Thermomechanical coupling effect of graphite electrodes upon the Electric Arc Furnace dynamics

Prediction of dynamic behavior of electrodes of the Electric Arc Furnace (EAF) fed by AC current is rather difficult because of several phenomena superposed, particularly during the first step of melting process, i.e. the so-called perforation, and in case of melting of scrap. Unexpected ruptures of electrodes are often observed as a consequence of vibration. Dynamic excitation is applied by the vertical position control of the mast supporting the electrodes and by the Lorentz's forces generated by the magnetic flux provided by each electric phase. Moreover, the irregular distribution of stiffness along the electrode, being due to the sensitivity of material properties upon temperature, affects quite a lot the dynamic response of this structure. To identify the origin of the observed ruptures and to suitably predict the dynamic behavior of the whole system a modeling activity was performed. A numerical model of the EAF structures was built, by resorting to an integrated approach based on the Finite Element Method and on the Multi Body Dynamics, then it was preliminarily validated on an existing plant. It demonstrated that stiffening effect upon the graphite electrode induced by temperature distribution makes dangerous the action of the vertical position control, when it is applied too fast and excites the flexural modes of the electrode. Numerical model allowed refining the design of the electrode and improving the safety factor as well as finding some design requirement to suitably limit the operation of the position control system.