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The effect of alloy elements on the microstructure and properties of austempered ductile irons
ScriptaMetsllmgica ef Matuiaha, Vol. 32. No. 9, pp. 1363-1367, 1995 copyright 8 1995 BlsevIer Science Ltd Ptinted in the USA. All rlgbts mserved 0956-716x195 $9.50 + .OO
0956-716X(95)00172-7
THE EFFECT OF ALLOY ELEMENTS ON THE MICROSTRUCTURE AND PROPERTIES OF AUSTEMPERED DUCTILE IRONS B.Y. Lia
E.T. Chen
T.S. Lei
Department of Mechanical Engineering National Taiwan Institute of Technology Taipei, Taiwan 106, R.O.C.
(Received August 23, 1994) (Revised November 18, 1994)
Ductile cast iron has already demonstrated excellent mechanical properties. If given proper austempering, it can exhibit even more outstandiug characteristics. The process of austempering for ductile cast iron is similar to steel, and requires an adequate period of time to be maintained, first at the austenitizing temperature allowing the matrix of ductile iron to be austenitized completely, and then rapidly quenching the austenitized ductile iron down to 300 “C - 400 “C Caution is required to prevent austenite from transforming into proeutectoid ferrite or pearhte. Finally, tlte ductile iron must be kept in an isothermal condition for a proper length of time. During the process of isothermal holding, the following two stages of the bait&e reaction will occur. In the first stage, the Iremaining austeuite quenched from high temperature will decompose into ferrite and carbon rich austenite. In the second stage, the high carbon content austenite will eventually decompose into ferrite and carbide. After completing the first stage, the microstructure of the matrix in austempered ductile iron (ADI) contains acicular ferrite and carbon rich austenite, giving ADI the outstanding mechanical properties of high strength and good ductility. If the isothermal holding time is long enough to pernit the reaction to reach the second stage, the carbon rich austenite will decompose into ferrite and carbide. Since the carbon rich austenite is eliminated continuously and the carbide particles form nearly continuous films on interfaces, this provides relatively convenient crack paths during the second stage reaction, and ADI dexases in ductility. Therefore, the isothermal holding time should be contrdled at the completion of the first stage and the reaction of the second stage should be avoided(l,2). Many kinds of experimental techniques such as quantitative metallography, magnetic change, dilatometry, X-ray diffraction, electrical resistivity change etc., may be used to measure the phase transformation during the austempering af ductile irons. However, the method of the change of electrical resistivity, not only measuring provides continuous and complete data , but also the time to start and to tirdsh for both stages of the reaction can be significantly determined (3,4). Figure 1 illustrates the variation of resistance and austenite volume fraction with austempering time for ductile iron austempcrcd at 400°C (4). It can be seen that the first stage of decreasing resistance stabilizes at approximately 700 , and should represent the time of the reaction when the first stage is completed. After 10,000 , the curve once again exhibits a significant drop, indicating the start of the second stage. The time region between the end of the first and the beginning of the second stage reaction is the stabilized maximal value in relation to the curve of austenite volume fraction, meaning that while the austempering is controlled within the processing window, ADI will achieve the optimal mechanical properties(5,6,7).
I ,,,1,,,, I ,,,,,,,
3.0
1.o B 2
4w’C Auatempering
1
10
lo2
10'
10'
lo5
TIME, sec.
Fig. 1. The variations of electrical resistance and austenite volume fraction with austempming hokiing time for ductile iron(4).
In this paper, the effect of alloy elements on the microstructure and property of ADI was investigated. First, the specimens containing Mn, Cu, Ni and MO were made separately, then :s PC-controlled vacuum heat treating system
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Vol. 32, No. 9
DUCTILE IRONS
1364
was used to detect the electrical resistivity change during austempering, and these results were used to select the optimum isothermal holding time ( i.e. the time at the interval of the processing window ) for the austempering treatment of impact specimens. Finally, impact and hardness tests and microscopy examination were performed to compare the difference subject to alloy elements.
In this experiment, four different compositions of ductile irons containing 0.4% Mn, 0.4%Mo, l%Cu and 15%Ni individually were melted. The chemical analysis of each heat is presented in table 1. For reasons of accurate control of the composition in each heat, ingots with determined compositions were pre-cast in a high frequency induction melting furnace using charge materials comprised of pig iron, steel scrap, ferrosiliam, and carbon. Then a 20kg pre-cast ingot was placed again in the furnace, wherein, ferromanganese, or electrolytic nickel or pure copper or ferromolybdenum was added, and heated to about 1530°C ,and the melted iron was treated using the sandwich technique and poured into a CO: mold at approximate 143O’C. Six casting bars 15mm in diameter and 150mm in length were cast from each heat. With the PC- controlled vacuum heat treating system, as the schematic diagram illustrates in Fig. 2, programmed software was used to control the DC current directly passing through the specimen, 3mm in diameter machined from the casting bar of each alloyed iron, to perform the process of austempering and to measure the changes of electrical resistivity. Specimens were first heated to and held at 900 “C for one hour to be austenitized, then quenched with the spray of liquid nitrogen tn 400 % and controlled at a desired isothermal level. The hardware and s&ware of this system has been illustrated in detail in a former report(4). Unnotched Charpy impact specimens with dimensions of lOxlOx15mm were made from the @ 15mm casting bars. The conditions of as-cast and 4OO’C austempering for the specimens were selected and impact tests were performed in a Tinius Olsen Charpy Impact Tester at room temperature. Austempering for impact specimens was carried out in salt baths. The heat treatment condition involved austenitization at 9OO’C for 1 hour, amtempering at 400°C for 50 minutes. The austempering holding time was determined from the processing windows of electrical resistivity records of each alloyed iron. After impact testing, Rockwell hardness readings were taken on each side of the impact specimens and the values averaged. Specimens for metallographic examination were taken from impact specimens and examined by an optical and scanning electron microscope.
TABLE 1 Chemical Composition of Heats
TABLE 2 Impact Strength and Rockwell Hardness of Austempered and As-Cast Ductile Irons. S~IIlple
No.
Impact Energy (kg-M)’
Hardness (HRc)”
Austempering As-Cast Austempring
As-Cast
14.4
4.8
32.2
28.4
15.5
4.5
32.3
31.6
Mn alloyed
157
51
315
21.7
Mo alloved
12
53
33.3
22 9
NI alloyed Cu alloyed
Fig.2.
Block diagram illustrating computer controlled vacuum system.
the personal heat treating
*Average of 3 readings. **Average of 6 readings.
Figure 3 illustrates the variations of electrical resistivity for Ni , Cu , Mn and MO alloyed irons relative to an austempeting holding time at 4OO’C. It can be seen that the time region of the processing windows for each alloyed iron is approximately from
Vol. 32, No. 9
DUCllLE IRONS
1000 seconds to 4000 seconds. Depending on the processing v.indow of each curve in Fig. 3, the isothermal holding time of 50 minutes was chosen for the 4OO’C austempering of impact specimens in this experiment.
1365
135
125
Touahness and I-[arenas Table 2 lists the unnotched Charpy impact strength and Rockwell hardness of the impact specimens of each alloyed iron in the as-cast condition and austempered at 4OO’C for 50 minutes. Figures 4 and 5 illustrate the variation of Rockwell hardness and impact strength in the aforesaid conditions relating with Ni, Cu, Mn and MO alloyed irons. It is seen that the hardness values of each austempered alloyed have no significant diffemnce. and are a little higher than that of each as-cast specimen. As for the impact strength, it is found that the values of all as-cast alloyed irons are approximately 5 kg-m: however, the values of austempered alloyed irons are nearly three times greater than that of the as-cast. The values of impact strength of Ni, Cu and Mn alloyed irons are around 15 kg-m, however the value of MO alloyed is only 12 kg-m, and is lower than that of otheralloyed irons,
$ & 115 i &lo5 5 3 .z B I%
95
a5 i
75b 1
Fig.3
?
I Ni ulloyed
Fig.4
I
CU alloyed
I Tli”,e.,
22
01
10
1
d 10’
1
0
5
&.
Electrical resistivity records of each alloyed irons during 400% austempering.
Oeeeo -AS
Au&m wing - LTost
I
MIl alloyed
Ni alloyed
MO alloyed
The effect of Ni, Cu, Mn and MO alloyed irons on Rockwell hardness. In as-cast condition and 4OO’C austempering for 50 minutes.
Fig.5.
CU alloyed
Mn alloyed
all&d
The effect of Ni, Cu, Mn and MO alloyed irons on impact strength. In as-cast condition and 400°C austempering for 50 minutes.
Microstntcture The microstnrctures of ah alloyed irons austempered at 4OO’C for 50 minutes reveal a typical upper bainite structure with acicular bainite ferrite and carbon rich austenite(7), as the photographs of Cu alloyed shown in Fig. 6. In most aspects the structure of Ni. Cu and Mn alloyed irons is relatively homogenous throughout the specimens, as shown in Figs. 7 (a) - 7 (c). As for the structure of MO alloyed it is clearly observed that light etching areas stand out in the intercellular regions ilhtstrating the existence of untransformed austenite as shown in Fig. 7(d). Figure 8(a), reveals a secondary electron image of a detailed microstructure of a light etching area of MO alloyed iron, and shows the presence of high levels of austenite associated with microshrinkage porosity and no martensite This area was further examined by imaging from Backscattered Electrons and EDS in SEM. Figure 8(b) shows the backscattered electrons image of the structure in Fig. 8(a). Figure 8(c) illustrates the EDS analysis of the white region in Fig. 8(b) ( indicated with arrow). From Figure 8, one sees that the segregation of MO is obviously apparent in the intercellular region. Comparing Figs. 7, 8 and 5, the Ni, Cu, and Mn alloyed irons show a homogenous matrix, so the impact values of these three alloyed irons have no significant difference. MO, with an extreme tendency of segregating to the last solidification area, retards the baiuite reaction and causes the microshrinkage porosity in the intercellular region. Consequently, the MO alloyed iron is found to have the lowest impact strength among all alloyed irons.
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DUCTILE?IRONS
Fig.6.
(a)
(c)
SEM photograph of Ni alloyed iron, a typical microstructure of ADI, austenitized at 9OO’C and austempered at 4OO’C for 50 minutes.
Ni alloyed
Mn alloyed
Fig.7.
Vol. 32, No. 9
(b)
Cu alloyed
(d)
MO alloyed
Microstructure of each alloyed ductile iron, austenitized at 9OO’C and austempered at 4OO’C for 50 minutes.
DUCTILE IRONS
Vol. 32, No. 9
(:a) secondary
electron image
The influence of alloy elements on the microstructure and mechanical properties of austempered ductile irons was investigated aad can be summarized as follows. 1. Given proper austempering treating, the Ni, Cu and Mn alloyed irons show a homogenous microstructure. However, the MO alloyed iron is found to have MO elements segregating to the intercellular regions, which retards the bainite reaction and causes the austenite to he stabilized, and is also associated with the microshrinkage porosity in this region. 2 .The hardness values of each of the austempered alloyed irons have no significant difference, and are a little higher than that of as-cast specimens. The impact strength of austempered specimens is nearly three times greater than that of as-cast. The values of the impact strength of Ni, Cu and Mn alloyed irons are around 15 kg-m, however the value of MO alloyed is only 12 kg-m.
The authors are grateful for the partial financial support of this work by the National Science Council of the R.O.C. under grant No. NSC SO-0405-EO11-08 and No. NSC 82-0405-E011-027.
1. M. Jonansson, AFS Trans., vol. 85, p. 117 (1977). 2. D. J. Moore, T. N. Rouns and K. B. Rundman, AFS Trans., vo1.93, p.705 (1985). 3. Y. J. Park, R. B. Gundlach and J. F. Janowark, AFS Trans., vol.95, p.411 (1987). 4. B. Y. Lin, E. T. Chen and T. S. Lei, Research Report for R.O.C., NSC the National Science Council of 80-0405-EOl l-08. 5. D. J. Moore, T. N. Rouns and K. B. Rundman, AFS Trans., ~01.94, p.255 (1986). 6. N. Darwish and R. Elliott, Materials Science and Technology, vo1.9, p.572 (1993). 7. R.C. Voig and C.R. Loper, JR., J. Heat Treating, Vol. 3, No. 4, p. 291 (1984).
(c) the EDS analysis of the white place ( indicated with arrow ) in fig.8 (b). Fig.8. Detailed microstructure of light etching area of Mo alloyed iron.
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0956-716X(95)00172-7
THE EFFECT OF ALLOY ELEMENTS ON THE MICROSTRUCTURE AND PROPERTIES OF AUSTEMPERED DUCTILE IRONS B.Y. Lia
E.T. Chen
T.S. Lei
Department of Mechanical Engineering National Taiwan Institute of Technology Taipei, Taiwan 106, R.O.C.
(Received August 23, 1994) (Revised November 18, 1994)
Ductile cast iron has already demonstrated excellent mechanical properties. If given proper austempering, it can exhibit even more outstandiug characteristics. The process of austempering for ductile cast iron is similar to steel, and requires an adequate period of time to be maintained, first at the austenitizing temperature allowing the matrix of ductile iron to be austenitized completely, and then rapidly quenching the austenitized ductile iron down to 300 “C - 400 “C Caution is required to prevent austenite from transforming into proeutectoid ferrite or pearhte. Finally, tlte ductile iron must be kept in an isothermal condition for a proper length of time. During the process of isothermal holding, the following two stages of the bait&e reaction will occur. In the first stage, the Iremaining austeuite quenched from high temperature will decompose into ferrite and carbon rich austenite. In the second stage, the high carbon content austenite will eventually decompose into ferrite and carbide. After completing the first stage, the microstructure of the matrix in austempered ductile iron (ADI) contains acicular ferrite and carbon rich austenite, giving ADI the outstanding mechanical properties of high strength and good ductility. If the isothermal holding time is long enough to pernit the reaction to reach the second stage, the carbon rich austenite will decompose into ferrite and carbide. Since the carbon rich austenite is eliminated continuously and the carbide particles form nearly continuous films on interfaces, this provides relatively convenient crack paths during the second stage reaction, and ADI dexases in ductility. Therefore, the isothermal holding time should be contrdled at the completion of the first stage and the reaction of the second stage should be avoided(l,2). Many kinds of experimental techniques such as quantitative metallography, magnetic change, dilatometry, X-ray diffraction, electrical resistivity change etc., may be used to measure the phase transformation during the austempering af ductile irons. However, the method of the change of electrical resistivity, not only measuring provides continuous and complete data , but also the time to start and to tirdsh for both stages of the reaction can be significantly determined (3,4). Figure 1 illustrates the variation of resistance and austenite volume fraction with austempering time for ductile iron austempcrcd at 400°C (4). It can be seen that the first stage of decreasing resistance stabilizes at approximately 700 , and should represent the time of the reaction when the first stage is completed. After 10,000 , the curve once again exhibits a significant drop, indicating the start of the second stage. The time region between the end of the first and the beginning of the second stage reaction is the stabilized maximal value in relation to the curve of austenite volume fraction, meaning that while the austempering is controlled within the processing window, ADI will achieve the optimal mechanical properties(5,6,7).
I ,,,1,,,, I ,,,,,,,
3.0
1.o B 2
4w’C Auatempering
1
10
lo2
10'
10'
lo5
TIME, sec.
Fig. 1. The variations of electrical resistance and austenite volume fraction with austempming hokiing time for ductile iron(4).
In this paper, the effect of alloy elements on the microstructure and property of ADI was investigated. First, the specimens containing Mn, Cu, Ni and MO were made separately, then :s PC-controlled vacuum heat treating system
1363
Vol. 32, No. 9
DUCTILE IRONS
1364
was used to detect the electrical resistivity change during austempering, and these results were used to select the optimum isothermal holding time ( i.e. the time at the interval of the processing window ) for the austempering treatment of impact specimens. Finally, impact and hardness tests and microscopy examination were performed to compare the difference subject to alloy elements.
In this experiment, four different compositions of ductile irons containing 0.4% Mn, 0.4%Mo, l%Cu and 15%Ni individually were melted. The chemical analysis of each heat is presented in table 1. For reasons of accurate control of the composition in each heat, ingots with determined compositions were pre-cast in a high frequency induction melting furnace using charge materials comprised of pig iron, steel scrap, ferrosiliam, and carbon. Then a 20kg pre-cast ingot was placed again in the furnace, wherein, ferromanganese, or electrolytic nickel or pure copper or ferromolybdenum was added, and heated to about 1530°C ,and the melted iron was treated using the sandwich technique and poured into a CO: mold at approximate 143O’C. Six casting bars 15mm in diameter and 150mm in length were cast from each heat. With the PC- controlled vacuum heat treating system, as the schematic diagram illustrates in Fig. 2, programmed software was used to control the DC current directly passing through the specimen, 3mm in diameter machined from the casting bar of each alloyed iron, to perform the process of austempering and to measure the changes of electrical resistivity. Specimens were first heated to and held at 900 “C for one hour to be austenitized, then quenched with the spray of liquid nitrogen tn 400 % and controlled at a desired isothermal level. The hardware and s&ware of this system has been illustrated in detail in a former report(4). Unnotched Charpy impact specimens with dimensions of lOxlOx15mm were made from the @ 15mm casting bars. The conditions of as-cast and 4OO’C austempering for the specimens were selected and impact tests were performed in a Tinius Olsen Charpy Impact Tester at room temperature. Austempering for impact specimens was carried out in salt baths. The heat treatment condition involved austenitization at 9OO’C for 1 hour, amtempering at 400°C for 50 minutes. The austempering holding time was determined from the processing windows of electrical resistivity records of each alloyed iron. After impact testing, Rockwell hardness readings were taken on each side of the impact specimens and the values averaged. Specimens for metallographic examination were taken from impact specimens and examined by an optical and scanning electron microscope.
TABLE 1 Chemical Composition of Heats
TABLE 2 Impact Strength and Rockwell Hardness of Austempered and As-Cast Ductile Irons. S~IIlple
No.
Impact Energy (kg-M)’
Hardness (HRc)”
Austempering As-Cast Austempring
As-Cast
14.4
4.8
32.2
28.4
15.5
4.5
32.3
31.6
Mn alloyed
157
51
315
21.7
Mo alloved
12
53
33.3
22 9
NI alloyed Cu alloyed
Fig.2.
Block diagram illustrating computer controlled vacuum system.
the personal heat treating
*Average of 3 readings. **Average of 6 readings.
Figure 3 illustrates the variations of electrical resistivity for Ni , Cu , Mn and MO alloyed irons relative to an austempeting holding time at 4OO’C. It can be seen that the time region of the processing windows for each alloyed iron is approximately from
Vol. 32, No. 9
DUCllLE IRONS
1000 seconds to 4000 seconds. Depending on the processing v.indow of each curve in Fig. 3, the isothermal holding time of 50 minutes was chosen for the 4OO’C austempering of impact specimens in this experiment.
1365
135
125
Touahness and I-[arenas Table 2 lists the unnotched Charpy impact strength and Rockwell hardness of the impact specimens of each alloyed iron in the as-cast condition and austempered at 4OO’C for 50 minutes. Figures 4 and 5 illustrate the variation of Rockwell hardness and impact strength in the aforesaid conditions relating with Ni, Cu, Mn and MO alloyed irons. It is seen that the hardness values of each austempered alloyed have no significant diffemnce. and are a little higher than that of each as-cast specimen. As for the impact strength, it is found that the values of all as-cast alloyed irons are approximately 5 kg-m: however, the values of austempered alloyed irons are nearly three times greater than that of the as-cast. The values of impact strength of Ni, Cu and Mn alloyed irons are around 15 kg-m, however the value of MO alloyed is only 12 kg-m, and is lower than that of otheralloyed irons,
$ & 115 i &lo5 5 3 .z B I%
95
a5 i
75b 1
Fig.3
?
I Ni ulloyed
Fig.4
I
CU alloyed
I Tli”,e.,
22
01
10
1
d 10’
1
0
5
&.
Electrical resistivity records of each alloyed irons during 400% austempering.
Oeeeo -AS
Au&m wing - LTost
I
MIl alloyed
Ni alloyed
MO alloyed
The effect of Ni, Cu, Mn and MO alloyed irons on Rockwell hardness. In as-cast condition and 4OO’C austempering for 50 minutes.
Fig.5.
CU alloyed
Mn alloyed
all&d
The effect of Ni, Cu, Mn and MO alloyed irons on impact strength. In as-cast condition and 400°C austempering for 50 minutes.
Microstntcture The microstnrctures of ah alloyed irons austempered at 4OO’C for 50 minutes reveal a typical upper bainite structure with acicular bainite ferrite and carbon rich austenite(7), as the photographs of Cu alloyed shown in Fig. 6. In most aspects the structure of Ni. Cu and Mn alloyed irons is relatively homogenous throughout the specimens, as shown in Figs. 7 (a) - 7 (c). As for the structure of MO alloyed it is clearly observed that light etching areas stand out in the intercellular regions ilhtstrating the existence of untransformed austenite as shown in Fig. 7(d). Figure 8(a), reveals a secondary electron image of a detailed microstructure of a light etching area of MO alloyed iron, and shows the presence of high levels of austenite associated with microshrinkage porosity and no martensite This area was further examined by imaging from Backscattered Electrons and EDS in SEM. Figure 8(b) shows the backscattered electrons image of the structure in Fig. 8(a). Figure 8(c) illustrates the EDS analysis of the white region in Fig. 8(b) ( indicated with arrow). From Figure 8, one sees that the segregation of MO is obviously apparent in the intercellular region. Comparing Figs. 7, 8 and 5, the Ni, Cu, and Mn alloyed irons show a homogenous matrix, so the impact values of these three alloyed irons have no significant difference. MO, with an extreme tendency of segregating to the last solidification area, retards the baiuite reaction and causes the microshrinkage porosity in the intercellular region. Consequently, the MO alloyed iron is found to have the lowest impact strength among all alloyed irons.
1366
DUCTILE?IRONS
Fig.6.
(a)
(c)
SEM photograph of Ni alloyed iron, a typical microstructure of ADI, austenitized at 9OO’C and austempered at 4OO’C for 50 minutes.
Ni alloyed
Mn alloyed
Fig.7.
Vol. 32, No. 9
(b)
Cu alloyed
(d)
MO alloyed
Microstructure of each alloyed ductile iron, austenitized at 9OO’C and austempered at 4OO’C for 50 minutes.
DUCTILE IRONS
Vol. 32, No. 9
(:a) secondary
electron image
The influence of alloy elements on the microstructure and mechanical properties of austempered ductile irons was investigated aad can be summarized as follows. 1. Given proper austempering treating, the Ni, Cu and Mn alloyed irons show a homogenous microstructure. However, the MO alloyed iron is found to have MO elements segregating to the intercellular regions, which retards the bainite reaction and causes the austenite to he stabilized, and is also associated with the microshrinkage porosity in this region. 2 .The hardness values of each of the austempered alloyed irons have no significant difference, and are a little higher than that of as-cast specimens. The impact strength of austempered specimens is nearly three times greater than that of as-cast. The values of the impact strength of Ni, Cu and Mn alloyed irons are around 15 kg-m, however the value of MO alloyed is only 12 kg-m.
The authors are grateful for the partial financial support of this work by the National Science Council of the R.O.C. under grant No. NSC SO-0405-EO11-08 and No. NSC 82-0405-E011-027.
1. M. Jonansson, AFS Trans., vol. 85, p. 117 (1977). 2. D. J. Moore, T. N. Rouns and K. B. Rundman, AFS Trans., vo1.93, p.705 (1985). 3. Y. J. Park, R. B. Gundlach and J. F. Janowark, AFS Trans., vol.95, p.411 (1987). 4. B. Y. Lin, E. T. Chen and T. S. Lei, Research Report for R.O.C., NSC the National Science Council of 80-0405-EOl l-08. 5. D. J. Moore, T. N. Rouns and K. B. Rundman, AFS Trans., ~01.94, p.255 (1986). 6. N. Darwish and R. Elliott, Materials Science and Technology, vo1.9, p.572 (1993). 7. R.C. Voig and C.R. Loper, JR., J. Heat Treating, Vol. 3, No. 4, p. 291 (1984).
(c) the EDS analysis of the white place ( indicated with arrow ) in fig.8 (b). Fig.8. Detailed microstructure of light etching area of Mo alloyed iron.
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