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  • CHINA FOUNDRY Vol.5 No.2

    82

    Recent development of ductile cast iron production technology in China

    *Cai Qizhou and Wei Bokang(State Key Lab of Material Processing and Dies & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074,

    China)

    Abstract: Recent progress in the production and technology of ductile cast iron castings in China is reviewed. The manufacture and process control of as-cast ductile iron are discussed. The microstructure, properties and application of partial austenitization normalizing ductile iron and austempered ductile iron (ADI) are briefl y depicted. The new development of ductile iron production techniques, such as cored-wire injection (wire-feeding nodularization) process, tundish cover ladle nodularizing process, horizontal continuous casting, and EPC process (lost foam) for ductile iron castings, etc., are summarized.

    Key words: ductile cast iron; production technology; recent developmentCLC Number: TP143.5/255 Document Code: A Article ID: 1672-6421(2008)02-0082-10

    With the constant growth of national economy, the castings output in China increases quickly. China has been ranked the top manufacturer of casting production in the world since 2000. The casting output had reached 24,420,000 tons in 2005, accounting for about 28.5% of the world's total output [1]. In addition, the output of ductile iron had also developed rapidly, which reached 5,838,000 tons in 2005, increasing to 2.46 times of output in 2000. The ratio of ductile iron castings to total castings increased from 18% in 2000 to 23.9% in 2005. Over the past fi ve years, the production of centrifugal-cast iron pipe had achieved an average 40% growth in China, to 1.9 million tons in 2004. Moreover, huge quantities of various centrifugal-cast iron pipes have been exported to Southeast Asian countries. However, the ratio of ductile iron castings to total castings in developed countries, such as France (40.8%), the United States (32.88%) and Japan (28.9%), is still beyond that of China in 2005.

    With the development of automotive and cast pipe industries, as well as the gradual improvement of ductile iron casting quality in China, the applications of ductile iron castings will be continuously expanded. In the "11th Five-Year Plan," the production of ductile iron in China will be greater than that of the world's average level, and will continue to be an important casting alloy for various engineering fields in the future. Therefore, further stabilizing ductile iron production process, improving ductile iron quality, exploiting and introducing new technologies for ductile iron production, are still an important task for Chinese casting manufacturing industry.

    1 Development of as-cast ductile ironsThe as-cast ductile irons own many advantages, such as energy-saving, equipment investment decreasing, production cycles shortening, production costs reducing and competitiveness promoting, compared with heat treated ductile irons. Moreover, some defects from heat treatment, such as high temperature oxidation and deformation, can be avoided in the as-cast ductile irons. About 80% ductile iron casting components in automotive applications aboard are manufactured in as-cast. Thus, the research and development in producing as-cast ductile irons for heavy-duty applications are the major tendency in future.

    1.1 Thin-wall as-cast ferrite ductile iron Thin-wall ferrite ductile iron is comprehensively utilized for making castings for some of safety critical automotive components, such as knuckle, front and rear control arms with overall section thicknesses about 10 mm. This kind of castings has the solidification characteristics, such as, high cooling rate, serious white tendency, formation of the intergranular carbide and pearlite microstructure under metastable system. So, appropriate measures should be adopted to control its metallurgical conditions.

    Eutectic solidification time can be effectively decreased by increasing the number of nodule graphite, which can promote the formation of eutectic graphite. The relationship between the number of white critical nodule count and the cooling rate is as follows [2]:

    N=0.58R2+19.07R+1.01 (1) Where, N white critical nodule counts per square mm; R

    cooling rate (/s). It is shown that there is the minimum

    graphite nodule count in a different thickness ductile iron under various cooling rate, where white cast iron would not be formed.

    Male, born in 1964, professor, Ph.D. Research interests: solidification of casting alloys and surface treatment of light metals.E-mail: caiqizhou@mail.hust.edu.cnReceived: 2007-12-12; Accepted: 2008-04-10

    *Cai Qizhou

  • Special ReviewMay 2008

    83

    Measures increasing nodule count mainly include:(1) The content of C and Si should be as high as possible

    when carbon equivalent (CE) is no more than 5.0%. However, ductile-to-brittle transition temperature of ductile iron will be signifi cantly increased by Si, and the brittleness of ductile iron will signifi cantly increase when Si content extends to a certain extent. Therefore, the fi nal content of Si ought to be controlled between 2.4% and 2.8% for thin-wall ferrite ductile iron.

    (2) Activity of C is weakened by some elements, such as Mn, Cr, V, Ti, etc., which should be as low as possible. It is generally considered that no more than 0.3% Mn is the key to produce ductile iron with high toughness [3]. Mn content should be 0.2% according to the trial conducted by BCIRA in Britain. But it is diffi cult to achieve such a low level to the domestic manufacturers because of poor quality pig iron resource.

    (3) Control of trace element content. It is as shown in the experimentation conducted by J. W. Cheng [4] that not only spheroidization rate and the toughness of ductile iron were decreased, but also graphite morphology was influenced by the trace elements Pb and Sb existed in raw materials. Disadvantageous effect of Pb and Sb on the microstructure and properties of the thin-walled ferrite ductile iron can be eliminated by both increasing RE content in spheroidizer and using inoculant containing Bi.

    (4) Ratio of RE to S in the molten iron should be 3 275 MPa, A > 12%, ak>12 J/cm

    2, HB156197. In recent years, many research and development works on as-cast thin-wall ductile iron were carried out by DFAC, and as-cast ductile iron parts with wall thickness of 3 mm and weight of 0.3 kg has been produced, which has high toughness [5]. QT400-18 as-cast ductile iron with spheroidizing rate 90% and ferrite content 90% was developed by Shandong Institute of Mechanical Design and Zhucheng Shilong Valve Ltd., using the molten iron containing Si 2.4%2.8%, Mn0.2%, low P and S content, and through appropriate spheroidization and inoculation operations. The technology was applied to produce the components of refl ux

    device assembly [6].At present, there is a misunderstanding in high toughness

    of ferrite ductile iron. It is thought that performance of ductile iron was strongly related to its toughness. Actually, that is inaccurate. The casting deformation is not generally permitted during its service. Therefore, certain strength can maintain precision of parts, prolong service life of castings, enhance deformation resistance, and reduce deformation during its service under appropriate toughness conditions since toughness is inversely proportional to strength. The casting components to export are usually requested with a mixed matrix in microstructure. For example, the spring bracket produced for Ford Company, only requested Rm > 525 MPa, A > 5%, and HB149187, without particular requirement for matrix [5].

    Some problems in casting production, specifi cally those with complex shape such as high cooling rate, poor graphitization ability, and higher tendency of shrinkage and porosity existed in thin-walled ferrite ductile iron. In addition to applying extensive detecting measures (such as X-ray and ultrasonic inspections), it is the fundamental and important approach to solve the problem through systematic study and integrated management in its solidification characteristic and process control. Besides general solidification characteristics of ordinary ductile iron, the ductile iron with thin sections also has the following characteristics:

    (1) High cooling rate due to thin wall, resulting in inadequate graphitization and formation of free carbide (reverse chill), makes the overall volume shrinkage tendency increase. Mr. Zhang Bo indicated that the actual contraction rate in solidifi cation was bigger than one from the calculation of equilibrium when the modulus of medium and small-sized ductile iron castings satisfy M < t/2=2.5 cm (t as casting thickness), and the total expansion volume caused by casting graphitization could not be enough to compensate the total shrinkage volume. This means that a riserless casting process is not suitable for thin section castings.

    (2) Austenitic dendrite developed. It is shown in the latest research [7] that developed divorced austenitic dendrite comes into in the course of the solidifi cation and growth process of ductile iron, and divorced dendrite becomes more developing with section thickness of casting decreasing. Moreover, small ductile iron casting usually has more complex shapes, which makes the liquid metal feeding channel choked easily, increasing the sensitivity to shrinkage and porosity and diffi culty to control. For production process of castings, shrinkage void and shrinkage porosity may be solved by comprehensive treatment involving both metallurgical factors and casting process factors.

    The metallurgical factors include: (1) Adjusting suitable chemical composition

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