diameter ductile iron pipe without excessive chill in the as-cast condition, additions of 0.016% cerium or 0.048% lanthanum were required. It is of interest to note ...
cast irons resulting in a lower chilling tendency. At higher levels, i. e., 0.046% La, compacted graphite is formed. The matrix is free of carbides until the carbide content reaches 0.075%. At higher values, the carbide content increases and is associated with the presence of irregular graphite.
Effect of Lanthanum and Cerium on the Structure Of Eutectic Cast Iron
Cerium did not exhibit a graphitizing effect in the irons studied. At cerium contents in excess of 0.014%, compacted graphite is formed but carbides may be present.
D. M. Stefanescu University of Alabama University, Alabama
Ferrosilicon postmoculation of lanthanum-treated cast iron extended the range of lanthanum over which compacted graphite could be obtained, as well as further increasing the graphitizing tendency of lanthanum.
C. R. Loper, Jr.
University of Wisconsin Madison, Wisconsin ABSTRACT A review of the literature demonstrated that lanthanum has a reduced tendency to form carbides in cast iron compared to cerium, suggesting that the presence of lanthanum in cast iron might be beneficial. Consequently, the effect of metallic lanthanum and cerium on graphite shape and carbideformation was studied in cast iron of near eutectic composition. It was proved that lanthanum, up to 0.06%, has a graphitizing effect in AFS Transactions
It was also shown that the cooling curve correlated well to the solidification structure obtained. In the absence of postinoculation, A T = TER - TEU values of more than 25° C (45° F) were associated with compacted graphite cast irons free of carbides and a maximum of 9% nodularity. Lower values of AT corresponded to either flake graphite, or irregular graphite and carbides, in the cast irons. INTRODUCTION Although lanthanum is present in most of the mischmetals, rare earth silicides and rare earth-containing ferroalloys used for the
81-29
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425
manufacture of ductile and/ or compacted-vermicular graphite cast irons, little is known about the specific influence of this element in the treatment of cast irons. Most of the literature concerning the effects of lanthanum in cast irons deals with ductile irons treated with lanthanum-containing rare earth alloys, with little information concerning the influence of metallic lanthanum in ductile irons and essentially no data related to either gray and/or compacted-vermicular graphite cast irons.
count increased and the amount of primary carbide decreased as the lanthanum to cerium ratio increased.
Over twenty years ago, Alexander1'2 recommended the use of rare earths containing a minimum of 30% lanthanum in order to obtain a softer ductile iron, with a residual lanthanum level in the iron of 0.004 to 0.020%. Lanthanum contents in excess of 0.020% resulted in the ductile iron becoming progressively harder.
PROCEDURE
A review of the literature demonstrated that lanthanum has a reduced tendency to form carbides in cast iron compared to cerium, suggesting that the presence of lanthanum in cast iron might be beneficial. In an attempt to gain a better understanding of the effect of lanthanum on cast iron structures, and to relate this to the effect of cerium, the following trials were conducted.
The effect of metallic lanthanum and cerium on graphite shape and carbide formation was studied in cast irons of near eutectic composition, Table 1. Three heats (A, B and C) were prepared in a 22.6 kg (50 lb) high frequency induction furnace, using ductile iron returns as the primary charge material (3.5% C, 2.4% Si, 0.4% Mn, 0.01% S).
Rice et al3 evaluated the influence of several individual nodularizing agents in a ductile iron base iron of 0.005 to 0.010% sulfur. It was found that in order to produce a 152 mm (6.0 in.) diameter ductile iron pipe without excessive chill in the as-cast condition, additions of 0.016% cerium or 0.048% lanthanum were required. It is of interest to note that a lanthanum addition of three times that of the cerium addition was required to produce a satisfactory spheroidal graphite structure. It should also be noted that carbides were present in the as-cast structure of all castings, and the pipe was subsequently annealed.
This base iron was heated to 1454C (2650F), a sample was tapped into a 5.6 kg (12.3 lb) graphite crucible (used as a pouring ladle) and several castings were poured. Sprue was then added to the melt in an amount equal to that tapped, and the base iron was reheated to 1454C (2650F). In the series referenced as Heat A, the base iron was then tapped into the 5.6 kg (12.3 lb) ladle containing metallic lanthanum. Sprue was then recharged to the furnace, and the process was repeated with various additions of lanthanum metal. The range of residual lanthanum contents obtained in Heat A is presented in Table 1.
Yamamoto et al4 produced a ductile iron in a cast iron containing 3.8% carbon and 2.1% silicon with the addition of 1.0% cerium. The addition of 1.0% lanthanum to the same base iron resulted in poor nodularity, bad nodule shapes and the presence of flake graphite.
Heat B was conducted in a manner similar to that described in Heat A, except that cerium metal was used in place of lanthanum metal. The range of residual cerium contents for Heat B is presented in Table 1.
It is evident from these reports that the nodularizing (or compacting) influence of lanthanum is much weaker than that of cerium.
Heat C was carried out similar to Heat A with metallic lanthanum additions. However, after the ladle containing the lanthanum was filled, an addition of 0.30% of 75% FeSi was thoroughly stirred in as a postinoculant.
Lalich5 has demonstrated that some lanthanum in cast iron is beneficial in improving graphitization. A single 0.023% sulfur base iron was treated with rare earth free 5% MgFeSi and split into several fractions. Each fraction was then treated with a rare earth addition of 0.017%, with the rare earth addition composed of mixtures of a 10% LaFeSi and of a 10% CeFeSi at different cerium to lanthanum ratios. It was reported that the nodule
Table 1. Chemical Composition of Cast Irons Studied
Each 5.6 kg (12.3 lb) ladle was used to pour the following castings: 1) Spectrographic analysis samples (copper mold); 2) Eutectometer thermal analysis samples; 3) Chill wedge, No. 4 (ASTM A 367) (air set);
Heat
Sample
C
Si
A
Al
3.44 IJA 3.59 3.36 3.47 3.56 NA NA 3.48 3.34 3.40
2.37
A2 A3 A4 A5 A6 A7 A8 A9 Al 0 All B
C
NA
NA 2.33 NA NA NA NA 2.29 2.56 2.66
Chemical Composi tion, °a P La S __ 0.33 IJA 0.010 NA NA NA 0.013 ;JA :JA 0.0O3 0.014 D.40 0.016 0.006 0.041 NA NA 0.007 0.046 NA 0.003 0.050 NA NA NA 0.012 0.060 MA NA 0.014 0.07 5 NA NA 0.010 0.110 NA NA 0.007 0.130 0.37 0.012 0.010 0.140 [-In
Bl B2 B3 B4 B5 B6 B7
3.49 3.47 3.62 3.60 3.61 3.57 3.61
2.50 2.62 2.45 2.28 2.62 2.60 2.60
0.41
0.007
NA NA NA NA NA iJA
NA NA NA NA NA NA
Cl C2 C3 C4 C5
3.36 NA 3.51 NA NA 3.50
2,60 NA
0.38
NA NA NA NA NA
C6
2.31 HA NA 2.67
NA NA NA NA NA
NA
0.008 0.010 0.010 0.007 0.008 0.010 0.003
-_ -_ — ----
0.006 0.010 0.010 0.012 0.010 0.008
0.039 0.052 0.061 0.077 0.130
Ce __ -.-_ -_-. .-_--
._ 0.016 0.031 0.083 0.150 0.150 0.220
Ti
Al
0.010
0.007
--_ --
--_
NA NA NA UA NA NA NA NA NA NA
IJA NA IJA MA NA NA NA NA NA
0.005
0.008 .
._
NA NA NA NA NA NA
._
NA NA NA NA NA NA
0.009 -_
TectiD
NA
0.01
NA NA NA MA NA
NA NA NA NA NA
4.14 4.32 4.20 4.32 4.30 4.30 4.03 4.17
4.32 4.30 4.30 4.28 4.28 ..
4.33 4.33 4.32 4.31 4.30
IJA - not analyzed
426
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TEU (temperature of eutectic undercooling), TER (temperature of the eutectic recalescence, i. e., maximum temperature of recalescence) and AT (difference between TEU and TER). Microstructural data presents an estimate of the ferrite, pearlite and carbide content and a brief notation of the type of graphite present at the mid-radius.
4) Chill pins (air set): a) 3.2 mm (0.125 in.) dia, b) 4.8 mm (0.188 in.) dia, c) 6.5 mm (0.250 in.) dia, d) 7.9 mm (0.313 in.) dia, e) 9.5 mm (0.375 in.) dia, f) 12.7 mm (0.500 in.) dia; 5) Bar casting, 25.4 mm (1,0 in.) dia (air set).
Table 3 illustrates the influence of lanthanum and cerium on the chilling tendency of the iron, as well as on the hardness as measured in the eutectometer sample and the 25.4 mm (1.0 in.) diameter bar. No attempt was made in the work to evaluate the mottled zone or the zone of mixed white and gray fracture, although this was observed to be affected in a manner similar to that of the clear white fracture region.
The chill wedge and chill pins were fractured and the visual evaluation of the amount of white fracture was used to estimate the chilling tendency of the iron. Solidification cooling curves were determined from the eutectometer samples, and the results correlated with the microstructure and Brinell hardness taken at the mid-radius of these castings. Microstmctural features and hardness were also determined in the 25.4 mm (1.0 in.) diameter bars.
Lanthanum additions to cast iron significantly changed the graphite shape, as well as affecting the matrix structure. With residual lanthanum up to 0.041% in cast iron, a variety of graphite forms occur but all of them are of a flake type (Fig. la, b and c). However, at 0.046% lanthanum in the cast iron 100% compacted graphite is present (Fig. Id) in a matrix which is predominantly ferrite with no carbides present. Only minor changes accompanied the increase in lanthanum to 0.075% (Fig. le). The graphite configuration remains essentially unchanged although a small amount of spheroidal graphite, less than 9%, was present. The matrix structure remains primarily ferrite and no carbides were observed. No increase in nodularity was observed with increased lanthanum contents. At 0.11% lanthanum another significant change occurs: the graphite shape changes from compacted to irregular (Fig. If), while a substantial amount of carbide is found in the matrix which contains an increased percentage of pearlite. A further increase in the residual lanthanum to 0.14% (Table 3) does not cause any other significant changes in the cast iron structure.
RESULTS AND DISCUSSION Chemical analyses of the series of heats cast are presented in Table 1. In addition, the estimation of carbon equivalent from the eutectometer is included with this data. While the carbon equivalent was in the range of 4.03 to 4.33%, most of the samples were about 4.3% CE. It may be noted that lanthanum contents in the cast irons ranged from nil to 0.140%, corresponding to lanthanum additions up to 0.5%. Residual cerium contents in the cast irons ranged from nil to 0.22%, corresponding to cerium additions up to 0.3%. In general, the recovery of lanthanum was about 21% while the cerium recovery was about 65%. A summary of the results obtained from the eutectometer samples is presented in Table 2. The thermal analysis data is presented in terms of TAL (temperature of austenite liquidus),
Accordingly, two critical levels of residual lanthanum in these cast irons can be defined:
Table 2. Data Obtained from Eutectometer Samples iperature, °C (° TAL
Sample Al A2 A3 A4 A5 Ab A7 A3 A9
Ain Ail
33
B6 B7
1173 1151 1166 1151 1155 1155 1134 1169
(214J) (2104) (2131) (2104) (2111) (2111) (2163) (2136)
a C3 C4 C5 C6
9
(17)
2 33 30 28 26 2
(4) (60) (54) (50) (47) (3)
1 2 8
(2098)
-1157 (2115)
1151 1158 1119 1122 1126 1129 1111 1115 1111
(2106) (2116) (2046) (2052) (2059) (2064) (2032) (2039) (2032)
1159 1160 1152 1152 1154 1155 1113 1116 1113
(2118) (2120) (2106) (2106) (2109) (2111) (2035) (2041) (2035)
(2098) (2102) (2043) (2048) (2030) (2035) (2030)
1156 1156 1145 1141 1113 1116 1111
(2113) (2113) (2093) (2086) (2035) (2041) (2032)
6 {ID 28 (50) 21 (38) 3 (5) 3 (6) 1
(2}
(2100) (2097) (2086) (2070) (2055) (2032)
1159 1149 1149 1156 1150 1119
(2118) (2100) (2100) (2113) (2102) (2046)
10 2 8 24 26 8
(18) (3) (14) (43) (47) (14)
1150 1155 1154 1157 1157
(2111) (2109) (2115) (2115)
1149 1149 1149 1153 1154
(2100) (2100) (2100) (2107) (2109)
1149 1147 1141 1132 1124 mi
Cl
AT (TEU-TER)
148
1148 1150 1117 1120 1110 1113 1110
(2102)
TER
TLJ
Graphite A,B,D, A A,D A,D CG CG ,9%SG CG.4XSG CG.9SSSG
MicrostrucLure* % Pearlite % Ferrite Yes 9 2 16 27 45 18
27
(2) (3}
IG IG IG
52 51 50
(15)
A,D
34 7 27 37 55
A,D,SG CG.92SG CG.23XSG IG
% Carbides
Yes 31 88 74 63 45 72 63 0 1 2
0 0 0 0 0 38
56 83 63 35
0 0 38
83 78 78 68 53 30
0 0 0 0 4 27
• G r a p h i t e s t r u c t u r e s noted a s : (A) - Type A g r a p h i t e p r e s e n t ; (B) Type B g r a p h i t e p r e s e n t ; (D) Type D g r a p h i t e p r e s e n t ; (CG) Compacted G r a p h i t e p r e s e n t ; (SG) S p h e r o i d a l G r a p h i t e p r e s e n t ; ( I G ) I r r e g u l a r G r a p h i t e p r e s e n t .
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427
Table 3. Chilling Tendency and Brinell Hardness of Cast Irons Studied
Sample
Chi 11inq Tendency Width o f the w h i t e Diameter of largest white pin, mm re g i o n on wedge,mm
-
A8 A9 A10 All
12.7 12.7 12.7 12.7 12.7 12.7 12.7
0 0 7 _ 5 17 32 32 32
121 37 106 161 167 174 495 477 -
Bl 32 B3 B4 B5 B6 B7
4.8 12.7 12.7 12.7 12.7 12.7 12.7
0 5 12 16 32 32 32
109 131 183 235 415 473
99 143 277 363 477 477 555
Cl C2 Cl C4 C5 C6
3.2 6.5 9.5 9.5 12.7 12.7
0 3 5
121 163 163 167 197 302
134 201 179 218
Al A2 A3 A4 A5 A6 A7
1) A transition from flake to compacted graphite, occurring at a residual lanthanum content of 0.041 to 0.046%; 2) A transition from a carbide-free structure to a heavily carbidic structure, occurring at a residual lanthanum content of 0.075 to 0.11%. In addition, this increase in carbide content is accompanied by an increase in the amount of irregular graphite and an increase in the pearlite content. While the influence of metallic cerium is similar to that of metallic lanthanum, the critical levels encountered are different (Table 2). The first critical level occurred at 0.016 to 0.031% residual cerium, where the graphite changed from a flake-type structure to one which was primarily compacted graphite. At 0.016% cerium a great diversity of graphite shapes was observed, ranging from type A flake to spheroidal graphite (Fig. 2a). Within this structure, a graphite type intermediate between that of Type D undercooled and coral graphite was also observed (Fig. 2b). At 0.031% cerium, compacted graphite is formed with about 9% spheroidal graphite present (Fig. 3a). This transition is at a lower residual level than was recorded for lanthanum. Increasing the residual cerium level results in an increase in the nodularity (Fig. 3b, 25% spheroidal graphite), but at 0.15% cerium, irregular graphite is formed (Fig. 3c). A gradual but significant increase in the carbide content can also be observed starting at 0.083% cerium residual (Fig. 3b). The second critical level occurs somewhere between 0.031 and 0.083% cerium residual. At 0.22% cerium the iron is completely white, Fig. 3d. As a general observation, irregular graphite was present in all cast irons where carbides in excess of 27% were present. These three structural regions were associated with three distinct types of cooling curves. In the first type, corresponding to a flake graphite structure, no liquidus arrest was noted. In this case, only the temperature of eutectic undercooling (TEU) and the temperature of eutectic recalescence (TER) were recorded, Fig. 4a. The second type corresponded to a typical compacted graphite cast iron where the temperature of the liquidus arrest was also present, Fig. 4b. A strong eutectic undercooling is usually associated with compacted graphite formation and can 428
S r i n e l l Ha rdness Eutectoneter 25.4i.in Sample "eaV n )
4.8 3.2 3.2 3.2
0 0
2.5
12 15
-
3 52
444
be seen here. Where irregular graphite and carbides are present, the third type of cooling curve shows all three characteristic temperatures: TAL, TEU and TER, Fig. 4c. Lanthanum and cerium were found to have essentially the same effect on the solidification cooling curve, that is, increasing either the lanthanum or cerium content resulted in changes in the cooling curve from Type I, to Type II, to Type III. However, the values of residual lanthanum or cerium at which these changes occurred were different. Figure 5 contains plots which demonstrate the effect of lanthanum, cerium or lanthanum plus FeSi on the TER and TEU. A very clear increase of both the recalescence and undercooling temperatures is evident as a result of an increase in residual lanthanum to 0.041%, the first critical level. This change is related to the graphitizing tendency of lanthanum, and is readily observed in both the hardness measurements (Fig. 6) and the chill pin fractures (Fig. 7). At the first critical level the chill pin fractures are entirely white, Fig. 8, but no chill was observed in the chill wedge until the first critical level was reached. As a result it may be concluded that no significant chilling tendency can be attributed to lanthanum up to the first critical level. The effect of cerium is somewhat different. There is no increase in the recalescence temperature up to the first critical level, Fig. 5. Also both the hardness, Fig. 6, and the chilling tendency of the iron are increased with cerium content, even from the first addition. No increase in the gray region can be observed on the chill pin fracture surfaces, Figs. 7 and 9, while chill is observed in the wedge at only 0.016% cerium, Figs. 7 and 9. Still, a common feature of both lanthanum and cerium is that there is a gradual decrease in the AT = TEU - TER interval with an increase of either lanthanum or cerium, Figs. 5 and 10. Increasing the lanthanum above the first critical level results in a sharp decrease of both recalescence and undercooling temperatures, Fig. 5. A further increase in lanthanum up to the second critical level (0.075% La) results in an increase of both TEU and TER. Between these levels the matrix structure
AFS Transactions AFS Library Copy: Page 4 of 12 Pages, Provided to User for Internal Use and Not Public Redistribution or Resale. Copyright © 2006 American Foundry Society.
V&&& a) No lanthanum added.
6; 0.015% La.
O.M«% La.
0.075% l a .
/-^. ;. £#ecr of lanthanum on the microstructure of eutectometer samples at their mid-radius. Etched, 2% nital. 100X. AFS Transactions AFS Library Copy: Page 5 of 12 Pages, Provided to User for Internal Use and Not Public Redistribution or Resale. Copyright © 2006 American Foundry Society.
429
4 a) WOX.
•JP^*
b
Fig. 2. Variation in graphite type observed in eutectometer samples of cast irons having a 0.014% Ce content. Etched, 2% nital.
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