Some studies on the impact behavior of banded microalloyed steel

Engineering Fracture Mechanics Vol. 53, No. 6, pp. 991-1005, 1996

Pergamon

0013-7944(95)00159-X

Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0013-7944/96 $15.00 + 0.00

TECHNICAL NOTE SOME S T U D I E S O N THE I M P A C T B E H A V I O R OF B A N D E D M I C R O A L L O Y E D STEEL P. SHANMUGAM and S. D. PATHAK Materials Testing Facility, Metal Forming Laboratory, Department of Metallurgical Engineering, I.I.T., Madras - 600 036, India Abstract--Microalloyed steels are used in automobile industries, offshore platforms and in structural applications. It is essential to establish a relation between service condition such as temperature, loading rate and fracture behavior of the steel. Impact study on new material is very handy to understand the mechanical properties in a rapid and inexpensive way. The present investigation aims to assess impact toughness (CVN), ductile brittle transition temperature (DBTT, 25J), and initiation dynamic fracture toughness (J~d*) of the indigenously developed microalloyed steel. The steel has shown banding with alternate layers of ferrite and pearlite. The banding concentration (ferrite bands per mm) has been altered by heat treatment. Presence of banding has given spikes and splits in impact fracture. Change in banding concentration has affected DBTT of the steel, upper shelf energy and the extent of splitting. A model of crack divider with respect to the present microstructure has been analyzed. Banding in divider orientation improves the impact as well as initiation dynamic fracture toughness of the steel. The effect of temperature on splitting is also discussed. Splits in fractured surface disappear with decreasing temperature and higher numbers of splits yield lower toughness. Further, initiation dynamic fracture toughness is calculated for all temperatures and correlated with impact toughness.

NOMENCLATURE JId* DBTT Y At CVN

Initiation dynamic fracture toughness, M/mL Ductile Brittle Transition Temperature, °C. Sampling rate (time interval between the recorded data), #s. Sampling number (number of data for the analysis). Charpy V-Notch impact toughness, J.

INTRODUCTION MICROALLOYEDSTEELSare used in automobile industries, off-shore platforms and in structural applications. It is essential to establish a relation between service condition such as temperature, loading rate and fracture behavior of the steels. Moreover, establishment of such relations would be useful in assessing the fracture mechanism during failure and would help to establish the performance limit of steel [1]. Toughness is the ability of a material to resist failure caused by crack development. The toughness and transition temperature are important parameters of steels used in structural applications and hence many toughness measurements are made in safety assurance and quality control schemes in addition to those made in research studies [2]. The most common toughness test is the Charpy V-notch (CVN) test detailed in ASTM E23-91 standard. Moreover, impact study on the new material is very handy to understand the mechanical properties in a rapid and inexpensive way. The present investigation aims to assess impact toughness (CVN), ductile brittle transition temperature (DBTT) and initiation dynamic fracture toughness (J~d*) of the indigenously developed microalloyed steel. The steel has shown banding as observed in hot rolled plate and characterized by microstructure of alternate layers of ferrite and pearlite. This kind of banding in steel sometimes deteriorates weldability and corrosion resistance [3]. However, banding in certain orientations imparts high toughness and lowers DBTT [4] which obviously helps to give added advantage in structural applications. Such a model of crack divider with respect to the present microstructure has been analyzed. Banded steels yield splits and spikes in impact fractures [5-7]. Heat treatment procedures have been developed to alter banding concentration. Effect of banding concentration on CVN, DBTT, JJd* and the extent of splitting have been studied. This paper also discusses the effect of temperature on splitting and the splitting mechanism. It is clear from the literature that very limited work has been carried out on HSLA steel for correlating the J~a* and CVN values. Hence, Zd* is calculated for all temperatures and correlated with impact toughness.

EXPERIMENTAL PROCEDURE The chemistry of the hot rolled microalloyed steel used for the present investigation is shown in Table 1. Vanadium and titanium are regarded as microalloying elements. The room temperature tensile properties are shown in Table 2. Suitable heat treatment experiments (Table 2) were conducted to alter the banding concentration of the steel. Charpy V-notch specimens (ASTM E23-91) were used for impact, DBTT and J)d* experiments. A Wolpert instrumented impact testing 991

Technical Note

992

Table 1. Chemical composition of the microalloyed steel C

Mn

Si

S

P

V

Ti

0.20

1.26

0.02

0.018

0.014

0.068

0.015

machine (300 J) has been used for these experiments. The load-time curve is stored in oscilloscope and transferred to computer to obtain load--displacement curves for further analysis. The smoothening of Ioad~lisplacement curves has been done by the moving average technique [8]. The efficiency of this method depends on the sampling rate (t). The load-time curve is recorded at the sampling rate of 5/~s. Climate chamber (Heraeus Votseh) with the temperature range of 180°C to - 7 0 ° C is used to determine DBTT temperature. Optical and scanning electron microscopes (Jeol JSM840) are used to characterize the microstructure and fractured surfaces.

RESULT

AND DISCUSSIONS

Banding Optical metallography reveals "banding" (alternate layers of ferrite and pearlite) in LT and TL directions of hot rolled as received plates. Typical photomicrographs for these orientations are shown in Fig. 1. Many researchers [3, 6, 9, 10] have confirmed that banding is primarily due to microsegregation of manganese, non-metallic inclusion, and hot rolling at low finishing temperature and cooling rates. The banding concentration (number of ferrite bands per mm) is altered by heat treatment. Furnace cooled conditions yield the banding concentration of 28 ferrite bands per mm (Fig. 2a), whereas air cooled condition yields no banding (Fig. 2b). In TL orientation (condition !), ferrite and pearlite layers are in continuous fashion, on the other hand, LT direction (condition 1) shows some amount of discontinuity of layers as ferrite grains are randomly distributed (Fig. t). Crack divider orientation and delamination induced toughening (DIT) process The present microstructures show a layered structure of ferrite and pearlite which typically resembles crack divider orientation when the notch is made in LT and TL direction as shown in Fig. !. The crack divider orientation has beneficial character in raising the toughness. Moreover, improvement in toughness is associated with delamination induced toughening process [4, 1 I]. Triaxial tensile stress state at the crack tip determines the extent of fracture toughness, which is shown to increase with decreasing tensile triaxiality. Moreover, tensile triaxiality is developed when plane strain conditions are present. Improvement of toughness is achieved by reducing the crack tip induced a~ or a.- stresses thereby reducing tensile triaxiality. Triaxiality can be reduced by relaxing a: stresses brought about by delamination of interfaces positioned normal to the thickness direction. When delamination occurs, the effective thickness of the sample is reduced and a: decreases to zero at each delamination. Consequently, the specimen acts like a series of thin plane stress samples instead of one thick plane strain sample. For this reason, the resulting shift in transition temperature will depend on the number of weak planes introduced in the specimen--the more planes introduced, the thinner the delaminated segments will be and the greater the tendency for plane stress response [4]. Each ferrite and pearlite interface acts like a weak plane and depending on the banding concentration the delaminated segments will be either thinner or thicker. Thus, higher banding density yields thinner segments and plane stress condition prevails. Consequently, higher toughness is obtained. Ductile-brittle transition Ductile-brittle transition in steels is associated with two different failure mechanisms. At high temperature in the upper shelf toughness range, fracture occurs by nucleation and coalescence of microvoids that produces ductile tearing. This process requires extensive plastic deformation and, therefore, large amounts of energy. At low temperature, fracture occurs by cleavage which is the sudden separation of atomic planes across the specimen.

Table 2. Room temperature tensile properties of the steel

Condition Hot rolled as received (I)

Specimen orientation

Yield strength MPa

Ultimate tensile strength MPa

Elongation (%)

Banding concentration (ferrite bands per mm)

LT

360

514

25

71

TL

340

500

20

62

930°C, 30 min Furnace cooled

LT

262

460

32

28

930°C, 30 rain Air cooled (!II)

LT

356

525

32

0

(ll)

Technical Note

993

(TL direction)

F

(LT direction) Fig. 1. A model of crack divider and optical microstructure in LT and TL directions.

994

Technical Note

m

_

_

m

Fig. 2. Microstructure of(a) furnace cooled (930 C, 30 min) sample; (b) air cooled (930 C, 30 min) sample

Technical Note

995

100

(a) 90

t 80

O O O"

8

70z

60Ot

o

5 0 -i

= O

o ct E

40i

L.

30-

20-

J

Oo• O O

10-

8

O

O. -40

-9-30

1

-20

I

I

I

t

I

-10

0

10

20

30

Ternperoture~ "C Fig. 3(a).

Caption ot,erh~a/~

.

40

996

Technical Note

90 (b) $0 O

70

8

.-%

O

A

60 O

g

O O

O

50

o =

z,0

E

O

30

20-

10-

0

-4(

T

i

I

1

-30

-20

-10

0

, I

10

1

I

20

30

L.O

Temperoturej *C Fig. 3. Impact toughness curve and the effect of temperature on splitting is shown for (a) LT direction; (b) TL direction.

Technical Note

997

/

=l

Pearlite f

f

g

Fig. 5. Splitting mechanism.

Ferrite

Separation at pearlite and ferrite interface

998

Technical Note

~ J

F

|

Fig. 10. Fracture morphology in (a) LT direction; (b) TL direction.

Technical Note

999

Impact toughness curves for LT and TL directions (condition 1) are shown in Fig. 3. Higher upper shelf energy and lower DBTT is noticed in LT direction, whereas TL orientation shows lesser upper shelf energy and higher DBTT. This can be explained on the basis of number of ferrite layers (banding concentration) and anisotropic nature of the rolled plates. Ferrite layers are more in the LT orientation and more weak interfaces have been introduced which aid to have lower DBTT. Figure 4 shows the effect of banding concentration on upper shelf energy and DBTT. It has been noticed that higher banding (condition 1) in steel shows lower WIT and lower upper shelf energy than conditions II and !II. According to the delamination induced toughening mechanism, a higher number of bands raises the fracture toughness. On the other hand, it leads to more splitting which decreases the fracture toughness. Thus, the two opposing factors i.e. DBTT mechanism and splitting are responsible for achieving low toughness in the uppershelf energy region where a large amount of splitting is observed (Fig. 3). At low temperature the harmful effects of splitting are not pronounced unlike at high temperature, and so at low temperature DBTT mechanism dominates. Evidently, lower banding (conditions 1I and III) in steel shows a higher upper shelf energy and higher DBTT, whereas steel with larger banding (condition 1) shows a lower DBTT and lower upper shelf energy. Embury et al. [12] conducted such tests with laminated samples in the divider orientation and found the transition temperature to decrease with increasing number of interfaces. DBTT for LT and TL directions of condition 1 are estimated to be - 5 'C and - 2 C , respectively, whereas furnace cooled (condition I!) and air cooled (condition II!) are T C and 11 'C, respectively.

Splitting phenomenon in controlled-rolled steels with finishing temperature in the two phase region, splits are often encountered in Charpy specimens [10, 13]. Tanaka [14] suggested that elongated MnS inclusions initiate a crack and intergranular failures occur along prior austenite grain boundaries by segregation of impurity atoms which may cause splitting[5]. Banded microstructure and texture can also be responsible for splitting. The splitting mechanism is illustrated in Fig. 5. The SEM fractography explicitly shows the occurrence of splits in fractured specimen for LT and TL orientation (condition l) as shown in Fig. 3a and b, respectively. However, intermittent splits are noticed in LT orientation. On the other hand, splits are more continuous with greater density in TL orientation. Intermittent splits are due to random distribution of ferrite grains across the layers and the spherical shape of non-metallic inclusion, while, continuous splits are due to the continuous ordered layer of ferrite and pearlite, and the elongated non-metallic inclusion. Splits seem to disappear with decreasing temperature in both directions. It is due to the fact that at low temperature not much time is available for the splitting micromechanisms to operate. It has been found that splitting phenomenon is undesirable for achieving higher impact toughness and a higher number of splits yields inferior toughness.

175

150

e-..e-*-e

71 f e r r i t e

bonds

per mm

0---43-----0

62 f e r r i t e

bonds

per mm

~-----d~-.~

28 f e r r i t e

bonds

per mm

0

bands

per mm

O--O--O

ferr;te

/ I

A

Z

125 -

~J v

0

J¢

I/ o

100

/

o

o

/

75o e,i

E

50

25-

0 -40

-3()

-20

-10

0

10

20

30

Temperature (*C) Fig. 4. Effect of banding concentration on DBTT and upper shelf energy. E F M 53/(P-K

1000

Technical Note

14 12 10

0.002 0,004 0.006 0.008 0,01 0,012 0.014 Displacement~ m

(a)

14 12 10

I

0

0.001

0.002

0.003

Dlsplocemen~ m

(b) Fig. 6(a,b),

0.004

14[-0 ,.

~ e o~

~o~ ~ o s~ g

I

I

0.001

0

I

I

I

0.002

I

0.003

I

0.004

Displacementj m

(c)

14~0 12

^

O

~

~Or~o~ ~ 81-/_o

~l ~ Ov

I

0

0.001

I

1

I

I

0.002

I

0.003

Displacement~

I

0.004

m

(d) 14 12 10 8

z

6

o

4

o ..J

2

0

; 0

0~04

0.008

0~)12

' ' 0~16

' I 1 0,02 0.024

D|$placementj m

(e) Fig. 6. Smoothening process of load-displacement curve, (a) unsmoothened curve, smoothening with (b) m = 5, (c) m = 10, (d) m = 10, (e) typical smoothened load displacement curve.

1002

Technical Note

160

0

N

E 11,0

*% •-~ 120 o cc'oa 100 o

E ~ 80 U

60-

o

o

,_u

E

o e-

"o ¢.. o

4 0 --o

o °~

e.-

20 0

I

I

I

20

40

60

Impact

toughness

(a) Fig. 7(a).

(CVN), J

80

Technical Note

1003

120

E "--

100 0

0

80 Ib e-

~

O0 60

U

~"

/.0

E

0

¢-

c

20

o °~ °~ C ""

0 0

I 20 Impac~

I 40 toughness

I 60

8o

(CVN)~ J

(b) Fig. 7. Relationship between initiation dynamic fracture toughness and impact toughness for (a) LT; (b) TL orientation.

1004

Technical Note ¢",4

"o

0--0---o TL direction ,%

~ooF 80 f

U

•~ "E

I -60

-40

I -20

I

I

0

20

40

Temperature,*C Fig. 8. Effect of temperature on initiation dynamic fracture toughness for LT and TL direction.

~4

E

:¢ "13

°--

=`%

?0! 0

65 6O

==

.=

55 50

"o

c o

45~r-

o

o-4~ °--

¢.

i-i

401 0

L 20 Banding

I 40 concentration

I 60 (Ferrite

bands

80 per

mm)

Fig. 9. Effect of banding concentration on initiation dynamic fracture toughness.

Technical Note

1005

Initiation dynamic .fracture toughness (Jhl*) Generally, impact toughness consists of initiation and propagation energy. Initiation dynamic fracture toughness (J~*) another parameter from Charpy V-notch sample, consisting of initiation energy only, could be used to assess the reliability of the material for particular application. Prior to the calculation of initiation energy, the crack initiation point has to be determined. Initiation dynamic fracture toughness (J~d*) is evaluated at all temperatures on unprecracked Charpy V-notch sample by a single specimen technique of compliance changing rate method [15]. Moreover, compliance changing rate technique gives better results for banded steel compared to the other methods [16]. Compliance changing rate technique requires a smooth load-displacement curve for accurate results. The steps involved in the smoothening of the load~lisplacement curve are schematically shown in Fig. 6a-d. The unsmoothened load-displacement curve has large oscillations causing inaccuracies in determining the yield and the maximum load (Fig. 6a). The large oscillations were attenuated periodically with the sequence of sampling number m = 5,10,10. With m = 5, little data was attenuated and large oscillations were still present (Fig. 6b). Subsequently, at m = 10, oscillations are considerably eliminated (Fig. 6c). Further, with m = 10, complete smoothening was obtained (Fig. 6d). The typical smoothened load~lisplacement curve is shown in Fig. 6e. Figure 7 shows the relation between initiation dynamic fracture toughness and impact toughness for LT and TL direction (condition I), respectively. However, J~d* and CVN follow a linear relation which is given below for the respective direction. Jl~* = 0,854"(CVN) + 40.92 [LT orientation]

(I)

J~o* = 1.463"(CVN) + 38.38 [TL orientation].

(2)

Initiation dynamic fracture toughness decreases moderately with decreasing temperature. Interestingly, like impact toughness curve, J~d* also shows transition behavior (Fig. 8). The effect of banding concentration on initiation dynamic fracture toughness is shown in Fig. 9. As the banding concentration increases the J~d* also increases. This is due to the delamination induced toughness process. It is well known that fracture morphology changes with transition temperature. The fracture mode appearance reflects the improvement in fracture toughness. At room temperature, fracture occurred by micro-void coalescence and results in dimple structure (Fig. 10a). In TL orientation, 100% shear fracture with microvoid coalescence is observed (Fig. 10b). Moreover, preferential crack propagation is also noticed. This might be the reason for inferior impact toughness of TL at room temperature compared to LT direction. It is expected that cleavage mode results from pearlite. On the other hand, fibrous mode fracture results from ferrite region. The fractograph (Fig. 10) is evidence of the splitting or delamination process. If any non-metallic inclusion occurred in the path of crack propagation, large sized splitting resulted.

CONCLUSIONS (1) Banding in divider orientation improves the impact as well as initiation dynamic fracture toughness of the steel. (2) LT orientation shows high impact toughness, initiation dynamic fracture toughness and lower DBTT. (3) Splits in fractured surface disappear with decreasing temperature. (4) As the number of bands increases, DBTT of the steel decreases at the expense of upper shelf energy. (5) Delamination induced toughening mechanism is responsible for the improvement of toughness in this steel. (6) Initiation dynamic fracture toughness is calculated by compliance changing rate method and exhibits a linear relationship with impact toughness. (7) A higher number of splits yields inferior toughness. Acknowledgements--The authors wish to thank Tata Iron and Steel Company Limited (TISCO), Jamshedpur, India, for the material and financial assistance. The authors also thank Director, I.I.T., Madras, for providing all the facilities and Defence Metallurgical Research Laboratory, Hyderabad, for SEM studies.

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