- 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.