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Thermomechanical coupling effect of graphite electrodes upon the electric arc furnace dynamics

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Abstract

Prediction of the 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 the melting process, i.e. the so-called perforation, and in case of the 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 the 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 was demonstrated that stiffening effect upon the graphite electrode induced by the 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.

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Abbreviations

AC:

Alternate current

C 0 :

Thermal expansion coefficient at 100 °C

C th :

Thermal expansion coefficient at current temperature

D :

Diameter of the electrode

E :

Elastic coefficient, Young’s modulus

EAF:

Electric Arc Furnace

EMF:

Electro-Motive Force

E ref :

Reference Young’s modulus (computation of the distribution along the electrode)

FEM:

Finite Element Method

G :

Tangential elastic modulus

I :

Transversal inertia of the electrode cross section

I p :

Polar inertia of the electrode cross section

K :

Elastic constant

M :

Torque applied to the electrode

MBD:

Multi Body Dynamics

T :

Temperature

T 21 °C :

Temperature at 21 °C

T ref :

Reference temperature for the computation of the distribution along the electrode

x :

Distance between the cross section and the gripper along the electrode length

x,y,z :

Coordinates of the global reference of the EAF system

ΔC th :

Increment of thermal expansion coefficient

ΔT a :

Increment of temperature along the axial direction of the electrode

ΔT c :

Increment of temperature along the circumferential direction of the electrode

ΔT r :

Increment of temperature along the radial direction of the electrode

εa :

Axial strain inside the electrode material

εc :

Circumferential strain inside the electrode material

εr :

Radial strain inside the electrode material

σa :

Axial strain inside the electrode material

σc :

Circumferential strain inside the electrode material

σr :

Radial strain inside the electrode material

θ:

Cross section rotation in torsion

v :

Poisson’s coefficient

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  1. Department of Mechanical and Aerospace Engineering, Politecnico Di Torino, Corso Duca degli Abruzzi, 24, 10129, Turin, Italy

    Eugenio Brusa

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  1. Eugenio Brusa
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Correspondence to Eugenio Brusa.

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Brusa, E. Thermomechanical coupling effect of graphite electrodes upon the electric arc furnace dynamics. Meccanica 49, 2979–2990 (2014). https://doi.org/10.1007/s11012-014-0023-x

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