Retained Austenite and Tempered Martensite Embrittlement

GARETH THOMAS

The p rob lems of detect ing the d i s t r ibu t ion of sma l l amounts (5 pet or less) of re ta ined aus ten i te f i lms around the m a r t e n s i t e in quenched and t empered expe r imen ta l medium carbon F e / C / X s tee l s a re d i scussed and e lec t ron opt ical methods of ana lys i s a re em- phas ized. These re ta ined aus teni te f i lms if s table s eem to be benef ic ia l to f r ac tu re toughness . It has been found that t h e r m a l ins tab i l i ty of r e t a ined aus teni te on t e mpe r i ng produces an e m b r i t t l e m e n t due to its decomposi t ion to in te r la th f i lms of MaC carb ides . The f r ac tu r e s a re thus i n t e r g r a n u l a r with r e spec t to mar t ens i t e but t r a n s g r a n u l a r with r e spec t to the p r io r aus ten i te . The t e m p e r a t u r e at which this occur s depends upon alloy content . The effect is not found in F e / M o / C for which no re ta ined aus teni te is detected af ter quenching, but is p r e sen t in all other a l loys invest igated.

I N a sys t ema t i c p rog ram of r e s e a r c h us ing vacuum mel ted high pur i ty F e / C / X s tee ls , the r e l a t ion be- tween mic roscop ic fea tu res and t ens i l e s t reng th and f r ac tu re p rope r t i e s has been cor re la ted , f rom which the subs t ruc tu re , whether dis located or twinned, has been shown to be very important.1

It i s now well known that the p r e s e n c e of d is located and twinned m a r t e n s i t e s u b s t r u c t u r e s found in quenched s tee l s depends upon composi t ion, espec ia l ly carbon 1-3 and the M s - M f t e m p e r a t u r e range . By proper des ign of these p a r a m e t e r s the s u b s t r u c t u r e and t ens i l e p rope r t i e s can be cont ro l led . In r e c e n t yea r s it has a lso become apparent that s m a l l amounts of very f inely d i spe r sed f i lms of r e t a ined aus teni te (~) oc- cupying about 0.01 to 0.05 volume f rac t ion can be r e - solved at the in te r faces between m a r t e n s i t e laths in ce r t a in quenched and quenched and t empered exper i - menta l and c o m m e r c i a l s tee l s by sophis t ica ted e lec- t ron mic roscop ic methods .4-6 The r e su l t is somewhat s u r p r i s i n g in cases where M s t e m p e r a t u r e is quite high (>200~ i .e. , in s tee l s containing most ly d i s lo- cated m a r t e n s i t e s . Th i s obse rva t ion of r e t a ined aus - teni te , f i r s t r epor ted for F e / C r / C expe r imen ta l s tee l s by McMahon and Thomas , 4 has been l inked to the f r a c - tu re toughness p rope r t i e s for which at leas t e m p i r i - cally it is now known that thermal ly stable re ta ined aus ten i te f i lms can be benef ic ia l . 4'6'7 McMahon and Thomas a a lso observed a loss in toughness upon t emper ing which they showed was co r re l a t ed with the decomposi t ion of the aus ten i te with format ion of i n - t e r l a th carb ide f i lms . This phenomenon has also been observed recent ly in 4340 and 300M s tee l s s but was not found in F e / M o / C al loys. 9

It is the purpose of this paper to emphas ize 1) that the detec t ion and unique ident i f ica t ion of these s m a l l amounts of in te r l a th aus ten i te is non t r iv i a l and in- volves ve ry carefu l e l ec t ron mic roscopy and d i f f rac- tion, 2) that such aus teni te f i lms , if s table , can be benef ic ia l to toughness and 3) that the t he rma l in s t a - b i l i ty of in te r la th aus teni te , leading to ca rb ides upon temper ing , is a fa i r ly genera l cause of t empe red m a r - t ens i t e e m b r i t t l e m e n t . This la t te r effect, to be de-

GARETH THOMAS is Professor of Metallurgy at the University of California, Department of Materials Science & Engineering, Hearst Mining Building, Berkeley, CA 94720.

Manuscript submitted May 3, 1977.

METALLURGICAL TRANSACTIONS A

reefed, c lea r ly depends on the abi l i ty to detect the o r ig ina l aus teni te .

IDENTIFICATION OF RETAINED AUSTENITE

When re ta ined aus teni te is p r e se n t in amounts g rea t e r than about 1 to 2 pet, it may be detected by c l a s s i c a l X - r a y methods, 1~ provided the gra in s ize of the p r i o r aus ten i te is not too large (~0.1 pan or more) or by ref ined magnet ic m e a s u r e m e n t s and other non- magnet ic techniques , e.g., ~ r ays . Even if these methods detect aus ten i te , they can not indicate any morphologica l c h a r a c t e r i s t i c s . However, in cases where such methods fai l to detect aus teni te , for exam- ple in medium or low carbon s tee l s in which the mi - c r o s t r u c t u r e cons i s t s of packets of d is loca ted m a r t e n - s i te laths, ca re fu l e l ec t ron mic roscopy and dark field ana lyses often show that these laths a re separa ted by thin f i lms of aus ten i te s

It is impor tan t to emphas ize , therefore , that such f i lms of aus ten i te may go undetected if X - r a y s a n d / o r magnet ic methods only a re employed, or if only b r igh t - field e lec t ron mic roscopy imaging* is c a r r i e d out.

*In this paper the abbreviations BF and DF will be used for bright field and dark field imaging, respectively.

It has a l r eady been pointed out by Rao et al s that carefu l se lec t ion of o r i en ta t ion and long exposure t imes a re needed if aus ten i te re f lec t ions a re to be r e - corded and appropr ia te dark field imaging* done on aus ten i te r e f l ec t ions . However, on-going r e s e a r c h on a va r i e ty of s t ee l s (see Tab le I), has indicated that even g rea t e r care mus t be exe rc i s ed because of the following:

1) In as -quenched and t empered m a r t e n s i t i c s tee ls the p r e s e n c e of carb ides gives added re f l ec t ions whose spacings coincide, or nea r ly so, with aus ten i te . Th i s wil l be espec ia l ly t rue in d is located m a r t e n s i t e s where M s is usual ly >300~ when a u t o - t e m p e r i n g wil l occur upon quenching. 1

2) In many cases adjacent laths a re twin re la ted which not only causes ex t ra m a r t e n s i t e r e f l ec t ions to appear but a lso double d i f f rac t ion spots which can add to the complexi ty of sor t ing out the s t r u c t u r e .

3) The a r e a cont r ibut ing to the se lec ted a r e a d i f f rac- t ion pa t t e rn for a given beam d i a m e t e r at the spec imen

ISSN 0360-2133 / 78/0310-0439500.75/0 �9 1978 AMERICAN SOCIETY FOR METALS AND VOLUME 9A, MARCH 1978-439

THE METALLURGICAL SOCIETY OF AIME

Table I. Range of Compositions (wt pct) of Experimental Steels Studied in This Research

Carbon Cr Mo Ni Co Mn B Temper Martensite Alloy Embrittlement Temp. Source t Ref.

0.3 4 . . . . . 300~ 1,2" 18 0.3 4 - - 0.5 300~ 2* 18 0.3 4 - - 2.0 - 300~ 2* l 8 0.3 4 5 - - - 400 ~ C 2 * 18 0.3 1 1 . . . . 350~ 3* 13 0.3 1 I - - - .0016 350~ 3" 13 0.41 - 4.2 . . . . Not Observed 1 * 24 0.43 - 2.2 - - - Not Observed 1 * 24 0.26 - 4.85 - 3.8 - 315~ 1 30 0.24 - - I2.5 3.8 425~ l* 31

*Vacuum melted. t 1)Republic Steel Research Labs; 2) Daido Steel Company; 3) Climax Molybdenum Company.

i s l i m i t e d by s p h e r i c a l a b e r r a t i o n and c a n n o t be c o m - p e n s a t e d f o r by u s i n g s m a l l e r s e l e c t e d a p e r t u r e s .

F o r c o n v e n t i o n a l 100 kV T E M i n s t r u m e n t s , t h i s i s abou t 1 to 2 /z but a t h igh v o l t a g e s i t i s c o n s i d e r a b l y r e d u c e d due to t he r e d u c t i o n in e l e c t r o n w a v e l e n g t h ( e . g . , ~0.2 /~at 650 kV). T h u s , the t y p i c a l s e l e c t e d a r e a d i f f r a c t i o n (SAD) p a t t e r n of l a th m a r t e n s i t e o b - t a i n e d wi th a 100 kV m i c r o s c o p e c o n t a i n s o v e r l a p p i n g d i f f r a c t i o n p a t t e r n s f r o m s e v e r a l c r y s t a l s ( l a ths ) and o f t en c o n s i d e r a b l e a r c i n g o c c u r s in t h e p a t t e r n a s a r e s u l t of t he i n t e r n a l S t r a i n s and m i n o r o r i e n t a t i o n d i f f e r e n c e s of a d j a c e n t l a t h s . A l l t h e s e f a c t o r s c o n - t r i b u t e to the d i f f i cu l t y in i d e n t i f y i n g any a u s t e n i t e r e - f l e c t i o n s . As s h o w n e a r l i e r w h e r e m u l t i p l e t w i n n i n g in p l a t e m a r t e n s i t e w a s found ~ h igh v o l t a g e e l e c t r o n m i c r o s c o p y i s of g r e a t a d v a n t a g e f o r i d e n t i f y i n g f ine d e t a i l s , e s p e c i a l l y a s m a r t e n s i t e l a t h s a r e f r e q u e n t l y 0.5 /~ o r l e s s in w id th and so s e v e r a l l a t h s n o r m a l l y c o n t r i b u t e to 100 kV e l e c t r o n d i f f r a c t i o n p a t t e r n s (e .g , s e e F i g . l (b ) ) .

A s a r e s u l t of t h e s e f a c t o r s a t y p i c a l 100 kv s e l e c t e d a r e a d i f f r a c t i o n (SAD) p a t t e r n can c o n t a i n t he f o l l o w - ing:

a) One o r m o r e m a r t e n s i t e o r i e n t a t i o n s i n c l u d i n g tw in r e l a t e d o n e s ,

b) Doub ly d i f f r a c t e d s p o t s . c) F a i n t a u s t e n i t e r e f l e c t i o n s . d) C a r b i d e r e f l e c t i o n s (e ,g . 3 out of 6 p o s s i b l e v a r i -

a n t s of M3C wi th {ll0}c~ hab i t o r 3 out of 12 p o s s i b l e v a r i a n t s wi th {112}c~ hab i t ) .

e) M u l t i p l e d i f f r a c t i o n due to i n t e r a c t i o n s f r o m the

a b o v e . T h i s l a t t e r p r o b l e m (e) h a s a l r e a d y b e e n e m p h a s i z e d

f o r i n t e r p r e t a t i o n of p h a s e s in m a r a g i n g s t e e l s by C h e n g and Thomas .12 A s an i l l u s t r a t i o n of t h e s e e f f e c t s F i g . l ( a ) s h o w s a c a l c u l a t e d c o m p o s i t e p a t t e r n of r e - t a i n e d a u s t e n i t e , c e m e n t i t e and m a r t e n s i t e . T h e c o m - m o n o r i e n t a t i o n r e l a t i o n s h i p s w h i c h have b e e n o b - s e r v e d in the s t e e l s , a r e t he K u r d ] u m o v - S a c h s f o r m a r t e n s i t e - a u s t e n i t e and 13agarya t sk i f o r c e m e n t i t e - m a r t e n s i t e and t h e s e have b e e n u t i l i z e d to p lo t F ig . l ( a ) . F i g u r e l (b) s h o w s an o b s e r v e d t y p i c a l e l e c t r o n

oi~ ~ i f o #.]i

2 I I "/l l~ �9 []11] MARTENSITE

(OLT&x II (dl)>. o [(311] AUSTENITE x [Oi'O] C E M E N T I T E

{ T l i } ~ II (011) 7

(oTI)~x II (BOO) c

(hl)~ II (O[O)c 12r l )~ II (OOl)c

(a) l v) Fig. 1--(a) Calculated e lec t ron diffraction pat tern for mar tens i t e in [i111orientation with retained austenite and Widmanstat ten cement i te . (b) Indexed e lec t ron diffraction pat tern of austenite (a) and mar tens i te (nz) ref lect ions in Fe/1Cr/1Mo/0.3C s teel quenched from 870~ into ice water (see Fig. 2).

440-VOLUME9A, MARCH 1978 METALLURGICAL TRANSACTIONS A

Fig. 2--Bright field image (a) and dark field image (b) of an Fe/1Cr/1Mofl).3C steel quenched from 870~ into ice water. Note that the interlath retained austenite films do not show good contrast in the bright field image, but are very clear in the (200)'y dark-field image. The corresponding diffraction pattern is shown in Fig. l(b).

d i f f ract ion pa t t e rn at 100 kV obtained f rom an a r e a containing only re ta ined aus ten i te and lath ma r t e ns i t e . The re a re two lath m a r t e n s i t e o r i en ta t ions cont r ibu t - ing to the pa t te rn , viz, (100)~ and (111)~ and the sym- m e t r i c a l {110}7 is su i tably indexed.

The p rob lem to prove the exis tence of aus teni te by da rk - f i e ld imaging is the choice of the co r r ec t r e f l ec - t ion af ter ident i fying the pa t t e rn and d i s t ingu ish ing aus ten i te r e f l ec t ions f rom al l o thers . Obviously, the choice of an aus ten i te re f lec t ion fur thes t r emoved f rom any others is needed espec ia l ly in view of the p rob l em of sphe r i ca l abe r ra t ion . F r o m Fig. 1 it is c lear that (200)V has a r e l a t ive ly la rge angular s e pa r a - t ion f rom other re f lec t ions and is the re f lec t ion to be u t i l ized. This re f lec t ion is also far enough removed f rom spots due to double d i f f ract ion which commonly occur close to the t r a n s m i t t e d beam. It is n e c e s s a r y to use long exposure t imes taken at appropr ia te con- dense r lens defocuss ing se t t ings for both the SAD pa t t e rn and da rk - f i e ld (DF) image. Imaging exposures of 30 s or so a re requ i red . F igure 2 shows an example of such an ana lys i s in which the (200)~ da rk field image c lea r ly r evea l s the in te r la th aus teni te f i lms

which a re more or less continuous throughout the packet, is Complete ana lys i s of the m i c r o s t r u c t u r e in fact r e q u i r e s s e v e r a l DF images of the same a r ea to be made in o rder to d i s t inguish f i lms of aus teni te , M3C or in te r face double diffract ion. For example , Fig. 3 shows an image where an in te r l a th M3C is found in an i so the rma l ly t r a n s f o r m e d s t ee l# 4 It can be desc r ibed morphologica l ly as upper baini te . Compar ing Fig . 3(b) with Fig . 2(b) it is c lear that one can not deduce the p r e s e n c e of r e t a ined aus teni te just f rom the con t ras t and morphology alone but a deta i led d i f f rac t ion and da rk - f i e ld ana lys i s is r equ i red . Another example to emphas ize this point is shown in Fig . 4, which shows that double d i f f ract ion gives in te r l a th boundary con- t r a s t ~3 which is not an i n t r i n s i c s t r u c t u r a l fea ture . F igu re 4(a) is the B F image of twin r e l a t ed laths in a packet m a r t e n s i t e the two va r i an t s of which a re r e - vealed in F igs . 4(b) and (c). F igu re 4(d) is obtained f rom a doubly diff racted re f l ec t ion whereby the i n t e r - faces between the two va r i an t s a re imaged. The cor - r e spond ing di f f ract ion pa t t e rn Fig. 4(e) is ana lyzed in Fig. 4(f). F i g u r e s 3 and 4 emphas ize that da rk - f i e ld imaging in t e rp re t a t i on r equ i r e s unique ident i f ica t ion

METALLURGICAL TRANSACTIONS A VOLUME 9A, MARCH 1978-441

of the o r ig in of the di f f ract ion con t r ibu t ing to that con- t r a s t . Thus , ve ry carefu l t r a n s m i s s i o n e l ec t ron mi - c roscopic ana lys i s is r equ i r ed in o rde r to unambigu- ously identify the p r e sence of thin f i lms of r e t a ined

Fig. 3- -Bain i te in Fe/4Cr/0 .4C s tee l i s o t h e r m a l l y t r a n s - f o r m e d at 400~ for 5 rain. B F (a) and DF (b) i m a g e s of a ca rb ide re f lec t ion .

442-VOLUME 9A, MARCH 1978

aus teni te in s t ee l s when the volume f rac t ion is of the o rder of 0.02 to 0.05.

RESULTS ON EXPERIMENTAL STEELS

1) F a c t o r s Affect ing Reta ined Austeni te and Its Signif icance on Toughness

The obse rva t ions suggest that at leas t qual i ta t ive ly the f r ac tu re toughness p rope r t i e s of m a r t e n s i t e s tee l s a re improved due to the p r e s e n c e of s table in te r la th f i lms of aus ten i te . An impor t an t des ign p a r a m e t e r is the re fo re the control of such aus teni te as outl ined in the following.

a) Effect of Composi t ion. It is well known that in- c r ea s ing the carbon content of s tee ls above 0.5 pct in- c r e a s e s the volume f rac t ion of r e t a ined aus ten i te following quenching to room t e m p e r a t u r e . The de t r i - men ta l effects of r e t a ined aus teni te in high carbon s tee l s where in it t r a n s f o r m s to plate (twinned) m a r t e n - s i te under the inf luence of an applied load or i so the r - mal ly a re well recognized, (see e.g. Ref. 15). However, in the expe r imen ta l s tee ls developed over the past 12 ye a r s (Table I) where in the composi t ion of the s tee l is adjus ted so as to produce dis located m a r t e n s i t e , such danger emana t ing f rom undes i rab le t r a n s f o r m a t i o n products of uns table aus ten i te is avoided. The r e t e n - t ion of aus ten i te in the as -quenched m a r t e n s i t e can thus be benef ic ia l .

An in t e r e s t i ng example of the effect of composi t ion on the amount of re ta ined aus teni te and the consequent mechan ica l p rope r t i e s is obtained f rom the sys t ema t i c study which has been c a r r i e d out of m i c r o s t r u c t u r e - mechan ica l p roper ty r e l a t ionsh ips of expe r imen ta l Fe/C/X t e r n a r y al loys where X is a subs t i tu t iona l a l loying e lement . These al loys a re l i s ted in Table I. F igu re 5 shows that for a given s t rength level, the plane s t r a i n f r ac tu r e toughness values of F e / C r / C s tee l s a re higher than those of F e / M o / C al loys . In both cases the m a r t e n s i t e is dis located. Careful e lec- t ron meta l lography and dif f ract ion had revea led 4,9 the exis tence of in t e r l a th f i lms of aus teni te in the f o r m e r whereas aus teni te could not be detected in the la t te r . The observed d i f fe rences in the amounts of r e - ta ined aus ten i te in Fe/Cr/C s tee ls vs Fe/Mo/C s tee l s can be par t ly reconc i led with the fact that Cr is much less potent than Mo in l imi t ing the aus teni te phase field, is S imi la r ly , extens ive amounts of i n t e r l a th r e - ta ined aus ten i te has been detected in F e / 1 C r / 1 M o / 0.3C s tee l s (Fig. 2) and again these s tee l s showed 13 much s u p e r i o r Charpy impact ene rg ie s compared to the F e / M o / C s tee l s .

Mn and Ni a re s t rong aus ten i te s t a b i l i z e r s and a re expected to promote re t a ined aus ten i te in the a s - quenched s t r u c t u r e s if added to a s teel . In the p re sen t s tudies on s e v e r a l spec imens and s e v e r a l d i f fe rent reg ions within each spec imen, re ta ined aus teni te is obse rved to i n c r e a s e monotonical ly with Mn addit ion (up to 2 pct Mn studied) to the t e r n a r y F e / 4 C r / 0 . 3 C alloy. An example is shown in Fig. 6. A 5 pct Ni addi- t ion to this t e r n a r y alloy also inc reased the volume f rac t ion of re ta ined aus teni te about 10 t imes compared to the base alloy, as indicated in Fig. 6(e).* Ana lys i s

*These results have been independently corroborated by the x-ray analyses of our alloys by Dr. R. L. Miller, (U.S. Steel). More details will be published later.

of F igs . l(b) and 7~) shows that the Kurd jumov-Sachs

METALLURGICAL TRANSACTIONS A

Fig. 4--As-quenched mar tens i te in Fe/1Cr/1Mo/0.3C alloy. (a) B F image of the twin related mar tens i te laths, (b) D F image of one twin orientat ion using ref lect ion A in ( f ) , (c) D F image of the other twin orientation using reflect ion B in ( f ) , and (d) D F image of a doubly diffracted beam C in ( f ) . (f) is the analysis of SAD pat tern in (e).

METALLURGICAL TRANSACTIONS A VOLUME 9A, MARCH 1978-443

~22

li'2 DO " ~ - ~ , 222

/ . J,;~176 I / ! - - /

- - - , i z

o [110] MARTENSITE �9 [110] TWIN-RELATED M. x DOUBLE DIFFRACTION

f Fig. 4-Continued.

o r ien ta t ion re la t ionsh ip between fcc aus ten i te and m a r t e n s i t e is obeyed. The s igni f icance of this in- c r ea se in pct aus teni te with r e spec t to mechan ica l p rope r t i e s is i l l u s t r a t ed in Fig. 7 which shows the in-

E 16C

Z

~2C

o H- ~ 8C

u~ c

u ~

o

E O,~oo

Ult imate Tensile Strength, ksi 250 300 _~

I I I ] i E Experimentol Steels :'---

A: Fe/4 Cr/O 35 C 120 ~ - - I hr at B: Fe/2 Mo/04 C ~400OC l hr OI �9 Untempered

80 ~

4 Commercial Steels ? ~ @-"./" > Quenched and / ~.~<11 j I.~

" J J - / / ' " c ~ < Tempered. / 4 0 5

o

t I I i I ~ I I 0 16(30 1800 2000 2200 2400

Ultimote Tensile Strength, MN/m 2

Fig. 5--Relationship between plane strain fracture toughness and ultimate tensile strength. Note that the Fe/Cr/C ailoy has superior properties to the Fefl~io/C steel and to other com- mercially available high strength steels.

c r ea se in Charpy impact energy with Mn addit ion to the t e r n a r y alloy. T e r n a r y al loys modified with Ni a lso showed improved impact toughness p rope r t i e s . Thus, the composi t ion of the s teel , in addit ion to in- f luencing m a r t e n s i t e subs t ruc tu re , plays a pa ramount ro le in cont ro l l ing the amount of aus teni te in the al loy and consequent ly the mechan ica l p rope r t i e s .

b) Influence of H e a t - T r e a t m e n t and Gra in -S ize . H e a t - t r e a t m e n t a lso plays a key role in de t e rmin ing the amount of re ta ined aus ten i te . Aus ten i t i z ing tem- pe r a t u r e s high enough to d issolve a l l the carb ides so keeping all the a l loying e lements in solut ion without undue g ra in growth a re observed to inc rease the amount of r e t a ined aus teni te . 4'6'17 F r o m a study of the effect of aus ten i t i z ing t r e a t me n t on the mic ro - s t r u c t u r e and p rope r t i e s of t e r n a r y F e / C r / C s tee ls , 17 it was concluded that r e ta ined aus teni te occurs in g rea te r amounts in the high t e m p e r a t u r e (>1000~ aus ten i t i zed s tee l s as compared to low t e m p e r a t u r e (~870~ t rea ted s t ee l s . These r e su l t s a re expected as a r e su l t of d isso lu t ion of M7C3 type carb ides at higher t e m p e r a t u r e s . If the a l loying e lements a re par t ly locked up in ca rb ides and other p rec ip i t a t e s , they wil l obviously have less effect on the amount of aus ten i te following quenching. In the above s tee ls the KIC f r ac - ture toughness improved with aus ten i t i z ing t e m p e r a - tu re unt i l l l00~ although it could not be a t t r ibu tab le s ingly or s igni f icant ly to the i n c r e a s e in the amount of aus teni te . Exper imen ta l double t r e a tmen t s which combine the benef i ts of d i sso lv ing al l the al loy ca r - bides and a fine g ra in size showed the highest amounts of r e t a ined a u s t e n i t e ) 3'18 The s tab i l i za t ion of aus ten i te by closely spaced boundar ies in a fine gra ined s tee l a re well documented in the l i t e r a tu re , z9 Thus , if a l l carb ides a re in solut ion, a fine gra ined s tee l would be expected to contain more re ta ined aus teni te compared to a coarse gra ined s t r uc t u r e as is now observed.

It was also found that subs tan t i a l amounts of re ta ined aus teni te can be in t roduced in the m i c r o s t r u c t u r e by holding the s teel in the mar t ens i t e fo rmat ion reg ion (Ms-Mf) dur ing quenching. 2~ The combined effects of f iner g ra in s ize and higher volume f rac t ion of r e - ta ined aus teni te yielded super io r impact toughness and s t reng th p rope r t i e s following gra in r e f i nemen t in F e / 1 C r / 1 M o / 0 . 3 C steels. /3

444-VOLUME 9A, MARCH 1978 METALLURGICAL TRANSACTIONS A

Fig. 6- -Var ia t ion of re ta ined aus ten i te in Fe/Cr/C t e rna ry al loys modified with Mn and Ni (a) D F image of aus ten i te in Fe / 4Cr/0.3C (b) D F image of aus teni te in s tee l modified with 0.5 pct Mn (c) DF of aus teni te in s teel modified with 1.0 pct Mn (d) DF image of aus ten i te in s tee l modified with 2.0 pet Mn (e) D F image of auster~ite in s tee l modified with 5.0 pct Ni {f) indexed SAD pat te rn with re ta ined aus teni te ref lec t ions {a) of modified s tee l with 2 pct Mn.

METALLURGICAL]RANSACTIONS A VOLUME 9A, MARCH 1978-445

Fig. 6-Cont inued.

2) T h e r m a l S tab i l i ty and T e m p e r M a r t e n s i t e E m b r i t t l e m e n t

In o r d e r to i m p r o v e the toughness to s t r eng th r a t i o (which in t u rn i n c r e a s e s the c r i t i c a l f law s ize) s t e e l s a r e usua l ly t e m p e r e d fol lowing quenching al though in 5s - many c a s e s the e x p e r i m e n t a l s t e e l s a r e des igned for good f r a c t u r e toughness p r o p e r t i e s in the a s - q u e n c h e d condi t ion 1 (see F ig . 5). However , upon t e m p e r i n g mos t of these s t e e l s exhib i t a t e m p e r m a r t e n s i t e e m b r i t t l e - ment a round 350~ t e m p e r i n g ( s o m e t i m e s r e f e r r e d to a s " t he 500~ e m b r i t t l e m e n t " in c o m m e r c i a l s t e e l s 2z) ~ 40 as s u m m a r i z e d in Tab le I , Such e m b r i t t l e m e n t i s a c - ~- companied by a lo s s of toughness a s shown in F ig . 8. ~, F i g u r e 8(a) shows a m i n i m u m in Charpy impac t ene rgy ~, in Fe/ICr/IMo/0.3C s t e e l fol lowing 350~ t e m p e r i n g while F ig . 8(b) shows e m b r i t t l e m e n t at 300~ in the t e r n a r y F e / 4 C r / 0 . 3 C s t e e l and q u a t e r n a r y F e / 4 C r / ~. 3C

E 0.3C + 0.5 pct a l l oys . Th i s e m b r i t t l e m e n t is not the c l a s s i c i n t e r g r a n u l a r e m b r i t t l e m e n t due to i m p u r i t y s e g r e g a t i o n a n d / o r p r ec ip i t a t i on a t the g r a i n bound- a r i e s , 22 i.e., the f r a c t u r e s a r e not n e c e s s a r i l y i n t e r - g r a n u l a r , and occu r even in the ca r e fu l l y vacuum me l t ed e x p e r i m e n t a l s t e e l s whe re in the i m p u r i t y con- 20 tent is much lower than mos t c o m m e r c i a l s t e e l s . The e m b r i t t l e m e n t in these s t e e l s is t r a n s g r a n u l a r and i s a t t r i bu t ed to the decompos i t i on of r e t a i n e d aus t en i t e

0 with the f o r m a t i o n of i n t e r l a t h cemen t i t e (or M3C) f i l m s as shown by the de t a i l ed e l e c t r o n m i c r o s c o p y a n a l y s i s of F ig . 9. Such t e m p e r e d s t r u c t u r e s a r e v e r y much l ike those of upper ba in i t e and a r e c l e a r l y un- d e s i r a b l e ( c o m p a r e to F ig . 3). T y p i c a l f r a c t o g r a p h s

T I f I ,,t.

~ hed

tempered

- 7 O

- 60

W

4o ~. E

- 30

@ 5 Ni olloy 200~ temper

5 Ni" As quenched

1.0 2.0

Wl % Mn

/II 20

Fig. 7- -Var ia t ion of Charpy impac t energy with Mn addition to the t e rna ry Fe/Cr/C s t ee l s . The data for 5 pct Ni addition is also included. All the s t ee l s are gra in ref ined by double t rea t ments .

446-VOLUME 9A, MARCH 1978 METALLURGICAL TRANSACTIONS A

140, I I I [ I I - ~ 1 8 0

120 - ] l 60

=" #~140 ~_ IOC

120 i

u5 80 ! ~, ilO0

o e~ W

e 60 8o "G o

~ e i.

= 60 ~ 40

2O

2O

0 I L ] I I [ 0 0 I00 200 300 400 500 600 700

Tempering Temperature (~ (a)

4O I I I I ]

3O

I

t o A (Fel4Cr 10.26C) [] B (Fe14Cr]0.24C I 0.54 Mn)

0 I I I I $ 200 400 As Quenched Tempering Temperature (~

(b) Fig. 8--Charpy impact energy variation with tempering in (a) Fe/1Cr/1Mo/0.3C steels (b) Fe/4Cr/0.3C and with 0.5 pct Mn alloys.

5O

#0

30 ~

w

2o ~

- I 0

I 0 60O

are shown in Fig. 10 for F e / C r / C s tee l s modified with Mn. F igu re s 10(a) and (c) a re the f rac tographs follow- ing 200~ t emper ing of 0.5 pct Mn and 2 pct Mn qua t e rna ry al loys while (b) and (d) a re the f rac to - graphs, r espec t ive ly , of the same al loys following 300~ temper ing . At 200~ the p redominan t f r ac tu re

mode is dimpled rup ture while at 300~ the f r ac tu re is main ly quas ic leavage . The f r ac tu r e behavior of these al loys co r r e l a t e s d i rec t ly with the decompos i - t ion c h a r a c t e r i s t i c s of aus teni te on t emper ing . At 300~ when most of the in te r l a th aus ten i te was found to be decomposed, the f rac tographs (Figs . 10(b) and (d)) can be in t e rp re t ed in t e r m s of f r ac tu re paths along the la th /packet boundar ies as a consequence of the p r e s e n c e of carbide pa r t i c l e s at these boundar ies .23 Thus the f r ac tu r e path is i n t e r g r a n u l a r with r e spec t to m a r t e n s i t e but of course t r a n s g r a n u l a r with r e spec t to p r io r aus ten i te .

It appears that although gra in r e f inemen t could de- c r e a s e the sever i ty of embr i t t l emen t , 18 the t h e r m a l s tab i l i ty of the r e t a ined aus ten i te was unaffected and the actual o c c u r r e n c e of the toughness m i n i m a could not be a l levia ted even in fine gra ined a l loys .

Thus, whatever the benef i t s of r e t a ined aus teni te a r e to toughness, these a re lost if the aus teni te be- comes uns table . Alloying addit ions which favor aus - teni te s tab i l i ty should, therefore , improve f r ac tu re p rope r t i e s upon t emper ing . In the F e / 4 C r / 0 . 3 C alloy modified with qua t e rna ry addit ions of 5 pct Ni, r e ta ined aus teni te appears to be more s table and apprec iab le amounts of un t r a ns f o r me d aus teni te a re not iceable even up to 400~ Consequent ly, the t empe red m a r - tens i te e m b r i t t l e m e n t in this case is ve ry mild. Nickel is thus ve ry benef ic ia l in this r ega rd (Table I). At the p r e s e n t t ime the mechan i sm of the decompos i - t ion of aus teni te to form carb ides is not known.

It was pointed out e a r l i e r that no re ta ined aus teni te could be detected in the F e / M o / C al loys . Cons is ten t with this observa t ion , Fig. 11 shows that these al loys do not undergo t empered m a r t e n s i t e embr i t t l emen t . 24

DISCUSSION

The aus teni te s tab i l i ty with r e spec t to t r a n s f o r m a - t ion to m a r t e n s i t e on quenching mus t be re la ted to two main fac tors : a) between M s and Mf, the solute e le- ments , e spec ia l ly carbon wil l segregate to the aus- teni te lower ing i ts M s t empera tu re , so d r iv ing M f

local ly fu r the r and fu r the r down, eventual ly leading to reg ions of un t r a ns f o r me d aus teni te r e m a i n i n g between m a r t e n s i t e laths (chemical s tabi l iza t ion) , b) accommo- dation of t r a n s f o r m a t i o n s t r a i n s by p las t ic de fo rma- t ion genera t ing d is loca t ions in aus teni te could also s tab i l ize the aus ten i te through mechanica l s t ab i l i za - tion. 25'26 Observa t ions show that r e ta ined aus teni te is invar iab ly heavily defomred (Figs . 6(a) through (e)).

On the other hand, the aus teni te s tab i l i ty with r e - spect to decomposi t ion into a + M3C will dec rea se as the carbon content in aus ten i te i n c r e a s e s due to a higher chemica l d r iv ing force for carbide nuclea t ion . Likewise d i s loca t ions in aus teni te could a s s i s t he te ro- geneously in nuclea t ion of carb ides and the reac t ion 7 ~ a + M3C. The problem, therefore , is that the fac tors that m in imize the V ~ M s t r ans i t i on will favor the V ~ a + M3C t rans i t ion . There is a se r ious lack of expe r imen ta l evidence to document how the t r a n s f o r - mat ion of r e t a ined aus teni te is inf luenced by al loying-- whether it t r a n s f o r m s to f resh ma r t e ns i t e , or ba in i t i c f e r r i t e + M3C or n o r m a l f e r r i t e + M3C. It appears that a l loying addit ions which d iscourage cement i te nuc lea - t ion and growth can postpone the onset of t empered

M E T A L L U R G I C A L T R A N S A C T I O N S A V O L U M E 9A, M A R C H 1 9 7 8 - 4 4 7

oiT

Too ~

oTI

IoT

0

o [III} FERRITE �9 [0i2] CEMENTITE

d Fig. 9--Example of the decomposi t ion of re ta ined austeni te into blocky ]V[3C and f e r r i t e , (a) B F (b) D F (c) SAD (d) analys is in Fe/Cr/C s tee l s modified by Mn addition.

448-VOLUME 9A, MARCH 1978 METALLURGICAL TRANSACTIONS A

Fig. 10--Fractographs of Fe/4Cr/0.3C + Mn alloys (a) and (b) are of 0.5 Mn alloy at 200 and 300~ tempering respectively and (c) and (d) are of 2 pct Mn alloy at 200 and 300~ tempering respectively.

ma r t ens i t e e m b r i t t l e m e n t to a higher t e m p e r a t u r e . Examples of these e l ements f rom c u r r e n t r e s e a r c h at Berke ley include Ni, ~8 Si and A1. 27 The carbon diffu- s iv i ty in aus ten i te and m a r t e n s i t e as affected by the chemica l affinity between carbon and other a l loying e lements is perhaps impor tant . Mn i n c r e a s e s the ca r -

bon diffusivi ty and p romotes rapid nuclea t ion and growth of cement i te . Ni, A1 and Si do otherwise by d i scourag ing M3C nuclea t ion and growth to a higher t e m p e r a t u r e and thus, postpone the onset of t empered m a r t e n s i t e embrittlement.18,z7 F u r t h e r r e s e a r c h along these l ines wil l be f ru i t fu l in success fu l ly exploi t ing

METALLURGICAL TRANSACTIONSA VOLUME 9A, MARCH 1978-449

12 I I I I I r

15

10

A

~ s

~ 6

O O.

E , - , 4

2 - - V Fe [ 4 . 2 M o [ 0 . 4 1 C

o Fe P 2.2Moi0.45C

Ol I i J L t i I O As 200 400 600 Quenched

Tempering Temperoture, *C (I hr)

Fig. l l - - C h a r p y - V - n o t e h impac t toughness as a function of t e m p e r i n g t e m p e r a t u r e in Fe/lVIo/C a l loys .

z 0

t t J

E - - 5 w

t h e b e n e f i t s of r e t a i n e d a u s t e n i t e on m e c h a n i c a l p r o p -

e r t i e s . I n r e l a t e d r e s e a r c h , r e t a i n e d a u s t e n i t e in s t e e l

h a s a l s o b e e n s h o w n to b e b e n e f i c i a l f o r s t r e s s c o r - r o s i o n c r a c k i n g a n d it r e d u c e s t he t e n d e n c y f o r e n - v i r o n m e n t p r o n e e m b r i t t l e m e n t , 2s'29

SUMMARY AND CONCLUSIONS

1) Retained austenite exists in most as-quenched dislocated martensitic medium carbon alloy steels ex- cept Fe/Mo/C as stabilized fine interlath films. These films are usually detected only by detailed electron metallography.

2) These retained austenite films, provided they are stable, seem to be beneficial to fracture toughness.

3) Temper martensite embrittlement occurs when retained austenite decomposes at the lath boundaries to form M3C. Alloying elements which disfavor M3C precipitation and its growth appear to postpone the onset of the embrittlement to a higher tempering tem- perature. Thus, Mn, Cr promote its decomposition at a lower temperature while Si, A1 and Ni postpone it to a higher temperature.

4) Those factors which promote austenite retention on quenching also favor its thermal instability on tem- pering.

5) These results on retained austenite indicate that the terminology, lath martensite, is probably incor- rect and that each martensite crystal within a packet is independently nucleated. This aspect is discussed elsewhere 32

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

I a m e s p e c i a l l y g r a t e f u l to t h e g r a d u a t e s t u d e n t s w h o

h a v e c o n t r i b u t e d s o m u c h to t h i s a l l o y d e s i g n p r o g r a m

o v e r t h e y e a r s . I p a r t i c u l a r l y t h a n k B . V . N . R a o f o r

h i s a s s i s t a n c e w i t h t h i s p a p e r . T h e c o n t i n u e d f i n a n c i a l

s u p p o r t of E R D A t h r o u g h t h e L a w r e n c e B e r k e l e y

L a b o r a t o r y a n d t h e s t e e l c o m p a n i e s w h o h a v e p r o d u c e d

t h e a l l o y s to o u r s p e c i f i c a t i o n s ( T a b l e I) i s g r a t e f u l l y

a c k n o w l e d g e d .

R E F E R E N C E S

1. G. Thomas: Iron Sreel IntL, 1973, vol. 46, p. 451. 2. J. Nutting: J. Iron SteelInst., 1969, vol. 207, p. 872. 3. G. Thomas: Met. Trans., 1971, voi. 2, p. 2373. 4. J. McMahon and G. Thomas: Proc. International Conference on The Micro-

structure andDesign o f Alloys, Cambridge, vol. 1, p. 180, Inst. of Metals, London, 1973.

5. B. V. Narasimha Ran, J. Y. Koo, and G. Thomas: Proceedings Electron Microscopy SocieO, o f America, 33rd Annual Meeting, p. 30, Claitors Publish- ing Division, Baton Rouge, 1975.

6. G. Y. Lai, W. E. Wood, R. A. Clark, V. F. Zackay, and E. R. Parker: Met. Trans., 1974, vol. 5, p. 1663.

7. D. Webster:Met. Trans., 1971,vol. 2, p. 2097. 8. R. O. Ritchie and R. Horn: University of California, Berkeley, unpublished

research, 1977. 9. R. A. Clark and G. Thomas: Met. Trans. A, 1975, vol. 6A, p. 969.

10. R. L. Mille r: Trans. ASM, 1964, vol. 57, p. 892;ibid., 1968, vol. 61, p. 592. 11. G. Thomas and S. K. Das: Z Iron SteelInst., 1971, vol. 209, p. 801. 12. L-Lin Cheng and G. Thomas: Trans ASM, 1968, vol. 6 i, p. 14. 13. Y. L. Chen: Ph.D. Thesis, 1976, Univ. of California, Berkeley, LBL Report

Number 5423. 14. R. W. Miller: M.S. Thesis, 1974, Univ. of California, Berkeley, LBL Report

Number 3104. 15. G. Krauss and A. R. Marder: Met. Trans., 1971~ vol. 2, p. 2343. 16. E. C. Bain and H. W. Paxton: Alloying Elements in Steel, 2nd Edition, Ameri-

can Soc. for Metals, Metals Park, 1966. 17. M. F. Carlson: M.S. Thesis (in progress), Univ. of California, Berkeley. 18. B. V. Narasimha Rao: Ph.D. Thesis (in progress), Univ. of California, Berkeley. 19. W. C. Leslie and R. L. Miller: Trans. ASM, 1964, vol. 57, p. 972. 20. E. P. Klier and A. R. Troiano: Metals Technol., 1945, vol. 12, p. 1. 21. B. V. Narasimha Rao, R. W. Miller, and G. Thomas: Proc. o f l d t h International

Heat Treatment Conference on Heat Treatment '76, The Metals Society (London), p. 75.

22. J. R. Rellick and C. J. McMahon: Met. Trans., 1974, vol. 5, p. 2439. 23. T. B. Cox and J. R. Low: Met. Trans., 1974, vol. 5, p. 1457. 24. R. A. Clark: M. S. Thesis, 1973, Univ. of California, Berkeley, LBL Report

Number 1801. 25. P. M. Kelly and J. Nutting: J. Iron Steellnst., 1961, vol. 197, p. 199. 26. B. Edmondson and T. Ko: ActaMet., 1954, vol. 2, p. 235. 27. M. S. Bhat: Ph.D Thesis, 1977, Univ. of California, Berkeley, LBL Report

Number 6046. 28. D. Webster: AFC 77, 1969, Boeing Document D6-23973. 29. M. Castro: M S. Thesis, 1977, Univ. of California, Berkeley, LBL Report

Number 6012. 30. D. Huang and G. Thomas: Met. Trans., 1971, vol. 2, p. 1587. 31. S. K. Das and G. Thomas: Trans. ASM, 1969, vol. 62, p. 659. 32. G. Thomas and B. V. N. Rao: International Conference on Martensitic Trans-

formation, Kiev, USSR, LBL-6242, May, 1977, (in press).

450-VOLUME9A,MARCH 1978 METALLURGICAL TRANSACTIONS A