and 2 would have mainly originated from the misalignment

of the thermocouples and the possibility of the base thermocouple

impinging on the copper stool surface and becoming detached from the

casting (e.g. thermocouple 1, cast B 2 ) . Therefore, actual thermo­

couple displacements (Table 18) were determined utilising an optical ted

microscope with a gradua/ travelling stage. Inaccuracies in these

measurements (due to sample alignment problems) are likely, possibly

in the order of + 1mm. In the region nearer to the chill, where cool­

ing rates are changing rapidly with position, any slight inaccuracy

in measurement of position will, be much more significant and thus

account for the greater variability in cooling rates obtained from

the output monitored from thermocouples in this region.

There is a slightly greater spread of cooling rate for thermo­

couple number 8 than for thermocouple number 7 which is accentuated

by the logarithmic scale. The likely cause of this variation is the

occasional lack of total insulation above the feeder head permitting

slight radiation heat loss. This phenomenon is illustrated on Figures

51, 55/ 57 by juxtapositioning of the readings from thermocouples 7

and 8.

To carry out the statistical analysis (the results of which appear

in Table 19 and Figures 61 and 62) twenty or more readings were required

ment, readings from a range of positions had to be assigned to the

nominal thermocouple positions (Table 19). There are therefore, small

inaccuracies inherent in the mean and standard deviation figures and,

hence, the limits of significance ( | -1.96<J | ).

The aim of this work was to develop the technique used by Gadgil

and K o n d i c ^ 0 ^ to produce a variety of cooling rates to investigate

the effect of compositional variation on the structure of ingot

mould type iron. The variability in cooling rates exhibited by thermo­

couples number 1 and 2 was, therefore, not particularly significant

since, at these positions (Table 8b and Figure 9), white iron struct­

ures were produced.

(68)

Information indicates that complete solidification of an

ingot mould may take as much as ten hours, suggesting that the metal

within the mould wall cools almost uniformly at an extremely slow rate.

The technique adopted in the present investigation has clearly produced,

even at the greatest distances from the chill, cooling rates significantly

greater than those encountered in commercial ingot mould practice.

During the development of the experimental method, measures were

taken to reduce the minimum cooling rate produced in the castings by

increasing the section of the castings (see Appendix 1), reducing the

water flow rate and increasing the mould preheating furnace temperature.

Further reduction in cooling rates was not undertaken so as not to risk

significant heat input occurring to the casting within the furnace,

thereby incurring non-unidirectional heat flow conditions. To have

reduced the rate of heat abstraction still further would also have

In spite of the differences in cooling rate between the present

work and commercial casting and solidification of ingot moulds, the

work carried out here does, however, provide a useful general insight

into the interaction of sulphur, nitrogen and titanium in ingot mould

type irons.

5.2 THE MICROSTRUCTURES OF THE INGOT MOULD TYPE IRONS

\

During the investigation of the effects of varying sulphur,

titanium and nitrogen contents on ingot mould type iron, a variety of

structures were obtained. A number of features however, were found to

be common to the majority of the melts. These were:-

1 . pro-eutectic austenite.

2 . the formation of the carbidic eutectic in close proximity

to the chill.

i

3. the formation of 'directional' or 'streamer' graphite

(Gadgil and K o n d i c ^ 0 ^).

4 . the formation of a graphitic area between impinging streamers

and/or dendrites which has been termed the 'grey' region in

the present work, and

5. the formation of graphitic regions towards the top of the

5.2.1 Proeutectic Austenite

The proeutectic austenite dendrites are a pre-requisite of

(1 2 4 8 9)

solidification in hypoeutectic irons ' ' '

'

cooling the austenite underwent transformation to pearlite, which is

seen in all the photomicrographs, but, in some cases, there was partial

transformation at the dendrite arm extremities to ferrite and graphite.

At the chill interface the dendrite orientation was random due to

nucleation and growth of dendrites of various orientations on contact

with the chill. This random orientation only existed over a short

distance as those dendrites with a favourable orientation for growth

rapidly crowded out those less favourably aligned (Plates 36 and 66).

The dendrites were.very directional over the majority of the casting

lengths (Plates 44, 81) and often were clearly visible in graphitic

areas towards the top of the castings. In these regions the dendrites

tended to be more randomly orientated. It is thought that this was due

to lower temperature gradients and the influence of growing eutectic

cells rather than significant departure from unidirectional heat

abstraction (Plates 69, 85, 88, 114, 127 and 128). It is clearly visible

in Plates 176 and 186 that cells have interfered with growth.

In close proximity to the chill the dendrites were very fine

(Plates 36, 37, 66, 90 and 161) but, as expected with the lower cooling

rates prevalent at greater distances from the chill, the dendrites

coarsened (Plates 44, 60, 67, 81, 105, 113 and 130). In all of the

castings, except for Gl, the primary dendrites were very prominent.

However, in Gl, the dendritic appearance is not as marked (Plates 90