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Surface Integrity of Cemented Carbide Tool and Its Brittle Fracture
Surface Integrity of Cemented Carbide Tool and Its Brittle Fracture H. Takeyama ( l ) , N. lijima, Tokyo University of Agriculture and Technology, Tokyo, and Toshiba Tungaloy Co.,LtdJJapan
K. Uno,
In this study, the effect of the surface properties of WC-Co cemented carbide tools, which are &rmined by the grain size of the tool grinding diamond wheel, on their brittle fracture is investigated experimentally, and the fractographical analysis is done in order to clarify the mechanism of fatigue type of brittle fracture of cemented carbide subjected to the cyclic impact tests and the cutting tests. It is found by the intermittent turning test and the cyclic impact test that the tool life due to brittle fracture decreases with a decrease in the diamond grain size of the grinding wheel. The Xray diffraction analysis shows that compressive residual stress is produced within the WC grains of the surface layer by grinding, and the binder phase is plastically deformed or work-hardened. Both the absolute values of residual stress and the degree of work-hardening in the surface layer increase with an increase of the diamond grain size. The successive microscopic observation of cemented carbides with the progress of the cyclic impact tests reveals that plastic deformation in the binder phase is likely to initiate microcracks mostly in the grain boundaries. The initiation of microcracks is suppressed by work-hardening. and che propagation of microcracks is decelerated by compressive residual stress.
1. Introduction Complex-structural tool materials such as sintered carbide and cermet have been widely utilized and in the future too they will be leadina tool materials. Nevertheless, the mechanism of their chipping or fracture has been little clarified. In this study the relation between the surface processinas of carbide tool and its brittle Fracture is experimentally investigated from the standpoint of surface inteqrity and the mechanism of structural deterioration of the tool material, which is resarded as the origin of low cycle fatique type of fracture. is microscopically analized.
In this study straight tungsten carbides of 4.0-20.0 wt5 Co, of which qrain size is approximately 2 ;m, are tested for simplicity. Every face of the tip specimens (12.7~12.7~4.76mm) is qround under the condition shown in Table 1 with diamond wheels of resin bond. Care was taken so that heat generated by tool qrindinq could be minimized by reducing the grindins pressure and applying abundant qrindina Fluid. The specimens are chamfered (0.14x0.14 nun) under the same condition as in Table 1. 3 . Result in interrcittent cuttinq test
Here, surface inteqrity 111 denotes overall assessment of surface and subsurface in view of topoloaical, chenical, physical, mechanical and metalluroical prouerties, and low cycle fatique type of fracture, which is most freouently encountered in intermittent cuttinq with sintered carbide tools, indicates brittle fracture by cyclic impacts nuch less than the fatique limit of a common structural metal. Cuttina tools or tiDs are usually surface-treated by qrinding or polishing before their services. It is very much conceivable that the arindinq condition will affect the surface intearity and conseouently the brittle fracture of cuttina tool, because the surface asperities produced by qrindino miqht have notch effect, and residual stress or work-hardenino in the surface of sintered carbide, which is produced by qrinding 121, [ 3 ] may affect the low cycle fatique type of fracture. Research works on the relation between tool verformance. especially, brittle fracture and surface integrity have been little conducted. In this study how the tool qrindina condition, particularly, the diamond qrain size of qrindinq wheel affects the behaviour of brittle fracture of a sintered carbide is experimentally analized. In addition to it, the mechanism of initiation of microcracks, which is resarded as the oriain of brittle fracture, is microscouically investiaated.
Firstly the relation between probability of brittle fracture and diamond qrain size of the qrindinq wheel was obtained in intermittent cuttinq tests. The cutting condition was chosen as shown in Table 2 so that low cycle fatigue type of brittle fracture can be emphasized while suppressing flaking type of fracture due to metal adhesion at the cutting edqe. Typical types of brittle fracture in intermittent cutting are demonstrated in Fio.1, in which chippinq at the corner (type 1): fracture at the major cuttina edqe (type 2) and flakinq on the face (type3) are observed. Types 2 and 3 which are reqarded a s brittle fracture due to metal adhesion are characterized by the tools qround with a grinding wheel of finer grain sizes. Especially, with the tool qround with a prinding wheel of el500 qrain size only flakinq (type 1 ) occurs at the very initial stage of cutting. However, with the tools ground by a qrindina wheel of qrain sizecoarserthan t1000, only chippinq at the corner (type 1 ) occurs, and types 2 and 3 are rarely observed.
Type of cutting
Intermittent turning
2. Test specimens and condition of tool grindinq
Cutting speed
V=lEOm/min
Complex-structural tool .materials consist of hard qrains such as WC, Tic, TiN, etc. and binders as CO, Ni, etc.
Depth of cut
a=l .Omm
Feed rate
S=0.32mm/rev
Tool
WC-4.O%Co cemented carbide (-5,-6,5,6,15,15,0.14C)
Type of grinding Grinding pressure
Constant pressure type of grinding 0.12 MPa
Work traverse speed
10 m/min
Grinding speed
700 m/min
Grinding wheel
Cutting fluid Lathe
Cr-Mo alloy steel (SCM3) Dry 5 0 0 X 1 0 0 0 1 ~ 1 , 5-15kW
Diamond cup wheel
Bond
Resin
Grade
N
Grain size
Work material
# lOO(150um)
t 270( t 280( # 500( t 700( #1000( t1500(
Annals of the CIRP Vol. 31/1/1982
57m) 5211m) 34vm) 22um) 15umI 10um)
Work p iece
D=llOmm
Engagement : B to A DiSengagement:A to B
Focussing at chippinq at the corner (type 1). which is most freauently encountered, the relation between cumulative probability of chippinq and number of impact cycles in intermittent cutting is illustrated in Fiq.2 takinq the qrain size of the diamond grindinq wheel applied as the parameter. This result apparently indicates that the snoother the tool surface, the shorter
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ir. whicn :he i?i]i?cL a n r i l e e is sc': 3 0 G O ( : . in or tn sir*iil;itcthe im:i,ict i s i i i c ? . i s m s s i l - l c to thy t e.Trnt in Lntcrwittcrit ciittinc:. Tie s is feel 2ftcr e:.'er\' iynict so that the tool t i r ) m;l:; hit its y : i v i n si!rfacc-, 2nd the steel ~jlitc i s clxmc8:l c?owrw.>rclnrior to the i?:mct in crder . The innact Yorce c m be n the imn?ct :'e13cit?: o r the :iteric\l to bc hit, a n d the 0 . 4 5 c carbon steel ( 2 3 0 [I..) rcxim,.it(,l:: 2 0 R V .
4,
,Air
cylinder
I
Flq.1 Types of brittle fracture of wC-4.0?Co carbide tool (K10) Three types of brittle fracture at early stanr (1) Chippinas around corner (2) Brittle fracture of cuttina edcr ( 3 ) Brittle fracture (flakinq) on Face
Air cyli
,
Number of Impact Cycles until Brittle Fracture
Drivinq wire
Pulse motor
Fig.2 Effect o f diamond qrain size of qrindinq wheel on probability of brittle fracture of 1%'C-4.0PCo cemented carbide tool in intermittent turninq
Fig.4
Schema of drop impact testing machine
the tool life in view of brittle fracture i s . 4 . Result in cyclic impact test
Cuttinq test, in which various types of brittle fracture take place simultaneously, is inconvenient i n order to clarify the mechanism o f low cycle fatique typc of brittle fracture independently and its relation with the surface intcqrity. Thus, a cyclic impact testina machine has becn developed in order to abstract only low cvcle fatinue tvpe of brittle fracture. 4.1 Cyclic impact testinq machinc and its reliability
g:
" O
I I
'1 1I0
I
I
,
, ,
,.,,,I100
,
, ,
, ,,,,I loo0
A13.04
, , ,
, ,~
5000
Nunber of Impact Cycles until Failure 01 Cemented carbide
Workpiece1
Bar stock y :RJ;ngle e:Impact angle a:Clearance angle t:Depth of indentation c:Engage angle a:Depth Of Cut
(a) Tool-workpiece enqaqe- (b) Tool tip-testpiece ment in intermittent enqagement in drop cuttinq impact test Fiq.3 Schematic view of engagement in intermittent cutting and drop impact test Tool-workpiece enaagement in interaittent cuttinq is schematically illustrated in Fiq.3fa). The cutting action from the tool-workpiece engaqement to disenaaqement gives rise to thermal stress within the tool and metal adhesion on the Face as wcll, so that it is difficult to purely abstract the mechanical brittle fracture. IF the relative velocity of tool and workpiece can be zero before chip formation takes place, alriost purely mechanical stress can be aDplied onto the tool without metal adhesion. This can be realized by a setup shown in Ficl.l(b), and a cyclic impact testinq machine developed for this purposc is illustratcd in Fir!.
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Fiq.5 Effect of C o content on cumulatlve prohability of failure of WC-Co cemented carbide in drop impact test Impact anqle e:30°, steel bar r,tock:S45C 240Av. impact velocity v:145n/min, drop mass m: 3.3k9, diamond qrain size of qrindinq wheel:lO:im(t1500) corner radius:0.14C Applying a certain number of drop imoacts to the carbide tiu in thc aforementioned testinq machine, brittle fracture takes !lace within the contact area of tool tio and steel plate. The relation between the number 0 5 cvclic impacts till the brittle fracture and the tool life in view of low cycle faticue type of brittle fracture (cumulative probability of fracture) is shown in fiq.5 takinc thc cobalt content of the sintered carbide as the parameter. The test result that the more the cobalt content, the lonqer the tool life is well coincides to the result in actual intermittent cuttinu tests, and the Veibull's parameter is also fairly close to that in the actual cuttinq tests. Thus, the proposed testina method can be justified fairlv well as a simulator of intermittent cuttino as Far as low cycle Eatiaue type O F brittle Fracture is concerned. 4.2 Effect of diamond qrain size of nrindinn wheel on low cvcle fatinue type of brittle fracture The effect of diamond arain size o f arindina whcel on low cycle fatiuue type of brittle fracture of WC-Co sintered carbidc subjected to the cvclic imDact test is shown in f'in.6, i n which the ordinate indicates
-
--y
r8 -
caused by the tool crindinn action. Fio . 7 i ll,.istrates thc distribution of rcsidual stress ! c o n p r e s s i v e ) cjf b’C Grains in the s~.hsiirfacn nE the sintcrcd carbide nround with t270 ( 5 2 ; J J ) a n d 31500 ( 1 C ‘ m ) oriin size. This was obtained frorr the S-ray diffrJction iCr-K ray) of 11021 plane of WC 2nd the stress , . i l i i c w . 3 ~ calculatrrl by nezns of si2’ 7c)thod.
Ground with diamond wheel1 A Ground with G.C.whee1 Polished with dianor.6 paste
0
0
I
1
0
I
1-m below surface
0
-
8
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I
8
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I
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,
l
1
,
1
1
Grain Size of Grinding Wheel (um) Fig. 6 Effect of grain size of grindin wheel on characteristic life of WC-13.0%&~ cemented carbide in drop impact test Impact angle e:30°, steel bar stock:S45C 240Hv, impact velocity v:133m/min, drop mass m:3.3kg, corner radius:O.OC the characteristic tool life, i.e., the number of imDact Cycles corresoondinq to 6 3 % cumulative nrobabili-. ty of brittle fracture, and the abssisa indicates the averaqe qrain diameter of the qrinding wheels aoplied. Each ooint in Pic.6 was obtained from eiuht Weibull’s plots. In Fio.6 the data when qrindinq the tip with a Sic wheel and polishinq i t with diamond paste are also DlOted for reference. This result clearly demonstrates that the finer the ?rain size aDplied in grinding the carbide tip, the shorter the tool life is. Since metal adhesion was observed neither on the tool face nor at the cuttina edge in the cvclic impact test, the chemical activity between the carbide tip and the matinn material would not affect the brittle fracture of the tip. The exocrimental fact that the asperity heiqht of tool surface and cuttinq edoe, which i s determined mostly by the qrain size of qrindinq wheel applied, apparentlv affects the tool life favourably, indicates that the notch effect resulted from the surface or edge aspcritics does not seem to be a positive factor to brittle fracture of tool tip. A conceivablc factor to affect the low cycle fatique type of brittle fracture is physical or mctallurqicnl one such a s residual stress, work-hardenins o r structural transformation in the surface. 5. Surface alteration due to qrindino and its effect on low cycle fatique type of brittle Fracture An apparent surface alteration of sintercd carbide due to qrindinq with a diamond wheel is =raomentation of WC grains in the surface, this beinrf niorc emphasized with the coarser qrain sizes.
-1.0.
\. l O u m 01500)
Cr-Ki WC(102) Diamond grain size, of grindinq wheel 0 52um (8270) A
Cr-K I
lOilm
-
(#l500)
0-
Fi9.7 Residual stress distribution below qround surface surface of WC-20.04Co cemented carbide However. even though the carbide tip is qround with coarse qrains as t270, the depth of qrain fragmentation is as shallow as 1 iim which corresponds to a half of V C grain diameter. It is hardly conceivable that such the thin layer of qrain fracture directly affects the low cycle fatinue type of tool fracture, and moreover this effects Favourably upon the tool life. What is worthy of attention is residual surface stress
x
U .r(
J
5 C c(
I (20um below surface
I
50’
L I
I I
1
I
I
60’
70’
80’
20 Fiq.3 Diffraction output for varied depths in reference to diffraction anule in WC-20.0?,Co cemented carbidc It is pointed out that in the case of carbidc tip qround with 3270 qrain size the compressive rcsidual stress increases sharply towards the sround surface .2nd the absolute value is a l s o very larile, whereas in the case of *1500 orain size the stress value is extrerrely small. This is ascribable to the fact that only the surface layer i s extended two-dimensionally by the mechanical energy of qrindinn, and this trend is more emohasized with the coarser grain sizes. Althouqh the residual stress within the binder phase has not bcen neasured in this study, within the binder phase too compressive residual stress i s oenerated by qrindinn accordinq to Suzuki, et a 1 1 4 1 . It is known that the binder phase of a b’C-Co sintered carbide, which is; phase of f.c.c. under an annealed condition, is partly transformed into Dlnte-like ;’ phase of h.c.p. by external stress 1 5 1 . If the stressinduced transformation has thus occurred in the binder phase, it must be somewhat accompanied with plastic flow there. So that the plastic deformation could be identified by detectinq rJ phase with X-ray. Fiq.B(a). fb) and ( c ) are the X-ray diffraction outputs of the surface 1 :m, 8 ‘im and 20 Ilm, respectively, beneath the surface of thc sintered carbide nround with 1270 diamond grains. In the subsurface immediately beneath the around surface fFiq.8 ( a ) ) an appreciable output of F ’ phase cienerated by the stress-induced transformation is observed in the background of ’, phase, this indicetinu the esistence of nlastic deformation. However. in the subsurface 8 I m beneath the around surface (Fiq.a(b))the diffraction intensity of .‘phase is fairly reduced, and 20 : ~ mbeneath the ground surface it is hardly recognized. From the above fact it is concluded that tool qrindinq qives rise to plastic flow in the binder phase, and the depth of plastic flow reachcs 10 :in aproximately. Accompanied with the plastic flow in the binder phase, a work-hardened layer will be formed. Fiq.9 shows the distribution of microhardness measured downwardly from the oround surface of VlC-5.5% Co and VC-209 Co sintered carbides taking the arain size of orindlinq wheel as the parameter. This result demonstrates that the carbide surface qround with #270 qrain size is workhardened down to 10 i:m beneath the qround surface, whcreas it is little work-hardened with ‘1500 qrain size.
61
t I
I
1
I
1
0
10
20
30
I
I
I
F i g . 3 % a r d n e s s 6 i s t r l b u t i o n of rrr.;ur.d si:rf?ce W-Co ccnented carbide il;.rdness t e s t i n r j l o ? . d : 5 . 5 ? C r 2000, 2 0 . 0 '
c y c l c s . F i q . 1 1 i s o n e e x a m p l e showr'ret n e a r t h c c n i - n c r 0 4 3 c , i r h i r i c t i p , tt!c s t r i , c t i l r e iprior- to t h e c : . c l i c i r 1,) is t h e s t : - u c t u r 1 ~ os r i t v z c t . C o i n t i o n 01- r o t a t i o n o This i n A i c a t e s t h a t p l a s t i c i n t h c !>intlt.r p h ~ i:wzcts. Durin-: t h e p l a s t i c f l o w t .,i-ies a r e ? , i c r ~ s c n p i c a l l yb r o k e n i n t!ir ex.t:-e:w c z s e t h e I T ? r ; l i n s t h e T s e l v e s f a l l o u t .? i n T i c . l l ) , and i n sor1.e c a s e s new WC m a i n s c e dJe t o i h e p l a s t i c f l o w oT t h e hin?r-r ';rFm; 1 i n F i q . 1 1 ) .
0:
1000
;li~i
c
e d o u t ?rwn
nic.7
2nd F i r l . 9 .
t h e ;Ibo.;v < , x p c r i i r i c n r a l I-esults, it i s ob:-ious to urindino increoses le ' ? t i a i i e t y p e of s oric a s u c c t o f s u r i s i'irns i d e i - e d t n .;iipprcss t h c s t r i i c t on o r i n i t i ' i t i o n o f r n i c r o d c E c c t s ir. t h e c a r b i d e s u r f a c e , and cormrcssive resiAual s t r e s s , which i s a n o t h e r a s ? e c t , t o s u n n r e s s crowth o r pro: > . i c ~ ~ i t i c )on f t h e 3icrndefects.
Fi-or1
3,,,,,
( a ) N?lmbcr o f i m p a c t cycles : 0
€'is.l i
( b ) Number of i m p a c t c y c l e s : 40
M i c r o s c o p i c s t r u c t u r a l d e t e r i o r a t i o n of WC20.OTco c e m e n t e d c a r b i d e i n d r o p i m p a c t t e s t
Khat i s w o r t h y o f a t t e n t i o n i s t h a t t h e i n i t i a t i o n o f m i c r o d e f e c t s a t t h e I
I
100
I--
cF
Impact direction
Measuring a r e a : 10Ox251im J
I
I
WC G r a i n S i z e
D e p t h of Removed L a y e r f r o m Ground S u r f a c e ( m ) F i q . 1 0 E f f e c t of s u c c e s s i v e s t o c k r e m o v a l from c r o u n d s u r f a c e o n c h a r a c t e r l s t i c l i f e o f WC-20.0% Co cenented c a r b i d e i n d r o p impact test I m p a c t a n g l e e : 3 0 ° , s t e e l b a r s t o c k : S45C 2 8 5 H v , i m p a c t v e l o c i t y v:145m/min, d r o p m a s s m : 3.3kq, corner radius: 0.14C
6 . M i c r o s c o p i c o b s e r v a t i o n of s t r u c t u r a l d e t e r i o r a t i o n of s i n t e r e d c a r b i 6 e A f i x & tarciet o f t h e cnrhiclc s u r f a c e s u b j e c t e c i t o t.hc cvcl i c i m n a c t s h a s h e e n o b s e r v e d c o n t i n u a l l y a f t c r
a d e f i n i t e number of c y c l i c i m p a c t s w i t h a n e l e c t r o n s c a n n i n q m i c r o s c o r ) c w i t h 10000 t i m c s m a c i n i f i c a t i o n . I t has heen rcvcald t h d t t h e s t r u c t u r a l d e t e r i o r a t i o n s t a r t s a f t e r . i p n r o x i m a t c l y 4 0 c y c l e s o f i m p a c t under an i m p a c t c o n d i t i o n cori-esoondinq t o t h p c h a r a c t e r -
62
I
I
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I
(!Jm)
F i g . 1 2 I n f l u e n c e of WC g r a i n s i z e of VC-20% Co c e m e n t e d c a r b i d e o n c l e a v a g e a t WC/binder i n t e r f a c e i n c y c l i c impact test Number of i m p a c t c y c l e s : 4 0
lire i s l i k e l y t o o c c u r . T h i s i n d i c a t e s t h a t t h e l a r g er t h e VC g r a i n , t h e h i c l h o r t h e P r o b a b i l i t y of microf r a c t u r e a t t h e q r a i n b o u n d a r y i s . T h i s t r e n d i s ass o c i a t e d w i t h t h e f a c t t h a t t h e l a r s e r t h e VC a r a i n , t h e h i q h e r t h e stress i n t h e b i n d e r p h a s e o r q r a i n b o u n d a r y close t o t h e a f o r e m e n t i o n e d i"C n r a i n , a s Hirao, e t a1 [ h l pointed o u t . F r o n t h e h i s t o r i c a l o b s e r v a t i o n of t h e c a r b i d e s u r f a c e i t i s u n d e r s t o o d t h a t p l a s t i c f l o w of t h e b i n d e r phase Lrirrqers t h e m i c r o d e f e c t s s u c h as m i c r o f r a c t u r e a t t h e q r a i n b o u n d a r i e s , f a l l - o f f of t h e PC q r a i n s , e t c . W h i l e t h e g r o w t h r a t e o f m i c r o d e f e c t s or microc r a c k s i s c o m p a r a t i v e l y slow a t t h e i n i t i a l s t a a e , t h e r a t e of c r a c k p r o p a g a t i o n becomes e x t r c m e l v f a s t a t t h e F i n a l s t a q e . One e x a m p l e of c r a c k p r o o a o a t i o n a t t h e f i n a l s t a g e is shown i n Fig.13, i n w h i c h how t h e c r a c k p r o p a a a t e s by o n l y s i n q l c i m p a c t ( t h e 2 4 1 s t i m p a c t ) is v i s > . i a l i z e d , and at t h i s s t a q c n o p l a s t i c
l i f e i n :-icw o f h r i t t l ?
Ci-;3cture i n t h e c : , c l i c im-
( 3 ) The s L r ? a c e i l t e r a t i o n of s i n t c r r d c > r t ) i d e s u c h a s wnrk-hardeninri and c o r n r e s s i v e r e s i d u a l s t r e s s s ~ i l j ~ r c s s et sh e h r i t t l e f r a c t u r e . 2r.d t h i s is emnhnsizcS w i t h I i r q c r c r ~ i ns i z c s n i d i a m c n d o r i n d i n q w h e e l a r m l i e d . C l o s e l : ~ s p e , i k j r . 4 , w o r k - h a r d e n i n q of c a r b i d e s ' n - f a c e S ' J P D T ~ S S E S ocnci-ati n n 06 :iicrcc r - a c k s , 2nd c w m r e s s i - e rcs:-i.:?l q t v ~ nS ~ I I ~ D ~ C S S C S ijrr,wth ,3f t h e m i c r o c r a c k s . 'd
Thc n r i a i n o f low c]-cie f a t i n u e t-::ze of b r i t t l e G r ~ c t u r ei s p l a s t i c f l r w i n t h e b i n d e r p h a s e a n d -iccorSincrl:: r c s u l t e d v i c r o c r a c k s a t o r n e a r t h a 7r71n S n l ; n d ? r i c s . r ' d r t h e r ? ' c r c , t h e l a r c c e r t h e d i a n e t e r of :KC : ? r a i n s . t h e h i s h e r t h e 3 r o b a h i l i t : : o f b r i t t l e ' r D c t i i r c i r m i n r l t h e a r q r e r e n t i o n e d !.C ' r:ra:ns i s .
Qeferences
f!l
F i q . 1 3 Exarrple of c r s c k p r o p d o a t i 2 n of I v C - 2 O . r ) ~ C o cemented c a r b i d e I n d r o p i n p a c t test
Elow o f t h e b i n d e r o h a s e i s r e c o n n i z e d . A t t h i s s t a q e t e n s i l e s t r e s s o l n v s a i a j o r D a r t a n d t h e c r - c k s nrooaaates mainlv alonn t h e q r a i n houndarics i n i b r i t t l e m a n n e r . C o n s i d e r i n u t h a t t h e o v e r a l l n u n b e r of c \ ' c l i C i m p a c t s t h r o u q h q e n e r a t i o n of m i ' z r o c r a c k s , t h e i r qrowt h and c r ; l c k p r o p a q a t i o n t o t h e f:n,il b r i t t l e :r;lctu r e i s f a i r l y l a r n e i n most c a s 5 s . 7. Conclusions ( 1 ) A c y c l i c i n u a c t t e s t i n o n a c h i n c t o e-:itlwtf
toi1c:hn c s s of t o o l m a t e r i a l s h a s been c?e~~r:oucd,a n d t h e t e s t i n a r e l i a b i l i t v and t h e c o r r e l a t i o n w i t h i n t c r m i t t e n t c u t t l n n t e s t haTfe b e e n zssesse?. As a result t h e t e s t i n s i..achine h a s b e e n v e r i f i e d to s a t i s f a c t o r i l v represent the strenath anainst low c y c l e f a t i n u e t v p c of b r i t t l e f r a c t u r e .
'8. p i e l ? , t'. K o s t e r , et a l : .'"itchininn c C ? i c h Str c n n t h R t c e ! s w i t h F r r n h a s i s c)n F u r f a c e I n t e m i t y , ' Center, Zktcut Research
! 2 ] A. I i a r a , >l.Mecat?, S . Yasu: X-Fa:' St,.d?; 0 ' Resid u a l S t r e s s i n G r o u n d Cer..cntecl Ca % t a l l u r n - I n t e r n a t i o n a l , 2 , 2 !19 ( 3 1 A. Miyoshi, A. IIara, c t a i : Stud qllnc? V i c k e r s I n d e n t a t i o n r;f Cerien ur. J o n a n S o c . M e t a l s , 3 1 , 1 0 1 1 9 6 7 ) , 1 1 2 3 . [ a ] F . S u z u k i , H . Hn:iashi. E6'ect of G r i n d i n n oE C e m e n t e l C a r b i 2 e c n I t s Mechdnic-1 P r w e r t i e s , J o u r . J a p a n SOC. W e t a l s , 3 8 , 7(1974), 6 0 4 . 1 5 : 7 7 . S u z u k i , e t al; Phase T r a n s f o r r a t i n n n i B i n d e r i n !,'C-Co Cemented C r l r l j i d e s d u e t o G r i n d i n n , J c u r . J a p a n Soc. M e t a l s , [fil M. Firac, et n l : F n e s s thrniiqh Xen-Cuttinc T n s t , Annals Q? C I R P , 2 8 , 1(1979), 29.
( 2 ) I t h a s been d i s c l o s e d t h a t a c a r b i d e t i p -7rrJmd w i t h l a r q e r dianond c t r i i n s i z e e x h i b i t s l o n n e r t a o l
63
K. Uno,
In this study, the effect of the surface properties of WC-Co cemented carbide tools, which are &rmined by the grain size of the tool grinding diamond wheel, on their brittle fracture is investigated experimentally, and the fractographical analysis is done in order to clarify the mechanism of fatigue type of brittle fracture of cemented carbide subjected to the cyclic impact tests and the cutting tests. It is found by the intermittent turning test and the cyclic impact test that the tool life due to brittle fracture decreases with a decrease in the diamond grain size of the grinding wheel. The Xray diffraction analysis shows that compressive residual stress is produced within the WC grains of the surface layer by grinding, and the binder phase is plastically deformed or work-hardened. Both the absolute values of residual stress and the degree of work-hardening in the surface layer increase with an increase of the diamond grain size. The successive microscopic observation of cemented carbides with the progress of the cyclic impact tests reveals that plastic deformation in the binder phase is likely to initiate microcracks mostly in the grain boundaries. The initiation of microcracks is suppressed by work-hardening. and che propagation of microcracks is decelerated by compressive residual stress.
1. Introduction Complex-structural tool materials such as sintered carbide and cermet have been widely utilized and in the future too they will be leadina tool materials. Nevertheless, the mechanism of their chipping or fracture has been little clarified. In this study the relation between the surface processinas of carbide tool and its brittle Fracture is experimentally investigated from the standpoint of surface inteqrity and the mechanism of structural deterioration of the tool material, which is resarded as the origin of low cycle fatique type of fracture. is microscopically analized.
In this study straight tungsten carbides of 4.0-20.0 wt5 Co, of which qrain size is approximately 2 ;m, are tested for simplicity. Every face of the tip specimens (12.7~12.7~4.76mm) is qround under the condition shown in Table 1 with diamond wheels of resin bond. Care was taken so that heat generated by tool qrindinq could be minimized by reducing the grindins pressure and applying abundant qrindina Fluid. The specimens are chamfered (0.14x0.14 nun) under the same condition as in Table 1. 3 . Result in interrcittent cuttinq test
Here, surface inteqrity 111 denotes overall assessment of surface and subsurface in view of topoloaical, chenical, physical, mechanical and metalluroical prouerties, and low cycle fatique type of fracture, which is most freouently encountered in intermittent cuttinq with sintered carbide tools, indicates brittle fracture by cyclic impacts nuch less than the fatique limit of a common structural metal. Cuttina tools or tiDs are usually surface-treated by qrinding or polishing before their services. It is very much conceivable that the arindinq condition will affect the surface intearity and conseouently the brittle fracture of cuttina tool, because the surface asperities produced by qrindino miqht have notch effect, and residual stress or work-hardenino in the surface of sintered carbide, which is produced by qrinding 121, [ 3 ] may affect the low cycle fatique type of fracture. Research works on the relation between tool verformance. especially, brittle fracture and surface integrity have been little conducted. In this study how the tool qrindina condition, particularly, the diamond qrain size of qrindinq wheel affects the behaviour of brittle fracture of a sintered carbide is experimentally analized. In addition to it, the mechanism of initiation of microcracks, which is resarded as the oriain of brittle fracture, is microscouically investiaated.
Firstly the relation between probability of brittle fracture and diamond qrain size of the qrindinq wheel was obtained in intermittent cuttinq tests. The cutting condition was chosen as shown in Table 2 so that low cycle fatigue type of brittle fracture can be emphasized while suppressing flaking type of fracture due to metal adhesion at the cutting edqe. Typical types of brittle fracture in intermittent cutting are demonstrated in Fio.1, in which chippinq at the corner (type 1): fracture at the major cuttina edqe (type 2) and flakinq on the face (type3) are observed. Types 2 and 3 which are reqarded a s brittle fracture due to metal adhesion are characterized by the tools qround with a grinding wheel of finer grain sizes. Especially, with the tool qround with a prinding wheel of el500 qrain size only flakinq (type 1 ) occurs at the very initial stage of cutting. However, with the tools ground by a qrindina wheel of qrain sizecoarserthan t1000, only chippinq at the corner (type 1 ) occurs, and types 2 and 3 are rarely observed.
Type of cutting
Intermittent turning
2. Test specimens and condition of tool grindinq
Cutting speed
V=lEOm/min
Complex-structural tool .materials consist of hard qrains such as WC, Tic, TiN, etc. and binders as CO, Ni, etc.
Depth of cut
a=l .Omm
Feed rate
S=0.32mm/rev
Tool
WC-4.O%Co cemented carbide (-5,-6,5,6,15,15,0.14C)
Type of grinding Grinding pressure
Constant pressure type of grinding 0.12 MPa
Work traverse speed
10 m/min
Grinding speed
700 m/min
Grinding wheel
Cutting fluid Lathe
Cr-Mo alloy steel (SCM3) Dry 5 0 0 X 1 0 0 0 1 ~ 1 , 5-15kW
Diamond cup wheel
Bond
Resin
Grade
N
Grain size
Work material
# lOO(150um)
t 270( t 280( # 500( t 700( #1000( t1500(
Annals of the CIRP Vol. 31/1/1982
57m) 5211m) 34vm) 22um) 15umI 10um)
Work p iece
D=llOmm
Engagement : B to A DiSengagement:A to B
Focussing at chippinq at the corner (type 1). which is most freauently encountered, the relation between cumulative probability of chippinq and number of impact cycles in intermittent cutting is illustrated in Fiq.2 takinq the qrain size of the diamond grindinq wheel applied as the parameter. This result apparently indicates that the snoother the tool surface, the shorter
59
ir. whicn :he i?i]i?cL a n r i l e e is sc': 3 0 G O ( : . in or tn sir*iil;itcthe im:i,ict i s i i i c ? . i s m s s i l - l c to thy t e.Trnt in Lntcrwittcrit ciittinc:. Tie s is feel 2ftcr e:.'er\' iynict so that the tool t i r ) m;l:; hit its y : i v i n si!rfacc-, 2nd the steel ~jlitc i s clxmc8:l c?owrw.>rclnrior to the i?:mct in crder . The innact Yorce c m be n the imn?ct :'e13cit?: o r the :iteric\l to bc hit, a n d the 0 . 4 5 c carbon steel ( 2 3 0 [I..) rcxim,.it(,l:: 2 0 R V .
4,
,Air
cylinder
I
Flq.1 Types of brittle fracture of wC-4.0?Co carbide tool (K10) Three types of brittle fracture at early stanr (1) Chippinas around corner (2) Brittle fracture of cuttina edcr ( 3 ) Brittle fracture (flakinq) on Face
Air cyli
,
Number of Impact Cycles until Brittle Fracture
Drivinq wire
Pulse motor
Fig.2 Effect o f diamond qrain size of qrindinq wheel on probability of brittle fracture of 1%'C-4.0PCo cemented carbide tool in intermittent turninq
Fig.4
Schema of drop impact testing machine
the tool life in view of brittle fracture i s . 4 . Result in cyclic impact test
Cuttinq test, in which various types of brittle fracture take place simultaneously, is inconvenient i n order to clarify the mechanism o f low cycle fatique typc of brittle fracture independently and its relation with the surface intcqrity. Thus, a cyclic impact testina machine has becn developed in order to abstract only low cvcle fatinue tvpe of brittle fracture. 4.1 Cyclic impact testinq machinc and its reliability
g:
" O
I I
'1 1I0
I
I
,
, ,
,.,,,I100
,
, ,
, ,,,,I loo0
A13.04
, , ,
, ,~
5000
Nunber of Impact Cycles until Failure 01 Cemented carbide
Workpiece1
Bar stock y :RJ;ngle e:Impact angle a:Clearance angle t:Depth of indentation c:Engage angle a:Depth Of Cut
(a) Tool-workpiece enqaqe- (b) Tool tip-testpiece ment in intermittent enqagement in drop cuttinq impact test Fiq.3 Schematic view of engagement in intermittent cutting and drop impact test Tool-workpiece enaagement in interaittent cuttinq is schematically illustrated in Fiq.3fa). The cutting action from the tool-workpiece engaqement to disenaaqement gives rise to thermal stress within the tool and metal adhesion on the Face as wcll, so that it is difficult to purely abstract the mechanical brittle fracture. IF the relative velocity of tool and workpiece can be zero before chip formation takes place, alriost purely mechanical stress can be aDplied onto the tool without metal adhesion. This can be realized by a setup shown in Ficl.l(b), and a cyclic impact testinq machine developed for this purposc is illustratcd in Fir!.
60
Fiq.5 Effect of C o content on cumulatlve prohability of failure of WC-Co cemented carbide in drop impact test Impact anqle e:30°, steel bar r,tock:S45C 240Av. impact velocity v:145n/min, drop mass m: 3.3k9, diamond qrain size of qrindinq wheel:lO:im(t1500) corner radius:0.14C Applying a certain number of drop imoacts to the carbide tiu in thc aforementioned testinq machine, brittle fracture takes !lace within the contact area of tool tio and steel plate. The relation between the number 0 5 cvclic impacts till the brittle fracture and the tool life in view of low cycle faticue type of brittle fracture (cumulative probability of fracture) is shown in fiq.5 takinc thc cobalt content of the sintered carbide as the parameter. The test result that the more the cobalt content, the lonqer the tool life is well coincides to the result in actual intermittent cuttinu tests, and the Veibull's parameter is also fairly close to that in the actual cuttinq tests. Thus, the proposed testina method can be justified fairlv well as a simulator of intermittent cuttino as Far as low cycle Eatiaue type O F brittle Fracture is concerned. 4.2 Effect of diamond qrain size of nrindinn wheel on low cvcle fatinue type of brittle fracture The effect of diamond arain size o f arindina whcel on low cycle fatiuue type of brittle fracture of WC-Co sintered carbidc subjected to the cvclic imDact test is shown in f'in.6, i n which the ordinate indicates
-
--y
r8 -
caused by the tool crindinn action. Fio . 7 i ll,.istrates thc distribution of rcsidual stress ! c o n p r e s s i v e ) cjf b’C Grains in the s~.hsiirfacn nE the sintcrcd carbide nround with t270 ( 5 2 ; J J ) a n d 31500 ( 1 C ‘ m ) oriin size. This was obtained frorr the S-ray diffrJction iCr-K ray) of 11021 plane of WC 2nd the stress , . i l i i c w . 3 ~ calculatrrl by nezns of si2’ 7c)thod.
Ground with diamond wheel1 A Ground with G.C.whee1 Polished with dianor.6 paste
0
0
I
1
0
I
1-m below surface
0
-
8
I
I
8
i
0 I
I
.
,
l
1
,
1
1
Grain Size of Grinding Wheel (um) Fig. 6 Effect of grain size of grindin wheel on characteristic life of WC-13.0%&~ cemented carbide in drop impact test Impact angle e:30°, steel bar stock:S45C 240Hv, impact velocity v:133m/min, drop mass m:3.3kg, corner radius:O.OC the characteristic tool life, i.e., the number of imDact Cycles corresoondinq to 6 3 % cumulative nrobabili-. ty of brittle fracture, and the abssisa indicates the averaqe qrain diameter of the qrinding wheels aoplied. Each ooint in Pic.6 was obtained from eiuht Weibull’s plots. In Fio.6 the data when qrindinq the tip with a Sic wheel and polishinq i t with diamond paste are also DlOted for reference. This result clearly demonstrates that the finer the ?rain size aDplied in grinding the carbide tip, the shorter the tool life is. Since metal adhesion was observed neither on the tool face nor at the cuttina edge in the cvclic impact test, the chemical activity between the carbide tip and the matinn material would not affect the brittle fracture of the tip. The exocrimental fact that the asperity heiqht of tool surface and cuttinq edoe, which i s determined mostly by the qrain size of qrindinq wheel applied, apparentlv affects the tool life favourably, indicates that the notch effect resulted from the surface or edge aspcritics does not seem to be a positive factor to brittle fracture of tool tip. A conceivablc factor to affect the low cycle fatique type of brittle fracture is physical or mctallurqicnl one such a s residual stress, work-hardenins o r structural transformation in the surface. 5. Surface alteration due to qrindino and its effect on low cycle fatique type of brittle Fracture An apparent surface alteration of sintercd carbide due to qrindinq with a diamond wheel is =raomentation of WC grains in the surface, this beinrf niorc emphasized with the coarser qrain sizes.
-1.0.
\. l O u m 01500)
Cr-Ki WC(102) Diamond grain size, of grindinq wheel 0 52um (8270) A
Cr-K I
lOilm
-
(#l500)
0-
Fi9.7 Residual stress distribution below qround surface surface of WC-20.04Co cemented carbide However. even though the carbide tip is qround with coarse qrains as t270, the depth of qrain fragmentation is as shallow as 1 iim which corresponds to a half of V C grain diameter. It is hardly conceivable that such the thin layer of qrain fracture directly affects the low cycle fatinue type of tool fracture, and moreover this effects Favourably upon the tool life. What is worthy of attention is residual surface stress
x
U .r(
J
5 C c(
I (20um below surface
I
50’
L I
I I
1
I
I
60’
70’
80’
20 Fiq.3 Diffraction output for varied depths in reference to diffraction anule in WC-20.0?,Co cemented carbidc It is pointed out that in the case of carbidc tip qround with 3270 qrain size the compressive rcsidual stress increases sharply towards the sround surface .2nd the absolute value is a l s o very larile, whereas in the case of *1500 orain size the stress value is extrerrely small. This is ascribable to the fact that only the surface layer i s extended two-dimensionally by the mechanical energy of qrindinn, and this trend is more emohasized with the coarser grain sizes. Althouqh the residual stress within the binder phase has not bcen neasured in this study, within the binder phase too compressive residual stress i s oenerated by qrindinn accordinq to Suzuki, et a 1 1 4 1 . It is known that the binder phase of a b’C-Co sintered carbide, which is; phase of f.c.c. under an annealed condition, is partly transformed into Dlnte-like ;’ phase of h.c.p. by external stress 1 5 1 . If the stressinduced transformation has thus occurred in the binder phase, it must be somewhat accompanied with plastic flow there. So that the plastic deformation could be identified by detectinq rJ phase with X-ray. Fiq.B(a). fb) and ( c ) are the X-ray diffraction outputs of the surface 1 :m, 8 ‘im and 20 Ilm, respectively, beneath the surface of thc sintered carbide nround with 1270 diamond grains. In the subsurface immediately beneath the around surface fFiq.8 ( a ) ) an appreciable output of F ’ phase cienerated by the stress-induced transformation is observed in the background of ’, phase, this indicetinu the esistence of nlastic deformation. However. in the subsurface 8 I m beneath the around surface (Fiq.a(b))the diffraction intensity of .‘phase is fairly reduced, and 20 : ~ mbeneath the ground surface it is hardly recognized. From the above fact it is concluded that tool qrindinq qives rise to plastic flow in the binder phase, and the depth of plastic flow reachcs 10 :in aproximately. Accompanied with the plastic flow in the binder phase, a work-hardened layer will be formed. Fiq.9 shows the distribution of microhardness measured downwardly from the oround surface of VlC-5.5% Co and VC-209 Co sintered carbides taking the arain size of orindlinq wheel as the parameter. This result demonstrates that the carbide surface qround with #270 qrain size is workhardened down to 10 i:m beneath the qround surface, whcreas it is little work-hardened with ‘1500 qrain size.
61
t I
I
1
I
1
0
10
20
30
I
I
I
F i g . 3 % a r d n e s s 6 i s t r l b u t i o n of rrr.;ur.d si:rf?ce W-Co ccnented carbide il;.rdness t e s t i n r j l o ? . d : 5 . 5 ? C r 2000, 2 0 . 0 '
c y c l c s . F i q . 1 1 i s o n e e x a m p l e showr'ret n e a r t h c c n i - n c r 0 4 3 c , i r h i r i c t i p , tt!c s t r i , c t i l r e iprior- to t h e c : . c l i c i r 1,) is t h e s t : - u c t u r 1 ~ os r i t v z c t . C o i n t i o n 01- r o t a t i o n o This i n A i c a t e s t h a t p l a s t i c i n t h c !>intlt.r p h ~ i:wzcts. Durin-: t h e p l a s t i c f l o w t .,i-ies a r e ? , i c r ~ s c n p i c a l l yb r o k e n i n t!ir ex.t:-e:w c z s e t h e I T ? r ; l i n s t h e T s e l v e s f a l l o u t .? i n T i c . l l ) , and i n sor1.e c a s e s new WC m a i n s c e dJe t o i h e p l a s t i c f l o w oT t h e hin?r-r ';rFm; 1 i n F i q . 1 1 ) .
0:
1000
;li~i
c
e d o u t ?rwn
nic.7
2nd F i r l . 9 .
t h e ;Ibo.;v < , x p c r i i r i c n r a l I-esults, it i s ob:-ious to urindino increoses le ' ? t i a i i e t y p e of s oric a s u c c t o f s u r i s i'irns i d e i - e d t n .;iipprcss t h c s t r i i c t on o r i n i t i ' i t i o n o f r n i c r o d c E c c t s ir. t h e c a r b i d e s u r f a c e , and cormrcssive resiAual s t r e s s , which i s a n o t h e r a s ? e c t , t o s u n n r e s s crowth o r pro: > . i c ~ ~ i t i c )on f t h e 3icrndefects.
Fi-or1
3,,,,,
( a ) N?lmbcr o f i m p a c t cycles : 0
€'is.l i
( b ) Number of i m p a c t c y c l e s : 40
M i c r o s c o p i c s t r u c t u r a l d e t e r i o r a t i o n of WC20.OTco c e m e n t e d c a r b i d e i n d r o p i m p a c t t e s t
Khat i s w o r t h y o f a t t e n t i o n i s t h a t t h e i n i t i a t i o n o f m i c r o d e f e c t s a t t h e I
I
100
I--
cF
Impact direction
Measuring a r e a : 10Ox251im J
I
I
WC G r a i n S i z e
D e p t h of Removed L a y e r f r o m Ground S u r f a c e ( m ) F i q . 1 0 E f f e c t of s u c c e s s i v e s t o c k r e m o v a l from c r o u n d s u r f a c e o n c h a r a c t e r l s t i c l i f e o f WC-20.0% Co cenented c a r b i d e i n d r o p impact test I m p a c t a n g l e e : 3 0 ° , s t e e l b a r s t o c k : S45C 2 8 5 H v , i m p a c t v e l o c i t y v:145m/min, d r o p m a s s m : 3.3kq, corner radius: 0.14C
6 . M i c r o s c o p i c o b s e r v a t i o n of s t r u c t u r a l d e t e r i o r a t i o n of s i n t e r e d c a r b i 6 e A f i x & tarciet o f t h e cnrhiclc s u r f a c e s u b j e c t e c i t o t.hc cvcl i c i m n a c t s h a s h e e n o b s e r v e d c o n t i n u a l l y a f t c r
a d e f i n i t e number of c y c l i c i m p a c t s w i t h a n e l e c t r o n s c a n n i n q m i c r o s c o r ) c w i t h 10000 t i m c s m a c i n i f i c a t i o n . I t has heen rcvcald t h d t t h e s t r u c t u r a l d e t e r i o r a t i o n s t a r t s a f t e r . i p n r o x i m a t c l y 4 0 c y c l e s o f i m p a c t under an i m p a c t c o n d i t i o n cori-esoondinq t o t h p c h a r a c t e r -
62
I
I
I
I
(!Jm)
F i g . 1 2 I n f l u e n c e of WC g r a i n s i z e of VC-20% Co c e m e n t e d c a r b i d e o n c l e a v a g e a t WC/binder i n t e r f a c e i n c y c l i c impact test Number of i m p a c t c y c l e s : 4 0
lire i s l i k e l y t o o c c u r . T h i s i n d i c a t e s t h a t t h e l a r g er t h e VC g r a i n , t h e h i c l h o r t h e P r o b a b i l i t y of microf r a c t u r e a t t h e q r a i n b o u n d a r y i s . T h i s t r e n d i s ass o c i a t e d w i t h t h e f a c t t h a t t h e l a r s e r t h e VC a r a i n , t h e h i q h e r t h e stress i n t h e b i n d e r p h a s e o r q r a i n b o u n d a r y close t o t h e a f o r e m e n t i o n e d i"C n r a i n , a s Hirao, e t a1 [ h l pointed o u t . F r o n t h e h i s t o r i c a l o b s e r v a t i o n of t h e c a r b i d e s u r f a c e i t i s u n d e r s t o o d t h a t p l a s t i c f l o w of t h e b i n d e r phase Lrirrqers t h e m i c r o d e f e c t s s u c h as m i c r o f r a c t u r e a t t h e q r a i n b o u n d a r i e s , f a l l - o f f of t h e PC q r a i n s , e t c . W h i l e t h e g r o w t h r a t e o f m i c r o d e f e c t s or microc r a c k s i s c o m p a r a t i v e l y slow a t t h e i n i t i a l s t a a e , t h e r a t e of c r a c k p r o p a g a t i o n becomes e x t r c m e l v f a s t a t t h e F i n a l s t a q e . One e x a m p l e of c r a c k p r o o a o a t i o n a t t h e f i n a l s t a g e is shown i n Fig.13, i n w h i c h how t h e c r a c k p r o p a a a t e s by o n l y s i n q l c i m p a c t ( t h e 2 4 1 s t i m p a c t ) is v i s > . i a l i z e d , and at t h i s s t a q c n o p l a s t i c
l i f e i n :-icw o f h r i t t l ?
Ci-;3cture i n t h e c : , c l i c im-
( 3 ) The s L r ? a c e i l t e r a t i o n of s i n t c r r d c > r t ) i d e s u c h a s wnrk-hardeninri and c o r n r e s s i v e r e s i d u a l s t r e s s s ~ i l j ~ r c s s et sh e h r i t t l e f r a c t u r e . 2r.d t h i s is emnhnsizcS w i t h I i r q c r c r ~ i ns i z c s n i d i a m c n d o r i n d i n q w h e e l a r m l i e d . C l o s e l : ~ s p e , i k j r . 4 , w o r k - h a r d e n i n q of c a r b i d e s ' n - f a c e S ' J P D T ~ S S E S ocnci-ati n n 06 :iicrcc r - a c k s , 2nd c w m r e s s i - e rcs:-i.:?l q t v ~ nS ~ I I ~ D ~ C S S C S ijrr,wth ,3f t h e m i c r o c r a c k s . 'd
Thc n r i a i n o f low c]-cie f a t i n u e t-::ze of b r i t t l e G r ~ c t u r ei s p l a s t i c f l r w i n t h e b i n d e r p h a s e a n d -iccorSincrl:: r c s u l t e d v i c r o c r a c k s a t o r n e a r t h a 7r71n S n l ; n d ? r i c s . r ' d r t h e r ? ' c r c , t h e l a r c c e r t h e d i a n e t e r of :KC : ? r a i n s . t h e h i s h e r t h e 3 r o b a h i l i t : : o f b r i t t l e ' r D c t i i r c i r m i n r l t h e a r q r e r e n t i o n e d !.C ' r:ra:ns i s .
Qeferences
f!l
F i q . 1 3 Exarrple of c r s c k p r o p d o a t i 2 n of I v C - 2 O . r ) ~ C o cemented c a r b i d e I n d r o p i n p a c t test
Elow o f t h e b i n d e r o h a s e i s r e c o n n i z e d . A t t h i s s t a q e t e n s i l e s t r e s s o l n v s a i a j o r D a r t a n d t h e c r - c k s nrooaaates mainlv alonn t h e q r a i n houndarics i n i b r i t t l e m a n n e r . C o n s i d e r i n u t h a t t h e o v e r a l l n u n b e r of c \ ' c l i C i m p a c t s t h r o u q h q e n e r a t i o n of m i ' z r o c r a c k s , t h e i r qrowt h and c r ; l c k p r o p a q a t i o n t o t h e f:n,il b r i t t l e :r;lctu r e i s f a i r l y l a r n e i n most c a s 5 s . 7. Conclusions ( 1 ) A c y c l i c i n u a c t t e s t i n o n a c h i n c t o e-:itlwtf
toi1c:hn c s s of t o o l m a t e r i a l s h a s been c?e~~r:oucd,a n d t h e t e s t i n a r e l i a b i l i t v and t h e c o r r e l a t i o n w i t h i n t c r m i t t e n t c u t t l n n t e s t haTfe b e e n zssesse?. As a result t h e t e s t i n s i..achine h a s b e e n v e r i f i e d to s a t i s f a c t o r i l v represent the strenath anainst low c y c l e f a t i n u e t v p c of b r i t t l e f r a c t u r e .
'8. p i e l ? , t'. K o s t e r , et a l : .'"itchininn c C ? i c h Str c n n t h R t c e ! s w i t h F r r n h a s i s c)n F u r f a c e I n t e m i t y , ' Center, Zktcut Research
! 2 ] A. I i a r a , >l.Mecat?, S . Yasu: X-Fa:' St,.d?; 0 ' Resid u a l S t r e s s i n G r o u n d Cer..cntecl Ca % t a l l u r n - I n t e r n a t i o n a l , 2 , 2 !19 ( 3 1 A. Miyoshi, A. IIara, c t a i : Stud qllnc? V i c k e r s I n d e n t a t i o n r;f Cerien ur. J o n a n S o c . M e t a l s , 3 1 , 1 0 1 1 9 6 7 ) , 1 1 2 3 . [ a ] F . S u z u k i , H . Hn:iashi. E6'ect of G r i n d i n n oE C e m e n t e l C a r b i 2 e c n I t s Mechdnic-1 P r w e r t i e s , J o u r . J a p a n SOC. W e t a l s , 3 8 , 7(1974), 6 0 4 . 1 5 : 7 7 . S u z u k i , e t al; Phase T r a n s f o r r a t i n n n i B i n d e r i n !,'C-Co Cemented C r l r l j i d e s d u e t o G r i n d i n n , J c u r . J a p a n Soc. M e t a l s , [fil M. Firac, et n l : F n e s s thrniiqh Xen-Cuttinc T n s t , Annals Q? C I R P , 2 8 , 1(1979), 29.
( 2 ) I t h a s been d i s c l o s e d t h a t a c a r b i d e t i p -7rrJmd w i t h l a r q e r dianond c t r i i n s i z e e x h i b i t s l o n n e r t a o l
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