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1 - CONTENTS 1 Effects of Alloying Elements on the Microstructures and
Mechanical Properties of Heavy Section Ductile Cast Iron 2 Austempered Ductile Iron Castings for Chassis Applications 3 Austempered ductile iron (ADI) is stronger per unit weight than
aluminium, highly wear resistant and easier to machine than free machining steel
4 Austemperng Ductle Iron 5 Fatigue Performance Comparison and Life Predictions of Forged
Steel and Ductile Cast Iron Crankshafts 6 Ductile Iron Microstructure by Thermal Analysis 7 To Study the Effect of Austempering Temperature on Fracture
Behaviour of Ni-Mo Austempered Ductile Iron (ADI) 8 Developments in Cast Iron Metallurgical Treatments 9 GGG 40.3 10 A Review of Common Metallurgical Defects in Ductile Cast Iron 11 Nodular Cast Iron Fatgue Lfetme In Ultra-Hgh-Cycle Regon 12 Nodulizing and Inoculation Approaches for Year 2000 and
Beyond - Part 1 13 Some Studies of Nodular Graphite Cast Iron 14 Suggestions for Improved Reliability in Thermal Analysis of Cast
J. Mater. Sci. Technol., Vol.23 No.l, 2007 97
Effects of Alloying Elements on the Microstructures and Mechanical Properties of Heavy Section Ductile Cast Iron
G.S.Cho1^, K.H.Choe^, K W.Lee 1) 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 section ductile 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 impact energy are presented as a function of alloy amounts within 0.25 to 0.75 wt pet range. Hardness is measured on the broken tensile specimens. The small additions of Mo, Cu, Ni and Cr changed the as-cast mechanical properties owing to the different as-cast matrix microstructures. The ferrite matrix of Mo and Ni alloyed cast iron exhibits low strength and hardness as well as high elongation and impact energy. The increase in Mo and Ni contents developed some fractions of pearlite structures near the austenite eutectic cell boundaries, which caused the elongation and impact energy to drop in a small range. Adding Cu and Cr elements rapidly changed the ferrite matrix into pearlite matrix, so strength and hardness were significantly increased. As more Mo and Cr were added, the size and fraction of primary carbides in the eutectic cell boundaries increased through the segregation of these elements into the intercellular boundaries.
K E Y W O R D S : Heavy section ductile cast iron; Alloying elements; As-cast microstructures; As-cast mechanical properties
Ductile cast iron exhibits good ductility and toughness because the graphite morphology is spher-ical. It has been reported that the strengthening and toughening of ductile cast iron result from the modification of the matrix structure when alloying elements'1] are added or heat treatment^ is applied. The austempered ductile cast irons have been studied to replace the forged or cast steel in the structural parts, because they have many advantages such as high strength and toughness, high machinability and good corrosion resistance I 3 - 6 ! . Recently, the heavy section drawing dies in pressing steel sheet for the automobile frame or panel are replaced by simple one body-type as-cast ductile cast iron, which yields low cost and shortened delivering time on producing press dies. The casting die is generally produced via a full mold process that uses the near-net shaped ex-panded polystyrene pattern replaced by the molten m e t a l A s - c a s t ductile cast iron should meet the re-quirements of material properties to be used in cold pressing dies including high strength and high ductil-ity, but both mechanical properties are very difficult to obtain within the same casting material. Thus, the amounts of alloying elements are controlled to achieve as-cast heavy section ductile cast having high strength and ductility. The purpose of this paper is to obtain experimental data for optimum alloy design of heavy section ductile cast irons and to present the effects of alloying elements on the as-cast microstructures and mechanical properties for using cold press die mate-rial.
f Ph.D., to whom correspondence should be addressed, E-mail: firstname.lastname@example.org.
The chemical compositions of ductile cast irons used for this work are presented in Table 1. The main 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 and 0.75 wt pet. The results are presented in terms of the nominal additions for convenience in discus-sion. The ductile irons were obtained by melting steel scrap, pig iron, graphite, Fe-75 wt pet Mn and Fe-75 wt pet Si in the high frequency induction fur-nace. Spheroidizing and inoculation practices were performed in a conventional sandwich method with 5.8 wt pet Mg-Fe-Si and 75 wt pet Si-Fe alloy, respec-tively. The metal was poured into furan resin molding sand molds to obtain Y-shaped 75 mm blocks. Ten-sile specimens with the dimensions shown in Fig.l and
98 J. Mater. Sci. Technol., Vol.23 No.l, 2007
Table 1 Chemical compositions of ductile cast iron (wt pet)
Melt C Si Mn P S Mo Cu Ni Cr Mg A 3.61 2.36 0.28 0.04 0.01 - - - - 0.06 B 3.57 2.37 0.28 0.04 0.01 0.19/0.38/0.58 - - - 0.06 C 3.60 2.40 0.28 0.04 0.01 - 0.24/0.47/0.70 - - 0.06 D 3.64 2.35 0.30 0.04 0.01 - - 0.24/0.48/0.70 - 0.06 E 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/jum A 10.2 141.0 90.4 30.3 B 13.1 153.3 88.5 33.0 C 13.0 154.6 87.5 32.8 D 12.2 152.6 87.3 31.8 E 11.4 148.0 86.0 31.5
Fig. 2 As-cast microstructure of melt A as a reference casting
unnotched Charpy specimens (55 mmx 10 mmx 10 mm) were machined from the bottom section of the Y-blocks. Tensile specimens were taken from the bottom section of Y-blocks in order to minimize casting de-fects such as microporosity. Three tensile specimens were tested in a 250 kN hydraulic Instron universal testing machine using a constant cross-head travel speed of 4 mm/min. Unnotched Charpy specimens were tested in a standard impact testing machine. Hardness measurements were made on a standard Brinell hardness machine with a load of 3000 kg and 10 mm steel ball. Metallographic specimens were obtained 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 melt
A as a reference cast iron given in Table 1. The mi-crographs show that the matrix structure is fully ferrite structure with a small fraction of pearlite near the eutectic cell boundaries. The ferrite structures are 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 distributed within the ferrite matrix. Two types of graphite in size are observed microscopically, because of the slight hypereutectic chemical composition of the reference cast iron. The bigger graphites, about 50 /xm in diam-eter, are crystallized at the graphite+liquid region in the Fe-C system. The smaller ones, under 50 /xm, are
formed through eutectic solidification range. Image analysis of volume fraction, nodule count, nodular-ity and average diameter of graphite on the different alloyed ductile cast irons are given in Table 2. The characteristic values of graphites in the alloyed irons are very similar to that of reference cast iron.
Figure 3 shows the as-cast microstructures of Mo alloyed cast iron. The Mo alloyed ductile cast iron of melt B exhibits substantial ferrite in the as-cast structure 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 the eutectic cell boundaries is slightly increased. The Ni alloyed ductile cast iron of melt D also has a mostly ferrite matrix having some pearlite structures in the cell boundaries. Adding up to 0.75 wt pet of Mo and Ni elements results in the same tendency on the for-mation of a ferrite matrix. The addition of Mo and Ni alloy will increase the hardenability of ductile cast iron by delaying the transformation of austenite to ferrite^8,9!. Also, the hardness of the ferrite matrix increased from about 169 HV to about 188 HV when more Mo was added. The Ni alloyed ductile cast iron also exhibited an increase in matrix hardness. This means that the addition of Mo and Ni strengthens the ferrite matrix via solid solution hardening.
Figure 4 shows microstructures of the as-cast melt C ductile cast iron. As more Cu is added, the amount of ferrite structure is significantly decreased with Cu content. For the melt C containing 0.5 wt pet Cu and more, the matrix was abruptly changed into pearlite
J. Mater. Sci. Technol., Vol.23 No.l, 2007 99
Fig.3 Microstructures of the as-cast melt B ductile cast iron with different Mo contents: (a) 0.25 wt pet Mo, (b) 0.5 wt pet Mo, (c) 0.75 wt pet Mo
Fig.4 Microstructures of the as-cast melt C ductile cast iron with different Cu contents: (a) 0.25 wt pet Cu, (b) 0.5 wt pet Cu, (c) 0.75 wt pet Cu
Fig.5 Microstructures of the as-cast ductile cast irons w