ductile transition temperature

Scripta

METALLURGICA

Vol.

23, pp. 2 1 3 7 - 2 1 4 2 , 1989 P r i n t e d in the U.S.A.

RESF_ARC~ INTO B R I I ~ ' L E ~ I L E

P e r g a m o n Press plc All r i g h t s r e s e r v e d

TRANSITION TI~M~,EKA93/RE

Z. Huang * M. Yao ** • Institute of Physics, Academia Sinica, Beijing $* Harbin Institute of Technology, Harbin, China (Received March (Revised October

7, 1989) 13, 1989)

Introduction The significant role of temperature has been given much attention and widely studied in low temperature brittle fracture of structure steel. Various " critical temperatures "(1-4) have been suggested according to empirical relationships or to phenomena in tests. However, a problem is simultaneously raised: which "critical temperature" is most closely related to the brittle fraotul~, or, in other words, which "critical temperature" can represent the property of brittle fracture at low temperatures. Therefore, the objective of the present paper is to investigate several temperature transitions in the fracture of mild steel and to gain a physical understanding of them by means of a cleavge model (5) and finally, to determine a characteristic temperature for low temperature brittle fracture of mild steel. Material and Procedures The chemical composition of steel used in the present investigation is ( wt-% ): 0.17C, 1.6Mn, 0.50Si, 0.009S and 0.027P. The steel was annealed for 2 hours at 1200"C. The microstructure was composed of ferrite with a grain size of about 0.042 mm and about 30~ pearlite. A three-point bending specimen was used to determine crack opening displacement (COD) of the steel from -70 to 60°C. The dimension of the specimen was 20 mm in thickness and width, 10 ,nn in crack depth, and the loading spen was 90 ram, which is in accord with the dimensions in the standard for COD testing (6). In order to investigate the feature of crack arrest in mild steel, the specimen, after precracking by fatigue, was treated by natural ageing for a year and a brittle zone was formed in the vicinity of the crack tip. A specimen used in a drop-weight test was used in a slow-bending test for the same purpose. The type of specimen is P2 ( ASTM-E208-81 ). The fracture appearances were observed by means of scanning electron microscopy (SEM). Experimental Results and Analysis Fracture Behaviour of Cracked-bending Specimen The variation of the fracture stress with temperature is shown in figure I. The fracture behaviour can be characterized by four temperature ranges (7,8): D' ,cleavage fracture prior to general yielding; C' ,cleavage fracture after general yielding; B' ,mixed fracture (fibrous and cleavage); and A' ,a ductile fracture range which is omitted here. In the D' range, the cleavage criterion can be expressed by (7,8): Q e . a y = Sco (1) where, Qe, w h i c h w a s d e f i n e d a s t h e e f f e c t i v e intensification of principal stress, bears a relation to the probabilistic aspect of cleavage and hence is different from the factor of principal stress intensification (4) w h i c h i s o n l y a m e c h a n i c a l p a r a m e t e r . Also, 6y is stress and Sco is defined as the cleavage characteristic stress. In this range, the fracture lom] is lower than the general yield strength. The plastic deformation concentrates in the vicinity of the crack tip and it is so small that it does not obviously affect cleavage mierocrack propagation. With temperature increasing, the load needed for fracture increases due to the decrease of yield strength until the t e m p e r a t u r e g e t s u p t o T~y. As s o o n a s t e m p e r a t u r e r e a c h e s T ~ y , the load

2137 0036-9748/89 $3.00 + .00 Copyright (c) 1989 P e r g a m o n Press

plc

2138

BRITTLE/DUCTILE

TRANSITION

Vol.

23,

No.

12

becomes higher than the general yield strength and hence general yielding takes place prior to fracture. That is to say, the fracture has the feature of the C' range in this ease. In the C' range,the plastic deformation and fracture behavior can be described as follows (9,]0). l) The plastic zone in the vicinity of the crack tip starts to expand greatly alone the crack section. 2) The increase of principal stress with load drops, which implies that plastic constraint decreases. Compared with the solution of the slip line field, the result of finite element analysis gives larger stress elevation at a given applied load due to the introdoction of the strain hardening, especially in the case that general yielding takes place, as seen from figure 2. That is, the role of strain hardening becomes more sii~nifieant after general yielding. 3) The strain has increased so much that the effect of primary cleavage microcrack blunting has to be considered and the cleavage criterion in this range becomes: Qe.ay : Sco +AS(~) (2) where AS( ~ ) i s the increment of the resistance to the primary microcrack propagation across the grain boundary. Obviously, a l l of these factors should a f f e c t the cleavage behavior in the C' range. The great expansion of the p l a s t i c zone will be favourable for the formation of an " effective yield zone"(5). The large increase of Qe with load in the lower temperature part of C' raz~e will assure a large increase of stress in the vicinity of the crack tip. However, in the higher temperature part, the increase of @e becomes so small, since the plastic constraint reduces greatly, that cleavage fracture occurs only when the load reaches a much higher level. Simultaneously, the obvious effect of blunting results from the high level of load and conversely results in the need for an even greater stress level. Consequently, the sharp increase of the fracture is brought about, as seen from figure ], in the temperature region above Tpm. The same analysis can be made from the microfractographs (Fig.3). Figure 3 shows that the cleavage appearance is flat in the temperature region below Tpm (Fig.3a,b), but exhibites many tear ridges at a temperature slightly higher than Tpm (Fig.3c) and the tear ridge becomes more and more evident with increasing temperature. As soon as temperature goes up to Tel, micro-voids appear in the vicinity of the crack tip (Fig.3d). Accordingly, the C'ranEe can be divided into two parts: in the part of temperature below Tpm, the plastic constraint plays an important role in the cleavage process; in the part above Tpm, strain hardening becomes a major factor in the final occurrence of cleavage. The temperature Tpm can be regarded as a parameter dealing with the transition in cleavage fracture, which actually expresses a change of cleavage resistance. When the temperature reaches Tcf, gross yielding occurs prior to fracture (10) and the plastic bending-deflection of the specimen before fracture reaches 1.6 ram. The deformation is so obvious that the fracture could be regarded as ductile. In the light of the above analysis, the fracture behavior of cracked-bending specimen can be specified in terms of different temperatures, such as T~y, ~ and Tel. Therefore, it is necessary to gain a physical understanding of these temperatures in fracture. Features of Fracture Behaviour at Tpm According to the above description of deformation and fracture behaviour of a cracked specimen, the meaning of T&y is closely related to plastic deformation of specimen; Tcf mainly represents aspects of plastic deformation and micro-void fracture although it includes the fracture factors; only Tpm involves the crucial factor in the cleavage process and deals with brittleness of mild steel. Hence, the focus of the following section is the discussion of the features of fracture 5ehaviour at Tpm. In figure I, the variation of the crack opening displacement 6c with temperature was shown, and a sharp increase of dc could be fot~d at temperatures above Tpm, that is, the transition of fracture toughness occurs at ~ also. In fact, this transition can be specified by means of the following relationship (II) :

• c = A,[(Soo +As(~)Y ~y]n+s %.Xe

(3)

where, A is a parameter related to a coefficient obtained by Shih (12) and elastic constants, n is the strain hardening index, and Xe is the distance from the crack tip to the edge of "effective yielding zone" (11). Figure 4 shows the fracture behaviour of the specimen with a brittle zone in the vicinity of the crack tip. Investigation of this kind of specimen for fracture behaviour would benefit the tmaderstandinE of the crack arrest property, since the initiation and propagation of brittle crack leads to a dynamic problem for the crack entering the non-brittle zone or matrix of the specimen. Figure 4 shows that a transition of the depth of crack propa~tion appears at -25 ° C, which corresponds to the t e ~ r a t u r e Tpm. Here, the definition of the depth/area of c ~ pror-_~_~Rtion is a distance/zone of crack propagation which is initiated by a ]m~ittle-oraek and arrested in the case of tazloadinE. In figure 4, the variation of the load at final fracture is presented to exhibit this phenomenon further. The final fracture load only represents the load bearing capacity

Vol.

of

the

23,

No.

12

specimen a f t e r

BRITTLE/DUCTILE

the brittle-crack

TRANSITION

p r o p a g a t e s i n t o t h e specimen and m a i n l y depends

2139

on

the

proportions of the unfracture part. Evidently, the variation of this capacity is the same as that of the depth. Figure 5 is a group of photogral~hs of the fracture appearanoes, in which the oxidized part is the area of brittle-crack propagation, and the same conclusion can be reached. In fact, in the slow-bending test in which the drop-weight specimen was used, a similar phenomenon was observed. The transition of area of brittle-crack propagation was also found at Tpm, as seen from figure 6. All of these phenomena demonstrate that the transition of the crack arrest property is closely related to the characteristic value of Tpm, and they may have the same essential causes as the previous phenomena although the velocity of crack propagation is a matter of considerable importance. On the basis of a relationship of crack velocity and crack resistance (13) and the suppositions that the loading system has enough rigidity and that the elastic energy stored in the specimen is within a certain level, the higher the resists.nee to cleavage is, the lower the velocity of the crack, which results in a greater relaxation of the stress in the vicinity of the crack tip and a shorter deceleration stage (14). This inference is supported by some results, such as those for a Rebertson-type test (15), a full-scale pipe test, in which the crack velocity not only depends on the temperature but also exhibits a transition (16). In the present investigation, the crack resistance is low and its variation with temperature is ~qm~]1 at temperatures lower than Tpm, so that a high stress level is maintained and the crack propagation cannot be caused to cease; in the temperature region higher than Tpm, however, the crack resistance sharply increases and the crack velocity greatly decreases so that the stress in the vicinity of the crack tip obviously relaxes and deceleration stage becomes obviously short. In summary, it is at Tpm that the transitions occur both for the toughness and for the crack arrest property of steel. Both the transitions are associated with the characteristic value of Tpm and express a change in the influence of plastic constraint and strain hardening in the cleavage process. Morever, with respect to mechanics, Tpm is mainly related to the stress and strain fields in the vicinity of the crack tip according to the previous (17) and the present works, in which the measured values of ~ are almost the same for specimens with different crack depth or shape in the same material. That is to say, Tpm is related to a transition in the fracture behaviour of a steel if the specimens meet a certain d ~ n d of plastic constraint, and is mainly determined by characteristic cleavage stress See and yield stress (ii). Therefore, Tpm can be regarded as a characteristic parsEeter describing fracture behaviour of mild steel at low temperature. Naturally, because the yield strength is related to the rate of loading, Tpm would be rate dependent, similar to other transition temperatures. The problem of influence of rate on Tpm will be discussed in a companion work. Conclusions In the light of the variation of fracture behaviour of cracked-bending specimen with temperature; a characteristic temperature Tpm was defined for low t e ~ r a t u r e brittleness of steel. It was demonstrated that Tpm is the temperature at which the resistance to cleavage starts to sharply increase because the plastic constraint decreases and the effect of microcraek blunting increases, lit was discovered that at Tpm transitions occure in both the toughness and the crack arrest property of steel. Morever, Tpm is arepresentative the property of a material if the test specimens meet a certain demand of plastic constraint and hence, it can be used to describe the brittleness ,of mild steel at low tem~oerature. Re~rences I. 2. 3.

W. J. Hall, Fracture An Advanced Treatise, 5, 1 (1969). ft. M. Wilkowski, R. J. Eiber, WRC, Bulletin, 239, ( 1978 ). W. S. Pellini, Principles of Structure Integrity Technology, Arlington, Office of Naval Research, (1976). 4. J. F. Knott, Fundemental of Fracture Mechanics, Butterworth, London, (1973). 5. Z. Huang, M. Yao, An Approach to Microcraek Propagation, Scripta Metall., 8, ( 1989 ). 6. National Standard ( Chinese ), The Method for COD Testing, GB 2358-80, Beijing, ( 1982 ). 7. M. Yao, Y. K. He, et al, Iron and Steel Sinica, 12, I0 ( 1 9 8 4 ) . 8. M. Yao, Y. K. He, et al, Advanoes in Fracture Research , Oxford: Pergs~Don, 2, 1423( 1984 ). 9. Z. Huang, M. Yao, Mat. Sei. and Technol. Sinica, 2, 1 ( 1987 ). I0. Z. h~mr~, M. Yao, Iron and Steel Sinica, 12, 45 ( 1987 ). II. Z. Huang, Doctorial Thesis, Harbin Institute of Technology, Rarbin, China, ( 1987 ). 12. C. F. Shih, M. D. German, Int. J. Fraoture, 17, 27 ( 1981 ).

13. J . J . Gilman, J . Appl. l ~ . y s . , 27, 1262 ( 1956 ).

2140

14. 15. 16. 17.

BRITTLE/DUCTILE

TRANSITION

Vol.

23, No.

12

E. Smith, Proc. Int. Conf. on Dynamic Crack Propagation, 63 ( 1972 ). T. S. Robertson, Engineering, 172, 445 ( 1951 ). A. R. Dully, G. M. McClure, et al, Fracture (ed. H. Liebowitz), Academic Press,5,159 (1969). D. M. Li, M. Yao, Scripta Metall., 21, 593 ( 1987 ).

--D'

17

]

I, B,--~ 0.4

C'

•--FRACTURE LOAD /o °--COD \\G....Y/.~d~.d

16 0.5 15 14 0.2

0

O c)

ll I •

lO 9 8 -80

Fig. I

(COD)

/o

~ .....~ ,~.~.... ~-

o /

Jo

T~7 ~ r

r°%l

,

0.I O.08

Tcf I

O.O6 0.04

,I

o.o O. O0

-60 -40-20 O +20 TEMPERATURE, C

+40 +60

Variations of fracture load and crack opening displacement of cracked-bendinE specimen with temperature

~93.4

-1.4 \

~D

c~3.0

~2.6

-1.2

\\\

1.0

CM

N2.2

0.8 cy

~1.8

0.6

~1.4

0.4

Z

~l.O

i

0.5

I

1.0

i

It

i

1.5 2.0 2.5 LOAD, 5-n /O~y

i

3.0

I

o~

0.2

5.5

Fig.2 Variations of effective stress intensification Qe and intensification Q with load

Vol.

23, No.

12

BRITTLE/DUCTILE

TRANSITION

214

Fig.3 Microfractographs of specimen fractured at different temperature (a) Tpm-SC; (b) Tpm ( -2SC ); (c) Tpm+SC; (d) Tcf.

E E Z O

(a)

o

O

r.)

m o 4 o

O

o

~a

o

~5

(b)

10

d

I

0 -80

-60

I

f l

I

-40 -20 0 TEMPERATURE, "C

I

l

+20

+40

+60

Fig.4 Variation of depth of brittle crack propagation (a) and load of final fracture ( b ) with temperature.

2142

BRITTLE/DUCTILE

-50C

-40C

-30C

TRANSITION

*-25C

-20C

Vol.

-15C

-10C

Fig.5 Fracture appearances of specimen with brittle zone in crack tip.

~]00 o

.~ 8c o o a~ 6 0

~

40

o a~ c~

2o I

r - 30

I Tpm

K ~

l

-20

TEMPERATURE

- ]0

( ° C)

Fig. 6 Variation of area of brittle crack propagation of drop-weight specimen with temperature in slow-bending test.

23,

No.

12