Review of methods of obtaining hollow bar by electroslag remelting. Part II

Current state, possibilities and limitations of the electroslag piercing technology are reviewed. Features of process of forming a hollow bar and methods of external influence on the melting process and solidification of the metal providing high metallurgical quality of hollow bars are considered. Industrial development of the piercing method encounters some difficulties related to the durability of tooling. First, the maximum length of the hollow bar produced in this way is limited by the stiffness of the stem preserving the required accuracy of the mandrel location relative to the outer mold. Secondly, the disadvantage of this method is that due to the lack of special protective measures part of the mandrel located above the level of the metal bath is subjected to considerable destruction. One of the reasons for this phenomenon is the electrical discharge erosion of the mandrel surface as a part of the operating current passes through it during the direct electrode-ingot connection scheme. The destruction of the mandrel is also due to the falling of drops of molten metal on it, as the area of drop formation is located in the axial part of the electrode just above the mandrel. This in turn determines the high thermal load on the mandrel head. The working part of the mandrel made of copper is exposed to strong electro-erosive wear and requires changing after a few heats. To prevent the destruction of the mandrel one can apply external effects on dropping processes, drop movement path in the slag bath and change of the delivery location of the electrode metal. Also with the help of external effects on can change the shape of the metal bath to achieve size reduction of crystallites, avoid crystallization defects, and increase refining capacity and productivity of the process.