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Identification and quantification of active manufacturing factors for graphite formation in centrifugally cast Nihard cast irons
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Identification and quantification of active manufacturing factors for graphite formation in centrifugally cast Nihard cast irons ´ J. Asensio-Lozano a,∗ , J.F. Alvarez-Antol´ ın b , G.F. Vander Voort c a
´ Departamento de Ciencia de los Materiales e Ingenier´ıa Metalurgica, Escuela T´ecnica Superior de Ingenieros de Minas, ˜ Spain Universidad de Oviedo, c/Independencia 13, E-33004 Oviedo, Asturias, Espana, b Departamento de Ciencia de los Materiales e Ingenier´ıa Metalurgica, ´ Escuela Universitaria de Ingenier´ıa T´ecnica Industrial, ´ Asturias, Espana, ˜ Spain Universidad de Oviedo, Campus Universitario, E-33203 Gijon, c Buehler, Ltd., 41 Waukegan Road, Lake Bluff, IL 60044, USA
a r t i c l e
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a b s t r a c t
Article history:
The present research arises from the need to gain knowledge to control the precipitation
Received 30 May 2007
of interdendritic dispersed graphite in mottled Nihard cast irons used in the outer ring
Received in revised form
layer of industrial duplex work rolls. Industry desires a volume fraction of 2.5–4% graphite,
24 November 2007
distributed homogeneously across the layer thickness, with a reasonably constant, high
Accepted 4 December 2007
number of particles across the layer, with a predominantly short flake-graphite shape. The objective was to ascertain the role of metallurgical manufacturing parameters and their interaction with graphite precipitation in the layer, and in the outer and inner sections of
Keywords:
the roll collar. These rolls, used in the finishing stands of hot strip steel mills, were processed
Fractional Design of Experiments
by a double pour method using vertical centrifugal casting machines. Analysis was conducted using a fractional Design of Experiments (DOE) program consist-
(DOE) Graphite
ing of 16 industrial trials based on the study of 7 manufacturing factors at 2 levels. The
Nihard cast iron
analyzed responses, obtained by quantitative metallographic techniques, were: the volume
Alloy indefinite chill
fraction, VV , number per unit area, NA , and the graphite morphology across the layer thick-
Quantitative metallography
ness. The results confirmed that an increase from 0.5 to 0.8 kg/T of SiCaMn added as a bath
Centrifugal casting
inoculant in the ladle increased the fraction of precipitated graphite without affecting NA .
Duplex rolls
An outstanding result was that the addition of 1 kg/T of FeTi as a bath inoculant did not modify the amount of precipitated graphite but significantly affected the morphology, favouring the development of elongated graphite shapes. © 2007 Elsevier B.V. All rights reserved.
1.
Introduction
The purpose of this research was to development knowledge regarding the influence of metallurgical manufacturing parameters, and their interactions, on the occurrence of the graphite phase, in the outer ring layer of alloy indefinite chill
∗
Corresponding author. Tel.: +34 985104302; fax: +34 985104242. E-mail address: [email protected] (J. Asensio-Lozano). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.12.015
(AIC) work rolls for the steel rolling industry, used in the last stands of hot strip mills. The work was a collaboration between a Spanish roll manufacturer and the Materials-Pro research group of the Materials Science and Metallurgical Engineering Department, Oviedo University, Spain. The industrial scale study was performed
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Table 1 – Chemical composition range of the outer ring layers expressed in weight percent C 3.2–3.3
Mn
Si
S
P
Cr
Ni
0.9–1.0
0.9–1.0
<0.015
<0.035
1.7–1.8
4.2–4.4
Mo 0.25
on duplex work rolls with compositions falling within the specification range (Table 1). The rolls were processed by the double pour method in vertical centrifugal casting machines. The duplex-type rolls consist of a peripheral outer layer and a core of differing compositions, processed consecutively by vertical centrifugal casting. The outer ring layer is a hypoeutectic white cast iron with interdendritic graphite embedded in the ledeburitic matrix constituent, in which the austenite becomes partially transformed into martensite (Pero-Sanz Elorz, 1994a; Asensio et al., 2003). The roll core contains graphite in either flake, vermicular or spheroidal morphology within a matrix composed of homogeneously distributed ferrite and pearlite (British Rollmakers, 2007). Graphite in the roll shell greatly improves the ability of the roll to withstand the thermal shock associated with hot rolling steel strip, decreases friction between the roll and the strip, and greatly reduces the potential for fusing of the strip to the roll (Nycen and Adams, 2000). On the negative side, however, graphite reduces hardness and lowers wear resistance, which shortens the usable life between regrinds compared with other roll types, such as HSS steel types and high Cr cast irons. The goal of this work was to investigate the influence of the aforementioned manufacturing parameters, both in the entire layer and at two locations in the shell layers with different solidification conditions: the outer part of the ring layer in contact with the die wall and the inner portion of the ring in contact with the roll core. By controlling the morphology of the precipitated graphite, it is possible to optimize thermal shock spalling resistance of the roll layer, and thus an increase of its service life, benefiting from the excellent heat diffusiv´ ity of this constituent (Asensio-Lozano and Alvarez-Antol´ ın, 2007; Pero-Sanz-Elorz, 1994b). Analysis was performed with a fractional Design of Experiments (DOE) program consisting of 16 industrial trials based on the study of 7 manufacturing factors at 2 levels. Fractional Factorial DOE allows a high number of factors to be studied with a considerably lower number of experiments executed in comparison to Complete Factorial DOEs. The loss of information derived from the possible interactions among the chosen factors, known as “confusion of the factors”, is assumed on conducting a fractional DOE. This variant is often employed where the unknown possible interactions are thought to have ´ et al., 1997). The facminimal or no influence (Prat Bartes tors studied in the former DOE included, among others, the active variables found to have a significant effect on the development of C(g) in a first saturated DOE (Asensio-Lozano and ´ Alvarez-Antol´ ın, 2007). It was designed to the highest fractioning degree possible for 7 factors resulting in 8 experiments. Thus, the first set of variables in the new study was: the total Si percent after inoculation, the liquidus temperature, and the amount of inoculation added either as SiCaMn and FeSi. The new factors incorporated in this study were: the centrifugal cast machine employed either with isolating sleeve or without
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it, which due to intrinsic characteristics influences the solidification rate; the weight of the outer ring layer being poured, equivalent to the layer thickness in case of rolls with similar barrel length (∼2000 mm) and roll diameter (∼700 mm) as was the case; and, finally the inoculation with FeTi. The responses being measured were the volume fraction, number per unit area, and the morphology of the graphite phase across the layer thickness by means of quantitative metallographic techniques using digital images from the light optical microscope. k−p Fractional designs are usually denoted by 2N , where 2 represents the number of levels studied for every factor, k represents the number of factors or metallurgical variables to study, p is the degree of fractioning, and N represents the degree of resolution. The present DOE can be represented by 27−3 IV , yielding a total of 16 experiments. The degree of resolution in this DOE is 4, which indicates the degree of confusion that is present in the estimation of the factors. Therefore, for the present analysis it is possible to estimate the principle effects, as well as the second-order interactions (Box et al., 2002a). There are many publications regarding precipitation of graphite in grey cast irons. However, the originality of this study resides in the investigation of the metallurgical manufacturing factors affecting precipitation of graphite in white cast irons without a definite chill zone; that is, in white cast irons in which graphite occurs by a critical balance between alloying elements such as C, Ni and Si which promote the formation of graphite, and carbide forming elements such as Cr, Mn and Mo. The present analysis determined the metallurgical variables and their interactions in the shells of the AIC rolls that have a significant influence on the graphite formation. The main conclusions of this study arose from the addition of SiCaMn and FeTi added as bath inoculants. An increase in the amount of SiCaMn added from 0.5 to 0.8 kg/T raised the amount of graphite without altering the count number. Inoculation of the bath with 1 kg/T of FeTi, although it did not modify the graphite volume fraction (despite its marked carbide forming nature), significantly altered the graphite number, lowering the final value at the two opposite sections of the roll layer studied. FeTi is known to modify the morphology of graphite nodules to produce vermicular graphite in grey cast irons. In this investigation conducted on mottled Nihard cast irons, FeTi inoculation promoted the development of elongated graphite shapes.
2.
Experimental procedure
The compositional range used for obtaining the desired outer layer is indicated in Table 1; the type, chemistry and weight of the different additions added as bath inoculants are shown in Table 2. The manufacturing method consisted of a melting stage conducted in medium frequency induction furnaces followed by stream-ladle treatment with bath inoculants. The pouring temperature of the melt from the furnace to the ladle was 60 ◦ C above the casting temperature of the layer, Tcast . The usual value for Tcast was ∼1320 ◦ C. After ladle inoculation, the melt was poured into the rotating mould through a ceramic hollow duct with three orifices at its lower end that project the
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Table 2 – Chemical composition of inoculants expressed in weight percent with an indication of the regular amounts added to the treatment ladle expressed in kg/T Inoculants
Base chemistry Si
A-type FeSia B-type FeSi FeMn SiCaMn FeTia a
75.0 75.4 2.0 58.3 0.39
Ca 2.5 0.5 – 16.4 –
Amount added
Al
Zr
Ti
C
S
P
1.4 1.0 – 1.1 3.45
1.6 – – – –
– 0.031 – 0.030 74.33
– 0.2 5.8 0.6 0.19
– 0.001 0.014 0.030 0.006
– 0.012 0.130 0.030 0.001
Mn – – 69.4 14.8 –
Fe rem. rem. rem. 7.4 rem.
1.6–2.2 0.6 0.8 0.36 0–1
Both A-type FeSi and FeTi were added as bath inoculants in variable amounts depending on the chosen levels for the studied factors included in the DOE.
melt into the metallic mould corresponding to the roll barrel. A protective granular flux was drawn by air suction into the mould cavity, impacting the rotating metallic shell plus layer melt. The flux melts and helps to prevent oxidation of the inner liquid free surface in contact with air. Once the specified amount of Nihard cast iron was poured, the casting of the roll core melt began with a small part of the outer layer still in the mushy state, the composition of which is designed to produce an SG cast iron. After core casting, the mould with its content were gently moved to the cooling bay and allowed to complete solidification and the subsequent solid-state transformations by cooling to RT + 50/70 ◦ C. This took place over a period of 4–5 days, and only then was the roll removed from its metallic mould, to avoid crack formation on cooling due to premature stripping. Table 3 shows the factors and chosen levels for the present DOE. It is worth noting that factor C refers to the type of centrifugal cast machine used, out of the two centrifugal cast machines available at the factory where the research was conducted. The main difference between these machines is the presence of a movable insulating sleeve placed around the mould in centrifugal cast machine No. 2, promoting slower solidification rates in the outer ring roll metal due to the lower heat diffusivity (The Open University, 1990). Factors F and G are, respectively, the amount of inoculation with FeTi and Atype FeSi added to the transfer ladle prior to pouring the melt into the mould. It was decided at this stage of the research to conduct a fractional Design of Experiments (DOE) consisting of 16 industrial trials based on the study of 7 manufacturing factors at 2 levels. Table 4a presents the DOE matrix, while Table 4b shows the generators, effects and confusions.
Table 3 – Factors and level description for DOE Code
Metallurgical parameter correspondence factors
Level (−1)
A B C
Liquidus temperature SiCaMn Centrifugal cast machine
1235–1240 ◦ C 0.5 kg/T No. 1
D E F G
Ring roll layer weight Si% FeTi A-type ferrosilicon
2300–2600 kg 1.0–1.1 None 1.6 kg/T
Level (+1) 1250–1255 ◦ C 0.8 kg/T No. 2 (with isolating sleeve) 3000–3300 kg 1.1–1.2 1 kg/T 2.2 kg/T
Table 4a – (27−3 IV ) array for DOE Number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
A
B
C
D
E
F
−1 +1 −1 +1 −1 +1 −1 +1 −1 +1 −1 +1 −1 +1 −1 +1
−1 −1 +1 +1 −1 −1 +1 +1 −1 −1 +1 +1 −1 −1 +1 +1
−1 −1 −1 −1 +1 +1 +1 +1 −1 −1 −1 −1 +1 +1 +1 +1
−1 −1 −1 −1 −1 −1 −1 −1 +1 +1 +1 +1 +1 +1 +1 +1
−1 +1 +1 −1 +1 −1 −1 +1 −1 +1 +1 −1 +1 −1 −1 +1
−1 −1 +1 +1 +1 +1 −1 −1 +1 +1 −1 −1 –1 −1 +1 +1
G −1 +1 −1 +1 +1 −1 +1 −1 +1 −1 +1 −1 −1 +1 −1 +1
Table 4b – Generators and confusions in the (27−3 IV ) matrix consisting of 16 experiments and 7 factors Generators
Confusions
E = ABC F = BCD G = ACD
A B C D E F G AB+CE + FG AC + BE + DG AD + CG + EF AF + BG + AG BC + AE + DF BD + CF + EG CD + BF + AG ABD
The experimental procedure that describes the metallographic techniques leading to the measured responses specified above is summarized as follows. First, samples were cut from the upper part of the roll barrel, corresponding to the position while the role was kept vertical in the centrifugal cast machine. The cutting process was conducted using an abrasive disc cutting machine. The dimensions of the specimens for metallographic inspection were approximately
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Fig. 1 – Selected light optical micrographs of the mid section or working zone (Zone II and/or Zone III), of the ring roll layers for each experiment in the DOE analysis: (1)–(16). Microstructures presented in the as-polished state.
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Fig. 1 – (Continued ).
3.23 0.89 1.00 0.015 0.027 1.81 4.32 0.30 1.6 1 1 0.8 1231 1343 3200 2 3.01 0.91 1.03 0.012 0.030 1.81 4.32 0.25 2.2 0.8 0 0.5 1249 1361 3020 2 3.1 0.89 1.16 0.011 0.025 1.78 4.29 0.25 1.6 1.6 0 0.5 1235 1348 3020 2 3.02 0.91 0.98 0.008 0.021 1.75 4.13 0.26 1.6 1 0 0.8 1248 1353 3260 1 3.14 0.91 1.17 0.010 0.020 1.79 4.27 0.27 2.2 0.8 0 0.8 1233 1334 3360 1 2.99 0.87 1.11 0.011 0.022 1.76 4.31 0.25 1.6 1.6 1 0.5 1252 1350 3360 1 3.21 0.90 0.98 0.014 0.023 1.80 4.36 0.29 2.2 0.8 1 0.5 1241 1332 3100 1 3.02 0.92 1.16 0.013 0.034 1.78 4.30 0.28 1.6 1.6 0 0.8 1247 1352 2340 2 3.17 0.93 1.10 0.010 0.022 1.818 4.23 0.28 2.2 0.8 0 0.8 1237 1348 2660 2 3.11 0.89 0.96 0.020 0.030 1.78 4.30 0.28 1.6 1 1 0.5 1248 1359 2660 2 3.14 0.91 1.16 0.009 0.022 1.80 4.32 0.26 2.2 0.8 1 0.5 1235 1347 2660 2 3.01 0.89 1.01 0.011 0.035 1.76 4.25 0.28 2.2 0.8 1 0.8 1245 1342 2380 1 3.15 0.92 1.20 0.020 0.040 1.74 4.07 0.35 1.6 1.6 1 0.8 1238 1344 2380 1 3.04 0.91 1.18 0.010 0.020 1.79 4.35 0.26 2.2 0.8 0 0.5 1248 1350 2340 1 % % % % % % % % kg/T kg/T kg/T kg/T ◦ C ◦ C kg No. C Mn Si S P Cr Ni Mo Type A-FeSi Type B-FeSi FeTi SiCaMn Liquidus temperature Casting temperature Outer layer weight Centrifugal cast machine
3.17 0.90 1.08 0.014 0.030 1.81 4.37 0.27 1.6 1 0 0.5 1237 1338 2340 1
9 8 5 4 3 2 1
Representative micrographs corresponding to the as-polished state at the mid section or work zone (Zones II and III), in the outer shell for each experiment are presented in Fig. 1. The casting parameters for each of the 16 experiments are shown in Table 5. The results of the analysis are presented in Tables 6–8c Tables 6-8; responses, effects and significant second-order or higher interactions are also presented. Figs. 2–4 illustrate the standardized effects, i.e., the ratio of the effect to its standard deviation, presented using normal probability plots in which the significant effects are noted with their corresponding identifying letters. Presentation of data with a normal distribution using a normal probability plot will reveal an alignment of the data points. The straight line so defined should pass through the point of coordinates (0, 50%) for the standardized effects and probability percent, respectively, thus indicating that the mean variability of the distribution is zero. The slope of the line represents the magnitude of experimental error. Every experimental response is subjected to random variations. This variation follows a normal law, and its standard deviation represents the experimental error. The effects are linear combinations of the responses; hence, by application
Units
Results and discussion
Casting parameters
3.
Table 5 – Casting parameters measured for each experiment in the DOE
1. The volume fraction of graphite, denoted by VV . 2. The number of graphite particles per unit area, NA , represented by #/mm2 . 3. The Feret quotient expressed by: FMAX /FMIN , where FMAX is the maximum Feret diameter and FMIN is the minimum Feret diameter.
6
7
Experiment number
10
11
12
13
14
15
16
15 mm × 15 mm × 40 mm; the major length coinciding with the layer thickness in the direction of the roll radius. Mechanical preparation of the specimens for metallographic inspection was conducted by the classical grinding techniques using 60–120–240 and 600 grit SiC papers. The specimens were then polished in two consecutive stages, first with 6 m and then with 1 m oil-based diamond paste. For the sake of simplicity in the analysis of the results, the samples were divided into four zones comprised between the periphery and the ring-core roll interface. Of particular interest in this study are the results in Zone I, corresponding to the outer section of the ring roll in contact with the die and in Zone IV, corresponding to the inner section of the roll layer in contact with the roll core. Five micrographs were randomly chosen for each of the four zones in the as-polished state, totalizing 20 micrographs per roll. Micrographs were taken with an Olympus PMG3 light optical microscope (LOM) connected to an OmniMet® Enterprise device for electronic image data acquisition and archiving. The quantitative metallographic assessment of the graphite phase was performed with Image ProPlus (version 4.5.0.29) coupled to a Materials-Pro module imaging software. In order to conduct the DOE analysis and the derivation of the significant effects, the commercial statistics software package Statgraphics Plus (version 5.1) was employed. The responses studied in this DOE were (Vander Voort, 1984; Box et al., 2002b):
3.03 0.89 1.13 0.011 0.023 1.77 4.12 0.25 2.2 0.8 1 0.8 1246 1347 3200 2
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Table 6a – Calculation of the effects by the application of Yates’ algorithm for the volume fraction of interdendritic graphite in Zone I (periphery), Zone IV (inner layer portion) and layer average Experiment number
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Vv (graphite) (vol.%) Zone I
Effect
Zone IV
Effect
Layer average
Effect
3.42 4.53 4.25 4.82 4.58 0.85 4.98 4.94 5.73 5.11 7.02 5.25 6.87 5.43 8.12 4.74
5.04 −1.16 0.95 0.01 0.05 −0.99 0.31 0.43 1.99 −0.64 −0.45 −0.78 0.47 0.38 −0.53 −0.63
7.19 6.14 7.52 6.09 6.39 1.04 5.49 5.01 7.4 7.15 9.57 6.95 9.75 6.95 7.16 6.21
6.63 −1.87 0.25 0.50 −1.25 −0.53 −0.31 1.18 2.03 0.21 −0.59 −0.63 1.00 0.31 −1.01 −0.13
4.27 5.32 6.36 5.49 5.91 2.08 5.7 4.81 5.71 6.14 8.14 5.37 8.04 6.11 8.01 5.66
5.82 −1.40 0.75 −0.33 −0.06 −0.86 −0.24 0.96 1.66 −0.26 −0.45 −0.58 0.68 0.37 −0.30 −0.26
Identification Mean A B AB C AC BC E D AD BD ABD CD G F AF
Table 6b – Joint significant effects given by second-order interactions, resulting from the confusions AC + BE + DG and AG + BF + CD for the average volume fraction of graphite in the layer AC + BE + DG
A
C
−1 +1
−1
+1
6.1 5.6
6.9* 4.7
AC + BE + DG
B
E
−1 +1
−1
+1
4.5 5.6
6.4* 6.2
AC + BE + DG
D
G
−1 +1
−1
+1
5.0 6.9*
5.6 6.4
CD + BF + AG
C
D
−1 +1
−1
+1
5.4 4.6
6.3 7.0*
CD + BF + AG
B
F
−1 +1
−1
+1
5.9 6.0
6.0 6.4*
CD + BF + AG
G −1
A
−1 +1
+1 *
6.7 4.6
6.4 5.6
The effects marked with an asterisk indicate the most probable relation between interrelated factors that generate the highest percentage in volume of graphite.
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Table 7 – Calculation of the effects by the application of Yates’ algorithm for the count number of graphite particles per square millimeter in Zone I (periphery), Zone IV (inner layer portion) and layer average Number of graphite/mm2
Experiment number Zone I 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
84 160 91 72 103 16 108 155 165 84 319 99 188 100 174 98
Effect 126 −56 27 −11 −17 5 5 48 55 −60 11 −21 −10 29 −51 −10
Zone IV
Effect
226 183 211 121 227 30 247 133 209 111 262 84 227 162 142 177
of the central limit theorem, they follow a normal law. If all the effects were non-significant, they would follow an N (0,) distribution, thus appearing aligned describing a straight line when represented on a normal probability plot. The significant effects follow an N (,) distribution, represented by points which are off the straight line defined by the non-significant effects. The effects which are potentially significant tend to accumulate at both ends of the straight line. Those effects which are not aligned with the straight line and also become apart from the mean zero are thus significant. As a result, each effect can be considered as a random variable; the value so determined is an estimate of its arithmetic mean. Accordingly, this value is accompanied by its standard deviation. The significance of a given effect is based on the ratio of its mean value to its error, using the Student-t statistic at the 95% confidence level (Chatfield, 1991).
Layer average
172 −94 0 7 −8 9 13 39 −1 17 −11 −2 18 53 −37 6
116 163 191 118 168 31 156 124 154 82 288 95 208 104 180 126
Effect 144 −77 32 −11 −14 −4 −12 49 21 −28 4 −7 14 31 −26 −7
Identification Mean A B AB C AC BC E D AD BD ABD CD G F AF
4. Significant effects in the graphite volume fraction Table 6 shows the calculated effects after applying Yates’ algorithm to the volume fraction of interdendritic graphite in Zone I (periphery), Zone IV (inner layer portion) and the layer average.
4.1.
Layer average
Fig. 2a illustrates of the significant factors influencing the graphite volume fraction in the layer average. The values so obtained have low experimental error considering the steepest slope of the straight line where the non-significant effects are aligned. These are the liquidus temperature; the weight of the outer roll layer, which is proportional to its thickness and
Table 8a – Calculation of the effects by the application of Yates’ algorithm for the quotient of Feret diameters FMAX /FMIN in Zone I (periphery), Zone IV (inner layer portion) and layer average Experiment number
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
FMAX /FMIN Zone I
Effect
1.83 2.09 2.25 2.02 2.30 1.94 2.10 2.14 2.36 2.02 2.36 1.83 2.09 1.86 2.38 2.21
2.11 −0.20 0.10 −0.03 0.03 0.02 0.06 0.14 0.05 −0.12 0.01 −0.01 −0.04 0.10 0.15 −0.08
Zone IV 1.93 2.18 3.21 2.70 3.90 2.26 1.80 2.31 3.26 2.09 3.16 1.93 2.87 1.99 2.53 2.98
Effect 2.57 −0.53 0.02 0.33 0.02 0.14 −0.37 0.54 0.07 −0.18 0.08 −0.02 −0.04 0.36 0.60 −0.19
Layer average 1.89 2.16 2.83 2.47 2.81 2.09 2.04 2.15 2.51 2.06 2.61 1.89 2.48 2.02 2.52 2.47
Effect 2.31 −0.30 0.12 0.04 0.02 0.02 −0.18 0.27 0.02 −0.12 −0.02 −0.01 0.09 0.15 0.32 −0.10
Identification Mean A B AB C AC BC E D AD BD ABD CD G F AF
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Table 8b – Joint significant effects given by second-order interactions, resulting from the confusions AD + CG + EF for the quotient of Feret diameters in the layer section adjacent to the mould wall AD + CG + EF
A
D
−1 +1
−1
+1
2.1 2.0
2.3* 2.0
AD + CG + EF
C
G
−1 +1
−1
+1
2.0 2.1
2.2* 2.1
AD + CG + EF
E
F
−1 +1
–1
+1
1.9 2.2
2.2 2.2
The effects marked with an asterisk indicate the most probable relation between interrelated factors that generate the most elongated shape of graphite.
Table 8c – Joint significant effects given by second-order interactions, resulting from the confusions BC + AE + DF and AB + CE + FG for the quotient of Feret diameters in the layer section adjacent to the roll core BC + AE + DF
B
C
−1 +1
−1
+1
2.4 2.8*
2.8* 2.4
BC + AE + DF
A
E
−1 +1
−1
+1
2.6 2.1
3.0* 2.5
BC + AE + DF
D
F
−1 +1
−1
+1
2.1 2.5
2.8 3.1*
AB + CE + FG
A
B
−1 +1
−1
+1
3.0* 2.1
2.7 2.5
AB + CE + FG
C
E
−1 +1
−1
+1
2.5 2.1
2.7 3.0*
AB + CE + FG
F
G
−1 +1
−1
+1
2.3 2.5
2.3 3.2*
The effects marked with an asterisk indicate the most probable relation between interrelated factors that generate the most elongated shape of graphite.
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Fig. 2 – (a) Normal probability plot for the volume fraction of graphite (%) in the layer average. (b) Normal probability plot for the volume fraction of graphite (%) in the periphery of the layer. (c) Normal probability plot for the volume fraction of graphite (%) in the periphery in the layer section adjacent to the roll core.
Fig. 3 – (a) Normal probability plot for the graphite count number per square millimetre in the layer average. (b) Normal probability plot for the graphite count number per square millimetre in the periphery of the layer. (c) Normal probability plot for the graphite count number per square millimetre in the periphery in the layer section adjacent to the roll core.
212
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therefore conditions the heat released during solidification; the percentage of silicon; and inoculation with SiCaMn. The weight of the layer, the Si weight percentage, the amount of SiCaMn added, and the interactions of two factors given by AG + DF + CD appear detached to the right of the straight line, withdrawing from the mean zero. On the other hand the liquidus temperature, and the second order interactions given by AC + BE + DG have a tendency to detach to the left of the straight line, again separating from the mean zero. Table 6b shows the significant joint interactions of 2 factors found to have influence on the graphite volume fraction in the layer average: AC + BE + DG and CD + BF + AG. The first one, given by AC + BE + DG, corresponds to the interaction of the liquidus temperature with the type of centrifugal casting machine; the interaction of the SiCaMn inoculation with the percentage of silicon; and, the interaction of the layer weight with the A-type FeSi inoculation. On the other hand, the joint interactions CD + BF + AG, translates into: the interaction of the type of centrifugal casting machine with the layer weight; the interaction of SiCaMn inoculation with the addition of FeTi; and, the interaction of the liquidus temperature with A-type FeSi inoculation. In summary, the metallurgical parameters contributing to an increase in the graphite volume fraction over the layer are: an increase of the Si content, the use of low liquidus temperature melts and the addition of SiCaMn inoculant at the highest level chosen. The former results are consistent with the physical metallurgy grounds for stable solidification of hypoeutectic compositions during the eutectic reaction in grey cast irons. In effect, they all contribute to increase the equivalent carbon and thus enhance the development of graphite. The layer weight and, therefore, its thickness also seem to have a key role, because the higher weights being poured allow for lower cooling rates during solidification and solid-state transformation of the roll shells. This might favour a reduction of the undercooling through the stable eutectic reaction, thus promoting a higher proportion of the liquid evolving through this reaction with the consequence of a higher proportion of graphite being formed. The results obtained from the interaction of the factors also showed that the use of isolating sleeves during solidification and the inoculation of the melt with A-type FeSi at the highest level studied could have an influence on the increase of the fraction of graphite precipitated. Because the aim of this research is to restrict the volume fraction of graphite to 4% or less, our strategy should be to control these metallurgical factors accordingly.
4.2.
Outer and inner sections of the roll collar
Fig. 2b shows that the only significant effect in the outer portion of the ring roll layer is the layer weight; increasing the weight favours a higher volume fraction of graphite being formed during solidification. Fig. 2c shows that an increase of the layer weight and a reduction of the liquidus temperature, are the significant effects that increase the proportion of graphite in the layer section adjacent to the roll core. It can also be observed that the slope of the straight line in Fig. 2c is somewhat smaller than those in Fig. 2b and c, thus indicating that the values so obtained have a higher experimental error.
The significant effects in controlling the amount of graphite formed in the most distant sections of the thickness in the ring roll, representing two extremes in the solidification of the layer, are, simply, the liquidus temperature and layer weight.
5. Significant effects in the number of graphite particles per unit area Table 7 shows the calculated effects after applying Yates’ algorithm for the graphite number per unit area in Zone I (periphery), Zone IV (inner layer portion) and in the layer average. Fig. 3a–c shows the significant factors influencing the total number of counts for the graphite phase.
5.1.
Layer average
In Fig. 3a, it can be seen the significant effects for the number of graphite particles in the layer average were found to be: the liquidus temperature; the layer weight; the Si content; and, inoculation with A-type FeSi. The percentage of Si, the inoculation of the melt with A-type FeSi, and the layer weight appear separated to the right of the straight line, thus withdrawing from the mean equal zero. In summary it is concluded that in order to obtain a high graphite NA , melts with low liquidus temperature and high Si content, should be poured. The addition of A-type FeSi at the highest level tested in this work, also increased the graphite counts. However, inoculation with SiCaMn revealed no significant effect on NA . A possible explanation for these results may be that casting low-liquidus temperature melts usually increases the number of graphite cells. Perhaps the addition of substantial inoculants counteracts the high depletion of homogeneously formed graphite nuclei being destroyed at the high casting temperatures demanded by industrial processing concerns. However, it is surprising that inoculating with SiCaMn at the levels added in the DOE did not enhance nucleation. It may be reasonable to increase the addition of SiCaMn beyond 0.8 kg/T to benefit from its inherent role as promoter of heterogeneous nucleation of graphite in the ladle.
5.2.
Outer and inner sections of the roll collar
Fig. 3b and c shows that a reduction in the amount of FeTi added as bath inoculant and the reduction of the melt liquidus temperature had a significant effect on increasing the graphite count number both in the outer and inner sections of the ring roll layer while it had no effect on the layer average. Indeed, FeTi is a shape-control element of graphite in grey cast irons, but it has never been reported to inhibit the graphite count. The present study showed the deleterious effect that Ti had when the objective is to increase the graphite counts.
6. Significant effects in the morphology of graphite Table 8a shows the calculated effects after applying Yates’ algorithm on the ratio of Feret’s diameters in Zone I (periphery), Zone IV (inner layer portion) and layer average. Fig. 4
213
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Fig. 4 – (a) Normal probability plot for the ratio of Feret diameters in the layer average. (b) Normal probability plot for the ratio of Feret diameters in the periphery of the layer. (c) Normal probability plot for the ratio of Feret diameters in the periphery in the layer section adjacent to the roll core.
depicts the significant factors influencing the graphite morphology given by the quotient of Feret diameters (FMAX /FMIN ).
6.1.
Layer average
The significant effects for the whole layer (Fig. 4a) are the liquidus temperature, the Si content, and FeTi inoculation. The percentage of silicon and the inoculation with FeTi are detached to the right of the straight line, thus separating from
the mean zero. Also, the liquidus temperature shows a tendency to separate from the same straight line to the left, also withdrawing from the mean zero. A decrease of the liquidus temperature and the elaboration of melts with high Si contents have been found to increase the development of elongated graphite morphologies in these mottled Nihard irons, i.e., with high FMAX /FMIN ratios. The addition of FeTi also promotes the development of elongated graphite geometries a result that seems consistent with its
Table 9 – Significant factors and levels providing an increase in the selected responses Response
Code
Significant factors
Steepest ascent level
Metallurgical parameter investigated
Level
Corresponding value
Vv (%)
A B E AG AC BF CD
Liquidus temperature SiCaMn Si% Liquidus temperature/A-type FeSi Liquidus temperature/centrifugal cast machine SiCaMn/FeTi Centrifugal cast machine/ring roll layer weight
−1 +1 +1 −1/+1 −1/+1 +1/+1 +1/+1
1235–1240 ◦ C 0.8 kg/T 1.1–1.2 1235 – 1240 ◦ C/2.2 kg/T 1235–1240 ◦ C/No. 2 (with isolating sleeve) 0.8 kg/T/1.1 kg/T No. 2 (with isolating sleeve)/3000–3300 kg
#/mm2
A E G D
Liquidus temperature Si% A-type FeSi Ring roll layer weight
−1 +1 +1 +1
1235–1240 ◦ C 1.1–1.2 2.2 kg/T 3000–3300 kg
FMAX /FMIN
A E F
Liquidus temperature Si% FeTi
+1 +1 −1
1250–1255 ◦ C 1.1–1.2 None
214
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role in the formation of vermicular graphite for the production of compacted cast irons by conversion of nodular cast irons (Karsay, 1994).
6.2.
Outer and inner sections of the roll collar
Fig. 4b, corresponding to the periphery and Fig. 4c to the inner section of the roll outer shell, show the significant effects of the liquidus temperature in both extremes of the layer thickness, and that of the Si content and the inoculation with FeTi in the inner section adjacent to roll core. The joint effect of the second order interactions, defined by AD + CG + EF, i.e., the liquidus temperature with the layer weight; the type of centrifugal casting machine with the A-type FeSi inoculation; and, the silicon content with FeTi inoculation for the layer section adjacent to the mould is analyzed in Table 8b. Similarly, Table 8c shows the analysis of the joint effect of the second order interactions given by AB + CE + FG and BC + AG + DF. The former second-order interaction corresponds to the liquidus temperature with the SiCaMn inoculation; the type of centrifugal casting machine with the silicon content; and, FeTi inoculation and A-type FeSi inoculation. The latter corresponds to SiCaMn inoculation with the type of centrifugal casting machine; the liquidus temperature with A-type FeSi inoculation; and, the layer weight with FeTi inoculation. Fig. 4b shows one effect that detaches from the straight line and at the same time, seen in conjunction with the effects close to it, shows a tendency to approach the straight line from the left, i.e., to the mean equal zero. This effect is then considered as to have no significant effect. This is so because in samples of reduced size following a normal law, the extremes tend to be over weighted. Then an effect appearing off the straight line but approaching the mean zero is considered as non-significant. In summary, the interactions reveal that a heavy layer weight in combination with low liquidus temperature melts enhance the development of graphite with elongated geometries capable of providing higher heat conductivities in the layer, thus enhancing the thermal shock resistance.
7.
Conclusions
Table 9 summarizes the results obtained in the DOE and presents the significant factors and factor interactions. It also presents the levels that define an increase in the studied responses as indicated hereinafter: - A decrease of the volume fraction of graphite can be achieved with melts of low equivalent carbon contents, based on compositions of low Si content (∼1 wt.% or less). To reduce the VV (C(g)) the layer thickness should be minimized for a given geometry. These results are pointed in relation to the mottled nature of the Nihard cast iron studied. - The factors found to have a significant effect on the graphite count number are consistent with the physical metallurgy of grey cast iron. If the goal is to increase the number of counts of the graphite phase, melts with high equivalent C and high Si content should be poured. Alternatively, the inoculation with A-type FeSi at a level of 2.2 kg/T promotes intensive
nucleation of this phase. SiCaMn inoculation showed no significant effect on the increase of graphite counts at the levels being tested in the DOE, and it seems logical that the amount added should be increased above 0.8 kg/T to gain benefits from its power as heterogeneous nucleant of graphite. - The addition of FeTi as a bath inoculant decreased the graphite count number in both the outer and inner sections of the roll shell. FeTi’s main contribution, at a level of 0.1 kg/T added to the ladle, was to promote the formation of graphite with an elongated shape, while it did not affect the volume fraction of graphite—despite being a carbide forming element. - It was found that the periphery of the rolls with thicker layers, i.e., with higher weights being poured for a similar roll geometry, in combination with low liquidus temperature melts, promoted the development of graphite with more elongated shapes.
Acknowledgements The authors wish to thank the University of Oviedo ViceRectorade for Research for providing economic support to carry out the correction of the grammar and syntax of the first draft manuscript. We likewise want to thank Eng. Dipl. ´ Nodular, S.A., for Celedonio Rodriguez, manager of Fundicion having granted his permission to publish the results derived from the research work developed within the Materials Pro research group.
references
Asensio, J., Pero-Sanz, J.A., Verdeja, J.I., 2003. Microstructure selection criteria for cast irons with more than 10 wt.% chromium for wear applications. Mater. Charact. 49, 83–93. ´ Asensio-Lozano, J., Alvarez-Antol´ ın, J.F., 2007. Saturated fractional design of experiments: toughness and graphite phase optimizing in Nihard cast irons. J. Mater. Eng. Perform., doi:10.1007/s1165-007-9138-8. British Rollmakers (China) Ltd., 2007. Alloy, Alloy indefinite chill (AIC) in Finishing Stand Rolls, Hong kong (China), http://www.brchina.com/about brc.htm. Box, G.E.P., Hunter, W.G., Hunter, J.S., 2002a. Concepto de ´ de un diseno. ˜ In: Estad´ıstica pa ra Investigadores. resolucion ´ Reverte´ Ediciones S.A., Mexico (pp. 396–401, in Spanish). ´ Box, G.E.P., Hunter, W.G., Hunter, J.S., 2002b. Apendice 10A-Algoritmo de Yates. In: Estad´ıstica para Investigadores. ´ Reverte´ Ediciones S.A., Mexico (pp. 355–360, in Spanish). Chatfield, C., 1991. The Design and Analysis of Experiments. 2. Factorial Experiments Statistics for Technology—A Course in Applied Statistics. Chapman and Hall, Bristol, pp. 257–287. Karsay, S.A.I., 1994. Elements Affecting Graphite Formation Ductile Iron Production Practices. American Foundrymen’s Society Inc., Des Plaines (IL), pp. 78–88. Nycen, B.T.C., Adams, T.P., 2000. Cast iron indefinite roll produced by the addition of Nb. US Patent 6,013,141, US. Pero-Sanz Elorz, J.A., 1994a. Fundiciones Ni-hard. In: Fundiciones ´ Ferreas. Dossat S.A., Madrid, pp. 124–129 (in Spanish). Pero-Sanz Elorz, J.A., 1994b. Propiedades generales de las ´ y grafito fundiciones grises no aleadas (o de baja aleacion) ´ laminar. In: Fundiciones Ferreas. Dossat S.A., Madrid, pp. 49–69 (in Spanish).
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´ A., Tort-Martorell Llabres, ´ X., Grima Cintas, P., Prat Bartes, ´ ´ de disenos ˜ Pozueta Fernandez, L., 1997. Construccion ´ fraccionales y calculo de las confusiones introducidas. In: ´ Metodos Estad´ısticos. UPC, pp. 170–172 (in Spanish). The Open University, 1990. Casting in insulating moulds. In: Edwards, L., Endean, M. (Eds.), Manufacturing with
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Materials–Materials in Action Series. The Open University, London, pp. 100–101. Vander Voort, G.F., 1984. Quantitative Microscopy Metallography Principles and Practice. McGraw-Hill Inc., New York, pp. 410–502.
journal homepage: www.elsevier.com/locate/jmatprotec
Identification and quantification of active manufacturing factors for graphite formation in centrifugally cast Nihard cast irons ´ J. Asensio-Lozano a,∗ , J.F. Alvarez-Antol´ ın b , G.F. Vander Voort c a
´ Departamento de Ciencia de los Materiales e Ingenier´ıa Metalurgica, Escuela T´ecnica Superior de Ingenieros de Minas, ˜ Spain Universidad de Oviedo, c/Independencia 13, E-33004 Oviedo, Asturias, Espana, b Departamento de Ciencia de los Materiales e Ingenier´ıa Metalurgica, ´ Escuela Universitaria de Ingenier´ıa T´ecnica Industrial, ´ Asturias, Espana, ˜ Spain Universidad de Oviedo, Campus Universitario, E-33203 Gijon, c Buehler, Ltd., 41 Waukegan Road, Lake Bluff, IL 60044, USA
a r t i c l e
i n f o
a b s t r a c t
Article history:
The present research arises from the need to gain knowledge to control the precipitation
Received 30 May 2007
of interdendritic dispersed graphite in mottled Nihard cast irons used in the outer ring
Received in revised form
layer of industrial duplex work rolls. Industry desires a volume fraction of 2.5–4% graphite,
24 November 2007
distributed homogeneously across the layer thickness, with a reasonably constant, high
Accepted 4 December 2007
number of particles across the layer, with a predominantly short flake-graphite shape. The objective was to ascertain the role of metallurgical manufacturing parameters and their interaction with graphite precipitation in the layer, and in the outer and inner sections of
Keywords:
the roll collar. These rolls, used in the finishing stands of hot strip steel mills, were processed
Fractional Design of Experiments
by a double pour method using vertical centrifugal casting machines. Analysis was conducted using a fractional Design of Experiments (DOE) program consist-
(DOE) Graphite
ing of 16 industrial trials based on the study of 7 manufacturing factors at 2 levels. The
Nihard cast iron
analyzed responses, obtained by quantitative metallographic techniques, were: the volume
Alloy indefinite chill
fraction, VV , number per unit area, NA , and the graphite morphology across the layer thick-
Quantitative metallography
ness. The results confirmed that an increase from 0.5 to 0.8 kg/T of SiCaMn added as a bath
Centrifugal casting
inoculant in the ladle increased the fraction of precipitated graphite without affecting NA .
Duplex rolls
An outstanding result was that the addition of 1 kg/T of FeTi as a bath inoculant did not modify the amount of precipitated graphite but significantly affected the morphology, favouring the development of elongated graphite shapes. © 2007 Elsevier B.V. All rights reserved.
1.
Introduction
The purpose of this research was to development knowledge regarding the influence of metallurgical manufacturing parameters, and their interactions, on the occurrence of the graphite phase, in the outer ring layer of alloy indefinite chill
∗
Corresponding author. Tel.: +34 985104302; fax: +34 985104242. E-mail address: [email protected] (J. Asensio-Lozano). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.12.015
(AIC) work rolls for the steel rolling industry, used in the last stands of hot strip mills. The work was a collaboration between a Spanish roll manufacturer and the Materials-Pro research group of the Materials Science and Metallurgical Engineering Department, Oviedo University, Spain. The industrial scale study was performed
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 6 ( 2 0 0 8 ) 202–215
Table 1 – Chemical composition range of the outer ring layers expressed in weight percent C 3.2–3.3
Mn
Si
S
P
Cr
Ni
0.9–1.0
0.9–1.0
<0.015
<0.035
1.7–1.8
4.2–4.4
Mo 0.25
on duplex work rolls with compositions falling within the specification range (Table 1). The rolls were processed by the double pour method in vertical centrifugal casting machines. The duplex-type rolls consist of a peripheral outer layer and a core of differing compositions, processed consecutively by vertical centrifugal casting. The outer ring layer is a hypoeutectic white cast iron with interdendritic graphite embedded in the ledeburitic matrix constituent, in which the austenite becomes partially transformed into martensite (Pero-Sanz Elorz, 1994a; Asensio et al., 2003). The roll core contains graphite in either flake, vermicular or spheroidal morphology within a matrix composed of homogeneously distributed ferrite and pearlite (British Rollmakers, 2007). Graphite in the roll shell greatly improves the ability of the roll to withstand the thermal shock associated with hot rolling steel strip, decreases friction between the roll and the strip, and greatly reduces the potential for fusing of the strip to the roll (Nycen and Adams, 2000). On the negative side, however, graphite reduces hardness and lowers wear resistance, which shortens the usable life between regrinds compared with other roll types, such as HSS steel types and high Cr cast irons. The goal of this work was to investigate the influence of the aforementioned manufacturing parameters, both in the entire layer and at two locations in the shell layers with different solidification conditions: the outer part of the ring layer in contact with the die wall and the inner portion of the ring in contact with the roll core. By controlling the morphology of the precipitated graphite, it is possible to optimize thermal shock spalling resistance of the roll layer, and thus an increase of its service life, benefiting from the excellent heat diffusiv´ ity of this constituent (Asensio-Lozano and Alvarez-Antol´ ın, 2007; Pero-Sanz-Elorz, 1994b). Analysis was performed with a fractional Design of Experiments (DOE) program consisting of 16 industrial trials based on the study of 7 manufacturing factors at 2 levels. Fractional Factorial DOE allows a high number of factors to be studied with a considerably lower number of experiments executed in comparison to Complete Factorial DOEs. The loss of information derived from the possible interactions among the chosen factors, known as “confusion of the factors”, is assumed on conducting a fractional DOE. This variant is often employed where the unknown possible interactions are thought to have ´ et al., 1997). The facminimal or no influence (Prat Bartes tors studied in the former DOE included, among others, the active variables found to have a significant effect on the development of C(g) in a first saturated DOE (Asensio-Lozano and ´ Alvarez-Antol´ ın, 2007). It was designed to the highest fractioning degree possible for 7 factors resulting in 8 experiments. Thus, the first set of variables in the new study was: the total Si percent after inoculation, the liquidus temperature, and the amount of inoculation added either as SiCaMn and FeSi. The new factors incorporated in this study were: the centrifugal cast machine employed either with isolating sleeve or without
203
it, which due to intrinsic characteristics influences the solidification rate; the weight of the outer ring layer being poured, equivalent to the layer thickness in case of rolls with similar barrel length (∼2000 mm) and roll diameter (∼700 mm) as was the case; and, finally the inoculation with FeTi. The responses being measured were the volume fraction, number per unit area, and the morphology of the graphite phase across the layer thickness by means of quantitative metallographic techniques using digital images from the light optical microscope. k−p Fractional designs are usually denoted by 2N , where 2 represents the number of levels studied for every factor, k represents the number of factors or metallurgical variables to study, p is the degree of fractioning, and N represents the degree of resolution. The present DOE can be represented by 27−3 IV , yielding a total of 16 experiments. The degree of resolution in this DOE is 4, which indicates the degree of confusion that is present in the estimation of the factors. Therefore, for the present analysis it is possible to estimate the principle effects, as well as the second-order interactions (Box et al., 2002a). There are many publications regarding precipitation of graphite in grey cast irons. However, the originality of this study resides in the investigation of the metallurgical manufacturing factors affecting precipitation of graphite in white cast irons without a definite chill zone; that is, in white cast irons in which graphite occurs by a critical balance between alloying elements such as C, Ni and Si which promote the formation of graphite, and carbide forming elements such as Cr, Mn and Mo. The present analysis determined the metallurgical variables and their interactions in the shells of the AIC rolls that have a significant influence on the graphite formation. The main conclusions of this study arose from the addition of SiCaMn and FeTi added as bath inoculants. An increase in the amount of SiCaMn added from 0.5 to 0.8 kg/T raised the amount of graphite without altering the count number. Inoculation of the bath with 1 kg/T of FeTi, although it did not modify the graphite volume fraction (despite its marked carbide forming nature), significantly altered the graphite number, lowering the final value at the two opposite sections of the roll layer studied. FeTi is known to modify the morphology of graphite nodules to produce vermicular graphite in grey cast irons. In this investigation conducted on mottled Nihard cast irons, FeTi inoculation promoted the development of elongated graphite shapes.
2.
Experimental procedure
The compositional range used for obtaining the desired outer layer is indicated in Table 1; the type, chemistry and weight of the different additions added as bath inoculants are shown in Table 2. The manufacturing method consisted of a melting stage conducted in medium frequency induction furnaces followed by stream-ladle treatment with bath inoculants. The pouring temperature of the melt from the furnace to the ladle was 60 ◦ C above the casting temperature of the layer, Tcast . The usual value for Tcast was ∼1320 ◦ C. After ladle inoculation, the melt was poured into the rotating mould through a ceramic hollow duct with three orifices at its lower end that project the
204
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Table 2 – Chemical composition of inoculants expressed in weight percent with an indication of the regular amounts added to the treatment ladle expressed in kg/T Inoculants
Base chemistry Si
A-type FeSia B-type FeSi FeMn SiCaMn FeTia a
75.0 75.4 2.0 58.3 0.39
Ca 2.5 0.5 – 16.4 –
Amount added
Al
Zr
Ti
C
S
P
1.4 1.0 – 1.1 3.45
1.6 – – – –
– 0.031 – 0.030 74.33
– 0.2 5.8 0.6 0.19
– 0.001 0.014 0.030 0.006
– 0.012 0.130 0.030 0.001
Mn – – 69.4 14.8 –
Fe rem. rem. rem. 7.4 rem.
1.6–2.2 0.6 0.8 0.36 0–1
Both A-type FeSi and FeTi were added as bath inoculants in variable amounts depending on the chosen levels for the studied factors included in the DOE.
melt into the metallic mould corresponding to the roll barrel. A protective granular flux was drawn by air suction into the mould cavity, impacting the rotating metallic shell plus layer melt. The flux melts and helps to prevent oxidation of the inner liquid free surface in contact with air. Once the specified amount of Nihard cast iron was poured, the casting of the roll core melt began with a small part of the outer layer still in the mushy state, the composition of which is designed to produce an SG cast iron. After core casting, the mould with its content were gently moved to the cooling bay and allowed to complete solidification and the subsequent solid-state transformations by cooling to RT + 50/70 ◦ C. This took place over a period of 4–5 days, and only then was the roll removed from its metallic mould, to avoid crack formation on cooling due to premature stripping. Table 3 shows the factors and chosen levels for the present DOE. It is worth noting that factor C refers to the type of centrifugal cast machine used, out of the two centrifugal cast machines available at the factory where the research was conducted. The main difference between these machines is the presence of a movable insulating sleeve placed around the mould in centrifugal cast machine No. 2, promoting slower solidification rates in the outer ring roll metal due to the lower heat diffusivity (The Open University, 1990). Factors F and G are, respectively, the amount of inoculation with FeTi and Atype FeSi added to the transfer ladle prior to pouring the melt into the mould. It was decided at this stage of the research to conduct a fractional Design of Experiments (DOE) consisting of 16 industrial trials based on the study of 7 manufacturing factors at 2 levels. Table 4a presents the DOE matrix, while Table 4b shows the generators, effects and confusions.
Table 3 – Factors and level description for DOE Code
Metallurgical parameter correspondence factors
Level (−1)
A B C
Liquidus temperature SiCaMn Centrifugal cast machine
1235–1240 ◦ C 0.5 kg/T No. 1
D E F G
Ring roll layer weight Si% FeTi A-type ferrosilicon
2300–2600 kg 1.0–1.1 None 1.6 kg/T
Level (+1) 1250–1255 ◦ C 0.8 kg/T No. 2 (with isolating sleeve) 3000–3300 kg 1.1–1.2 1 kg/T 2.2 kg/T
Table 4a – (27−3 IV ) array for DOE Number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
A
B
C
D
E
F
−1 +1 −1 +1 −1 +1 −1 +1 −1 +1 −1 +1 −1 +1 −1 +1
−1 −1 +1 +1 −1 −1 +1 +1 −1 −1 +1 +1 −1 −1 +1 +1
−1 −1 −1 −1 +1 +1 +1 +1 −1 −1 −1 −1 +1 +1 +1 +1
−1 −1 −1 −1 −1 −1 −1 −1 +1 +1 +1 +1 +1 +1 +1 +1
−1 +1 +1 −1 +1 −1 −1 +1 −1 +1 +1 −1 +1 −1 −1 +1
−1 −1 +1 +1 +1 +1 −1 −1 +1 +1 −1 −1 –1 −1 +1 +1
G −1 +1 −1 +1 +1 −1 +1 −1 +1 −1 +1 −1 −1 +1 −1 +1
Table 4b – Generators and confusions in the (27−3 IV ) matrix consisting of 16 experiments and 7 factors Generators
Confusions
E = ABC F = BCD G = ACD
A B C D E F G AB+CE + FG AC + BE + DG AD + CG + EF AF + BG + AG BC + AE + DF BD + CF + EG CD + BF + AG ABD
The experimental procedure that describes the metallographic techniques leading to the measured responses specified above is summarized as follows. First, samples were cut from the upper part of the roll barrel, corresponding to the position while the role was kept vertical in the centrifugal cast machine. The cutting process was conducted using an abrasive disc cutting machine. The dimensions of the specimens for metallographic inspection were approximately
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 6 ( 2 0 0 8 ) 202–215
205
Fig. 1 – Selected light optical micrographs of the mid section or working zone (Zone II and/or Zone III), of the ring roll layers for each experiment in the DOE analysis: (1)–(16). Microstructures presented in the as-polished state.
206
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 6 ( 2 0 0 8 ) 202–215
Fig. 1 – (Continued ).
3.23 0.89 1.00 0.015 0.027 1.81 4.32 0.30 1.6 1 1 0.8 1231 1343 3200 2 3.01 0.91 1.03 0.012 0.030 1.81 4.32 0.25 2.2 0.8 0 0.5 1249 1361 3020 2 3.1 0.89 1.16 0.011 0.025 1.78 4.29 0.25 1.6 1.6 0 0.5 1235 1348 3020 2 3.02 0.91 0.98 0.008 0.021 1.75 4.13 0.26 1.6 1 0 0.8 1248 1353 3260 1 3.14 0.91 1.17 0.010 0.020 1.79 4.27 0.27 2.2 0.8 0 0.8 1233 1334 3360 1 2.99 0.87 1.11 0.011 0.022 1.76 4.31 0.25 1.6 1.6 1 0.5 1252 1350 3360 1 3.21 0.90 0.98 0.014 0.023 1.80 4.36 0.29 2.2 0.8 1 0.5 1241 1332 3100 1 3.02 0.92 1.16 0.013 0.034 1.78 4.30 0.28 1.6 1.6 0 0.8 1247 1352 2340 2 3.17 0.93 1.10 0.010 0.022 1.818 4.23 0.28 2.2 0.8 0 0.8 1237 1348 2660 2 3.11 0.89 0.96 0.020 0.030 1.78 4.30 0.28 1.6 1 1 0.5 1248 1359 2660 2 3.14 0.91 1.16 0.009 0.022 1.80 4.32 0.26 2.2 0.8 1 0.5 1235 1347 2660 2 3.01 0.89 1.01 0.011 0.035 1.76 4.25 0.28 2.2 0.8 1 0.8 1245 1342 2380 1 3.15 0.92 1.20 0.020 0.040 1.74 4.07 0.35 1.6 1.6 1 0.8 1238 1344 2380 1 3.04 0.91 1.18 0.010 0.020 1.79 4.35 0.26 2.2 0.8 0 0.5 1248 1350 2340 1 % % % % % % % % kg/T kg/T kg/T kg/T ◦ C ◦ C kg No. C Mn Si S P Cr Ni Mo Type A-FeSi Type B-FeSi FeTi SiCaMn Liquidus temperature Casting temperature Outer layer weight Centrifugal cast machine
3.17 0.90 1.08 0.014 0.030 1.81 4.37 0.27 1.6 1 0 0.5 1237 1338 2340 1
9 8 5 4 3 2 1
Representative micrographs corresponding to the as-polished state at the mid section or work zone (Zones II and III), in the outer shell for each experiment are presented in Fig. 1. The casting parameters for each of the 16 experiments are shown in Table 5. The results of the analysis are presented in Tables 6–8c Tables 6-8; responses, effects and significant second-order or higher interactions are also presented. Figs. 2–4 illustrate the standardized effects, i.e., the ratio of the effect to its standard deviation, presented using normal probability plots in which the significant effects are noted with their corresponding identifying letters. Presentation of data with a normal distribution using a normal probability plot will reveal an alignment of the data points. The straight line so defined should pass through the point of coordinates (0, 50%) for the standardized effects and probability percent, respectively, thus indicating that the mean variability of the distribution is zero. The slope of the line represents the magnitude of experimental error. Every experimental response is subjected to random variations. This variation follows a normal law, and its standard deviation represents the experimental error. The effects are linear combinations of the responses; hence, by application
Units
Results and discussion
Casting parameters
3.
Table 5 – Casting parameters measured for each experiment in the DOE
1. The volume fraction of graphite, denoted by VV . 2. The number of graphite particles per unit area, NA , represented by #/mm2 . 3. The Feret quotient expressed by: FMAX /FMIN , where FMAX is the maximum Feret diameter and FMIN is the minimum Feret diameter.
6
7
Experiment number
10
11
12
13
14
15
16
15 mm × 15 mm × 40 mm; the major length coinciding with the layer thickness in the direction of the roll radius. Mechanical preparation of the specimens for metallographic inspection was conducted by the classical grinding techniques using 60–120–240 and 600 grit SiC papers. The specimens were then polished in two consecutive stages, first with 6 m and then with 1 m oil-based diamond paste. For the sake of simplicity in the analysis of the results, the samples were divided into four zones comprised between the periphery and the ring-core roll interface. Of particular interest in this study are the results in Zone I, corresponding to the outer section of the ring roll in contact with the die and in Zone IV, corresponding to the inner section of the roll layer in contact with the roll core. Five micrographs were randomly chosen for each of the four zones in the as-polished state, totalizing 20 micrographs per roll. Micrographs were taken with an Olympus PMG3 light optical microscope (LOM) connected to an OmniMet® Enterprise device for electronic image data acquisition and archiving. The quantitative metallographic assessment of the graphite phase was performed with Image ProPlus (version 4.5.0.29) coupled to a Materials-Pro module imaging software. In order to conduct the DOE analysis and the derivation of the significant effects, the commercial statistics software package Statgraphics Plus (version 5.1) was employed. The responses studied in this DOE were (Vander Voort, 1984; Box et al., 2002b):
3.03 0.89 1.13 0.011 0.023 1.77 4.12 0.25 2.2 0.8 1 0.8 1246 1347 3200 2
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207
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Table 6a – Calculation of the effects by the application of Yates’ algorithm for the volume fraction of interdendritic graphite in Zone I (periphery), Zone IV (inner layer portion) and layer average Experiment number
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Vv (graphite) (vol.%) Zone I
Effect
Zone IV
Effect
Layer average
Effect
3.42 4.53 4.25 4.82 4.58 0.85 4.98 4.94 5.73 5.11 7.02 5.25 6.87 5.43 8.12 4.74
5.04 −1.16 0.95 0.01 0.05 −0.99 0.31 0.43 1.99 −0.64 −0.45 −0.78 0.47 0.38 −0.53 −0.63
7.19 6.14 7.52 6.09 6.39 1.04 5.49 5.01 7.4 7.15 9.57 6.95 9.75 6.95 7.16 6.21
6.63 −1.87 0.25 0.50 −1.25 −0.53 −0.31 1.18 2.03 0.21 −0.59 −0.63 1.00 0.31 −1.01 −0.13
4.27 5.32 6.36 5.49 5.91 2.08 5.7 4.81 5.71 6.14 8.14 5.37 8.04 6.11 8.01 5.66
5.82 −1.40 0.75 −0.33 −0.06 −0.86 −0.24 0.96 1.66 −0.26 −0.45 −0.58 0.68 0.37 −0.30 −0.26
Identification Mean A B AB C AC BC E D AD BD ABD CD G F AF
Table 6b – Joint significant effects given by second-order interactions, resulting from the confusions AC + BE + DG and AG + BF + CD for the average volume fraction of graphite in the layer AC + BE + DG
A
C
−1 +1
−1
+1
6.1 5.6
6.9* 4.7
AC + BE + DG
B
E
−1 +1
−1
+1
4.5 5.6
6.4* 6.2
AC + BE + DG
D
G
−1 +1
−1
+1
5.0 6.9*
5.6 6.4
CD + BF + AG
C
D
−1 +1
−1
+1
5.4 4.6
6.3 7.0*
CD + BF + AG
B
F
−1 +1
−1
+1
5.9 6.0
6.0 6.4*
CD + BF + AG
G −1
A
−1 +1
+1 *
6.7 4.6
6.4 5.6
The effects marked with an asterisk indicate the most probable relation between interrelated factors that generate the highest percentage in volume of graphite.
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Table 7 – Calculation of the effects by the application of Yates’ algorithm for the count number of graphite particles per square millimeter in Zone I (periphery), Zone IV (inner layer portion) and layer average Number of graphite/mm2
Experiment number Zone I 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
84 160 91 72 103 16 108 155 165 84 319 99 188 100 174 98
Effect 126 −56 27 −11 −17 5 5 48 55 −60 11 −21 −10 29 −51 −10
Zone IV
Effect
226 183 211 121 227 30 247 133 209 111 262 84 227 162 142 177
of the central limit theorem, they follow a normal law. If all the effects were non-significant, they would follow an N (0,) distribution, thus appearing aligned describing a straight line when represented on a normal probability plot. The significant effects follow an N (,) distribution, represented by points which are off the straight line defined by the non-significant effects. The effects which are potentially significant tend to accumulate at both ends of the straight line. Those effects which are not aligned with the straight line and also become apart from the mean zero are thus significant. As a result, each effect can be considered as a random variable; the value so determined is an estimate of its arithmetic mean. Accordingly, this value is accompanied by its standard deviation. The significance of a given effect is based on the ratio of its mean value to its error, using the Student-t statistic at the 95% confidence level (Chatfield, 1991).
Layer average
172 −94 0 7 −8 9 13 39 −1 17 −11 −2 18 53 −37 6
116 163 191 118 168 31 156 124 154 82 288 95 208 104 180 126
Effect 144 −77 32 −11 −14 −4 −12 49 21 −28 4 −7 14 31 −26 −7
Identification Mean A B AB C AC BC E D AD BD ABD CD G F AF
4. Significant effects in the graphite volume fraction Table 6 shows the calculated effects after applying Yates’ algorithm to the volume fraction of interdendritic graphite in Zone I (periphery), Zone IV (inner layer portion) and the layer average.
4.1.
Layer average
Fig. 2a illustrates of the significant factors influencing the graphite volume fraction in the layer average. The values so obtained have low experimental error considering the steepest slope of the straight line where the non-significant effects are aligned. These are the liquidus temperature; the weight of the outer roll layer, which is proportional to its thickness and
Table 8a – Calculation of the effects by the application of Yates’ algorithm for the quotient of Feret diameters FMAX /FMIN in Zone I (periphery), Zone IV (inner layer portion) and layer average Experiment number
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
FMAX /FMIN Zone I
Effect
1.83 2.09 2.25 2.02 2.30 1.94 2.10 2.14 2.36 2.02 2.36 1.83 2.09 1.86 2.38 2.21
2.11 −0.20 0.10 −0.03 0.03 0.02 0.06 0.14 0.05 −0.12 0.01 −0.01 −0.04 0.10 0.15 −0.08
Zone IV 1.93 2.18 3.21 2.70 3.90 2.26 1.80 2.31 3.26 2.09 3.16 1.93 2.87 1.99 2.53 2.98
Effect 2.57 −0.53 0.02 0.33 0.02 0.14 −0.37 0.54 0.07 −0.18 0.08 −0.02 −0.04 0.36 0.60 −0.19
Layer average 1.89 2.16 2.83 2.47 2.81 2.09 2.04 2.15 2.51 2.06 2.61 1.89 2.48 2.02 2.52 2.47
Effect 2.31 −0.30 0.12 0.04 0.02 0.02 −0.18 0.27 0.02 −0.12 −0.02 −0.01 0.09 0.15 0.32 −0.10
Identification Mean A B AB C AC BC E D AD BD ABD CD G F AF
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Table 8b – Joint significant effects given by second-order interactions, resulting from the confusions AD + CG + EF for the quotient of Feret diameters in the layer section adjacent to the mould wall AD + CG + EF
A
D
−1 +1
−1
+1
2.1 2.0
2.3* 2.0
AD + CG + EF
C
G
−1 +1
−1
+1
2.0 2.1
2.2* 2.1
AD + CG + EF
E
F
−1 +1
–1
+1
1.9 2.2
2.2 2.2
The effects marked with an asterisk indicate the most probable relation between interrelated factors that generate the most elongated shape of graphite.
Table 8c – Joint significant effects given by second-order interactions, resulting from the confusions BC + AE + DF and AB + CE + FG for the quotient of Feret diameters in the layer section adjacent to the roll core BC + AE + DF
B
C
−1 +1
−1
+1
2.4 2.8*
2.8* 2.4
BC + AE + DF
A
E
−1 +1
−1
+1
2.6 2.1
3.0* 2.5
BC + AE + DF
D
F
−1 +1
−1
+1
2.1 2.5
2.8 3.1*
AB + CE + FG
A
B
−1 +1
−1
+1
3.0* 2.1
2.7 2.5
AB + CE + FG
C
E
−1 +1
−1
+1
2.5 2.1
2.7 3.0*
AB + CE + FG
F
G
−1 +1
−1
+1
2.3 2.5
2.3 3.2*
The effects marked with an asterisk indicate the most probable relation between interrelated factors that generate the most elongated shape of graphite.
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Fig. 2 – (a) Normal probability plot for the volume fraction of graphite (%) in the layer average. (b) Normal probability plot for the volume fraction of graphite (%) in the periphery of the layer. (c) Normal probability plot for the volume fraction of graphite (%) in the periphery in the layer section adjacent to the roll core.
Fig. 3 – (a) Normal probability plot for the graphite count number per square millimetre in the layer average. (b) Normal probability plot for the graphite count number per square millimetre in the periphery of the layer. (c) Normal probability plot for the graphite count number per square millimetre in the periphery in the layer section adjacent to the roll core.
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therefore conditions the heat released during solidification; the percentage of silicon; and inoculation with SiCaMn. The weight of the layer, the Si weight percentage, the amount of SiCaMn added, and the interactions of two factors given by AG + DF + CD appear detached to the right of the straight line, withdrawing from the mean zero. On the other hand the liquidus temperature, and the second order interactions given by AC + BE + DG have a tendency to detach to the left of the straight line, again separating from the mean zero. Table 6b shows the significant joint interactions of 2 factors found to have influence on the graphite volume fraction in the layer average: AC + BE + DG and CD + BF + AG. The first one, given by AC + BE + DG, corresponds to the interaction of the liquidus temperature with the type of centrifugal casting machine; the interaction of the SiCaMn inoculation with the percentage of silicon; and, the interaction of the layer weight with the A-type FeSi inoculation. On the other hand, the joint interactions CD + BF + AG, translates into: the interaction of the type of centrifugal casting machine with the layer weight; the interaction of SiCaMn inoculation with the addition of FeTi; and, the interaction of the liquidus temperature with A-type FeSi inoculation. In summary, the metallurgical parameters contributing to an increase in the graphite volume fraction over the layer are: an increase of the Si content, the use of low liquidus temperature melts and the addition of SiCaMn inoculant at the highest level chosen. The former results are consistent with the physical metallurgy grounds for stable solidification of hypoeutectic compositions during the eutectic reaction in grey cast irons. In effect, they all contribute to increase the equivalent carbon and thus enhance the development of graphite. The layer weight and, therefore, its thickness also seem to have a key role, because the higher weights being poured allow for lower cooling rates during solidification and solid-state transformation of the roll shells. This might favour a reduction of the undercooling through the stable eutectic reaction, thus promoting a higher proportion of the liquid evolving through this reaction with the consequence of a higher proportion of graphite being formed. The results obtained from the interaction of the factors also showed that the use of isolating sleeves during solidification and the inoculation of the melt with A-type FeSi at the highest level studied could have an influence on the increase of the fraction of graphite precipitated. Because the aim of this research is to restrict the volume fraction of graphite to 4% or less, our strategy should be to control these metallurgical factors accordingly.
4.2.
Outer and inner sections of the roll collar
Fig. 2b shows that the only significant effect in the outer portion of the ring roll layer is the layer weight; increasing the weight favours a higher volume fraction of graphite being formed during solidification. Fig. 2c shows that an increase of the layer weight and a reduction of the liquidus temperature, are the significant effects that increase the proportion of graphite in the layer section adjacent to the roll core. It can also be observed that the slope of the straight line in Fig. 2c is somewhat smaller than those in Fig. 2b and c, thus indicating that the values so obtained have a higher experimental error.
The significant effects in controlling the amount of graphite formed in the most distant sections of the thickness in the ring roll, representing two extremes in the solidification of the layer, are, simply, the liquidus temperature and layer weight.
5. Significant effects in the number of graphite particles per unit area Table 7 shows the calculated effects after applying Yates’ algorithm for the graphite number per unit area in Zone I (periphery), Zone IV (inner layer portion) and in the layer average. Fig. 3a–c shows the significant factors influencing the total number of counts for the graphite phase.
5.1.
Layer average
In Fig. 3a, it can be seen the significant effects for the number of graphite particles in the layer average were found to be: the liquidus temperature; the layer weight; the Si content; and, inoculation with A-type FeSi. The percentage of Si, the inoculation of the melt with A-type FeSi, and the layer weight appear separated to the right of the straight line, thus withdrawing from the mean equal zero. In summary it is concluded that in order to obtain a high graphite NA , melts with low liquidus temperature and high Si content, should be poured. The addition of A-type FeSi at the highest level tested in this work, also increased the graphite counts. However, inoculation with SiCaMn revealed no significant effect on NA . A possible explanation for these results may be that casting low-liquidus temperature melts usually increases the number of graphite cells. Perhaps the addition of substantial inoculants counteracts the high depletion of homogeneously formed graphite nuclei being destroyed at the high casting temperatures demanded by industrial processing concerns. However, it is surprising that inoculating with SiCaMn at the levels added in the DOE did not enhance nucleation. It may be reasonable to increase the addition of SiCaMn beyond 0.8 kg/T to benefit from its inherent role as promoter of heterogeneous nucleation of graphite in the ladle.
5.2.
Outer and inner sections of the roll collar
Fig. 3b and c shows that a reduction in the amount of FeTi added as bath inoculant and the reduction of the melt liquidus temperature had a significant effect on increasing the graphite count number both in the outer and inner sections of the ring roll layer while it had no effect on the layer average. Indeed, FeTi is a shape-control element of graphite in grey cast irons, but it has never been reported to inhibit the graphite count. The present study showed the deleterious effect that Ti had when the objective is to increase the graphite counts.
6. Significant effects in the morphology of graphite Table 8a shows the calculated effects after applying Yates’ algorithm on the ratio of Feret’s diameters in Zone I (periphery), Zone IV (inner layer portion) and layer average. Fig. 4
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Fig. 4 – (a) Normal probability plot for the ratio of Feret diameters in the layer average. (b) Normal probability plot for the ratio of Feret diameters in the periphery of the layer. (c) Normal probability plot for the ratio of Feret diameters in the periphery in the layer section adjacent to the roll core.
depicts the significant factors influencing the graphite morphology given by the quotient of Feret diameters (FMAX /FMIN ).
6.1.
Layer average
The significant effects for the whole layer (Fig. 4a) are the liquidus temperature, the Si content, and FeTi inoculation. The percentage of silicon and the inoculation with FeTi are detached to the right of the straight line, thus separating from
the mean zero. Also, the liquidus temperature shows a tendency to separate from the same straight line to the left, also withdrawing from the mean zero. A decrease of the liquidus temperature and the elaboration of melts with high Si contents have been found to increase the development of elongated graphite morphologies in these mottled Nihard irons, i.e., with high FMAX /FMIN ratios. The addition of FeTi also promotes the development of elongated graphite geometries a result that seems consistent with its
Table 9 – Significant factors and levels providing an increase in the selected responses Response
Code
Significant factors
Steepest ascent level
Metallurgical parameter investigated
Level
Corresponding value
Vv (%)
A B E AG AC BF CD
Liquidus temperature SiCaMn Si% Liquidus temperature/A-type FeSi Liquidus temperature/centrifugal cast machine SiCaMn/FeTi Centrifugal cast machine/ring roll layer weight
−1 +1 +1 −1/+1 −1/+1 +1/+1 +1/+1
1235–1240 ◦ C 0.8 kg/T 1.1–1.2 1235 – 1240 ◦ C/2.2 kg/T 1235–1240 ◦ C/No. 2 (with isolating sleeve) 0.8 kg/T/1.1 kg/T No. 2 (with isolating sleeve)/3000–3300 kg
#/mm2
A E G D
Liquidus temperature Si% A-type FeSi Ring roll layer weight
−1 +1 +1 +1
1235–1240 ◦ C 1.1–1.2 2.2 kg/T 3000–3300 kg
FMAX /FMIN
A E F
Liquidus temperature Si% FeTi
+1 +1 −1
1250–1255 ◦ C 1.1–1.2 None
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role in the formation of vermicular graphite for the production of compacted cast irons by conversion of nodular cast irons (Karsay, 1994).
6.2.
Outer and inner sections of the roll collar
Fig. 4b, corresponding to the periphery and Fig. 4c to the inner section of the roll outer shell, show the significant effects of the liquidus temperature in both extremes of the layer thickness, and that of the Si content and the inoculation with FeTi in the inner section adjacent to roll core. The joint effect of the second order interactions, defined by AD + CG + EF, i.e., the liquidus temperature with the layer weight; the type of centrifugal casting machine with the A-type FeSi inoculation; and, the silicon content with FeTi inoculation for the layer section adjacent to the mould is analyzed in Table 8b. Similarly, Table 8c shows the analysis of the joint effect of the second order interactions given by AB + CE + FG and BC + AG + DF. The former second-order interaction corresponds to the liquidus temperature with the SiCaMn inoculation; the type of centrifugal casting machine with the silicon content; and, FeTi inoculation and A-type FeSi inoculation. The latter corresponds to SiCaMn inoculation with the type of centrifugal casting machine; the liquidus temperature with A-type FeSi inoculation; and, the layer weight with FeTi inoculation. Fig. 4b shows one effect that detaches from the straight line and at the same time, seen in conjunction with the effects close to it, shows a tendency to approach the straight line from the left, i.e., to the mean equal zero. This effect is then considered as to have no significant effect. This is so because in samples of reduced size following a normal law, the extremes tend to be over weighted. Then an effect appearing off the straight line but approaching the mean zero is considered as non-significant. In summary, the interactions reveal that a heavy layer weight in combination with low liquidus temperature melts enhance the development of graphite with elongated geometries capable of providing higher heat conductivities in the layer, thus enhancing the thermal shock resistance.
7.
Conclusions
Table 9 summarizes the results obtained in the DOE and presents the significant factors and factor interactions. It also presents the levels that define an increase in the studied responses as indicated hereinafter: - A decrease of the volume fraction of graphite can be achieved with melts of low equivalent carbon contents, based on compositions of low Si content (∼1 wt.% or less). To reduce the VV (C(g)) the layer thickness should be minimized for a given geometry. These results are pointed in relation to the mottled nature of the Nihard cast iron studied. - The factors found to have a significant effect on the graphite count number are consistent with the physical metallurgy of grey cast iron. If the goal is to increase the number of counts of the graphite phase, melts with high equivalent C and high Si content should be poured. Alternatively, the inoculation with A-type FeSi at a level of 2.2 kg/T promotes intensive
nucleation of this phase. SiCaMn inoculation showed no significant effect on the increase of graphite counts at the levels being tested in the DOE, and it seems logical that the amount added should be increased above 0.8 kg/T to gain benefits from its power as heterogeneous nucleant of graphite. - The addition of FeTi as a bath inoculant decreased the graphite count number in both the outer and inner sections of the roll shell. FeTi’s main contribution, at a level of 0.1 kg/T added to the ladle, was to promote the formation of graphite with an elongated shape, while it did not affect the volume fraction of graphite—despite being a carbide forming element. - It was found that the periphery of the rolls with thicker layers, i.e., with higher weights being poured for a similar roll geometry, in combination with low liquidus temperature melts, promoted the development of graphite with more elongated shapes.
Acknowledgements The authors wish to thank the University of Oviedo ViceRectorade for Research for providing economic support to carry out the correction of the grammar and syntax of the first draft manuscript. We likewise want to thank Eng. Dipl. ´ Nodular, S.A., for Celedonio Rodriguez, manager of Fundicion having granted his permission to publish the results derived from the research work developed within the Materials Pro research group.
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