= ko" ~,n (n; s t r a i n hardening exponent) , the r e v e r s e yield s t r e s s at p r e s t r a i n Vp can be s imply calcula ted by

4"rR[ = TF - TB = ko "~ '~-k .y~n [31

The above equat ion is the e m p i r i c a l equation under the uniaxia l s t r e s s condi t ion, but it is useful as the one which gives a boundary condit ion of the genera l yield c r i t e r i o n of p r e s t r a i n e d m a t e r i a l s under a mul t i ax ia l s t r e s s condit ion.

The authors a re gra tefu l to Prof . Miyagawa and Dr . N i s h i m u r a of Tokyo Metropol i tan Un ive r s i ty for expe r imen t ,

I. R. L. Wooley: Phil. Mag., 1953, vo]. 353, pp. 597-618. 2. D. V. Wilson: Acta Met., 1965, vol. 13, pp. 807-16. 3. S. Kumakura: Bull. Japan Soc. Mech. Eng., 1968, vol. 11, pp. 427-36. 4. J. R. Hancock and J. C. Grosskreutz: ActaMet., 1969, vol. 17, pp. 77-97. 5. T. Kishi and I. Gokyu: Suppl. Trans. Iron Steellnst. Japan, 1971, vol. 11, pp.

1044-48. 6. R. M. Jamieson and J. E. Hood: J. Iron Steellnst., 1971, vol. 209, pp. 46-48. 7. T. Kishi and T. Tanabe: J. Mech. Phys. Solids, in print.

Effect of Grain Refinement on the Microstructure and Mechanical Properties of 4340M

DONALD WEBSTER

PREVIOUS papers 1'2 have desc r ibed a technique for re f in ing the aus ten i te g ra in of a h igh-s t r eng th s t a in l e s s s t ee l , AFC 77, and the r e s u l t a n t i m p r o v e m e n t in m e - chanica l p rope r t i e s . The technique, can be employed on a wide va r i e ty of s t ee l s . The p r e s e n t work de - s c r i b e s the g ra in r e f i n e m e n t obtained in a h igh- s t r eng th , low-a l loy s tee l , 4340M, and i ts effect on the mechan ica l p r o p e r t i e s of that s tee l .

The 4340M used in this study was rece ived as a 2�88 in. square b i l le t . Composi t ion by weight pe rcen t of the v a c u u m - m e l t e d s tee l (as repor ted by the manufac tu re r ) is as follows:

C Mn Si S P Ni Cr Mo V 0.43 0.84 1.72 0.004 0.010 1.72 0.77 0.39 0.08

The 2~ in. square b i l l e t was hot roi led at 2100~ to 1.12 in. thick plate . Mate r i a l for spec imens of coarse g ra in s ize was then fu r the r hot rol led to 0.56 in. thick plate (normal heat t r ea tmen t ) . Mate r i a l for spec imens to be used for evaluat ing the aus teni te g ra in ref in ing technique was t empered at 1275~ for 1 h and then cold ro l led 50 pct to 0.56 in. thick pla te , with two i n t e r m e d i - ate annea l s at 1275~ A f inal aus ten i t i z ing t r e a t me n t of ~ h at t e m p e r a t u r e s between 1500 ~ and 1600~ was c a r r i e d out in vacuum. These spec imens were then oil quenched and t empered at 400 ~ or 550~ for 2 + 2 h. Spec imens removed at var ious t e m p e r a t u r e s for m i c r o -

DONALD WEBSTER, formerly with Boeing Company, Seattle, Wash., is now Research Scientist, Metallurgy and Composites Labora- tory, Lockheed Missiles and Space Company, Palo Alto, Calif. 94304.

Manuscript submitted January 14, 1972.

s t r u c t u r a l examina t ion f rom m a t e r i a l given e i ther heat t r e a t m e n t were water quenched.

Tens ion tes ts were conducted at room t e m p e r a t u r e on 0.25 in. d iam spec imens tes ted at a s t r a i n ra te of 0.005 i n . / i n . / m i n through yield and then loaded at a c rosshead speed of 0.100 i n . / m i n to f a i lu re .

F r a c t u r e toughness tes ts were c a r r i e d out on s i ng l e - edge-notched spec imens 7.5 by 1.5 by 0.5 in. tested in t h r ee -po in t bending.

S t r e s s - c o r r o s i o n tes t ing under p l a n e - s t r a i n condi - t ions was conducted on fatigue p r ec r acked s ing le -edge notched bend spec imens tested in can t i l eve r bending. 3 Tes t ing was c a r r i e d out in a 3.5 pet NaC1 solut ion, with f resh solut ion cons tant ly dr ipping into the upturned notch. The c rack was i m m e r s e d in the solut ion before the load was applied. Specimens were held 1 week at each s t r e s s - i n t e n s i t y level before unloading. Crack growth ra te under p l a n e - s t r a i n condi t ions was ca l cu - lated by dividing the length of the c rack by the total t ime elapsed s ince the in i t i a l loading. Since incubat ion per iod is ignored, r a t e of c r ack growth is plotted as an apparen t c rack growth ra te . Rate of c r ack growth is ze ro at K l s c c .

Austeni te g ra in s ize was de te rmined by me asu r ing the mean l inea r in te rcep t of 200 g r a i n s . F o r g ra in s i zes below 20 ~ , m e a s u r e m e n t was made on ca rbon r ep l i ca s examined in the e lec t ron mic roscope .

Meta l lography. At t e m p e r a t u r e s jus t below Acl , both deformed and undeformed spec imens cons i s t of pa r t i a l ly r e c r y s t a l l i z e d t empered m a r t e n s i t e . The r e c r y s t a l l i z e d f e r r i t e g ra ins tend to be c o a r s e r and more equiaxed in the deformed m a t e r i a l . At the Acl t e m p e r a t u r e , the

Fig. 1--4340M: Undeformed tempered martensite austenitized I h at 1480~ and water quenched. Arrows point to new austenite grain boundaries between the blocks of aligned ferrite lathes. Scanning microscope picture. Magnification 450 times.

392-VOLUME 4, JANUARY 1973 METALLURGICAL TRANSACTIONS

morpho logy of the aus t en i t e that beg ins to f o r m in the d e f o r m e d and unde fo rmed s p e c i m e n s is m a r k e d l y d i f - f e r en t . Aus ten i te f o r m e d in the unde fo rmed m a t e r i a l is a c i c u l a r and s i m i l a r in morpho logy to the t e m p e r e d m a r t e n s i t e f rom which i t f o r m s . C a r b i d e s at the a u s t e n i t e - m a r t e n s i t e i n t e r f ace can be seen to be e x e r t -

AUSTENITIZING TIHE=�89 HR . . . . 4 - . . . .

30

/

& r~ ~ 20 0'~o REDUCTI OH "-~

i /

~ I0: < /

~ 50~o REDUCTI OH

I

1300 IqO0 1500 1600

AUSTENITIZIHG TEHPERATURE (~ Fig. 2--Effect of austenitizing temperature on austenite grain size of deformed and undeformed tempered martensite in 4340M. Drop in grain size at 1550 ~ and 1600~ is not observed in slowly heated specimens.

300

280

26O

2,,o

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200

TEHSILE STRENGTH

0.2~ YIELD STRENGTH

F I N ~ O W L Y

/ ~I1~__. COARSE GRA IN / d

TEHPERED AT qO0 ~ 2 + 2 HR

F I N ~ HEATED SLOWLY

/

A

450 v

2 qo

20

F I N E ~ ~ ~ .,,,...

/ ~ C~RSE ~ IN

0 I I i IqSO 1500 1550 1600

AUSTENITIZING TEHPERATURE (~ Fig. 3--Strength and toughness of fine- and coarse-grained 4340M tempered at 400~

ing some d e g r e e of pinning that wi l l have the ef fec t of s t ab i l i z i ng what is b a s i c a l l y an uns tab le s t r u c t u r e . Examina t i on of the s t r u c t u r e at a lower magn i f i ca t ion with the scanning e l e c t r o n m i c r o s c o p e r e v e a l s that t he re i s a high d e g r e e of a l i g n m e n t of the u n d i s s o l v e d f e r r i t e l a thes within each p r i o r aus t en i t e g r a i n , F ig . 1. At some t e m p e r a t u r e be tween Ac~ and Ac3, the s m a l l a c i c u l a r aus t en i t e g r a i n s g row r a p id ly unt i l the new g r a i n s i ze is the s a m e as that of the p r i o r aus t en i t e g r a i n s i z e , and the new aus t en i t e g r a i n b o u n d a r i e s l ie be tween the d i f f e r en t l y a l igned g roups of f e r r i t e l a t he s .

In m a r k e d c o n t r a s t , the newly f o r m e d aus t en i t e in the d e f o r m e d m a t e r i a l e x i s t s as fine (~1 ~) equiaxed g r a i n s , and the und i s so lved f e r r i t e is p r e s e n t in a l m o s t equ iaxed b locks along the aus t en i t e g r a i n b o u n d a r i e s . With i n c r e a s i n g aus t en i t i z ing t e m p e r a t u r e , the r e s i d u a l f e r r i t e d i s s o l v e s and a s m a l l amount of g r a i n growth o c c u r s , unt i l a t 1500~ the s t r u c t u r e c o n s i s t s e n t i r e l y of f i n e - g r a i n e d aus t en i t e and und i s so lved c a r b i d e s that a r e p r o b a b l y V4C3. At 1525~ these c a r b i d e s a r e s t i l l und i s so lve d , but a t 1550~ a r e a l m o s t c o m p l e t e l y in so lu t ion .

The e f fec t of aus t en i t i z ing t e m p e r a t u r e on the a u s - ten i te g r a i n s i ze of d e f o r m e d and unde fo rmed 4340M is shown in F ig . 2. The a n o m a l o u s l y c o a r s e g r a i n s i ze p roduced in the r eg ion of 1500~ in the unde fo rmed m a t e r i a l i s commonly o b s e r v e d in l o w - a l l o y s t e e l s conta in ing vanad ium 4 and is thought to be due to the p r e f e r e n t i a l unpinning of aus t en i t e g r a i n b o u n d a r i e s that o c c u r s dur ing the g r a dua l so lu t ion of vanad ium c a r - b ide in the r ange 1450 ~ to 1500~ The s p e c i m e n s t r e a t e d at 1550 ~ and 1600~ were hea ted r a p i d l y through the c r i t i c a l r ange so that a l l the g r a i n b o u n d a r i e s w e r e r e l e a s e d at the s a m e t ime and d i scon t inuous g r a i n growth was p r e v e n t e d . Howeve r , in l a r g e componen t s ,

300

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I ~ 2~

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:.F, u qo

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FINE G R A ~ O N L Y TENSIIF STRENGTH

TE]',4PERED AT 550 ~ 2+2 NR FINE GRAIN -'-'-x.

0.2~o YIELD ~ I ~ L Y STRENGTH

FINE GRAIN r..,,,- " " O,,....""vl ,.~ """~= HEATED SLOWLY

f ~ COARSE GRA IN ,:,,,,-

:} I I I

1'~50 1500 1550 1600

AUSTENITIZING TEHPERATURE (~ Fig. 4--Strength and toughness of fine- and coarse-grained 4340M tempered at 550~

METALLURGICAL TRANSACTIONS VOLUME4, JANUARY 1973-393

~L

t~

I OOC

I OC

AUSTENIZING GRAIN TEMPERING TEMPERATURE(~ SIZE TEMPERATURE(OF~ '15oo 15so i~|

o ~ 0 COARSE ~00 �9 ~ �9 COARSE 550

~ ~ FINE ~00 ~ ~ FINE 550

(HEATED SLYLY)

t

�9 Q

I0 I I 12 13 Iq 15 16 17 18 19 20

INITIAL STRESS-INTENSITY FACTOR, Kl i (ksi ~ ' ~ . )

Fig�9 5 - - A p p a r e n t c r a c k g r o w t h r a t e in 3�9 pc t NaCI so lu t ion in f i n e - and c o a r s e - g r a i n e d 4340M t e m p e r e d a t 400 ~ and 500~ 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 i s no t a p p r e c i a b l y a f f e c t e d by e i t h e r g r a i n s i z e o r t e m p e r i n g t e m p e r a t u r e � 9

in which slow heating (<500~ is unavoidable, grain growth occurs in the c r i t i ca l range and a coarse grain s ize is obtained at al l hea t - t r ea tmen t t empera tu res . In deformed ma te r i a l , a fine gra in s ize is maintained even at slow heating r a t e s .

Mechanical P r o p e r t i e s . Grain ref inement resu l t s in an inc rease in s trength and toughness in ma te r i a l tempered at 400~ Fig. 3, and at 550~ Fig . 4. This inc rease is most noticeable at an austenit izing t empera tu re of 1500~ where the effect of grain s ize is most marked. As in most cases of grain ref inement , the yield strength is inc reased more than the ul t imate s trength.

The optimum strength of 4340M is obtained at about 1550~ At lower t empe ra tu r e s , undissolved vanadium carb ides remain which lower the carbon content of the mat r ix and hence the s trength. At higher t empe ra tu r e s , gra in coarsening occurs which a lso reduces s t rength. Grain ref inement r e su l t s in improvement in both s t rength and toughness at 1500~ but at 1550~ the gra in size difference is too smal l to produce a m e a s u r - able difference in the mechanical p rope r t i e s . To achieve optimum mechanical p rope r t i e s with maximum grain ref inement in 4340M would requi re alloying to e i ther lower the Ac3 t empera tu re or r a i s e the t empera tu re at which signif icant grain ref inement is achieved.

The apparent c r ack growth ra t e s in 3.5 pct NaC1 solu- tion for f ine- and c o a r s e - g r a i n e d specimens are shown in Fig . 5. The threshold l ies between 12 and 15 ksi vrin. and does not seem to be markedly affected by ei ther gra in s ize or temper ing t empera tu re . The f r a c - ture path in both f ine- and c o a r s e - g r a i n e d specimens is predominant ly in te rg ranu la r , F ig . 60

The following conclusions can be drawn from the above data:

1) Deformation of tempered mar tens i te in 4340M s tee l r e su l t s in a marked reduction in austenite gra in s ize at austeni t izing t empe ra tu r e s between Acl and 50~ above AC3.

Fig. 6 - - I n t e r g r a n u l a r s t r e s s - c o r r o s i o n f r a c t u r e pa th in f i n e - g r a i n e d 4340M a u s t e n i t i z e d -~ h a t 1500~ and t e m p e r e d 2 + 2 h a t 550 ~149 Scann ing m i c r o s c o p e p i c tu r e . M a g n i f i c a t i o n 940 t i m e s .

2) This gra in ref inement r e su l t s in a modest increase in s trength and toughness but no increase in s t r e s s - cor ros ion threshold (Kiscc) af ter the austeni t izing t e m - pe ra tu re of 1500~ At higher austenit izing t e m p e r a - tu re s , there is l i t t le effect on ei ther gra in s ize or me- chanical p rope r t i e s .

3) Slow heating produces an increase in austeni te grain s ize in undeformed m a t e r i a l by allowing d i scon- tinuous grain growth to occur. Slow heating does not coarsen the austenite grain s ize of deformed ma te r i a l .

4) To achieve the optimum benefit f rom the grain ref inement p roces s studied, it is n e c e s s a r y to design a s tee l for which the austenit izing t empera tu re for opt i - mum mechanical p rope r t i e s is coincident with the t em- pe ra tu re of maximum grain ref inement . In the case of 4340M, this would mean alloying to e i ther lower the Ac3 tempera tu re or r a i s e the t empera tu re at which significant gra in ref inement is achieved.

This work was sponsored in par t by the Advanced Resea rch P ro j ec t s Agency of the Depar tment of Defense, ARPA Order No. 878, under Contract No. N00014-66- C0365.

i. D. Webster: Trans. ASM, 1969, vol. 62, p. 470. 2. D. Webster: Trans. ASM, 1969, vol. 62, p. 759. 3. B. F. Brown: ASTMMater. Res. Stand., 1966, vol. 6, p. 129. 4. D. Webster and G. B. Allen: J. Iron Steel. Inst., 1962, vol. 200, p. 520.

394 -VOLUME 4, JANUARY 1973 METALLURGICAL TRANSACTIONS