The failure of graphite arc-furnace electrodes
Graphite electrodes used in steelmaking are joined together by threaded, tapered connectors (nipples) of similar material. The jointed regions are subjected to arduous thermal and mechanical stresses during use. Mechanical stresses arise from electrode self-weight and tightening torque, and the thermal stresses from the high furnace operating temperatures which ensure a high radiative surface cooling rate as the electrode is removed from the furnace. This thermal shock effect is thought to contribute to particular types of electrode failure. In this computer-aided analysis of the stresses induced by the above effects, a commercial finite element program is used in conjunction with a purpose-written finite difference program. Mechanical loads due to electrode self-weight and pretightening torque are evaluated and applied with suitable restraints to an axisymmetric finite element mesh, to obtain a mechanical stress analysis. The finite difference program is then used to calculate the time-variant temperature field experienced by an electrode on being removed from the furnace. An interpolation program is used to assign temperatures at the nodes of the same finite element mesh, the thermal stresses then being evaluated by the commercial finite element program. A 'failure envelope' analysis of the results identifies the critically-stressed regions of the joint and shows that in some such areas the thermal-shock stresses act to relieve the mechanically-induced stresses. A statistical analysis based on Weibull theory predicts a high incidence of crack formation due to thermal stresses. Finally, consideration is given to the effect of thermal orthotropy and temperature-dependent material properties.