Metal Casting Technologies : MCT-DEC-2014
32 www.metals.rala.com.au METAL casting Technologies December 2014 33 BacktoBASICS Introduction ost alloys behave in a consistent and predictable manner on cooling from pouring through to the solidus temperature. Two separate phases of shrinkage occur. Firstly, as the alloy cools from the pouring temperature to the liquidus, this is commonly termed liquid or superheat shrinkage. Secondly, as the alloy cools from the liquidus to the solidus point, which is generally referred to as solidification shrinkage. On the other hand, the cooling and solidification of graphitic iron castings, which includes grey, ductile and malleable iron alloys, 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. By far the most significant aspect of designing feeding and gating systems for graphitic cast irons is the requirement to maintain positive pressure on the liquid throughout the solidification process. Initially, atmospheric pressure must be allowed to act on the liquid in the feeder, for this to happen, the feeders must pipe. Once expansion begins, well-designed feeding systems control expansion pressure and ensure castings are self-feeding during the remainder of solidification. This contrasts with steel, aluminium, copper etc, where feed metal must be supplied to the casting throughout solidification because no expansion is involved. pressure control The feeder contact is perhaps the most critical component of the feeding system design in that it usually determines the magnitude of the residual pressure on the liquid. The feeder contact must be large enough to allow liquid transfer of feed metal from the feeder to the casting for sufficient time, and if necessary relieve excessive pressure from the mould cavity, yet small enough to maintain positive pressure on the liquid at the end of solidification and facilitate easy removal of the feeder from the casting. The feeder contact can be considered as the “safety valve” on a pressure container in that it should be designed to maintain pressure at manageable levels within the casting. The mould material, or more particularly the ability of the mould material to withstand the pressure of expansion without dilating generally determines manageable levels. If the mould material is weak, such as would be expected with a soft greensand mould, the feeder contact should be designed to relieve some of the expansion pressure in order to avoid mould dilation. This is achieved by designing the feeder contact so that it solidifies at a relatively late stage of freezing, thus allowing some of the pressure to be relieved through the feeder contact to the feeder body. With stronger, more rigid mould binders such as resin systems, the feeder contacts can be designed to solidify earlier in the expansion phase, by making them smaller, thus retaining a higher level of residual pressure on the liquid. Whilst feeder contacts that are too small can result in retaining excessively high residual pressure within the casting causing porosity related to mould dilation. Feeder contacts that are too large will often cause a loss of positive pressure on the liquid before the completion of solidification resulting in porosity related to solidification contraction and gasses expelled from solution. design rules The dimensions of the feeder neck are often based on the geometric modulus of the casting (Mc). Typical values for iron castings produced in green sand moulds are between 0.6(Mc) and 0.9(Mc). The exact value is determined by the rigidity of the mould material, the chemistry and degree of inoculation of the iron and the cooling rate of the casting. If the feeder is moved closer to the casting, the heating effect on the sand between the casting and feeder will make it possible to reduce the geometric modulus of the contact whilst still maintaining equivalent thermal modulus. If the neck is make short enough, such that its length is equal to or less than the lesser cross sectional dimension of the contact, the geometric modulus can safely be reduced by a factor of 0.6 the modulus of the longer neck (Mn (short) = 0.6 Mn (long) ). This represents a reduction in contact area of approximately 65%. Figure 1 shows a long feeder neck with a geometric and thermal modulus of 1cm. Figure 2 shows a neck of reduced length with a geometric modulus at the contact of 0.6cm, but with a thermal modulus of approximately 1cm. The design as illustrated in Figure 2 is one which has proven reliable in practice. The length of the neck should be equal to or less than the lesser cross sectional dimension of the contact. Figure 3 shows the neck is “notched” at an angle of 45 degrees to aid removal and is taken back at an angle of approximately 30 degrees to the feeder body. Figure 4 shows an example of a ductile iron casting where the feeder contacts are designed according to the above rules. An actual example of a feeder contact designed according to the above rules is shown in figure 4. Conclusion Successful feeding of graphitic cast irons involves the maintenance and control of positive pressure on the liquid iron throughout the solidification process. Correct design of feeding and gating systems, as well as good control of metallurgical and pouring practice is essential to the production of porosity free graphitic iron castings. n Graphitic iron casting feeder design - feeder contacts M J. f. Meredith, casting solutions Pty Ltd “iN ORDER TO OPTiMisE cAsTiNG sOUNDNEss iT is NEcEssARY TO MAiNTAiN POsiTiVE PREssURE ON THE LiQUiD METAL THROUGHOUT THE sOLiDificATiON PROcEss” Figure 1. Feeder with “long” neck. Figure 2. Feeder with “short” neck of reduced size. Figure 4. Example of feeder with well designed contact. Figure 3. detail of reduced contact neck.