Optimization of Strength and Toughness Two High-Strength Stainless Steels

in

DONALD WEBSTER

It has been found poss ib le to i n c r e a s e the s t reng th and toughness of two h igh-s t reng th s ta in- less s t ee l s , AFC 77 and AFC 260, by aus ten i t i z ing at t e m p e r a t u r e s that a r e in the range where both aus teni te and 5 f e r r i t e a r e s table . The 5 f e r r i t e is then r e m o v e d by i s o t h e r m a l t r ans fo rma t ion in the range 1800 ~ to 2000~ This technique r e su l t s in a g r e a t e r solution of ca rb ides and i n t e r m e t a l l i c pa r t i c l e s and consequent ly in a g r e a t e r amount of re ta ined aus - teni te than is poss ib le at aus ten i t iz ing t e m p e r a t u r e s below the 5 - f e r r i t e range. In addit ion, the technique p e r m i t s opt imum mechan ica l p r o p e r t i e s to be obtained o v e r a wider c o m p o s i - t ional range.

PREVIOUS work ~ has demons t r a t ed the i n c r e a s e d toughness and s t r e s s - c o r r o s i o n r e s i s t a n c e that may be produced in AFC 77 by con t ro l l ed amounts of re ta ined aus ten i te . The high aus ten i t iz ing t e m p e r a t u r e s used to insu re the p r e s e n c e of aus ten i te in the f inal s t r u c t u r e a lso i n c r e a s e the al loy content of the ma t r ix through g r e a t e r solut ion of ca rb ides and in t e rme ta l l i c p a r t i c l e s . A l imi t on the maximum aus ten i t iz ing t e m p e r a t u r e that can be used is set by the fo rmat ion of 5 f e r r i t e , which is known to lower duct i l i ty and toughness in s t a in l e s s s t e e l s . However , in AFC 77 and AFC 260 it has been found poss ib le to r e m o v e the f e r r i t e i so the rma l ly at t e m p e r a t u r e s in the range 1800 ~ to 2000~ This a l lows use of aus teni t iz ing t e m p e r a t u r e s that a r e higher than n o r m a l , g r e a t e r re ten t ion of aus ten i te and, in g e n e r a l , i m p r o v e d mechan ica l p r o p e r t i e s . The object of the p r e sen t work is to document the changes in m i c r o s t r u c - tu re and mechan ica l p r o p e r t i e s produced by this t ech- nique.

MATERIAL

Two steels--AFC 77 and AFC 260--were studied in this investigation. Both steels were developed by Crucible Steel Company under Air Force contracts. 2'3

9 The AFC 260 was obtained as -~ in. thick plate and the AFC 77 both as 3 in.-diam bar and 0.090 in.-thick sheet. Analysis showed the AFC 77 sheet, nominally 0.15 pct C, to be only 0.115 pct C, which is outsiJe the recommended range (0.13 to 0.18 pet). However, in this study the low carbon content was an advantage in that it provided a more severe test of the 5-ferrite removal technique than would be provided by material within the AFC 77 composition range. The chemical compositions of the materials tested are given in Table I.

EXPERIMENTAL PROCEDURE

Heat T r e a t m e n t

Specimens f rom the AFC 77 bar and the AFC 260 plate w e r e aus ten i t i zed 1 hr in argon or vacuum at s e v e r a l t e m p e r a t u r e s between 1800 ~ and 2200~ how-

DONALD WEBSTER is Senior Engineer, Commercial Airplane Group, Boeing Company, Seattle, Wash.

Manuscript submitted September 21, 1970.

e v e r , those f rom the AFC 77 sheet w e r e aus ten i t i zed at 1900 ~ and 2200~ only. 5 f e r r i t e was fo rmed at a u s - teni t iz ing t e m p e r a t u r e s above 2100~ in the AFC 77 s p e c i m e n s and above 1900~ in the AFC 260 spec imens . The amount of 5 f e r r i t e was reduced in these spec imens by i s o t h e r m a l t r e a tm en t s at 1900~ using i s o t h e r m a l holding t imes of 1 hr for the AFC 77 bar and AFC 260 plate and 20 hr for the AFC 77 sheet . 5 - f e r r i t e s t r i ng - e r s that had been p re sen t in the AFC 77 sheet in the a s - r e c e i v e d condition and had not been r em oved by aus ten i t i z ing 1 hr at 1900~ w e r e r e m o v e d by this two- s tage aus teni t iz ing t r ea tmen t .

Af t e r the aus teni t iz ing t r e a t m e n t s , the AFC 77 bar and sheet spec imens w e r e cooled to -100~ for 1 hr and then t empe red at t e m p e r a t u r e s between 700 ~ and l l 0 0 ~ for 2 + 2 hr . The AFC 260 plate spec imens w e r e a lso cooled to -100~ for 1 hr , but w e r e then cooled to -320~ for 1 hr be fore t emper ing at t e m p e r - a tu re s between 800 ~ and l l 0 0 ~ for 2 + 2 hr. Some AFC 260 spec imens w e r e given an extended subzero t r e a t - ment of 48 hr at -100~ followed by 1 hr at -320~

I s o t h e r m a l - t r a n s f o r m a t i o n d i a g r a m s for the AFC 77 sheet and AFC 260 plate w e r e de t e rm ined at t e m p e r a - t u r e s between 2000 ~ and 1600~ for t imes between 15 min and 100 hr. Specimens w e r e t r a n s f e r r e d f rom a fu rnace at 2200~ to a second furnace at the i s o t h e r m a l holding t e m p e r a t u r e . Af t e r the i s o t h e r m a l t r e a tm en t , the spec imens w e r e wa te r quenched and examined m e t - a l log raph ica l ly . The amount of 5 f e r r i t e was de t e rmined by point counting, using 1000 points . Because of the t ime r e q u i r e d for the spec imens to cool f rom 2200 ~ to 1700 ~ and 1600~ some f e r r i t e t r ans fo rma t ion p rob- ably o c c u r r e d during cool ing; this has been taken into account in drawing the i s o t h e r m a l - t r a n s f o r m a t i o n d i a g r a m s .

M e c h a n i c a l - P r o p e r t y Tes t s

M e c h a n i c a l - p r o p e r t y t es t s w e r e p e r f o r m e d on s p e c i - mens of the AFC 77 bar and sheet and the AFC 260 plate a f t e r heat t r ea tmen t . Some m e c h a n i c a l - p r o p e r t y t es t s w e r e also conducted on spec imens of AFC 77 shee t f rom which the f e r r i t e f o r m e d during aus ten i t iz ing at 2200~ had not been r e m o v e d .

Tens ion tes t s on the AFC 77 bar and AFC 260 plate w e r e conducted at room t e m p e r a t u r e on 0.25 in. diam spec imens tes ted at a s t ra in r a t e of 0.005 in. pe r in.

METALLURGICAL TRANSACTIONS VOLUME 2, JULY 1971-1857

Table I. Composition, wt pct

Material Heat C Si Mn S P Ni Cr Mo V Co Cb N

AFC 77 64555 0.16 0.13 0.18 0.021 0.015 0.21 14.0 5.02 0.23 13.41 - 0.04 3 in. diam bar

AFC 77 46073 0.115 0.07 0.10 0.008 0.010 0.080 14.22 4.62 0.30 12.93 - 0.03 0.090 in. thick sheet

AFC 260 063573 0.08 0.30 0.34 0.012 0.015 1.94 15.42 4.30 - 13.03 0.14 0.04 9 in. thick plate

o 1400 [ AUSTEHITE OrlLY

Fig. 1--Two-stage austenitiz[ng t rea tment for removal of 5 fer r i te .

p e r m i n t h r o u g h y i e l d a n d then l oaded a t a c r o s s h e a d r a t e of 0.10 in. p e r m i n to f a i l u r e . T e n s i o n t e s t s on the A F C 77 s h e e t w e r e c o n d u c t e d at 0.02 in. p e r in. p e r m i n to f a i l u r e .

F r a c t u r e - t o u g h n e s s t e s t s on the A F C 77 b a r and A F C 260 p l a t e w e r e c o n d u c t e d on f a t i g u e - p r e c r a c k e d , s i n g l e - e d g e - n o t c h e d b e n d (1.5 by 7.5 by 0.48 in.) and C h a r p y s p e c i m e n s . F o r Kic v a l u e s a b o v e 100 k s i i~n-., the C h a r p y s p e c i m e n s gave v a l u e s abou t 10 pc t l o w e r than the n o t c h e d b e n d s p e c i m e n s . S o m e of the n o t c h e d b e n d s p e c i m e n s in the t o u g h e s t h e a t - t r e a t m e n t c o n d i t i o n s w e r e too th in to m e e t ASTM r e q u i r e m e n t s .

F r a c t u r e - t o u g h n e s s t e s t s on A F C 77 s h e e t w e r e c o n - d u c t e d on p r e c r a c k e d , s u b s i z e C h a r p y s p e c i m e n s t e s t e d in t h r e e - p o i n t b e n d i n g , u s i n g m a x i m u m load to c a l c u l a t e a K va lue . A l t h o u g h the s p e c i m e n s w e r e too th in to p r o v i d e va l id Kic v a l u e s , it i s thought tha t the r e s u l t s w i l l r e l i a b l y i n d i c a t e r e l a t i v e t o u g h n e s s . The r e s u l t s on t h e s e s u b s i z e s p e c i m e n s show the s a m e t r e n d of i n c r e a s i n g f r a c t u r e t o u g h n e s s w i th i n c r e a s i n g a u s t e n i t i z i n g t e m p e r a t u r e and d e c r e a s i n g t e m p e r i n g t e m p e r a t u r e a s the l a r g e r s p e c i m e n s f r o m the A F C 77 b a r .

S t r e s s - c o r r o s i o n t e s t s on the A F C 260 p l a t e w e r e c a r r i e d out on f a t i g u e - p r e c r a c k e d C h a r p y and s i n g l e - e d g e - n o t c h e d b e n d s p e c i m e n s t e s t e d in c a n t i l e v e r b e n d i n g . T e s t i n g w a s c a r r i e d out in 3.5 pc t s a l t s o l u - t ion ( sod ium c h l o r i d e ) w i t h f r e s h s o l u t i o n c o n s t a n t l y d r i p p i n g in to the u p t u r n e d n o t c h . The c r a c k w a s i m - m e r s e d in the s a i l s o l u t i o n b e f o r e the load w a s a p p l i e d .

20 i o~Fc 77

O IFC 260

~ ' " " " " D " ISOTHERMALLY lO ~ _ . ~ . ~ . ~ " ~ , . ~ TREATED AT:

I11 ~.-

0 ,-.-L. to

1900~

1800~

1700~

i '"""E)'- ' - - - - - - -"E) 1600OF Oi ~ I

10 102 103 104 I05 106 LOG TIME (SECONDS)

Fig. 2--Rate of 5 fe r r i t e removal in AFC 77 sheet and AFC 260 plate austenitized 1 hr at 2200~

RESULTS

Maximization of Retained Austenite Content

It has previously been shown i that the fracture tough-

ness of AFC 77 is a function of the retained austenite content, which can be increased by raising the austeni- tizing temperature. The 5 ferrite formed at high austen- itizing temperatures is unstable at lower temperatures and can be isothermally transformed to austenite, Fig. i. The rate of this transformation for AFC 77 sheet and AFC 260 plate austenitized at 2200~ is shown in Fig. 2. If allowance is made for the ferrite transformation that probably occurs during cooling to the lower transfor- mation temperatures, a classical C-curve can be con- structed for both materials, Figs. 3 and 4. Only in the range 1800 ~ to 2000~ can the 5 ferrite be completely removed. Transformation curves for AFC 77 of normal

1858-VOLUME 2, JULY 1971 METALLURGICAL TRANSACTIONS

carbon content would be moved to the left of that shown in Fig. 3.

The large amount of 5 ferrite present in a sample of AFC 77 sheet that was austenitized at 2200~ is shown in Fig. 5. After holding I00 hr at 1900~ the sample is virtually free of 5 ferrite, Fig. 6. The 5 ferrite effec- tively pins the austenite grain boundaries at high tem- peratures, resulting in a fine austenite grain size. As

the f e r r i t e is i so the rm a l ly r emoved , a m a r k e d i nc r ea se in aus teni te gra in s ize is o b s e r v e d , com pa re F igs . 5 and 6.

The morphology of the 5 f e r r i t e changes with t r ans - fo rmat ion t e m p e r a t u r e . At 2000 ~ and 1900~ the 5 f e r - r i t e i s lands shr ink un i formly while maintaining approx- ima te ly the s a m e shape. At 1800~ the f e r r i t e r emova l

_ . ISOTHERMAL TRANSFORMATION OF OELTA FERRITE IN. . A LOW CARdON AFC 77 AUSTENITIZEO 2200~ 1 HR.

2200

~2100

/ \ AUSTEN/TE + FERRITE

,~ A / 15% FERRITE~.." ~ _ 10% 2000 ~ ~ , L . ~ . '~

190C

taD: x

170~

10 102 103 104 105 106

LOG TIME (SECONDS)

F~g. 3 - - I s o t h e r m a l t r a n s f o r m a t i o n of 5 f e r r i t e in low c a r b o n

ISOTHERMAL TRANSFORMATION OF DELTA FERRITE IN AFC 260 AUSTENITIZEO 2200~ 1HR,

2200

10 102 103 I04 105 106 LOG TIME (SECONOS)

Fig. 4 - - I s o t h e r m a l t r a n s f o r m a t i o n of 5 f e r r i t e in AFC 260 p la te a u s t e n i t i z e d 1 h r a t 2200~

AFC 77 s h e e t a u s t e n i t i z e d 1 h r at 2200~ ~.~.p, ~ - . ~ E f l ~ , ~ r : ~ . , . ~ : ~ l ~ . ~ ~ . q ~ ; : ~

~ r ~ t r ~ , .Z~ '~ .~ . *.~..:~"~.-~,

100 h r at 1900~ The 5 f e r r i t e shown in Fig. 5 h a s been r e - Fig. 5--AFC 77 s h e e t a u s t e n i t i z e d I h r at 2200~ and wa t e r m o v e d and the a u s t e n i t e g r a i n s i z e h a s i n c r e a s e d . M a g n i f i c a - quenched , showing 5 f e r r i t e . Magn i f i ca t i on 550 t i m e s , t ion 550 t i m e s .

METALLURGICAL TRANSACTIONS VOLUME 2, JULY 1971-1859

occurs mainly by an irregular advance of the austenite- fertile interface during which fingers of austenite ad- vance into the fertile. At 1700 ~ and 1600~ the above mechanism for ferrite removal is supplemented by the precipitation of austenite in the ferrite. The skeletal remains of a large ferrite block held 4 hr at 1600~ can be seen in Fig. 7. The morphology of the austenite precipitated in 6 ferrite on cooling bears a marked resemblance to the austeniLe precipitated in the ~ fer-

Fig. 7--Euteetic-like remains of a large block of 6 ferrite in AFC 77 sheet held 4 hr at 1600~ Magnification 2900 times.

rite of low alloy steels during the healing. 4'5 A consid- erable amount of work on the transformation of 5 ferrite has been published by Kuo. e-z~ In most of the steels in- vestigated by Kuo the 6 ferrite transformed to a eutec- toid-like mixture of austenite and carbide which was not observed in AFC 77 and AFC 260. However, Kuo

300

I

UTS --

AFC 77 TEMPEREO 700-1100~

AUST. TEMP. ~F) 0.2~ Y.S.

1800 ,,oo : 2000 �9 2200+ 1900 �9 v

200OF

2000oF 250

~225 V 1000~

I 200 ~ I ~ ~ 2000 ~

175 ~ $O0~

0 50 I00 150 KIc (Ks i%/-~. )

Fig. 8--Summary of strength-toughness values obtained on AFC 77 bar austenitized at temperatures between 1800 ~ and 2200 ~ F.

INOICATED TEliPERATURE

BOOOF ~ ?

/ ] ,,o0~

I o | ] - - ~ ~ 7oo~ I ouu-r ~ I I

- - - - - - ' - " ' - ' - ~ "' I

U 1900 2000 21 OO 2200 + 1900

AUSTENITIZING TEMPERATURE (OF)

Fig. 9--Ductility of A FC 77 bar as a func- tion of austenitizing and tempering temper- atures.

1 8 6 0 - V O L U M E 2 , J U L Y 1971 METALLURGICAL TRANSACTIONS

300

250

w Q:

-- 200

[ i z A U S T . 2200 F 1HR + 20 HR 19OO~

fry I �9 0

, ,0 I I ,ODD~ , ,R 2200~ I HR I

INEO AUSTENITE

700 800 900 I000 1100 1200

TEMPERING TEMPERATURE (OF)

Fig. 10--Stren~h and tou~ness of low carbon AFC 77 sheet.

125

-tOtl ~ 75

~ 2~

did o b s e r v e the t r a n s f o r m a t i o n of 5 f e r r i t e t o a u s t e n i t e w i thou t c a r b i d e in a low c a r b o n 1 8 / 1 0 / 3 C r - N i - M o s t a i n l e s s s t e e l . 9 The m o r p h o l o g y of t h i s newly f o r m e d a u s t e n i t e w a s s i m i l a r to tha t o b s e r v e d in A F C 77 and A ~ C 260 below 1800~

Mechanical Properties

AFC 77 BAR

A summary of the tensile and toughness values for AFC 77 bar is shown in Fig. 8. In general there is a continuous improvement in the strength-toughness com- bination as austenitizing temperature is increased, with the most dramatic improvements being obtained in the 1800 ~ to 2000~ range. The ductility of the material does not seem to be markedly dependent on austenitizing temperature, Fig. 9. However, the low ductility obtained by tempering at 900 ~ and 1100~ after austenitizing at 2000 ~ and 2100~ seems to be eliminated by the two- stage (2200 ~ + 1900~ austenitizing treatment.

AFC 77 SHEET

The strength of AFC 77 sheet, Fig. i0, is similar to that of the bar stock, although it is somewhat lower as a result of the low carbon content. Raising the austen- itizing temperature from 1900 ~ to 2200~ produces a marked drop in strength due to the formation of 5 fer- rite. Removal of the 5 ferrite produced at 2200~ by isothermal transformation at 1900~ increases the

28[

26C

24~

i 22~

2U~

I M

160

j e t AUSTENITIZING 0.2~ ULTIMATE

_ _ TREATMENT YIELD STRENGTH TENSILE fry STRENGTH, f tu

2200~ I HR + 1 HR 1900~ m o 2IOO~ I HR + I NR 10ooOF �9 19OO~ 1HR �9 o

- - 18OO~ 111R & 2100~ 1HR} I fiR 48 HR-IOO~ �9 o

+1000~ I HR HR -320~

2200OF [

140 DOO 900 lOOO 1100

TEMPERING TEMPERATURE (OF)

Fig. l l - -St rength of AFC 260 as a function of austenitizing and tempering tempera tures .

strength to that obtained by austenitizing directly at 1900~ while maintaining a much higher toughness level. A comparison of the toughness values of speci- mens austenitized at 2200~ shows that the removal of 5 ferrite increases the toughness at all tempering tem- peratures. Little difference in toughness and strength was observed between specimens taken in the trans- verse and in the longitudinal grain directions.

This same 0.090 in.-thick AFC 77 sheet material was extensively investigated by Ault et al .,11 who used a single austenitizing temperature of 1900~ The strength properties reported by these workers is in general agreement with the results reported here for the same austenitizing temperature.

AFC 260 PLATE

The effect of austenitizing temperature on the tensile properties of AFC 260 plate is shown in Fig. ii. Small but significant increases in tensile strength are ob- served for each increase in austenitizing temperature. However, the yield strength is somewhat lower in spec- imens austenitized at 1900~ than in those austenitized at 1800~ An extended subzero treatment of 48 hr at -100~ produces a significant increase in yield strength but little change in ultimate strength. The toughness of specimens austenitized at 1800~ is markedly lower than that of specimens austenitized from 1900 ~ to 2200~ Fig. 12. The stress-corrosion threshold Kiscc decreases slightly as the tempering temperature is in-

METALLURGICAL TRANSACTIONS VOLUME 2, JULY 1971-1861

c r e a s e d . T h e r e w a s no m e a s u r a b l e c h a n g e in f r a c t u r e t o u g h n e s s p r o d u c e d by the e x t e n d e d s u b z e r o t r e a t m e n t .

DISCUSSION

A significant improvement in mechanical properties can be achieved in AFC 77 and AFC 260 by austenitizing at temperatures high enough to form 5 ferrite. Although

125

~ 1 0 0

~ 75

AUSTENITIZING TEMPERATURE

(OF) RIc XIscc

1800 z,. 1900 o 2100+ 1900 '?

_ 2200+ 1900 ca �9

000 900 I000 1100

TEMPERING TEMPERATURE (OF)

Fig. 12--Toughness and s t r e s s - c o r r o s i o n res i s t ance of AFC 260 as a function of austenit izing and temper ing tempera tures .

HiGH RATIO OF AUSTEHITE/FERRITE FHHIIHG (LEHENT$

SINGLE-STAGE TWO-STAGE AUSTENITIZIHO AHSTEHITIZINS

FORMED OPTllUll HEAT- HEAT - TAEATIIENT THE ATIAEIIT RANGE HINGE

LOW HATIG OF AUSTENLTE/FERAIT[ FOHHINO ELEHEHTS

~ STJaLs rEHHITE

II

SINGLE-STAGE TNG-STAOE AUGTENITIZIHG AUGTENITtZING

Fig. 13--Schematic i l lustration showing how a two-stage austenit izing t reatment i nc reases the tempera ture range in which optimum mechanical p roper t i e s can be obtained.

the 5 f e r r i t e t hus f o r m e d is d e t r i m e n t a l to s t r e n g t h , t o u g h n e s s , and d u c t i l i t y , i t s r e m o v a l by i s o t h e r m a l t r a n s f o r m a t i o n r e s u l t s in a f ina l m i c r o s t r u c t u r e that p o s s e s s e s a s u p e r i o r c o m b i n a t i o n of s t r e n g t h and t o u g h n e s s .

A f u r t h e r a d v a n t a g e of the t w o - s t a g e a u s t e n i t i z i n g t r e a t m e n t i s tha t it a l l o w s a w i d e r c o m p o s i t i o n r a n g e to be s p e c i f i e d fo r t h e s e h i g h - s t r e n g t h s t a i n l e s s s t e e l s . T h i s i s i l l u s t r a t e d in F i g . 12. If the r a t i o of a u s t e n i t e - f o r m i n g e l e m e n t s to f e r r i t e - f o r m i n g e l e m e n t s i s h igh , t h e r e w i l l be a r e g i o n a r o u n d 2000~ w h e r e the a u s - t e n i t i z i n g t e m p e r a t u r e i s h igh enough to c a u s e a s i g n i f i - can t a m o u n t of a u s t e n i t e to be r e t a i n e d a f t e r hea t t r e a t - m e n t , but low enough to a v o i d the f o r m a t i o n of G f e r r i t e , F i g . 13(a). T h i s i s the n o r m a l cond i t i on f o r A F C 77 w i t h i n the s p e c i f i e d c o m p o s i t i o n r a n g e . H o w e v e r , if t he r a t i o of a u s t e n i t e - f o r m i n g e l e m e n t s to f e r r i t e - f o r m i n g e l e m e n t s is low, t h e r e m a y be no r a n g e of a u s t e n i t i z i n g t e m p e r a t u r e in w h i c h o p t i m u m p r o p e r i t e s can be o b - t a i n e d , F i g . 13(b). A F C 260, b e c a u s e of i t s low c a r b o n c o n t e n t and in s p i t e of i t s h i g h e r n i c k e l c o n t e n t , ha s a g r e a t e r t e n d e n c y to f o r m 5 f e r r i t e t han A F C 77.

CONCLUSIONS

1) 5 ferrite produced by austenitizing AFC 77 and AFC 260 at high temperatures can be isothermally transformed to austenite in the range 1800 ~ to 2000~

2) A two-stage austenitizing treatment designed to take advantage of this effect has been shown to produce a better combination of strength and toughness in these steels.

3) 5 ferrite reduces the strength and toughness of AFC 77, especially after tempering temperatures above 700~ are used.

A C K N O W L E D G M E N T

T h i s w o r k w a s s p o n s o r e d in p a r t by the A d v a n c e d R e s e a r c h P r o j e c t s A g e n c y of the D e p a r t m e n t of D e f e n s e , A R P A O r d e r No. 878, u n d e r C o n t r a c t No. N 0 0 0 1 4 - 6 6 - CO365.

R E F E R E N C E S

1. D. Webster: Trans. ASMQuart., vol. 61, December 1968, p. 816. 2. A. Kasak, V. K. Chandhok, J. H. Moll, and E. J. Dulls: Development of High

Strength, Elevated Temperature, Corrosion Resistant Steel, ASD-TDR-63-766. Crucible Steel Company of America, Contract AF33(657)-8458 Sept. 1963.

3. J. H. Moll, W. Stasko, and A. Kasak: Development of a New High Strength Semi-austenitic Precipitation Hardenable Stainless Steel, AFML TR 67-286 Crucible Steel Company of America, Contract AF 33(615)-2201, Sept. 1967.

4. D. Webster and G. B. Allen: J. Iron Steellnst., 1962, vol. 200, p. 520. 5. D. Webster: The Effect of Grain Refinement on the Microstructure and Me-

chanical Properties of 4340M, Boeing Document D6-25220, unpublished re- search.

6. K. Kuo: J. Iron Steel lnst., 1954, vol. 176, p. 433. 7. K. Kuo: J. Iron Steellnst., 1955, vol. 181, p. 128. 8. K. Kuo: J. Iron Steellnst., 1955, vol. 181, p. 134. 9. K. Kuo: J. lronSteellnst., 1955, vol. 181, p. 213.

10. K. Kuo: J. Iron Steellnst., 1955, vol. 181, p. 218. 11. R. T. Ault, R. B. Holtman, and J. R. Myers: Trans. ASM Quart., vol. 61,

March 1968, p. 75.

1862-VOLUME 2, JULY 1971 METALLURGICAL TRANSACTIONS