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.

Date of Award	Jul 1972
Original language	English
Awarding Institution	Aston University