J. Mater. Sci. Technol., Vol.23 No.1, 2007 97

Effects of Alloying Elements on the Microstructures and Mechanical

Properties of Heavy Section Ductile Cast Iron

G.S.Cho1)†, K.H.Choe1), K.W.Lee1) and A.Ikenaga2)

1) Advanced Material Processing Team, Korea Institute of Industrial Technology, 994-32, Dongchun-Dong, Yeonsu-Ku,Incheon 406-130, South Korea

2) Department of Metallurgy and Materials Science, Graduate School of Engineering, Osaka Prefecture University, 1-1,

Gakuen-Cho, Sakai, Osaka 599-8531, Japan

[Manuscript received February 17, 2006, in revised form May 29, 2006]

The effects of alloying elements on the as-cast microstructures and mechanical properties of heavy sectionductile cast iron were investigated to develop press die material having high strength and high ductility.Measurements of ultimate tensile strength, 0.2% proof strength, elongation and unnotched Charpy impactenergy are presented as a function of alloy amounts within 0.25 to 0.75 wt pct range. Hardness is measuredon the broken tensile specimens. The small additions of Mo, Cu, Ni and Cr changed the as-cast mechanicalproperties owing to the different as-cast matrix microstructures. The ferrite matrix of Mo and Ni alloyed castiron exhibits low strength and hardness as well as high elongation and impact energy. The increase in Mo andNi contents developed some fractions of pearlite structures near the austenite eutectic cell boundaries, whichcaused the elongation and impact energy to drop in a small range. Adding Cu and Cr elements rapidly changedthe ferrite matrix into pearlite matrix, so strength and hardness were significantly increased. As more Mo andCr were added, the size and fraction of primary carbides in the eutectic cell boundaries increased through thesegregation of these elements into the intercellular boundaries.

KEY WORDS: Heavy section ductile cast iron; Alloying elements; As-cast microstructures;

As-cast mechanical properties

1. Introduction

Ductile cast iron exhibits good ductility andtoughness because the graphite morphology is spher-ical. It has been reported that the strengtheningand toughening of ductile cast iron result from themodification of the matrix structure when alloyingelements[1] are added or heat treatment[2] is applied.The austempered ductile cast irons have been studiedto replace the forged or cast steel in the structuralparts, because they have many advantages such ashigh strength and toughness, high machinability andgood corrosion resistance[3–6]. Recently, the heavysection drawing dies in pressing steel sheet for theautomobile frame or panel are replaced by simple onebody-type as-cast ductile cast iron, which yields lowcost and shortened delivering time on producing pressdies. The casting die is generally produced via afull mold process that uses the near-net shaped ex-panded polystyrene pattern replaced by the moltenmetal[7]. As-cast ductile cast iron should meet the re-quirements of material properties to be used in coldpressing dies including high strength and high ductil-ity, but both mechanical properties are very difficultto obtain within the same casting material. Thus, theamounts of alloying elements are controlled to achieveas-cast heavy section ductile cast having high strengthand ductility. The purpose of this paper is to obtainexperimental data for optimum alloy design of heavysection ductile cast irons and to present the effects ofalloying elements on the as-cast microstructures andmechanical properties for using cold press die mate-rial.

† Ph.D., to whom correspondence should be addressed,

E-mail: gscho@kitech.re.kr.

2. Experimental

The chemical compositions of ductile cast ironsused for this work are presented in Table 1. Themain difference lies on the Mo, Cu, Ni and Cr al-loy contents. The nominal compositions of alloy-ing element additions are given as 0.25, 0.5 and0.75 wt pct. The results are presented in termsof the nominal additions for convenience in discus-sion. The ductile irons were obtained by meltingsteel scrap, pig iron, graphite, Fe-75 wt pct Mn andFe-75 wt pct Si in the high frequency induction fur-nace. Spheroidizing and inoculation practices wereperformed in a conventional sandwich method with5.8 wt pct Mg-Fe-Si and 75 wt pct Si-Fe alloy, respec-tively. The metal was poured into furan resin moldingsand molds to obtain Y-shaped 75 mm blocks. Ten-sile specimens with the dimensions shown in Fig.1 and

Fig.1 Y-shaped block and tensile specimen dimensions

98 J. Mater. Sci. Technol., Vol.23 No.1, 2007

Table 1 Chemical compositions of ductile cast iron (wt pct)

Melt C Si Mn P S Mo Cu Ni Cr MgA 3.61 2.36 0.28 0.04 0.01 – – – – 0.06B 3.57 2.37 0.28 0.04 0.01 0.19/0.38/0.58 – – – 0.06C 3.60 2.40 0.28 0.04 0.01 – 0.24/0.47/0.70 - - 0.06D 3.64 2.35 0.30 0.04 0.01 – – 0.24/0.48/0.70 – 0.06E 3.66 2.31 0.33 0.04 0.01 – – – 0.26/0.49 0.06

Table 2 Typical characteristics of spherical graphite in ductile cast iron

Melt Area fraction/% Nodule count/(N·mm−2) Nodularity/(%) Ave. diameter/µmA 10.2 141.0 90.4 30.3B 13.1 153.3 88.5 33.0C 13.0 154.6 87.5 32.8D 12.2 152.6 87.3 31.8E 11.4 148.0 86.0 31.5

Fig.2 As-cast microstructure of melt A as a reference casting

unnotched Charpy specimens (55 mm×10 mm×10 mm)were machined from the bottom section of the Y-blocks. Tensile specimens were taken from the bottomsection of Y-blocks in order to minimize casting de-fects such as microporosity. Three tensile specimenswere tested in a 250 kN hydraulic Instron universaltesting machine using a constant cross-head travelspeed of 4 mm/min. Unnotched Charpy specimenswere tested in a standard impact testing machine.Hardness measurements were made on a standardBrinell hardness machine with a load of 3000 kg and10 mm steel ball. Metallographic specimens wereobtained from broken impact samples.

3. Results and Discussion

3.1 As-cast microstructures of ductile cast iron

Figure 2 shows the as-cast microstructure of meltA as a reference cast iron given in Table 1. The mi-crographs show that the matrix structure is fully fer-rite structure with a small fraction of pearlite nearthe eutectic cell boundaries. The ferrite structuresare selected to serve as a base material by minimiz-ing Mn content in 75 mm heavy section Y-block.The spheroidal graphite nodules are well distributedwithin the ferrite matrix. Two types of graphite insize are observed microscopically, because of the slighthypereutectic chemical composition of the referencecast iron. The bigger graphites, about 50 µm in diam-eter, are crystallized at the graphite+liquid region inthe Fe-C system. The smaller ones, under 50 µm, are

formed through eutectic solidification range. Imageanalysis of volume fraction, nodule count, nodular-ity and average diameter of graphite on the differentalloyed ductile cast irons are given in Table 2. Thecharacteristic values of graphites in the alloyed ironsare very similar to that of reference cast iron.

Figure 3 shows the as-cast microstructures of Moalloyed cast iron. The Mo alloyed ductile cast ironof melt B exhibits substantial ferrite in the as-caststructure with some pearlite at intercellular regions.As more Mo is added, the matrix becomes fully fer-rite and the area fraction of pearlite structure in theeutectic cell boundaries is slightly increased. The Nialloyed ductile cast iron of melt D also has a mostlyferrite matrix having some pearlite structures in thecell boundaries. Adding up to 0.75 wt pct of Mo andNi elements results in the same tendency on the for-mation of a ferrite matrix. The addition of Mo andNi alloy will increase the hardenability of ductile castiron by delaying the transformation of austenite toferrite[8,9]. Also, the hardness of the ferrite matrixincreased from about 169 HV to about 188 HV whenmore Mo was added. The Ni alloyed ductile cast ironalso exhibited an increase in matrix hardness. Thismeans that the addition of Mo and Ni strengthensthe ferrite matrix via solid solution hardening.

Figure 4 shows microstructures of the as-cast meltC ductile cast iron. As more Cu is added, the amountof ferrite structure is significantly decreased with Cucontent. For the melt C containing 0.5 wt pct Cu andmore, the matrix was abruptly changed into pearlite

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Fig.3 Microstructures of the as-cast melt B ductile cast iron with different Mo contents: (a) 0.25 wt pct Mo,(b) 0.5 wt pct Mo, (c) 0.75 wt pct Mo

Fig.4 Microstructures of the as-cast melt C ductile cast iron with different Cu contents: (a) 0.25 wt pct Cu,(b) 0.5 wt pct Cu, (c) 0.75 wt pct Cu

Fig.5 Microstructures of the as-cast ductile cast irons with different alloying elements: (a) 0.75 wt pct Mo(b) 0.75 wt pct Cu, (c) 0.75 wt pct Ni, (d) 0.5 wt pct Cr

with a bull′s-eye ferrite rim around the nodules. Fig-ure 4 presents the ability of Cu to hinder the diffu-sion of carbon into graphite at the graphite-austeniteinterface during the solid state transformation aftersolidification. The Cu will increase as-cast strengthand hardness through increased pearlite formation[8].

Figure 5 shows the as-cast microstructures of duc-tile cast iron with different alloying elements. While

the 0.75 wt pct Mo and 0.75 wt pct Ni alloyed ductilecast irons exhibit a full ferrite matrix, the 0.75 wt pctCu and 0.5 wt pct Cr added ductile cast irons ex-hibit a bull′s-eye structure within the pearlite matrix.Carbide-like phases are observed in the middle of theeutectic cell boundaries of the 0.75 wt pct Mo and0.5 wt pct Cr alloyed ductile cast iron. These phasesare not observed at the intercellular regions of

100 J. Mater. Sci. Technol., Vol.23 No.1, 2007

Fig.6 EDX line scan profiles measured on as-cast ductile cast irons: (a) 0.75 wt pct Mo, (b) 0.5 wt pct Cr

Fig.7 Variation in tensile strength and elongation of as-cast ductile cast irons having different alloying contents:(a) tensile strength, (b) elongation

0.25 wt pct Mo and 0.25 wt pct Cr alloyed ductilecast iron. Figure 6 presents the EDX line scan profilesmeasured on the as-cast ductile cast irons. Figure 6(a)shows the sharp increase in Mo peak at the eutecticcell boundary. This means that the segregation of Moduring solidification forms Mo-rich carbides in the cellboundaries[10¯12]. Figure 6(b) shows the segregatedCr-rich phase in the as-cast ductile cast iron. Mn waspositively segregated in the cell boundaries but Si wasnegatively segregated[13]. The increased Mo and Mnlevels in the intercellular boundary in the alloyed ironattributed to segregation caused considerable pearliteformation near the cell boundaries. Carbide forma-tion will deteriorate the ductility of as-cast ductilecast iron[14,15]. Thus, the amount of Mo and Cr addi-tions should be limited to a certain level consideringthe thickness of castings.

3.2 Mechanical properties of as-cast ductile cast iron

Figure 7 shows the variation in tensile strengthand elongation of as-cast ductile cast iron having dif-ferent alloying contents. The trends in terms of tensileproperties of Ni added ductile cast iron resemble thatof Mo added ductile iron. With the increase in theamount of Mo and Ni additions, tensile strength isslightly increased while elongation is decreased. Thestrength of Mo and Ni alloyed ductile cast iron ismainly dependent on the solid solution hardening offerrite matrix. It was also considered that the de-crease in elongation was caused by the increase inpearlite formation in the cell boundaries. As more Cuelement is added, tensile strength is steeply increased.The tensile strength of 0.5 wt pct Cu added ductilecast iron approaches at a maximum of 700 MPa, butelongation quickly decreases below 5%. The trendsin terms of tensile properties of Cr added ductile iron

resemble that of Cu added ductile cast iron. Addi-tion of up to 0.5 wt pct Cr could not reach a maxi-mum of 700 MPa because of the formation of a largebull′s-eye ferrite structure. It is confirmed that thetensile strength of as-cast ductile cast iron is stronglydependent on the area fraction of pearlite, while elon-gation is inversely proportional to the pearlite frac-tion in the as-cast microstructure[16¯18]. Figure 8presents the SEM micrographs of the fractured sur-face of tensile specimens for comparing two typicalfracture modes. Figure 8(a) shows the brittle fracturesurface of 0.75 wt pct Cu alloyed iron having pearlitematrix with cleavage river patterns. The ductile frac-ture surface of 0.75 wt pct Ni alloyed iron is shownin Fig.8(b). Plastic deformation of ferrite around theboundary of graphite nodule is observed. Some cleav-age brittle fracture area having a pearlite phase nearthe intercellular region is also shown.

Figure 9 shows the variation in hardness and im-pact energy with different alloying additions. As morealloying contents are added, the Brinell hardness ofMo and Ni alloyed ductile cast iron is linearly in-creased, but that of Cu and Cr alloyed ductile castiron increased very steeply. The Brinell hardness ofductile cast iron refers to the resistance of the matrixand graphite to plastic deformation. The variationin hardness with the alloying element is strongly de-termined by the as-cast microstructures. Generally,the hardness of cast iron is increased by the volumefraction of pearlite structure[19,20]. In this study, theaddition of Mo and Ni slightly increased the matrixhardness via solid solution hardening. As more Cuand Cr element is added, the hardness increased sig-nificantly owing to the high fraction of pearlite. TheCharpy impact energy of Mo and Ni alloyed iron isgradually decreased, but that of the Cu and Cr

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Fig.8 SEM micrographs showing the fractured surface of tensile specimens: (a) 0.75 wt pct Cu, (b) 0.75 wt pct Ni

Fig.9 Variation in hardness and impact energy of as-cast ductile cast irons having different alloying contents:(a) Brinell hardness, (b) Charpy impact energy

alloyed iron begins to decrease fast. This indicatesthat ferrite matrix provides highly ductile cast ironwhile pearlite yields highly strong cast iron. Mo wasfound to reportedly segregate at cell boundaries andaggravate the toughness of ductile cast iron[21]. How-ever, the toughness of ductile cast iron was more de-pendent on the area fraction of ferrite than the areafraction of intercellular carbide-like phases.

4. Conclusions

(1) The as-cast microstructures are strongly de-pendent on the alloying elements. The ferrite matrixis mostly formed as Mo and Ni elements are added.The addition of Cu and Cr rapidly changed the ferriteinto pearlite matrix.

(2) The Mo and Cr element are segregated at theeutectic cell boundaries and caused the formation ofcarbide-like phases. Therefore, the amount of Mo andCr added should be controlled considering the thick-ness of castings.

(3) The ferrite matrix of Mo and Ni alloyed duc-tile cast iron exhibited low strength and low hardnessbut high elongation and high impact energy. The me-chanical properties of pearlite cast iron alloyed withCu and Cr element were the exact opposite of thoseof ferrite cast iron.

(4) Ferrite matrix provides highly ductile cast ironwhile pearlite provides highly strong cast iron. Thetoughness of ductile cast iron was more dependent onthe area fraction of ferrite than the area fraction ofintercellular carbide-like phases.

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