Computer-Aided Differential Thermal Analysis of Spheroidal and Compacted Graphite Cast Irons

1. SEMINAR
ON
Computer-Aided Differential
Thermal Analysis of
Spheroidal and Compacted
Graphite Cast Irons
BY :-
SANDIP GOPE
44107
Foundry Technology
UNDER THE GUIDANCE OF :-
Dr. NANDITA GUPTA
ASSOCIATE PROFFESSOR
Dept. of Foundry Technology

2. INTRODUCTION
One of the primary fields of application for this concept is the
prediction of the microstructures of casting alloys from their
cooling curves, especially for alloys whose processing involves
liquid treatment. Typical alloys falling in this category are
spheroidal graphite (SG) and compacted graphite (CG) cast irons.
If a cooling curve could be recorded and interpreted within 2 to 3
minutes after pouring, it would be possible to estimate the
microstructure of the iron before pouring it into castings, the
result being considerable savings in materials and work power

3. Cooling curves of nodular, compacted, and flake
graphite cast iron.

4. EQUIPMENTS USED
• The equipment used for recording and processing data from the
cooling curves is a specially designed microcomputer.
• The complete experimental set consisted of a Syscon 6800 data
acquisition and recording computer with 5'/4-in, disk drive
• Three signal conditioners, a televideo terminal, a digital plotter, and a
printer.
• To collect the data, three Quik-Cups produced by Electro-Nite,
mounted on separate holders and connected to the microcomputer,
were used. The Quik-Cup is a shell sand cup into which is inserted a
type-K chromel-alumel thermocouple. The internal dimensions of the
cup are approximately 4.13 cm (1.625 in.) in height, 3.81 cm (1.5 in.)
in width in upper square and 3.175 cm (1.25 in.) in width in lower
square.
• The type-K thermocouple is placed 1.75 cm (0.687 in.) from the
bottom of the cup.

5. COMPUTATION OF THE DERIVATIVES
OF THE COOLING CURVE
Computer program written in BASIC to calculate the first and second
derivatives of the cooling curves.
A typical cooling curve and its first and second derivative for a
hypoeutectic cast iron.
There are several critical points on the first and second derivatives of
the cooling curves

6. All the critical points detected on the derivatives of the cooling curves have
corresponding times. The timer starts to count after the temperature reaches its
maximum. Thus, it is also possible to correlate the durations of certain critical
points with the as-cast microstructures.

7. LATENT HEAT MEASUREMENTS BY
COMPUTER-AIDED THERMALANALYSIS (CA-DTA)
It is possible to calculate the latent heat of solidification by
subtracting the heat evolved during the solidification of the neutral
body from the heat evolved during the solidification of the cast iron
sample. This is differential thermal analysis (DTA). Using the
computer, it is possible to calculate the same latent heat through
integration of the area between the first derivative curve (heat
evolved during the solidification of the sample) and the zero curve
(heat evolved during the solidification of the neutral body). It’s
called as computer-aided differential thermal analysis (CA-DTA).

8. Influence of the Amount of Graphite
The summary of latent heat values as a function of the carbon equivalent of
the SG iron is given below:
Composition of SG iron Hypoeutectic Hypereutectic
Latent heat of solidification, CE = 4.00 CE = 4.68
Btu/lb°F 81.2 102.6
It is evident that as the CE increases (that is, as the amount of graphite in
the structure increases), the latent heat increases.

9. CORRELATION BETWEEN CRITICAL
POINTS OF THE CA-DTA CURVES AND
NODULARITY IN SG IRONS
The first derivative follows three different paths, with three different slopes,
between NPAE and NPE.
The average slope of these segments can be determined from the second
derivative curve by simply selecting the corresponding maximum and
minimum values of MSE, ASE II, and ASE III, respectively.
The scattering can be attributed to differences in carbon equivalent, pouring
temperature, nodule count, and the presence or absence of small amounts of
carbides. The pouring temperature greatly influences the cooling process of
the system..
The cooling rate of the whole system depends on the thermal conductivity
coefficient, which in turn depends on the temperature of the heat source
and the temperature of the surroundings
Both the carbon equivalent and the pouring temperature had a direct
influence over the critical points of the second derivative.1 For example, it
was found that a higher pouring temperature resulted

11. CORRELATIONS BETWEEN THE GENERAL
SHAPE OF THE CA-DTA CURVES AND THE
AS-CAST MICROSTRUCTURE OF CAST IRONS
Although the first derivative curve of a white iron exhibits three
paths, exactly as for SG and FG irons, the shape is quite
different, and has almost no visible negative peak. With regard
to white iron, it can be seen that, due to the lower eutectic
temperature, a small amount of pro eutectic austenite will
solidify, resulting in a completely different shape of the eutectic
region in the first derivative curve.
As for hypoeutectic irons, the FG iron exhibits the deepest
negative peak, the SG iron exhibits an intermediate negative
peak, and the CG iron shows the least deep negative peak. It
must be noted that the eutectic irons have deeper negative
peaks than the hypoeutectic ones.

12. CONCLUSION
This study has demonstrated that Computer-Aided
Differential Thermal Analysis (CA-DTA) is a viable
method to be used for the prediction of structure and
graphite shape in cast irons. It seems to be relatively
easy to predict graphite shape (SG, CG, or FG)from
the shape of the CA-DTA curves for both hypoeutectic
and eutectic irons. The same principles apply to
hypereutectic irons.
Also, it is easy to distinguish between irons solidifying
with gray or white eutectic.