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Fatigue crack propagation of bainitic nodular cast iron
IntJ FatiguelO No 4 (1988) pp 219-226
Fatigue crack propagation of bainitic nodular cast iron Ji-Liang D o o n g and Shyolng Yu
The effects of austempering temperature and isothermal transformation time on fatigue crack growth rate in a ductile iron with a bainitic structure have been studied. Crack growth rates in austempered samples were compared with those in materials with a 'bullseye' casting structure. Using scanning electron microscopy, the mechanism of the fatigue crack growth can be understood by observing the fracture surface of a fatigue specimen. X-ray diffractometry was used to determine the volume fraction of retained austenite. It can be concluded that the volume fraction of retained austenite, the fracture mode and the matrix microstructure are closely related to the fatigue crack propagation rate and the fracture mode. Key words: fatigue crack propagation; austempering temperature; ductile iron; bainitic structure; fracture mode
Austempered ductile iron exhibits a combination of high strength and ductility which makes it competitive with steel forgings in many applications. 1=3 The material has been used in components such as crankshafts, camshafts, railway wagon wheels and particularly gears of various types. The mechanical properties of austempered ductile iron including tensile properties, 4~7impact toughness s and fracture toughness 9-10 have been evaluated and the microstructural characteristics of a number of the irons have been characterized and correlated to chemical composition and heat treatment. The influence of the microstructure on the properties of the material has also been studied lt-14 but limited information on the fatigue properties of austempered ductile iron is available. Sharma 15 studied the contact fatigue properties of austempered nodular iron transformed at 230°C to Rc45-47 by using rolling contact fatigue tests. He suggests that additional research into the material is needed for new engineering applications. The most frequently quoted work on the fatigue properties of the material is by Johansson 16 but his results lack full details of the heat treatments carried out. He studied the fatigue strength of austenitic-bainitic ductile iron using beam fatigue tests. Fracture toughness and fatigue crack growth in nodular cast iron for various matrix microstructures has been studied by the first author of this paper. 17-19 However, the influence of isothermal bainitic transformation temperature on fatigue crack growth of the four specimens studied is still not clear. The present study investigates the effects of different transformation temperatures and times on the crack growth rate in austempered ductile irons.
Experimental procedure Samples of the cast irons (Table 1, Fig. 2) were melted in a 50 kg high-frequency induction furnace, magnesium treated in a ladle and cast into sand moulds. The dimensions of the Y-block castings are shown in Fig. la.
To compare the influence of austempering temperature on the fatigue crack propagation rates, specimens were austenitized at 900"C, quenched between 250"C and 450"C, and held at this temperature for 4 h. The amount of bainite produced can be controlled by varying the holding time at the isothermal treatment temperature. Holding times from 2 min up to 33 h at isothermal transformation temperatures of 300 and 400"C were chosen to cover almost the whole bainitic transformation range (Table 2). Fatigue tests were performed using a computerized closed-loop servo-controlled hydraulic testing system (MTS). Compact-tension specimens were used to measure the fatigue crack growth rate, da/dN (specimen configurations are shown in Fig. l b). The experimental method meets the ASTM E647-83 standard. A sinusoidal load was applied at a frequency of 40 Hz and the ratio of minimum to maximum load was 0.1 throughout. The growing fatigue crack length was calculated from the crack opening displacement, measured by a clip-on gauge. The fatigue cycles and the cyclic stress intensity could be automatically recorded by the printer connected to the computer. Fatigue crack growth rates in all hainitic nodular cast irons can be described by the Paris equation as
da/dN= C(AIO" Using the least-squares method, the experimental crack length and stress intensity factor data were correlated using a microcomputer system (Seiko 9000). The calculated values of C a n d , are shown in Table 3. X-ray diffraction was used to determine the volume percentage of retained austenite in the samples. A Rigaku diffractometer with a strip chart recorder scanned the angular 20 range from 30° to 39°, the scan rate was 0.01" s -t. The amount of retained austenite was calculated by the method proposed by Cullity20 and Rundman21. The fatigue c r a c k surfaces were studied by SEM analysis for variations in the fracture mechanism. For accurate comparison, the locations
0142-1123/88/040219-08 $3.00 © 1988 Butterworth & Co (Publishers) Ltd Int J Fatigue October 1988
Table 1. Chemical composition (weight %)
Furnace Furnace
No 1 No 2
C
Si
Mn
P
S
Mg
Cu
Mo
Ni
3.16 3.27
2.34 2.40
0.094 0.096
0.043 0.040
0.008 0.013
0.041 0.035
0.54 0.52
0.256 0.235
0.54 0.53
Table 2. Specimen number and austempering treatment conditions
i< ¢
175
7O i
Specimen
a / ~ 9 . 0 + 0 . 0 5 x2
1
~ - - , ~ -
21.6-0.I
--
_-¢.--
.-.-I
43.2
L ~5.4 36 + 0 . 1 - ~ 45+_0.15
>
~ 5 ~
number
Austenitizing temperature (°C) and time (h)
Austempering temperature (°C) and time
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
900,1 9001 9001 9001 9001 9001 9001 9001 9001 9001 9001 9001 9001 9001 9001 9001 9001 9001 9001 As-cast alloy
250,4 h 300, 4 h 350, 4 h 400, 4 h 450, 4 h 300, 2 min 300, 9 min 300, 30 min 300, 1.5 h 300, 4 h 300, 10 h 300, 33 h 400, 2 min 400, 9 min 400, 30 min 400, 1.5 h 400, 4 h 400, 10 h 400, 33 h
45°
b Fig. 1 Speccmen configurations for (a) Y-block casting and (b) fatigue crack growth. All dimensions in mm
which have the same stress intensity factor were chosen for examination. The amount of spheroidization was analyzed and a nodular count was carried out using an imaging analyser (Hewlett Packard).
Results and discussion According to Dorazil et al, 22 the isothermal transformation of austenite in the bainitic region can be divided into three stages: (1)
(2)
(3)
Fig. 2 Microstructure of as-cast nodular cast iron
220
A considerable amount of martensite is formed. If the isothermal transformation time is held both the amount of bainite and carbon-enriched, untransformed austenite increase and the amount of martensite is reduced. Transformation continues mainly by the lateral growth of bainitic ferrite plates. The additional carbon produced diffuses into the austenite as additional ferrite forms. This carbon enrichment of the austenite is sufficient to stabilize fully the austenite even when cooled rapidly to room temperature. The best combination of tensile strength and ductility are reached at this stage. The austenite decomposes to form additional ferrite and plate-like carbide precipitates. A mixture of ferrite and carbide is found in areas where bainitic ferrite plates have grown into each other. It is suggested that at this stage the austenite transformation is in the form of the eutectoid-type reaction, T --~ a + c. The decrease
Int J Fatigue October 1988
T a b l e 3. M e c h a n i c a l p r o p e r t i e s and r e t a i n e d a u s t e n i t e Specimen number
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Ultimate strength (kg mm -=)
Hardness (H Rc)
122.5 125.0 93.7 81.8 79.7 53.1 74.6 121.2 119.9 125.0 127.3 124.3 65.7 76.0 75.5 75.7 81.8 74.1 76.4 56.5
42.4 43.8 32.1 29.3 25.9 49.2 44.9 43.2 36.3 43.8 45.0 44.5 41.4 25.3 24.5 27.3 29.3 27.8 27.7 19.2
C 5.18 6.15 1.40 1.86 1.29 8.39 6.70 1.39 1.82 6.15 1.36 2.09 7.14 6.32 8.02 3.77 1.86 2.22 7.03 2.94
Int J Fatigue October 1988
x x x x x x x x x x x x x x X
X X X X X
10 -1° 10 -1° 10 -1° 10 -11 10 -is 10 -1= 10 -13 10 -9 10- lo 10 -1° 10-' 10-' 10 -11 10 -11 10 -11 10 ÷11 10 -11 10 -14 10 -1" 10 -13
n
(%)
2.87 2.74 3.02 3.72 5.15 4.69 4.78 2.59 3.02 2.74 2.66 2.56 3.69 3.31 3.20 3.42 3.72 5.57 5.92 4.65
17 18 22 8 9 23 24 25 27 18 16 14 33 35 30 15 8 7 6
10-3
in the amount of austenite from approximately 35% to less than 10% during austempering can be seen in specimens 17-19 in Table 2. The result of crack growth rate measurements on specimens with different austempering temperatures is shown in Fig. 3, and their microstructures in Fig. 4. By comparing the crack growth rates in Fig. 3, it can be seen that with a 4 h isothermal transformation time, the lower hainitic structure produced at the 350"C austempering temperature (Fig. 4c) has the slowest fatigue crack growth rate. Good toughness and medium strength are obtained at the 350"C austempering temperature. Because of high tensile strength and hardness, the resistance to fatigue crack growth of the £mer lower bainitic structures obtained at austempering temperatures of 250°C and 300°C are not as good as those of the structures produced at 350°C. The upper bainitic structure produced by austempering at 400°C and 450°C (Figs 4d and e), has low tensile strength and less retained austenite beacuse of the longer transformation time and the resistance to fatigue crack growth is worse. In this case, it is believed that the transformation of upper bainite had proceeded to the third stage described above. The formation of carbide and ferrite from the austenite significantly reduces the toughness of the material. Although the as-cast 'bullseye' structure has the lowest tensile strength and the lowest hardness, its resistance to crack growth is between that of the upper and lower bainitic structures. The influence of various isothermal holding times on crack propagation rate and microstructure of upper bainitic ductile irons are shown in Figs 5 and 6. Upper bainitic transformation times from 9 min to 1.5 h have the best resistance to the crack growth. This is because bainite trans-
Retained austenite
Parameter in Paris equation
....
As cast
450°C 400oc
>. u
E 10 -q
3 x 1 0 -5
I 30
I
I 50
I
I 70
I I [ , Ill 100 140
200
L~K ( k g mm - 3 / 2 )
Fig. 3 Effect of austempering temperature on fatigue crack growth rate
221
Fig. 4 Optical micrographs of specimens 1-5 illustrating effects of austempering temperature on bainite morphology at: (a) 250°C, (b)
300°C, (c) 350°C, (d) 400°C, (e) 450°C 10 -3 900 - 400°C
33 h
B U >, l.J
E E "D
10 -4
"D
2m 3 x 1 0 -5 0.5h I
30
i
I
50
i
I 70
AK{kgmm
I
I
] , I,I 100 140
200
-3/2)
Fig. 5 Effect of austempering time at 400"C on fatigue crack growth rate
formation is just in the second stage, and enough retained austenite is present to affect the properties of the material. This conclusion agrees with Dorazil et al's results which indicated that structures with 20-40% retained austenite have the optimum mechanical properties. 22 If the transformation time is too short (k the bainite transformation is in the first stage), ~
martensite will cause the crack growth rate to increase quickly. If the transformation time is too long (the bainite transformation is in the third stage) the amount of ferrite and carbide will increase and lower the toughness and increase the fatigue crack propagation rate of the material. Plots of fatigue crack growth rate da/dN versus stress intensity factor range AK for bainitic iron austempered at 300°C are shown in Fig. 7. The austempering transformation time was varied from 2 min to 33 h. From Fig. 7 it can be seen that the specimens with transformation times from 30 min to 1.5 h have the best resistance to the crack growth. If the transformation time is shorter than 30 min, the transformed structure will become martensitic (see Figs 8a and b). This accounts for a drop in the toughness and the increase in crack growth rate experienced. If the transformation time is held over 15 h, the amount of retained austenite in the microstructure decreases slowly (Figs 8c and d). These specimens had higher tensile strengths and slower crack propagation rates than specimens austempered for nine minutes. Fig. 8 illustrates typical microstructures for specimens austempered at 300°C. The structure was much finer than those formed by austempering at 400°C. Although ferrite nucleated more rapidly at lower austempering temperatures, its growth slowed considerably and the completion of the austenite transformation required a long time. 23 This is why the amount of retained austenite was still 14% after 33 h at the 300°C austempering temperature. The slow transformation of the retained austenite into ferrite and carbide, gave the specimen a crack growth rate similar to the specimen austempered for 30 min. Comparing the properties of as-cast, lower bainitic and upper bainitic structures (from Figs 3, 5 and 7 and Table 2) it can be seen that materials containing between 20% and 40% retained austenite with the lower bainitic structure produced at 350°C have the best resistance to crack propagation. The as-cast structure has the worst resistance. The crack growth rates of the upper hainitic structure (produced at 400°C) and the lower bainitic structures (produced at 250°C and 300°C) are quite similar. Both are inferior to those of the lower hainitic structures produced at 350°C. Fractographs of the crack growth area of the as-cast
Int J Fatigue October 1 9 8 8
Fig. 6 Optical micrographs of specimens 13-19 illustrating effects of austempering time at 400°C on bainite transformation. (a) 2 min, (b) 9 rain, (c) 30 min, (d) 1.5 h, (e) 10 h, (f) 33 h 10-3
lower bainitic irons have the mixed intergranular and transgranular rupture patterns shown in Fig. 10a. This explains their reduced resistance to crack growth. After second stage transformation, the crack growth areas of the upper and lower bainitic structure have the transgranular rupture with some of the intergranular type rupture (Fig. 10b). In the tear area, the fracture mode is transgranular and brittle quasi-cleavage (Fig. 10c). If the transformation time is too long (for example, 33 h), the fracture surface of both the upper and lower bainitic irons is of the mixed transgranular and quasi-cleavage types (Fig. 10d). The surface of the tear area of the sample austempered at 350°C shows dimple rupture (Fig. 11) and provides an indication of why irons treated in this way have good toughness and resistance to crack growth. Voids tend to form at graphite modules when irons are subjected to tensile loads. Fig. 12a illustrates this and shows the rupture in this area. Some cleavage due to the concentration of stress can also be observed. Sometimes, a crack may seem to have been initiated and propagated by this stress concentration effect. However, the propagation of cleavage cracks can be retarded at other graphite modules (Fig. 12b). The crack induced from the front graphite module has stopped at the next module although secondary crack growth can be observed (Fig. 12b).
900 - 300°C
>.
E E "O
"~
10-4
9 min
oh
3x10 -5
_
/
o///
m,o
"22"/
1.5h
I 30
I
I 50
=
I 70
J
= [ J I=1 I00 140
Conclusions 200
1)
~ K ( k g m m -3/2) Fig. 7 Effect of austempering time at 300°C on fatigue rate
crack
growth
2) nodular iron show that rupture extends along the direction of the pearlite (Fig. 9a). The tear area has the river pattern as in Fig. 9b. This result coincides with Griswold's investigation.24 With short transformation times, both the upper and
Int J Fatigue October 1988
3)
Crack growth rates increase in the following order: lower bainitic nodular cast iron, upper bainitic nodular cast iron, and as-cast mainly pearlitic iron with bullseye ferrite surrounding the nodules. During isothermal transformation to upper or lower bainite, the crack growth rates initially decrease with increased holding times. However, with extended holding times, the reverse occurs. The slowest crack growth rate is obtained with an austempering temperature of 350°C with holding times in excess of 0.5 h.
223
Fig. 8 Optical micrographs of specimens 6-12 illustrating effects of austempering time at 300°C on bainite morphology. (a) 2 min, (b) 9 rain, (c) 30 min, (d) 1.5 h, (e) 10 h, (f) 33 h
References
Fig. 9 Fractograph of as-cast nodular iron: (a) crack propagation region, (b) tear rupture region
224
I.
Rossi, F.A. and Gupta, B. Ko 'Austempering of nodular cast iron automobile components" Met Progr (April 1981) pp 25-31
2.
Dodd, J 'High strength, high ductility, ductile iron' Modern Casting (May 1978) pp 60-66
3.
Harris, D. A and i a i t i a n d , R. J. 'The products of isothermal decomposition of austenite in a spheroidal graphite cast iron" Iron and Steel (February 1970) pp 53-60
4.
Cox, G. J. 'Tensile properties of spheroidal graphite acicular irons" Brit Foundryman 75 (January 1982) pp 1-7
5.
Pivovarov, V. M., Levchenko, A. A. and Tanako, I. A. 'Characteristics of the microstructure of Cr-Ni-Mo cast iron after isothermal quenching' Met Sci Heat Treat (January 1987) pp 67-68
6.
Bevan, J. E. and Scholz, W. G. "Effect of molybdenum on transformation characteristics and properties of highstrength ductile irons' AFS Trans (1977) pp 271-276
7.
Dorazil, E., Barta, B., Crhak, J. and Munstarova, E. 'High-strength bainitic cast iron with spheroidal graphite' Met Sci Heat Treat (1978) pp 532-535
8.
Voigt, R. C. and Loper, C. R. 'Austempered ductile iron process control and quality assurance' J Heat Treat (December 1984) pp 291-309
9.
Ostensson, B. 'Fracture toughness and fatigue crack growth in nodular iron' ScandJ Metall2 (1973) pp 194-196
10.
Lazaridis, A., Worzalz, F. J. Loper, Co R. and Heine, R. W. 'Fracture toughness of ductile cast iron' AFS Trans (1971) pp 351-360
11.
Gagne, i . and Fallon, P. A. 'Microstructural characteristics of bainitic ductile irons' Can Metal/ Quart 25 1 (1986) pp 79-90
12.
Moore, D. J., Rouns, T. N. and Rundman, K. B. 'The effect of heat treatment, mechanical deformation and alloying element additions on the rate of bainite formation in austempared ductile irons' J Heat Treat4 1 (1985) pp 7-24
13.
Rouns, T. N., Rundman, K. B. and Moore, D. M. "On the structure and properties of austempered ductile cast iron" AFS Trans (1984) pp 815-840
14,
Gagna, M. 'The influence of manganese and silicon on the microstructure and tensile properties of austempered ductile iron" AFSD Trans (1985) pp 801-812
Int J F a t i g u e O c t o b e r 1 9 8 8
Fig. 10 Typical fracture surface of isothermal transformation of austenite in the bainite region: (a) stage 1, (b) stage 2 (crack Propagation region), (c) stage 2 (tear region), (d) stage 3.
Fig. 12 (a) Interface of nodular voids and transgranular cleavage. (b) Secondary crack growth.
Fig. 11 Fractograph of 35o’C austempering bainitic structure
Int J Fatigue October 1988
15.
Sharma, V. K. ‘Roller contact fatigue study of austempered ductile iron’ J Heat Treat3 4 (1984) pp 326334
16.
Johansson, M. ‘Ductile-bainitic ductile iron’ AFS Tram (1977) pp 117-122
17.
Doong, J. L. Hwang, J. R. and Chen. H. S. ‘Effect of ferrite and pearlitic distribution on fracture toughness in nodular cast iron’ J Mater Sci Letf (1983) pp 737-740
18.
Doong, J. L., Hwang, J. R. and Chen, H. S. ‘The influence of pearlite fraction on fracture toughness and fatigue crack growth in nodular cast iron’ J Mater Sci (1986) pp 871-678
19.
Doong, J. L., Ju, F. C., Chen, H. S. and Chen, L. W. ‘The influence of austempering temperature on fracture toughness in bainitic nodular cast iron’ J Mater Sci Len (1986) pp 555-558
225
20.
Cullity, B. D. 'Elements of Xoray diffraction' (November 1977) pp 411-417
21.
Rundman, K. B. and Klug, R. C. 'An X-ray and metallographic study of an austempered ductile cast iron' AFS Trans (1982) pp 499-508
22.
Dorazil, E., Barta, B., Munsterova, E., Stransky, L. and Huvar, A., "High-strength bainitic ductile cast iron" AFS Int Cast MerJ (1982) pp 52-62
23.
226
Voigt, R. C. 'Microstructural analysis of austempered ductile cast iron using the scanning electron microscope" AFS Trans (1983) pp 253-262
24.
Griswold, F. D. Jr and Stephens, R. I. 'Comparison of fatigue properties of nodular cast iron production and Y-block castings'/ntJ Fatigue9 1 (1987) pp 3-10
Authors Ji-liang Doong is an Associate Professor and Shy-lng Yu is a Research Assistant, in the Department of Mechanical Engineering, National Central University, Chung-li, Taiwan, ROC.
Int J Fatigue O c t o b e r 1988
Fatigue crack propagation of bainitic nodular cast iron Ji-Liang D o o n g and Shyolng Yu
The effects of austempering temperature and isothermal transformation time on fatigue crack growth rate in a ductile iron with a bainitic structure have been studied. Crack growth rates in austempered samples were compared with those in materials with a 'bullseye' casting structure. Using scanning electron microscopy, the mechanism of the fatigue crack growth can be understood by observing the fracture surface of a fatigue specimen. X-ray diffractometry was used to determine the volume fraction of retained austenite. It can be concluded that the volume fraction of retained austenite, the fracture mode and the matrix microstructure are closely related to the fatigue crack propagation rate and the fracture mode. Key words: fatigue crack propagation; austempering temperature; ductile iron; bainitic structure; fracture mode
Austempered ductile iron exhibits a combination of high strength and ductility which makes it competitive with steel forgings in many applications. 1=3 The material has been used in components such as crankshafts, camshafts, railway wagon wheels and particularly gears of various types. The mechanical properties of austempered ductile iron including tensile properties, 4~7impact toughness s and fracture toughness 9-10 have been evaluated and the microstructural characteristics of a number of the irons have been characterized and correlated to chemical composition and heat treatment. The influence of the microstructure on the properties of the material has also been studied lt-14 but limited information on the fatigue properties of austempered ductile iron is available. Sharma 15 studied the contact fatigue properties of austempered nodular iron transformed at 230°C to Rc45-47 by using rolling contact fatigue tests. He suggests that additional research into the material is needed for new engineering applications. The most frequently quoted work on the fatigue properties of the material is by Johansson 16 but his results lack full details of the heat treatments carried out. He studied the fatigue strength of austenitic-bainitic ductile iron using beam fatigue tests. Fracture toughness and fatigue crack growth in nodular cast iron for various matrix microstructures has been studied by the first author of this paper. 17-19 However, the influence of isothermal bainitic transformation temperature on fatigue crack growth of the four specimens studied is still not clear. The present study investigates the effects of different transformation temperatures and times on the crack growth rate in austempered ductile irons.
Experimental procedure Samples of the cast irons (Table 1, Fig. 2) were melted in a 50 kg high-frequency induction furnace, magnesium treated in a ladle and cast into sand moulds. The dimensions of the Y-block castings are shown in Fig. la.
To compare the influence of austempering temperature on the fatigue crack propagation rates, specimens were austenitized at 900"C, quenched between 250"C and 450"C, and held at this temperature for 4 h. The amount of bainite produced can be controlled by varying the holding time at the isothermal treatment temperature. Holding times from 2 min up to 33 h at isothermal transformation temperatures of 300 and 400"C were chosen to cover almost the whole bainitic transformation range (Table 2). Fatigue tests were performed using a computerized closed-loop servo-controlled hydraulic testing system (MTS). Compact-tension specimens were used to measure the fatigue crack growth rate, da/dN (specimen configurations are shown in Fig. l b). The experimental method meets the ASTM E647-83 standard. A sinusoidal load was applied at a frequency of 40 Hz and the ratio of minimum to maximum load was 0.1 throughout. The growing fatigue crack length was calculated from the crack opening displacement, measured by a clip-on gauge. The fatigue cycles and the cyclic stress intensity could be automatically recorded by the printer connected to the computer. Fatigue crack growth rates in all hainitic nodular cast irons can be described by the Paris equation as
da/dN= C(AIO" Using the least-squares method, the experimental crack length and stress intensity factor data were correlated using a microcomputer system (Seiko 9000). The calculated values of C a n d , are shown in Table 3. X-ray diffraction was used to determine the volume percentage of retained austenite in the samples. A Rigaku diffractometer with a strip chart recorder scanned the angular 20 range from 30° to 39°, the scan rate was 0.01" s -t. The amount of retained austenite was calculated by the method proposed by Cullity20 and Rundman21. The fatigue c r a c k surfaces were studied by SEM analysis for variations in the fracture mechanism. For accurate comparison, the locations
0142-1123/88/040219-08 $3.00 © 1988 Butterworth & Co (Publishers) Ltd Int J Fatigue October 1988
Table 1. Chemical composition (weight %)
Furnace Furnace
No 1 No 2
C
Si
Mn
P
S
Mg
Cu
Mo
Ni
3.16 3.27
2.34 2.40
0.094 0.096
0.043 0.040
0.008 0.013
0.041 0.035
0.54 0.52
0.256 0.235
0.54 0.53
Table 2. Specimen number and austempering treatment conditions
i< ¢
175
7O i
Specimen
a / ~ 9 . 0 + 0 . 0 5 x2
1
~ - - , ~ -
21.6-0.I
--
_-¢.--
.-.-I
43.2
L ~5.4 36 + 0 . 1 - ~ 45+_0.15
>
~ 5 ~
number
Austenitizing temperature (°C) and time (h)
Austempering temperature (°C) and time
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
900,1 9001 9001 9001 9001 9001 9001 9001 9001 9001 9001 9001 9001 9001 9001 9001 9001 9001 9001 As-cast alloy
250,4 h 300, 4 h 350, 4 h 400, 4 h 450, 4 h 300, 2 min 300, 9 min 300, 30 min 300, 1.5 h 300, 4 h 300, 10 h 300, 33 h 400, 2 min 400, 9 min 400, 30 min 400, 1.5 h 400, 4 h 400, 10 h 400, 33 h
45°
b Fig. 1 Speccmen configurations for (a) Y-block casting and (b) fatigue crack growth. All dimensions in mm
which have the same stress intensity factor were chosen for examination. The amount of spheroidization was analyzed and a nodular count was carried out using an imaging analyser (Hewlett Packard).
Results and discussion According to Dorazil et al, 22 the isothermal transformation of austenite in the bainitic region can be divided into three stages: (1)
(2)
(3)
Fig. 2 Microstructure of as-cast nodular cast iron
220
A considerable amount of martensite is formed. If the isothermal transformation time is held both the amount of bainite and carbon-enriched, untransformed austenite increase and the amount of martensite is reduced. Transformation continues mainly by the lateral growth of bainitic ferrite plates. The additional carbon produced diffuses into the austenite as additional ferrite forms. This carbon enrichment of the austenite is sufficient to stabilize fully the austenite even when cooled rapidly to room temperature. The best combination of tensile strength and ductility are reached at this stage. The austenite decomposes to form additional ferrite and plate-like carbide precipitates. A mixture of ferrite and carbide is found in areas where bainitic ferrite plates have grown into each other. It is suggested that at this stage the austenite transformation is in the form of the eutectoid-type reaction, T --~ a + c. The decrease
Int J Fatigue October 1988
T a b l e 3. M e c h a n i c a l p r o p e r t i e s and r e t a i n e d a u s t e n i t e Specimen number
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Ultimate strength (kg mm -=)
Hardness (H Rc)
122.5 125.0 93.7 81.8 79.7 53.1 74.6 121.2 119.9 125.0 127.3 124.3 65.7 76.0 75.5 75.7 81.8 74.1 76.4 56.5
42.4 43.8 32.1 29.3 25.9 49.2 44.9 43.2 36.3 43.8 45.0 44.5 41.4 25.3 24.5 27.3 29.3 27.8 27.7 19.2
C 5.18 6.15 1.40 1.86 1.29 8.39 6.70 1.39 1.82 6.15 1.36 2.09 7.14 6.32 8.02 3.77 1.86 2.22 7.03 2.94
Int J Fatigue October 1988
x x x x x x x x x x x x x x X
X X X X X
10 -1° 10 -1° 10 -1° 10 -11 10 -is 10 -1= 10 -13 10 -9 10- lo 10 -1° 10-' 10-' 10 -11 10 -11 10 -11 10 ÷11 10 -11 10 -14 10 -1" 10 -13
n
(%)
2.87 2.74 3.02 3.72 5.15 4.69 4.78 2.59 3.02 2.74 2.66 2.56 3.69 3.31 3.20 3.42 3.72 5.57 5.92 4.65
17 18 22 8 9 23 24 25 27 18 16 14 33 35 30 15 8 7 6
10-3
in the amount of austenite from approximately 35% to less than 10% during austempering can be seen in specimens 17-19 in Table 2. The result of crack growth rate measurements on specimens with different austempering temperatures is shown in Fig. 3, and their microstructures in Fig. 4. By comparing the crack growth rates in Fig. 3, it can be seen that with a 4 h isothermal transformation time, the lower hainitic structure produced at the 350"C austempering temperature (Fig. 4c) has the slowest fatigue crack growth rate. Good toughness and medium strength are obtained at the 350"C austempering temperature. Because of high tensile strength and hardness, the resistance to fatigue crack growth of the £mer lower bainitic structures obtained at austempering temperatures of 250°C and 300°C are not as good as those of the structures produced at 350°C. The upper bainitic structure produced by austempering at 400°C and 450°C (Figs 4d and e), has low tensile strength and less retained austenite beacuse of the longer transformation time and the resistance to fatigue crack growth is worse. In this case, it is believed that the transformation of upper bainite had proceeded to the third stage described above. The formation of carbide and ferrite from the austenite significantly reduces the toughness of the material. Although the as-cast 'bullseye' structure has the lowest tensile strength and the lowest hardness, its resistance to crack growth is between that of the upper and lower bainitic structures. The influence of various isothermal holding times on crack propagation rate and microstructure of upper bainitic ductile irons are shown in Figs 5 and 6. Upper bainitic transformation times from 9 min to 1.5 h have the best resistance to the crack growth. This is because bainite trans-
Retained austenite
Parameter in Paris equation
....
As cast
450°C 400oc
>. u
E 10 -q
3 x 1 0 -5
I 30
I
I 50
I
I 70
I I [ , Ill 100 140
200
L~K ( k g mm - 3 / 2 )
Fig. 3 Effect of austempering temperature on fatigue crack growth rate
221
Fig. 4 Optical micrographs of specimens 1-5 illustrating effects of austempering temperature on bainite morphology at: (a) 250°C, (b)
300°C, (c) 350°C, (d) 400°C, (e) 450°C 10 -3 900 - 400°C
33 h
B U >, l.J
E E "D
10 -4
"D
2m 3 x 1 0 -5 0.5h I
30
i
I
50
i
I 70
AK{kgmm
I
I
] , I,I 100 140
200
-3/2)
Fig. 5 Effect of austempering time at 400"C on fatigue crack growth rate
formation is just in the second stage, and enough retained austenite is present to affect the properties of the material. This conclusion agrees with Dorazil et al's results which indicated that structures with 20-40% retained austenite have the optimum mechanical properties. 22 If the transformation time is too short (k the bainite transformation is in the first stage), ~
martensite will cause the crack growth rate to increase quickly. If the transformation time is too long (the bainite transformation is in the third stage) the amount of ferrite and carbide will increase and lower the toughness and increase the fatigue crack propagation rate of the material. Plots of fatigue crack growth rate da/dN versus stress intensity factor range AK for bainitic iron austempered at 300°C are shown in Fig. 7. The austempering transformation time was varied from 2 min to 33 h. From Fig. 7 it can be seen that the specimens with transformation times from 30 min to 1.5 h have the best resistance to the crack growth. If the transformation time is shorter than 30 min, the transformed structure will become martensitic (see Figs 8a and b). This accounts for a drop in the toughness and the increase in crack growth rate experienced. If the transformation time is held over 15 h, the amount of retained austenite in the microstructure decreases slowly (Figs 8c and d). These specimens had higher tensile strengths and slower crack propagation rates than specimens austempered for nine minutes. Fig. 8 illustrates typical microstructures for specimens austempered at 300°C. The structure was much finer than those formed by austempering at 400°C. Although ferrite nucleated more rapidly at lower austempering temperatures, its growth slowed considerably and the completion of the austenite transformation required a long time. 23 This is why the amount of retained austenite was still 14% after 33 h at the 300°C austempering temperature. The slow transformation of the retained austenite into ferrite and carbide, gave the specimen a crack growth rate similar to the specimen austempered for 30 min. Comparing the properties of as-cast, lower bainitic and upper bainitic structures (from Figs 3, 5 and 7 and Table 2) it can be seen that materials containing between 20% and 40% retained austenite with the lower bainitic structure produced at 350°C have the best resistance to crack propagation. The as-cast structure has the worst resistance. The crack growth rates of the upper hainitic structure (produced at 400°C) and the lower bainitic structures (produced at 250°C and 300°C) are quite similar. Both are inferior to those of the lower hainitic structures produced at 350°C. Fractographs of the crack growth area of the as-cast
Int J Fatigue October 1 9 8 8
Fig. 6 Optical micrographs of specimens 13-19 illustrating effects of austempering time at 400°C on bainite transformation. (a) 2 min, (b) 9 rain, (c) 30 min, (d) 1.5 h, (e) 10 h, (f) 33 h 10-3
lower bainitic irons have the mixed intergranular and transgranular rupture patterns shown in Fig. 10a. This explains their reduced resistance to crack growth. After second stage transformation, the crack growth areas of the upper and lower bainitic structure have the transgranular rupture with some of the intergranular type rupture (Fig. 10b). In the tear area, the fracture mode is transgranular and brittle quasi-cleavage (Fig. 10c). If the transformation time is too long (for example, 33 h), the fracture surface of both the upper and lower bainitic irons is of the mixed transgranular and quasi-cleavage types (Fig. 10d). The surface of the tear area of the sample austempered at 350°C shows dimple rupture (Fig. 11) and provides an indication of why irons treated in this way have good toughness and resistance to crack growth. Voids tend to form at graphite modules when irons are subjected to tensile loads. Fig. 12a illustrates this and shows the rupture in this area. Some cleavage due to the concentration of stress can also be observed. Sometimes, a crack may seem to have been initiated and propagated by this stress concentration effect. However, the propagation of cleavage cracks can be retarded at other graphite modules (Fig. 12b). The crack induced from the front graphite module has stopped at the next module although secondary crack growth can be observed (Fig. 12b).
900 - 300°C
>.
E E "O
"~
10-4
9 min
oh
3x10 -5
_
/
o///
m,o
"22"/
1.5h
I 30
I
I 50
=
I 70
J
= [ J I=1 I00 140
Conclusions 200
1)
~ K ( k g m m -3/2) Fig. 7 Effect of austempering time at 300°C on fatigue rate
crack
growth
2) nodular iron show that rupture extends along the direction of the pearlite (Fig. 9a). The tear area has the river pattern as in Fig. 9b. This result coincides with Griswold's investigation.24 With short transformation times, both the upper and
Int J Fatigue October 1988
3)
Crack growth rates increase in the following order: lower bainitic nodular cast iron, upper bainitic nodular cast iron, and as-cast mainly pearlitic iron with bullseye ferrite surrounding the nodules. During isothermal transformation to upper or lower bainite, the crack growth rates initially decrease with increased holding times. However, with extended holding times, the reverse occurs. The slowest crack growth rate is obtained with an austempering temperature of 350°C with holding times in excess of 0.5 h.
223
Fig. 8 Optical micrographs of specimens 6-12 illustrating effects of austempering time at 300°C on bainite morphology. (a) 2 min, (b) 9 rain, (c) 30 min, (d) 1.5 h, (e) 10 h, (f) 33 h
References
Fig. 9 Fractograph of as-cast nodular iron: (a) crack propagation region, (b) tear rupture region
224
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2.
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Gagna, M. 'The influence of manganese and silicon on the microstructure and tensile properties of austempered ductile iron" AFSD Trans (1985) pp 801-812
Int J F a t i g u e O c t o b e r 1 9 8 8
Fig. 10 Typical fracture surface of isothermal transformation of austenite in the bainite region: (a) stage 1, (b) stage 2 (crack Propagation region), (c) stage 2 (tear region), (d) stage 3.
Fig. 12 (a) Interface of nodular voids and transgranular cleavage. (b) Secondary crack growth.
Fig. 11 Fractograph of 35o’C austempering bainitic structure
Int J Fatigue October 1988
15.
Sharma, V. K. ‘Roller contact fatigue study of austempered ductile iron’ J Heat Treat3 4 (1984) pp 326334
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Johansson, M. ‘Ductile-bainitic ductile iron’ AFS Tram (1977) pp 117-122
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Doong, J. L. Hwang, J. R. and Chen. H. S. ‘Effect of ferrite and pearlitic distribution on fracture toughness in nodular cast iron’ J Mater Sci Letf (1983) pp 737-740
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Doong, J. L., Hwang, J. R. and Chen, H. S. ‘The influence of pearlite fraction on fracture toughness and fatigue crack growth in nodular cast iron’ J Mater Sci (1986) pp 871-678
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Doong, J. L., Ju, F. C., Chen, H. S. and Chen, L. W. ‘The influence of austempering temperature on fracture toughness in bainitic nodular cast iron’ J Mater Sci Len (1986) pp 555-558
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Cullity, B. D. 'Elements of Xoray diffraction' (November 1977) pp 411-417
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Rundman, K. B. and Klug, R. C. 'An X-ray and metallographic study of an austempered ductile cast iron' AFS Trans (1982) pp 499-508
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Griswold, F. D. Jr and Stephens, R. I. 'Comparison of fatigue properties of nodular cast iron production and Y-block castings'/ntJ Fatigue9 1 (1987) pp 3-10
Authors Ji-liang Doong is an Associate Professor and Shy-lng Yu is a Research Assistant, in the Department of Mechanical Engineering, National Central University, Chung-li, Taiwan, ROC.
Int J Fatigue O c t o b e r 1988