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Metal Casting Technologies : September 2006
89 GAS DEFECTS Gas defects in iron castings always leave telltale signs that aid in their identification . The first step in identification is to note the shape and location of the defect. If the gas holes are confined to the cope surface and are generally rounded in shape, then the most likely cause is manganese sulphide inclusions. Manganese sulphides are lighter than the liquid iron and tend to float to the cope surface where they react with the precipitating graphite giving off carbon monoxide gas. Microscopic examination of the matrix surrounding the blowhole will usually show a high concentration of the characteristic dove grey manganese sulphides. Fig. 3. Low pouring temperatures are the primary cause of manganese sulphide blowholes; the incidence increases markedly as metal temperature drops below about 1380oC. Often only the last few castings from a particular ladle will be affected, which is information that can be gained from an occurrence survey. If the gas holes are again rounded and smooth, but appearing on all surfaces of green sand moulded iron castings, then hydrogen pinholing is to be suspected. This can be supported by a visual examination of the defects surface, which may have a lustrous appearance with no evidence of non-metallic inclusions in the voids. It may also be noted there is a higher incidence of the defect in areas of the casting more remote from the ingates. Microscopic examination will provide the conclusive evidence of hydrogen pinholing, if the sample is carefully prepared then the gas hole can be observed to have a graphite lining and corresponding depletion of graphite in the surrounding matrix. Nitrogen fissure defects are often confused with shrinkage cavities as they can both appear as irregular voids in the thermal centres of a casting. Metallographic examination will often reveal the presence of compacted graphite in the near vicinity of a nitrogen fissure. Fig. 4. Chemical analysis can also be used to confirm if nitrogen is responsible for a gas defect. SHRINKAGE DEFECTS With graphitic cast irons, the shrinkage on freezing varies according to many variables. The liquid contracts from the pouring temperature, however at some stage during freezing there is a volume increase. This expansion is due to graphite precipitation during the eutectic reaction. The temperature of and amount of expansion are not a constant and vary according to many influencing factors including; metal composition, the degree of superheat, the inoculation practice, the pouring speed, the mould material, the solidification time and cooling rate of the casting. The consistent production of shrinkage free castings in graphitic cast irons requires strict control of mould and metallurgical variables. Generally, successful feeding requires compensation of the volume loss up to the start of the eutectic expansion phase. The feeder, or more particularly, the feeder neck should then solidify some time between the start and end of expansion. The primary factor which influences when the neck should solidify is the mould material, or more particularly, the ability of the mould material to withstand the pressure of the expansion phase without undergoing excessive dilation. If the feeder neck solidifies too early, then the residual pressure may be excessive and cause the mould to dilate resulting in Figure 2. Cast-on time date code on high volume iron casting Figure 3. Gas defect on cope surface of iron casting. Figure 4. Compaction of graphite flakes, an indicator of possible high nitrogen WHO'S WHO OF METALS -- ANNUAL 2006/7