Solidification and Cooling of Grey Cast Iron in Shell Moulds with Particular Reference to the Sub-surface Structure

Abstract

Shell moulded grey iron castings exhibit excessive amounts of ferrite and 'D' type graphite in the surface structure when compared to conventional green sand castings. A review of the literature does not reveal any satisfactory explanation for the structures which pieee although this gives some indication of the undesirability of the phenomenon and some means of overcoming it. For this reason, to enable a better assessment of the structures arising, the solidification, of grey Gast iron in shell moulds for sections up to 2.5 cm in thickness is studied and discussed in detail. The thermal properties of various moulds, in terms of chilling power, were measured. It was found that the chilling power of silica shell moulds was 10% less than for a green sand mould with a nominal wall thickness of 5cm. The rate of heat extraction: dQ/dt for times less than 3 minutes, however, is marginally higher for silica shell moulds. It was found that resin content has no effect on chilling power value and it is suggested that the principal reason for the difference in chilling power between clay bonded and silica shell moulds is the difference in mould thickness. It was found that the generally accepted longer solidification times for shell moulds opposed to green sand moulds do not exist at depths of 1 mm and 3 mm from the surface of 0.5 cm section plate castings. The solidification times were found to be the same although 'D' type graphite was formed only in the shell casting. The existence of ferrite in the structure arisesdue to the graphite form being 'D' type, although the rate of geheing after @lidification was shown to be very slow for shell moulds, when castings are left in the mould. The depth of chill measured by wedge testing was the same for silica shell and clay bonded moulds. It is illustrated, however, that the mode of solidification is different and that a casting skin occurred on shell moulded specimens. Experiments showed that the eutectic temperature was not influenced by the type of mould material, although a greater degree or undercooling occurs with shell moulds. It ‘is suggested that the occurrence of D' type graphite in shell mould castings is due to the mode of solidification modifying the graphite form, other than increasing the rate of solidification or reducing the eutectic temperature. Zircon and Olivine shell moulds are shown to behave in an identical manner to similar silica moulds.