EFFECT OF S AND C SEGREGATION ON GRAIN BOUNDARY PROPERTIES IN ULTRA HIGH PURITY IRON

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EFFECT OF S AND C SEGREGATION ON GRAIN BOUNDARY PROPERTIES IN ULTRA HIGH

PURITY IRON

M. Tacikowski, A. Kobylanski, M. Grabski

To cite this version:

M. Tacikowski, A. Kobylanski, M. Grabski. EFFECT OF S AND C SEGREGATION ON GRAIN

BOUNDARY PROPERTIES IN ULTRA HIGH PURITY IRON. Journal de Physique Colloques, 1990,

51 (C1), pp.C1-653-C1-658. �10.1051/jphyscol:19901103�. �jpa-00230010�

EFFECT OF S AND C SEGREGATION ON GRAIN BOUNDARY PROPERTIES IN ULTRA HIGH PURITY IRON

M. TACIKOWSKI, A. KOBYLANSKI* and M.W. GRABSKI

Institute of Materials Science and Engineering, Warsaw University of Technology, Narbutta 8 5 , 02-524 Warsaw, Poland

Ecole Nationale Superieure des Mines de Saint-Etienne, 158 Cours Fauriel, F-42023 Saint-Etienne, France

RBsume

-

L ' e f f e t de segregation de S e t C sur les proprietks des joints de grains (JDG) dans le f e r de t r e s haute purete a k t e examine du point de vue de leur r6le dans la rupture B. chaud. A basse teneur en S la f r a g i l i t e interganulaire e s t controlee par la prksence des prgcipites de AIN. L'addition de C ameliore la ductilitk deteriorke par la segregation du S. Le carbon change le mecanisme de la rupture. La presence de C e t de S decroit la d i f f u s i v i t e des JDG e t emp@che leur migration. Cependant L'addition de C attenue I ' e f f e t particulierement f o r t du S sur la migration des JDG.

Abstract

-

U l t r a high p u r i t y iron doped with S and C was investigated from the point o f view o f the influence o f S and C segregation on grain boundary (GB) properties related t o high temperature fracture. At low S contents intergranular f r a c t u r e was found t o be controlled by GB precipitates presence (AIN). C addit ion improves high temperature ductility deteriorated by S segregation. C and S e f f e c t on grain boundary properties seems t o be complex. Both elements were found t o decrease GB d i f f u s i v i t y and inhibit GB migration. S e f f e c t on migration is particularly strong, but it is healed by C presence. C addition seems t o change fracture mechanism from mechanical decohes ion on GB towards diffusional voids format ion and growth. This e f f e c t combined with C influence on migration and cohesion are favourable f o r good high temperature ductility.

I - INTRODUCTION

In numerous steels and other alloys certain elements segregate t o the grain boundaries (GB) o f t e n modifying their properties and in consequence influencing polycrystal properties. The classical cases o f practical importance are the phenomena o f high temperature intergranular fracture induced by segregation. Although in the past t h i s problem was widely investigated and d i f f e r e n t models o f fracture mechanism were proposed [l-31, the phenomena is probably much more complex and certain important quest ions s t i l l remain unanswered. In general most o f the proposed models explain hot-brittleness in terms o f voids nucleation on GB precipitates, as well as by GB cohesion loss provoked by segregation o f certain elements.

Hence, more sophisticated approach should be developed, taking into consideration the contribution o f individual GB properties t o the f r a c t u r e phenomena (41. In f a c t GB could be more o r less susceptible t o segregation and fracture. It seems essential t o study what kind o f features determine GB population character formed in polycrystal and especially, what is the role of segrega t ion.

There a r e also other possible segregation e f f e c t s t h a t should be considered: segregation can a f f e c t not only GBs cohesion, but also t h e i r d i f f u s i v i t y and consequently all d i f fusional processes related t o high temperature intergranular fracture. The examination o f the segregation influence on GB d i f f u s i v i t y may help t o explain why certain elements have positive whereas the other detrimental e f f e c t on f r a c t u r e susceptibility o f GB. Furthermore, such phenomena as cosegregation o r s i t e competition o f d i f f e r e n t acting elements have t o be taken into account.

Present work is an attempt t o give a contribution t o some o f the mentioned questions concerning relation between d i f f e r e n t possible segregation e f f e c t s on GB properties and high temperature fracture. To reveal the specific e f f e c t of examined elements the Investigations were performed on model alloys of u l t r a high p u r i t y iron prepared in Ecole Nationale Superieure des Mines de Sa int-Et ienne.

2

-

EXPERIMENTAL

The iron alloys were prepared by doping the u l t r a high p u r i t y iron with controlled quantities o f carbon and/or sulphur (Table 1 ). Those elements are known t o segregate on GB and have an opposite e f f e c t on high temperature ductility 151. Some alloys were also doped with AI and N t o

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19901103

Cl-654 COLLOQUE DE PHYSIQUE

examine grain boundary precipitates effect. In order t o study separate and then simultaneous e f f e c t o f t h i s mtcrostructure elements three group o f alloys were prepared:

-

alloys doped with sulphur and carbon (segregation);

-

alloys containing

AN

(precipitation);

-

alloys containing simultaneously AIN, C and S (precipitation and segregation).

It has t o be stressed t h a t the employment o f high p u r i t y alloys assure t h a t the investigations a r e not affected by an eventual e f f e c t o f uncontrolled trace impurities. The alloys were cold forged o r hydrostatically extruded and then heat treated as indicated in Table l and 2.

The high temperature ductility in f e r r i t e region was investigated by hot tensile t e s t s in vacuum o r argon atmosphere a t 8 = 2 ~ 1 0 - ~ and 2x10-". The reduction o f area a t f r a c t u r e (RA) was used as a ductility measure. Then f r a c t u r e surface as well as microstructure were examined by SEM, TEM and optical microscope. In order t o characterize the influence o f C and S segregation on GB properties the method based on TEM observation o f trapped lattice dlslocat ion (TLD) temperature stability C61 was applied.

3

-

RESULTS AND DISCUSSION 3.1 High temperature ductility.

in most o f published works h o t ductiltty o f steels was studied in y o r a-y range.

However as the y/y GB cannot be preserved t o room temperatures and thus couldn't be subjected t o direct exam~nations in TEM, then consequently t h i s work concerns a-phase reglon.

One can notice from Fig.1 t h a t f o r S content

<

50ppm (FS30, FSSO), alloys display quite high ductility. However, simultaneous presence o f AIN, even f o r S content as low as about loppm

(AINSIO), leads t o tne drastic ductility loss. It is due t o intergranular f r a c t u r e which occur by GB voids formation and growth. It is important t o note t h a t AIN presence without simultaneous S segregation is neutral f o r h o t ductility. Thus brittleness results from combined actton o f AIN and sulphur. As brlttleness induced by combined action was already revealed in other works in austenite range f o r d i f f e r e n t kinds o f precipitates C3,71, the actual result demonstrates the general character o f t h i s phenomenon. The same f r a c t u r e model, as proposed in our previous work E31, can be adopted t o explain the results (Fig.2). In fact, AIN precipitates were observed on a/a GB and sulphur segregation was revealed. Therefore, fracture can be explained by the interference o f stress concentration on the precipitates and GB cohesion loss provoked by sulphur segregation. Both factors are necessary f o r voids format ion.

At higher sulphur content (100ppm) intergranular brlttleness IS observed even when AIN precipitates are absent in the alloy (FS100). In t h i s case GB boundary must be strongly weakened by sulphur segregation in spite o f relatively high carbon content in atloy. George e t al. 151 reported t h a t adequate carbon addit ion can improve hot duct il lty. The mechanlsm o f t h IS favourable lnf luence o f carbon on h o t ductility is not clear and it may be more complex than the simple C cosegregation e f f e c t on GB cohesion usually suggested. In f a c t the high temperature deformation involves d i f f e r e n t diffusional processes and in consequence e f f e c t o f both segregated species should be reviewed in t h i s aspect. Further p a r t o f t h i s work aims a t the determination o f different posslble e f f e c t s o f sulphur and carbon segregation on GB properties whtch are related t o high temperature ductility.

In order t o reveal specific and combined influence o f C and S, second group o f iron microalloys was tested Fig.3. &re iron exhibits good hot ductility. Carbon alone (FC200) has no e f f e c t on hot ductility and it is only slightly a f f e c t e d a t low sulphur content (FS20CO) and thereby a t low segregation level. Both sulphur

-

iron (FSIOOCO) and sulphur

-

carbon doped iron (FS100C200) are b r i t t l e a t s t r a i n r a t e 2=2x10--, however s t r a i n r a t e increase t o t=2x10-3 provokes significant ductility improvement when carbon is present in alloy.

3.2.Fracture surface character

A t low sulphur content, also in samples with no carbon (FS20CO1, f r a c t u r e is of dimple

-

ductile type. It originated from voids formation on GB weakened by sulphur atoms. However, GB destruction by voids formation and growth is probably too slow t o produce intergranular fracture.

Furthermore, it may be modified by compet it ive healing phenomenon cons 1st ing in voids separation from GB in consequence o f their migration. For both alloys with high S content (FSIOOCO, FS100C200) fracture is intergranular a t 0=2x1 0-4, however, there is a noticeable difference in fracture surface appearance - in FSlOOCO it is smooth even when observed a t high magnifications Fig.4, whereas when carbon is present grain surface is corrugated, Fig.5. It may suggest d i f f e r e n t f r a c t u r e mechanlsm:

-

mainly mechanlca! decohesion on weakened GB in absence o f C and diffusional void formation and growth when C 1s present. Thls thesis seems t o be confirmed by the lack o f essential ductility improvement f o r FS1OC)CO alloy with s t r a i n r a t e increase t o 6=2x10-" (Fig.3) and visible change f o r FS100C200 associated with t r a n s i t ion t o mixed dimple-ductile and intergranular f r a c t u r e F ig.6.

S segregation was revealed by etching with specific reagent. Presence o f intergranular carbides precipitated during cooling a f t e r treatment in solid solution range, shows t h a t carbon also segregate t o the GB. When both elements were simultaneously present in alloy the s i t e competition was observed (FS30): a f t e r recrystallizat ion carbon atoms, which were the f i r s t t o segregate were successively displaced by sulphur. However, small carbides remaining on GB indicated the existence o f a certain level o f cosegregation. C and S strongly inhibit grain growth in recrystallized iron previously hydrostat icaly estruded (Fig.7). The grain size is extremely stable when sulphur is present a t about 100ppm content (Fig.7c,e). I t ' s interesting t o point out t h a t carbon addit ion improves slightly the capability o f GB f o r migration (Fig.7d). S t i l l the mechanism o f C and S e f f e c t on GB migration is n o t clear. One o f the possible explanation may be related t o the modification o f GB properties by segregation itself. Other interpretation, in terms o f the selection of low mobility GB during recrystallization, may be also considered.

3.4.Grain boundary d i f f u s i v i t y

This p a r t o f work resumes our preliminary attempt t o investigate C and S e f f e c t on GB diffusivity. GB d i f f u s i v i t y modif icat ions were estimated from TLO s t a b i l i t y observations, which, as it is commonly admitted depend directly on GB d i f f u s i v i t y 161. TLD spreading during short annealing was observed on recrystallized and then slightly deformed (€=l%) samples o f iron and FC200, FSlOOCO, FSlOOC2OO alloys. In all samples the same C 1 l 0 1 recrystallization texture was found. The percentage o f GB free from TLD was taken as a s t a b i l i t y measure. The TLD s t a b i l i t y data disposed on present stage o f our work is collected in Table 2 and compared on Fig.8. This comparison nas rather qualitative character and represents r e i a t ive change o f TLD stability, taking as a reference the u l t r a high p u r i t y iron behaviour. It is clearly visible t h a t both S and C strongly increase TLD stability, which is related t o the GB d i f f u s i v i t y decrease. Carbon e f f e c t seems t o be very strong.

The later result is contradictory with those o f other authors C8,93 however the comparison is not direct because o f the important difference o f chemical composition and p u r i t y o f investigated materials. The tendency f o r simultaneous S and C presence (FSlOOC200) is n o t clear and f u r t h e r systematic observat~on are in project. A t present stage o f the work it is d i f f i c u l t t o explain if the d i f f u s i v i t y decrease is produced by segregation itself o r rather it results from high f r a c t i o n o f low d i f f u s i v i t y GB in the GB population produced by S o r C presence in alloy during recrystallization and grain growth. The determination o f GB population character could be useful there.

4 - CONCLUSIONS

1. During high temperature deformat ion, f r a c t u r e mechanism and therefare hot ductility will result from the competition o f two processes: GB destruction by mechanical decohesion o r voids formation and growth and healing phenomena a t GB. The kinetics o f those processes must depend on GB properties. Our preliminary results let us corroborate t h a t carbon seems t o exercise a complex positive influence on GB properties in iron, which can explain the ductility improvement observed when C was present in alloy. Carbon segregation increase GB cohesion (as reported in literature) and decreases also GB diffusivity. Both e f f e c t s result in the decrease o f GB destruction r a t e by voids formation. Parallely the capability o f GB f o r migration, being potential healing factor, is also improved when carbon is present in alloy containing sulphur. On the contrary, sulphur decrease GB cohesion and strongly deteriorate the capability f o r GB migration both features being favourable f o r brittleness. Sulphur decrease also GB diffusivity, however f u r t h e r quantitative investigations should show if it is modif ied by a simultaneous carbon addition and how t h i s could contribute t o the ductility improvement. Finally it may be suggested t h a t carbon and sulphur could influence, in different manner the character o f GB population in polycrystal. The investigation o f the problem may be interesting f o r polycrystal design based on G8 properties control.

2. A t low sulphur segregation level intergranular brittleness o f iron appears only when precipitates o f AIN are present on GB. The suggested explanat ion could be the interference o f stress concentrat ion on precipitates with GB cohesion loss due t o sulphur segregation.

BIBLlOGRAPHY

1. H.SUZUK1, S.NISHIMURA, 1.lMAMURA and LNAKAMURA: Trans. iron Steel Inst. Japan, 24, 170, 1984.

2. P.P.MESSMER and C.L.BRIANT: Acta Metall. 32, 2043, 1984.

3. M.TACIKOWSK1, G.A.OSW0LU and A.KOBYLANSK1: Acta Metal/., 36, 995,1988.

4. T.WATANABE: Res. Merhanica, 11, 47, 1984.

5. E.P.GEORGE, P.L.LI and 0 . P . m : Acta Metall. 35, 2460, 1987.

6.

W.SWI~TNICKI,

W.COJKDWSK1 and M.W.GRABSK1: Acta Metall. 34, 599, 1986.

7. P.H.YI: thesis, Saint-Etienne, 1989.

8. S.LARTIGUE and L.WIESTER : Acta Metall., 31, 1809, 1983.

9. W.A.SWI~TNICKI and M.W.GRABSK1 : Acta ~Yeterll., 34, 81 7, 1986.

Cl-656 COLLOQUE DE PHYSIQUE

Tab1 e 1. Chemi c a l c o m p o s i t i o n (ppm) a n d t r e a t e m e n t o f a l l o y s .

ALLOY C ' S AI N TREATMENT

F P u l t r a p u r e F e 5 0 0 ° C / 0 , 5h

F C 2 0 0 2 0 0

-

-

-

h y d r o s t a t i c 7 3 0 0 C / 0 , 5 h

FS2OCO

-

2 0 - - 6 0 0 ° C / 0 , 5 h

F S l OOCO 1 0 0 - - e x t r u s i o n 7 5 0 0 C / 0 , 5 h F S 1 0 0 C 2 0 0 2 0 0 1 0 0 -

-

7 5 0 ° C / 0 , 5 h F S 3 0 2 0 0 3 0 -

-

^{c o l} d f o r g i ng

+

F S 5 0 2 0 0 SO - -

F S l O O 2 0 0 1 0 0 - - 1 3 0 0 ° C / l h

+

q u e n c h i n g

+

AI N 2 0 0

-

^{3 0 0 1 0 0}

AI N S l 0 2 0 0 1 0 3 0 0 1 0 0 + 8 5 0 0 C / l h AI NS30 2 0 0 30 3 0 0 1 0 0

R.Ab I '

1

500 600 70'0 800 900 Tpc l

Fig.1 Influence of S and A1N on hot ductility at C=ZXIO-~ .

100

40 20

RA[%

o- - -0-

-

^{- 0}

FP, FC 200

FS O ; CO 8 0

'01

^{@ \}

^{2 IO-~}

. ALN .-,-,x=

= - X

~ ~ 3 0 " '

^/'

ao-FSSOg, T,/

..

'

_\

^\

I

\ '

FSIOO*,

^A,

.mALNS10

..

.\

_+----+

^{- ALN S30}

500 600 700 800 900 T[yl Fig.3 Influence of S and C on hot ductility at &=2x10'4 .

xqt50 Fig.5 Corrugated grain surface For intergranular fracture of FSlOOC200 alloy tested at 700°C and &=2x10'~ .

Fig.2 The model fracture induced by combined R1N-S action.

x'iooo Fig.9 Smooth grain surface for intergranular Fracture of .FS100CO alloy tested at 700°C and &=2xl0'~ .

x500

Fig.6 Dlrnple-ductile region of

Fracture surface for FS100C200

tested at 700°C and&=2xloJ .

Cl-658 COLLOQUE DE PHYSIQUE

Fig.7 EFfect OF C and S on grain size after recrystallization at 750°/30rnin:

a )

FP-pure iron, b) FC200, c > FSlOOCO, d > FSlOOC200, el FSlOOCO after 29h

annealing at 800°C. Magnification 100x.

Table 2. TLD spreading data

...

FILLOY HEFIT TLD SPRERDING eb TLD Free

Meth. - methode of spreading

TREATnENT TIeCI t I s I n e t h . No NF L

... observation

FP ~ t s o * i i h 300 3 ~ n S A ~ U 99 2% 99

NO - number of observed g b

FP ' f s o * / l h 300 30 ~ n s i t u 25 20 80 N f

- number of TLD Free gb

FC200 6 0 0 5 ' l h 300 30 l n s i t u 27 0 0 FSlOOCO 885 */%h 300 30 i n s l t u 59 7 B FP 500 *i0,5h 300 60 ex p a s t 23 1 7 7Y FC200 730 * / 0 , 5 h 300 60 ex p o s t 7 0 0 FSlOOCO B85 *iO,Sh+ 300 60 ex p o s t 52 1 3 25

7 U 4 */a 4k

. .-

¹

-,-..

FS100C200 750 */0,5h 300 60 ex p o s t 25 7 28 FP 500.10.5h 325 300 ex p o s t 2 1 1 8 8 6 FC200 730 */O, Sh 325 300 ex p o s t 6 0 0 FSIOOCO BBS*/O, s h + 325 ' 300 ex p o s t 59 32 6 1

7 9 s * i 3 , 5 h

FSIOOCZOO 750-io, 5h 325 300 ex p o s t 18 6 3 3

...

Fig.8 EFFect of C a n d , S on TLD

stability -

%

of TLD free gb

at 300°C and 325'C Cdata from

Table 2).