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Materials Transactions, Vol. 49, No. 9 (2008) pp. 2163 to 2165 #2008 The Japan Institute of Metals RAPID PUBLICATION Relation between Surface Tension and Graphite Shape in Cast Iron D. Shi* , D. Li, G. Gao and L. Wang Department of Material Science and Engineering, Harbin University of Science & Technology, Harbin, 150040, P.R. China A relationship between the surface tension of cast iron and the graphite shape has been proven by experimental analysis and thus the graphite shape can be forecast by the surface tension. A specific relation between the surface tension and the graphite shape has been established by experiments. When the surface tension is above 1385 mN/m, nodular graphite occurs. When the surface tension is between 1283 mN/m and 1385 mN/m, vermicular and nodular graphite occurs. When the surface tension is between 1108 mN/m and 1283 mN/m, vermicular graphite occurs. When the surface tension is between 990 mN/m and 1108 mN/m, flake and vermicular graphite occurs. When the surface tension is below 990 mN/m, flake graphite occurs. [doi:10.2320/matertrans.MRP2008165] (Received May 19, 2008; Accepted July 4, 2008; Published August 13, 2008) Keywords: surface tension, graphite shape, specific relation, cast iron 1. Introduction It is well known that graphite shape has an important influence on mechanical properties of cast iron.1,2) To effectively improve the quality of the castings and reduce the amount of rejected materials, rapidly monitoring the graphite shape of cast iron is becoming very important.3) As a consequence, graphite shape monitoring is strongly recommended. Although many methods have been invented or developed,4–7) the most widely used ones are metallographic observation and thermal analysis. As accurate they are, their measuring time is too long and their operation is too complicated. Therefore, neither metallographic observation nor thermal analysis can meet the requirement upon the testing speed and the operative convenience. Hence, to effectively monitor the graphite shape and thus improve mechanical properties of cast iron, there is a need for a new and better method. Surface tension is one of important physicochemical properties of cast iron.8) It is the foundation of studying the interface dynamics.9) In solidification phenomena, surface tension plays important roles in optimizing and simulating the crystal growth parameters,10) forecasting the degree of the modification,11) porosity formation,12) mould filling capacity and surface roughness.13,14) In the present study, experimental analysis has shown that a relationship between the surface tension of cast iron and the graphite shape does occur. A specific relation has been also established by experiments. It will provide an effective method and convenience for rapidly monitoring the graphite shape of cast iron. 2. Relationship Between Surface Tension and Graphite Shape Early in 1951, Buttner had concluded that high interfacial tension of the liquid cast iron could promote the formation of nodular graphite, which has been approved by many researches.15,16) However, the measurement of the interfacial tension is very difficult, and so the method for forecasting the graphite shape by the interfacial tension is unfeasible. According to the interfacial tension theory and the crystal *Corresponding author. E-mail: [email protected] growth theory,17,18) one of conditions for the graphite growing into nodular shape is that the interfacial tension between the graphite and the liquid cast iron must be higher than between others, and it is the interfacial tension that decides final shape of the graphite. For the graphite’s aeolotropy, however, the interfacial tension between various crystal faces and the liquid cast iron differs. After surface active elements (such as oxygen and sulfur) are adsorbed to the edge plane between the graphite and the liquid cast iron, the interfacial energy and the contact angle will reduce, and the growth speed of the graphite will increase. In the end, the graphite will grow into the flake along the edge plane. Contrarily, because the modifying agents have very strong affinities for oxygen and sulfur, surface active elements on the edge plane will combine with the modifying agents and thus their quantity will reduce. Accordingly, the interfacial energy of the edge plane is much bigger than that of the base plane, and the interfacial tension and the surface tension increase simultaneously. As a consequence, the graphite will change from flake shape into vermicular or nodular shape. Whenever graphite particles occur in the liquid cast iron, there are three tensions among the graphite, the liquid cast iron and the air. Their relationships can be expressed by the eq. (1).19) LS ¼ SG þ LG cosð180 Þ ð1Þ where is the contact angle between the graphite and the liquid cast iron. Because the interfacial tension SG between the graphite and the air retains the same, the change of the interfacial tension LS between the graphite and the liquid cast iron can be indicated by LG cosð180 Þ, and the bigger is the interfacial tension LS , the greater is the surface tension LG between the liquid cast iron and the air. Furthermore, the graphite shape has close relation with the change of the interfacial tension LS . Hence, the graphite shape could be forecast instantly if the surface tension of the liquid cast iron were measured. 3. Experimental and Establishment of Specific Relation A modified maximum bubble pressure method20) is used to fast measure the surface tension of the liquid cast iron 2164 D. Shi, D. Li, G. Gao and L. Wang and the basic principle is as follows. A capillary is vertically dipped in the liquid cast iron and the dipping depth can change at the range of 10–20 mm. Pressurized inert gas is fast blown in the liquid cast iron through the capillary and bubbles will generate and vanish continuously at the speed of 2–3 bubbles per second. During the course of blowing gas, the pressure in the bubble is monitored by computer and the pressure curve is proved to be a sinusoid. According to the Laplace’s formula,21) the amplitude and the period of the sinusoid reflect the additional pressure caused by the surface tension and the speed of generating bubbles respectively. To simplify the calculation, the period of the sinusoid is replaced by the number of bubbles in five seconds. In addition, the temperature of the liquid cast iron and the capillary diameter will affect the calculation of the surface tension. Therefore, if the amplitude P, the number N of bubbles in five seconds, the capillary diameter x , and the temperature T of the liquid cast iron are fast measured, the surface tension can be calculated instantly by the following equation. (a) σ =782mN/m, flake (b) σ =894mN/m, flake (c) σ =1082mN/m, flake +vermicular (d) σ =1203mN/m, vermicular (e) σ =1360mN/m, vermicular+nodular (f) σ =1465mN/m, nodular ¼ 1:6641P þ 19:5022 N þ 0:2038 ðx 600Þ 0:3581 T þ 292:9447 ð2Þ According to this method, a device has been constructed to fast measure the surface tension of the liquid cast iron in five seconds. Many experiments have shown there are small deviations between the surface tension measured by this device and the reference values. Experimental materials for studying the relation between the surface tension and the graphite shape are pig iron Z14, modifying agent 1RETiMg10-5 and inoculant 75% FeSi. The composition of the liquid cast iron after modification is as follows: C 3.7–4.2%, Si 2.2–3.0%, Mn 0.3–0.5%, P < 0:06%, S < 0:02%, Re > 0:035%, Ca < 0:02%. To obtain the liquid cast iron with different surface tension and different graphite shape, the following melting process is adopted. Pig iron Z14 is melted in a high frequency induction furnace. Over the 1500–1520 celsius degree temperature range, the liquid cast iron is modified by the modifying agent 1RETiMg10-5, and the mass fraction of the modifying agent is varied from 0.0% to 3.0% in 0.025% steps. Then the liquid cast iron is inoculated by 75% FeSi. After inoculation treatment, the surface tension of the liquid cast iron is fast measured by the self-developed device over the 1340–1410 celsius degree temperature range. At the same time, the metallographic samples are poured and standard graphite shape can be got by microstructural analysis. When the temperature of the liquid cast iron changes over small temperature range, its surface tension will change slightly.22) Moreover, as shown in eq. (2), the surface tension has been amended by the temperature when the surface tension is fast measured by the self-developed device. So, the temperature can be neglected when the relation between the surface tension of cast iron and the graphite shape is established. Many experiments have been performed. When the quantity of the modifying agent differs, the microstructure and the surface tension are shown in Fig. 1. The modifying agent has a great effect on the microstructure and the surface tension of cast iron. With the increase of the modifying agent, Fig. 1 Microstructure and corresponding surface tension at different quantity of the modifying agent. the surface tension will change from the minimum to the maximum, and the graphite will change from flake to nodular shape correspondingly. Figure 1(a) shows the microstructure of unmodified cast iron and its surface tension. The graphite is flake shape and the surface tension is lower than the others, and it is 782 mN/m. When a small quantity of the modifying agent is added, although the surface tension increases and it is 894 mN/m, the graphite shape is still flake, as shown in Fig. 1(b). Then, from Fig. 1(c) to Fig. 1(e), the surface tension increases continuously with the increase of the modifying agent, and the graphite will change from flake shape to vermicular shape. When the modifying agent continues to be added, as shown in Fig. 1(f), the graphite will be nodular shape and the surface tension will reach the maximum, which is 1465 mN/m. Through detailed comparison and statistical analysis between the microstructure and the surface tension, a specific relation between the surface tension and the graphite shape of cast iron has been obtained, which is shown in Table 1. When the surface tension is below 990 mN/m, the flake graphite will occur. When the surface tension is between 990 mN/m and 1108 mN/m, the flake and vermicular graphite will occur. When the surface tension is between 1108 mN/m and 1283 mN/m, the vermicular graphite will occur. When the surface tension is between 1283 mN/m and 1385 mN/m, the vermicular and nodular graphite will occur. When the surface tension is above 1385 mN/m, the nodular graphite will occur. According to the specific relation, if the surface tension of cast iron were measured, Relation between Surface Tension and Graphite Shape in Cast Iron Table 1 Specific relation between the surface tension and the graphite shape. Surface tension (mN/m) Graphite shape 990 flake 990 < 1108 1108 < 1283 flake + vermicular vermicular 1283 < < 1385 vermicular + nodular 1385 nodular the graphite shape could be got quickly. 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