Abstract

Porous plugs are used in secondary metallurgy for the production of high purity steels. It can improve steel cleanliness, homogenization of composition and temperature. The porous plug is usually assembled on a metal can that is connected to a gas inlet pipe. Then this porous plug is fitted in the bottom of the ladle and through it nitrogen or argon is blown into the molten metal bath. To ensure a good performance of the stirring system, the plugs are tested for leakage and outflow behavior. Nitrogen is used to test the leak-proofness of the entire stirring system on the ladle and a hydrocarbon gas like propane and natural gas can be used to check the flame pattern of the porous plug. Several wear mechanisms of the plugs have already been studied and reported in the literature, like abrasion due to back attack of the gas, slag and FeO corrosion, peeling and cracking due to thermal shock and infiltration of molten steel. But little has been said about the decrease in gas permeability due to carbon deposition in the porous structure of plug. According to this study it was confirmed that during the life of the plug, when it is submitted to gas flame test pattern, the propane and natural gas can be cracked and cause carbon deposition, what can decrease the efficiency and as a consequence the life of the plug. After conducting a test with a hybrid plug in a customer, it was removed from the ladle and sent to the lab for a post-mortem analysis. The test results shown a carbon deposition throughout the entire extension of the plug. The characterization of the plug was performed using chemical analysis, optical microscopy, density, porosity, permeability and total carbon. After checking the quality control parameters of the plug and making sure it was produced according to the specification, comparative measurements of total carbon and permeability have been performed along the used plug. It was observed by this analysis that there is a direct correlation between carbon deposition and permeability decrease. The greater the carbon deposition, the lower the permeability of the plug. The carbon deposition has been originated from the hydrocarbon gas cracking used to check the flame pattern on the porous plug. It was also influenced by the long time spent for the flame test. According to the results, the carbon deposition directly influences the plug performance by reducing its permeability and therefore decreasing the gas injection efficiency.