The stress corrosion resistance and fatigue crack growth rate of a high strength martensitic stainless steel, AFC 77

The Stress Corrosion Resistance and Fatigue Crack Growth Rate of a High Strength Martensitic Stainless Steel, AFC 77

DONALD WEBSTER

The p l a n e - s t r a i n s t r e s s co r ros ion threshold Klsec and fa t igue crack growth r a t e have been de t e rmined for a high s t reng th m a r t e n s i t i c s t a i n l e s s s teel , AFC 77, in both convent ional ly p r o c e s s e d and s t r a i n - a g e d condi t ions. The Kiscc (in 3.5 pct sodium chlor ide solut ion) is marked ly affected by both the t emper ing t e m p e r a t u r e and the degree of s t r a in aging. The highest Kisee of 105 ks i ~ was obtained by t e mpe r i ng at 500~ and the lowest Kisce of 10 ksi ~ by t e m p e r i n g at l l 0 0 ~ Retained aus teni te r a i s e d Kisec at t emper ing t e m p e r a - t u r e s up to 1000~ which was the highest t e mpe r i ng t e m p e r a t u r e at which aus ten i te could be main ta ined . Fatigue c rack growth ra tes in both d ry a i r (< 10 pct r e l a t i ve humidity) and 3.5 pct sodium chloride so lu t ion were at a ma x i mum for m a t e r i a l t empered at 700~ Over the r ange of s t r e s s in tens i ty s tudied, r e ta ined aus teni te reduced fatigue c rack growth ra te in sal t so l u - t ion but i nc rea sed it in d ry a i r .

A p rev ious paper I desc r ibed th ree techniques for i m - proving the f r ac tu re toughness of a high s t rength m a r - t ens i t i c s t a in l e s s s teel , AFC 77. These techniques were : cont ro l of r e t a ined aus ten i te content, s t r a i n ag - ing, and aus forming . Samples produced by the f i r s t two of these techniques have been used to invest igate the s ta t ic and fatigue loaded c rack growth ra tes in e n v i r o n m e n t s of dry a i r (< 10 pct humidity) and 3.5 pct sodium chlor ide solut ion.

EXPERIMENTAL TECHNIQUE

Mate r i a l

The m a t e r i a l used in this inves t iga t ion was rece ived f rom the manufac tu re r in the fo rm of 3- in . diam round ba r . The m a t e r i a l was a i r mel ted and of the chemica l composi t ion (wtpct) shown in Table I.

Heat T r e a t m e n t

Aus ten i t iz ing t r e a t m e n t s were of 1 hr dura t ion at 2000~ and were c a r r i e d out in vacuum or argon. Spe- c imens were oil quenched af ter aus ten i t i z ing and then cooled to - 100~ for 30 min to reduce the re t a ined aus ten i te content . Specimens were t empe red at t e m - p e r a t u r e s between 500 ~ and l l 0 0 ~ for 2 -p lus -2 hr (2 hr at t e m p e r a t u r e followed by an a i r cool and a fu r the r 2 hr at t empe ra tu r e followed by a second a i r cooling). The m a t e r i a l had a coa r se aus teni te g ra in s ize of about 60 ~ (ASTM ~5) . To produce the s t r a i n - aged condit ion, ma te r i a l that had been aus ten i t ized at 2000~ and t empe red at 500~ was cold ro l led to p r o - duce 10 or 20 pct reduct ion in th ickness and then t e m - pe red at e i ther 700 ~ or 1000~ The se lec t ion of 2000~ as the aus ten i t i z ing t e m p e r a t u r e e n s u r e s that r e t a ined aus ten i te is p r e sen t in the s t r a i n - a g e d condition, and this is e s s en t i a l to ensu re the op t imum mechan ica l p r o p e r t i e s . 1

DONALD WEBSTER is Reasearch Engineer, The Boeing Co., Com- mercial Airplane Group, Seattle, Wash.

Manuscript submitted March 2, 1970.

Mechanica l Test ing

The techniques used to de t e rmine f r a c t u r e toughness and t ens i l e p r o p e r t i e s were fully desc r ibed in a recen t publ ica t ion I and will not be repeated he re .

S t ress c o r r o s i on t es t ing under p l a n e - s t r a i n condi- t ions was c a r r i e d out on fatigue p r e c r a c k e d Charpy and p rec r acked s ing le -edge notch bend spe c imens (7.5 by 1.5 by 0.48 in.) t es ted in can t i l ever bending. 2 It should be noted that for the t empered 500~ condit ion the spec imen th ickness fal ls below the r e q u i r e m e n t s of the ASTM Standards for f r ac tu re toughness tes t ing, a l - though no s t anda rds have yet been se t for s t r e s s co r - ros ion tes t ing. Tes t ing was c a r r i e d out in a 3.5 pct sa l t solut ion (sodium chloride) with f resh solut ion con- s tant ly dr ipping into the upturned notch. The crack was i m m e r s e d in the sa l t solut ion before the load was ap- pl ied. Specimens were held 1 week at each s t r e s s in - t ens i ty level before unloading. If c rack growth was less than 0.1 in., the s p e c i m e n was again fatigue cracked and r e t e s t ed at a d i f fe rent s t r e s s in tens i ty level . The extent of s t r e s s c o r r o s i o n growth at each s t r e s s in tens i ty level could eas i ly be s een between the two fatigue c racks . The p l a n e - s t r a i n s t r e s s co r ros ion th resho ld KIsec is the highest s t r e s s in tens i ty level that could be m a i n - ta ined without c r ack growth.

The growth ra te of the s t r e s s c o r r o s i on c rack was calcula ted by dividing the c rack length by the total t ime at that p a r t i c u l a r s t r e s s in tens i ty level . Since the in - cubation per iod is ignored, the ra te of c rack growth is plotted as an apparen t c rack growth ra te . This plot t ing method does not change the KIsec value, because the c rack growth ra te is ze ro at the threshold .

After a spec imen had broken in s t r e s s cor ros ion , a value KI5 was ca lcula ted using the total c rack length (including the length of c rack that had grown in s t r e s s

Table I. Chemical Composition, Weight Percent

Heat C Si Mn S P Ni Cr Mo V Co N No.

64555 0.16 0.13 0.18 0.021 0.015 0.21 14.0 5.02 0.23 13.41 0.04

METALLURGICAL TRANSACTIONS VOLUME 1, OCTOBER 1970-2919

500 ~ F TEMPER 7000 F TEMPER >- >-

I -

- - .

�9 KI. ~ ~o -J 1 0 0 1 - K I - ~ i -- 100 o3 u.i ~ e ~ = > i ' - KZ~T (z:: LU I l ~ I I

~ 00L-~ ~ 10 100 ~ OOL~ ~ 10 100

APPARENT CRACK GROWTH APPARENT CRACK GROWTH RATE (IN. X 10 .3 PER HR) RATE (IN. X 10 .3 PER HR)

~) (b)

900 ~ F TEMPER 1100 o F TEMPER

> 150 F > ~F

,oo t IZ Kic AND KI~

_ 40

k- - E ~ K~: ~ ~1- �9 "

~ so - - �9 , oL~ ~ K~cc ~,,>, . . . . . . . ~: ,,>, _ . _ . ~ r K~c ~

0L-~. , I 0 , , 0 1 10 100 10 100

APPARENT CRACK GROWTH APPARENT CRACK GROWTH RATE (IN. X 10 .3 PER HR) RATE (IN. X 10 -3 PER HR)

(d) (e)

>..

800 ~ F TEMPER

150

KI6 100

KIc

50 I - Ki.~c

0 I I i - - 1 10 100

APPARENT CRACK GROWTH RATE (IN. X 10 -3 PER HR)

(c)

F i g . 1 - - A p p a r e n t c r a c k g r o w t h r a t e s i n 3 . 5 p o t s o d i u m c h l o r i d e s o l u t i o n u n d e r static loading for AFC 77, austenitized 2000~ for 1 hr, and tempered for 2 + 2 hr at: (a) 500~ (b) 700~ (c) 800~ (d) 900~ (e) 1100~

! look , --o 5~ - - "

=ol 500 700 900 1109

T E M P E R I N G T E M P E R A T U R E ( ~

Fig. 2--Variation in Kiscc , KI6 , and Kic as a function of tempering temperature.

c o r r o s i o n ) and the l a s t s u s t a i n e d load l eve l . The r a t i o of KI6 to Kic ind ica tes the amount of blunt ing at the c r a c k t ip . Blunting is c o n s i d e r e d he re to be any m e c h - a n i s m tha t r educes the e f fec t ive s h a r p n e s s of the c r a c k .

Fa t igue c r a c k growth r a t e s we re m e a s u r e d on 9 -by 3 in. p a n e l s , 0.080 and 0.030 in. thick, in both d r y a i r (< 10 pc t r e l a t i v e humidi ty) and 3.5 pc t s a l t so lu t ion . The m a x i m u m s t r e s s was be tween 18 and 25 ks i and the r a t i o of m in imum to m a x i m u m s t r e s s (R) was kep t a t 0.05. In i t ia l c r a c k length was 0.32 in. , and the c r a c k was a l lowed to g* ow unti l it r e a c h e d a length of 1 in.

RESULTS AND DISCUSSION

S t r e s s C o r r o s i o n Res i s t ance

The appa ren t c r a c k growth r a t e s in 3.5 pc t s a l t s o l u - t ion for s t a t i c a l l y loaded, a u s t e n i t e - c o n t a i n i n g m a t e r i a l a r e shown in Fig . 1. The s t r e s s in tens i ty l eve l a t which

2920-VOLUME 1, OCTOBER 1970

the c r a c k g rowth r a t e is z e r o (Kiscc) v a r i e s m a r k e d l y with t e m p e r i n g t e m p e r a t u r e .

T e m p e r i n g at 500~ Fig. l (a) , p r o d u c e s the h ighes t s t r e s s c o r r o s i o n t h r e s h o l d (105 ks i ~ ), al though this is c o n s i d e r a b l y be low the Kic va lue . Ra i s ing the t e m - p e r i n g t e m p e r a t u r e to 700~ Fig. l (b) , c a u s e s the value to d rop to 50 k s i ~ .

I n c r e a s i n g the t e m p e r i n g t e m p e r a t u r e fu r the r to 800~ Fig. l (c ) , and 900~ Fig. l (d) , ha s l i t t l e effect on the s t r e s s c o r r o s i o n th re sho ld , a l though t e m p e r i n g at l l 0 0 ~ Fig . l ( e ) , p r o d u c e s a m a r k e d d rop in the Kiscc va lue to about 10 ks i ~ . The v a r i a t i o n in toughness and s t r e s s c o r r o s i o n t h r e s h o l d as a function of t e m p e r i n g t e m p e r a t u r e is s u m m a r i z e d in Fig. 2, where i t can be s e e n that the l a r g e d rop in Kic p r o - duced by t e m p e r i n g at t e m p e r a t u r e s above 700~ is not r e f l e c t e d in e i t h e r the KIscc o r KI5 �9

The f r a c t u r e mode in s p e c i m e n s f a i l ed in s t r e s s c o r r o s i o n is m a r k e d l y dependent on the t e m p e r i n g t e m p e r a t u r e . The d imp le d rup tu r e which is o b s e r v e d in s p e c i m e n s t e m p e r e d at 500~ F ig . 3(a), changes p r o g r e s s i v e l y to include i n c r e a s i n g amounts of q u a s i - c l eavage and g r a i n bounda ry f a i l u r e a s the t e m p e r i n g t e m p e r a t u r e is i n c r e a s e d , F igs , 3(b), 3(c), 3(d), and 3(e).

The a p p a r e n t c r a c k growth r a t e s fo r s t a t i c a l l y loaded s t r a i n - a g e d m a t e r i a l a r e shown in Fig . 4. M a t e r i a l t e m p e r e d at 700~ shows a Kiscc va lue c lo se to Kic a f t e r 10 pc t cold r educ t ion ; but a f te r 20 pc t co ld r educ t ion , the t h r e s h o l d d r o p s to the s a m e l eve l a s unde fo rme d m a t e r i a l t e m p e r e d a t the s a m e t e m p e r a t u r e . Kiscc va lues c o n s i d e r a b l y be low Kic a r e shown by s p e c i m e n s cold r e d u c e d 10 pc t and t e m - p e r e d at 1000~ al though a l l the s t r a i n - a g e d s p e c i - mens have a h ighe r s t r e s s c o r r o s i o n t h r e s h o l d than o ther s t e e l s in the s a m e s t r eng th r a n g e ( f t u ~ 300 ks i ) such as 4340M and the m a r a g i n g s t e e l s .

The s t r e s s c o r r o s i o n t h r e s h o l d as a function of s t r eng th is shown in Fig. 5 for a u s t e n i t e - c o n t a i n i n g and s t r a i n - a g e d m a t e r i a l . The KI5 v a l u e s above Kic ,

M I ~ T A L L U R ( ; I C A L I R A N S A ( ' T I O N S

(a) (d)

{b)

Fig. 3 - - F r a c t o g r a p h s of AFC 77 aus ten i t ized at 2000~ Magnif icat ion 1600 t imes . (a) T e m p e r e d at 500~ showing complete ly dimpled f r ac tu re ; (b) t e m p e r e d at 700~ showing mix tu re of dimpled and quas ie leavage fa i lure ; (c) t e m p e r e d at 800~ showing an a r e a of quas ic leavage fa i lure ; (d) t e m - pe red at 800~ showing an a r ea of g ra in boundary fa i lure ; (e) t e m p e r e d at l l 00~ showing an a r e a of g ra in boundary fa i lure .

(e)

shown for some heat t r e a t m e n t s in Fig. 5, indicate that, for these condi t ions , the s t r e s s c o r r o s i o n crack is cons ide rab ly l e s s sharp than the fatigue c rack used in the Kic de t e r mi na t i ons . There a re at l eas t two ways in which the s t r e s s concen t ra t ion may be reduced at the tip of the growing s t r e s s c o r r o s i on c rack . C o r r o - s ion may occur at the tip of the crack so that the sharp tip is rounded off, or the c rack may b ranch and divide the load with one or more secondary c r acks . A clue to which m e c h a n i s m is opera t ing is obtained f rom Fig. 6 where the KI5 to K i c ra t io is compared for a va r i e ty of heat t r e a t m e n t condi t ions . Only two va lues a r e ob- ta ined for the r a t io : a value of unity co r r e spond ing to no crack blunt ing and a value of two that would c o r r e - spond to a s imple b ranched c rack having each b ranch the same length. It should be noted that no obvious b ranch ing occurs in the samples having a KI5/Kic ra t io of two. Branching is v i s ib le only on a m i c r o sca le , with only a few m i c r o n s sepa ra t ing the b r a n c h e s . In low a l - loy s tee l s where b ranch ing is v i s ib le on a m a c r o scale , the KI6/KIe ra t io v a r i e s f rom 1.2 to 2.2 (private com- munica t ion with C. S. Car te r ) . Values of l e s s than two can be expected when the b ranched a r m s a re of unequal length. In such a case , fa i lu re will occur along the long- es t a r m of the b r a n c h although some load wil l be taken up by the secondary a r m to give a KI6 h igher than KIe.


~-" 250 v . j 200 UJ > ILl . j 150 >-

100 z uJ I.- z 50

~ o W

ee 0 p-

10% SA, 700 o F

TEMPER

- Kig

K~cc

1 10 100 0

20% SA, 700eF

TEMPER

KZc - K ~ 6"

m m , ~ m ~

| KIscc

1

10% SA 1000 ~ F

TEMPER

KZ6

KZc /

I I ISCC , �9 ~ I I I

10 1000 1 10 100

APPARENT CRACK GROWTH RATE (IN. X 10 -3 PER HR)

Fig. 4--Apparent crack growth rates under static loading in 3.5 pot sodium chloride solution, AFC 77 in the strain-aged condition.

HT KIc K~cc O

K I6 O

~ 200

J O uJ KI(~ 10% SA

150 F b~'~-- -- O-- ........ __

K 0 100

z LU z_ .,~ 50 F o ~ o --'~o~ rr

| I I I I ~ m 0 240 250 260 270 280

NORMAL SA, 700 e F TEMPER

2>

,% \\~16

\

KIc X 4, \ * l -~

I I I

290 300 310

TENSILE STRENGTH (KSI)

Fig. 5--Variation in Kic, Klscc , and KI6 as a function of strength for conventionally treated and strain-aged material.

There a r e some cases in which a high value of KI6 could be of p rac t i ca l impor tance . In p r e s s u r e v e s s e l s and s u b m a r i n e hul ls , which a re designed so that a c rack can grow through the wall th ickness and cause leakage before ca tas t rophic fa i lu re can occur , a KI6 higher than / f i e gives a higher safety factor .

It is poss ib le to de t e rmine the inf luence of r e t a ined aus ten i te on Klsee by compar ing the data obtained h e r e for an aus ten i t i z ing t e m p e r a t u r e of 2000~ with Kisee data obtained p rev ious ly 3 for an aus ten i t i z ing t e m p e r - a tu re of 1800~ which r educes the amount of r e t a ined aus ten i te . The change in aus ten i t i z ing t e m p e r a t u r e p roduces l i t t le change in s t reng th , i The above t ech - nique allows the inf luence of r e t a ined aus teni te at each t e m p e r i n g t empe ra tu r e to be compared, Fig. 7.

The Kiscc va lues obtained at the two aus ten i t i z ing t e m p e r a t u r e s a r e compared in Fig. 7. The samples aus teni t ized at 2000~ have higher KIscc values at a l l t e m p e r a t u r e s where r e t a ined aus ten i te is p r e sen t (i.e. l e s s than l l 0 0 ~ The high Kisce value obtained at 1400~ is thought to be due to the low s t reng th ( f tu 195 ksi) in this condit ion.

A s u m m a r y of the p rope r t i e s obtained with ma t e r i a l s containing r e t a i n e d aus teni te in both the convent ional ly hea t - t r ea t ed and s t r a i n - a g e d condit ions is given in Table II.

Fat igue Crack Growth

Fig. 8 shows the fatigue crack growth ra tes for 0.080 in. thick spe c i me ns aus teni t ized at 2000~ and t empered

2.0

1.5

KI6 KT c 1.0

NORMAL HT O

10% SA

20% SA �9

O ~ O ~ BRANCHED CRACK ~

- 1/

UNBRANCHED CRACK A O-- - -

o s - o =o

0 I I I I I I I I 400 600 800 1 6O0 1200

TEMPERING TEMPERATURE (OF)

Fig. 6--Variation of KIa to Kic ratio with tempering temperature.

16O

AUSTENITIZED 2000 ~ F

O AUSTENITIZED 1800 ~ F

RETAI NE D AUSTE N ITE 80 h " ~ ~ ~.~

--• 6O

v

r (J ~ 4 0

v - \ t

0% 0%

0 I I I I I 500 700 900 1100 1300 1500

TEMPERING TEMPERATURE (~ Fig. 7--Variation of Klscc with tempering temperature as a function of retained austenite content.

2922-VOLUME 1, OCTOBER 1970 METALLURGICAL TRANSACTIONS

Table II. Summary of the Properties Obtained on Specimens Austenitized at 2000~ F in Both the Conventionally Treated and Strain-Aged Conditions

Heat Pet Treatment, ~ Cold Work fty, ksi ftu, ksi Klc, ksi~/in. Klscc, ksix/in.

2000 T 500 0 169 242 200 105 2000 T 700 0 180 245 160 50 2000 T 800 0 207 259 70 40 2000 T 900 0 214 278 56 35 2000 T 1100 0 221 284 43 10 2000 T 1400 0 150 195 116 80 2000 T 700 10 277 288 106 90 2000 T 700 20 297 303 107 48 2000 T 1000 10 252 307 80 30

v v

v, =- O I - {..) < LL

>. I-.

z u.i I.- z

w o: I -

30 500 ~ F TEMPER R = 0.05

0.080-1N. SHEET

25

20

15

DRY A IR O

3.5% NaCI SOLUTION �9

700 ~ F TEMPER 800 ~ F TEMPER

2 F yf / 20

15 i i I I I I

9 0 0 ~ F T E M P E R 1 1 0 0 ~ F T E M P E R

2 2o! 0 5 5 10 50

CRACK GROWTH R A T E , A 2 a / A N (/21N./CYCLE)

Fig . 8 - - F a t i g u e c r a c k g r o w t h r a t e in d r y a i r and 3.5 pc t s o d i u m c h l o r i d e so lu t ion fo r 0.080 in. shee t .

between 500 ~ and l l 0 0 ~ The d i f ference between the r a t e s in d ry a i r and 3.5 pet sa l t solut ion is sma l l for most t e m p e r i n g t e m p e r a t u r e s , for the l imi ted s t r e s s in tens i ty range studied. The l a rge s t env i ronmen ta l effect is obtained in the t e m p e r e d l l 0 0 ~ condition, which is the only condit ion where the Kiscc is below the s t r e s s in tens i ty level used in tes t ing . The use of higher R values than 0.05 would probably have r e su l t ed in a g r e a t e r env i ronmen ta l effect although the crack growth r a t e s a re no rma l ly reduced by inc reas ing the value of R.

Fig. 9 shows fatigue c rack growth data over the s t r e s s in tens i ty range 10 to 14 ks i ~ for 0.030 in. thick shee t aus ten i t ized at 2000~ Compar i son of the

s t r a i n - a g e d m a t e r i a l in Fig. 9(c) with the uns t r a ined m a t e r i a l in Fig. 9(a) indica tes that for the t empe red 700~ condit ion the c rack growth ra te in both dry a i r and 3.5 pet sa l t so lu t ion is unaffected by s t r a i n aging.

The effect of a reduced re ta ined aus ten i te content on c rack growth ra te was de te rmined by us ing fatigue c rack growth data obtained for an aus t en i t i z ing t e m - p e r a t u r e of 1800~ s The c rack growth r a t e s for t e m - per ing t e m p e r a t u r e s between 500 ~ and l l 0 0 ~ a r e shown in Fig. 10 for aus teni t iz ing t e m p e r a t u r e s of 2000 ~ and 1800~

Specimens aus ten i t i zed at 1800~ have l i t t le or no re ta ined aus ten i te at t e mpe r i ng t e m p e r a t u r e s above 500~ while spe c i me ns aus ten i t ized at 2000~ contain r e t a ined aus ten i te af ter t emper ing at t e m p e r a t u r e s up to 1000~ The spe c i me ns containing high aus teni te contents show m o r e rapid c rack growth r a t e s in dry a i r but much s lower ra tes in sa l t solut ion. There is l i t t le va r i a t ion in c rack growth ra tes in e i the r d ry a i r or 3.5 pet sa l t so lu t ion for spec imens t e m p e r e d at 500~ and it may be s igni f icant that at this t emper ing t e m p e r a t u r e , s a m p l e s aus teni t ized at both 1800 ~ and 2000~ contain subs t an t i a l amounts of r e t a ined aus t e - ni te . Samples with and without aus teni te show the fas tes t c rack growth ra tes at 700~ for both types of env i ronment .

E lec t ron f rac tographs f rom the su r f aces of c racks grown under fatigue loading show s t r i a t ed a r e a s typ i - cal of this type of tes t ing . Specimens t es ted in dry a i r show s t r i a t i ons cover ing the m a j o r i t y of the f r a c -

14

12

J• lO , /

v'

E 0 8 I - < LL >- I.-

14

12

10

700 ~ F TEMPER R = 0.05 0.030-1N. SHEET

DRY A I R �9

3.5% SALT SOLUTION t'l

I I I

(b) 1100 ~ F TEMPER

.// I I I i 2 5 10 20

(c) 1 0 % SA,

700 ~ F TEMPER

I I I I 2 5 10 20

CRACK GROWTH R A T E , A 2 a / A N (~. IN. /CYCLE)

Fig . 9 - - F a t i g u e c r a c k g r o w t h r a t e in d r y a i r and 3.5 pe t s o d i u m c h l o r i d e s o l u t i o n fo r 0.030 in. s h e e t .


Fig. 10--Fatigue crack growth rate at a Kma x = 20 ksi i ~ . for material austenitized at 1800 ~ and 2000~

tu re su r f ace . Fig. 11 shows a f rac tograph f rom a s p e c i m e n tes ted in dry a i r a f ter aus ten i t iz ing at 2000~ and t e m p e r i n g at 700~ The s t r e s s in tens i ty level in the a r e a of the f rac tograph is about 17 ksi i~m., which co r r e sponds to a crack growth ra te (AZa/AN) of about 10 ~ in. pe r cycle, Fig. 8. The width of one step is t he re fo re about 5 ~ in. , which is approximate ly the width of the s teps v i s ib le in Fig. 11. In dry a i r the re is l i t t le d i f ference between the f r ac tu re appearance of s p e c i m e n s with or without r e t a ined aus teni te , whereas in 3.5 pct sa l t solut ion, there is cons ide rab le quas ic leavage fa i lure in spec imens that do not conta in aus ten i t e , Fig. 12. This p robab ly accounts for the m o r e rap id c rack growth ra te observed under these condi t ions , Fig. 10.

CONCLUSIONS

The p l a n e - s t r a i n s t r e s s c o r r o s i o n threshold (KIscc) for AFC 77 is s t rongly dependent on t emper ing t e m - p e r a t u r e , re ta ined aus ten i te content , and degree of s t r a i n aging.

For some hea t - t r ea t ed condi t ions , a value of KI5 cons ide rab ly above Kic is obtained, indicat ing that some c rack blunt ing is occu r r i ng . The actual value of KI5 obtained suggests that fine sca le b ranch ing of the s t r e s s co r ros ion c rack is occu r r i ng .

Fat igue crack growth r a t e s in s t r u c t u r e s containing r e t a ined aus teni te a re i n c r e a s e d only s l ight ly at 20 ks i ~ by changing the e n v i r o n m e n t f rom dry a i r to 3.5 pct sa l t solut ion, in con t r a s t to the sharp i n c r e a s e in c rack growth ra te that occur s in s t r u c t u r e s that do not conta in r e t a ined aus ten i te .

LIST OF SYMBOLS

f tu Tens i l e u l t imate s t reng th , ks i fry Tens i le yield s t rength , ks i

2924-VOLUME 1, OCTOBER 1970

Fig. l l - -Fat igue striations on fracture surface of specimen austenitized at 2000~ and tempered at 700~ Test conducted in dry air. Magnification 4700 times.

Fig. 12--Area of quasicleavage failure in specimen austenitized at 1800~ and tempered at 700~ Fatigue test conducted in 3.5 pct sodium chloride solution. Magnification 1650 times.

KIc

~scc

P l a n e - s t r a i n c r i t i ca l s t r e s s in tens i ty factor, ksi P l a n e - s t r a i n s t r e s s in tens i ty factor at which rapid fa i lu re occurs in s p e c i m e n s where the s t r e s s concen t ra t ion is a s t r e s s co r ros ion crack , ks i P l a n e - s t r a i n s t r e s s c o r r o s i on threshold , ksi

METALLURGICAL TRANSACTIONS

K m a x Maximum s t r e s s in tens i ty f ac to r in fat igue, k s i

A 2 a / A N Crack growth r a t e , /~in. p e r cyc le

ACKNOWLEDGMENTS

The au thor would l ike to acknowledge Mr. A. Ross fo r a s s i s t a n c e with the e x p e r i m e n t a l work. The s t r e s s c o r r o s i o n por t ion of this r e s e a r c h was suppor ted by

the Advanced R e s e a r c h P r o j e c t s Agency of the Depa r t - ment of Defense (ARPA O r d e r 878) under con t rac t N00014-66-C0365.

RE FERENCES

I. D. Webster: Trans. ASMQuart., 1968, vol. 6I, p. 816. 2. B. F. Brown: ASTMMaterials Research and Standards, 1966, voI. 6, p. 129. 3. D. Webster: Trans. ASMQuart., 1969, vol. 62, p. 759.


Documents

The stress corrosion resistance and fatigue crack growth rate of a high strength martensitic stainless steel, AFC 77