HIGH TEMPERATURE MECHANICAL BEHAVIOUR OF CEMENTED CARBIDES WC - (6.5, 12, 15 AND 25) % Co

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HIGH TEMPERATURE MECHANICAL BEHAVIOUR OF CEMENTED CARBIDES WC - (6.5, 12, 15 AND

25) % Co

N. Bouaouadja, G. Orange, Gilbert Fantozzi, F. Thevenot, P. Goeuriot

To cite this version:

N. Bouaouadja, G. Orange, Gilbert Fantozzi, F. Thevenot, P. Goeuriot. HIGH TEMPERATURE MECHANICAL BEHAVIOUR OF CEMENTED CARBIDES WC - (6.5, 12, 15 AND 25) % Co.

Journal de Physique Colloques, 1986, 47 (C1), pp.C1-739-C1-743. �10.1051/jphyscol:19861112�. �jpa-

00225508�

HIGH TEMPERATURE MECHANICAL BEHAVIOUR OF CEMENTED CARBIDES WC - (6.5, 12, 15 AND 25) % CO

N. BOUAOUADJA, G . ORANGE, G . FANTOZZI, F. THEVENOT* a n d P. GOEURIOT'

Groupe d i E t u d e s de Metallurgie Physique et de Physique des Materiaux, C.R.R.A.C.S., U.A. 341, I.N.S.A., Bat. 502, F-69621 Villeurbanne Cedex, France

"E.N.S.M.S.E. - C.R.R.A.C.S., F-42023 Saint-Etienne, France

&sume - ^Le d u l e e l a s t i q u e e t la c o n t r a i n t e B l a rupture s o n t determines pour q u a t r e nuances : W- (6,5 - ¹² - 1 5 e t 25) % Co jusqu'a 1000°C. La v a r i a t i o n du module de W!dIBULL est mesuree en t e e r a t u r e pour la nuance W-ti,5 % Co. Les mesures de t h n a c i t e s o n t rCalisQs jusqu'h 800°C sur t r o i s nuances : WC - (6,5 - 1 5 et 25)% Co et la v a l i d i t 6 d e ces mesures est d i s c u t k en u t i l i s a n t les cr iteres de r u p t u r e classique.

Aostract - E l a s t i c modulus and f r a c t u r e s t r e n g t h have been determined up t o 1000°C i n t h e case of XCo m a t e r i a l s with d i f f e r e n t c a r p s i t i o n (6.5 - ¹² - 1 5 and 25) % w t Co. Temperature dependence of WEIBULL modulus has been observed for WC - ^{6.5 % w t} Co conposition. k a c t u r e toughness has been c h a r a c t e r i z e d up to 800eC f o r WC - ^(6.5 - ^{1 5 and 25)s w t 8 and the}

v a l i d i t y of these r e s u l t s is discussed according t o f r a c t u r e mechanics c r i t e r i a .

Cemented c a r b i d e s a r e widely used m a t e r i a l s f o r i n d u s t r i a l a p p l i c a t i o n s i n metal c u t t i n g and rock d r i l l i n g . Tfiese composites m t e r i a l s , made of hard tungsten c a r ~ i d e and s o f t e r c o b a l t binder phase, combine high h a r d n e s s , s t r e n g t h and toughness a s we11 a s wear resistance. mst of s t u d i e s devoted to these m a t e r i a l s focused on room temperature behaviour /l/. Several attemps have been made to explain the room temperature f r a c t u r e behaviour and d i f f e r e n t t h e r o r e t i c a l models have been advanced. Only a few papers d e a l with temperature behaviour /2, 3, 4/ and so t h e r e is a l a c k of knowledge about deformation mechanisms a t high temperature and s p e c i a l l y a b v e 600°C. I n usual a p p l i c a t i o n s , high s t r e s s e s and high terrperatures a r e generated l o c a l l y a t t h e tool surface and temperatures up t o 1000°C have been recorded i n metal c u t t i n g operations /5/. In the present work, t h e f r a c t u r e s t r e n g t h

, toughness and e l a s t i c modulus of t h r e e i n d u s t r i a l WC+ grades a r e determined up to 1000°C. Iiesults a r e discussed on t h e b a s i s of microstructure.

...

(U) PEDEEEN - W S I E U 69680 (France)

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19861112

JOURNAL DE PHYSIQUE

11-1- Materials

Studied m a t e r i a l s a r e i n d u s t r i a l grades (*) with d i f f e r e n t contents o f c o b a l t phase. The mean s i z e of W g r a i n s is about 2.5 p. A l l specimens have been made by p r e s s u r e l e s s s i n t e r i n g b e t w e e n (1340-1440) OC i n h y d r o g e n . Main c h a r a c t e r i s t i c s are l i s t e d i n t a b l e 1.

Table 1 : Main c h a r a c t e r i s t i c s of studied materials.

h

Grade

% CO(&) Density

I n c a s e of low CO content, t h e c o n t i g u i t y between carbide g r a i n s is important and the c o b a l t phase is present i n some p l a c e s a s t h i n f i b . Residual p o r o s i t y has been observed f o r each grade (A, B and sometimes C type (AS'IM recomnendation)).

Gl G2 G3 G5

6.5 12 1 5 25

14.9 14.1 14.03 13.1

11-1-111--1----_--___---------------------------------

Mechanical p r o p e r t i e s have been measured by four p o i n t bending tests (with a span dimension 24 / 8 m) under c o n t r o l l e d atmasphere (95 % N2 + 5 % H2) with a high temperature apparatus /6/. A l l tests were performed a t a crosshead speed of 0.1 mm/min. Fracture s t r e n g t h (Q) and e l a s t i c modulus (E) measurements were c a r r i e d o u t with c a r e f u l l y polished specimens ( 2 x 4 ~ 3 0 mn3) up to 1000°C. For f r a c t u r e toughness (KIc) values determination, a "V" notch was introduced with a low speed d i m d saw on 4 x 6 ~ 3 0 mn3 specimens (S.E.N.B. type) ; the pre-notch was extended to a very sharp notch by spark erosion using a 25 Jbn width tungsten f o i l . l b v e r i f y t h e KIC v a l i d i t y measurements, we have applied t h e mechanical f r a c t u r e c r i t e r i o n . Results show t h a t Linear E l a s t i c F r a c t u r e Mechanics theory cannot be applied above 800°C f o r G1 corrppsition and above 60U°C f o r G3 and G5. So we have used t h e J i n t e g r a l f r a c t u r e c r i t e r i o n w i t h compliance method /7/. I n o r d e r t o c h a r a c t e r i z e t h e f r a c t u r e toughness of G3 grade up to 1000°C we have determined the c r i t i c a l parameter "JICr' which l e a d s to a s i g n i f i c a n t "KIC" value

At room temperature, t h e r e is a decrease of e l a s t i c modulus with an increase o f f r a c t u r e s t r e n g t h a s t h e c o b a l t content increases (from 6.5 % to 25 8 ) . Cemented c a r b i d e s room temperature f r a c t u r e mode is mainly o f b r i t t l e type. Thus t h e mechanical behaviour can be explained i n terms of G r i f f i t h ' s theory : f o r a given grade, t h e s t r e n g t h is determined by the f r a c t u r e energy and the s i z e of c r i t i c a l defects. Weibull s t a t i s t i c a l a n a l y s i s has been applied to o b t a i n a q u a n t i t a t i v e e s t h a t i o n of observed s t r e n g t h dispersion : G1, G3 and G5 grades e x h i b i t Similar W i b u l l modulus (m) between 5 and 8. This modulus is q u i t e law, but same values have reported f o r s i m i l a r s i n t e r e d cemented c a r b i d e s ; higher rrpdulus needs s p e c i f i c s i n t e r i n g processes a s HIP /8/. I n case of Gl grade, Weibull d u l u s t e p r a t u r e dependence has been observed: m is nearly constant up to 600°C and is s l i g h t l y increased a t higher t e q e r a t u r e s . The decrease of e l a s t i c d u l u s (E) and f r a c t u r e s t r e n g t h (C£) values ( s e e fig.1,2) is observed from (300-400) OC f o r a l l grades /9/.While f o r t h e low CO content grade ( G l ) f r a c t u r e s t r e n g t h is lower than t h e values f o r the other grades, t h e decrease w i t h tenperature is not so l a r g e for G1

(up to 600°C). This can be c o r r e l a t e d to t h e absence of major p l a s t i c i t y i n G 1 grade a t low and medium t e q e r a t u r e s .

A t room t e q e r a t u r e , the f r a c t u r e toughness (KIc) increases with the cobalt content

; t h i s p o i n t has Deen observed by d i f f e r e n t authors /10/. Fracture energy (e.g.

c r i t i c a l s t r a i n energy r e l e a s e r a t e : GIG) is obtained from E and KIC experimental

values.

*--- *--

-O\ 0---

WC-12 X CO

"----

.-. _*,

\. _*\ ^{.c--- WC*. 5 X CO}

\ " ^\ *,

-4.-,

* \

5 _m-'-. 1

C

'F?

C H -4.-

i.- 8..

fa *

0 100 200 900 400 500 800 700 800 8 0 0 1000 TEMPERATURE

T <*C)

Figure 1: 6f versus temperature of WC-CO grades.

Figure 2 : E versus temperature of WC-CO grades.

a-.

8..

g ^p--.

- ^B--

W

8..

5..

0 .,

. +----.. WC-B. 5 X CO

*--- WC-12 X CO +--- WC-15 X C0 a---

+-- ^{WC-25 X C0}

* - ^---a=-*.

'*.>*,

*

.

+- - - ^+-

^{,* 'S;} _---*---

+ \'*

-- -- ^{- t , + h---\}

W-

\ ' \A,

=-. ^\ *\, -?

*L A U \

' ^r '. _'. ^\ _Y ' ^,\+

2 \ L*

'R '+

\

' t \ t \ +

' r

0 100 200 300 400 500 800 700 800 8 0 0 1000

TEMPERATURE

T (*C)

JOURNAL DE PHYSIQUE

Figure 3: G l c versus tenperature f o r G1, G 3 , and G5.

Cm f i g u r e 3, we have r e p r t e d t h e GIC temperature dependence : we observe nearly constant values up to 300°C, a maximum a t 40U°C, and a gradual increase a t higher t-ratures.

I V - MS(IUSSION

RDom temperature s t r e n g t h is c o n t r o l l e d by processins d e f e c t s : predominant d e f e c t s

8 ...-

l..

p "-

a!

8"

"W..

8 N..

0 ,

i n a l l studied grades e l a r g e s pores of A type ( > ^{10 W ) , B type (} > 40

s o m e t h s C type (Eta phase "R"). This have been confirmed by S.E.M. observa C"" ions o f f r a c t u r e o r i g i n s /9/ and is c o n s i s t e n t with flaw s i z e s (+) .The equivalent c r i t i c a l flaw s i z e , is c a l c u l a t e d from s t r e n g t h and toughness experimental values :

S---

G1 +--- G3

*--- _G5

/ 7

/ /

7

, -*.

\

L*' /'

I 7'-

/ /

/ /+

--*-/*'

*--

/

/ +..

.+--+'

/ /.

/

+----+--+l / /

,W

--* '

. :

Fracture mode is intergranular f o r smaller g r a i n s and transgranular i n case of l a r g e r c a r b i d e grains. There is a s l i g h t increase of a, with tenperature : t h i s can be explained by s u b c r i t i c a l crack growth mechanisms with coalescence o r linking of pre-existing m a l l e r defects. Microcracks network can e f f e c t i v e l y be observed on high temperature f r a c t u r e faces.

0 100 200 300 400 500 600 700 800 Q00 TEMPERATURE

T<*C>

25OC 600°C 700°C 800°C

Weibull npdulus

m -

Table 2 : C r i t i c a l flaw s i z e s and Weibullmodulus a t d i f f e r e n t tenperatures

According to f i g u r e 3, t h e high temperature behaviour of cemented c a r b i d e s can be divided i n t o t h r e e tenperature f i e l d s :

i) Between room temperature and 3UU°C (zone I) : GIC is n e a r l y constant, and

f r a c t u r e is of brittle type f o r -11 c o b a l t content grade. I n c a s e o f c x h l t

However, we don't observe any difference i n the f r a c t u r e i n i t i a t i o n d e f e c t s for each grade ; that means processing d e f e c t s (pores) c o n t r o l t h e l o w temperature mechanical behaviour of a l l studied materials.

ii) between 300 and 800°C (zone 11) : t h e d u c t i l i t y of c o b a l t phase increases. The f r a c t u r e energy GIG is increased by reduction of crack t i p stress f i e l d i n the p l a s t i c zone by energy d i s s i p a t i o n through p l a s t i c relaxation occuring i n the s o f t phase (cobalt phase). This pher~menon is accentuated for higher Co contents. I n t h i s tenperature zone, a l l grades present important p l a s t i c deformation by g r a i n boundary s l i d i n g ( i n the c o b a l t phase ) leading to the observed decrease of elastic rrodulus and f r a c t u r e strength.

A f i r s t maximum of "GIc" is observed Eor a l l grades a t about 400°C. This behaviour can be i n t e r p r e t e d i n terms of toughening e f f e c t s of the wbalt phase produced by d i f f e r e n t phenomena such a s the change i n magnetic anisotropy ( HCP Co ), t h e a l l o t r o p i c transformation ( HCP to FCC ), the hardening of Co phase by p r e c i p i t a t i o n or formatian of secondary phases /11, 12,'. This p i n t has to be confirmed and d i f f 6 r e n t e x p e r i m e n t s a r e now r u n n i n g to i d e n t i f y t h e e f f e c t i v e toughening mechanism. High temperature X.R. a n a l y s i s have been p e r f o r d recently ( i n H2 ) with our specimens; f i r s t r e s u l t s show s i g n i f i c a n t formation of secondary conpounds a s OojW. The auplitude of t h i s lw tenperature G l C m a x h is increased with cobalt volumic f r a c t i o n . U n t i l now t h i s phenomenon h a s never been observed ( t o c u r knowledge) i n cemented carbides. Extensive experiments have to be performed to confirm t h i s toughening e f f e c t .

iii) A t higher temperatures ( >800°C) zone 111 : A l l the p r o p e r t i e s show a d r a s t i c decrease probably due to the low y i e l d s t r e n g t h of the c o b a l t above 8 0 ° C and t h e materials have a tendancy to e x h i b i t creep deformations. The Linear E l a s t i c Fracture Mechanics c r i t e r i a a r e not v a l i d / g / .

F r a c t u r e toughness ( o r c r i t i c a l s t a i n e n e r g y r e l e a s e r a t e G l c ) t e m p e r a t u r e dependence, define d i f f e r e n t f r a c t u r e modes :

. ^{Zone 1} (20-300°C) : b r i t t l e f r a c t u r e node, with s t r e n g t h controlled by processing defects.

. ^{Zone I1} (300-800°C) : increase of f r a c t u r e energy by p l a s t i c relaxation i n the c o b a l t phase with an important decrease of f r a c t u r e strength. An apparent toughening e f f e c t has been observed a t 400°C.

. ^{Zone 111} (>80U°C) : a l l p r o p e r t i e s decrease.

/l/ Chermant, J.L. e t al., Mechanical c h a r a c t e r i s t i c s of carbides metal composites and the c o r r e l a t i o n with microstructure parameters - Proc. B r i t . Ceram. Soc. (1975) 25 p. 197-208

/i/ Gurland, J. and h r d z i l , P. Trans. AILvlE (1975) , 203, p. 311 /3/ Murray, M.J. and Smith, D.C., J. Mater-Sci., 8 (1973) pp. 469-484 /4/ S i Mohand, H., 3 h cycle t h e s i s - ^{INSA LYON} 71983) ,France.

/5/ Trent, &M., Wear Processes.. . ^Iron and steel I n s t . Publ. 126, fandon (1970) /6/ Orange. G . , Doctor Ing. t h e s i s INSA LYON (1976) ,France

/7/ G i r d , S.J. e t al., Int. ^J. F r a c t . , g (1975). p. 528

/ 8 / M S L , U. L. F. HWNEX, H. I n s t . Fiir ~ r k s t o f f u r w i s s e n c h a f t e n University o f Erlanger, iWrnberg, 8520 - Martenstrasse, Germany

/Y/ ~ u a o u a d j a , N., Dxtor Ing. t h e s i s INSA LYON (1985) ,France

/10/ Osterstock, F., Doctor Ing. thesis University o f Caen (1975),France /ll/ Jeanjean, K., Doctor Ing. t h e s i s INSA LYON (1972) ,France

/U/ le FOux, H., High Temperatures- High Pressures, 13 (1981) pp. 503-506