MEASUREMENTS OF INTERFACIAL HEAT TRANSFER COEFFICIENTS DURING THE CASTING OF HIGH TEMPERATURE ALLOYS IN SAND MOLD AND CERAMICS MOLD

摘要

This study is conducted to measure the interfacial heat transfer coefficient (h) between high temperature casting alloys and molds during the casting due to gap formation, which is caused by thermal expansion of mold as well as metal shrinkage during solidification. The high temperature casting alloys of concern are Inconel-600, SKD-61 and SKT-4. The mold materials include sand and ceramics. The interfacial heat transfer coefficient is determined by using the Inverse Method with measured temperature data and known thermo-physical properties. To comply with the requirements of the Inverse Method, one-dimensional heat transfer system from casting to mold needs to be devised. Due to the high melting temperatures of the casting alloys, one-dimensional heat transfer system is very difficult to be approached by directly using the heat insulating materials. Therefore, a spherical casting system is designed and constructed in the study. With its radiative heat transfer characteristics, the temperatures under one-dimensional condition along the radial direction are measured. Also, the Inverse Method under Cartesian coordinate system has been transformed into the Spherical coordinate system. Six spherical casting experiments for various combinations of casting alloys and mold materials are conducted. Temperature variations in casting alloys and molds are recorded. By using the Inverse Method, the interfacial heat transfer coefficients between various alloys and mold materials are obtained and compared. The results obtained from this study indicate that, for all combinations of casting alloys and mold materials, the profiles of h values have the same tendency. A high value of interfacial heat transfer coefficient is generally obtained at the start of the casting. The value drops abruptly and then rises to a certain value. From that on, the value gradually decreases. For castings with ceramics molds, the h values are in the range between 200W/m{sup}2. K and 350W/m{sup}2. K. For castings with sand molds, the h values are in the range between 40W/m{sup}2. K and 90W/m{sup}2. K. It is also observed that the h value is not considerably affected by the casting alloys but rather by the mold materials. The h values for casting in ceramics mold are three to four times higher than those in sand mold.