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Metal Casting Technologies : September 2006
90 BacktoBASICS porosity. If the feeder neck is too large, secondary shrinkage defects could result due to loss of positive pressure on the last metal to solidify. The object in designing a feeding system for graphitic cast irons should be to control the pressure generated in the liquid between a minimum level, which will prevent the occurrence of secondary porosity, and a maximum after which mould dilation occurs. Most shrinkage related defects in iron castings are the result of incorrect pressure management. The diagnosis of the cause of shrinkage defects in graphitic cast irons can be rather complicated, as metal, mould and design factors can all contribute to shrinkage. Initially when shrinkage occurs a visual examination of the feeder pipe, or lack of pipe should be made, along with an examination of the casting walls to check for dilation. Ideally, feeders should show pipe, and in the case of weak moulds, some sign of bleed back of iron, Fig. 5, which is an indication of pressure relief from the casting. If shrinkage at the feeder neck occurs and the feeder shows deep pipe with no sign of bleed back, then the feeder contact is probably too small and is freezing at or before the onset of expansion. If there is shrinkage at the neck and the feeder shows no sign of pipe or perhaps even a "mushrooming" effect on the upper surface, then the feeder contact is too large and the loss of positive pressure on the liquid has caused porosity. A common problem experienced with iron castings is variability of pipe in feeders. Sometimes feeders will pipe and the casting is sound, and on other occasions there is no evidence of pipe and shrinkage at the contact is evident. This problem is more common with ductile irons than grey irons and is usually caused by one or more of the following: ■ Pouring temperature too low ■ Ingates too large ■ Placing more than one feeder in the same feeding zone Excessively high pouring temperatures can also cause shrinkage defects in iron castings by increasing the amount of liquid shrinkage prior to eutectic precipitation. When a number of castings are poured from a single ladle, the initial temperature is often elevated to avoid miss run defects at the end of the ladle, the castings poured first may show shrinkage defects. This can be highlighted by an occurrence survey over the course of the ladle. High levels of proprietary inoculants can cause shrinkage in iron castings by increasing eutectic cell count. Inoculants should be kept to the minimum that will achieve the required chill removal and graphite structure. If random shrinkage defects occur, the level of inoculation should be checked. The carbon equivalent value of an iron will affect the volume of initial liquid shrinkage and the amount of expansion. Generally, low carbon equivalent irons are more prone to shrinkage. If random shrinkage problems are encountered in iron castings, the carbon equivalent value should be checked. Phosphorus can cause great difficulty in achieving high degrees of soundness in iron castings. Due to its low melting point, it is virtually impossible to compensate for the shrinkage, which occurs during solidification of the phosphide eutectic. INCLUSIONS Inclusions in iron castings can be of an exogenous or indigenous nature. Exogenous inclusions originate from sources outside the melt and are typically particles of sand, refractory, ladle slag, dross from treatment alloys and undissolved treatment alloys. Indigenous inclusions originate from chemical reaction between elements within the melt during melting and treatment. They are generally silicates, oxides, nitrides and sulphides, or more often, complexes of these. A visual examination is usually sufficient to distinguish between an exogenous inclusion and a blowhole or shrinkage cavity. More accurate diagnosis of the inclusion origin may require the use of a microscope, which can usually distinguish between inclusions such as sand, slag and undissolved alloys. Oxide inclusions may require techniques such as SEM analysis to fully characterise the oxide and potentially identify its source. Indigenous inclusions will usually require at least microscopic examination for identification; inclusions such as manganese sulphides in cast irons, or magnesium silicates in ductile irons, are quite distinctive under the microscope. Generally however, SEM or electron microprobe analysis is required for accurate diagnosis of indigenous inclusions. CONCLUSION Defects are an undesirable but largely unavoidable consequence of grey and ductile iron casting production. Accurate diagnosis and the implementation of appropriate corrective actions can be a tedious and expensive exercise. If a systematic approach to defect diagnosis is used, then the time taken to implement corrective actions can be minimised. ● Figure 5. Cast iron feeder piping REFERENCES: 1. Hornung M J, A Systematic Approach to Cast Iron Defect Analysis, Modern Casting, pp 33-36, April 1990. 2. Fallon M J, Surface Defects and Abnormal Surface Structures in Grey and Ductile Iron Castings, Transactions of Australian Foundry Institute National Convention, pp152-164, 1990 www.metals.rala.com.au