MATADOR SPECIAL ISSUE. Effect of Orientation, Thickness and Composition on Properties of Ductile Iron Castings

MATADOR SPECIAL ISSUE Effect of Orientation, Thickness and Composition on Properties of Ductile Iron Castings Vasudev D. Shinde 1, +, B. Ravi 2 and K. Narasimhan 3 1) PhD Research Scholar, Department of Metallurgical Engineering & Materials Science, Indian Institute of Technology Bombay, Mumbai-400076 (India) shinde@iitb.ac.in, vasu.metal@gmail.com + Corresponding author. Tel.:+912225764399; Fax: +912225726875; 2) Professor, Department of Mechanical engineering, Indian Institute of Technology Bombay, Mumbai-400076 (India) b.ravi@iitb.ac.in 3) Professor, Department of Metallurgical Engineering & Materials Science, Indian Institute of Technology Bombay, Mumbai-400076 (India) nara@iitb.ac.in Abstract In this work, the effects of casting orientation (horizontal, side and vertical), section thickness (4-16 mm) and composition (Cu, Mn) were investigated on the cooling rate, microstructure and mechanical properties (tensile strength, yield strength, elongation, hardness) of hypereutectic ductile iron castings. Overall, horizontal castings were found to cool faster than side and vertical oriented castings. Thermal analysis (using cooling curves) showed a wide difference among the four sections. Thinner sections exhibited significant undercooling and thereby carbide formation, leading to poor ductility. The combined effect of Cu and Mn showed an increase in amount of pearlite to 82% and nodularity to 94% along with a reduction in nodule count to 323 and amount of ferrite. Also, increased tensile strength (659 MPa) and hardness (264 BHN) were observed along with a drop in ductility to 2.5% in 4 mm thin section, which helps offset carbide formation. Thermal analysis was found to be a useful tool in understanding the combined effect of orientation, thickness variations and processing parameters. Keywords: Thin wall, ductile iron, solidification, inoculation, microstructure 1. Introduction: Ductile iron provides a range of mechanical properties, often comparable to steels, while having good castability [1]. It is therefore gradually replacing forged and welded components in automobiles [2]. Since castings now constitute a significant proportion of vehicle weight, manufacturers are increasingly redesigning the parts with thinner 1

walls to reduce the total weight [3]. The challenge is to obtain the desired combination and values of mechanical properties [4]. The properties of as-cast ductile iron are largely controlled by chemical composition, melt treatment and cooling rate [5]. Melt treatment includes the addition of magnesium alloy followed by inoculation to increase the nodule count and to suppress carbide formation [6]. The magnesium treatment eliminates oxide bifilms and produces multiple nuclei in the melt. Post inoculation treatment too is beneficial, especially in thin wall ductile iron castings, since it further increases the active number of nucleation sites. Graphite nucleates on these particles, and their further growth is controlled by austenite dendrites [7]. Austenite formed during solidification undergoes solid state transformation at eutectoid temperature, which modifies the solidified structure and leads to other complexities in solidification morphology [8]. Since early nucleation of graphite nodules helps prevent carbides in thin sections, ductile irons with hypereutectic composition are preferred for such castings [9]. However, due to higher cooling rates in thin wall ductile iron castings, sufficient pre-eutectic graphite nucleation is required [10]. Thin wall ductile iron castings require an optimal combination of composition and melt processing to be free of carbides that occur due to chilling. Hypereutectic ductile iron, with CE ranging from 4.45 to 4.9 %, is usually recommended for plates of thickness below 5 mm [11]. To avoid primary carbides in ductile iron, the eutectic temperature should be greater than 1140 o C [12]. Further, more homogenous microstructures are obtained by using multiple gates instead of risers [13]. The alloying elements such as Cu, Mn, Sn, Sb and Cr are known to increase tensile strength and hardness with subsequent decrease in ductility and impact energy. This is due to an increase in the amount of pearlite with subsequent decrease in ferrite. Even small changes in the amount of the above elements show significant increase or decrease in mechanical properties of ductile iron. Silicon is a strong solid solution strengthener; it reduces undercooling and avoids carbide formation by nucleating graphite. It increases volume fraction of ferrite and nodule count. Copper is a strong 2

pearlite promoter; its addition up to 1% converts ferritic structure into pearlitic [14]. Manganese increases hardness and strength by stabilizing pearlite but promotes carbides in heavy sections. It segregates at grain boundaries and thus increases hardenability [15]. Arsenic, tin and antimony promote pearlite and carbides, and are hence kept to lower limits; their effect can be counteracted by cerium additions [16]. The ratio of ferrite to pearlite in the matrix and the morphology of graphite decide the mechanical properties of ductile iron castings [17]. This depends upon the cooling rate during eutectoid transformation, nodule count and alloying elements [18]. The ferrite being softer gives higher ductility but lower tensile strength than pearlite. Also, the graphite morphology plays an important role; deviation from spheroidal shape reduces the ductility and impact properties [19]. The time span between spheroidal treatment and pouring has a significant effect on elongation, but less effect on the tensile strength and hardness of castings [20]. The cooling curve generated by inserting suitable thermocouples in the casting cavity reflects the effect of solidification variables such as chemical composition, inoculation and its effectiveness [21]. There is a high temperature drop in liquid metal due to heat transfer between flowing metal stream and mould walls [22]. The casting orientation affects the solidification behaviour, and thereby leads to variations in graphite nodules as well as deviation from nodularity. The top portions of the castings were observed to have a higher nodule count but lower values of nodularity compared to bottom portions [23]. Further, as solidification proceeds more rapidly in thin wall ductile iron castings, the feeding pattern in these section influences the final microstructure and thereby mechanical properties [24]. Previous work shows that for improving the strength of a casting, copper is an important constituent. Manganese is also used in the present work for strengthening the casting by promoting pearlitic matrix, but in limited amounts, since it can alter the structure by promoting carbides in different section thicknesses. An attempt is made to balance the strength and ductility of ductile iron by varying the amount of manganese and copper. Further, the effects of different section thickness and casting orientation on the 3

microstructure and properties of ductile iron castings, which have not been reported earlier, have been included in the present investigation. 2. Experimental work Experiments were designed and conducted to study the solidification behaviour in varying thickness ductile iron castings, as shown in Table 1. A step casting was designed with four sections having thickness 4, 8, 12 and 16 mm, respectively. Each step is 50 mm long, making the total length of casting 200 mm. The width of the casting is 100 mm, so as to avoid end freezing effects in all sections. Four gates were provided for rapid and uniform filling. Total four melt compositions with code A-D were used to pour a total of 12 castings. Initially, four castings with melt composition A were produced in vertical and side orientation (two AV and two AS). Another eight were produced in horizontal orientation, with melt composition A to D (two castings of each composition). The moulds were prepared in green sand using a wooden pattern of the step casting. The casting is moulded in drag box whereas runner and sprue were in cope. The gating systems for the vertical and horizontal orientation of the casting are shown Fig.1. The total mould height of vertical casting was 250 mm whereas in horizontal castings it was 200 mm. K-type thermocouples were inserted in the middle of each step to record the thermal history of casting solidification. A DAQ-3005 (MCC-USA) data logger for data acquisition synchronised with Desylab 12.0 software was used. The melt charge consisted of 50 kg pig iron, 150 kg cold rolled steel scrap and balance foundry returns with suitable chemical composition. The charge mix was melted in 300 kg capacity coreless medium frequency induction furnace in a production foundry. The molten metal was tapped into a preheated ladle containing Ferro-silicon- magnesium (FeSiMg) alloy granules of size 10-15 mm at the bottom covered with steel scrap (sandwich process). The tapping temperature was 1450 o C. 4

Table 1: Experimental castings and study parameters Casting code Casting orientation Composition Study parameters AV 1, AV 2 Vertical Trace Cu, 0.2% Mn Under-cooling and AS 1, AS 2 Side Trace Cu, 0.2% Mn delay in solidification AH 1, AH 2 Horizontal Trace Cu, 0.2% Mn BH 1, BH 2 Horizontal 0.2% Cu, 0.3% Mn Phase and variation CH 1, CH 2 Horizontal 0.4% Cu, 0.4% Mn in properties DH 1, DH 2 Horizontal 0.5% Cu, 0.5% Mn Fig. 1: Castings with gating system in (a) horizontal, (b) side and (c) vertical orientation The inoculant was added in the melt stream while transferring metal into pouring ladle of 50 kg capacity for proper mixing. Inoculant particles were of 6 to 10 mm in size so as to dissolve easily and dust free to avoid oxidation losses. The spectroscopic melt samples was taken just before pouring into the mould, and analysis was carried out using a spectrometer (BRUKER, model Q-4 Tasman). The treated iron was poured into mould cavity at a temperature of 1380 o C for all castings. Six experiments (two vertical, two side, and two horizontal castings) were conducted for studying the effect of casting orientation with the same chemical composition (A). The thermocouples were inserted in one casting of each orientation (AH2, AS2 and AV2). 5

Another six castings were poured to study the effect of chemical composition for a given orientation (horizontal). In all the melts (labelled A, B, C, and D), 3.6% carbon and 2.5-2.78% silicon gave a carbon equivalent of 4.44-4.57, which is in hyper-eutectic range. Cu varied from 0.035 to 0.512% and Mn varied from 0.216 to 0.518% as shown in Table 2. Other elements present in the melt were Pb<0.01, Al=0.006, Cr=0.015, Mo<0.002, Ni<0.002 and Ti=0.02. Table 2: Chemical analysis of the melts A, B, C and D Melt code C Si Cu Mn P S Mg A 3.62 2.51 0.035 0.216 0.005 0.011 0.035 B 3.63 2.69 0.214 0.310 0.004 0.009 0.034 C 3.68 2.68 0.401 0.392 0.004 0.010 0.032 D 3.61 2.78 0.512 0.518 0.006 0.010 0.039 3. Results The solidification temperature history recorded using thermocouples and stored in the data logger was used to plot the cooling curves. The cooling curves in the four sections (different thicknesses) of castings AV and AH are shown in Fig. 2. 6

Fig. 2: Cooling curve at the middle section (different thicknesses) of horizontal, side and vertical castings. 7

Fig. 2: Cooling curve at the middle section (different thicknesses) of horizontal, side and vertical castings. (new) Fig. 3: Cooling curve and its first derivative (cooling rate) in 8 mm side oriented casting The first derivative of the cooling rate indicates the evolution of cooling rate during solidification. Its interpretation can reveal microstructural information that can not be easily obtained from standard metallographic techniques. The typical cooling curve and cooling rate in 8 mm side oriented ductile iron casting are shown in Fig.3. The graphite nucleation starts at liquidus temperature (TL), followed by horizontal eutectic portion. 8

The cooling rate curve passes through zero indicating end of eutectic solidification (TEend), but due to presence of trace elements end of freezing (EOF) extends further. Key parameters of the cooling curves are shown in Table 3. The solidification temperature range is approximately 1170 o C to 1120 o C. It has been observed that the cooling rates during solidification in different sections range from 6.4 to 0.4 o C/s in horizontally oriented castings, 3.6 to 0.7 o C/s in side orientation and 2.6 to 1.1 o C/s in vertically oriented castings. In other words, horizontal castings have nearly twice the cooling rates of side oriented castings, owing to increased rate of heat transfer from the larger surfaces in horizontal orientation. A wide range of under-cooling was observed in horizontal (1-4 o C) and side (2-8 o C) castings due the presence of both thick and thin sections. The comparatively slower cooling rates in side oriented castings gave wider total solidification times. The difference in solidification times of horizontal, side and vertical orientated castings is found to increase with increasing casting wall thickness. Table 3: Thermal analysis of cooling curves in different sections of casting (Melt A) Thickness Cooling rate ( o C/s) Undercooling ( o C) Solidification time (sec) (mm) Hori Side Vert Hori Side Vert Hori Side Vert 4 1.8 1.0 5.00 18 16 0 40 57 20 8 0.8 0.4 1.42 11 7 10 70 95 50 12 0.7 0.3 0.83 6 9 0 120 150 60 16 0.5 0.2 1.43 5 7 0 145 180 70 Thickness Cooling rate ( o C/s) Undercooling ( o C) Solidification time (sec) (mm) Hori Side Vert Hori Side Vert Hori Side Vert 4 6.4 3.63 2.63 2 2 1 25 33 38 8 1.11 0.97 1.03 1 2 4 63 72 77 12 0.64 0.61 0.51 4 4 6 125 110 118 16 0.41 0.68 1.1 4 8 6 170 100 160 9

Fig. 4: Microphotographs of 12 mm section castings with composition A-D The samples for microstructure studies were taken from the middle portion of the casting and polished. These were etched with 2% Nital (2% concentric Nitric acid and 98 ml Methanol solution). Optical micrographs were taken using a camera attached to a Leintz microscope (Fig. 4). The polished samples were studied using an Image Analyzer (Pro-metal-11) for microstructural studies to compare fraction of pearlite content and nodule count in each casting sections; these images are shown in Fig.5. Tensile test specimens were prepared from each casting as per ASTM standard E8M-04. The Brinell hardness is measured on the samples taken from the middle portion of each casting. The average values (derived from two samples of each composition) of tensile and hardness are shown in Fig. 6. More undercooling that is observed in thin sections, indicates the possibility of carbides in these sections. This also indicates the failure of inoculation in generating a sufficient number of nucleating sites for graphite, resulting in 5-6% carbides observed in 4 mm thin sections. 10

Fig. 5: Values of (a) percentage pearlite and (b) nodule count in the castings Fig. 6: Values of (a) tensile strength and (b) hardness of the castings 4. Discussion The factors influencing the solidification process, such as metal composition (including trace elements), melt modification, nodularization treatment and inoculation influence the shape of the cooling curve too. The part of the cooling curve from the liquidus temperature to the end of eutectic solidification represents the solidification range. Two separate cooling curves can be compared in terms of their shape and temperature values. The thermal analysis of the four sections within the horizontal castings indicates faster cooling rate compared to corresponding sections in side oriented castings. Solidification time in horizontal castings ranged between 25-170 seconds for various thicknesses, compared to 33-110 seconds for side orientation. The longer solidification time of side oriented castings can be attribed to hot metal continuously feeding from the the top. The 11

amount of undercooling observed in 4 and 8 mm sections of horizontal castings was more than in side orientated castings. The nucleation potential depends on the number of potential heterogeneous nuclei, which can be indirectly assessed by carbide content (chill depth). A high nucleation potential will result in low formation of carbides. The nodule count is a direct measure of the nucleation potential of ductile iron but it can be measured only after completion of melt processing. The amount of Mg residual needs to be maintained above a critical value of 0.03% to achieve the desired nodule count [1]. Nodule count can be maximized by oxide-free base iron melting along with good inoculation practice. Nodule count and nodularity, both are affected by cooling rate. Thin sections (due to rapid cooling) result in better nodule shape than slower cooled sections for the same magnesium residuals. High nodule count associated with the increased cooling rates of thin wall castings is a major factor in matrix evolution. It was found that because of high nodule count produced by rapid cooling rate, the pearlite content was higher in the thinner plates. In 4 mm section approximately 4-5% primary carbides are found in casting where eutectic undercooling temperature is below 1140 o C. The cooling curve shown in Fig. 3 indicates undercooling (minimal) temperature of 1143 o C, which is above the critical limit (1140 o C) [12] and therefore gives carbide free casting. The castings BH-DH were found to be free of carbides even after increased manganese up to 0.5 %. The overall nodule count observed in side oriented castings was found to be higher compared to horizontal and vertical castings produced in the same composition indicating continual graphite nucleation throughout solidification. The simultaneous increase of both Cu and Mn enhances both tensile and yield strengths without a significant decrease in ductility (as compared to that observed by an increase in Cu alone). The microstructure study indicates no traces of carbides in 16, 12 and 8 mm sections and only 4 to 5% carbides in 4 mm thick sections. The nodule size distribution affects shrinkage tendency, since it reflects graphite formation and expansion throughout the entire solidification sequence. Small nodules and uniform distribution indicate early graphite nucleation and wider range of graphite nodule sizes, indicating continuous nucleation of graphite during solidification. In side and vertical 12

oriented castings, the metal is more turbulent, which delays solidification and results in less carbide formation, which is reflected in the cooling curves of the respective castings. The tensile strength and hardness variations due to thickness variations are less in vertical orientation compared to horizontal and side orientation, with the same chemical composition. 5. Conclusions The properties of ductile iron as indicated by their grades are largely determined by their microstructure, which in turn is affected by section thickness of the casting and chemical composition of the melt. It is found that the casting orientation also alters the nodule count and final microstructure in the casting. The difference in solidification times of horizontal and vertical orientated castings is found to increase with increasing casting wall thickness. Thin wall (4 mm) sections are more prone to deep undercooling and carbide formation, especially in horizontal orientation. The microstructure and thereby mechanical properties (especially tensile strength and hardness) can be improved in thin wall ductile iron castings by simultaneously increasing the amount of copper and manganese. The combined addition of Cu and Mn varying from 0.1 to 0.5% increased the amount of pearlite from 10 to 82% in the ductile iron castings, which in turn increased the strength from 467 to 659 MPa. The corresponding fall in ductility (% elongation) was 15 to 3 in 4 mm thick castings. Thus, ductile iron castings with Cu and Mn upto 0.5 % with produce carbide free structures in side oriented 4 mm wall castings. Acknowledgement This work is partially supported by the E-Foundry project, funded by the National Knowledge Network Mission of the Ministry of Communications and Information Technology, New Delhi. The authors gratefully acknowledge the assistance of Ganesh Foundry and S.S. Industries, Ichalkaranji for arranging melting trials. The first author acknowledges the support of his parent organization, Textile and Engineering Institute, Ichalkaranji for carrying out the present research work. References 13

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