13 G

l

... ...... . . r y f4AaTI'!T ....... ..

As Receivec 7317J

a. Button-Head Surface i

.~ -

.. .,y A. ) ', -

t

,, ) . ,q s . - {y l

b' '.>..,- .

4., ; ;, >

e ;Q _ ,.

. yarA u - .'

t$ .,=. s'

n. - s. ,

.4-~ .j.;< -

o l ,3

.gs..}[s

^

.-l~.?

.[3,. -

• f "

. ,~

As Received 7317-9 As Received 7317-8

b. Fracture Surface on Left S' ide c. Fracture Surface on Right Side FIGURE 2.

Q PHOTOGRAPHS OF PIECE 5 FROM ANCHOR HEAD HV016 I

l_ . _ . . _ _ _ _ _ , _ , _ _ . . _ _ , - _ _ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ' - -

O O Q I.

ag:as C

7. ,, e g .:. ,

l id:.Y!)

1 m., .- s. .

u sr~

1 i.. .n!C, f c,y:f.'$ fj '

3. i

\

' 5,: ..;. cb,h.N)U f.! ..

y

, i.,?;'ir:.J. 6 YUY. n .. , ' .

J r [. .y

.yy. : .Si;;. 2.' O'. f '* , . -

• 1 ..-. ,.;;.> c j

):; ' ' . , , fG,~,f *

- ~

~2 i . -

(@;;ln&;'l' , .- g: : ;. ,.y . . , .-l'

, - k, , t .

}'.??)blr?l.ijh i; :: 8. " ' u I.T ..

?k.I. '

$1 ,'

b,,'f'

, .fo r .. ,

( ,

l h'SkN b. f $. '

I j

r;...y...u

/ ., ',.., f 7..['./

. .L'

~-y'N'I-;,{;kW l:~

yC

%* '... ~R. ' e'l,;G_

I

.' * '.r. c. .~ ., O . . . '_

':) . .@Q, s s

} .

r ,C- N' ". .-

_, f~ , ~ ; _

j _,

l

! 1X As Received 9L828 l

j FIGURE 3. FRACTURF SURFACE OF PIECE FROM ANCHOR HEAD HV038 l The button-head surface is at the top of the j figure, and the shim-side surface is at the bottem.

The portion of the surface contained within the out-i lined area was examined in the SEM. The arrows point to suspected fracture-origin regions. Circled Regions A and B indicate where SEM frdctographs discussed in i subsequent sections of this report were taken. Note the whi te-appearing deposit on the surface of the tendon hole in circled Region C.

l l li 15

'O i

No evidence of significant corrosion (rust) was present on the l- fracture surfaces or on the surfaces of either anchor head. In addition,

, no evidence was present that would indicate that quench cracks had formed prior to tempering of the anchor heads during their heat treatment. How-ever,'some small pits and whitish desposits were present on the surfaces of the tendon holes. One particularly noticeable deposit"was located on the surface of a tendon hole in Region C circled on Figure 3.

Examination of the shim-side surfaces of the ligaments between tendon holes revealed the presence of cracks in several of those likments

in Anchor Head HV038. The locations of those cracks are shown in Figure 4a; Figure 4b shows two of those cracks at higher magnification.

1 Scanning-Electron-Microscope Examination i

Anchor Head HV016 -

The majority of the fractographic examination of the pieces from Anchor Head HV016 in the scanning electron microscope (SEM) was conducted on a fracture surface from Piece 5. That fracture surface is illustrated in Figure 5. The bulk of the other fracture surfaces were destroyed dur-ing sectioning to obtain the mechanical-property specimens. The predominant

, fracture mode observed on the lower portion of the fracture surface shown in Figure 5 was intergranular fracture. Figures 6a and 6b illustrate the predominantly intergranular fracture mode in the regions identified as 5 and 4, respectively, in Figure 5. In Region 3, the fracture mode consisted of a mixture of intergranular fracture and ductile fracture, as is shown in Figure 7. In that region, the intergrar,ular fracture occurred in bands on slightly different planes, and the ductile fracture appeared to result from shear between those bands, forming steps on the fracture surface. In Regions 1 and 2 and the upper portion of the fracture surface shown in Figure 5, the fracture mode consisted of a mixture of ductile and cleavage fracture with varying amounts of cleavage fracture. The typical appearance of the fracture surface in Region 2 is shown in Figure 8, and Figure 9 shows an area of predominantly cleavage fracture in Region 1.

L l

a o Q I

2 N -

t 1

! 1X 1 9L829

!' d. LoCdtions Of Cracks Between Tendon lloles. Arrows Indicate Ligaments That Were Cracked; the Crack in Circled Region 2 was Opened to Allow Examination of its Surfaces.

b ,,

l*;.ffC+j , N % .'s.Y % $

- - '~,',,iV'4g{$yi:.}.

,,Qpt " .

' ' ' A ' '

?.

o , . * * ,

/ v.  : >, .

'a .

l  ;. .5

.t -:i-0, l

I I

,_ = ..

i 10X 9L827

] b. Cracks in Circled Region 1 from (a) l

. FIGURE 4. Pil0TOGRAPilS Sil0 WING IllE I.0 CAT 10flS Of CRACKS Ill Tile LIGAMENTS BETWEErl TErlD0tl ll01.ES !!1 ANCil0R llFAD llV038 j

l I

2 17 O

1 i

l

1. ~g' .

};,3 -

-i ,

l

,[N ' ~# -

g  ?,, / <.'k . ,

ll '*'.' k$ .$4'pk1lh.~;Q.'

_f Y. 5$' *h'hh. h':.'$'5f-T;.s g

' Wh*h.,-)f*5W,&.::.'i..'( .

r  ;?! .* i~.{.g .g%$?f,&,'._

,3 ,

.~.:+glp,

. r- ,A./ , .; -

1 9 l

.. y y, ; 9. ,.. -

4 2 i

~

.. j W,56fi:

^

4', a ,btIf.[.?:;!'.,_i-N._

d. - > r . ,

"~

A,S?g CV ~ , .gr. .-A% . , [

. v4. :.m. 3

..r*%h' '~ "l,-

('%k

+

1.5X As Received 9[509 FIGURE 5. ONE TRACTURE SURFACE ON PIECE 5 FROM ANC.HCR PEA HV016 The numbered regions indicate the loca-icns where SE.*1 fractographs described in sucse-cuent figures were taken.

i Q

l 1

2 l

. - - - . . ~.__.-- . -. . - - - - . _ - - - - _ _ . - . . _ _ - . . . _ - - . _ .

18

?#

q -

~

.,n  :-

- ~ -

^

4 A

~

-p

, . ' 'p'..h  !

h~..,. $

4 *' s *

. 3 e,-

n, 4 . - ? g**%,m(*

~

Q, .

~

mT'

,t -

k, *

(l

\

~

g 4 s . ..

753X

3. Region 5 O

g x

y

.4

( e y . ,..

.] :. i-

• ., . *~

8.

M*, f-5

~

$ ^-

ll*

500/ ,

>,. n FIGURE 6. PRECC 4!;A TLY IrlTERGRAtiULAR FRACTURE 0' THE LOWER PORTI0i; 0F THE FRACTURE SURFACE m0?.

j A!;CHOR HEAD HV016 SHOWil lil FIGURE 5 Q

• I l 19 1

%c-

\

lR

~:.4 %

e l ,

- .JW$

4 .f. y p

..ro p n s ,

.-l 'N u

{ ) 4.$t

;r.

500X 50035 FIGURE 7. BANDS OF INTERGRANULAR FRACTURE AND DUCTILE FRACTURE (SHEAR STEP) OBSERVED IN REGION 3 ON THE FRACTURE SURFACE SHOWN IN FIGURE 5 O ,

.. . ~~

r:' -a%f

['!

a 1

1 l

l

'yi'

. .g,

'l 1500X 50033 FIGURE 8. MIXTURE OF CLEAVAGE AND DUCTILE FRACTURE OBSERVED IN REGION 2 ON FIGURE 5 l l

l

i l

! 20 1

!O i

i i

in .

~ ~. -~

g .

w _ ec

,a,

-E J

Qsh, Tg.*.*; . g, - -

.~ _

m. -

f ~~

l J

1000X 50035

FIGURE 9. PREDOMINANTLY CLEAVAGE FRACTURE OBSERVED IN REGION 1 ON THE FRACTURE SURFACE SHOWN IN FIGURE 5 i

i 1

l 1

1 i

l

' 21

' O' All three modes of cracking were observed on the fracture surfaces t of the ligaments between the tendon holes. In addition, numerous manganese sulfide inclusion stringers were present on all of the fracture surfaces examined.

Anchor Head HV038 -

The f actographic examination in the SEM of the fracture surfaces from Anchor Head HV038 was concentrated on the suspected fracture-origin regions illustrated previously in Figure 3. In addition, one of the cracks in the ligaments between the tendon holes was opened so that its surfaces also coulo be examined (see Figure 4a). Also, a section that containe'd ligaments becween tendon holes from a region where no cracks were apparent was broken in three-point bending in the laboratory, and the surfaces of that freshly produced fracture were examined in the SEM.

Figure 10 illustrates the typical fracture appearance of the suspected fracture-origin regions; that SEM fractograph was taken in Region A shown on Figure 3. As is shown in Figure 10, the fracture surfaces in those regions exhibited predominantly intergranular fracture. Similarly, the predominant fracture mode within the region outlined in Figure 3 was intergranular; however, some regions with mixtures of intergranular and ductile fracture modes also were present in that region as is shown in Figure 11. Figure 11 also shows manganese sulfide inclusion stringers that often were present on the fracture surfaces of many regions examined.

Portions of the fracture surface within the region examined in

.; the SEM exhibited a dark appearance, perhaps indicating the presence of corrosion products. Region B in Figure 3 illustrates that feature. Exami-nation of the surfaces in that region nivealed again that the predominant

, fracture mode was intergranular, as is shown in Figure 12. Energy-  ;

dispersive analysis of X-rays (EDAX) in that region revealed the presence fI of calcium and zine on the surfaces of that region, in addition to iron, i chromium, sulfur, and silicon; t e latter four elements were present in the steel. The corrosion-preventative grease contains calcium, and, thus, it  ;

l I

! O O Q l

f g

.?  ;

TN l

U

~

P' 500X 50200 500X 50206 FIGURE 10. TYPICAL INTERGRANULAR FRACTURE OBSERVED FIGURE 11. MANGANESE SULFIDE INCLUSION STRINGERS AT THE SUSPECTED ORIGIN REGIONS OF Tile AND MIXED FRACTURE MODES ALSO PRESENT FRACTURE IN ANCil0R llEAD llV038 IN REGION A IN FIGURE 3 The fractograph was taken in Region A shown in Fig- Anchor llead liv 038.

ure 3. The fracture modes present were intergranular and ductile. This fractograph was taken from a region at the right side of the circle labeled A.

I 23 0

l

' =

1 A i i

l0 '

i J

, my .

~ '

500X ~ 50207 i

FIGURE 12. TYPICAL INTERGRANULAR FRACTURE IN THE DISCOLORED REGION B IN FIGURE 3

)

lQ 4

I 24

'O it was presumed to be the source of the calcium detected on the fracture ,

f surface. The zinc detected in that region most likely came from corrosion of particles of zine that may have been transferred to that region when 1 the tendon wires were strung through the galvanized tendon conduit.

Figures 13a and 13b illustrate the appearance of the surfaces of the pre-existing crack in a ligament that was opened in the laboratory.

As is shown in Figure 13a, there was a faint thunbnail-shaped region that extended inward from the tendon-hole surface on the left side of the figure, thereby indicating a fracture-origin in that region. As is shown in Figure 13b, the predominant fracture mode adjacent to the tendon-hole sur-face in that region was intergranular. Toward the tendon surface opposite the origin region, shown in Figure 13a, the fracture surface exhibited a mixture of intergranular and ductile fracture similar to that shown pre-viously in Figure 11.

Figures 14a and 14 illustrate the appearance of the surface of the fracture of a ligament in which no pre-existing crack was present, which O was fractured by three-point bending in the laboratory. Figure 14a shows that numerous manganese sulfide inclusion stringers were present, and Figure 14b shows that the fracture mode was a mixture of ductile and cleavage fracture; no intergranular fracture was observed.

EDAX Analyses of Deposit on Tendon Hole Surfaces

'l As was shown in Figure 3, circled Region C, there was a white deposit on the surface of one of the tendon holes in Anchor Head HV038.

'! Some of that deposit was collected on a nonmetallic fiber and analyzed by EDAX in the SEM. A similar whitish colored, but much smaller, deposit ij observed on the surface of a tendon hole in Anchor Head HV016 also was is

analyzed by EDAX in the SEM along with an adjacent region that did not contain a deposit. Those results of those analyses were as follows:

L -

A i! O i

n I

i,

. -4 . , .,_ _. v, , - _ _ , . , ~ , _ , , _ _ _ _ _.,,,m-.. _ _ _ _ . _ _ _ . . _ _ _ _ _ _ , _ _ _ _ _ _ _ _ _ _ - _ , , ,

]

O O O l

l l

l 8 .'I-

. . ,4 '

4yd \-Q

" 4e, tag %

= .;g .

h*l[$kQ

.9, ..

's, ,

g,,,gl,g

'{,7 [s .:e *>

... L_

.l %pk. Q'kl".'L'"Q?

1 6

t- . >

)]

.h.,s

'y.; ,f,4 - ..

• 0 U0 .'g{

, . .k 1,, <

',! r .

ti' ,

l

~

-  ; k. ' l-

, , 3 .

, M-I > -

[

sg-t. -

• 1 11X 5021 500X 502 i

a. General Appearance of Crack Surface Indicating b. Intergranular Fracture Mode at the Origin l an Origin Adjacent to the Tendon-ilole Surface Region in (a) l (Origin Region is Indicated by the Bracket) l FIGURE 13. SURFACE OF T}iE CRACK IN TiiE LIGAMENT BETWEEN TENDON ll0LES Sil0WN IN REGION 2 0F FIGURE 4a l TilAT WAS OPENED IN Tile LABORATORY l

4

26 G '

1 i -

e.

~'

. :n . . -

.;,) . ~.-.:*

.'1. ... -

5. ..;

W

.mW ~ % esc (). %.. idj?..:7,u$5,'

~ - -

s ;Y~hs f !$ h Na$ $

M c

• f., . .:>*\ , R% .:.t % % t .n.-

, .- _ r .- .

c ,, :.

. d ,,p,.;"

.x -- -

vf .

wgy - _ _ - .

. i 100X 5029

a. Typical Fracture Appearance Showing Presence of Numerous Manganese Sulfide Inclusions l l

0 .

.wy-T: . j' g-e c,p sg-n, o . x, s l

3 g i V ' s .'"fg'D",' - -

y i y ,%

,# C -

s l

_ q m

• m* M_ '+

1. y_g4,e , ,- -

st ce  !

3, ,

t$f

' ~

',. -L ,

.A A -

, f $'h .

~

1 h E _,p I

I -+ v. 4g (}I .,;.., , , . ,

q i a . .' c' 4( l afreEx,.fw wvm 1000X 50294 D. mxture of Ductile and Cleavage Fracture FIGURE 14 TYPICAL APPEARANCE OF THE FRACTURE SURFACE OF LIGAMENT BETWEEN TENDON HOLES THAT DID NOT CONTAIN A CRACK Q. AND WAS BROKEN IN THREE-POINT BEND-ING IN THE LABORATORY

)

l t

27 O

U l

Elements Detected,_ relative weight percent a)

Identification Zn Fe Ca S Si Al Na Cr Deposit, HV038 3.53 8.45 60.67 9.26 4.44 5.19 8.46 ND ID)

Deposit, HV016 4.47 86.4 2.19 1.46 2.34 ND 'ND 2.27

" Clean" Surface, ND 94.8 ND 0.30 1.09 ND ND 3.06 HV016 i

(a) Based on total nunter of X-ray counts detected.

(b) ND = not detected.

4 As is shown in the above tabulation, the deposit that was removed from the surface of the tendon hole in Anchor Head HV038 contained a significant

j amount of Ca (present in the grease), lesser amounts of Fe, S, and Si (present in the steel) as well as Na (present in the grease), A1, and Zn.

The zinc presumably was present as an oxide (white rust) and its presence O indicates that preferential corrosion of zinc particles entrapped in the tendon holes occurred. The source of the aluminum is not known.

The deposit on the surface of Anchor Head HV016 contained Zn, Fe, Ca, S, Si, and Cr and the adjacent " clean" surface region that did not contain a deposit contained only Fe, Si, Cr, and S. The differences in ll the relative weight percents of those various elements in the deposit removed from the surface of Anchor Head HV038 with a fiber and the deposit on the surface of Anchor Head HV016 was caused by the electrons pene-trating the deposit on HV016 and exciting X-rays from the steel substrate.

Fracture Surfaces of the Tensile and Impact Specimens From the Anchor Heads As has been described in the previous sections of this report, the predominant mode of failure of many of the surfaces of the fractures that resulted from the service failures of Anchor Heads HV016 and HV038 was intergranular fracture. That fracture mode suggests: (1) the failures were caused by envir nmentally induced failed mechanisms, such as hydrogen-O stress cracking or stress-corrosion cracking, that are characterized by

I i

28 O' intergranular fracture of high-strength steels, and/or (2) the steel was enbrittled during processing such that it failed mechanically by inter-granular fracture.

The examination of the fracture surfaces produced.in the laboratory by three-point loading of a section that did not contain a pre-existing crack indicated that the steel was not embrittled severely during processing.

To provide further assessment of whether the anchor heads had been embrittled during processing, the surfaces of the tensile and impact specimens from both anchor heads tested in the laboratory were examined with the SEM to detennine the fracture mode. Examination of those fracture surfaces revealed that all of those specimens exhibited mixtures of ductile and cleavage fracture.

. Examples of those fracture modes were shown previously in Figures 8 and 14b.

No intergranular fracture was observed on any of those mechanical-test-i specimen fracture surfaces. The absence of intergranular fracture on the mechanical-test specimens indicates that the anchor heads were not em-brittled as a result of their chemical composition and/or processing.

O Metallographic Examination Microstructure i Two sections for metallographic examination, one longitudinal and one transverse, were prepared from each anchor head. The transverse section from each anchor head traversed the 2-inch-wide solid rim section and intersected several of the tendon-wire holes. Examination of those sections revealed that the microstructures of the steel from both anchor i

heads consisted of mixtures of tempered martensite and/or bainite with some patches of ferrite in the midregion of the rim. Near the periphery of each anchor head and in the ligaments between the tendon holes, the microstructure consisted essentially of all tempered martensite, as is shown in Figure 15a. With increasing distance from the periphery, more bainite and some patches of ferrite were observed, as is shown in Figure 15b. Slight partial decarburization of the steel was evident at the outer O.

I l

29 R  %. * * '?'hi.K "' :.sy* A C s. . ..': .,.r .

' *('} t W. ~

~ n- .

[t f"i.3 5 ??yys. =': *

[*' %p . ,..l.~%- ; . n;4'.',

. ,i2 , 3 c-

ly,..

+> -

_~;q' *i' . . .1

.f.'y,Q:g:.

' ;qw

.- y ;

,{ ': *

.? '.

!glG

.N' ' '

g. -

.- r - ' ,"Q ', ;,.e d.g[t

+ ~

l1,i 1

,.y .. a

~

gg

, y -

, %r ** -

7, .

.  % . .. .:t , .,

500X Picral Etcn 3L506

a. Typical Microstructure Near the Circum.ferenti41 Surface and Adjacent to the Tendon Holes. Tre microstructure consists of tempered martensite ana/ r sain to nP 'd ':i t ' O X. -Q' .

. , ay . . ?

L , ..r '~4C,'.f{.b.,

i

.~' 1. -

4* 9 ,... f,.

)

, ,. : At$1?+

-@?j;G - J 2

tg :p l

p.v .$'g.. ~/'. '

, %. -;.W. U.s?

ifm of. y.,9

, -. e

.Rfh

• • ','&y.~.f<;sg

>82

~ .la.Sp h.3 q" . T. v,e. x i f ,y7 ,W T..

F.- ,

)

3 V-

?W Q-c  ?.

~ '

Q" g..; E .

$. } h5 5'

~.U . , kR,. * .'.

-? Y' . $:. . . lk

';. . '.Y 500X Picral Etch '

b. Typical Mi <ed Micros tructure Appro i"ito:,

From the f.i rumferential Surface of tho '

.r1 *.'.:n 4 Head. I t <.on',i s t s o f C ini s ture o f te ; r eu

• Cdrtensi te .ind bilini te wi th ;0"e 5%11 I . '.t t i '

e ,

Q w

of ferri te.

FIGURE 15. TYPICAL MICR0 STRUCTURES OF THE STEEL FROM ANCHOR HEADS HV016 AND HV038

f I

30 i

1 i

1 l

4

-- ^* W h-.  % ,%.. .

i **1X .';a h

^

y), t h ~ W. U.{$r' h-d.{. 'e E

.h i.ff*h,',.,*.4[Q.yff,@_{$;&h?fff b't k.A $ % $ l~'k

$f;<  ! *'z,}Q.t'.a.h*.%.fl}.,.QQ;

%f*.1,'.,*p_iynj. .l f. .% G . . o . .

.Q.*Q y75

....,,.:;..:. .r gv. . .u,_,W._y% t

1. . .

A... " . s

. ~- %

- , ;u ..'.,e. ,s

.w; ,c.

~.q c.a.4.e:.s.y

. c a .< u .: e .. .,%.....n *:. M . s.y#.:,,,s.

3 . " , ,.f, ,,

..,g p,,

-  %.?.4 p i,'.T::,r .;M.. Q :2G% v.. i

, i. -  ;

, , ..$.w Q. c... .. , r,.ww . Q.

'.2-[II

...s..+,

t. o.

. .; .  ?.

.f v,. . - v. ;c , , . ~

's.1 i

, .s. . ;.w . 9+, ; :,;,,rg.j:.~.. ,r . . .S

-..~ ~ u. . . d*rc . ,-c-.epp'y.:M: . .e' ':Q. :mf.;r.qq a .-.r. .u y:,4 ; y .;

- ,.a..'f/ d,.t 7-W.%  ;

• ,Y ' .. ; w.-:'+T, t .. i. ,1,..f.^; y * .; g,':

= g , 'n. 9,.i'$'p..

.e * .Lg n m,$1

'U 1:-t. * %..i'.%' ,7 %.g4 t}. e . a,., >%.  %.' om.Q. r,( -wp N !P'j -

t . . - * * .

... . . - . . . .m,.A .

,  ?.  : .v. *

. .E.,.>.a.

,+.,<.. A ,.$,9 ~ er ai .,g.4. i. r.. .t ..sn.

• ,h.s.es

• . s ,. s.:-0m. s

> .t v. o

.r?,: . s

.'^ ~.f*.,,,. 4.t. . . . . . . m .

. ?;-; ,l:$z. e: a-.%::;o.a * ' ., . .:.g.t;1 ? < %. *

\

' Q.p'Qgg'i'.y"?

j f -

. . . , . m . y. ' ;.C. ( '. ;, .i, ..y, .,, .. , ,Y 'c .' {.y/ff~$ypf  ;-

f ..

j.L

.+ u:. d, ,. . %.

.< x ,s.s .g ,. 2 .- 3. .

, uh , . , ,

. . , q. , e. . .-

s~ .- ,s. . . .

. . . e.

1 .

. ..u. -.1 ...~.,; -. .

.y.. .w s f . g. ,. .w,~.

f y . s.  : '. . . ..

r

.- * . , -4. ,.. ,, +n,- . r. r...,. J- [.s,

, : *m a ,> , p ; v.

, . . . . . . , s -J .' v . ~.

- e s

. . . r ..

. ?g, e L. 7. ) ;

• s ': ..-i '. Q,1. ,

- . ]. '

• m . .

f *s..m

,6,, --5. . ,,

4 * .

500X Picral Etch 9L508 i c. Partial Decarburization at the Circumferential

] Surface of the Anchor Head l FIGURE 15. (Continued)

.w-+--,.w.- .mweg,---

, 31 l

surface of the anchor heads and at the surfaces of the tendon holes. That

!O partial decarburization is shown in Figure 15c; the light-etching patches of ferrite that extended inward from the surface to a depth of about 0.001 1

' inch indicate the partial decarburization.

l Cleanliness Ratings l

l The cleanliness of the steels was rated on the longitudinal sections from the two anchor heads in accorda. ice with the procedures in

! ASTM E45, Method A. The results are listed in the following tabulation:

1

i. Cleanliness Rating i Type A Type B Type C Type D Identification (Sulfides) (Silicates) (Alumina) (Globular Oxides)

HV016 4H 2H --

lH HV038 4H --

3H 3H .

I f

Those results indicate that the steels from both anchor heads were relatively l dirty with respect to the Type A (sulfide) inclusions. In addition, the section examined from Anchor Head HV038 was somewhat dirty with respect to

(

Type C (alumina or aluminates) and Type D (globular oxide inclusions).

Examination of Failed Tendon Wires

'l The sections of failed Anchor Head HV016 provided to Battelle for this study, contained portions of eight failed tendon wires. It was mutually decided among representatives of INRYCO, Inland Steel Company, and

,, Battelle to examine the fracture surfaces of those wires to determine if 1 they failed in a brittle manner. The surfaces of all eight tendon wires

, were examined visually and with a low-power stereomicroscope. The regions

] adjacent to all the fractures showed evidence of necking, and all the fractures appeared to be the result of overstress. Subsequently, the fracture surfaces on three of those wires were examined in the SEM.

'I

. O I

e

t l

t 32

!O Figure 16 illustrates the typical appearance of the fractures l in the tenden wires at low magnification and shows the very apparent nec'k ing of the wire prior to rupture. Examination of the fracture surfaces of thi three tendon wires at higher magnifications revealed that the i fracture modes present were ductile and cleavage fracture.- Figure 16b illustrates a mixture of ductile and cleavage fracture in Region A of

, Figure l'6a, and Figure 16c illustrates predominantly ductile fracture in 4 , Region B of Figure 16a. These observations suggest that the tendon wires examined failed by overstress, most likely as a result of the changes in i .

1; stress distribution when the anchor head failed, rather than prior to the failure of the anchor head. However, only a limited number of tendon ' wire

, fractures were available for examination. On the other hand, none of the tendon wires that passed through cracked Anchor Head HV038 reportedly had '

failed. That observation suggests that the anchor heads fail prior to failure of the tendon wires.

i DISCUSSION OF RESULTS 1

The results obtained from the metallurgical evaluation of 4 the pieces of Anchor Head HV016 have shown that the steel met the chemical-composition requirements for ASTM A3222, Grade 4140/4142, steel in all respects. The average carbon content from the piece of Anchor Head HV038 examined was found to be 0.48, whereas the maximum allowable carbon content for a product analysis is 0.47 percent.

I However, it is not likely that the slightly higher carbon content of i

the steel from Anchor Head HV038 had a significant effect on the failure.

The sulfur contents of the steels from both anchor heads were relatively high, but well within the specification limits, and both anchor heads j.I contained numerous manganese sulfide inclusion stringers. The contents

of the nonspecified residual elements that can promote brittleness of j steel under appropriate processing or testing conditions were low and, thus, most likely those elements did not contribute to these failures.

L I

I4

+y-- - - - ys--- -----=&-,-%gm ,,y,p7 m ---r, =79.,, ,*g-y-.--.,,w--,,r,-o9,y rm -% ,m y,e,-w. err e r -y ,w w---y-y gy- w mwwwwwe Twe--s==<we=ewt-w-r=- .,w ,y w-yw-w

33 0

• *t i .

. Ja b,e. ~. , < .

.') .

d?;4 h . . . . a;[

x. ~ . ,j"RQt. a. fg.'- /

.-T' ~ b- :, . ..*W,.1'" *"

[..,,1 ,? % - W)5;f T , {, .

. s _x w(--Cl-^

u , , , .. .- .,

.5'_-

- w.

o .'~r~ - ' -

y -L f, '_K,

'W - 'a -

v .,

l}_

4 . , . .

^

?

0 Y 7

.1 '

~

f' .

15X 50155

d. General Appearance of Failed Tendon Wire FIGURE 16.

TYPICAL APPEARATJCE OF THE FRACTURES OF THE TENDON WIRES EXAMINED

i

[ i I

34 l l 1

0

~

4:-

\

, f ? '.. -- ' ,

• Q ...%

A

)

i R', 9 ~ Y., ': a'; , A l

l

• i ..h ' ! 5

. % *&'$ .'.. .{. ?. ,

,' h . '

f*f l

ff' _

'~

/, ,,

.* \

' ' . * 's .-

l .

f,

~"

.': 4, -

7 , Ar l

~

s

g y' e% . i

- l

~~~P ,

.\ ,

"y- g

. ,[A- ^ '. - , I >

. . .. . 's < '

>~ '

3 1000X 50164

b. Ductile and Cleavage Fracture in Region A in Figure 15a

O

'O,'

MJ 4 1

\'pg e e , .

h r 1. 4?

7 pi '

g,t.'(@f ch"j

~ ~

e .. .. : - f, 1000X 501o3

c. Predominantly Ductile Fracture in Region B in Figure 15a FIGURE 16. (Continued)

I

, 35 O

The hardness of the steel from Anchor Head HV016 ranged from 41 to 44 Rockwell C and, thus, was within the specified range of 40 to 44 Rockwell C. The hardnesses measured on various sections from Anchor Head'HV038 ranged from 39 to 45 Rockwell C and averaged 42.5 Rc. Of the 27 hardness readings, there was one at 39 Rc and one at 45 Rc; all of the others were within the specified range of 40 to 44 Rc. The hard-

, nesses measured on the button head and the shim-side surfaces ranged from 42 to 44 Rockwell C.

i The ultimate tensile strengths of the steel from both anchor

, heads were consistent with the hardnesses, and there was relatively little difference in the average strength properties of the specimens frnm the two anchor heads or with respect to specimen orientation. In addition, the ductilities and impact properties of the steel from the two anchor heads were similar. The ductilities and impact properties were significantly lower for the transverse specimens from both anchors heads than for the longitudinal specimens. The differences in those properties with

,O respect to specimen orientation are not unusual for wrought low-alloy engineering steels heat treated to the strength of the anchor heads.

At least part of the lower ductility and toughness of the transverse specimens can be attributed to the relatively high sulfur contents and the presence of the manganese sulfide inclusion stringers.

l The examination of the fracture surfaces produced during the failures of both anchor heads during service revealed the presence of considerable amounts of intergranular fracture. That fracture mode was present on the surfaces of the fractures in the ligaments between the tendon holes as well as in various regions of the 2-inch-thick rim section. In addition, that fracture mode was present on the surfaces of the pieces that appeared to have spalled off the button-head surface of Anchor Head HV016. In other regions of the field failures, the fracture mode consisted of mixtures of ductile and cleavage fracture modes. Intergranular and cleavage fracture modes are characteristics of brittle fracture. Cleavage fracture usually indicates that fracture occurred at a temperature below the ductile-to-brittle fracture-transition i

i

7 36

'A U

temperature of the steel. Cleavage fracture also can occur under complex loading conditions that lead to biaxial or triaxial stresses in the part.

[

Analysis of the service loading conditions of an anchor head has shown that complex loading does exist. In addition, examination of fracture surfaces on the impact specimens from both anchor heads tested at room temperature and 212 F revealed that cleavage fracture was present on the specimens. For those reasons, regions of cleavage fracture would be expected on the service failures.

The presence of intergranular fracture, however indicates that the steel was embrittled either from a combination of its composition and processing or as the result of interactions between the steel and the;

. service environment. Embrittlement mechanisms that cause intergranular fracture that result from the combination of steel composition and pro-cessing include temper enbrittlement and "500 F", or tempered-martensite, embrittlement. In addition, the formation of grain-boundary sulfide films during the processing of the steel can result in intergranular fracture.

Descriptions of those embrittlement mechanisms obtained from the Metals Q Handbook **are as follows:

Temper Embrittlement. Embrittlement that results in steels that have been heated in the temperature range. Temper em-brittlement usually is detected by an upward shift in the ductile-to-brittle transition temperature in a notched-bar test, such as a Charpy V-notch impact test. The tensile ll properties of the steel generally are not influenced by this embrittlement mechanism, except in cases of extreme enbri ttlement. It is caused by the segregation of impurity l[

elements (phosphorus, arsenic, tin, antimony, and possibly l others) to the prior-austenite grain boundaries.

-i

• Pecknold, D. A., and Presswalla, H. H., "Elastoplastic Analysis of Thick Perforated Plates With Application to Prestressing Anchor Heads", Computers a & Structures, U (4), 539-553 (1983).

• • Metals Handbook, 9th Ed., Volume 1, Properties and Selection: Irons and pV Steels, B. P. Bardes, Editor, American Society for Metals, Metals Park, Ohio (1978), pp 684-685.

h

I 1

i 37 o' 500 F Entrittlement. Occurs when steels with tenspered-martensite and/or lower bainite microstructures are tempered f .

in the temperature range fro'm approximately 400 to 700 F. It is detected by measuring the effect of tempering temperature on room-temperature impact energy; this is in contrast to temper embrittlement, which is evaluated by meariuring the effect of tempering {emperature on the ductile-to-brittle fracture-transition temperature. 500 F embrittlement is believed to be caused by ferrite networks resulting from precipitation of cementite platelets along prior austenite grain boundaries. However, some investigators believe that the precipitation of grain-boundary cementite platelett

> as such is responsible for 500 F embrittlement.

The low levels of residual elements that promote temper embrittle-ment present in the two-anchor heads and the absence of intergranular frac-ture on the surfaces of the tensile specimens and the impact specimens tested in the laboratory strongly indicates that neither temper embrittle-ment nor 500 F embrittlement were responsible for the intergranular fracture observed on the fracture surfaces of these anchor heads.

Failure mechanisms that involve interaction of a high-strength steel part and its environment that can result in intergranular fracture are hydrogen-stress cracking (HSC) and stress-corrosion cracking (SCC).

Both of these failure mechanisms are time dependent and, thus, appear to be more consistent with the fact that the anchor failed some time after 6 years of service. Stress-corrosion cracking can occur without signifi-cant general corrosion; however, usually when SCC occurs some corrosion products are present on the surface of the part or on the fracture surfaces of the part. No significant corrosion products were detected on the frac-ture surfaces of the pieces from either anchor head examined at BCL.

Hydrogen-stress cracking, also referred to as hydrogen-induced, delayed, brittle failure, is a cracking mechanism that results in a brittle fracture in nominally ductile steel at applied tensile stresses below the yield strength while the steel is subjected to a sustained load. Steels with hardnesses below about 20 Rockwell C are essentially immune to hydrogen-

I P

38

.Q b stress cracking, but with increasing hardness above 20cR the susceptibilities of the steels to hydrogen-stress cracking increases. The applied stress level that will result in hydrogen-stress cracking varies inversely with the strength level of the steel; that is as the strength level (hardness) increases, the threshold applied stress decreases. For this failure mecha-nism to be operative, the steel must contain atomic hydrogen from processing or absorb-hydrogen from its service environment. The amount of hydrogen required to cause cracking is dependent upon the strength level of the steel and the applied stress level. The higher the strength level and the applied stress level, the lower the hydrogen content or hydrogen charging rate from the environment required for cracking. If the conditions for hydrogen-stress cracking are satisfied, failures will occur after some period of time during which cracks nucleate and grow to critical size. The time for failure to occur is related to the strength level, applied stress level, and the hydrogen content or severity of the environment. At high values of those three factors, failure can occur in very short times such as minutes or C) hours. At low values of one or more of those factors, the time for failure may be years.

The anchor head satisfies two of the conditions for hydrogen-stress cracking: high-strength steel that is subjected to sustained tensile stresses. It was reportept by other investigators working on the failure of Anchor Head HV016 that water that contained chlorides and other ionic species was detected in the grease from the grease can surrounding the anchor heads. Also, it was reported that the concentrations of certain ionic species in the grease were higher than the specified amounts. However,

., there is some question as to whether the grease was contaminated during or after removal of the grease can. Subsequently, water was found in the I

grease from the can surrounding Anchor Head HV038. Thus, it is possible to have a potential source of atomic hydrogen in the vicinity of the anchor head. That source of hydrogen would be corrosion of the steel in the water.

Furthermore, small corrosion pits and deposits were observed on the surfaces

of some of the tendon holes in the pieces from Anchor Heads HV016 and HV038 examined at Battelle. EDAX analysis revealed the presence of calcium and O ziac ia those deposits. Silicoa. chromium. iron, and nickei aiso were detected in those deposits, however, those elements are present in the steel

,1

's 4

l 39 O and may have been detected because X-rays penetrated the deposits. The grease reportedly contains significant amounts of calcium and, thus, the calcium detected most likely came from it. The zinc may have been washed into that region by water, or small flakes of zinc may have been picked off by the tendon wires when they were threaded through the galvanized conduit when the posttensioning system was installed. Since zinc ,is anodic to steel, local galvanic corrosion between the zine and the steel may have resulted in atomic hydrogen being deposited on the steel. Some of that hydrogen may have entered the steel and caused cracks to initiate and grow by hydrogen-stress cracking. Such a cracking mechanism would result in intergranular fracture in the regions where the hydrogen-stress cracks were growing. Once the cracks reached a critical size, the remaining. ,

section would fail rapidly and, most likely, in these anchor heads the regions of fast fracture would exhibit cleavage and/or ductile fracture l modes. Furthermore, hydrogen-stress cracking is a time-dependent failure mechanism. That is, time is required for the cracks to initiate and grow.

.; Also, it may have taken time for the conditions that allowed water to be O present in the anchor-head region to develop. Those time-related factors are consistent with the fact that the failure occurred some time after 6 years of service. Thus, based upon the infonnation obtained from the studies of the failures of Anchor Head HV016 and HV038, it appears that hydrogen-stress cracking is the most probable cause of failure.

To prevent similar failures of anchor heads processed to this t hardness range in the future, methods must be employed to prevent water

from contacting the surfaces of the anchor heads. Those steps should include thoroughly coating the anchor-head surfaces with corrosion-prevention grease and eliminating the source (s) of water in the post-tensioning system. In addition, the design of the posttensioning system should be reviewed and considerations given to modifications to permit the anchor heads to be made of a lower strength steel that will be less susceptible to hydrogen-stress cracking and other environmentally induced failure mechanisms.

1 O

~

4

. .- . _. .- .