effect of si content on the deterioration of vibration fracture resistance of ferritic sg cast iron...

7/30/2019 Effect of Si Content on the Deterioration of Vibration Fracture Resistance of Ferritic SG Cast Iron Under an Aqueou

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Effect of Si Content on the Deterioration of Vibration Fracture Resistance

of Ferritic SG Cast Iron under an Aqueous Environment

Yu-Hua Song*1, Truan-Sheng Lui*2 and Li-Hui Chen

Department of Materials Science and Engineering, National Cheng-Kung University, Tainan 701, Taiwan, R.O.China

Ferritic spheroidal graphite cast irons generally exhibit intergranular fracturing even if the specimens are strengthened by silicon in solid

solution when tested under wet conditions. The maximum area fraction of intergranular fracture is closely related to the microstructural feature

and ambient aqueous environment that caused the deterioration in vibration fracture resistance. In spite of the increasing silicon content,

experimental evidence confirms the overall D-N curves can be generalized into three characteristic regions. Intergranular cracks initiated in the

vicinity of the nodular graphite; the existence of nodular graphite acting as porosity should be considered as a dominant microstructural factor on

the initiation of intergranular fractures. It should be noted that the intergranular cracks can be prevented, resulting in better vibration fracture

resistance, when a specimen is covered with oil film. Based on experimental results, when the silicon content is increased, the vibration fracture

resistance in general will be improved for all specimens whether they are tested in oil mist, air or aqueous environments. In particular, when the

deflection amplitude is fixed, the vibration fracture resistance can be significantly raised for a high silicon sample (4.4Si).

(Received March 18, 2004; Accepted June 4, 2004)

Keywords: spheroidal graphite cast iron, damping capacity, resonant vibration, vibration life, aqueous ambient environment.

1. Introduction

Water-assisted deterioration of fatigue properties is likely

to occur when spheroidal-graphite (SG) cast iron is exposed

to an aqueous environment.13) Concerning the tensile

properties of water-immersed SG cast irons, it has been

reported that water-assisted embrittlement occurs only in the

case of high strength cast irons such as ADI (austempered

ductile iron). In our previous investigations on the vibrational

fracture behavior of ferritic nodular graphite cast iron under

aqueous conditions, vibration cracks initiating from the

surface took place either from nodular graphite or sequently

along the ferritic boundary, also causing embrittlement,

although microstructural refinement or damping capacity

improved the vibration fracture resistance.46)

On the other hand, it has been recognized that increasing

silicon content will improve thermal and weather-resistance

of SG cast iron, the solid solution strengthening effect of Si

atoms often were found to be the main controlling factor. As

to the silicon affected microstructural evolution, however,

why or how these metallurgical factors affected vibration

fracture behavior has still not been clarified. Therefore,

ferritic SG cast iron with different silicon content rangingfrom 2.1 mass% to 4.4 mass% were used to assess the effect

of silicon as well as the environmental effect on the

occurrence of intergranular embrittlement with regard to

the deterioration of vibration fracture resistance.

2. Experimental Procedures

2.1 Material preparation

Ferritic spheroidal graphite cast irons were prepared by

melting pig iron, silicon steel scrap and ferrosilicon in a high

frequency induction furnace. Small amounts of Fe-75 mass%

Si alloy and Fe-45 mass% Si-5 mass% Mg alloy were applied

as inoculator and spheroidizer, respectively. Ferritizing heat

treatment involved soaking at 1198 K for 3 hours prior to

furnace cooling to 1023 K for another holding period of 5hours before the final furnace cooling to room temperature.

Table 1 presents the chemical compositions, where 2.1Si,

3.0Si and 4.4Si denote SG cast irons containing 2.0, 3.0 and

4.4 mass % Si, respectively. Figure 1, which shows the

optical microstructures of the test materials, demonstrates

that the hypoeutectic cell structure can be changed into a

hypereutectic cell structure as the Si content is increased to

4.4 mass%. Table 2 presents the results of quantitative

microstructural analysis of the specimens used.

2.2 Resonant vibration, tensile tests and damping ca-

pacity measurementA simple cantilever beam vibration system, as shown

schematically in Fig. 2(a), was used for the vibration

experiment. The test specimen, rectangular with dimensions

15mm 100mm 2mm, as shown in Fig. 2(b), was

clamped on end to the vibration shaker. An acceleration

sensor monitored the vibration force, and a deflection sensor

measured the deflection amplitude at the end of the specimen.

Vibration tests were conducted at their resonant frequen-

cies and a fixed vibration force, with a detected acceleration

of 1.7 g, where g denotes acceleration due to gravity (9.8 m/

s2). In addition, a constant deflection amplitude test (around

13.2 mm) was also performed to clarify the effect of silicon

solid solution strengthening. The observed resonant frequen-

cy is obtained from the frequency leading to the largest

deflection and can be examined by varying the vibration

frequency continuously.

Table 1 Chemical Composition (mass%).

C Si Mn P S Mg Fe

2.1Si 3.37 2.10 0.062 0.044 0.008 0.043 Bal.

3.0Si 3.39 3.01 0.040 0.053 0.010 0.035 Bal.

4.4Si 3.32 4.43 0.059 0.042 0.007 0.044 Bal.

*1Graduate Student, National Cheng-Kung University*2Corresponding author, E-mail: [email protected]

Materials Transactions, Vol. 45, No. 7 (2004) pp. 2463 to 2470#2004 The Japan Institute of Metals EXPRESS REGULARARTICLE


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Three ambient environmental conditions were applied; air,

an aqueous and an oil environment were introduced by

spraying water and oil mist (using a drop device) on the

surface of the specimens during vibration testing, denoted as

Air Aq and Oil, respectively. The mechanical properties of

the samples were examined by tensile testing, with an initial

strain rate of6:67 104 s1, for verification.

Damping capacity is a materials ability to measure the

dissipated strain energy during a vibration test. The loga-

rithmic-decrement value (), derived from the deflection-

amplitude decay of a specimen under a free vibration test, isone method to measure damping capacity. In this study, the

specimen was placed under a resonant vibration condition

with a fixed push force of 0.8 g, and then the shaker was

stopped abruptly after 5 seconds of vibration. Thus, the decay

of the deflection amplitude could be measured. The loga-

rithmic-decrement value can be expressed as follows.

1=n lnAi=Ain

where Ai and Ain are the deflection amplitude of the ith and

(i n)th cycles after the load was removed, as shown in

Fig. 2(c).

3. Results

3.1 Relation between silicon content and logarithmic-

decrement value with respect to initial deflection

amplitude

Based on our previous report,5) the vibration life (Lx) is

affected primarily by crack propagation resistance and initial

deflection amplitude (DI). The former is directly proportional

to its yield strength (YS), and this can be correlated with

silicon content and recognized as a positive effect. It can be

expressed as Lx / YS. But the latter is controlled by damping

capacity, which is inversely proportional to deflection

amplitude. Both factors are closely related to silicon content,therefore, we can consider the maximum deflection ampli-

tude (Dmax) as a factor of deflection effect. To clarify the

relation between YS and with respect to Dmax, the data

points are shown in Table 2.

3.2 Relation between silicon content, initial deflection

amplitude, and logarithmic-decrement value with

respect to vibration life

From Fig. 3, it is clear that the overall D-N curves are

strongly ambient environmentally dependent and can be

generalized into three characteristic regions. The feature can

C.E.

=4.07

C.E.

=4.40

C.E.

=4.80

Fig. 1 Typical optical microstructure of electrochemical etching samples:

(a) 2.1Si (b) 3.0Si (c) 4.4Si.

(a)

1: vibration controller. 5: specimen.

2: acceleration sensor.3: vibration shaker. 7: recorder.

4: specimen clamp.

6: deflection sensor.

(c) 5 A5

A0

,A0

A5

(b)

fixture

unit: mm

Fig. 2 (a) Vibration apparatus (b) dimensions of test specimens (c) measurement of logarithmic decrement .

2464 Y.-H. Song, T.-S. Lui and L.-H. Chen


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be described from the beginning as follows: Region I, with

increasing deflection amplitude, Region II, with a constant

deflection amplitude and Region III with descending de-

flection amplitude. Based on our previous investigations7,8)

and observed evidence as shown in Fig. 3(a), strain-harden-

ing results in an ascending deflection in the early stage of the

D-N curve (Region I). The deformation is mainly localized in

the vicinity of graphite nodules due to the stress concen-

tration effect under cyclic loading on the surface (Fig. 4(a)).Region II, the constant deflection amplitude, can be attributed

to the strain hardening effect in competition with crack

generation and linking, is shown in Fig. 4(b). Region III, with

a descending trend, resulting from the actual vibration

frequency deviating from the resonant frequency, can be

attributed to the rapid inward propagation of major cracks.

Therefore, the quantitative data for vibration fracture

resistance can reasonably be defined as the total number of

vibration cycles within the interval of Region I and II.

On the other hand, the specimens tested in ambient oil mist

conditions commonly exhibited a significant improvement invibration fracture resistance compared to the specimens

tested under identical air and ambient water mist environ-

ments. This implies that the ambient environmental factor

plays an important role in the deterioration of fracture

resistance, and this can be correlated with the silicon content

of SG cast iron.

Figure 3 show the D-N curves of the specimens tested with

various ambient environmental conditions for specimen 2.1Si

vs. 3.0Si (similar DA value) and 2.1Si vs. 4.4Si (different DAvalue), respectively. The vibration life of 3.0Si specimens is

better than 2.1Si specimens when the deflection amplitude is

similar. But, as shown in Fig. 3(b), the high silicon specimen

(4.4Si) cannot acquire further improvement due to higher

deflection amplitude. In order to clarify the effect of the

damping capacity factor caused by silicon solid solution

strengthening, a fixed constant deflection amplitude test was

(a)

8

10

12

14

16

18

De

flec

tion,

d/mm

2.1Si-Oil2.1Si-Air2.1Si-Aq.3.0Si-Oil3.0Si-Air3.0Si-Aq.

3.0Si-Oil

2.1Si-Oil2.1Si-Aq

3.0Si-Aq

0 5000 10000 15000

Number of Cycles

8

12

16

20

De

flec

tion,

d/mm

2.1Si-Oil

2.1Si-Air

2.1Si-Aq.

4.4Si-Oil4.4Si-Air

4.4Si-Aq.

4.4Si-Oil

2.1Si-Oil4.4Si-Aq

2.1Si-Aq

(b)

Fig. 3 Effect of silicon content on the D-N curves with a fixed push force

of 1.7 g under different ambient conditions, (a) comparison of 2.1Si and

3.0Si (b) 2.1Si vs. 4.4Si samples.

Fig. 4 (a) The typical crack initiation sites for 3.0Si specimens are mainly

in the vicinity of graphite nodules, (b) evidence of vibration crackgeneration and linking behavior.

Table 2 Results of quantitative microstructural analysis and mechanical

properties of the specimens used, (: logarithmic decrement, DA:

deflection amplitude).

MaterialsAg Dg Counts YS Elong.

DA

(%) (mm) (/mm2) (MPa) (%) (mm)

2.1Si 12.8 50 106 317 21 0.131 13.2

3.0Si 14.2 46 128 341 21 0.128 13.5

4.4Si 13 40 197 421 12 0.108 16.1

Effect of Si Content on the Deterioration of Vibration Fracture Resistance of Ferritic SG Cast Iron under an Aqueous Environment 2465


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performed, as shown in Fig. 5. It is clear that the vibration

life of 4.4Si specimens is profoundly longer than 2.1Si

specimens.

To gain a better understanding of the predominant factors

of vibration life, it is reasonable to take YS and into

consideration at the same time when tested in a fixed push

force. Since Dmax is inversely proportional to the logarithmic-

decrement value (), it can be recognized as a negative effect,and can be expressed as Lx / 1=1=. That is Lx / .

Combining both of the above-mentioned terms, finally we get

the relation Lx / YS . Figure 6 shows the vibration life

increased at the beginning then decreased as YS increas-

ed. On the other hand, for constant deflection amplitude tests,

since they have isolated the effect of damping capacity, it is

reasonable to only take YSinto consideration. Figure 7 shows

the vibration life increased as YSincreased. This implies that

if we can eliminate the effect of damping capacity, then the

solid solution strengthening of silicon will lead to better

vibration fracture resistance for all specimens regardless of

whether they are tested in an oil mist, air or aqueous

environments.

Moreover, to quantitatively assess how the silicon solid

solution strengthening affects the vibration life, since there is

very little intergranular fracture evidence when tested in an

oil environment, it is reasonable to define the degradation

rates (D.R.), and calculate as follows:

D.R. 1Lair;Laq=Loil 100%

where Lair, Laq and Loil are the vibration lives of the

specimens when tested in air, aqueous, and oil environments,

respectively. For the specimens tested with fixed push force,

Fig. 8 reveals that the ones tested in an aqueous environment

are more serious intergranular fracture than those tested in

the air, regardless of whether they were tested with fixed push

force or constant deflection amplitude. Figure 8(a) shows

that the degradation rates kept constant at the beginning then

improved as YS increased for the specimens tested with

fixed push force. Figure 8(b) shows the degradation ratesgenerally can be improved as YSincreased for the specimens

tested with constant deflection amplitude.

3.3 Effect of silicon content on the initiation of inter-

granular fracture under ambient aqueous environ-

ment

As mentioned above and marked by arrow A indicated in

Fig. 4(a), it is clear that the cracks initiated in the vicinity of

graphite nodules due to the stress concentration effect

regardless of whether the specimens were strengthened by

silicon solid solution when tested under wet conditions.

Moreover, the typical fractography of each specimen, tested

with fixed push force and constant deflection amplitude, was

examined. Figures 9 and 10 show that many intergranular

cracks can be observed in the vicinity of graphite nodules.

However, the cracks initiated from the surface of the test

0 6000 12000 18000 24000 30000

Number of Cycles

6

8

10

12

14

16

Deflection,d

/mm

2.1Si-Oil

2.1Si-Air

2.1Si-Aq.

4.4Si-Oil

4.4Si-Air

4.4Si-Aq.

2.1Si-Aq

4.4Si-Oil

4.4Si-Air

Fig. 5 Effect of silicon content on the D-N curves of ferritic spheroidal

graphite cast iron under a constant deflection amplitude during a resonant

vibration test.

42 44 46YS MPa

3

6

9

12

Num

bero

fCyc

les

toFa

ilure,

103 Oil

Air

Aq.

2.1SiD=13.2

3.0SiD=13.5

4.4SiD=16.1

P.F.=1.7g

Fig. 6 Effect of silicon content on the vibration fracture resistance with a

fixed push force (PF) of 1.7 g tested in oil, air and aqueous environments.

320 360 400 440YS MPa

6

9

12

15

18

21

Num

bero

fCyc

les

toFa

ilure,

103

Air

Aq.

Oil

2.1Si 3.0Si

4.4Si

DA:13.2mm

Fig. 7 Effect of silicon content on the vibration fracture resistance with a

fixed deflection tested in oil, air and aqueous environments.



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piece. It should be noted that the penetration area fraction of

intergranular cracks was significantly intensified when the

specimens were tested in an aqueous ambient environment

regardless of whether they were tested with fixed push force

or constant deflection amplitude.

4. Discussion

Concerning the vibrational application, it is clear that a

significant deterioration of vibration fracture resistance

commonly occurs in ferritic SG cast iron containing different

silicon contents when tested in an aqueous environment.

(a)

42 44 46YS MPa

0

20

40

60

80

Degra

da

tion

Ra

te(%)

D.R.=[1-(Lair, Laq/ Loil)] 100 %

Air(g)Aq.(g)

2.1SiD=13.2

3.0Si

D=13.5

4.4SiD=16.1

(b)

280 320 360 400 440YS MPa

0

20

40

60

DegradationRate(%)

D.R.=[1-(Lair,Laq/ Loil)] 100 %

Air(D)Aq.(D)

2.1Si 3.0Si

4.4Si

Fig. 8 Degradation rate (D.R.) of three specimens tested with (a) fixed push force: (g) and (b) constant deflection: (D), respectively

(Aq: Aqueous).

Fig. 9 Fractograph of 2.1Si specimens with a fixed push force of 1.7 g tested in: (a) air (b) aq. and 4.4Si specimens tested in: (c) air (d) aq.

environments, (aq: aqueous).



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Based on our earlier investigations,5,6) the resonant vibration

samples, resembling the reverse bending fatigue test, suffer

from alternate bending stress. It can be deduced that the

alternate tensile-compressive stresses should be correlated to

the existence of nodular graphite which actually plays an

important role in causing deterioration in vibration fracture

resistance.

As for the specimen tested in the simple cantilever beam

vibrating system, Fig. 3 shows the deflection increases as the

number of vibration cycles increases in the initial stage.

According to our previous examination, this should be

associated with increasing work hardening.4,7,8)

Furthermore, in order to verify the effect of silicon solid

solution strengthening, a constant deflection amplitude test

was also performed. From Fig. 5, it is clear that the vibration

life increased as the silicon content increased. It is reasonable

to suggest that if we eliminated the effect of damping

capacity, then the solution strengthening of silicon will

improve the vibration fracture resistance for all specimens

regardless of whether they are tested in an oil mist, air or

aqueous environment. On the other hand, if we cannot isolate

the effect of damping capacity, it still can be decreased

through mechanical design by placing a cushion or high

damping alloy at certain places. Then the solution strength-

ening of silicon can also acquire the actual application.

From Fig. 8, it is clear that the degradation rate can be

Fig. 10 Fractograph of three specimens with a fixed deflection tested in: 2.1Si (a) air (b) aq., 3.0Si (c) air (d) aq. and 4.4Si (e) air (f) aq.environments, (aq: aqueous).



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improved if the silicon content is increased, especially when

tested with constant deflection amplitude. It is reasonable to

suggest that silicon strengthening is active even in the 4.4Si

specimen (DA 16:1mm) which suffered from much more

induced push force than the 2.1Si specimens (DA

13:2mm) when tested with a fixed push force. Moreover,

from our previous results, the cracks tended to nucleate fromand propagate toward the prior-eutectic cell-wall regions

where a fair amount of MgO inclusions embedded in-

between the nodular graphite.5,9) Figure 1, also reveals that

the silicon content will affect the microstructural feature, and

although the MgO inclusion clustered10,11) region is an

inevitable route for vibration crack propagation, little

influence can be recognized in this study. However, more

detailed investigation is needed.

As for the fracture behavior of ferritic SG cast irons during

vibration testing, it must be pointed out that many surface

cracks generally exhibit intergranular cracks which initiated

from the ferrite rim of nodular graphite regardless of whetherthe specimens were tested in air or an aqueous environment.

The intergranular fracture formed around the nodular graph-

ite as a rim shape, however also occurred in the vicinity of

nodular graphite as shown in Fig. 11. According to previous

investigations,12) the effect of the plastic strain rate and stress

concentration at the tip of a notch will encourage a cleavage

fracture to occur. Moreover, the SG cast iron exposed to an

aqueous environment shows a larger area fraction of

intergranular fracture. A generally accepted opinion is that

the wet environmental effect on fatigue crack propagation

should be considered closely related to hydrogen-induced

cracking.13,14) For SG cast iron, graphite nodules acting as

porosity will lead to the stress concentration in the vicinity ofgraphite nodules during the elastic deformation stage.15,16)

Concerning the hydrogen resolving from water molecules

contained in the air or aqueous environments, the effect of

stress concentration around nodular graphite can be assumed

to be responsible for the initiation of intergranular fracture.17)

In addition, the preferential absorption of atomic hydrogen

along metalloid-segregated grain boundaries, and subsequent

intergranular cracking were related to the deterioration of

vibration fracture resistance.

With regard to the vibration fracture process, cyclic

loading with an alternate tensile-compressive stress will

promote the diffusion of interstitial atoms.15) From previous

investigations,18) particularly for ferritic spheroidal graphite

cast iron, Si showed negative segregation at the old austeniteshells associated with spheroidal graphite. Furthermore,

silicon solution in matrix will decrease the strength of the

ferritic grain boundary.19) The graphite nodules are believed

to play an important role in promoting the hydrogen-induced

fracture effect on grain boundary decohesion.17) In the case of

the degradation rate, a most likely possibility for the effect of

silicon content of SG cast iron is through the governing

fracture behavior.

5. Conclusions

(1) The vibration fracture resistance of ferritic SG cast ironis reduced through contact with water molecules

regardless of silicon solid solution strengthening.

Samples, tested in an aqueous environment, exhibit a

larger inward extended intergranular fracture area

fraction than those tested in other conditions. In

particular, the specimens tested in an oil mist environ-

ment exhibited very few intergranular fractures.

(2) It is reasonable to suggest that the reaction between

water molecules, containing hydrogen, and ferritic SG

cast iron is responsible for the deterioration of vibration

fracture resistance. In particular, the cracking behavior

of ferritic SG cast iron is probably correlated to the

existence of nodular graphite, which is the stressconcentration site during testing and results in the

induced intergranular cracks in the earlier stages of

vibration.

(3) The solid solution strengthening of silicon will decrease

the damping capacity when tested with fixed gravity.

Solid solution strengthening of silicon will enhance the

vibration fracture resistance for the specimens with

higher silicon content regardless of whether they were

tested in an oil mist, air or aqueous environment, when

the vibration test was conducted under constant initial

deflection amplitude. The degradation rate can be

improved, as YS and YS increases for a fixedgravity push force and constant deflection amplitude,

respectively.

Acknowledgments

The authors would like to thank the National Science

Council of the R.O.C. (Taiwan) for financial support of this

research under Contract No. NSC 92-2216-E-006-040-.

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