A study on inhomogeneous distribution of temper graphite particles in strip-cast Fe-C-Si alloys
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I. INTRODUCTION
THE decomposition of brittle metastable carbide by heating white cast iron at temperatures above Ac1 point, the critical temperature for the transformation from ferrite (or pearlite) to austenite, is what is called first-stage graphitization (FSG) accompanied by nucleation and growth of temper graphite.[1,2] Cast iron strips can be directly produced from molten metal by using the unequal-diameter twin-roller (UDTR) strip casting process.[3,4] The cooling rate of the strips is estimated to range from 102 ⬚C/s ⬃103 ⬚C/s, which is obviously faster than conventional gravity casting. A fine and laminate solidification structure comprising a large quantity of eutectic cementite can be observed. To attain acceptable workability, graphitization treatment of cast iron strips should be performed. Our previous investigation[5] demonstrated that the fine solidification structure of white cast iron strips promotes a finer graphite distribution and faster graphitization rate. In particular, the structure of the cast iron strip grows along the direction of heat transfer due to the cooling effect of the upper and main rollers. Differences in cooling rates and solidification careers resulted in different structural morphology in different parts of the strip.[4] In addition, a change in the chemical composition of the strips will also give rise to morphology changes in eutectic cementite. Considering the lack of studies on the graphitization of cast iron strips, the effects of the characteristics of the solidification structure and composition changes (including carbon and silicon content) on the FSG properties of cast iron strips J.M. SONG, Postdoctoral Researcher, and T.S. LUI and L.H. CHEN, Professors, are with the Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan, Republic of China. Contact e-mail: [email protected]. Manuscript submitted June 7, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
were quantitatively investigated in this study. In addition, the nucleation of temper graphite was also examined in terms of the morphology and crystallographic textures of eutectic cementite. II. EXPERIMENTAL PROCEDURES Cast iron strips with different carbon and silicon compositions were prepared for this study. Their chemical composition and carbon equivalent (CE) (CE ⫽ pct C ⫹ (pct Si/3)) data are given in Table I. Each strip in the table is designated according to its carbon and silicon contents. Experimental materials were melted in a high-frequency furnace, and were then cast into an UDTR strip caster and Y-shaped chill molds for comparison. Figure 1 schematically depicts the setup of the strip caster in which the two rollers were made of a Cu1 wt pct Cr alloy. The casting speed (i.e., the tangential speed of the main roller) was maintained at a constant 16.8 m/min to obtain the white cast iron strips with a thickness of about 2 mm and a width of 100 mm. Due to the result that the cooling rate is most stable in the central area of the strips in the lengthwise direc
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