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Effect of copper content on the microstructure and mechanical properties of a modified nodular iron
Materials Characterization 51 (2003) 219 – 224
Effect of copper content on the microstructure and mechanical properties of a modified nodular iron M.A. Neri *, C. Carren˜o Advanced Materials Research Center (C.I.M.A.V.), Miguel de Cervantes No 120, Complejo Industrial Chihuahua, C.P. 31109, Chihuahua, Chihuahua, Mexico Received 3 April 2003; accepted 7 September 2003
Abstract During the drilling operation of two crankshafts, both made of modified nodular iron, the drilling tool failed in one of them and a failure analysis was conducted on the two crankshafts. Results indicate that in the crankshaft where the drilling tool failed had a finer pearlitic microstructure with a higher hardness compared with the other. This difference in the microstructure and hardness was due to the higher content of carbon and copper in the matrix of the first crankshaft analyzed. D 2003 Published by Elsevier Inc. Keywords: Failure analysis; Pearlitic nodular iron; Microstructure; Fine pearlite; Machinability
1. Introduction Ductile iron is a cast iron with spheroidal graphite. Controlled processing of the molten iron precipitates graphite as spheroids rather than flakes. The round shape of the graphite eliminates the material’s tendency to crack and helps prevent cracks from spreading. Graphite flakes in gray cast irons act as stress risers that initiate and propagate cracks, making the material weaker. Nodular iron is a member of the family of cast irons that includes gray, malleable, white, and compacted graphite irons. The chemical and metallurgical qualities of ductile iron make it the strongest and
* Corresponding author. Tel.: +52-614-439-1102; fax: +52-614439-1112. E-mail address: [email protected] (M.A. Neri). 1044-5803/$ - see front matter D 2003 Published by Elsevier Inc. doi:10.1016/j.matchar.2003.09.001
toughest cast iron, and it also has the highest endurance limit. A review of steel and iron production makes it easy to understand the benefits of ductile iron. Pearlitic ductile iron has graphite spheroids in a matrix of pearlite. Pearlite is a fine lamellar aggregate of ferrite and cementite (Fe3C). The alloy is relatively hard with moderate ductility and high strength. Though easy to machine, castings made from pearlitic ductile iron resist wear and impact, have reduced thermal conductivity, and have low magnetic permeability and high hysteresis loss [1]. In order to treat large sections successfully, the chemical composition is modified with element additions that increase the hardenability of the alloy. Among the elements more widely used are nickel, copper, and molybdenum. It is also essential to control the austenitization stage because it influences the isothermal treatment and therefore the final properties.
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Fig. 1. Whole crankshaft without machined condition; the clear zone shows the site where the drilling tool broke (M-1).
Fig. 2. Whole crankshaft in the machined condition showing the site where the samples were obtained.
M.A. Neri, C. Carren˜o / Materials Characterization 51 (2003) 219–224 Table 1 Sample identification Sample No.
Description
M-1
Bad sample 1, without machined condition, in the zone where the cutting tool fractured. Bad sample 2, without machined condition, on the opposite side of the crankshaft where the cutting tool did not fail. Good sample 1 in the machined condition in the same position of the sample M-1, but the cutting tool did not fail. Good sample 1 in the machined condition on the opposite side of the crankshaft, but the cutting tool did not fail.
M-2
B-1
B-2
The common alloying elements used to control ferrite and pearlite contents in as-cast grades of ductile iron are Si, Mn, and Cu. Mn and Cu are used to promote pearlite; Si is used to promote ferrite and to strengthen ferrite. Si is generally held below 2.5% when producing the pearlitic grades and between 2.5% and 2.8% when producing ferritic grades. Mn is generally held between 0.4% and 0.6% when making pearlitic grades and below 0.3% when making ferritic grades. Cu is never added when making ferritic grades and is generally held between 0.4% and 0.8% when making pearlitic grades. Regarding hardness and machinability, a higher hardness usually means poorer machinability. Likewise, greater amounts of pearlite and harder pearlite also cause more difficulty in machining. Annealing usually produces partial decomposition of the pearlite structure and improves machinability [2].
221
Machinability is determined by microstructure and hardness. The graphite particles in gray, malleable, and ductile irons are responsible for the free-machining characteristics of these materials and their superior machinability when compared to steels. Within the cast irons, graphite morphology plays an important role in machinability, with the graphite flakes found in gray iron providing superior machining characteristics. While the graphite particles influence cutting force and surface finish, the matrix is the primary determinant of tool life [3]. The aim of this work is to present the results of a failure analysis conducted on two crankshafts made of pearlitic nodular iron, one of them broke the drilling tool during the drilling process, while the other one did not damage the drilling tool during the same operation.
2. Experimental procedure The two crankshafts were inspected visually and were sectioned in the zones indicated in Figs. 1 and 2, obtaining two samples from each crankshaft. One sample was obtained from the zone were the cutting tool fractured (M-1), and the other sample was obtained from the opposite side of the same crankshaft (M-2). For the other crankshaft, the samples were obtained from the same positions (B-1 and B-2, see Figs. 1 and 2). They were identified according to the Table 1. Chemical analysis of the crankshafts was conducted for the four samples, two taken from the zone where the drilling tool failed (M-1 and B-1) and two taken from the other extreme of the crankshafts where the drilling tool did not fail (M-2 and B-2). A metallographic analysis was carried out on the four samples
Table 2 Chemical composition (wt.%) of the samples Sample
C%
Si%
Mn%
P%
S%
Cu%
Cr%
Al%
Range M-1 M-2 B-1 B-2
3.2 – 4.10 3.7733 2.4869 3.7728 2.0670
1.80 – 3.00 2.38 2.20 2.87 2.93
0.10 – 1.00 0.4426 0.3664 0.6642 0.5977
0.015 – 0.10 N.D. N.D. N.D. N.D.
0.005 – 0.035 N.D. N.D. N.D. N.D.
0.80 – 1.2 1.13 1.13 0.74 0.76
0.06 – 0.13 0.098 0.097 0.099 0.101
0.001 – 0.018 0.01 0.0095 0.0039 0.004
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Fig. 3. Microstructure of sample M-1, a fine pearlitic matrix with some graphite nodules surrounded by ferrite.
in order to compare the microstructures of the zones where the drilling tool failed and the zones where the drilling tool did not fail. Microhardness measurements
were made with the Vickers hardness scale for the four samples in order to detect some difference between the zones that failed and did not fail.
Fig. 4. Microstructure of sample M-2, a pearlitic matrix with some graphite nodules surrounded by ferrite.
M.A. Neri, C. Carren˜o / Materials Characterization 51 (2003) 219–224
Fig. 5. Microstructure of sample B-1, a coarse and fine pearlitic matrix with graphite nodules without ferrite surrounded them.
Fig. 6. Microstructure of sample B-2, a coarse and fine pearlitic matrix with graphite nodules without ferrite surrounded them.
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3. Results and discussion The four samples were chemically analyzed in order to determine the content of carbon, silicon, manganese, phosphorus, sulfur, copper, chromium, and aluminum, and the results were shown in Table 2 with the range for a nodular iron of the type B3 modified composition. From this table, we can observe that the carbon content in the sample M-1 that broke the drilling tool was greater (3.77% C) than that of sample M-2 that did not break the drilling tool (2.48% C). The copper content in both samples M-1 and M-2 is 1.13%, which is greater than the copper content in the samples B-1 and B-2, 0.74 and 0.76%, respectively. The microstructure observed in samples M-1 and M-2 consisted of a fine pearlitic matrix with graphite nodules and some nodules surrounded by ferrite (see Figs. 3 and 4). The microstructure observed in samples B-1 and B-2 consisted of a coarse pearlitic matrix with graphite nodules and some nodules surrounded by ferrite (see Figs. 5 and 6). Measurements of Vickers microhardness were taken from the samples analyzed in different zones to detect some possible variations among them. Table 3 shows the results obtained from the different samples M-1, B-1, M-2, and B-2. The highest value of microhardness was measured in sample M-1 in the fine pearlite zone (314 –304 HV) and in sample M-2 in the fine pearlite zone (293 –294 HV). In samples B-1 and B-2, the microhardness values obtained were 267 and 269 HV, respectively. Sample M-1 that broke the drilling tool had the highest carbon and copper content, promoting a higher hardness in the pearlitic matrix. The microstructure exhibited a finer pearlitic matrix [4]. Regarding hardness and machinability, higher hardness usually cause poorer machinability. Greater amounts of pearlite and harder pearlite also cause more difficulty in machining.
Table 3 Vickers microhardness measured in the matrix of the different samples of the crankshafts Sample No.
Measured zone
Microhardness Vickers (Rockwell B)
M-1 M-1 B-1 B-1 M-2 M-2 B-2 B-2
Fine pearlite Fine pearlite Coarse pearlite Coarse pearlite Fine pearlite Fine pearlite Coarse pearlite Coarse pearlite
314.3 304.7 267.8 267.2 294.1 293.4 269.1 269.8
(106.5) (105.6) (101.8) (101.7) (104.5) (104.4) (102) (102.1)
The differences in the chemical composition and microstructure promote a higher hardness in sample M-1, lowering the machinability of the crankshaft and causing the drilling tool to break.
4. Conclusions Sample M-1, where the drilling tool broke during the machining operation, had the highest carbon and copper contents and the highest value of Vickers hardness. In addition, this sample had the finest pearlitic microstructure compared to the other three locations. It had the highest carbon and copper contents, which promote the fine pearlite. The higher hardness of the fine pearlite caused poorer machinability in the crankshaft. References [1] Ductile Iron Marketing Group. Getting more out of cast parts using ductile iron. Mach Des 1998 (August 6);96 – 7. [2] Gundlach R. Ductile Iron Society. Private communication, 2002. [3] Ductile Iron Society. Ductile Iron Data for Design Engineers, Section VI Machinability. Montreal, Quebec, Canada: Rio Tinto Iron & Titanium Inc., 2002. [4] Porter DA, Easterling KE. Phase transformations in metals and alloys. UK: Van Nostrand Reinhold; 1984. p. 332.
Effect of copper content on the microstructure and mechanical properties of a modified nodular iron M.A. Neri *, C. Carren˜o Advanced Materials Research Center (C.I.M.A.V.), Miguel de Cervantes No 120, Complejo Industrial Chihuahua, C.P. 31109, Chihuahua, Chihuahua, Mexico Received 3 April 2003; accepted 7 September 2003
Abstract During the drilling operation of two crankshafts, both made of modified nodular iron, the drilling tool failed in one of them and a failure analysis was conducted on the two crankshafts. Results indicate that in the crankshaft where the drilling tool failed had a finer pearlitic microstructure with a higher hardness compared with the other. This difference in the microstructure and hardness was due to the higher content of carbon and copper in the matrix of the first crankshaft analyzed. D 2003 Published by Elsevier Inc. Keywords: Failure analysis; Pearlitic nodular iron; Microstructure; Fine pearlite; Machinability
1. Introduction Ductile iron is a cast iron with spheroidal graphite. Controlled processing of the molten iron precipitates graphite as spheroids rather than flakes. The round shape of the graphite eliminates the material’s tendency to crack and helps prevent cracks from spreading. Graphite flakes in gray cast irons act as stress risers that initiate and propagate cracks, making the material weaker. Nodular iron is a member of the family of cast irons that includes gray, malleable, white, and compacted graphite irons. The chemical and metallurgical qualities of ductile iron make it the strongest and
* Corresponding author. Tel.: +52-614-439-1102; fax: +52-614439-1112. E-mail address: [email protected] (M.A. Neri). 1044-5803/$ - see front matter D 2003 Published by Elsevier Inc. doi:10.1016/j.matchar.2003.09.001
toughest cast iron, and it also has the highest endurance limit. A review of steel and iron production makes it easy to understand the benefits of ductile iron. Pearlitic ductile iron has graphite spheroids in a matrix of pearlite. Pearlite is a fine lamellar aggregate of ferrite and cementite (Fe3C). The alloy is relatively hard with moderate ductility and high strength. Though easy to machine, castings made from pearlitic ductile iron resist wear and impact, have reduced thermal conductivity, and have low magnetic permeability and high hysteresis loss [1]. In order to treat large sections successfully, the chemical composition is modified with element additions that increase the hardenability of the alloy. Among the elements more widely used are nickel, copper, and molybdenum. It is also essential to control the austenitization stage because it influences the isothermal treatment and therefore the final properties.
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M.A. Neri, C. Carren˜o / Materials Characterization 51 (2003) 219–224
Fig. 1. Whole crankshaft without machined condition; the clear zone shows the site where the drilling tool broke (M-1).
Fig. 2. Whole crankshaft in the machined condition showing the site where the samples were obtained.
M.A. Neri, C. Carren˜o / Materials Characterization 51 (2003) 219–224 Table 1 Sample identification Sample No.
Description
M-1
Bad sample 1, without machined condition, in the zone where the cutting tool fractured. Bad sample 2, without machined condition, on the opposite side of the crankshaft where the cutting tool did not fail. Good sample 1 in the machined condition in the same position of the sample M-1, but the cutting tool did not fail. Good sample 1 in the machined condition on the opposite side of the crankshaft, but the cutting tool did not fail.
M-2
B-1
B-2
The common alloying elements used to control ferrite and pearlite contents in as-cast grades of ductile iron are Si, Mn, and Cu. Mn and Cu are used to promote pearlite; Si is used to promote ferrite and to strengthen ferrite. Si is generally held below 2.5% when producing the pearlitic grades and between 2.5% and 2.8% when producing ferritic grades. Mn is generally held between 0.4% and 0.6% when making pearlitic grades and below 0.3% when making ferritic grades. Cu is never added when making ferritic grades and is generally held between 0.4% and 0.8% when making pearlitic grades. Regarding hardness and machinability, a higher hardness usually means poorer machinability. Likewise, greater amounts of pearlite and harder pearlite also cause more difficulty in machining. Annealing usually produces partial decomposition of the pearlite structure and improves machinability [2].
221
Machinability is determined by microstructure and hardness. The graphite particles in gray, malleable, and ductile irons are responsible for the free-machining characteristics of these materials and their superior machinability when compared to steels. Within the cast irons, graphite morphology plays an important role in machinability, with the graphite flakes found in gray iron providing superior machining characteristics. While the graphite particles influence cutting force and surface finish, the matrix is the primary determinant of tool life [3]. The aim of this work is to present the results of a failure analysis conducted on two crankshafts made of pearlitic nodular iron, one of them broke the drilling tool during the drilling process, while the other one did not damage the drilling tool during the same operation.
2. Experimental procedure The two crankshafts were inspected visually and were sectioned in the zones indicated in Figs. 1 and 2, obtaining two samples from each crankshaft. One sample was obtained from the zone were the cutting tool fractured (M-1), and the other sample was obtained from the opposite side of the same crankshaft (M-2). For the other crankshaft, the samples were obtained from the same positions (B-1 and B-2, see Figs. 1 and 2). They were identified according to the Table 1. Chemical analysis of the crankshafts was conducted for the four samples, two taken from the zone where the drilling tool failed (M-1 and B-1) and two taken from the other extreme of the crankshafts where the drilling tool did not fail (M-2 and B-2). A metallographic analysis was carried out on the four samples
Table 2 Chemical composition (wt.%) of the samples Sample
C%
Si%
Mn%
P%
S%
Cu%
Cr%
Al%
Range M-1 M-2 B-1 B-2
3.2 – 4.10 3.7733 2.4869 3.7728 2.0670
1.80 – 3.00 2.38 2.20 2.87 2.93
0.10 – 1.00 0.4426 0.3664 0.6642 0.5977
0.015 – 0.10 N.D. N.D. N.D. N.D.
0.005 – 0.035 N.D. N.D. N.D. N.D.
0.80 – 1.2 1.13 1.13 0.74 0.76
0.06 – 0.13 0.098 0.097 0.099 0.101
0.001 – 0.018 0.01 0.0095 0.0039 0.004
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Fig. 3. Microstructure of sample M-1, a fine pearlitic matrix with some graphite nodules surrounded by ferrite.
in order to compare the microstructures of the zones where the drilling tool failed and the zones where the drilling tool did not fail. Microhardness measurements
were made with the Vickers hardness scale for the four samples in order to detect some difference between the zones that failed and did not fail.
Fig. 4. Microstructure of sample M-2, a pearlitic matrix with some graphite nodules surrounded by ferrite.
M.A. Neri, C. Carren˜o / Materials Characterization 51 (2003) 219–224
Fig. 5. Microstructure of sample B-1, a coarse and fine pearlitic matrix with graphite nodules without ferrite surrounded them.
Fig. 6. Microstructure of sample B-2, a coarse and fine pearlitic matrix with graphite nodules without ferrite surrounded them.
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3. Results and discussion The four samples were chemically analyzed in order to determine the content of carbon, silicon, manganese, phosphorus, sulfur, copper, chromium, and aluminum, and the results were shown in Table 2 with the range for a nodular iron of the type B3 modified composition. From this table, we can observe that the carbon content in the sample M-1 that broke the drilling tool was greater (3.77% C) than that of sample M-2 that did not break the drilling tool (2.48% C). The copper content in both samples M-1 and M-2 is 1.13%, which is greater than the copper content in the samples B-1 and B-2, 0.74 and 0.76%, respectively. The microstructure observed in samples M-1 and M-2 consisted of a fine pearlitic matrix with graphite nodules and some nodules surrounded by ferrite (see Figs. 3 and 4). The microstructure observed in samples B-1 and B-2 consisted of a coarse pearlitic matrix with graphite nodules and some nodules surrounded by ferrite (see Figs. 5 and 6). Measurements of Vickers microhardness were taken from the samples analyzed in different zones to detect some possible variations among them. Table 3 shows the results obtained from the different samples M-1, B-1, M-2, and B-2. The highest value of microhardness was measured in sample M-1 in the fine pearlite zone (314 –304 HV) and in sample M-2 in the fine pearlite zone (293 –294 HV). In samples B-1 and B-2, the microhardness values obtained were 267 and 269 HV, respectively. Sample M-1 that broke the drilling tool had the highest carbon and copper content, promoting a higher hardness in the pearlitic matrix. The microstructure exhibited a finer pearlitic matrix [4]. Regarding hardness and machinability, higher hardness usually cause poorer machinability. Greater amounts of pearlite and harder pearlite also cause more difficulty in machining.
Table 3 Vickers microhardness measured in the matrix of the different samples of the crankshafts Sample No.
Measured zone
Microhardness Vickers (Rockwell B)
M-1 M-1 B-1 B-1 M-2 M-2 B-2 B-2
Fine pearlite Fine pearlite Coarse pearlite Coarse pearlite Fine pearlite Fine pearlite Coarse pearlite Coarse pearlite
314.3 304.7 267.8 267.2 294.1 293.4 269.1 269.8
(106.5) (105.6) (101.8) (101.7) (104.5) (104.4) (102) (102.1)
The differences in the chemical composition and microstructure promote a higher hardness in sample M-1, lowering the machinability of the crankshaft and causing the drilling tool to break.
4. Conclusions Sample M-1, where the drilling tool broke during the machining operation, had the highest carbon and copper contents and the highest value of Vickers hardness. In addition, this sample had the finest pearlitic microstructure compared to the other three locations. It had the highest carbon and copper contents, which promote the fine pearlite. The higher hardness of the fine pearlite caused poorer machinability in the crankshaft. References [1] Ductile Iron Marketing Group. Getting more out of cast parts using ductile iron. Mach Des 1998 (August 6);96 – 7. [2] Gundlach R. Ductile Iron Society. Private communication, 2002. [3] Ductile Iron Society. Ductile Iron Data for Design Engineers, Section VI Machinability. Montreal, Quebec, Canada: Rio Tinto Iron & Titanium Inc., 2002. [4] Porter DA, Easterling KE. Phase transformations in metals and alloys. UK: Van Nostrand Reinhold; 1984. p. 332.