THE INFLUENCE OF MIXED FERRITE-BAINITE MICROSTRUCTURE ON CRACK INITIATION UNDER DYNAMIC LOADING

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THE INFLUENCE OF MIXED FERRITE-BAINITE MICROSTRUCTURE ON CRACK INITIATION

UNDER DYNAMIC LOADING

M. Holzmann, J. Buchar, Z. Bílek

To cite this version:

M. Holzmann, J. Buchar, Z. Bílek. THE INFLUENCE OF MIXED FERRITE-BAINITE MI- CROSTRUCTURE ON CRACK INITIATION UNDER DYNAMIC LOADING. Journal de Physique Colloques, 1985, 46 (C5), pp.C5-155-C5-161. �10.1051/jphyscol:1985520�. �jpa-00224750�

JOURNAL DE PHYSIQUE

Colloque C5, suppl6ment au n08, Tome 46, aoOt 1985 page C5-155

THE INFLUENCE OF MIXED FERRITE-BAINITE MICROSTRUCTURE ON CRACK INITIATION UNDER DYNAMIC LOADING

M. Holzmann, J. Buchar and Z. ~ i l e k

I n s t i t u t e o f PhysicaZ MetaZZurgy, CzechosZovak Academy of Sciences, Ziikova 2 2 , 616 6 2 Bmo, CzechosZovakia

Resum6 - Nous Gtudions l'influence de la microstructure mixte ferrite-bainite de l'acier C-Cr-Mo sur l'amorpage des fissures lors de chargements dynamiques. Trois techniques expgrimentales diff6- rentes ont GtG employges pour Gvaluer le comportement en rupture' dynamique de cet acier.

Abstract

-

A study of the influence of mixed ferrite-bainite microstructure of C-Cr-Mo steel on crack initiation under dyna- mic loading was performed. Three different experimental techni- ques were used to evaluate the dynamic fracture behaviour of the given steel.

I

-

INTRODUCTION

In many engineering applications, there is the increasing necessity of understanding the mechanical behaviour of materials at high stra- in rates corresponding to different kinds of dynamic loading. It was recognized long time ago that strain rate, as well as temperature, affect many material properties including the fracture initiation.

The rate effect can occur due to either mechanical (inertia) or me- tallurgical reasons (ageing, change of fracture mode et~.).

This paper deals with the second source affecting the fracture beha- viour. The main attention is focused on the study of the influence of initial microstructure of C-Cr-Mo steel on crack initiation. In previous papers /1

-

3/ it was shown that various types of micro- structure of the given steel strongly affected its mechanical and fracture behaviour. The microstructure studied in /1

-

^3/ was formed by tempered martensite or bainite. In the present paper, the fractu- re behaviour of steel with mixed ferrite-bainite microstructure is studied. The fracture properties are characterized by temperature dependence of dynamic fracture toughness K (T). The dependence KId (TI was evaluated by the instrumented char$$ test and by the modi- fied Hopkinson split pressure bar (HSPB). In order to describe the influence of higher loading rates than those obtained by the methods mentioned above, the results of studies on crack stability at stress pulsu loading are presented. The influence of main microstructural parameters on the observed fracture behaviour is also briefly outli- ned.

I1

-

^{MATERIALS AMI} TESTING PROCEDURES

The material studied was C-Cr-Mo steel with 0.12 % C, 0.57 % Mn, 0.27 % Si, 0.015 % P, 0.018 % S, 2.3 % Cr and 1.04 % Mo. Three va- rious types of microstructure in the given steel were prepared:

a ) Bainite ( Austenitizing temperature 940 OC/I hour/air ).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985520

C5-156 JOURNAL DE PHYSIQUE

b) Mixture fgrrite (24.74 %)

-

bainite. (940 OC/I hour/ and then bath 700 C/ 3 minutes (air).

C) Mixt~re ferrite (54.43 %)

-

bainite. (940 OC/I hour and then bath 700 C/18 minutes/air 1.

The following experimental procedures were applied:

1. The instrumented Charpy test. The results of these experiments were evaluated according tg prop0779 ASTM E 24.03.03 standard.

The loading rate KI was 10 MPa

8

s.

2. The higher loading rate, K 10 MPa m1l2s was obtained by SHPB method. The details of this procedure in which wedge loaded com- pact tension (WLCT) specimens are applied are given, e.g., in /4/, 3. The crack instability studies were performed according to the

procedure described in /3/.

I11

-

^{RESULT AND} DISCUSSION

The temperature dependences of the dynamic fracture toughness K obtained for all the microstructure stages a

-

c by means of me4flods ad 1 and 2 are given in Figs. 1

-

3. The values K obtained by the instrumented Charpy test are plotted, for the temhgratures limited above, by temperature T which determines the cleavage initiation region. When using the ~ P H B method, this temperature is marked T It turns out that for all the. studied microstructural stages, thg' increase of the loading rate KI leads up to the decrease of K va- lues and up to the increase of temperatures at which the cleabflge initiation takes place. From the point of view of the K intensity in can be stated that the lowest resistance against thelPracture initiation is shown by a steel the microstructure of which is formed by mixture of ferrite (-25 %) and bainite. A higher resistance is shown by the microstructure of ferrite (-55 %) and bainite; the most required is the steel with the bainitic structure. This order

is identical for both loading processes. At the same time, the in- crease of K rate leads up to the most outstanding decrease of K for the mic&ostructure containing -25 % of ferrite; sirnultaneouHqy there occurs the maximal increase of temperature T

.

When evaluating the differences of values K it 1s necessary to consider that the SPHB method is affected wfeh a larger error in comparison with the method using the instrumented Charpy test. This indeterminateness is due to the neglection of wave processes in the WLCT specimen, the stress state of which is discribed by the rela- tions based on the assumption of equilibrium in the specimen /4/. It turns out that this conception is right only for the stress pulses with the duration timeA2 100 us with /2/. However the loading rate K is thus limited.

d e decrease of A leads not only up to the limiting of validity of the assumption concerning the stress equilibrium, but also to the limiting of the validity of the crack stability criterion developed within the framework of the classical conception of fracture mecha- nics /6/. To appreciate that effect, an experimental process was used which makes it osible to determine the surfaces of the defect

stability 6 ( A, 1

7,

where 1 is the crack length and 6 is the stress pulsg amplit%de determifled so that the crack i n i t i h o n occurs /3/ when the pulse with 6 > 6 effects.

In Figs. 4

-

6, the dependences 6

('f

are given,. determined for the steel with bainitic structuremusiRg sbress pulses with A = 18, 43 and 68 ps at the temperature T = -120 C. At the same time, the curve 6, is plotted determined by the relation:

~,(t) = 6;- f(u) =

Kid,

( 1 ) which is based on the normal form of the crack stability criterion, where f(u) is the compliance function connecting the crack size with

body dimensions / 5 / and K is the dynamic fracture toughness deter- mined by the instrumentedlf?harpy test for the given temperature. At the same time, the dependence 6 (1 is plotted, determined by the relation. ( 1 ) where, on the lef$ siae, there stands out the quanti- ty max /K (tj/, K (t) being the course of the stress intensity coe- ff icient Jetermingd numerically, by the method given, e .g., in /6/.

It turns out, similarely to /6/, /8/, that the statical criterion of crack stability is valid only for definite values of crack length 1 limited by the constant 1 being the increasing function of A .

18

/8/ a modification of the08efect stability criterion was proposed within the framework of so-called "short pulse fracture mechanics", where, for a crack, that criterion has the form:

where K~ = l/A

f

~ ~ ( t ) d t and c l the longitudinal wave velocity (c, =

= 5000&/s). o

In Fig. 7, the results obtained by the criterion (2) are plotted showing that its validity is limited only to a narrow interval of crack lengths. Analogous results were obtained for remaining struc- tural stages of the given steel. The reason of this disagreement is apparently the fact that the method loading up to relations (2) res- pects only one of the effects of the high-rate loading, namely the influence of the material inertia. For the description of the influ- ence of material properties, the results for studied microstructural stages were treated analogically to /3/ where the validity of the expression is demonstrated:

c1 A h o c = const. = A. ( 3 ) The values A are given in Tab. 1. In Tab. 1 there are given, at the same time, the parameters of rate sensitivity cx

/lo/,

the values of static yield strength and the constant

P

giving the maximally po- ssible increase 6 at dynamic loading / l o / .

The increasing sefisitivity of material to the deformation rate des- cribed by the constants cc and

fl

leads up to the validity limiting of the statical criterion of crack stability, see eq. ( 1 ) , in the direction of the lower values of crack size.

This connection makes it possible, qualitatively at least, to des- cribe the influence of microstructure formed, in the given case, by a mixture of bainite and ferrite. Considering the relation for cc /1 o/

where 6 is deformation stress determined for 0.5 % deformation and 6B the 6ack stress,cx can be expressed in the form / I I / :

where B is the drag coefficient and N is the mobile dislocation density. 8 describes the influence ofmboth the stress deviations and the dislocation density from mean values due to local stress fields.

The course of these internal stresses is influenced by the character of distribution of bainitic laths and ferritic grains. Since the si- ze B is in both structural components nearly identical /1/ the ob- served rate sensitivity cr is given by the product N ( 1 + 8J.

The incorporating of the correction expressed by th'& constant i)' /11/

enables thus a more precise way of interpretation in comparison with former processes when the dependence of type (4) was unambiguosly interpreted as a consequence of the effect of typical high-rate me-

(25-158 JOURNAL DE PHYSIQUE

chanisms of plastic deformation, where cc-B/N

/lo.

This simplifica- tion would lead up to a rather problematic coRclusion concerning the highest density of mowing dislocations in a purely bainitic micro- structure. The constant 8 can then be applied for describing the in- fluence of further significant features of the considered micro-

structures, namely of a mutual distribution of both components, i.e., bainite and ferrite.

IV

-

CONCLUSION

From the performed experiments it fallows:

-

From the point of view of the temperature dependence of dynamic fracture toughness K

,

the lowest crack initiation resistance is demonstrated by the Beeel with the microstructure containing 25 % of bainite.

-

In the mentioned steel characterized by the highest sensitivity rate there occurs the most significant limiting of validity of the static criterion of crack stability.

-

On the basis of the study on defect stability at pulse loading, it can be stated that the decrease of values K

,

when using the men tioned HSPB method is connected with inaccuJ4cies in evaluating the results rather than with the influence of loading rate. The value K determined by means of the instrumented Cherpy test can thus reagesent a ~ i n i m u m at the curve K I a ( K I ) , for the given tem- perature of -120 C at least.

The results of studiyng the stability of the stress pulse loaded defect are in agreement with the hitherto knowledge. Analogously to /3/, the influence of mechanical characteristic (dynamical yield point, flow stress), on the observed behaviour was shown. The connections found, expressed by the parameter A

-

see (31, make it possible to interprete

-

qualitatively at least

-

the influence of parameters of the given types of microstructures on the fracture behaviour

.

V

-

REFERENCES

/1/ Buchar, J. and Bilek, Z., Proc. 2rd Cairo University MDP confe- rence, Cairo ( 1 982 695.

/2/ Buchar, J., Bilek, 2 . and Holzmann, P., Proc. 4th International Conference on Mechanical Behaviour of Materials, Stockholm (1983)

1 1 1 1 .

/3/ Buchar, J., Bilek, Z., Kotoul, M. and Bucki M., Proc. 3rd Conf. Mech. Prop. High Rates of Strain, Oxford 11984) 237.

/4/ Klepazko, J. R., J. Eng. Mat. and Technology

104

( 1982) 29.

/5/ Knott, J. F., Fundamentals of Fracture Mechanics, Butter-Worths, London ( 1 973 1.

/6/ Homma, K., Shockey, D. A. and Muryayma, Y., J. Mech. Phys. Sol.

1 (1983) 261.

7/ Kalthoff, J. F. and Shockey, D. A., J. Appl. Phys.

48

( 1977)

f

984.

/8/ Shockey, D. A., Kalthoff, J. F. and Ekrlich, D. C., Int. J. of Fracture

22

(1983) 217.

/9/ Holzmann, M., Research Report No 537/646, Institute of Meta- llurgy C.A.S., Brno (1983).

/ l o / Buchar, J. and Bilek, Z., Behaviour of metals at hi h rates of

strain, Academia Publ. Bo. Czech. Ac. Sci., Prague ( 19847.

/ 1 1 / Clifton, J. R., Gilat, A. and Li, C. H., In: Material Behaviour under high stress and ultrahigh loading rates (J. Mescal1 and

V. Weiss, Eds.) Plenum, New York (1983) 1.

Tab. 1 Material properties of C-Cr-Mo steel

MICROSTRUCTURE FERRITE ( 2 5 % ) FERRITE (55%) BAINITE

6,

pal

65 1 3 27 603

a [k Pa s] 32.1 2 4 24 .3

A [ I ] 62 49 5 5

@ [I

1

^2.4 1.4 1.6

1 /2] 34 -8 40.7 37.8

T E M P E R A T U R E T L O C I e°C

-- --

lNSTRUMENTED CHARPY T E S T

- a A SHP8 METHOD

6 0 -

7

E 0 - a

4 0 -

U n * Y

Fig. 1

-

Temperature dependence of fracture toughness.

2 0

roo

r - - -

^{I !} I

- I ^I I -

1 I

r ^I I

1

C-Cr-Mo(25% F E R R I T E )

I

----

INSTRUMENTED CHARPY TEST

a A S H P B METHOD

/ I ^-I

/ /

a /

'

^{I ; i}

/'

Y-

--.--

/ / - / ^a

T E M P E R A T U R E T ^{[ O C ]}

Fig. 2

-

Temperature dependence of fracture toughness.

JOURNAL DE PHYSIQUE

l o o - -

.. i ^{--- -}

I N S T R U M E N T E D CHARPY T E S T

A L S H P D METHOD

n

2 0 I

I I ,

I I

0 ~ 1 i .. .A L

2 0 0 100 0

TEMPERATURE T ['C ]

Fig. 3

-

Temperature dependence of fracture toughness.

C R A C K L E N G T H i,[rnrn]

5 0 0 , , , , ,

Fig. 4

-

Instability stresses under short pulse loading conditions

400 n

0

a

-

E X P E R I M E N T

----

STATIC EVALUTION 16; I

-.-.- NUMERICALI~: I 0 CRACK GREW

n m

500-1

^A CRACK DID NOT GROW

-

EXPERIMENT I

----

STATIC-EVALUATION IS:]

- N U M E R I C I A L 16; 1 0 CRACK GREW A CRACK DID NOT GROW

C R A C K LENGTH Lo [mm]

Fig. 5

-

Instability stres- ses under short pulse loa- ding conditions.

-

h = 1 8 ~ s

iT I- ur

. '

, ^-

0

'-I>.

lY=>.

~ I I I 8 1 8---z-I ~ I '

0 2 4 6 8 1 0 1 2

-

E X P E R I M E N T

----

S T A T I C . E V A L U T I O N I S ; I

-.-.- N U M E R I C A L I B ; I

0 CRACK GREW

a ^A CRACK 010 NOT GROW

i

0 2 6 8 1 0 1 2

C R A C K LENGTH lo[rnrn]

Fig. 6

-

Instability stresses under short pulse loading conditions.

LLI-~_U.IId

0 5 10

C R A C K L E N G T H l,[rnrn]

Fig. 7

-

Comparison of experimental data on instability stresses (6,) determined by Eq. ( 2 ) .