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Metal Casting Technologies : March 2008
52 www.metals.rala.com.au BacktoBASICS T Feederless design of graphitic iron castings INTRODUCTION he cooling and solidification of most alloys is accompanied by volume reductions which must be compensated for in order to avoid shrinkage defects in castings. Contraction occurs as the liquid metal cools to the liquidus, or start of freeze temperature, and further contraction takes place during the solidification process as the metal cools to the solidus, or end of solidification temperature. It is accepted practice that these volume changes are compensated for by the positioning of feeders on the casting. The cooling and solidification of grey, compacted graphite and ductile iron castings in sand moulds is accompanied by an unusual phenomenon, the metal starts to expand. This expansion is generally attributed to the precipitation of the less dense graphite phase overcoming and exceeding the contraction associated with cooling liquid and the solidification of austenite. Figure 1 shows graphite spheroids precipitated in ductile iron. The pressure build-up in the liquid iron generated by expansion can be more than adequate to compensate for contraction so that feeders can be greatly reduced in size and number and under favourable conditions even eliminated entirely. Practice shows that when the cooling rate of the casting is relatively slow and with proper control of metallurgical, pouring and mould conditions, castings completely free from internal porosity defects can be produced without the need for feeders. FEEDERLESS DESIGN PRINCIPLES Conditions necessary for the successful production of feederless graphitic iron castings include: 1. The cooling rate of the casting needs to be relatively slow. In a sand mould, it is generally accepted that a casting with a solidification modulus equal to or exceeding 2.5cm will cool slowly enough to allow adequate expansion pressure to accumulate in the liquid to compensate for austenite contraction. 2. Pour within the temperature range of 1300 to 1350oC. A low pouring temperature is necessary so that liquid contraction is minimized. Liquid iron contracts approximately 1% for every 60oC of superheat so reducing the amount of superheat reduces liquid contraction. 3. Maintain a high fill rate during pouring. A high fill rate is desirable so that expansion which may be starting during filling is not wasted. It is also beneficial to gate through multiple thin gates which freeze quickly so that expansion pressure is transmitted to the liquid as early as possible. 4. Maintain high mould rigidity. Generally this means at least a resin bonded mould, however, on its own such a mould may not be sufficiently rigid to contain expansion pressure without yielding. The mould may also need to be contained in a strong steel box fitted with bars and adequately weighted and clamped so that the sand mass can not move. Figure 2 is an example of an iron casting produced in a box-less resin bonded mould. The large cracks visible in the mould were caused by expansion pressure; such a mould cannot be considered as being sufficiently rigid for feederless design. 5. Maintain high metallurgical quality of the iron. Almost every aspect of producing irons has an effect on metallurgical quality. Of particular significance is the choice of charge materials, metal chemistry, melting practice, treatment and inoculation practices. Suffice to say that maintaining good metallurgical quality has the effect of maximizing expansion potential and reducing shrinkage tendency in the iron. 6. Provide generous venting off the mould cavity. Venting enables the mould to be filled quickly and completely by avoiding air-locks in the cavity. As a rule, vents should be J. F. Meredith Solutions - Casting Solutions Pty Ltd Fig. 1. Graphite spheroids in ductile iron.