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The fracture toughness behaviour of a 18 Ni (300 grade) maraging steel in various solution treated and aged conditions
Scripta
METALLURGICA
Vol. 18, pp. 373-378, 1984 Printed in the U.S.A.
Pergamon Press Ltd. All rights reserved
T H E F R A C T U R E T O U G H N E S S B E H A V I O U R OF A 18 Ni (300 GRADE) M A R A G I N G S T E E L IN V A R I O U S S O L U T I O N T R E A T E D AND A G E D C O N D I T I O N S H. C h a n d r a - H o l m , M. Bichsel and P. U g g o w i t z e r Institute for M e t a l l u r g y Swiss F e d e r a l Institute of T e c h n o l o g y E T H - Z e n t r u m , 8092 ZUrich, S w i t z e r l a n d (Received (Revised
December 5, 1983) January 24, 1984)
INTRODUCTION E x p e r i m e n t a l e v i d e n c e i n d i c a t e s that in m a r a g i n g steels the m e c h a n i c a l p r o p e r t i e s as m e a s u r e d by u n i a x i a l tensile tests and the fracture t o u g h n e s s beh a v i o u r are i n f l u e n c e d to d i f f e r e n t extents by v a r i o u s m e t a l l u r g i c a l parameters. For example, the p r i o r a u s t e n i t e grain size exerts c o n s i d e r a b l e i n f l u e n c e on the tensile properties, but n e g l i g i b l e i n f l u e n c e on f r a c t u r e t o u g h n e s s - as shown by KIC values (i). On the o t h e r hand, the fracture toughness was found to be g r e a t l y i m p a i r e d by the p r e s e n c e of Ti (C,N) and Ti2S p a r t i c l e s on the prior a u s t e n i t e grain b o u n d a r i e s (2,3). In this study, the i n f l u e n c e of v a r y i n g the m i c r o s t r u c t u r e on the tensile p r o p e r t i e s and fracture t o u g h n e s s b e h a v i o u r will be reported. V a r i o u s m i c r o s t r u c t u r e s were p r o d u c e d by a g e i n g at d i f f e r e n t temperatures. The i n f l u e n c e of v a r y i n g the s o l u t i o n t e m p e r a t u r e is also reported. MATERIAL
AND E X P E R I M E N T A L
METHODS
The c o m p o s i t i o n of the steel studied was 0.009 C, 18.51 Ni, 8.76 Co, 5.09 Ti, 0.16 Cr, 0.i AI, 0.09 Si, 0.07 Cu, 0.01 (max) S, < 0.01 Mn, 0.004 P and 0.006 S, all wt %. The m a t e r i a l was s u p p l i e d by the V e r e i n i g t e n E d e l s t a h l w e r k e AG, in bar form, of t h i c k n e s s 70 x 70 mm. U n i a x i a l tensile tests were c o n d u c t e d on c y l i n d r i c a l s p e c i m e n s (gage diam e t e r = 6 ram, gage length = 35 mm) on an Instron m a c h i n e w i t h a c r o s s h e a d speed of 1 mm/min. F r a c t u r e t o u g h n e s s tests were c o n d u c t e d on CT-I specimens u n l o a d i n g J integral m e t h o d for d u c t i l e m a t e r i a l s (as-solution KIC m e t h o d for the less d u c t i l e m a t e r i a l s (aged condition).
using the partial treated) and the
S p e c i m e n s p r e p a r e d from the a s - r e c e i v e d m a t e r i a l were heat treated for 3 hrs at 500oc, to ensure similar basic m i c r o s t r u c t u r e in all samples. S o l u t i o n heat t r e a t m e n t was c a r r i e d out for 1 hr at 770, 820 or 870°C. Some samples in each s o l u t i o n - t r e a t e d c o n d i t i o n were aged at s e l e c t e d t e m p e r a t u r e s in the range 400 600oc for 3 hours. The samples were always air cooled. The basic m i c r o s t r u c t u r a l features of the m a t e r i a l s in the v a r i o u s c o n d i t i o n s were d e t e r m i n e d u s i n g both o p t i c a l and Philips EM 400 t r a n s m i s s i o n e l e c t r o n m i c r o s c o p e s . Thin foils were p r e p a r e d using a m i x t u r e of 88 ml perchloric acid, 130 ml butylcellosolve in 1 litre of a b s o l u t e alcohol, at a t e m p e r a t u r e of -40oc, using a Struers unit. Prior to e l e c t r o t h i n n i n g , the samples were thinned c h e m i cally in a s o l u t i o n of 5 ml HF and 95 ml H202.
373 0 0 3 6 - 9 7 4 8 / 8 4 $3.00 + .00 Copyright (c) 1984 P e r g a m o n Press
Ltd.
374
FRACTURE TOUGHNESS OF 18 Ni M A R A G I N G STEEL
Vol.
18, No.
4
RESULTS The m i c r o s t r u c t u r a l features as r e v e a l e d by optical and t r a n s m i s s i o n e l e c t r o n m i c r o s c o p e i n v e s t i g a t i o n s and the SEM m i c r o g r a p h s of the f r a c t u r e surfaces, are shown in figures 1 (a-c) and 2 (a-c) for the various s o l u t i o n - t r e a t e d and aged conditions. The m a i n features can be summarized as follows: Optical m i c r o s c o p y observations: W h e n etched w i t h a solution of 1 gm NaOH, 4 gms KMnO~ in i00 ml H20, particles, p r e s u m b l y carbides, are found d i s t r i b u t e d on prior a u s t e n i t e grain b o u n d a r i e s and w i t h i n grain interiors, in both solutiontreated and aged conditions. Etching w i t h V i l e l l a ' s etchant (5 ml HCI, 1 gm picric acid in i00 ml ethanol) reveals the basic structure in all samples (except the o v e r a g e d ones) to be lath martensite. The structure in the o v e r a g e d c o n d i t i o n is aged martensite. TEM observations: In the three solution treated c o n d i t i o n s the basic structure is lath m a r t e n s i t e c o n t a i n i n g dislocations. Particles, p r e s u m a b l y carbides, are also o b s e r v e d on prior a u s t e n i t e boundaries. The o v e r a g e d l a t h spacing is about 0.3 ~m. The basic structure after ageing at 400°C is still lath martensite. No new p r e c i p i t a t i o n is detected. A g e i n g at 500°C results in p r e c i p i t a t e s forming on lath b o u n d a r i e s and on the d i s l o c a t i o n s w i t h i n the laths. These give rise to streaks in the SAD pattern. After ageing at 600OC, p r e c i p i t a t e s w i t h spherical and plate like cross section are observed. The structure is no longer lath martensite. SEM observations: coalescence.
Fracture,
in all cases,
involves t r a n s g r a n u l a r m i c r o v o i d
The m e c h a n i c a l p r o p e r t i e s m e a s u r e d in the u n i a x i a l tensile tests - ~0.2 yield stress, u l t i m a t e tensile stress, e strain at fracture - and the KIC or KjC values are summarized in table i. TABLE 1 Summary of M e c h a n i c a l P r o p e r t i e s
Property measured
Solution treatment temperature
Ageing temperature §
770Oc
820oc
870oc
914
870
886
1485
2195
1454
1121
1054
1009
1587
2274
1806
6.2
4.5
7.4
2.9
2.1
1.6
Kic**or KjC* (MN m-3/~)
64.3
78.0
79.2
48.4
Nature of crack propagation
stable
00.2 (MN m -2) UTS (MN m -2) efracture (%)
§ * **
stable
stable
400oc
unstable
500oc
600oc
35
48.2
unstable
unstable
S o l u t i o n treated at 820°C A g e i n g at 500Oc c o r r e s p o n d s to the peak aged c o n d i t i o n S o l u t i o n treated c o n d i t i o n Aged c o n d i t i o n
The o b s e r v a t i o n s can be summarized as follows: (i) V a r y i n g the solution t r e a t m e n t t e m p e r a t u r e b e t w e e n 770 and 870°C has negligible influence on the m e c h a n i c a l p r o p e r t i e s both on tensile p r o p e r t i e s and on the level of fracture toughness.
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F R A C T U R ETOUGHNESS OF 18 Ni MARAGING STEEL
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(2) The ageing t r e a t m e n t selected, p r o d u c e s various m i c r o s t r u c t u r e s . This v a r i a t i o n in the m i c r o s t r u c t u r e exerts c o n s i d e r a b l e i n f l u e n c e on the tensile behaviour, but n e g l i g i b l e i n f l u e n c e on fracture toughness. (3) The aged m a t e r i a l s are s t r o n g e r than the s o l u t i o n treated materials, a KIC of only 35 - 50 MN m-S/2, are about half as tough.
but, w i t h
(4) Samples in the s o l u t i o n treated c o n d i t i o n have quite a low KjC level, 80 K N m - S / 2 . (5) I r r e s p e c t i v e of the m i c r o s t r u c t u r e , coalescence.
70 to
fracture involves t r a n s g r a n u l a r m l c r o v o l d
DISCUSSION The range of the s o l u t i o n t r e a t m e n t t e m p e r a t u r e selected in the p r e s e n t study does not lead to a p p r e c i a b l e v a r i a t i o n in the prior a u s t e n i t e grain size (4). The m a r t e n s i t e lath spacing, is t h e r e f o r e almost c o n s t a n t for the various heat treatm e n t s and this e x p l a i n s the n e g l i g i b l e influence of s o l u t i o n t e m p e r a t u r e on the m e c h a n i c a l properties. It is w e l l - k n o w n that d u r i n g the ageing of m a r a g i n g steels r o d - l i k e NisMo p r e c i p i t a t e s form and these act as p r i n c i p l e h a r d e n i n g agents (5). The spherical p r e c i p i t a t e s are r e c o g n i z e d to be FeTi (3). Similar p r e c i p i t a t e s were found in the p r e s e n t study also. The o b s e r v e d i n f l u e n c e of the ageing t r e a t m e n t on the tensile p r o p e r t i e s was as expected. The m o s t striking o b s e r v a t i o n s are that (a) the v a r i o u s ageing t r e a t m e n t s have no a p p r e c i a b l e i n f l u e n c e on the fracture t o u g h n e s s and that (b) the m a x i m u m level of fracture toughness r e a c h e d is as low as 40 MN m-3/2 in the aged c o n d i t i o n and 80 MN m -s/~ in the a s - s o l u t i o n treated condition. The m i n o r influence e x e r t e d by the m i c r o s t r u c t u r a l d i f f e r e n c e on the fracture p r o c e s s e s can also be seen in the s i m i l a r i t y in the nature of the fracture surfaces (Figs 1 (3) and 2 (3)). Dimples w e r e found m o s t l y to be a s s o c i a t e d w i t h large p a r t i c l e s w h i c h were a n a l y s e d to be t i t a n i u m carbide particles. These were found in all samples, s o l u t i o n - t r e a t e d and aged. That it is the p r e s e n c e of the t i t a n i u m carbide p a r t i c l e s w h i c h is r e s p o n s i b l e for the low level of fracture t o u g h n e s s can be confirmed by a p p l y i n g the model of St~we (6) for the c a l c u l a t i o n of KIC. A c c o r d i n g to this model, w h e n Y o u n g ' s m o d u l u s E, the dimple size d and the level of the ultimate. tensile s t r e n g t h o of the m a t e r i a l are known, KIC is given as
The values of KIC so c a l c u l a t e d are given in table 2. TABLE 2 C a l c u l a t i o n of KIC accordin= Temperature oc UTS (MN m -2) Dimple size,
d (pro) KIC (MN m-3/z)** range, ealc. KIC (MN m-S/z ) experimental *
Solution treatment temperature 770oc 820oc 870oc 1121
1054
1009
to St~we model: A~eing temperature * 500oc 600oc 400oc
1587
2274
1806
6.8 + 0.5
11.3 -+ i
7 + 0.3
7.2 + 2
8 + 2
7.9 -+ 2
26.24 to 28.24
32.45 to 25.48
26.28 to 26.39
29.4 to 39.1
36.47 to 47.08
30.9 to 40.0
64.3 +
78.0+
79.2+
48.4
Solution treatment temperature: 820°C
35
48.2
** Calculated according to equation (i)
KIC is converted from measured value of JIC as KjC = ~ JIcE/(I --l)2) where E is the Young's modulus and ~ is the Poisson's ratio.
376
FRACTURE TOUGHNESS OF 18 Ni M A R A G I N G STEEL
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There is good a g r e e m e n t b e t w e e n the values so c a l c u l a t e d and the e x p e r i m e n t ally m e a s u r e d values for the aged materials. This shows that in the aged samples, the t i t a n i u m carbide p a r t i c l e s are m a i n l y r e s p o n s i b l e for the fracture behaviour. Data for samples in solution treated condition, given in table 2, show that such a c a l c u l a t i o n is not valid for ductile materials, as was also p o i n t e d out by St~we (6). Thus, the v a r i a t i o n of the s o l u t i o n or ageing t e m p e r a t u r e has no a p p r e c i a b l e influence, because the d i s t r i b u t i o n and the size of the t i t a n i u m carbide p a r t i c l e s are similar in all c o n d i t i o n s and the p r e s e n c e of Ni~Mo and FeTi p r e c i p i t a t e s in the aged c o n d i t i o n s becomes unimportant. K a l i s h and Rack (3) o b s e r v e d that the rate of cooling after heat t r e a t m e n t had an important influence on the fracture t o u g h n e s s of m a r a g i n g steels. They o b s e r v e d a p p r e c i a b l e i m p r o v e m e n t in w a t e r - q u e n c h e d materials, as c o m p a r e d to aircooled ones, w h e n solution treated above 1000°C. Cairns and N o v a k (2) also r e ported that d u c t i l i t y can be r e s t o r e d by solution t r e a t m e n t s above 980oc followed by rapid cooling. This p r o c e d u r e should e l i m i n a t e the t i t a n i u m carbide particles. In order to check this, samples were s o l u t i o n treated at i100°C and w a t e r quenched. A n o t h e r set was solution treated at 820°C and w a t e r quenched. The JIC values m e a s u r e d for these c o n d i t i o n s were 80.7 and 62.7 KJ m -2 r e s p e c t i v e l y . That either w a t e r q u e n c h i n g or s o l u t i o n t r e a t i n g at a h i g h e r t e m p e r a t u r e did not lead to i m p r o v e m e n t s in the level of fracture toughness, is c o n t r a r y to expectations. O b s e r v a t i o n s of the fracture surface indicate that such t r e a t m e n t s do not lead to the c o m p l e t e d i s s o l u t i o n of t i t a n i u m carbide particles. W a t e r q u e n c h i n g may not be fast enough to hinder the formation of new c a r b i d e s in the 25 mm thick CT-I samples. CONCLUSIONS The p r e s e n c e of t i t a n i u m carbide p a r t i c l e s is very d e t r i m e n t a l to the toughness of the 18 Ni m a r a g i n g steels and o v e r r i d e s any influence the other m i c r o structural features may have. E x p e r i m e n t s are in p r o g r e s s to reduce the carbon content so as to m i n i m i z e the amount of t i t a n i u m c a r b i d e s that form and also to choose more rapid m e t h o d s of q u e n c h i n g the samples, than e m p l o y e d here. REFERENCES i. 2. 3. 4. 5. 6.
H.J. Rack, Scr. Met., 13, 577 (1979) R.L. Cairns and CJ. Novak, Met. Trans., 2, 1837 (1971) D. Kalish and H.J. Rack, Met. Trans., 2, 2665 (1971) H.J. Rack, Metals Sci. and Eng., 34, 263 (1978) K. Shimizu and H. Okamoto, Trans. J. I. M., 12, 273 (1971) H.P. StOwe, Eng. F r a c t u r e Mechanics, 13, 231 (1980)
Vol.
18, No.
4
FRACTURE T O U G H N E S S OF 18 Ni M A R A G I N G STEEL
ii!i iliiii!i!ii!ciii!i iiiiillii!i!ii i!ii!ii iill~
FIG.
1
M i c r o s t r u c t u r e and fracture surface of s o l u t i o n - t r e a t e d samples. S o l u t i o n t r e a t m e n t temperatures: a) 770°C, b) 820°C, c) 870°C Time: 1 hr. (i) Optical micrographs, (2) T r a n s m i s s i o n e l e c t r o n m i c r o g r a p h s and (3) SEM m i c r o g r a p h s of the fractured surfaces of CT-I samples.
377
378
FRACTURE
TOUGHNESS
OF 18 Ni MARAGING
a
b
STEEL
Vol.
c
1
2
3
FIG.
2
Microstructure and fracture surface of aged samples. Solution treated for 1 hr at 820°C. Ageing temperature: (a) 400°C, (b) 500°C, (c) 600oc. (i) Optical micrographs, (2) Transmission electron micrographs and (3) SEM micrographs of the fractured surfaces of CT-I samples.
18, No. 4
METALLURGICA
Vol. 18, pp. 373-378, 1984 Printed in the U.S.A.
Pergamon Press Ltd. All rights reserved
T H E F R A C T U R E T O U G H N E S S B E H A V I O U R OF A 18 Ni (300 GRADE) M A R A G I N G S T E E L IN V A R I O U S S O L U T I O N T R E A T E D AND A G E D C O N D I T I O N S H. C h a n d r a - H o l m , M. Bichsel and P. U g g o w i t z e r Institute for M e t a l l u r g y Swiss F e d e r a l Institute of T e c h n o l o g y E T H - Z e n t r u m , 8092 ZUrich, S w i t z e r l a n d (Received (Revised
December 5, 1983) January 24, 1984)
INTRODUCTION E x p e r i m e n t a l e v i d e n c e i n d i c a t e s that in m a r a g i n g steels the m e c h a n i c a l p r o p e r t i e s as m e a s u r e d by u n i a x i a l tensile tests and the fracture t o u g h n e s s beh a v i o u r are i n f l u e n c e d to d i f f e r e n t extents by v a r i o u s m e t a l l u r g i c a l parameters. For example, the p r i o r a u s t e n i t e grain size exerts c o n s i d e r a b l e i n f l u e n c e on the tensile properties, but n e g l i g i b l e i n f l u e n c e on f r a c t u r e t o u g h n e s s - as shown by KIC values (i). On the o t h e r hand, the fracture toughness was found to be g r e a t l y i m p a i r e d by the p r e s e n c e of Ti (C,N) and Ti2S p a r t i c l e s on the prior a u s t e n i t e grain b o u n d a r i e s (2,3). In this study, the i n f l u e n c e of v a r y i n g the m i c r o s t r u c t u r e on the tensile p r o p e r t i e s and fracture t o u g h n e s s b e h a v i o u r will be reported. V a r i o u s m i c r o s t r u c t u r e s were p r o d u c e d by a g e i n g at d i f f e r e n t temperatures. The i n f l u e n c e of v a r y i n g the s o l u t i o n t e m p e r a t u r e is also reported. MATERIAL
AND E X P E R I M E N T A L
METHODS
The c o m p o s i t i o n of the steel studied was 0.009 C, 18.51 Ni, 8.76 Co, 5.09 Ti, 0.16 Cr, 0.i AI, 0.09 Si, 0.07 Cu, 0.01 (max) S, < 0.01 Mn, 0.004 P and 0.006 S, all wt %. The m a t e r i a l was s u p p l i e d by the V e r e i n i g t e n E d e l s t a h l w e r k e AG, in bar form, of t h i c k n e s s 70 x 70 mm. U n i a x i a l tensile tests were c o n d u c t e d on c y l i n d r i c a l s p e c i m e n s (gage diam e t e r = 6 ram, gage length = 35 mm) on an Instron m a c h i n e w i t h a c r o s s h e a d speed of 1 mm/min. F r a c t u r e t o u g h n e s s tests were c o n d u c t e d on CT-I specimens u n l o a d i n g J integral m e t h o d for d u c t i l e m a t e r i a l s (as-solution KIC m e t h o d for the less d u c t i l e m a t e r i a l s (aged condition).
using the partial treated) and the
S p e c i m e n s p r e p a r e d from the a s - r e c e i v e d m a t e r i a l were heat treated for 3 hrs at 500oc, to ensure similar basic m i c r o s t r u c t u r e in all samples. S o l u t i o n heat t r e a t m e n t was c a r r i e d out for 1 hr at 770, 820 or 870°C. Some samples in each s o l u t i o n - t r e a t e d c o n d i t i o n were aged at s e l e c t e d t e m p e r a t u r e s in the range 400 600oc for 3 hours. The samples were always air cooled. The basic m i c r o s t r u c t u r a l features of the m a t e r i a l s in the v a r i o u s c o n d i t i o n s were d e t e r m i n e d u s i n g both o p t i c a l and Philips EM 400 t r a n s m i s s i o n e l e c t r o n m i c r o s c o p e s . Thin foils were p r e p a r e d using a m i x t u r e of 88 ml perchloric acid, 130 ml butylcellosolve in 1 litre of a b s o l u t e alcohol, at a t e m p e r a t u r e of -40oc, using a Struers unit. Prior to e l e c t r o t h i n n i n g , the samples were thinned c h e m i cally in a s o l u t i o n of 5 ml HF and 95 ml H202.
373 0 0 3 6 - 9 7 4 8 / 8 4 $3.00 + .00 Copyright (c) 1984 P e r g a m o n Press
Ltd.
374
FRACTURE TOUGHNESS OF 18 Ni M A R A G I N G STEEL
Vol.
18, No.
4
RESULTS The m i c r o s t r u c t u r a l features as r e v e a l e d by optical and t r a n s m i s s i o n e l e c t r o n m i c r o s c o p e i n v e s t i g a t i o n s and the SEM m i c r o g r a p h s of the f r a c t u r e surfaces, are shown in figures 1 (a-c) and 2 (a-c) for the various s o l u t i o n - t r e a t e d and aged conditions. The m a i n features can be summarized as follows: Optical m i c r o s c o p y observations: W h e n etched w i t h a solution of 1 gm NaOH, 4 gms KMnO~ in i00 ml H20, particles, p r e s u m b l y carbides, are found d i s t r i b u t e d on prior a u s t e n i t e grain b o u n d a r i e s and w i t h i n grain interiors, in both solutiontreated and aged conditions. Etching w i t h V i l e l l a ' s etchant (5 ml HCI, 1 gm picric acid in i00 ml ethanol) reveals the basic structure in all samples (except the o v e r a g e d ones) to be lath martensite. The structure in the o v e r a g e d c o n d i t i o n is aged martensite. TEM observations: In the three solution treated c o n d i t i o n s the basic structure is lath m a r t e n s i t e c o n t a i n i n g dislocations. Particles, p r e s u m a b l y carbides, are also o b s e r v e d on prior a u s t e n i t e boundaries. The o v e r a g e d l a t h spacing is about 0.3 ~m. The basic structure after ageing at 400°C is still lath martensite. No new p r e c i p i t a t i o n is detected. A g e i n g at 500°C results in p r e c i p i t a t e s forming on lath b o u n d a r i e s and on the d i s l o c a t i o n s w i t h i n the laths. These give rise to streaks in the SAD pattern. After ageing at 600OC, p r e c i p i t a t e s w i t h spherical and plate like cross section are observed. The structure is no longer lath martensite. SEM observations: coalescence.
Fracture,
in all cases,
involves t r a n s g r a n u l a r m i c r o v o i d
The m e c h a n i c a l p r o p e r t i e s m e a s u r e d in the u n i a x i a l tensile tests - ~0.2 yield stress, u l t i m a t e tensile stress, e strain at fracture - and the KIC or KjC values are summarized in table i. TABLE 1 Summary of M e c h a n i c a l P r o p e r t i e s
Property measured
Solution treatment temperature
Ageing temperature §
770Oc
820oc
870oc
914
870
886
1485
2195
1454
1121
1054
1009
1587
2274
1806
6.2
4.5
7.4
2.9
2.1
1.6
Kic**or KjC* (MN m-3/~)
64.3
78.0
79.2
48.4
Nature of crack propagation
stable
00.2 (MN m -2) UTS (MN m -2) efracture (%)
§ * **
stable
stable
400oc
unstable
500oc
600oc
35
48.2
unstable
unstable
S o l u t i o n treated at 820°C A g e i n g at 500Oc c o r r e s p o n d s to the peak aged c o n d i t i o n S o l u t i o n treated c o n d i t i o n Aged c o n d i t i o n
The o b s e r v a t i o n s can be summarized as follows: (i) V a r y i n g the solution t r e a t m e n t t e m p e r a t u r e b e t w e e n 770 and 870°C has negligible influence on the m e c h a n i c a l p r o p e r t i e s both on tensile p r o p e r t i e s and on the level of fracture toughness.
Vol.
18,
No.
4
F R A C T U R ETOUGHNESS OF 18 Ni MARAGING STEEL
375
(2) The ageing t r e a t m e n t selected, p r o d u c e s various m i c r o s t r u c t u r e s . This v a r i a t i o n in the m i c r o s t r u c t u r e exerts c o n s i d e r a b l e i n f l u e n c e on the tensile behaviour, but n e g l i g i b l e i n f l u e n c e on fracture toughness. (3) The aged m a t e r i a l s are s t r o n g e r than the s o l u t i o n treated materials, a KIC of only 35 - 50 MN m-S/2, are about half as tough.
but, w i t h
(4) Samples in the s o l u t i o n treated c o n d i t i o n have quite a low KjC level, 80 K N m - S / 2 . (5) I r r e s p e c t i v e of the m i c r o s t r u c t u r e , coalescence.
70 to
fracture involves t r a n s g r a n u l a r m l c r o v o l d
DISCUSSION The range of the s o l u t i o n t r e a t m e n t t e m p e r a t u r e selected in the p r e s e n t study does not lead to a p p r e c i a b l e v a r i a t i o n in the prior a u s t e n i t e grain size (4). The m a r t e n s i t e lath spacing, is t h e r e f o r e almost c o n s t a n t for the various heat treatm e n t s and this e x p l a i n s the n e g l i g i b l e influence of s o l u t i o n t e m p e r a t u r e on the m e c h a n i c a l properties. It is w e l l - k n o w n that d u r i n g the ageing of m a r a g i n g steels r o d - l i k e NisMo p r e c i p i t a t e s form and these act as p r i n c i p l e h a r d e n i n g agents (5). The spherical p r e c i p i t a t e s are r e c o g n i z e d to be FeTi (3). Similar p r e c i p i t a t e s were found in the p r e s e n t study also. The o b s e r v e d i n f l u e n c e of the ageing t r e a t m e n t on the tensile p r o p e r t i e s was as expected. The m o s t striking o b s e r v a t i o n s are that (a) the v a r i o u s ageing t r e a t m e n t s have no a p p r e c i a b l e i n f l u e n c e on the fracture t o u g h n e s s and that (b) the m a x i m u m level of fracture toughness r e a c h e d is as low as 40 MN m-3/2 in the aged c o n d i t i o n and 80 MN m -s/~ in the a s - s o l u t i o n treated condition. The m i n o r influence e x e r t e d by the m i c r o s t r u c t u r a l d i f f e r e n c e on the fracture p r o c e s s e s can also be seen in the s i m i l a r i t y in the nature of the fracture surfaces (Figs 1 (3) and 2 (3)). Dimples w e r e found m o s t l y to be a s s o c i a t e d w i t h large p a r t i c l e s w h i c h were a n a l y s e d to be t i t a n i u m carbide particles. These were found in all samples, s o l u t i o n - t r e a t e d and aged. That it is the p r e s e n c e of the t i t a n i u m carbide p a r t i c l e s w h i c h is r e s p o n s i b l e for the low level of fracture t o u g h n e s s can be confirmed by a p p l y i n g the model of St~we (6) for the c a l c u l a t i o n of KIC. A c c o r d i n g to this model, w h e n Y o u n g ' s m o d u l u s E, the dimple size d and the level of the ultimate. tensile s t r e n g t h o of the m a t e r i a l are known, KIC is given as
The values of KIC so c a l c u l a t e d are given in table 2. TABLE 2 C a l c u l a t i o n of KIC accordin= Temperature oc UTS (MN m -2) Dimple size,
d (pro) KIC (MN m-3/z)** range, ealc. KIC (MN m-S/z ) experimental *
Solution treatment temperature 770oc 820oc 870oc 1121
1054
1009
to St~we model: A~eing temperature * 500oc 600oc 400oc
1587
2274
1806
6.8 + 0.5
11.3 -+ i
7 + 0.3
7.2 + 2
8 + 2
7.9 -+ 2
26.24 to 28.24
32.45 to 25.48
26.28 to 26.39
29.4 to 39.1
36.47 to 47.08
30.9 to 40.0
64.3 +
78.0+
79.2+
48.4
Solution treatment temperature: 820°C
35
48.2
** Calculated according to equation (i)
KIC is converted from measured value of JIC as KjC = ~ JIcE/(I --l)2) where E is the Young's modulus and ~ is the Poisson's ratio.
376
FRACTURE TOUGHNESS OF 18 Ni M A R A G I N G STEEL
Vol.
18, No.
4
There is good a g r e e m e n t b e t w e e n the values so c a l c u l a t e d and the e x p e r i m e n t ally m e a s u r e d values for the aged materials. This shows that in the aged samples, the t i t a n i u m carbide p a r t i c l e s are m a i n l y r e s p o n s i b l e for the fracture behaviour. Data for samples in solution treated condition, given in table 2, show that such a c a l c u l a t i o n is not valid for ductile materials, as was also p o i n t e d out by St~we (6). Thus, the v a r i a t i o n of the s o l u t i o n or ageing t e m p e r a t u r e has no a p p r e c i a b l e influence, because the d i s t r i b u t i o n and the size of the t i t a n i u m carbide p a r t i c l e s are similar in all c o n d i t i o n s and the p r e s e n c e of Ni~Mo and FeTi p r e c i p i t a t e s in the aged c o n d i t i o n s becomes unimportant. K a l i s h and Rack (3) o b s e r v e d that the rate of cooling after heat t r e a t m e n t had an important influence on the fracture t o u g h n e s s of m a r a g i n g steels. They o b s e r v e d a p p r e c i a b l e i m p r o v e m e n t in w a t e r - q u e n c h e d materials, as c o m p a r e d to aircooled ones, w h e n solution treated above 1000°C. Cairns and N o v a k (2) also r e ported that d u c t i l i t y can be r e s t o r e d by solution t r e a t m e n t s above 980oc followed by rapid cooling. This p r o c e d u r e should e l i m i n a t e the t i t a n i u m carbide particles. In order to check this, samples were s o l u t i o n treated at i100°C and w a t e r quenched. A n o t h e r set was solution treated at 820°C and w a t e r quenched. The JIC values m e a s u r e d for these c o n d i t i o n s were 80.7 and 62.7 KJ m -2 r e s p e c t i v e l y . That either w a t e r q u e n c h i n g or s o l u t i o n t r e a t i n g at a h i g h e r t e m p e r a t u r e did not lead to i m p r o v e m e n t s in the level of fracture toughness, is c o n t r a r y to expectations. O b s e r v a t i o n s of the fracture surface indicate that such t r e a t m e n t s do not lead to the c o m p l e t e d i s s o l u t i o n of t i t a n i u m carbide particles. W a t e r q u e n c h i n g may not be fast enough to hinder the formation of new c a r b i d e s in the 25 mm thick CT-I samples. CONCLUSIONS The p r e s e n c e of t i t a n i u m carbide p a r t i c l e s is very d e t r i m e n t a l to the toughness of the 18 Ni m a r a g i n g steels and o v e r r i d e s any influence the other m i c r o structural features may have. E x p e r i m e n t s are in p r o g r e s s to reduce the carbon content so as to m i n i m i z e the amount of t i t a n i u m c a r b i d e s that form and also to choose more rapid m e t h o d s of q u e n c h i n g the samples, than e m p l o y e d here. REFERENCES i. 2. 3. 4. 5. 6.
H.J. Rack, Scr. Met., 13, 577 (1979) R.L. Cairns and CJ. Novak, Met. Trans., 2, 1837 (1971) D. Kalish and H.J. Rack, Met. Trans., 2, 2665 (1971) H.J. Rack, Metals Sci. and Eng., 34, 263 (1978) K. Shimizu and H. Okamoto, Trans. J. I. M., 12, 273 (1971) H.P. StOwe, Eng. F r a c t u r e Mechanics, 13, 231 (1980)
Vol.
18, No.
4
FRACTURE T O U G H N E S S OF 18 Ni M A R A G I N G STEEL
ii!i iliiii!i!ii!ciii!i iiiiillii!i!ii i!ii!ii iill~
FIG.
1
M i c r o s t r u c t u r e and fracture surface of s o l u t i o n - t r e a t e d samples. S o l u t i o n t r e a t m e n t temperatures: a) 770°C, b) 820°C, c) 870°C Time: 1 hr. (i) Optical micrographs, (2) T r a n s m i s s i o n e l e c t r o n m i c r o g r a p h s and (3) SEM m i c r o g r a p h s of the fractured surfaces of CT-I samples.
377
378
FRACTURE
TOUGHNESS
OF 18 Ni MARAGING
a
b
STEEL
Vol.
c
1
2
3
FIG.
2
Microstructure and fracture surface of aged samples. Solution treated for 1 hr at 820°C. Ageing temperature: (a) 400°C, (b) 500°C, (c) 600oc. (i) Optical micrographs, (2) Transmission electron micrographs and (3) SEM micrographs of the fractured surfaces of CT-I samples.
18, No. 4