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AD-758 754

Stress Corrosion Cracking and Hydrogen Embrittlement of High-Strength Fasteners

Aerospace Corp.

APRIL 1973

Distributed By:

National Technical Information Service U. S. DEPARTMENT OF COMMERCE

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AIR FORCE REPORT NO SAMSO-TR-73-131

AEROSPACE REPORT NO I R-0073(341 301) - 1

Stress Corrosion Cracking and Hydrogen Embriltlement of

H igh-Slrenglh Fasteners

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Prepared tor SPAM AM) MISSILI sV^IIMS ORCiAM/ATluN

AIR i:{)R( I SV.STIMS ( OMMAM)

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Ins .Atiics. ( i! i; < • t ii i .i

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LABORATORY OPERATIONS

The Laboratory Operations of The Aerospace Corporation is conducting

experimental and theoretical investigations necessary for the evaluation and

application of scientific advances to new military concepts and systems. Ver-

satility and flexibility have been developed (o a high degree by the laboratory

personnel in dealing with the many problems encountered in the nation's rapidly

developing space and musile systems. Expertioe in the latest scientific devel-

opments is vital to the accomplishment of tasks related to these problems. The

laboratories that contribute to this research are;

Aerodynamics and Propulsion Research Laboratory: Launch and reentry aerodynamics, heat transfer, reentry physics, propulsion, high-temperature chemistry and chemical kinetics, structural mechanics, flight dynamics, atmo- spheric pollution, and high-power gas lasers.

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Materials Sciences Laboratory: Development of new materials; metal matrix composites and new forms of carbon; test and evaluation of graphite and ceramics in reentry; spacecraft materials and components in radiation and high-vacuum environments; application of fracture mechanics to stress corrosion and fatigue-induced fractures in structural metals; effect of nature of material surfaces on lubrication, photosensitization, and catalytic reaction«, and development of prosthesis devices.

Plasma Research Laboratory; Reentry physics and nuclear weapons effects; the interaction of antennas with reentry plasma sheaths, experimenta- tion with thermonuclear plasmas; the generation and propagation of plasmn waves in the magnetosphere; chemical reactions ot vibrationally -sxciud species in rocket plumes; and high-precision .sser ranging.

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THE AEROSPACE CORPORATION El Segundo, California

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Air Force Report No. SAMSO-TR-7 3-131 Aerospace Report Nu.

TR-0073(3413-01)-1

STRESS CORROSION CRACKING AND HYDROGEN

EMBRITTLEMENT OF HIGH-STRENGTH

FASTENERS

Prepared by

.1. K. Stanley- Materials Sciences Laboratory

Laboratory Operations

73 APR 30

Systems Engineering Operations THE AEROSPACE CORPORATION

Prepared for

SPACE AND MISSILE SYSTEMS ORGANI7A PIOiV AIR FORCE SYSTEMS COMMAND

LOS ANGELES AIR FORCE STATION Los Angeles, California

Approved for public release; distribution unlimited.

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FOREWORD

This report is published by The Aerospace Corporation, El Segundo,

California, under Air Force Contract No. F04701-72-C-0073.

This report, which documents research carried out from June 1971

to January 197Z, was submitted on 11 November 1972 to Colonel Frank B,

Alford, Jr., LVCA, for review and approval.

Approved

Mft W. C. Riley, director Materials Scvences Laboratory Laboratory Operations

S. 'Lafäzan, Group Director Titan III Directorate / Veiiicle Systems Divisiqn Systems Engineering Operations

Publicatinn of this report does not constitute Air Force approval of

the report's findings or c> nciusions. It is published < nly for the exchange

and stinuil.ition of ide.iis.

J^faii**& & Frank B. Alford, Jr.^ Col. , USAF Director, Titan Engineering and

Test Space Launch Vehicles SPO

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1 ORIGINATING ACTIVITY (Corporatö authur)

The Aerospace Curporation El Segundo, California

i ti REPORT SECURITY CLASSIFICATION

Unclasoified 2 ft GROUP

3 RCPORT TITLE

Stress Corrosion Cracking and Hydrogen EmbriUlement of High-Strength Fasteners

4 DESCRIPTIVE NorE5(TVp»o( report and Inctuatve date»)

5 Au THORISI fF/r«r mm«, mtddla Inltlcl, Imal . a)

James K. Stanley

6 REPO RT DATE

73 APR 30 8/1 CONTRACT OR GRANT NO

F04701-72-C-0073 h PROJEC T NO

7o TOTAL NO OF PACES

49 7b NO OFREF5

39 9^1 ORIGINATOR'S RETPOHT NUMBER'S;

TR-0073(3413-01)-1

SAMSO-TH-7-.-131

10 DISTRIBUTION STATEMENT

Approved for public release; distribution unlimited.

II SUPPLEMENTARY NOTES 12 SPONSORING MILITARY ACTI

Space and Missile Systems Organization Air Force Systems Command United States Air Force

13 ABSTRACT

Unexpected, brittle failures of high-strength fasteners on aerospace vehicles have been caused by stress corrosion cracking (SCC) and by hydrogen stress cracking (HSC). Despite extensive study, much remains to be learned about the phenomena. The poorly understood failure mechanisms are difficult to differ- entiate, especially in the field. There is a growing .-.se of the term SCC to describe failures by both mechanisms.

Data are given to characterize the classes. For low alloy carbon steels heat- treated to yield strengths below ~160 ksi, stress corrosion is not a problem, nor is hydrogen embrittlement (delayed cracking) very common. Above 160 ksi, difficulties can occur. The high-strength, precipitation-hardening, stain- less steels have varying degrees of resistance to stress-corrosion cracking and hydrogen embrittlement, depending upon strength level and heat-treating pro- cedures that influence the microstructure.

Consideration of plane strain fracture toughness K^Q and stress corrosion threshold Kjcrr; parameters allows selection of optimum bolting materials for a specific environment. The advantage of plane strain fracture toughness analysis is that it does not differentiate between failure mechanisms; failure can be by either SCC or HSC.

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UNCLASSIFIED Set un i\ < LihMÜi .itmii

KEY WORDS

Bolt failures Brittle fracture Fracture mechanisms in bolt selection High-strength fasteners Hydrogen embrittlement Stress corrosion cracking

Distribution Statement (Continued)

Abstract (Continued)

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CONTENTS

FOREWORD ii

ABSTRACT iii

I. INTRODUCTION 1

II. DEFINITION AND DIFFERENTIATION OF STRESS CORROSION CRACKING AND HYDROGEN STRESS CRACKING 5

III. MATERIALS FOR HIGH-STRENGTH FASTENERS . . . .13

A. Classifiralion by Strength Level M

B. Classification by Campositiun and Metallurgical Structure 14

C. MartensiLic Steels 17

D. Stainless Steels (Austenitic and Ferritic) 17

E. Precipitation-Hardening Stainless Steels 19

F. Superalloys (Nickel-Base and Cobalt-Base) 20

G. Classification by Relative Resistance to SCC .... 20

IV. FAILURES DUE TO SCC OR HSC ON THE TITAN III FAMILY OF VEHICLES 2^

V. THE PROMISE OF FRACTURE TOUGHNESS CRITERIA 27

VI. RECOMMENDATIONS <1

A. Prevention of SCC and HSC 57

B, Data Collection and Test Standardization i:<

VII. CONCLUSIONS >)

REFERENCES 41

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TABLES

2.

3.

4.

5.

6.

Nominal Compositions of All Alloys Mentioned in the Text 8

Differences in Failures by HSC and SCC 10

Materials Highly Resistant to SCC Z2

Materials Highly Resistant to SCC if Used with Caution 23

Materials with Low Resistance to SCC 24

Fastener Failures on the Titan III Family of Vehicles 26

Fracture Toughness Rating of Alloys and Heat Treatments for Resistance to Saltwater Environments (Northrop Data) 34

Fracture Toughness Rating of Alloys and Heat Treatments for Resistance to Saltwater Environments (Boeing Data) 35

Results of Sustained-Load Experiments with Notched Specimens of Various Ultra-High- Strength Steels Exposed to a 5% Salt-Fog Atmosphere 36

-VI-

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FIGURES

1. Delayed Failure Characteristics of 4340 Steel at Various Strength Levels Resulting from Exposure to Distilled Water at 68 F 7

2. Typical Data Obtained from Potentiostatic Tests 12

3. Classification of Low Alloy Steels (Martensitic Types) by Strength Range 15

4. Classification of Stainless Steels and Superalloys by Strength Range 16

5. Typical Behavior of a High-Strength Steel in a Corrosive Environment Zr)

6. Delayed Fracture Characteristics of 4340 Steel at Various Strength Levels Resulting from Exposure to Distilled Water 30

7. Data of Figure 6 Normalized by Use of the Stress Intensity Ratio 31

-vii-

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I. INTRODUCTION

The design engineer may not always be fully aware of Lhe possibility

of brittle failures in fastener systems that could result from stress corrosion

cracking (SCC ) or hydrogen stress cracking (HSC ). The failure of high-strength

fasteners in aerospace equipment as a result of SCC or HSC occurs infrequently

and almost always unexpectedly. There is a growing acceptance of the term SCC

to co\er both mechanisms. The term 'environmental stress cracking1 failures

is also used sometimes.

Perhaps some of the reasons for occasional surprises from these phenom-

ena are that (1) the existing knowledge v.as no* available to the designer, (Z) the

possible ha/.ard was recognized but ignored, (i) the existing knowledge was not

believed applicable to the current situation, and (4) there was insufficient test-

ing on a new material before it was commercialized. Designers should not de-

sign fasteners by handbook only. There are no reliable handbooks for material

selection for possibly ha/.ardous situations.

Very few fastener failures can be attributed to deficiencies in the technol-

ogy of high-strength materials. Part of the difficulty can be traced to inadequate

dissemination or utilization of available information and i '■ the unrealistic assess

ments of technology by the improperly informed, failures generally occur from

inadequate knowledge of environmental conditions.

Any systems as sophisticated as those in aerospace application should use

a fastener system to realize the full benefit of high-strength fasteners. A sys-

tem requires proper material selection for bolt, washer, and nut. ikolt heads

(hexagonal or IZ-po'nt), boll-head fillets, and radii at base of threads are all

important aspects of good bolt design. Cart? should be exercised in selection

of nuts and washers. The use of uashers produces more uniform loads. Nuts,

if not properly selected, can fail by SCC or HSC.

HSC refers to hydrogen embrittlemenl, hydrogen cracking, and hydrogen induced delayed cracking.

-1

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l.illli- allention is preäetitly given to torque - tensicn relationships in fas-

tener systi-nih, althouuh ari appreciation of this relationship is essential to opti-

mum structural design, since the function of a fastener sy /turn is to furnish a

clamping; lorce or load, Oplimum design requires torquinji that will capitali/.e

on the full strength potential of the bolt. Use of preload indicating devices and

strain gages is often necessary, so that the loa.ds placed on a bolt are known.

Many engineers have no idea of the clamping forces in their structures, Effects

ol contact surface finish on components, coatings (if any), lubrication, asym-

metrical holes, and torquing anrl retorquing on the same boll are frequently not

appreciated. When 70 to 80 percent of the energy of torquing is expended in

overcoming friction on a dry bolt-nut combination, little energy is available

to develop a clamping force.

Fatigue of the fasteners in aerospace hardware has not been a problem,

ma:nly because acceptance testing time and flight times have been too short

(less than 1 hr) to develop failures with the stress loadings. However, on the

space shuttle that aims for 100 flights, fatigue (jf fasteners could become a prob-

lem. As parts subjected to higher stress are used, more attention must be

given to steel cleanliness (inclusions act as crack nuclei), to radiuserl threads

(to .-educe notch effect), and to larger head shank fillets (also to reduce notch

effect). High-strength fasteners should have threads and head shank fillets rolled

on after heat treatment, and shanks should be ground anrl polished. Rolling

threads after heat treatment builds up a residual stress to counttract a portion

of the applied tensile load, and it ensures unbroken grain flow lines through the

critical area created by the notch effect of the threads. Hood (Ref. 1) reports

experiments in which the threads and !"illets were rolled before and after heat

treatment. The 260 ksi (UTS) bolt rolled before heat treatment had a time to

failure in SCC of 2 hr, whereas if the threads and fillets uere rolled after heat

treatment, the bolt had a time to failure of 74 hr.

The incidence of fastener failure has been decreasing because engineers

have learned to avoid materials that have given difficulties in the past. Solutions

are not always as simple as a material substitution, but may be complex, involv-

ing material changes, use of coatings, redesign, or reduction of stress, or a

-2-

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cumbinaiion ul thesu ( Kel . ^ I. i he re a n-, ui cou r s<;, many (J1 he r I \ l)^■ .^ • i. 1.»■-

lener failures, L'.U., uve rt<; rqumji and stri;ris r.pti.rt-, but l.iilui'cs i.-- .1 ri'biilt

of stress-cor rosiun or hyflrogen enii;rill ICIIUMII .1 :■'■ most itibif.'ious.

E^erause ul a lack ol standa rdi/e'l le.sl ii.i-iho'l'- :or S('.(. .ind lor li~<-, sta-

tistical analyses ul data arc not feasible. A.S'J.M is aiii^i-i', v. ori-inu in iini . irca

to develop standa rrli/"d tests lor both phenomena."

As the st re njit 11 level ine reases ai)o\ e ~ 1 t.D k s 1, liot h the sens it 1 v 11 •. I 1 l)r il

lie fracture and the suseeplibilily lo SCC and ii.S(^ IIK ri'.ise. Althoai;!". •-t.i-l-

with st rengths in excess of 3 00 k s i ire aval 1 able, desi une r s a re !•■ 1 ' 1 ' to

push for this strength level and have settled on levels of ^00 to JnO ksi. .^ume

designers who have experienced either SCC or M.SC ha\ e even nickel •'.': '

strengths of 160 lo 180 ksi.

SCC and HSC have caused many serious and nne \ pec; e<. lailures m iiudi-

st-ength fasteners. Failures 'ia\ e occtirreri in .ippl ical luiis at slresses thai

appeared safe (below the yield strengthl 1 rom stress malvses, even with the

use of generous factors of safety. These failures have led designers '" use

materials far below their true capabilities, either by ustrit; less than optimum

strengths of a high- strength steel or by using steels heat -1 reated to the maxi-

mum strength at very low strenqth levels, say .' • uercenl ul the vu id strenulh,

Hoth SCC and HSC^ involve chennc il end n a.-t d hi ru 11 1! laitors tliat are

poorly understood. Much research ha been 'loin.' in various en-. 1 roi 'i.enl s lo

establish relative sensitivity ol mate n 1: s lo these 1 v> u n.ei h snisius. Media ri re

often chosen to give accelerated failure. Metallurgical slruci r. s have been

studied extensively so that crack initiation and propagalion ■• .in ije l-eller

understood. So far, this approach has not been parlicularlv : ••,. ';..l.

Some of the difficulty in understanding thi tvo fracture modes arises

because they are so much alike. Hydrogen appears to be '.he cuipnl in both

phenomena. It is only in the laboratory, by electrochemical means, that the

'The American Society for Testing and Materials is seeking to develop test procedures in the areas of stress lorrosioi. cracking and corrosioi :ati<4ue smooth test specimens, environments and materials, precrack uriv.l.h, sub critical crack growth, and hydrogen embrittlement.

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'v.n 11 u-e hamsnih can be uniquely differentiated. In the field, it is difficult, il

nut impussibic , to identity which cracking phenomenon was responsible for the

tailurc. St ress-corrosion theory is not sufficiently advanced to predict failure

limes. There is a need for a unifying theoretical mechanism for explanation

ol SCC and 1ISC.

However, despite many similarities, the basic mechanisms are differ-

ent. Although brittle failure in high-si rength steels exposed to aqueous envi-

ronments has been well documented, there is still controversy concerning the

mechanisms ol delayed failure. Relerence i contains a useful discussion ol

Hi- :. K-t hanisms of failure m SCC.

Shotpeening, plating, and painting of low alloy, high - st rength martensitu

laslcncrs as a means of preventing delayed tailures at ambient temperatures

has been largely unsatisfactory. Current interest lies in use of HSC- or SCC-

resistant stainless types and superalloys (nickel-base and cobalt-base).

Whuch of these two mechanisms is responsible for failure of a metal fas-

tener is generally of academic interest to the engineer. Because HSC and SCC

are so similar, the same type of solution will often suffice for both problems.

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II. DKl'lMIIO.X AND DII'KI-;KK;\ Tl ATIO.\ Ul' HTKLSS CORROSION

CRACKING AM) H i'hKOCiKA S.'RIiSS ClACKINCi

Excellent reviews of SCC and HSC arc to In- lound in Rets. 4 and ->.

Stress corrosion cracking is nucleation of a crack in a susrcptiblc metal

in a corrosive environment while the metal is stressed in tension: the crack

then propagates by stress - inrluced cor rosion of the arivancing crack, tip. Crack-

ing may occur inte rgranularly or t ransgranuiarly, depending on the metal

and its heat treatment, failure occurs v,ith little or no plastic deformation,

the fractures are termed brittle failures. The stresses are generally below

the yield stress.

The SCC occurs in specific environments and with environmentally sen-

sitive metals. In most cases, there is negligible loss of metal by general cor-

rosion, and at times the corrosion is imperceptible to the eye. Stress corro-

sion cracking requires highly anodic areas and a locali/.ed pll, such as may

exist in oxide film cracks, pits, crevices, and cold-worked areas.

HSC occurs because ■J hydrogen penetration into the lattice ui the pres-

ence of a tensile stress. It is generally agreed that corrosion plays no direct

role in this mechanism. However, corrosion often plays an indirect role as

the source of hydrogen.

In classic HSC, the hydrogen is introduced into solid solution by electro-

lytic charging, pickling, heat treatment, and corro:-ion reactions. Hydrogen

then causes delayed failure under static load in high-strength alloys, and the

embrittling effect increases with increase in severity of notch, i.e., stress

concentration (Refs. 6 and 7). Tests for HSC have not been slanna rdized; ihe\

should include both machine - not ched and precracked specimens.

Obviously, if no hydrogen is present in the lattice, no HSC can ocmr. if

hydrogen has entered during pickling, heat treating, or electroplating and is

removed by baking in air or vacuum, no HSC will occur when tensile stresses

are applied. The amount of hydrogen that will cause HSC is exceeding]^, small,

of the order of 4 or 5 ppm. Damage has been reported with hydrogen contents

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üi i'\en Ifss than I ppm (Rcl. -1). The v.ork oS Ma/.ancc and Sc-jnoha iRel. H)

HULim'sts that (tclayed traclurcs in as-Cjiicni hid sleula ^rc due* lo some pr<jper-

t u's öl ihc martensile transformation it.icli and not to hyfl rtjgcn, Delects in the

auslfiiili' gram boundary occur as a result ol dynainu eltt.'Cls caused by grov -

niL ii )a rl ens 111' plates. Thesu delects an1 a source ol weakness in the prior nus-

trnitc gr.iin boundaries and give rise to delayed liailurc.

The delayed lailure characteristics ol '1340 steel, which «vas exposed to

distillefl water anri failed presumably by 11SC, are shown in Figure 1. (Xonn-

nal compositions of all alloys mentioned in this report are given in Table 1.)

Similar failure characteristics are caused by SCC. Note in figure 1 the short

time to failure of the 4340 as the ultimate tensile strength (UTS) ol the steel

increases and the threshold stresses below which no lailure occurs (Ref. 7).

The important parameters in these delayed failures are strength levels,

steel composition and metallurgical structure (microstructure), tensile stresses,

environment (i.e., tendency toward corrosion or introduction of hydrogen), and

time. Increases in temperature seem to increase the likelihood of SCC more

than thai of IISC, the latter occurs around room lempe ral ure. The tensile stresses

can be applied stresses, residual stresses (as in roll-formed threads, heat

treatment, welding, straightening, or cold-rolling), or a sum of these stresses.

If residual stresses are present, they are very difficult to measure or estimate

so that one does not know their magnitudes. For either SCC or HSC, there

exists a threshold limit below which the stresses will not cause fracture.

In the laboratory, the electrochemical behavior of the metal offers per-

haps the best arguments that SSC and HSC are separate, distinct phenomena.

Delayed cracking that occurs under cathodic polarization (hydrogen polarization)

can hardly be attributed to SCC, Conversely, delayed fracturing where anodic

polarization is causing dissolution (cor rosion) of the metal can hardly be identi-

fied as HSC. The polarization vs time to failure curves identify the two mech-

anisms uniquely. Obviously, such methods would have limited field use. Brown

(Ref. 9) and Bhatt and Phelps (Ref. 10) have proposed this type of electrochem-

ical procedure for distinguishing between SCC and HSC. SCC occurs by defi-

nition when cracking involves corrosion at anodic areas of the advancing tip

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240

200

0 —

\ 10"

TEMPERED AT 800CF TO A 205ksi STRENGTH LEVEL

TEMPERED AT 400oF TO A 285ksi STRENGTH LEVEL

10 I02

TIME TO FAILURE, mm I05 ;o4

Figure 1. Delayed Failure Characteristics of 4340 Steel at Various Strength Levels Resulting from Exposure tu

Distilled Water at 68 F (Ref. 7)

-7-

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^■U-.UUiUjjg^ifgWl^ll|WW«-iW!llW!li'M#|i ',- i1 !JJ-.ai;j|^HgpjWHWMipw.^^MWiaw*>^.Mii...i i!iüi»if^^^ju,i|..J-..»jp-u^BW.^-i^^1,.'i^-'^»M*'-*-4^^^^,^^ IIMIJIJI iiminiijijj.i . M

I alile 1. Nominal Compositions of ALI Alloys Mentioned in the Text

1 I

-1 !

- ■ - ----- — -'— 1 ,

', il

M-, A in i . Al "Hi.. j

i'. 2.

4 Mu ■ 1 1.4I

4 1-»' - 1 :.1 1.1' 1...1

" ' 1 1 t.-il '•.^■■

in 11 1 . - • 1 -1 '. .1 ' ) . -l ! 1 t . 1 .Sj [

:K \. ■'.V I , DD 1,41 1 . ' n. ■!; \ |

in ■ t ".1 1 . -■ ■ •<. 1 " -1

1 -.

. HI i,ji

1 .

'- ' ( . 0. in

M.. r .»■' . ii. .1 • IH , , . ■ 1..1 4.H.II 11,-11, u. 1 'i j

v-'' "'■ "''

t v|.. !(■ . 1 '■■

1,.,

1 , •' ■ '

i * ,■■ i:: 1 ' II. h i,..i l.i.n 1. ,. ". .■ ■ \ ■

i s... -t i 1 ' i'. 1 • 1 < 1. - i M,. u. i I.I" i; i,.il ". ■

ii 11 11,-11 i 1. .1 1 . »H i. ■ V

1 .' 4 I'll

i; ■ rii i, .i .

ii

1 7 UH

■1. Il li.il 11. t

l.ll)

I'll 1 . Mr. II. in 1- II 1 ~. 1 h.ll 2 . ', 1 1. VII

I'll 1 1-N Mu II. n-, 1.' 7 ■> H.I' ll.ll .' . 2 .

1 I'll 1 1 - M.,

1 ^ ■ I'M

n.io

ii. U7

M

11

1 i h . 1

-1.

l,..l

l.dl

. . ',11

II. t

1.111

AM ii. 1 ,. It, 1. •> I...I 2. '( n. 1 ,N

AM > .-. il. 1J 1 •1. -i l,..l 2. ■• '). 1 N [

An .■ n.iil 1-1 i.. IMI II.Ml

In, „,,-l 7IH o. n ■, 1 ■> 11 2i,, 11 bal 1 .2i 2.1-. '). J 1). 1 \

Mj>ri.-»-:i o. i II in n In. ( 1 -> . 11

il <.ll' , Wl III

l.i stun. -1 -i n.n-, 1 1 -,-, li.il 11. S 11. > I.I J. ii C.u

A J.ii i). (i-. 11 7., li,il 1 . 1 2. 1 1 t). t S 0. ) v 1

Ml' l-.N Jn 11 1-, , V . tl 1 H. 11

II.1.-.1. Il.n X i). in H (1 l,.il 1 IH. ', '." U.i,

Kin. -11 ii.n t il 11 li.il 11 .11 1 11. 11 1. 1 1 •', W.,s|, .(„v II. 117 1 ' IMI It 2. 11 ■1. 1 2. '. (1.7

.m iin.-l X7 '.(1 ". IM 1 ', l.,.l 7.11 II.''^ 2.S 0.7

lyp,- 111 ll. 1 • Ih 0 '/. li ll.ll

1 VP'- KM 11. 1IH 1 1 s '*, '. IJ.II

lyp.' im 11. ill 2 i (1 1 1. H ll.ll

lypi- Ut, II. OH 17 (1 '2.11 I1..1 2.2-,

i yp.- m a. ab IH 0 1 l.ll lul bxc

Typ.- I-47 1). UH IK 11 1 1 .(1 l.,.l ioxr 1 nit. nip J 1 J il. OK 1 n 0 I'y.u li.il CI.SO 4.0 0.1^

iMM*jAiti^ia^^teVHV^-^a,ate-4taäaa^

WWj!!!W,.a».. 4 mMMm,W< IMWJL ■ ■■uti'.."' —-' tj^iuiiJ ..iy.in...iiij|i üt^iJiJMt^..J BP'.'!JI*!»Ml."S-MJ^ip-»*ll

(Kcl. I I ). I1SC t)\ (lc 11 ml ion is i ru 1; i .'ifj ni'lu' IM| by hydrogen ^ern-iatjun ,il

the 11 [5 l)\' icitho'ln i u r rcnl s , I )il I c ri-nvcs lli.it li.ivc Itct n (.cnr ral I y noted in

lailurrs dur to IlSCi or S('(' ,ii'c (jiwti in I'.ihli- .! .

Many tost proo'durus are USIMJ to evaluati- suscuptiljility to SCCand lISCl.

A common SCC It'sl to establish susi-eptibil ily ol ijar slock involves loading the

specimen to some high percentage ol it'.i vielt] stress, or sometimes UTS, and

exposing it to alternate immersion in a 3. i percent XaCl solution lor ! (J mm

and drying in forced air :or 50 mm. The cycling is continued until the speci-

men fails or the test is discontinued. The sodium chloride is usually dissolved

in distilled water or is acidified (to pll of ~l.5). Imposed stresses generally

vary from 75 to l*() percent of the tensile strength of fatigue-c racked notched

specimens and from T-> to 90 percent of the ".2 percent yield strengths of un-

notched specimens.

Austenitic steels are commonly checker] for SCC by exposure to -4 2 per-

cent boiling aqueous MgCI^ (I54C, JO'M') solution; lerritic stainless steels, by

contrast, are relatively immune to cracking in MgCl ,> (lief. 12).

Until recently, SCC tests were conducted on smooth specimens. These

tests are helpful in selection of materials for environments or in development

of coatings. They could not be used, however, lor establishing safe design

loads. Often there is large scatter in the data. The SCC tests on both smooth

and precracked specimens used in fracture toughness studies provide the designer

with tools for material selection on the basis of service environments that

can be simulated in the laboratory. The testing methods, however, need

standardization.

For the HSC test to establish susceptibility, a notched specimen is loaded

to 75 percent or more of its notched yield strength, and hydrogen is cathodi-

cally charged into the steel vhile it is under load. Sometimes a notched speci-

men is plated with electrolytic cadmium and then loaded. A notched specimen

is more susceptible to HSC than an unnotched one. Cathodically charging a

steel with hydrogen provides considerably more hydrogen than is necessary to

produce a delayed failure. Loadings are similar to those used for SCC tests.

■9-

••;i>ii-l--;',-'uV^"'^t:'" ., . itimimmmmmm^mmmmmmmmmmmmtmmmjmmm^ — ~~^-~~~^

able Z. 1 )il iC rt'iv.L'S iri [■"diluresj by 1ISC atvl SCC

use SCC

Rupture nucleation inside the Rupture nucleation at the metal- |

metal or at notches environment interface originating

from pits

Single crack v>.ith minimal Extensive branching and several

branching secondary cracks j

Electron microscopy may show No striations are observable on

strialions indicative of Hiscon- the fracture face 1

tinuous (.rack propagation. (Do

i not conlu.se v. itli latiguc

striations.)

Presence of flat dimples and Flat dimples and hairlines on

hairline lattices on the grains, grains are less numerous than in

as seen by electron microscopy HSC, as seen by electron |

microscopy j

No corrosion is ordinarily Corrosion products are evident

present by microscopy

Occurs mainly in martensitic Occurs in many alloys (ferritic,

steels at very high ultimate austenitic, martensitic) over a

j strength levels wide range of UTS levels

-10-

ljlltgMMBaMMttlBlMttBali^^

ip^^yujrJM^MW.^^^

High-strength bolts must also pass a si rcss-durability (static fatiguu)

lest. The bolls are slresseci lo 1 ^ percent ol Ihe Piininium t( nsile load .nd

are held lor 46 hr. Il eniliriltied by hyflrogen, they will fail.

The potenliotlatic proceriure is becoming more popular lor the study ol

both SCC and HSt'. Figure I shows that IS-HPII steel can be made to lail by

either phenomenoil if the potential is changed (Rel. 13). This type ol test can

probably be standardized to assess the relative susceptibility of a given steel

in various heat-treated conditions to either SCC or HSC. The lest is definitely

a laboratory tool and would yield guidelines rather than design data.

In environmental cracking failures at the launch ranges, it is not possible

to unequivocally separate HSC from SCC. Therefore, the failures are often

attributed lo (1) stress - cor rosion cracking (v.hich includes SCC and HSC) or

(2) environmental stress cracking. For engineering purposes, this may be

sufficient. Pinpointing the actual failure mech-.nism may be ol academic inter-

est only .

Fleet ron I ractography with the transmission electron microscope or the

newer scanning electron microscope cannot uniquely identify HSC. Flectron

microscopy is a very useful tool when used in conjunction with other tools. A

knowledge of circumstances leading to failure is a valuable adjunct to success-

ful failure analyses. The compilations of electron fractographs by AFMI. are

helpful in deciding what the nature of the failure mode may be (Ref. 14).

Intergranular and cransgranular cracking result from hydrogen, a suscep-

tible microstructure, the specimen geometry, and static or dynamic tensile

loading. It is not true that intergranular cracking is typical ol SCC and that.

transgranular cracking is characteristic of HSC. A review (Ref. 15) of 3'*

studies involving fractography of materials failed by HSC and SCC found ZZ

instances of fully or predominantly intergranular cracks, 8 instances of fully

or predominantly transgranular cracks, and 8 mixed cases. Intergranular crack

ing appears to be the most frequent type in both HSC and SCC.

11

■,..:-..■.,.;.. ■.. ..!.:■! ■-' ^x--:-:.::!^:^,i.\^I.:i..:L:{..^::. i&iiitiisi^'. ^^i^Hi^iSi'M^t ̂.„..^^^u^..*:^^,.^^ ^iiUUiM^^ IHMMiaMMMiifittJüiAiia&daiiaMsaaMfliiii ViiiWihiMWiiWitt^^

iBHP ii&i^wiwmm&jmivwi***1!™?'?* i^^WW!!tfP^''IW^IIJ-1JPU'"IM''1'|'lti?M-'j

Ul Q O

o

J UJ Sr O _J <r

Q QL <I

CO

CO

o

12"

08

UJ \— o CL

04

0

-04

-08

I 2

16

20

TESTS CONDUCTED w[ 31/2% NaCi LOADED TO 0 9 OF NOTCHED UTS

ANODIC CONDITION (METAL DISSOLUTION)

7-4PH AGED , , lhr900F(H-900)

I7-4PH AGED Ihr 950F(H-950)

CATH0DIC CONDITION HYDROGEN GENERATED

10 I02 I03

TIME TO FAILURE, mm

10^

Figure Z. Typical Data Obtained from Potentiostatic Tests (Ref. 13)

12-

u-xM--.«cimfntirwiftifr»nTi ''' I'HWTW

Ill JJ^IMI'llMIPMIipipilll»!!' ■JM-A»-!» »J5?™i>Jii|iswu<l-«.ni,i|-,«*<:,j.'':'" ;gppW^^B(.j«MjmtiPLii|jJMU»wyjJU4P.^^HJiJ!!:fl''lfH'>^wH^^^

III. MATERIALS FOR illi;il Si Ul.NCi ASTL'NKRS

The ncrosparc industry has i-mphasizcd Ihr systnas corn rpt uf lasten»-»

for Ihr saki- ol reliability. Desijjn has been improved so that btrnnger, lijjhti

and more sophisticated fastener systems are available. Environmentally resi

tant fasteners have recently been stutlii'd.

Steel composition is important, as is the metallurgical structure. Ihc

time for initiation of the first crack in either HSC or SCC depends on the

mic rest rue tu re resistance to hydrogen diffusion or pit corrosion.

Much is known empirically about what compositions and structures arc

susceptible to HSC or SCC, but little is known from first principles about the

relation of mic rost ructure to crack initiation and propagation. Most data are

basically phenomenological. Tiner and Galpin (Reference 16) are among the

few to attempt studies of mic roprocesses involved in SCC.

Nuts and washers must also be carefully selected for the fastener sys-

tem. Careful selection means that the nuts are also subjected to a SCC test,

generally torqued on a bolt and submerged in salt water. Washers are used

to distribute the load. They too should be compatible with the bolts so that

they do not produce galvanic corrosion. Because they are loaded in compres-

sion, they do not fail by HSC or SCC.

Materials for aerospace fasteners are categorized in several ways:

(1) by strength levels, (2) by composition and metallographic structure, and

(3) by relative susceptibility to SCC and HSC. In the future, the steels will

be categorized by fracture toughness parameters; some such compilations

are already appearing ano are discussed in Section V.

A. CLASSIFICATION BY STRENGTH LEVEL

One arbitrary classification of threaded fasteners is by ultimate str* ngth

range:

low strength

medium strength

< 125 ksi

125-160 ksi

-13.

- ....'..■ ^::..:l:ii- T- ,■-■,, nr ->^---•,> ^ ■-^ Vi. ■ I- r..^v.^-..V^V.w-t^..-;.Jv<^r;^ -„.. im m am aas^flsa

h i L;1I st rcntith

ult ra-hiuh st rciTM h

1H0-260 ksi

• Z « 0 ksi

i iuur( 5 ahovvs the strength ranges fur some popular alloy steel

fasteners. Fmure 4 shows the strength ranges lor some stainless steels

and superallnys.

Of course, stri-nglh luveis disclose neitfier the type, composition,

and mir rost ruclu re of the materials nor susceptibility to SCC anrl HSC

B. ("LASSIFICATION BY COMPOSITION AND METALLURGICAL

STRUCTURL

Another classification, by type of alloy, is useful to the understanding

of the behavior of these materials; alloys are

ma rtensit ic,

stainless steels (austenitic and ferritic),

precipitation-iia rdening stainless steels ( semi-austenitic '.nd martensitic), or

superalloys (nickel-base and cobalt-base)

Experience has shown that some types of high-st rength steels art.' quite

susceptible to SCC and HSC while others are not. Some change in degree of

susceptibility to SCC results from the precipitation of grain boundary carbides

and the presence of secondary phases. Corrosion theory is not sophisticated

enough to explain this phenomenon.

Steels used at or above 160 ksi UTS should meet AMS-2300A cleanli-

ness standards. Steels should be vacuum- or consumable arc-melted for

minimization of inclusions, which can cause pitting. Mechanical properties

sometimes can be higher and more uniform with greater toughness if steels

are vacuum melted.

•14-

.;....,-.„,.^..f, gaBgjaaaaeieMaiBiMaftBBiiaBiMMMiMi .it.-..!.;..^.,-,. ■- ■■■■■ I -....■■■la.lili.r.lrriill-li'iWr.i«- -■■ .■.«*!>*■.■■'- •■"'^■■•■^■■^C-U-AUi.

**z*™^*im'!mTt.T**"?',','r'''*''~'=''''~'J' y" ■ ■■ir'f'"-vMi-.-f"-'f7. i„,.a.iit-.;:L_w..jji<tfBjfe).!i,i,t.w IH:<J

ALLOY r"rrL5

STRCNGTH RANGL

ULTIMATE TLN5UF STRENGTH, ksi

i MEDIUM r* IÜÜ 150

-HIGH 200

-^[JLTRA-HIGH, 250 300

4130

4140

4340

8740

HY-TUF

D6AC

H-ll

HP9-4

MARAGING-200

MARAGING-250

MARAGING-300

□

I I

Figure 3. Classification of Low Alloy Steels (Martensitic Types) by Strength Range

(The wide ranges of strength of some of the steels are ob- tained by tempering at different temperatures. )

-15-

jjjayli^.s.».»;....,■.... , ^.,--..,.■■ ».^„i;i^i.-L,%-^.^,:-..v.aw:i^^^^

■^^■^■Mj'iy.lwi,iiiii!W|.^Hij|. ^w.;lMl^^M.vu!^Vll';°-■ YJ^W»'"^1''/1^.^ -r-nTs-'-r- ■Tg-j":?—--ix-r f-v!.-r--

STAiNLESS STEELS AND

STRENGTH RANGE ULTIMATE TENSILE STRENGTH, ksi

MEDIUM h- HIGH SUPERALLOYS ioo 150 200

-1 ULTRA-HIGH 250 300

TYPr 410

TYPE 416

TYPE 422

TYPE 431

I7-4PH

I7-7PH

PHI5-7Mo

PHI3-bMo

PHI4-8M0

I5-5PH

AM 355

INCONEL 718

HÄYNES-25 □ GUSTOM 455

1 l l

i 1 _ . ^

A286 i ;

MP35N HASTELLOY-X

RENE -41 i i

WASPALOY 1 1

Figure 4. Classification of Stainless Steels and Superalloys by Strength Range

(Dashed lines indicate strengths obtainable by cold-work and aging. )

16-

.,,v-...:. .■.^.. c..-/;:-..-^-^^;,^^.!^^.-^^^ ■ , i,.,.,,,.,.,^...,.., , ..|8M,--.w^,.^-^

rVT^V vw^^^^^p^^wtmm^r^^^J-Mf^i-iL^^ * ''jgVK y?f:i*rr,'&*Q

C. MARTENSITIC STEE_LS

For carbon and low alloy steels heat-treated to strength levels of

< 160 ksi, SCC is not a problem nor is 1ISC common. In the martensitic

steels, there is a ductile-to-brittle transition raniZ'.;, and crack growth

takes place differently above and below the range. Above the transition,

envii onmcntally induced slow crack growth occurs over a large area before

catastrophic failure occurs. Below the transition, significantly less crack

growth is observed before total failure occurs. Below the transition, a

martensitic steel has poor impact or impact-tension properties. Fasteners

made of 11 - 1 ' steel heat-treated to -w 2 00 ksi are usable to -65° F before the

transition is reached.

The austenit ic-type stainless steiVs and nickel-base alloys (Inconel

7IH, Waspaloy, AZH6, and MP ^N) have no such transition and actually

have enhancer! mechanical properties at cryjgenic temperatures.

In martensitic steels, the prior austcnite grain boundaries appear to

facilitate hydrogen diffusion. It is probably true that all martensitic steels

will fail by IISC under severe conditions. Retained austenite facilitates sus-

ceptibility to SCC (Ref, 17).

The maraging steels can develop strengths > 300,000 psi; thus far they

have had limited application at this level, but at lower strengths they are

usable alloys. These steels are cooled from the austenitizing temperature

to form a soft and weak martensite, which is hardened and strengthened by

aging a! S50-(l^0°F. The fracture toughness of these materials is good up

to yield strengths of -^ZSO ksi.

D. STAINLFSS STEELS (AUSTFNITIC AND FERRITIC)

The austenitic steels of the Type 300 series are much loo weak" to be

considered for use in high-strength fasteners. Only through cold work and

MIL Handbook 5A assigns Types 301, 302, 303, 304, 316, 321, and M7 yield strengths of 30 ksi and ultimate tensile strengths of 75 ksi.

-17

MBaäiJMkiaaafaaayBa^MgMäMttaia ^^wi-ji^iry^.Mi'itn^'.i.wj-^t.^ L-^^:^.^.^.../.'-/..^^-.^^:...!*^^.:^ ■.■-■■-::..•:■/^

^■jMm^'HJV-;,:|»'j-''''",'-w^

stress relief can high strengths be developed in these materials; half-hard

materials have a UTS of 150 ksi, while fully hard materials have a UTS of

185 ksi.

Even though the annealed materials have low strength levels, the alloys

can be fractured in environments containing chlorides. Suss (Ref. 17) reports

SCC failures with stresses as low as 2000 psi in an environment of 50 ppm

sodium chloride. The annealed materials are very resistant to HSC. Pitting

on these steels can occur at emergent sli- lines as well as at inclusions.

Precipitation-hardened austenitic r ?kel chromium is also available.

Alloy AZH6, one of the first, is now one of the most popular high-strength

stainless steels. If the alloy is cold-worked (60%) and aged (1200oF),

strengths > 200 ksi are attainable. Alloy A286 has outstanding resistance

to SCC and HSC. In potentiostatic experiments at The Aerospace Corp.

Material Sciences Laboratory, it has been impossible to fracture the alloy

under anodic or cathodic conditions. Field experience confirms the alloy's

high resistance to these phenomena.

Ferrite in an austenitic steel (Type 304) retards SCC that may have

originated in the austenite. Type 301, when cold-worked over 20%, shows

marked susceptibility to HSC because of the martensite formed from the

austenite by cold work (Ref. 12). Stainless steels containing a mixture of

austenite and martensite may fail by either HSC or SCC, depending on the

environment. Additions of silicon to austenitic steels in concentrations i 5

percent arc claimed to make the steels immune to SCC (Ref. 18).

The ferritic stainless steels. Type 400 (e. g. , 410, 416, 420, 422, and

431), are heat-treatable to strengths of ^200 ksi. Although these steels are

martensitic in structure, they are not generally considered in the martensitic

class because they have relatively high chromium contents, ^-13 percent.

These types have good general corrosion resistance but they are suscepti-

ble to SCC. This susceptibility can be removed by tempering at 1100°F or

higher, but high strength is sacrificed (Ref. 19). Reference 20 recommends

limiting the use of 13 percent chromium in corrosive service to ^140 ksi.

■18-

fca».^.,-.^i:....v,..,^^u.^;^ -.r,.-..>,o.^vJi...^.m.,.<v,,a^^^^^^ g riTiMlMJi

Piipippipppippipiipllpffi ^psppgH^^^ipraps^JWisssrrBB^^

Failure üf the ferritic stainless steels by SCC is intergranular or

transgranular, depending on the tempering temperature (Refs. ?A and 22).

When cracking is intergranular, the fracture follows the prior austenite

grain boundaries (Ref. 16). The high susceptibility to cracking in the prior

austenite grain boundary is ascribed to the presence of c-carbide. Delta fer-

rite, sometimes present in Type 410, increases susceptibility to SCC (Ref. 17).

The 13 percent chromium martensitic stainless steels, when tempered

in their secondary hardening range, have minimum resistance to SCC or HSC

(Ref. 23). Phelps and Loginow (Ref. 24) have shown that a 13 percent chro-

mium steel, when tempered above 900° F, has a minimum time to failure in

the Kure (N. Carolina) Beach atmosphere.

E. PRECIPITATION-HARDENING STAINLESS STEELS

The precipitation-hardening stainless steels offer high-st rength,

unplated, corrosion-resi stant alloys that can resist: HSC and SCC, which have

been experienced with plated and unplated martensitic, low-- and high-alloy

steels in the range from 200 to 250 ksi.

These precipitation-hardening steels have varying degrees of resistance

to SCC and KSC depending, of course, on strength level but also on the tem-

perature of the aging treatment. Aging below 1000° F may make these steels

susceptible to SCC and HSC, but aging above this temperature makes them

very resistant to these cracking phenomena. The reasons for this behavior

are not well understood.

The precipitation-hardening steels are of LWO basic classes, marten-

sitic and semi-austenitic. The martensitic types are 17-4 P1I, 15-5 PH,

and PH 13-8 Mo. In these alloys, the martensitic structure forms upon cool-

ing from a solution treatment; subsequent aging between 900 and 1 150ok

strengthens the martensite by precipitation hardening and tempering. Typi-

cal semi-austenitic steels are 17-7 PH, PH 15-7 Mo, PH 14-8 Mo, and

AM 350. • In these alleys, the composition has been adjusted so that the

■i9'

tew^jaAiaa^i^MMiäi^^ ££»

SS^PP^P^SI?!<Sf«jHai»Ä»?JW«W Wlfl!Ki^(^sp«M!5«^!'|!??w??,^T" TTW^m^m^^^

austenite forms upon solution treatment and is retained at room temperature.

In this condition it is readily fabricated (cold-worked). The hardening is

obtained by reheating the austenite to 1400 or 1750° F (called conditioning),

cooling, and finally aging at 950 or 1050oF.

If either of these steels is overaged beyond highest strength, both the

fracture toughness and the SCC resistance are improved.

The fracture toughness and SCC resistance of martensitic steels are

significantly higher than those of the semi-austenitic types, probably because

of the absence of delta ferritic and grain-boundary carlndes (Ref. 25).

F. SUPERALLOYS (NICKEL-BASE AND COBALT-BASE)

In general, the greater the nickel content in the austenitic steels, the

greater their resistance to SCC and to HSC.

Ultra-high-strength stainless fasteners are madd from superalloys

(high-strength nickel-base and cobalt-base alloys). The nickel-base Inco

718 superalloy, cold-worked and precipitation-hardened, has strengths in

excess of 200, 000 psi.

The new and highly alloyed MP-35N exhibits the corrosion resistance

of the best nickel-base alloys. It is highly resistant to SCC and to HSC in

salt environments and marine atmospheres. It is a fairly new material,

and little experience with it has been obtained.

G. CLASSIFICATION BY RELATIVE RESISTANCE TO SCC

A third classification (Ref. 26) rates materials according to relative

susceptibility to SCC and can be used as a rough guide to material selec-

tion based on experience and some laboratory work. This classification

comprises (1) alloys and heat treatments that can be used without restric-

tion, (2) alloys and heat treatments that must be used with caution, and

(3) alloys and heat treatments that should not be used.

-20-

-iliiiiLHi JJt- ^BA^ik; llgllllllljgjgll ■~ti ti ■:--'.-^i---'.-^i i /i-^Jrt -uj^^-re. M ^■n,^,,^^^,^^ -.,.. .....,^^,..i..-.^j;..a-..;::.„..- ^.....^...^

««"«Ä^^W^^W'-r^W^'W^^

These materials are clescriljetl in Tables 3-5. The tables apply only

to SCC in environments of sodium chloride solutions, salt sprays, alternate

immersion (wetting and drying), and marine atmospheres. Similar tables

for HSC are not available.

These ratings are not to be construed as exact, because no attempt

has been made to evaluate the effects of stress, environment, metallo-

graphic structure, and time. These tables represent the type of SCC data

that was available before the advent of fracture mechanics. No attempt has

been made here to incorporate data obtained by fracture mechanics and given

elsewhere. Other factors of concern to the aerospace materials engineer

are the metal-propcllanl compatibilities, which are not discussed here.

-21-

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Table 3. Material« lli^hly Resistant to SCC

M.lt ITl.ll^ Type Heat Treatment3 Remarks j

^00 scrii-s stain- less types 303, 3 04, 316, 32 1. 3-17

A u s'. e n i 11 c Annealing Stressed material can crack in chloride solutions. Annealed mate- rials are not of high strength. | Cold-worked materials can develop high strength but they must be j st ress- relieved. |

17-4 PH Martensit if H 1000 and above

17-7 PH Semi-austenit u CM (100 Strength is developed by cold- working (60%) and aping ((100oF).

Pll 13-K Mo M a r t e n s i 11 e H 1000 and above

l^.S PH Ma n ensil n MIOOO ami above

PH IS-7 Mo Si-nii - .unitemt n rn loo Strength is developed by told- \ working (60";,) ai.d agmp (,*00UF).

PH |4.H Mn S.-MII .uisremt it CM oOO Same is pll 1 ^.-7 Mr..

AM ^0 S<-iiii .uisr.-ml ii S( TI00O ami above

; AM l1^ Semi - aus! eint 11 SCTI000 and above

Cv]stiiin-4Sci Semi-a\ist enit ic HI 000 and above

A 2H6 Austenitic Solut ion-treated and a^ed

A 28b (CW and Aged)

Austenit ic High strength is developed by cold- working (60%) and aging ( UOO^F).

Inconel 7lH Face-centered cubic

Solution-treated and aged

Inconel X-7S0 Face-centered cubic

Solution-treated and aged

Rene-41 Face-centered cubic

Solution-treated and aped

MP 3CN Face-centered cubic

S<»lution-t reated and aged

Solut ion-annealed, cold-worked 60%, and aged.

V'aspaloy Face-centered Solut ion-1 reated

and aped

Low Alloy Steels 4 1 SO. 4 140, 4 340. S74Ü

Martensitic Quenched and tempered

High resistance to .SCC if tempered to 1 60 k si or lower.

Ma raping Steel Ma rtensitic Solution-treated and aped

High resistance if heat-treated to 200 ksi or lower.

For heat treatments, refer to Aerospace or steel producer's literature.

Structural Metals Handbook, Metals Handbook ASM.

-22-

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Table 4. Materials Highly Resistant to SCC if Used with Caution

Materials Type Heat Treatment Remarks |

Low-alloy steels 4130, 4140, 4340, 8740, D6AC, HY-TUF

Martensitic Quenched and tempered

Good resistance to SCC if tempered to ■v 160- 180 ksi

Maraging steel Martensitic Solution-treated and aged

Ma raging-200 and -250

400 series stain- less 410, 416, 422, 431

Martensitic Quenched and tempered

Not susceptible if tempered at 1 100° F or higher

15-5 PH Ma rtensitic H?50 to H1000

PH 13-8 Mo Martensit ic 119 50 to HI 000

17-4 PH Martensitic H950 to H1000

AM 355 Semi- austenitic

SCT 950 to H1000

-23-

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Table 5. Materials with Low Resistance to SCC

Materials Type Heal Treatment Remarks |

Low- alloy si eel s Marl ensit ie Quenched and Very susceptible to 41 -.O, 4 140, tempered SCC if tempered to 4340, 8740, 1H0 ksi and higher | D6AC, HY-TUF

Ma raging Martensitic Solution-treated and aged

Maraging-300

H-ll Martensitic Quenched and tempered

17-7 PH Semi- All heat- austenitic treatments

except CH900

PH 15-7 Mo Semi- All heat- austenitic treatments

except CH900

AM 355 S e m i - Heat-treatments austonitic below SCT ft00

400 series stain- Martensitic Quenched ant) Very susceptible in less 410, 416, tempi; red the secondary hard- 4ZZ. 431 ening range from 500

to 1000oF 1

-24-

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IV. FAILURES DUE TO SCC Oll HSC ON THE TITAN III FAMILY OF VEHICLES

The Air Force Titan III Frogram has had difficulties with high-strength

fasteners on the boosters during the past 6 or 7 years. Table 6 lists some

of the fastener steels and modes of failure. Failures were all in a marine

atmosphere. The precipitation-hardening steels have all been slowly

replaced by the cold-worked type of A286. The 440C and H-ll were continued

in service, but either their heat treatments were modified or protection by

organic coatings became a requirement, or both. Type 212 was eliminated,

but Type 431 was continued in service with organic coatings; long-range

solutions involved substitution of A286.

These failures occurred during the early years of Titan III development

despite a program of stress corrosion control. Tensile stresses (preloads)

on the fasteners are now minimized to 40 percent of yield, and materials are

heat-treated where possible to UTS of 160 ksi. The importance of stress

level, environment, and metallurgical structure of the metal in SCC and HSC

is recognized by the Program Office. Contact with dissimilar metals, the

most likely source of hydrogen from corrosion, is avoided or protected

against. Chemical conversion coatings and anodizing on aluminum, often

retard such corrosion.

NASA plans to initiate studies of service influence on fracture behavior,

i.e., use of fracture mechanics concepts (Ref. 27). NASA has had a few

failures by corrosion in 4330, 4340, AM 355, and 17-7PH.

-2 5-

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V. THE PROMISE OF FRACTURE TOUGHNESS CRITERIA

Improvement of analysis of fracture in various environments through

the use of fracture mechanics allows microscopic study of the fracture pro-

cess (whether by SCO or HSC) independent of the influence of specimen geo-

metry effects and dependent only on stress level and environment. The

fracture toughness approach gives, for the first time, a quantitative knowl-

edge of the effects of a particular environment on a steel stressed below the

yield stress. Such quantitative data v/ill be required by the designer once

he learns how to use it. This method reflects the behavior of a metal in an

environment that may lead to either SCC or IISC and does not differentiate

between mechanisms leading to failure.

The KjgQQ parameter, hence, indicates with good reproducibility the

stress-crack-size threshold below which subcritical cracks will not propa-

gate to a critical size leading to catastrophic failure in a gaseous, liquid or

complex environment in a period of usually 500 to 1000 hours. Both KIQ

and KTC,r,~ have units of ksi \ in. Both K's are also independent of specimen

geometry.

Because smooth test specimens require long times for crack nucleation,

Brown (Refs. 28 and 29) and others have used specimens with preexisting

cracks, thus eliminating the crack initiation periori during which surface films

break down and pitting starts. The use of these specimens reduces the likeli-

hood of erroneous conclusions that alloys are immune to SCC (they may not

pit in the test environment, and pitting is generally prerequisite to SCC) and

permits the use of fracture mechanics concepts. Brown introduced the con-

cept of the threshold, KTC,pp. Very quickly Brown's idea became popular,

and many investigations have shown the value of this approach. 4

The use of plane strain fracture toughness criteria, i.e., K.,, and

K „, makes it possible to select fastener materials that are not susceptible

The term plane strain conditions refers to the square fracture produced by SCC or HSC. Plane stress conditions would involve slant or shear fractures, which are not ordinarily observed with SCC or HSC.

■27-

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to cither SCC or HSC on the launch pad or in any other environment. The

fracture mechanics approach can show whether a metal is affected by the

stress environment and to what degree. The analysis also shows that where

degradation occurs there is a threshold stress below which no SCC or HSC

occurs, as shown in Figure 5. Note how severely the salt environment

reduces the stress intensity factor K (Ref. 30).

In tough alloys, failure occurs after longer time intervals. Crack propa-

gation may be slower, and the alloy may tolerate a longer crack before frac-

turing. A small crack may cause SCC in a material of low toughness, whereas

a larger crack may be required to fail a tougher material. The tougher mate-

rial may require a longer time to fail because crack growth is slower in a

tougher material and not because SCC or HSC is slower. Time to failure thus

can be used as a measure of SCC growth.

Relating the environmental applied stress intensity factor Krc,pp to the

plane strain fracture toughness K „ permits normalization of the differences

in toughness or heat treatments of alloys . The ratio KTC,rp/K,„ serves as a

normalizing parameter for comparison of steels and their heat treatments.

Figure 6 shows the delayed fracture characteristics of 4340 steel heat-treated

to various strength levels in distilled water (Ref. 31). Figure 7 shows the

same data of Figure 6 normalized by use of the stress intensity ratio

KISCC/K1C-

Stress corrosion tests using precracked specimens have assumed in-

creasing importance in the aerospace industry. These tests are useful to

the designer, the engineer, and the metallurgist. A knowledge of KTr, and

KTC,r„ is important for structural design.

Distilled water and moisture in the atmosphere are seldom considered

to constitute an aggressive environment, yet moisture can have a controlling

influence on the fracture behavior of high-strength steels. The growth of a

crack from an experimentally induced flaw (e.g., EDM-Electro-discharge-

machined slot) was studied by Johnson and Willner (Ref. 32), who found rela-

tionships between the crack growth rate and the crack tip under varying

-28'

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120

[^100

" 80

4340 STEEL HEAT TREATED TO 225 ksi UTS

PRECRACKED SPECIMEN EXPOSED TO 3 1/2% NaCl SOLUTION

UNTILEVER BEAM LOADING

T FAILURE CAUSED BY SUBCRITICAL CRACK EXTENSION BY STRESS ENVIRONMENTAL' CRACKING THRESHOLD

Li NO FAILURE

I02 t I03 I04

TIME TO FAILURE, mm

Figure 5. Typical Behavior of a High-Strength Steel in a Corrosive Environment (Ref. 3)

I0;

■29-

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4340 STEEL HEAT TREATED TO INDICATED LEVELS

PRECRACKED SPECIMEN EXPOSURE DISTILLED WATER

CANTILEVER BEAM LOADING

'0

■265ksi UTS

232 ksi UTS

10 10' 10- 10'

TIME TO FAILURE, mm

Figure 6. Delayed Fracture Characteristics of 4340 Steel at Var;jus Strength Levels Resulting from Exposure to

Distilled Water (Ref, 31)

-30-

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10 I02 I03

TIME TO FAILURE, mm

10'

Figure 7. Data of Figure. 6 Normalized by Use of the Stress Intensity Ratio (Ref. 31)

-31

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conditions oi'liquid water, water vapor, and temperature. The work was

conducted with an 11-11 steel heat-treated to a strength of 230 ksi. Water

vapor had a marked effect on crack growth at constant load; crack growth

changed as humidity was varied. The crack propagation in water or water

vapor is consistent with crack propagation mechanisms of either SCC or HSC .

It is highly probable that either mechanism can occur in high-strength steels

and that the chemistry of the aqueous environment determines which mech-

anism is operative. In this instance, however, it is very likely that corro-

sion occurs on this steel and hydrogen enters the metal (Ref. 33). Hanna,

Troiano, and Steigerwald (Ref. 7) have also presented evidence that HSC

occurs with distilled water; the hydrogen is supplied by the cathodic reaction

of a corrosion cell. In addition, the crack growth rate activation energy of

9000 cal/mole agrees fairly well with the known value for the diffusion of

hydrogen in iron (Q - 7800) (Rcf. 34), which also supports the findings of

Johnson and Wilner.

Since both SCC and HSC can occur in aqueous environments, it is sur-

prising that the operative mechanism appears to depend on the nature of the

steel. Benjamin and Steigerwald (Ref. 35), using fracture toughness analysis,

exposed two high-strength steels to distilled water and 1 . 5I\' and 3. ON NaCi

aqueous solutions. Distilled water was found to be a more agressive environ-

ment than the salt solutions for 4340, and the reverse was true for HP-9-4.

Using supplementary polarization experiments, they found that delayed failure

in 4340 was the result of HSC. The behavior of the HP-9-4 steel, however,

was consistent witli the SCC mechanism, with anodic dissolution occurring

along active paths.

The type of data obtained by fracture mechanics analysis is illustrated

by the work of Freedman (Ref. 36), who obtained K.,, and K „ data on

ferrous and nickel alloys. Single-edge notched and fatigue-cracked specimens

were tension-loaded in a salt solution for 1000 hr (accelerated test). Identical

specimens were tension-loaded in racks exposed at the seacoast (Playa del Rey,

California). Times to failure at seacoast varied from 49 to 7668 hr. Some

specimens were tested for 12, 843 hr without failure.

•32

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Freedman's data (Table 7) ^ive a rating of the susceptibility of various

alloys to accelerated (laboratory) and sraroast testing; the ratio KTC,rr/K r

for seawater ranks the alloys from Inconel 71K (most resistant) to sensitized

Type 304 (most susceptible). There are unexplained differences in the ratio

for the accelerated test, which can probably be attributed to the variability

of the aggressive environments.

Additional data on the stress-corrosion properties of the precracked

high-strength precipitation-hardening stainless steels were obtained by

Carter (Ref. 25). These data (see Table 8) showed that the precipitation-

hardening stainless steels (e.g., 17-7 FH and IJH 15-7 Mo) were particularly

sensitive to SCC. Most martensitic steels were very resistant, with

^ISrr^IC Vt""v r'OSl' io unity. Steels with low K' „„ values had low K ,r •

All failures were intergranular.

Overaging was found to increase fracture toughness and stress-corrosion

resistance. As might be expected, comparison of data for precracked speci-

mens and smooth specimens revealed discrepancies.

Comparison of Freedman's data with iho.se of Carter and ro-worker."-

is not possible because of the different alloys tested. In only two cases arc

the data comparable: AM355 SC J" 1000 and 17-4 lJil H900. There exist wide

and unexplained differences in the K.i.rr/K ratios.

The effect of high-pressure (5000 psi) hydrogen on Inconel 7 18, Inconel

625, A286, AISI 347, Ti-6A1-4V and Ti-5Al-2.5Sn has been studied by fracture

mechanics analysis (Refs. 37 and 38;. Of the group, Inconel 718 and A286

were niost resistant, while Inconel 62 5 was most embrittled.

-33.

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Table 7. Fracture Toughness Rating of Alloys and Heat Treatments lor Resistance to Saltwater Environments (Northrop Data, Ref. 36)

Seaco^s 1 OBt Accel erated Test

Material Heat Treatment UTS. ksi 'Sscc ^ SCC/K]C K1SCC KISCC/K1C

Inconel 1950,F AC, 8 hr 189 t 106 0. 87 130 0. 98 718a 1350' + FC to

1200'F for 24 hr

17-4 PHa H 1150 151. 6 93. 9 0. 77 I 10 0. 89

AISI 304 Annealed 84 0 53. 5 0. 77 59. 7 0. 86

4340 800^ .'empered 204 8 48. 3 0. 72 29. 7 0. 44

17-4 PH H 9C0 202 4 38. 5 0.69 40. 3 0. 72

H-l 1 (AM)b 1 lOO'F Tempered 232 6 39. 5 0.62 23 2 0. 24

410a 1 125*F Tempered 128 a 52. 4 0. 55 49, 6 0. 52

H-l 1 /AM)b 1000'F Tempered 300. 3 16. 7 0. 52 H. 6 0. 27

18 Ni USO) 900 "F 269 5 55. 6 Ü.50 72. 9 0.65

H-ll (VM)b 1000'F Tempered 114 0.40 10. 8 0. 38

4340 475^ Tempered 267 2 1 3 3 0 29 11.1 0. 24

AM 35S (FH)b SCT 1000 33. 1 0. 28 50. 3 0.42

AM 355 SCT 1000 169 4 24 5 0. 24 36. 7 0.43

410 650'F 7 empered 197 Ü 22. 0 0. 24 25. 8 0. 26

AM 355 SCT 850 195 9 10. 7 0.22 24. 9 0. 52

AM 355 (FH)b SCT 850 9.7 0. 15 6 2 0. 10

AISI 304 Sensitized 100 hr 1100'F

83 9 8. 5 0. 12 15. 2 0. 22

Plane strain conditions maintained only at low stress intensities. Therefore, values are approximate. True plane strain K.-^,- could not be obtained.

AM = air melt, VM = vacuum melt, FH = fully hardened.

34-

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Table 8. Fracture Toughness Rating of Alloys and Heat Treatments for Resistance to Saltwater Environments (Boeing Data, Ref. 2S)

Material Heat Treatment UTS, ksi Kiscc/Kr

AM 35.i SCT 1000 178.0 1.00

AM 351- SCT 1000 Modified 173.4 1.00

AM 364 H 950 191.5 1.00

17-4 PH li "no 194.6 1.00

17-4 PH H 1000 162.2 1.00

15-5 PH {AM)a H 1000 161.6 1.00

15.5 PH (VM)a H 1000 162.9 1.00

PH 13-8 Mo H 950 225. 1 1.00

Custom 455 H 950 247.0 1.00

15-5 PH (AM)a H 900 195,7 0.83

AM 362 11 1000 178.'» 0.77

15.5 I'll (VM)a 11 9ÜÜ rn.5 0.7 5

AM 364 11 850 188.7 0.71

17.7 PH RH 950 186.5 0.59

PH 15-7 Mo TH 1050 178.2 0.55

AM 355 SCT 850 213.5 0.55

PH 15-7 Mo RH 950 219.4 0.44

AM 362 H 900 200.5 0.41

17.7 PH TH 1050 197.2 0.41

AM - air melt, VM = vacuum melt.

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UI'HIIIIK öl SUHI .11 tii-d - Load Kxpc r I mmt S with Notclicrl Specimens of Various Uli rn-Hijjh-St rcngtli Steels Exposed to a 5% Salt-

'o(4 Atmosphere (Kef, SI)

UTS Appln-.l Numlie r of fa i hi res

it.-el Rant;c St rcsfi, 1 yp«* ..1 Coat ipy in 1000 hr/N. imiie r ksi kBia of Spec imens T< sted

4 (4 0h ^f,0-2H0 16 1 Not plated 1/1 A i-lO1' 26O-2H0 16 1 l.i.w H Cd pUtcd 1/3

1 MO"-' 2 60-?. K0 161 Not plated 1/3

■1 140 Zt,0.2H0 163 l.i.v.- II Cd platrd i/i 4 140 1 HO-200 120 N'.l plated 0/1 4 14 0 1HO-200 120 \'.ii iiutn- deposit cd Cd 0/1

4 UOM J20-240 142 Not plated 1/3

4 1 !0M 220-240 142 Low II Cd plated 2/1

11-11 2HO. iOO 17', Not plated 1/i II. 1 1 2K0- uin 17'- I.nw II C.tl plali'd 2/1

II 1 1 .'HO- 100 1 ;'• N 1 Id pl.ll e.| 2/1 II II .'i.O .'HO 11. Not plated 1/3

II 1 1 2 Ml- .'HO 16 i I.ow !l 1 d plated 6/1

II 1 1 .'(.0 .'Ml) 16 < N i ( d [ilat ed 0/3

l)i..i> 2H0- 100 171 Not plated 1/1 l)6ai 2H0- 100 1 71 \' at until - depo.s it ed id 3/3

Pba. 2(.0-2KO H.1 Not plated i/3

Dfcac 2(>0-2H0 It. 1 V aru im - deoos itcd Cd 2/3

D6ar 220-240 142 Not plat ed 3/3

n6ac 2 2 0-240 142 Vacuum-deposited Cd 3/1 D hat- 220-240 142 Ni-Cd plated 0/3

HP-r»-4- 4^ 260-2H0 M'1 163 Not plated 3/3

IIP-rf-4- 4^ 260-2H0 M'l 163 Vacuum-deposited Cd 1/3

IIP-9-4 4S 260-280 nr 163 Not plated 3/3

HP-'t-4. 4S 260-2K0 n1' 163 Vacuum-deposited Cd 3/3

C-45S 220-240 142 Not plated 0/3

C-4SS 220-240 142 Vacuum-deposited Cd 3/3

r-4ss 220-240 142 Ki-Cd plated 0/3

AFC-77 240-260 142 Not plated 2/3 A FC-77 240-260 142 Vamum-deposit ed Cd 3/3

IHNi (2H0) 2H0- )00 171 Not plated 3/3

ma ra^in B 2K0- 100 171 Vacuum-deposited Cd 3/3

2H0. 100 171 Ni-Cd plated 1/3 IHN,(2S0) 2SO-270 163 Not plated 0/3

ma ranin B 2S0-270 210-270

16 1

It. 1 Vacuum-deposited Cd Ni-Cd plated

0/3 0/3

d

The applied stress was equivalent to about 71"', of the design yield strengths of the steels, except for the marajjinK steels.

Air melted.

"Vacuum melted.

Martensitic heat treatment.

Rainitic heat treatment.

•36-

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VI. KKrOMMKN'DATIÜNS

Two major problems related u^ SCC and HSC confront the aerospace

industry: (1) prevention of SCC and HSC and (2) test standardization, data

accumulation, and dissemination of information.

A. PREVENTION OF SCC AND HSC

There are some obvious practical approaches lo the problem of failed

fasteners by SCC and HSC. The first is the use of alternative materials.

There are materialt with good resistance to SCC. On the basis of the frac-

ture toughness criteria, KTC.r„, and the ratio K ...p,,/KT(-, tempered with

experience, a selection could be made. Unfortunately, fracture toughness

criteria are only now being collected and not all fastener materials have been

tested. However, there are some highly co rrosion-resistant materials of

the stainless steel type available, e.g. , A2Hb, A2Hb C\V, Inconel 718, and

Inconei 7 18 CW.

The second obvious solution is to keep the agressive environment or

the hydrogen source away from the steel. Although aerospace engineers shy

away from coatings, platings of cadmium or aluminum may be helpful on

low-alloy steel martensites. Extreme care in electroplating and required

subsequent baking must be exercised at high strength levels to prevent HSC.

In some aerospace applications, organic coatings, paints, Lock-tite, Lock-

safe, greases, or even baked-on solid lubricants can protect against the

environments. Torquing and retorquing on the bolts can remove these' types

of coatings, and reappl icat ion ol the protection is then required.

The resistance to environmental cracking ( S"'. salt log) ol ten high-

strength steels, with and without cadmium or cadmium-nickel coatings,

was studied by Lauchner (Ref. 39); the results are given in Table u. Some

steels, AISl 4340 (180-200 ksi UTS), Custom-455 (220-240 ksi UTS), and

18 Ni (250 ksi UTS), had good resistance to SCC in this environment without

-37.

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i'oatings. Note the superiority of the cadmium-nickel coating to the other

types of cadmium coatings (low hydrogen plated and vacuum deposited).

A less obvious solution to the problem is the use of designs that elimi-

nate or minimize factors promoting SCC and HSC. One should, for instance,

avoid crevices, deep recesses, sharp corners, notches of ai.y kind, and

dissimilar metals unless one metal is insulated from the other.

In any design in which new alloys are to be tried and new environments

experienced, it is highly recommended that fracture toughness tests be con-

ducted to ascertain the possibility of some susceptibility to degradation, if

not complete failure.

L3 DATA COLLECTION AND TEST STANDARDIZATION

There is a need for standardization of fracture toughness tests using

the precrackod specimens. The ASTM already has a tentative specification

on testing (Ref. 3). Without standardization, the available dati have limited

usefulness in material selection or design. A standard test must be in exis-

tence long enough that sufficient data can be accumulated for statistical study.

The accumulated data should be compiled into usable form by industry,

the metallurgical profession, a government agency, or a technical society.

-38-

...^■.^.^.^^-.^^im^ - am i — —-;.-—

'Jf ^i»sT»fs. V* -^J *«J!^

vii. f:()K(:LrsK;::.s

The rurrfiit I ci luiolciu y ul lii uh - st ri'ni'lh Uiblcm-r.s in .(

ajjplications has lircn appraisefl.

While (lu'rc is ,i (hstinct rlil h-rciu i

on the laboratory scale, it is diftn ul

bet v-'ei-n the I \<,

er spat

I), tv/eeii SCC anrl 11S( ■ i>li

It , l f" not tllUOS.Ml,] '•. 1" 'iilNrnii at e " "lee In 111 Mils i n I )ic ! i ( 1,,'. ( III CDS [l HI

■^ • l!'.'| n.IHlien

■i - I1 ' II i I I , 1

• • ( n i n ; i ;,

eiil Iy aflvain ed tu p ri ii i( i tlarit'er.■■!; • I i

nat ions I hal c >.IIM Ie.id If i,111 u r.■.

file Ii i::!i - si '•. IM I Ii la sli-in i--. lh.il

f I I V I (lef 1 i ill ii t ll r i e I Vpi s ul i ,! I .■'."

i ,i I type, .i ml ( . 1 l)\ r i I

, I I I I M 1 Spll ( fl - s .

Kxa tuples ■ 11 I,I 11 ii r es un the 1 .tan III i ■ . i i ■,• •' '. >

na I ii re i I the p ml) lei i .

I'rait ii ri' tuuiihness analysis filers 'he lu ■ I I; 'pi

SCC or IISC. Tile analysis dm s mt " lien nt^.te ',.■!. ■

use of plan«' - st ra in I rai t ure tnut'lmess 1,. . .i IT' ■ ' I ■ .•- I

' h

K . e , ( r i I e r i a M 11 e r s [ i r o m i s < ■ 11 s e 1 e i i ,; ; i ;, •:, i ■ ■

SMS« ept lb le to «SCC .ir I ISC |,| i hires. I i . I , , . I ■ . ., '1.(1

hei om inu ' i M i r-1 ■ i in a lli n i.' I ii 1 to I hi' '1

1 '■ i ^ not .-,. j f f

' * r i ■ , 11,! i

i : ,ii ■ i i' ■ ,; n ■ ■ i

I r-'e Ml..-, , ,,| ,1,1:1.,, ,,, ,

•M, I, . I| ■'■ • i M tile

■ ' ! ' !'■ i ho , ,,l , •1 fea , bei oi: i I h,- ,. ii i ■.i s 11 r, ■- I lily lurina n<

!l I'L' 'i.it.i ,,n

:.-::■ s f J,

liil-l ' MJli

■;. ' . •■,• ,,.,1

■■'■. oi !,,.;, ,r

i si ion |,n

«^■-^-^-■■ -*. ■-■- —- ..-..--^■— ■■ --■*- -■ -■■ —"TirtnMiiH^tniii.i , ■■■- -— --.-■-. ..■--..~J.

ssssr«!^^;""'-^"? r^^i^'r-Tic-•>!> T^<^ii¥|nqrFl^«JiH.l'!W..'J'^^'^^

REFERENCICS

1. A.C. Hood, "Preventing Stress-Corrosion Cracking in Threacled Fasteners, " Metal Prog. Q_l_, HS-«8(1067).

2. J. K. Stanley, "Solutions to Some Stress Corrosion Cracking Problems in Aerospace Situations," Proc. Joint Aerospace Marine Corrosion Tech. Seminar, First, NACF, Houston, Texas ( j %9).

3. B. F. Brown, Stress Corrosion Cracking: A Perspective Review ul the Problem, NRL7I30, U.S. Naval Research Lab., Washington, DTT. (16 June 1070).

5.

F.K. Fletcher, W.F. Berry, and A. R. Klse.i, Si ress - To r ros iun Cracking and Hyd rogen-Sl ress Parking of High SI rcnglh Steel, I) MIC ZiZ, Battelle Memorial Insl. , Defi-nse Mi.'tal Iniurmation Center, Columbus, Ohio {29 July 1''()()).

H. L. Logan, The Stress Corrosion of Metals, John Wiley and St-ns, Inc. , New York ( 1066).

6. A. R. Elsea and E. E. Fletcher, Hydrogen Induced Delayed Brittle Failures of High Strength Steels, DMIC [9b, Battelle Memorial Inst. , Defense Metals Information Center, Columbus, Ohio (20 January 1(:'64).

7. G. L. Hanna, A.R. Troiano, and E.A. Steigerwald, "A Mechanism for Embrittlement of High Strength Steels by Aqueous Er.vi ronments," Trans. ASM 57, 658-671(1064).

H. K. Ma/.anec, and R. Seinoha, "Delayer] Fracture in Ma rt ens ite, " Trans. AIMEZV-), 1 602-1 (.OK ( 1 965).

10.

B. F. Brown, Si ress-Co r ros inn Cracking and Related Phenomena in High Strength Steels, NRL 6041, U. S Naval Research Lab. , Washington, D. C. ( 1063).

H. J. Bhatt, and E.H. Phelps, "Effect of Solution pH on the Mechanism of Stress Corrosion Cracking of a Martensitic Stainless Steel." Corro- sion 17, 430-434 (1061).

11. N. A Nielsen, "Observat ions and Thoughts un Stress Corrosion Mechanisms," Corrosion 27, 173-HO (1971).

Preceding page blank -41

a.t^a^^.^yiaabMaM^a^^ . ..-,...■... „„,BJ-^^^.....a- ^^-^■'- ^

lilKilPIPIIP SflflJFBj L-Wl^SiT^TTliT

12. I. Matsushiina, A. Dee^an, and H.II- Uhlig, "Stress Corrosion and Ilydroiicn Cracking of 17-7 Stainless Steel," Corrosion 22, 23-27 ( l'r'66).

13. D. L. Dull and L. Raymond, "A Method of Evaluating Relative Sus- ceptibility of Bolting Materials tu Stress Corrosion Cracking," Paper presented Westcc Conference (March 1^72).

14. A. Phillips, V. Kerlin.s, and R.V. Whiteson, Electron F ractographic Handbook, ML-TDR-64-4 1 6, Air Force Materials Lab. , Wright- Patterson Air Force Base, Ohio ( 1964).

15. J. P. Fidelle, J. Legan, and C. Couderc, "A Fractographic Study of Hydrogen Gas Embrittlement in Steels," Trans. AIME (1972)(in press).

16. N. A. Tiner and C. B. Gilpin, "Microprocesses in Stress Corrosion of Martensitic Steels," Corrosion 22, 271-279 (1966).

17. H. Suss, "Stress Corrosion - Causes and Cures," Mater. Design Eng. 6j_, 102-148 (1965).

18. J. Bourrat and J. Hochmann, New Austenitic Stainless Steels Resistant to Stress Corrosion in Chloride Media, Aciers Speciaux, Monographies Techniques, No. 9(1964).

19. F. R. Bloom, "Stress Corrosion Cracking of Hardenable Stainless Steels " Corrosion 11, 351-361(1955).

20. .I.E. Truman, R. Perry, and G. N. Chapman, "Stress Corrosion of Martensitic Stainless Steels," J. Iron Steel Inst. (London) 2 02, 745-756 (1964).

21. E.A Drey er. Investigation of Susceptibility of High Strength Marten- sitic Steels to Stress Corrosion, ASD-TDR-62-876, Aeronautical Sys- tems Div. , Wright-Patterson Air Force Base, Ohio (19b2).

22. R.T. Ault, R. B. Holtman, and J. R. Meyers, Heat Treatment of a Martensitic Stainless Steel for Optimum Combination of Strength, Toughness and Stress Corrosion Resistance, AFML-TR-68-7, Air Force Materials Lab. , Wright-Patterson Air Force Base, Ohio (1968).

23. P. Lillys and A. Nehrenberg, "Effect of Tempering Temperature on Stress Corrosion Cracking and Hydrogen Embrittlement and Marten- sitic Stainless Steels." Trans. ASM 48, 327(1956).

-42-

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PIPBWWWP!»!! wmm^wtmimm

E. H. Phelps and A. W. Loginow, "Slrcss-Corrosion of Steels for Aircraft and Missiles," Corrosion 16, 325-3351(1969).

25. C. S. Carter, D. G. Farwick, A.M. Ross, andJ.M. Uchida, "Stress Corrosion Properties of High Strength Precipitation Hardening Stain- less Steels," Corrosion Z7, 190-197(1971).

26. Design Guidelines for Controlling Stress Corrosion Cracking, NASA 10M33107, Marshall Space Flight Center, Huntsville, Alabama (1970).

27. W. R. Lucas, Chairman, Report of the NASA Ad Hoc Committee on Failure of High Strength Materials, Marshall Space Flight Center, Huntsville, Alabama ( 1971).

28. B. F. Brown, "A New Stress Corrosion Cracking Test for High Strength Alloys, " Mater. Res. Std. 1_6, 129-133(1966).

29. B. F. Brown, Stress Corrosion Cracking and Corrosion Fatigue of High Strength Steels, DMIC 210, Battelle Memorial Inst. , Defense Metals Information Center, Columbus, Ohio (1964).

30. B. F. Brown, "The Application of Fracture Mechanics to Stress- Corrosion Cracking," Mefjyjl_urj^£aJ_J^ 13, 17 1-18 3(1968).

31. J.H. Mulherin, "Stress Corrosion Susceptibility of High Strength Steel in Relation to Fracture Toughness," Trans. A5ME, J. Basic Eng. 88, 772-782(1966). —-._-.-——_— - —.

32. H. H. Johnson and A. M. Willner, "Moisture and Stable Crack Growth in a High Strength Steel," Appl. Mater. Res. 4, 34-40(1965).

33. N. J. Norton, "Diffusion of D2 from D2O Through Steel," J. Appl. Phys. 24, 499 (1953).

34. E. W. Johnson and M. H. Hill, "The Diffufivity of Hydrogen in Alpha Iron," Trans. AIME 218, 1104-1112(1960).

35. W. D. Benjamin and E. A. Steigerwald, Stress Corrosion Cracking Mechanisms in Martensitic High Strength Steels, AFML-TR-67-98, Air Force Materials Lab. , Wright-Patterson Air Force Base, Ohio (1967).

36. A. H. Freedman, Development of an Accelerated Stress-Corrosion Test for Ferrous and Nickel Alloys, NOR 68-58, Northrop Corp. , Norair Div. , Hawthorne, California (1968).

-43-

f,:Jt»JMaL^i,^,.^»rf.».,-:^,.«J,,a„-.^^ mj|gjagBaaaBjiMg|gM^||gjgggjg|jg||gg|g|i|y

SSa«?S«!BP5SäiHB?Hp JIJIIWJJWiMlWMIgiHBfW^^

37.

38.

39.

J. A. Harris Jr. anrl M. C. Van Wanderhan, Properties of Materials in High Pressure Hydrogen at Cryogenic, Room and Elevated Temper- atures, PWA FR-4566, NAS 8-26191. Pratt and Whitney Div Aircraft Corp. , West Palm Beach, Florida (1971).

United

R. J. Walter, H. G. Hayes, and W. T. Chandler, Influence of Gaseous Hydrogen on Metals, R-8719, NAS 8-25579, Rocketdyne Div. , North American Rockwell Corp. , Canoga Park, California ( 1971).

E. A. Lauchncr, The Stress Corrosion Resistance of High Strength Steels for Bolting Applications, NOR 69-91, Northrop Corp., Norair Div., Hawthorne, California (1969).

n

-44-

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