151 SOLIDIFICATION OF FLAKE, COMPACTED/VERMICULAR AND SPHEROIDAL GRAPHITE CAST IRONS AS REVEALED BY THERMAL ANALYSIS AND DIRECTIONAL SOLIDIFICATION EXPERIMENTS DORU MICHAEL STEFANESCU Professor of Metallurgical Engineering, The University of Alabama, Post Office Box G, University, AL 35486 INTRODUCTION When trying to understand the thermal behavior of solidifying cast irons, two experimental procedures seem to be especially helpful in gene- rating the expected type of information, which then can be used for a quantitative understanding of the phenomena. They are thermal analysis and directional solidification. The thermal analysis is based upon the principle that the thermal events on a cooling (or heating) curve are directly related to phase transformations occurring in the sample. Thus, a cooling curve is actually a record of the solidification history of the particular alloy for which the curve was obtained. On the other hand, directional solidification performed on Bridgeman type furnaces allows the control of the solidification process of the alloy under investigation by precisely monitoring the temperature gradient at the liquid-solid interface (G) and the solidification rate (R). Both G and R can be controlled independently. The results then are interpreted in terms of the G R product (which is the cooling rate) or of the G/R ratio. Thus, thermal variables are correlated to structural changes in the solidifying alloy. THERMAL ANALYSIS EXPERIMENTS Experimental Method and Equipment Used Quantitative thermal analysis of solidification has been done by means of traditional differential thermal analysis (DTA). When using DTA, the heat evolution or absorbtion due to phase transformation can be measured by taking the difference between the thermal events observed in the sample and in a neutral reference, which undergoes no physical or chemical trans- formation involving an absorbtion or evolution of heat within the tempera- ture range investigated. Unfortunately, the use of DTA has its limitations for foundry use, mainly due to the complicated and expensive instrumentation and the rigid requirements of sampling procedures. If the cooling curve data obtained by using a simple thermocouple-cup technique are stored in a computer, the first and second order derivatives of the cooling curve can be calculated and then used for a more complete interpretation of phase transformation. The derivatives of the cooling curve can also be used for quantitave thermal analysis, similar to DTA, if a neutral reference is simulated. This is normally done by using a portion of the cooling curve which is not affected by phase transformation to extrapolate the neutral reference (zero curve). The method, as described above, has been developed by Rabus and Wlodawer [1, 2]. Other researchers have used it to study the bulk solidi- fication of cast iron [3], to assess nodularity [4, 5], or to correlate mechanical properties of gray cast iron to parameters of the cooling curve [6]. Latent heat calculations have been attempted by either this method [2] or by a modified version [7]. In the present research the method has been used to investigate some possible correlations between data obtained from the cooling curve and its Mat. Res. Soc. Symp. Proc. Vol. 34 (1985) ( Elsevier Science Publishing Co., Inc.