ML19323D358

Acceptability for Svc of Midland Reactor Pressure Vessel Anchor Studs, Revision 1, Prepared for Bechtel

ML19323D358
Person / Time
Site:	Midland
Issue date:	05/20/1980
From:	TELEDYNE ENGINEERING SERVICES
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ML19323D353	List: ML19323D351 ML19323D357 ML19323D358
References
TR-3887-2, TR-3887-2-R1, NUDOCS 8005210480
Download: ML19323D358 (100)

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/~N U . W TELEDYNE ENGINEERING SERVICES l

30. 3E AR Hiii AOAD WALTHAM. MASSACHUSET7S C2154 (617) 890-3350 Twx (710) 324-7508 May 15, 1980 '

3887-24 Bechtel Professional Associates Corp.

777 Eisenhower Parkway Ann Arbor, MI 48106 i

Attn: Mr. L. H. Curtis 1 Project Engineer

Subject:

Consumers Power Co.

Midland Plant - Job. 7220 Failure Analysis of Broken Reactor Vessel Support Studs - Unit 2 Gentlemen:

Enclosed are 80 copies of our report TR-3887-2, Rev. I concerning the investigation of the subject studs.

Sincerely, TELEDYNE ENGINEERING SERVICES Wilson G. Dobson Project Engineer William E. Cooper Consulting Engineer WGD:WEC:dem l

• 600ggio 4 0

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BECHTEL ASSOCIATES PROFESSIONAL CORPORATION 777 EAST EISENHOWER PARKWAY ANN ARBOR, MICHIGAN TR-3887-2, Rev. 1 ACCEPTABILITY FOR SERVICE OF MIDLAND RPV ANCHOR STUDS MAY 20, 1980

'MTELEDYNE ENGINEERING SERVICES 303 BEAR HILL ROAD WA LTH AM, M ASSACHUSETTS 02154 617-890-3350

TN ENGNEERNG SEMCES TABLE OF CONTENTS PAGE 1.0 SCOPE 1 2.0 RECOMMENDATIONS 2 2.1 Unit 1 RPV Studs 2 2.2 Unit 2 RPV Studs 2

3.0 INTRODUCTION

3 3.1 Previous Work 3 3.2 Present Work 3 3.3 Failure Mode 4 4.0 MATERIAL HARDNESS 5 4.1 Specified Hardness 5 4.2 Statistical Data 6 4.3 Midland Stud Hardnesses 7 5.0 FRACTURE MECHANICS 8 5.1 Stress Intensity Factor and Fracture Toughness 8 5.2 Stress Corrosion Threshold 8&9 5.3 Summary of Toughness Data 9 6.0 DEVELOPMENT OF RECOMMENDATIONS 10 6.1 Unit 1 RPV Studs 10 6.1.1 Present Status 10&11 6.1.2 Property Ratio Method, Long-Term Loading ll&l2

, 6.1.3 Available Data Method, Long-Term Loading 12

6.1.4 Application of Factor of Safety, Long-Term Loading 12 6.1.5 Discussion of Conservatism, Long-Term Loading 13-16 6.1.6 Consideration of Load Duration, Short-Term Loading 16&l7 6.2 Unit 2 RPV Studs 17-19 i APPENDIX A - AVAILABLE FRACTURE TOUGHNESS DATA 40 4

A.1 ARPA Handbook Draft 40-42

-A.2 Other Data 42&43 A.3 Discussion 43-45 l

WTF1 ATVNE 8b , e 1 ENGNEERING SERVICES 1.0 SCOPE Teledyne Engineering Services (TES) under contract to Bechtel Associates Professional Corporation (BAPC) has studied the preservice failure of three Reactor Pressure Vessel (RPV) anchor studs in Consumers Power Company's (CPC) Midland Unit I and has performed a hardness survey on the remaining RPV studs in both Units 1 and Unit 2. The results of the investigation of the first two studs are reported in TR-3887-1 (Ref. 1).

Subsequent to presentation of that report, one additional stud failed. The metallurgical study of that stud will be reported in Addend I to TR-3887-1, however the results have been considered in the preparation of the present report.

The purpose of the present effort is to recommend acceptance criteria for continued service of the remaining Unit 1 and Unit 2 RPV studs. This report provides that recommendation and a review of the technical litera-ture upon which the recommendation is based.

The present report does not include a discussion of the root cause of t

the f ailure of the Midland Unit 1 RPV studs. It has been established that the failures resulted from stress corrosion cracking which propagated to the point that the st' ids failed by cleavage fracture. It is also known that the decreased resistance to stress corrosion resulted from surface hardnesses considerably in excess of expected values. What has not yet been established is the cause of the excessive hardness, the root cause of failure.

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l TN 8 2, ev 1 2.0 RECOMMENDATIONS Based upon the results of this effort, as reported herein, TES is of

, the opinion that the following acceptance criteria may be applied to the remaining Midland RPV anchor studs. The recommendations have been developed in the manner described by 6.0.

2.1 Unit 1 RPV Studs It is postulated that material composition and material strength requirements are met, based on supplier data, but that surface properties are such as to possibly reduce resistance to stress corrosion cracking and

, fracture.

The Unit 1 RPV support studs should be detensioned, with the existing I

preload determined during detensioning. Retensioning is permitted if the average tensile stress computed on the basis of the nominal net cross-l sectional area does not exceed 6 ksi. Short-term service loadings are j permitted if the stress does not exceed 43 ksi, subject to the restriction of the last two sentences of 6.1.6.

i i 2.2 Unit 2 RPV Studs The Unit 2 studs are acceptable for service in the manner originally planned.

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Technical Report WF WNE TR-3887-2, Rev. 1 ENGNEERING SERVICES

3.0 INTRODUCTION

3.1 Previous Work TES Technical Report TR-3887-1 contains i.he results of a metallurgical investigation of two reactor pressure vessel (RPV) anchor studs which failed preservice in Midland Unit 1 and hardness measurements on the remaining Unit I studs and the Unit 2 studs. Although both sets of studs were ordered to the same ASTM specification from the same supplier and at about the same time; the material, heat treatment subcontractor and methods, and properties differ.

Tne data reported for the first of the two failed studs (stud 3 inside) in TR-3887-1 is quite complete and indicates mechanical properties which are not fully consistent with those provided by the supplier. Suffi-cient material was not available for an extensive investigation of the second of the two failed studs (stud 36 outside), because the position of the fracture along the stud length was different. The third failed stud (stud 35 outside) has subsequently been examined by TES with the results to be reported in Addend 1 to TR-3887-1.

Because of the failure of these three studs there is a concern as to the remaining RPV support studs in Unit 1 and of the studs in Unit 2, the concern relating to both acceptability for future service and personnel safety during construction. Therefore a hardness survey was conducted of l the remaining RPV support studs. The results of this study are reported in l

l TR-3887-1, and are summarized in 4.3.

3.2 Present Work The obvious question is the acceptability of the RPV anchor studs for continued service. TES has reviewed the available literature on this

! subject in order to develop a recommendation on the basis of that litera-ture. Background data on the material are included in 4.0 and Appendix A, 5.0 sumarizes the fracture mechanics approach, and the recommended criteria are developed in 6.0.

Technical Report TR-3887-2, Rev. 1 WM 4- DIG #EstNG SERVCES 3.3 Failure Mode

, The metallurgical investigations reported in TR-3887-1 and to 'be

reported in Addend 1 to TR-3887-1 indicate that the three Midland Unit 1 i

stud, failed as a consequence of stress corrosion crack growth to such a size as to result in cleavage fracture.

The crack originated as a very small surface discontinuity, such as could result by cracking of the surface oxide film or from a corrosion pit.

The corrosive environment may have been humid air. The nominal applied stress level was 92 ksi. This combination of circumstances was sufficent to cause propagation of a stress corrosion crack in materials of high surface hardness. The crack propagated by this mechanism until it reached a critical size for cleavage fracture; 1.5 and 3 millimeters deep for studs 36 and 3, respectively, and about 6 millimeters deep for stud 35.

The only mechanical property which can be measured in situ is the surface, or near-surface material hardness. In a uniformly hard low alloy, quenched and tempered material, such that all test specimens are rep-resentative of the same material, the material hardness is a useful l

l indicator of resistance to stress corrosion and of fracture toughness. The l surface hardness is more important than the subsurface hardness when a ,

hardness gradient is present because stress corrosion occurs at the sur-face. The surface hardness will be a conservative indicator of the frac-ture toughness in studs which have a higher surface hardness than core hardness.

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Technical Report WTBEYNE TR-3887-2, Rev. 1 ENGNEERING SERVICES 4.0 MATERIAL HARDNESS 4.1 Specified Hardness Based upon specific sampling procedures, ASTM A354 establishes a max-imum acceptable hardness level. A-354 does not define the location of the hardness measurement, but refers to A-370, Methods and Definitions for '

Mechanical Testing of Steel Products. A-370-74 Supplement III covers steel f asteners. 510.2 describes the purpose of Supplement III as "to facilitate production control testing and acceptance testing with certain more precise tests to be used for arbitration in case of disagreement over test results". S13.1 covers hardness testing for bolts, and TES believes this supplement is applicable to studs, and it does provide for a "more precise test" as follows:

"For final arbitration the hardness shall be taken on a traverse section through the threaded section of the bolt at a point one-quarter of the nominal diameter from the axis of the bolt. This section shall be taken at a distance from the end of the bolt which is equivalent to the diameter of the bolt."

Therefore, for the subject 2 1/2" diameter Grade BD studs, the maximum permissible hardness measured at mid-radius one-diameter away from a quenched end is 38 HRC.

Even if maximum hardenability of these studs is assumed, some hard-ness gradient would be expected in all the Midland RPV studs. There would, l

of course, be considerably less gradient in the Unit 2 4340 studs than in the Unit 1 4140 studs. Since it is the surface property which controls resistance to stress corrosion cracking, the surface hardness is much more important to service behavior than is the as-specified mid-radius hard-ness.

There not being a materials specification requirement on surface hardness, TES consf dered the requirements of component support standards which address this concern. Specifically with respect to support bolting of the class of materials of interest, including 4140 and 4340, footnote (3) to ASME Section III Table I-13.3 and footnote 6 to Table 4 of Code Case N-71 (1644) read as follows:

Technical Report WTA MWNE TR-3887-2, Rev. 1 BIGINEstlNG SERVICES "The maximum tensile strength shall not exceed the minimum speci-fied tensile strength by more than 40 ksi. Where the specifica-tion does not limit hardness, the maximum surface hardness shall not exceed the hardness values corresponding to the maximum ten-sile strength, as determined from the applicable Tables in SA370."

For the material of interest, the specified minimum tensile strength was 150 ksi. Applying the footnote procedure, the maximum permissible surf ace hardness would be 41.3 HRC. Therefore, based on rounding to integer values I

in accordance with SA-370, TES concludes that a maximum surface hardness of 41 HRC is consistent with a specified maximum mid-radius hardness of 38 HRC, and that 41 HRC would be the proper value for surface hardness speci-fication.

4.2 Statistical Data What is the nature of the hardness variation which would be expected to result if a large number of studs were heat treated with the objective of meeting a specific hardness? Data have not been found for the specific materials of interest, but are available (Ref. 2) on a large number (8935) of 1/2" diameter AISI 1038 bolts. Because of this small diameter, the higher hardenability of the 41XX or 43XX materials is not required to obtain essentially uniform hardness. The carbon content 0.38% is suffi-cient to represent the type of data one would expect from the materials of inter t. Figure 1 contains data from Ref. 2. Appro.imately 1000 bolts were heat treated to each of 8 levels of nominal hardness. The results may be summarized as follows:

i Hardness, HRC Max. Variation Nominal Minimum Maximum Range Minus Plus 20 14 23 '9 6 3 22.5 19 27 8 3.5 4.5 25 21 29 8 4 4 30 25 32 7 5 2 l

32.5 26 , 35 9 6.5 2.5 35 30 38 8 5 3 37.5 33 41 8 4.5 3.5 40 38 44 6 2 4

Technical Report TR-3887-2, Rev. 1 1PTri rrWNE ENGNEERING SERVCES The average value of the range is 7.875, and the average plus variation is 3.312. Based on these data, it is reasonable to expect that material which has a nominal hardness based on limiting sampling in accordance with a specification of some value would have a maximum hardness 3 HRC higher if it were more extensively sampled. For example, uniformly ~ hard material with a nominal hardness of 38 HRC would be found to have a maximum hardness ,

of 41 HRC if a large number of samples were measured.

4.3 Midland Stud Hardnesses The data summaries contained in Ref. (1) are included here for refer-ence:

Present Report TR-3887-1 Distribution of measured near-surface hardnesses in:

Fig. 2 Fig. 11 Unit 1, Heat 0 Fig. 3 Fig. 12 Unit 1, Heat 00 Fig. 4 Fig. 13 Unit 1, Heat 000 Fig. 5 Fig. 15 Unit 2, Heat X Fig. 6 Fig. 16 Unit 2, Heat XX Table 1 Table VII Field Hardness Survey Results, Unit 1 Table 2 Table VIII Field Hardness Survey Results, Unit 2 I

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Technical Report SPTF1FrT(NE TR-3887-2, Rev. 1 ENGNEBW4G SERVICES 5.0 Fracture Mechanics 5.1 Stress Intensity Factor and Fracture Toughness Fracture mechanics provides an analytical relationship between the stress present in a structure, the dimensions of a crack in the structure, and the fracture toughness of the material. In general, one computes the stress intensity factor for the structure using an equation of the form K = CS6 y (1) where K y = stress intensity factor C = a constant depending upon the dimensions S = stress in the absence of a crack a = flaw depth and compares the calculated value with the material toughness which has been obtained from testing a cracked specimen. If the specimen is suffi-ciently large as compared to the crack size, the fracture toughness is independent of specimen dimensions and the property determined is the plane strain fracture toughness, K IC. If tiie dimensions have not been demonstrated to be sufficiently large, the subscript C is commonly re-placed by a different letter.

5.2 Stress Corrosion Threshold The concept of the threshold stress intensity for stress corrosion cracking, KIscc, is used in this report. The stress corrosion effect, if any, for a given material is dependent upon the stress level and the environment which are present at the tip of the crack. The magnitude of the stress level at the crack tip is measured in terms of the stress intensity factor calculated by fracture mechanics techniques. The environment at the crack tip is dependent upon the media in which the material resides, but is not necessarily definable in terms of the average

Technical Report if-P WNE TR-3887-2, Rev. 1 89GNEERING SERVICES properties of that media. For example, in humid air there may be water at the crack tip because of condensation. K g3 is determined experimentally by subjecting a specimen to a known stress intensity factor and placing it in the environment of interest. A curve of stress intensity factor vs.

time to failure is pl(tted. As the stress intensity factor is decreased, the times to failure inc.9ase. At some value of stress intensity factor, termed the threshold stress intensity and denoted by the symbol KIscc, the ,

failure time tends to infinity.

5.3 Summary of Toughness Data Figure 7 contains a summary of available plane strain fracture tough-ness, K IC, and stress corrosion cracking threshold, K Iscc, data from Appendix A for low alloy quenched and tempered steels plotted as a function of material hardness, HRC. The Su vs HRC correlation used on Figure 7 is that of SA370. The Sy vs HRC correlation is approximate in that a yield to ultimate strength ratio appropriate to tempered martensite has been used.

For other microstructures, the curves would shif t to the right relative to the hardness axis. Three curv a are given for KIscc, a most probable value and upper and lower bounds which include all but a few isolated data IC curve presented is intended to be an approximate oest fit points. The K curve to the available data, the majority of which were obtained using AISI 4340.

The plane strain fracture toughness, KIC, measured on material from stud 3 inside was 43 ksik and the measured surface hardness was 46 HRC.  ;

This value is indicated on Figure 7 by the numeral 3.

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Technical Report WT51 Fr?(NE TR-3887-2, Rev. 1 ENGNEBtlNG SERVICES 6.0 DEVELOPMENT OF RECOMMENDATIONS 6.1 Unit 1 RPV Studs 6.1.1 Present Status The remaining unbroken Unit 1 studs have been proof tested in a -

corrosive environment. Although this was certainly not the original in-tent, it is actually what has happened. The present plan is to detension the remaining studs and to modify the RPV support concept so that the studs are used, but are subjected to reduced service stress. That plan can be

, followed if sufficiently high (even if unusually low) values of allowable service stress can be justified. Because stress corrosion is a time-dependent process, and because the stress corrosion threshold can not be higher than the fracture toughness, higher allowable stresses can be per-mitted for short-term loading than for long-term loading.

There are several difficulties in obtaining a very precise statement as to the present condition of each individual stud. For example:

a. There are three heats of material present with field-measured near-surface hardness values ranging between 37 HRC and 49 HRC.

( ikinn tha ennvarsion tables of SA-370, this implies a variation of tensile strength between 166 ksi and 246 ksi,

b. The original applied preload stress was 0? ksi. In general, it is '

considered that the preload stress will relax to about two-thirds of yield. Applying this principal, the present stress level probably ranges between about 87 ksi and 92 ksi but may be lower.

This value will be measured during detensioning.

C. The studs have been subjected to a corrosive environment, and stress corrosion cracks may have formed. Nondestructive examin-ation techniques are not capable of assuring the absence of very small cracks, say less than 0.1" deep, in 2 1/2" diameter studs I

over 7' long which are embedded in concrete.

Technical Report TR-3887-2, Rev. 1 _11_ SPT51 STffNE ENGINEERING SERVICES However, a very important fact is known, the remaining studs have not yet failed.

The calculated stress intensity factor at failure for the first two studs was 42 ksi-J in. As would be indicated by Figure 7, initiation would be expected in those studs at an applied stress intensity factor above 8 ksikwith failure when the crack had propagated sufficiently to increase the stress intensity factor to about 42 ksi-J in. In stud 3 inside, the final flaw depth was measured as 0.118" (3 mm). These figures imply that the initial effective flaw depth was about 0.004", if the initial flaw had the same aspect ratio as did the final flaw. For a larger aspect ratio, a lesser depth would be computed.

With respect to the remaining studs, it must be assumed (to be conser-vative) that an initial flaw of about this size may exist in all studs.

Tnen, for the higher hardness studs, it must be assumed that a stress corrosion crack has propagated to a size which approaches that of a critical size flaw. To avoid further stress corrosion crack propagation, it is necessary to assure that the applied long-term stress is reduced to such a level that the stress intensity factor is smaller than the threshold stress intensity value for stress corrosion cracking in the presence of an existing flaw of near critical size. Also, to avoid cleavage fracture as a result of short-term loadings, the presence of the assumed crack must be considered.

6.1.2 Property Ratio Method, Long-Tcrm Loading From equations (1), it is noted that the applied stress intensity factor is linearly dependent upon the applied stress. Therefore, it is required that the applied stress be reduced by at least the ratio of the stress corrosion threshold value to the fracture toughness value. Note that in applying this method it is most conservative to use a low stress corrosion threshold value and a high fracture toughness value. Since our intent is to use all of the availabic data and to apply a nominal factor of safety on the results consistent with normal Code practice, we have chosen to use the lower bound K Iscc curve and the K curve given by Figure 7. We IC l

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Technical Report gg TR-3887-2, Rev. 1 g have identified two K Isa data points which lie below the lower bound K Iscc curve in the high hardness region and consider these two points with the i associated K IC and yield strength data in the last paragraph of 6.1.5.

From Figure 7, the K Iscc to K IC ratio has a minimum value of about 0.14. This requires the stud stress to be limited to less than (0.14) (87)

= 12 ksi to avoid further propagation.

6.1.3 Available Data Method, Long-Term Loading 4

It is of significance to compare the value of 12 ksi obtained by the above discussion and the results plotted on Figure A.4 for a fatigue precracked specimen. As is shown in figure A.4 no propagation of a stress corrosion crack is expected if the applied stress level is below 8% of the ultimate tensile strength. The ultimate tensile strengths for materials

on Figure A.4 which have this low threshold are in excess of 230 ksi, which would indicate a stress level in excess of 18 ksi is required for propaga-tion. However if in the conservative spirit of the previous discussions it is essumed that this low threshold value is appropriate for materials of the minimum specified tensile strength, 150 ksi, the threshold stress value becomes (0.08) (150) = 12 ksi.

6.1.4 Application of Factor of Safety, Long-Term Loading Finally, what is the allowable value of applied tensile stress in the stud? From Section III, Division 1, NF-3380 and Paragraph XVII-2461.1, the j allowable average tensile stress, computed on the basis of the actual tensile stress area available (independent of any initial tightening force) shall not exceed one-half of the ultimate tensile strength. Utiliz-ing 12 ksi as the tensile strength which can be applied to the stud without further stress corrosion cracking, and applying the same factor of safety,

. the allowable tensile stress in the remaining Unit 1 studs is 6 ksi. The future preload and long-term service stress should not exceed this value.

Technical Report TR-3887-2, Rev. 1 W TF1 FT?( E DJGINEERING SERVICES 6.1.5 Discussion of Conservatism, Long-Term Loading The procedures used to obtain the long-term allowable value of 6 ksi will now be reviewed for conservatism. There were two methods used to develop this value, the first based upon the fact that the remaining studs have not failed to date and the second based upon threshold stress corro-sion data for fatigue precracked specimens. In both cases, the normal Section III factor of safety of two was applied to the value of 12 ksi which was calculated as the maximum stress level which could be applied without further crack propagation.

The second method is more direct, but is based on a limited amount of data. A f atigue precrack and a stress corrosion crack have the same characteristics with respect to crack geometry. Further, if stress corro-sion cracking propagates an existing fatigue crack, the original source of the crack is of no interest. With respect to the present application, it is believed that the originating defect is the result of oxide film crack-ing or corrosion pitting, and the expected crack tip characteristics would be expected to be no worse than at a fatigue precrack. From Fig. A.4, the threshold stress at the bottom of the curve would be in excess of 18 ksi.

This value would be expected to be applicable to studs with a hardness of 47 HRC or higher, about 23 of the remaining 93 Unit 1 studs. For the lower hardness studs, a threshold stress higher than 18 ksi would be predicted.

The 12 ksi value used assumes that the minimum valJe applies even to studs having the minimum specified tensile strength of 150 ksi. Actually, the curve used on Fig. A.4 is not a lower-bound curve, as it is identified thereon, because all of the points are non-failure points. Use of a curve through the no break data points, ignoring data from materials other than 4340, would increase 12 ksi to at least 30 ksi.

The first method assumes that during the period between the time the studs were originally preloaded and the time that the preload was removed that a stress corrosion crack grew to just under critical size. Of course if this were really true, all of the studs would fail when the preload was removed, since removal of the preload involves a brief increase in stud tension to permit nut rotation. (For the reason just stated, some studs

Technical Report TR-3887-2, Rev. 1 WTA RVNE ggggg may fail during detensioning.) The conservatisms in the first method can best be demonstrated by reviewing each term in the mathematical process used to obtain the 12 ksi value for the maximum stress level which could be applied without further crack propagation, specifically:

12 = A x (B/C)

Where:

A= the proof stress applied to the studs. The nominal pre-load stress was 92 ksi. The value used in the calculation was 87 ksi, based on the common assumption that the pre-load stress will relax to two-thirds of yield and that the actual yield was the minimum srecified value, 130 ksi.

For the higher hardness studs, the yield strength will be higher than 1.5x 92 = 138 ksi, and no significant relaxa-tion would be expected.(92/87= 1.06)

B=

Iscc value indicated at high hardness levels.

the minimum K The value used in the calculation was 8 ksi M , based on the minimum curve of Figure 7. The minimum value from tests is usually stated as 10 to 20 ksi4 in (see A.l.1 and the last paragraph of A.3). (10/8 = 1.25)

C= The value of K IC such that the ratio of B/C is a minimum.

The value used was 57 ksi $ from Figure 7 at 46 HRC.

The actual measured toughness of stud 3, at this hardness, was 43 ksi-J in. (57/43 = 1.32).

Therefore, a more probable value is not 12 ksi, but is 92(10/43) = 21 ksi.

This value is still considered to be conservative because a minimum K Iscc curve has been used with measured K IC data. )

1 j Based on this discussion, a value of either 30 k'si or 21 ksi could be  !

t

) justified in lieu of the 12 ksi value actually used as the maximum stress '

l level which could be applied over a long-term without further stress corro-

Technical Report TR-3887-2, Rev. 1 WT51 prh'NE ENGNEERING SERVICES sion crack propagation. Using these values, the allowable long-term stress would be 1 creased from 6 ksi to 15 ksi or 11 ksi after application of the factor of safety of 2.

As mentioned in 6.1.2, two K p ints have been identified which lie Isa below the K Iscc lower bound curve plotted in Figure 7 in the high hardness region. These points are shown in Figure A.7. In dealing with such isolated. data points, the usual procedure, and the procedure used here, is to deal with the isolated data as a specific case without a f actor of safety to ensure that the data scatter are adequately covered by the factor of safety applied to the general data. Following this procedure with these two isolated paints:

Sy K Iscc K IC KIscc/KIC (92 ksi) (K Iscc N IC) 194 8.5 72 0.118 10.9 225 5 4.63 >0.079 > 7. 3 The "less than" symbol is used on the K value in the second row because IC the numerical value given would be lower if it were obtained in full conformance with ASTM Standards. The last column applies the same calcula-tional method that was used in 6.1.2, but using 92 ksi instead of 87 ksi since 2/3 of the yield strength of the material in question is greater than the nominal preload of 92 ksi and no relaxation would be expected. There {

is a point on the lower bound curve at 200 ksi yield which has a toughness  !

ratio of 10/59 = 0.169. Applying the same method, the valu in the last  !

column would become 15.6. Figure A.7 also indicates a relatively high K IC point from Pellini. Applying this method, with a toughness ratio of 15/82

= 0.183 gives a last column .value of 16.8.

The next two highest KIC points are included in the above tabulation. Since these values are higher than the long-term allowable value of 6 ksi established in 6.1.4, the procedure followed has been demonstrated to be conservative.

The property ratio method of 6.1.2 applied a preload value of 87 ksi in establishing the 12 ksi value. As previously noted the actual preload is to be measured during detensioning, and values lower than 87 ksi are expected (and have been experienced at the time this report is being written). Such data are considered to be isolated data points, in the same i

Technical Report TR-3887-2, Rev. 1 WTF1 F-TY(NE NBUNG SERVICES manner as are the two low K points discussed in the preceeding para-graph. Isolated prestress measurements as low as 43 ksi are covered by that reasoning. Of even more practical significance, however, is the fact that the initial crack size would have to have been increased by a factor of 4 in order to initiate stress corrosion cracking in such low-stressed studs, which is unlikely based upon the service record. This experience when coupled with the alternative methods used to justify the 12 ksi value are considered sufficient to indicate that no correction to the 6 ksi allowable value is required as a result of measured prestress values lower than 87 ksi.

6.1.6 Consideration of Load Duration, Short-Term Loading Additionally, the allowable stress discussed to this point is chosen so as to avoid further stress corrosion cracking under long-term loading.

Sirce stress corrosion cracking is a time dependent process, considerably higher stresses could be safely imposed for short periods of time. Since the significant service loading on the subject studs is that which occurs during the initial stages of a plant Faulted Condition, an allowable value for such short-term loadings must also be considered to avoid cleavage fracture in the presence of an existing flaw.

Based upon the experience to date, the argument could be made that the studs could be reloaded to the present preload stress value (92 ksi) for some short time period at any time in the future. To be conservative, the nominal ASME Code factor of safety of 2 will be applied to a value equal to two-thirds of the minimum specified yield strength (130 ksi), giving an allowable short-term service stress of 43 ksi.

The two-thirds of yield value is used in this evaluation as an estimate of the minimum value of the as-relaxed preload present in the studs during the proof-test period. When in detensioning a lower as-relaxed preload is measured, the short-term allowable stress value for all studs shall be reduced to one-half of the lowest measured detensioning load on any stud which is considered to contribute to load carrying capability in the new design concept. Note that the detensioning load may be in-

TechtlicblReport WP WNE TR-3887-2, Rev. 1 N SBMCES creased above the prestress load required for nut rotation in order to determine the allowable short-term service stress.

6.2 Unit 2 RPV Studs Figures 5 and 6 and Table 2 contain the results of field hardness measurements on the Unit 2 studs. In general, these studs did not exhibit any significant variation in hardness with location on the end of the stud, nor would they be expected to based upon the hardenability of AISI 4340

, material. It should be noted, however, that the absence of machining af ter heat treatment also minimizes the potential for hardness gradients on the end of the stud. Considering the higher of the surface hardness or the average hardness in the last two columns of Table 2, and rounding the values to integer values in accordance with SA-370, the following studs are found to have near-surface hardnesses in excess of 38 HRC:

Hardness, HRC Stud Number Inside Outside 39 2,3,9,10,15,19 1,7,8,9,11,12,14,17, 26,29,30,33,35, 19,27,33,41,44,46 45 i

40 12,17,21,27 6,10,15,18,20,21 <

41 28 2,3,38 Of the 96 studs; 42 have a hardness less than 38 HRC, 14 have 38 HRC, 26 have 39 HRC,10 have 40 HRC, and 4 have 41 HRC. Of the 40 studs tabulated, 25 are from heat X and 15 are from heat XX. There were a total of 32 studs from heat X and 64 from heat XX. All 4 studs with a hardness in excess of 40 HRC were from heat X.

The discussion in 4.2 concludes that a maximum surface hardness of 41 HRC is consistent with a specified maximum mid-radius hardness of 38 HRC,

Technical Report W TF1FrT(NE TR-3887-2, Rev. 1 ENGNEBWJG SERVICES and would be the proper value for surface hardness specification. The discussion in 4.3 concludes that a maximum surface hardness of 41 HRC would be found .if a large number of samples were measured of a uniformly hard h.aterial which had a nominal hardness of 38 HRC based on limiting sampling in accordance with a specification. The above tabulation indicates that the Unit 2 studs have a nominal hardness value of 38 HRC, in that 42 are sof ter and 40 are harder with the remaining 14 having a hardness of 38 HRC.

Based upon these findings, TES is of the opinion that Unit 2 studs having a hardness of 41 HRC or less are acceptable as normal material in accordance with the purchase specification. Therefore, all Unit 2 studs are consider-ed normal material, and there should be no unusual restrictions on the use of those studs.

In order to confirm this conclusion, the studs with hardness less than or equal to 41 HRC will be considered with respect to toughness properties.

It is first useful to compare the properties for studs having a hardness of 41 HRC or less with those having a hardness of 46 HRC or more, because the evaluation of 6.1 considered the latter:

Property Hardness HRC 41 or less 46 or more Minimum K 25 or more 8 or less Iscc Probable K 5 or more 16 or less Iscc Maximum K 69 or more 43 or less Iscc Mean K 69 or more 57 or less IC In 6.1, the 8 ksiJ in value was used to indicate that an initial crack 0.004" deep would be required to initiate stress corrosion cracking under

'~

the initial preload condition. With a minimum threshold value of 25 ksi. fin an initial crack 0.040" deep would be required, and depths of 0.076" or 0.30" would be required for the probable and maximum stress corrosion threshold, respectively. The latter value would also be the critical flaw depth for cleavage fracture. These values indicate why stress corrosion cracking is seldom a concern in AISI 4340 studs of normal hardness. The

Technical Report TR-3887-2, Rev. 1 SPTNE 94GNEERING SElWICES initial flaw size required to initiate stress corrosion cracking is larger than would be expected from initial cracking, and the stress corrosion threshold and fracture toughness are sufficently high to prevent propaga-tion and failure, respectively, in the absence of very large defects.

The conclusion that studs with surface hardness less than or equal to 41 HRC are acceptable as normal is also consistent with the experience discussed in A.3. That experience indicates that stress corrosion crack-ing becomes of concern in AISI 4340 materials for properties in excess of 180 ksi yield strength, 200 ksi tensile strength, or hardnesses above 43 HRC.

Technical Report TR-3887-2, Rev. 1 "#PTF1FnYNE ENGINEERING SERVICES References

1. Teledyne Engineering Services Technical Report TR-3887-1, Investiga-tion of Preservice Failure of Midland RPV Anchor Studs, January 25, 1980.

2. Metals Handbook Vol. 1 9th Ed. Page 281.

I i

l

TABLE 1 Field Hardness Survey Results Unit 1 Hardness Readings Unit I Inside Stud Avg. Surface Location Heat L)

L 5

Number- Number Edge + L Center L Ave Converted Converted 2 '3 '4 1 0 704 705 683 657 656 681 43.9 46.8 2 0 687 690 684 674 668 681 43.9 44.7 0 Failed 46.4 ,

4 .000 677 682 675 663 644 668 42.4 43.4 y 5  ? 681 654 651 641 C.D.* 657 40.9 43.9 6 0 677 675 664 659 650 665 42.0 43.4 7 00 625 636 648 636 633 636 38.2 36.9 8 000 707 704 702 686 692 CD 698 46.1 47.1 9 0 717 703 694 671 654 688 44.8 '48.3 10 00 640 626 604 587 CD 614 35.5 38.8 11 -0 644 642 640 641 CD 642 39.0 39.3 12 000 675 673 673 672 678 674 43.1 43.2 13 .000 '705 694 682 681 685 689 45.0 46.9

14. 0 686 696 694 695 689 692 45.3 44.5

• C.D. means Center Drill on end of stud >

making hardness measurements impossible ,

+1/8" from edge .

"#TELEDYNE ENGINEERING SERVICES

Stud Avg. ' Surface Location Heat L)

L 5

Number Number Edge L Center L Ave Converted Converted 2 '3 '4

.15 00 -640, 619 641 636 637 628, 624 632 37.7 37.4 16 0- 709 711 706 688 675 698 46.1 47.4

-17 0 705 697 687 676 670 687 44.7 46.9 18 0 713 706 700 689 692 700 46.3 47.9 19 00- 643 639 634 635 631 636 38.2 39.2 20 00 638 624 624 622 627 627 37.1 38.5 21 0 687 672 634 634 616 649 40.0 44.7 22 0 683 676 674 664 CD 668 42.4 44.1 23 00 639 637 631 629 CD 634 38.0 38.7 24 000 674 669 659 660 651 663 41.7 '43.1 L 25 0 665 657 647 659 CD 657 40.9 42.0 Y 26' O 723 716 714 712 703 714 48.0 49.1 27 000 683 679 658 650 645 663 41.7 44.2 28 0 668 669 667 670 644 664 41.9 42.4 29 0 675 676 668 692 651 672 42.8 43.2 30 0 -689 691 683 676 670 682 44.0 45.0 31 0 680, 672 686 683 679 672 679 43.7 43.3 32 000 708 707 695 660 643 683 44.2 47.3 33 0 703, 701 698, 698 672, 679 645, 656 627, 632 669, 673 42.5, 43.0 46.5 34 00 640 637 629 619 612 627 37.1 38.8 35 00 645 633 640 640 651 642 39.0 39.4

-36 0 70i 686 671 651 646 671 42.7 46.4 WTELEDYNE ENGINEERING SERVICES

Stud Avg. -Surface Location Heat L) L HRC HRC.

5 Number Number Edge L

'c Center L Ave Converted Converted 2 '3 i37 CK) 624 627 624 610 604 618 36.0 36.8 38 .000 688 698 691 687 690 691 45.2 -42.4 39 0 692 693 678 669 663 679 43.7 45.3

40 0. 646 641 638~ 640 CD 641 38.9 39.5

.41 00 638 635 630 624 615 628 37.3 38.5 42 0 697 696 684 680 CD 689 45.0 45.9 43 0 696 679 661 649 647 666 42.1 45.8

.44  ? 675 679 656 640 636 657 40.9 43.2 45' 00 644 642 642 641 646 643 39.2 39.3 $'

46 0 689. 685 684 675 654 677 43.4 45.0 47 0 682 688 690 688 . CD 687 44.7 44.0 48 00 635' 638 640, 698 642 638 639 38.7 38.1 WTELEDYNE ENGINEERING SERVICES

Unit 1 Outside Stud Avg. Surface Location Heat Lj L 5

Number Number Edge L Center L Ave Cor.verted Converted 2 '3 '4 1 0 691, 690 682' 668, 672, 652, 657 645, 651 668, 668 42.4, 42.4 45.1 2 0 683 693 691 687 682 687 44.7 44.2 3 00 622 629 616 611 602 616 35.7 36.4 4: 000 691, 697 698, 702 686, 691 675, 680 669, 663 684, 687 44.3, 44.7 45.6 5  ? 712 709 699 704 CD 706 47.0 47.7 6 0 677 666 663 666 641 663 41.7 43.4 7 0 667, 649 677, 678 679, 691 675, 668 671, 666 674, 670 43.1, 42.6 41.1 8 000 683, 677 667, 671 645, 653 633, 636 645, 656 655, 659 40.7, 41.2 43.8 9 000 715, 710 710, 711 703, 699 679, 687 652, 645 692, 690 45.3, 45.1 47.7 10 0 677, 681 678, 677 663, 661 649, 648 641, 645 662, 662 41.6 43.7 4 11 0 712 696 671 658 CD 684 44.3 47.7 i 12 000 697 695 696 676 671 687 44.7 45.9 13 000 671 673 673 681 673 674 43.1 42.7 14  ? 714 724 716 698 CD 713 47.9 48.0 15 00 648 649 649 668, 670 CD 654 40.6 39.8 16 0 715 717 714 696 CD 711 47.6 48.1 17 0 668 673 670 672 680 673 43.0 42.4 18 0 657 655 623 627 625 637 38.4 40.9 19 000 703 699 698 691 692 697 45.9 46.7 20 0 706 707 699 699 685 699 46.2 47.0

1. TELEDYNE ENGINEERINGSERVICES

Stud' ,

Avg. Surface-Location -Heat Lj L 5

Number Number Edge L Center L Ave Converted Converted 2 '3 '4

~ 21 0 712 714 696 673 CD 699 46.2 47.7' 22 0 668 677 675' 668 CD 677 43.4 42.4 23 0 690 683 674 672 CD 680 43.8 45.1

'24 000 702 690 666 669 CD 682 44.0 46.5 25 0 699 704 699 697 693 698 46.1 46.2

26. 0 713 712 703 701 695 705 46.9 47.9 27 0 701 646 681 672 659 672 42.8 46.4 28  ? 720, 695 709 706 705 694 704 46.8 47.2 29 000 689 683 678 672 679 680 43.8 45.0 30 000 671 673 664, 690 657 645 665 42.0 42.7 ,

31 0 696 699 696 682 676 690 45.1 45.8 y 32 0 660 670 666 657 648 660 41.3 41.3 33 0 670 694 694 704 691 691 45.2 42.6 34 0 668 671 659 647 644 658 41.1 42.4 35 000 686 680 670 680 662 676 43.3 44.6 36 Failed 690 681 678 672 660 676 43.3 45.1 37 0 695 693 694 699 694 695 45.7 45.7 38 0 661 680 681 673 684 676 43.3 41.5 39 00 623, 645 640 616 610 597 619 36.1 38.0 40 0 716 701 695 665 655 686 44.5 48.2

"#TELEWNE ENGNEERING SERVICES

' Stud Avg. Surface-Location Heat- Lj L 5

.HRC HRC

. Number. Number Edge L L l Center L Ave Converted Converted 2 3 4 41 000 682 683 684 713, 713 690 690 45.1- .44.0 42 -000 702 709 712 709 710 708 47.3 .46.5 43 0- 661 675 685 634, 623 CD 663 41.7 41.5 44 0 692 699 ,681 653 657 675 43.2 45.3 45 "O 698 694 694 692 681 692 45.3 46.1 46 000- 684 678 671 665 CD 675 43.2 44.3 -

47- 0- 699 696 692, 709 694 683 695 45.7 46.2

'. 48 0 695 696 676 671 656 679 43.7 45.7.

b?

E

~~

WTELEENNE ENGNEERINGSERVICES

TABLE 2 Field Hardness Survey Results

. Unit.2 Hardness Results Unit 2 Inside

-Stud Avg. Surface

. Location Heat' :L j L HRC HRC 5

Number Number . Edge- L L Center L Ave Converted Converted.

2 3 '4 1 ~X 631 625 629 636 633 631 37.6 37.6 2_ .X. 627 627 637 642 659, 668 639 38.7 37.1 3 .X. 638 632 626 631 CD 632 -37.7 38.5 4 XX 629 634 627 638 634 632 37.7 '37.4 b 5 XX 628 635 643 640 CD 637 38.4- 37.2 6 X 633 632 626 637 CD 632 37.7 37.9

'7' XX 629, 612 635 648 638 641 ~ 637 38.4 36.3 8 XX 625 635 642 638 CD 635 38.1 36.8 19 XX. 626 630 633 651 672, 662 641 38.9 37.0 10 .XX 641 640 633 646 CD 640 38.8 38.9

-11 XX 634 .629 634 638 635 634 38.0 38.0 12 XX 632 642 655 653 654 647 39.7 37.7 13 XX -627 641 640 644 633 637 38.4 37.1 14 X 596 596 601 606 602- 600 33.6 33.0 15- -X 630 634 637 653 CD 639 38.7 37.5 161 X 628 629 628 629 CD 629 37.4 .37.2 WTELEDGE ENWE!ERNGSGMCES

. Stud Aug. Surface Location Heat L)

L 5

Number Number Edge L L L Center L Ave Converted Converted-2 3 4 17 X 647 640 634 640 CD 640 38.8 39.6 18 X 623 639 633 641 CD 634 38.0 36.6

'19 X 643 629 620 633 CD 631 37.6 39.2 20 X 608 638 633 636 CD 629 37.4 34.6 21 X 649 637 636 637 CD 640 38.8 40.0 22 -XX 611 607 613 614 611 611 35.0 35.0 23 XX 612 608 608 607 597 606 34.3 35.2 24 XX 610 613 629 628 631 622 36.5 34.9 25 XX 608, 616 604, 604 613, 611 620, 622 623, 648 614, 620 35.5, 36.2 35.2 h 26 X 639, 624 645 648 652 CD 644 39.3 38.0 27 X 646 635 639 635 CD 639 38.7 39.5 28 X 659 647 640 646, 629 CD 646 39.5 31.2 29 X 632 648 648 651 CD 645 39.4 37.7 30 X 643, 627 645 637 645 CD 641 38.9 38.1 31 XX 625 627 632 631 630 629 37.4 36.8 32 XX 619 626 630 635 633 629 37.4 36.l' 33 XX 631 634 639 641 658 641 38.9 37.6 34 XX 627 632 631 635 637 632 37.7 37.1

, 35 X 639 633 627 632 CD 633 37.9 38.7 j 36 XX 608 605 602 605 601 604 34.1 34.6 37- XX 614 610 606 607 618 611 35.0 35.5 38 X- 614, 607 635 635 646 CD 632 37.7 35.0 39 XX 608 610 605 610 625 612 35.2 34.6 40 'XX 608 597 608 605 600, 595 603 34.0 34.6

^

WTELEDGE ENGDEERNGSERVICES

Stud Avg. Surface

. Location. Heat ;L j L HRC HK ,

5 Number. Number Edge L Center L Ave Converted Converted' 2 '3 '4 41 XX 602 601 608 617 635 613 35.3 33.8

. 42 XX. 593 597 603 597 636, 617 603 34.0 32.7

~

43 XX 596 599 616 609 601, 622 606 34.3 33.0 44' XX. 610 606 623 638 632 622 36.5 34.9 45 XX- 625 635 630 650, 651 641, 658 638 38.5 36'.9 46 XX 604 593 597 614 616 605 34.2 34.1 47 XX- 618 600 604 610 615 609 34.8 36.0

'48 XX 608 615 607 605 611 609 34.8 34.6, I .

em t.

i r

1

bnit2Outside Stud Aug. Surface Location Heat Lj L "

5 Number Number Edge' L L Center L Ave Converted Converted 2 '3 4 1 X ~638 642 640 640 CD 640 38.8 38.5 2 -X 659 642 642 644 CD 647 39.7 41.2'

'3 X 647,'674 632, 634 636, 636 677, 65a CD 648, 649 39.8, 40.0 - 41.4 4 -XX 604 602 609 620 C12, 660 617 35.8- 34.1

-5 XX 63'- 626 636 638 632 633 37.9 38.0-6 X 637- 647 649 655 CD 647 .39.7 38.4 7 XX 623 632 646 650 674 645 39.4 36.7 8 .XX 637 640 643 644 651 -643 39.2 38.4

'9- XX 637 637 638 649 650 642 39.0 38.4 g 10 XX 628 633 645 659, 677 671 649 40.0 37.3 11 XX 630 635 642 640 648 639 38.7 37.5 12 XX- 635 631 637 642, 636 656, 649 639 38.7 38.1

-13 XX '611, 609 600 606 618 607 608 34.6 34.9 14 X 642 646 643 641 CD 643 39.2 .39.0 15 X- 652 648 640 642 CD 646 39.5 40.3 16 X 629 633 635 632 CD 632 37.7 37.4 17 X 641 633 634 649 CD 639 38.7 38.9 18 X 645 640 645 655 CD 646 39.5 39.4

'19 X 639 637 642 647 CD 641 38.9 38.7 20 X 649 631 621 627 CD 632 37.7 40.0 WTELEDGE ENGDEERNG SERVICES

Stud Aug. Surface Location Heat Lj L 5

Number Number Edge L 2 '3 '4 Center 'l Ave Converted Converted 21 .X 653 641 639 633 CD 642 39.0 40.4 22 XX 612 619 622 618 621 618 36.0 35.2 23 XX 587, 597 597 619 613 617, 622 608 34.6 32.5

' 24 XX 613 606 610 633 616 '616 35.7 35.3

.25 XX 619 611 610 641 649, 642 625 36.9 36.1 26 XX 604 601 618 626 647, 641 619 36.'1 34.1 27 XX 631 636 638 640 648 639 38.7 37.6.

28 XX 598 601 606 604 607 603 34.0 33.3 29 XX- 618 615 630 610 628 620 36.2 36.0 30 XX 595 604 612 633 643 617 35.8 32.9 L, 31 XX 602 591 596 619 605, 616 604 34.1. 33.8- I 32 XX 617 604 611 611 616 612 35.2 35.8

'33 XX 616, 616 640 637 642 654 638 38.5 .35.7 34 XX 610 611 612 624 636 619 36.1 34.9 35 XX 591, 670 602 609 624 624 610 34.9 32.3 36 XX 608 599 607 624 634 614 35.5 34.6 37 XX 609 600 602 608 617, 611 607 34.5 34.8 38 X 659 653 673 653 CD 660 41.3 41.2 39 XX 610 612 616 616 628 616 35.7 34.9 40 XX 610 610 598 608 613 608 34.6 34.9

Stud-Avg. Surface:

Location' Heat -L j L 5

~

. Number.. Number. Edge L L Center L Ave Converted Converted 2 '3 4 i f;

.41 XX- '626 -633- 645 656 663. 645: 39.4 37.0

.42

-XX- 597 597 606 614 611 605 .34.2 33.2

;43- XX- 629 631 631 638 639 634- 38.0 37.4i

~

44. XX. '634 -637 638 643 655, 656 642 39.0- '38.0 45 -XX 600 608 618 619 619 613 '35.3  : 33.6-46- .XX 630 632 634 661 670 645 39.4 37.5 47 XX- 606 599 599 612 623 608 34.6 34.3-48 XX 610 604 612 616 610 610 34.9 34.9 1

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32 3S 40 45 50 SU RFACE HARNESS RC WN ENGNEERNG SERVICES

20-Figure 3 Distribution of Measured Hardnesses

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32 35 40 45 50 S U R FAC E HARDNEM RC wm ENGDEERNGSEMCES

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32 35 40 45 50 SU RFA CE HARDNESS RC wm ENGSEERNGSGMCES

20-Figure 5 Distribution of Measured Hardnesses in

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45 50 SU RFA C E HARDNESS R C W TEENNE ENGNERING SEWCES

Figure 6 Distribution of Measured Hardnesses in

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Technical Report TR-3887-2, Rev.1 DJGBEB8JNG SERVICES APPENDIX A AVAILABLE FRACTURE TOUGHNESS DATA A.1 ARPA Handbook Draft The following are from a chapter prepared by Clive S. Carter of the Boeing Commercial Airplane Co. for the ARPA Handbook on Stress Corrosion Cracking and Corrosion Fatigue which is entitled " Stress-Corrosion Cracking and Corrosion Fatigue of Medium-Strength and High-Strength Steels. When hardness values are not included in the quotation, hardness values obtained from the correlations indicated on Figure 7 are inserted and are underlined. Then;

a. "Most low-alloy, ultra-high-strength steels (that is, heat treated to yield strengths exceeding 1240MNm-2 (180 ksi)

(43 HRC) are susceptible to cracking in air. Suscept-ibility increases with yield strength and, in general, with increased relative humidity. Threshold stress values in humid air usually correspond to the value in water, although the rate of crack growth may be considerably slower in air." (Draft page 300)

b. " Low alloy steels (e.g. AISI 4340 and D6AC) are very sus-ceptible at tensile strength levels exceeding 1515 MNm-2 (220 ksi) (46 HRC), and service failures have occurred."

(Draft, page 300).

c. "The susceptibility to delayed failure of notched (Kt = 10) specimens of a variety of low-alloy steels in direct contact with aerated distilled water increases drastically for steels with strengths greater than about 1240MNm-2 (180 ksi) (40 HRC) tensile strength or 1070 MNm-2 (155 ksi) yield strength" (Draft page 77)

Technical Report TR-3887-2, Rev. 1 SERVICES

d. " Stress corrosion cracking was a contributing factor in a recent failure of a large pressure vessel _ _ _ weld heat-affected zone of high hardness (up to 444 HVN or 45 HRC),

probably as a result of stress relief heat treatment extended to critical size by stress corrosion under the combined action of residual stress and moisture that remained from hydrotesting." (Draft page 111)

e. "AISI 4340 exhibits crack growth under sustained load in dry argon. When heat treated to a yield strength of about 1475 MNm-2 (214 ksi) (48 HRC), the threshold stress inten-sity is about 0.78 K "

c (Draft page 310)

f. " Smooth specimens of low-alloy ultra-high-strength steels at yield strengths exceeding about 1450 MNm-2 (""1 ksi)

(47 HRC) _ _ are generally susceptible to stress-corrosion cracking in air." (Draft page 311).

g. " Sustained load tests on precracked specimens in air of controlled relative humidity (RH) show the following:

Air-melted AISI 4340 steel (1860 MNm-2 (270 ksi) (52 HRC) tensile strength exhibits crack growth at stress intens-ities as low as 43% K IC in 90% RH air.

The threshold stress intensity for AISI 4340 steel (1345 MNm-2 (195 ksi) yield strength (45 HRC) in 100% RH air (or 100% RH argon) is 30% K IC The threshold stress intensity for AISI 4340 steel (1240 MNm-2 (180 ksi) (43 HRC) yield strength in 54% RH air is ll4MNm -I (105ksi-(In'),i.e.75%K IC These observations illustrate that the 'ltra-high-strength steels can be susceptible to cracking in moist air, the susceptibility tending to increase with relative humidity and yield strength" (Draft page 314)

Technical Report TR-3887-2, Rev. 1 DJGtEstlNG SERVICES

h. "At the low end of moisture concentration, crack growth in AISI 4340 steel (1965 MNn-2 (285 ksi) (53 HRC) tensile strength) has been observed at dew-points higher than -20 C" (Draft page 318)

1. " Smooth specimens of most steels heat treated to yield strength levels exceeding 1380 MNm-2 (200 ksi) yield strength (HRC 46) are susceptible to SCC when stressed to 75% of their yield strength "in water (Draft page 350).

J. "For tensile strength levels exceeding 1515 MNM-2 (220 ksi) (HRC 46) the threshold stress can be less than 50%

tensile strength" in water (Draft page 350).

k. For AISI 4340, "the threshold stress intensity typically falls within the range 11-22 MNm- /2 (10 - 20 KsiM for yield strengths exceeding 1380 MNm-2 (200 ksi) (46 HRC)

(Draft page 355)

1. "There is no great difference in the SCC threshold K Iscc among any of the medium-carbon low-alloy steels within a given strength range". (Draft page 359).

Table A.1 and Figures A.1 through A.6 are copies of available data from Carter's report which are of interest to the present concern.

A.2 Other Data Data in addition to those contained in the ARPA draft report are presented by:

a. Stress Corrosion Cracking of 4140 steel, E. M. Mielnik and G. D.

Wang in Environmental Degradation of Engineering Materials Virginia Polytechnic Instit9te, Blacksburg 1977, pp. 217-234

Technical Report yg TR-3887-2, Rev. 1 gg

b. Deformation and Fracture Mechanics of Engineering Materials Richard W. Hertzberg, John Wiley & Sons, 1976, pp. 390-391

c. Inhibition of Environmentally Enhanced Crack Growth Rates in High Strength Steels, C. T. Lynch, F. W. Vahldick, F. J. Bhansali, R.

Sunni tt, in Environment Sensitive Fracture of Engineering Materials, Met. Soc. of AIME, 1979

d. Stress Corrosion Cracking of High Strength Steels and Titanium Alloys in Chloride Solutions at Ambient Temperatures, M. H.

Peterson, B. F. Brown, R. L. Mewbegin and R. E. Groover, Corrosion, Vol. 23 (1967), p. 142

e. Principles of Structural Integrity Technology. W. S. Pellini, Office of Naval Research, 1976, p. 185.

Although the Mielnik title indicates that the material tested was AISI 4140, telephone conversation with the author indicates that the material had excessive carbon and was otherwise unusual in behavior.

The data from those sources are summarized on Figure A.7. Because the behavior of Mielnik's material was unusual, the curve through these points is dashed.

A.3 Discussion The behavior of these low alloy, high-strength steels in air is depen-dent upon the relative humidity, with the threshold stress intensity value in humid air approaching the value in water. Further, the behavior in water is similar to that in a sodium chloride solution used to simulate seawater.

Data related to relative humidity effects on AISI 4340 are given in A.1, subparagraphs e and g as follows:

Technical Report TR-3887-2, Rev. 1 WTF1 PTT(NE ENGNEERING SERVICES Hardness Iscc in % K IC

% RH K 43 54 75 45 100 30 r

48 0 78 52 90 43 These data indicate substantial effects when the relative humidity is high. !Jote that A.I.d reports on a failure in 45 HRC material as a result of moisture remaining from hydrotesting.

The effects of applied stress level are considered by A.1 subpara-graphs i and j and Figure A.4. The subparagraphs and the upper curve in Figure A.4 relate to smooth specimens. The subparagraphs indicate t.do quite different failure stress levels (150 ksi and 110 ksi) for quite similar strength properties, since 200 ksi yield strength is generally associated with about 220 ksi tensile strength. Figure A.4 provides a better indication of the possible scatter. The lower curve in Figure A.4 is labeled the " lower limit of precracked specimen data", with the pre-cracking by fatigue. However this identification is very misleading, since the data points plotted are non-failure points not failure points.

Actually, a curve representing the lower limit of stress corrosion crack-ing would be a curve drawn above the data points rather than below the data peints. The lower curve actually plotted on Figure A.4 is used in 6.1 to be conservative, but an upper bound is also used in the discussion of 6.1 to indicate the degree of conservatism.

Figure A.2 provides a more detailed way of visualizing the effect of stress concentrations on delayed fracture for various low-alloy steels, although the data are for a very short test time. The behavior for KT=5 is approximately what one would expect for a threaded fastener. Although no data are plotted for ultimate strengths between 180 ksi and 230 ksi, it is evident that corrosive effects become significant at some intermediate stress level. The curves for K = 10, which are the source of the quota-t tion in A.1 subparagraph c, could represent the behavior at the first engaged thread. The lower pair of curves indicate the effects of precrack-ing. Here the effect becomes significant at even lower strength levels.

Technical Report TN TR-3887-2, Rev. 1 N SERVICES Remaining quotations have been chosen so as to indicate the AISI 4340 strength levels at which a concern is expressed with respect to stress corrosion. Since the hardness values added were from one specific correla-tion, the data are summarized as follows:

Subparagraph Stated strength Approx. HRC a yield above 180 ksi 43 b tensile above 220 ksi 46 f yield above 210 ksi 47 These data indicate a conservative level of concern for yield strengths above 180 ksi, tensile strength above about 200 ksi, or hardness above 43 HRC. The variation of stress corrosion resistance for AISI 4340 steels with yield strength is plotted in Figure A.5 and summarized in subparagraph 1 of A. I. As with the applied stress data plotted in Figure A.4, the effect saturates at higher strength levels with a lower limit of about K Iscc = 10 ksi 6 at yield strengths abvve 200 ksi.

There are but a limited amount of data available for AISI 4140. In addition to that of Mielnik which was previously discussed, the data on Figure A.6 and Table A.1 are of interest. These data indicate the follow-ing:

Tensile strength (ksi) K Iscc 202 22 200-220 15 232 14 260-280 11 These values are consistent with those available for AISI 4340.

Table St.-Threshold Stress intensity Values for Ultrahigh Strength Steels in Water.

Sodium Chloride Solution, and Synthetic Seasater Thresold MN m-3/2 (ksi E), at indicated tensite strength. MN m-2 (ksi) 1240 to 1380 1380 to 1515 1515 to 1G55 1655 to 1795 1795 to 1930 1930 to 2070 Steel (180 to 2006 (200 to 220) (220 to 240) (240 to 260) (260 to 280) (280 to 300)

AISI 4140 '16 (15) a12 (11)

A!SI 4330M b 27(25)

AISI 4340 C d h7.7 to 19 (7 to 18) u 28 to 42 (2G to 38) 9 to 47 (8 to 43) '11 to 30 (10 to 27) '11 to 16 (10 to 15) 911 to 16 (10 to 15) 300MI4340M i 512 to 21 (11 to 19) 16(15) rri D6AC k 3 23 (21) 13 to 22 (12 to 20) 57 .7 (7)

H 11 tD3 (30) "25(23) "22 (20) *8 to 10 (7 to 9) 835M30 12 (11)

NCMV P10 (9) 35NCD16 Y13 (11) Y13 (11) Y13 (11) _

HP 9Ni-4Co-0.2C 977 to 126 (70 to 115) i 5

HP 9Ni-4Co-0.25C '82 (75) 38 (35) f HP 9Ni-4Co 0.3C '29 (27) u< 26 (< 14)

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