Graphite Electrode Consumption Mechanism in EAF Steelmaking
Graphite electrodes serve as electricity conducting material in electric arc furnace steel smelting, their consumption rate is in direct proportion to EAF furnace power. Heat resources used for modern EAF steelmaking are electrical energy and chemical energy. During smelting process, dephosphorization, decarburization, deoxygenation, and desulfurization are carried out, gases and impurities are removed, temperature and ingredients are adjusted. Graphite electrode application performance indicators mainly include adaptation ability and consumption rate. Electrode consumption rate is directly related to the mass of electrode itself. The consumption of graphite electrode during EAF smelting process mainly includes the following factors:
End Part and Outer Periphery Consumption
Electric arcs generated by graphite electrode are classified into long arc, middle arc and short arc, while the melting of furnace charge and temperature raising rate are decided by electric arc power. Arc length is in direct proportion to secondary voltage, and is inversely proportionally to secondary current and temperature raising rate. To increase smelting speed and to shorten smelting time, compulsory oxygen blowing, a kind of high chemical energy operation is adopted in most steel mills, which requires higher oxidization resistance and thermal shock resistance of graphite electrode. During smelting, graphite electrode end part consumption includes high temperature sublimation, chemical reaction with steel liquid and slag. Oxidization loss accounts for 2/3 the consumption of graphite electrode. Oxidization loss is the product of unit oxidization rate and area, and it is in direct proportion with time. The longer the heating time, the larger the consumption of graphite electrode. So it is necessary to equip water cooling spraying system on EAF electrodes. In normal smelting, graphite electrode cause steel liquid carbon increase is about 0.01%, end part consumption usually appears non-cone tip shape.
Graphite Electrode Residual Body Consumption During Smelting
Residual body consumption refers to non-production consumption, i.e., electrodes fall into EAF furnace due to production malfunction and become waste electrode. Residual body quantity is not only related to nipple and electrode mass, but directly related to furnace charge distribution, furnace atmosphere, power switch on operation, and other factors. Main apparent phenomenon is, residual body end part have large longitudinal cracks or split cracks, shelling or rupture caused by the oxidization of loose connected joint part. Electrode suffers external force and causes nipples or hole bottom rupture, improper furnace charge distribution caused large area charge collapse or irrational power switch on curve operation caused severe electrode rupture, the quality of electrode itself is not good and so on. This part of electrode consumption is not large during normal production, but the great importance is attached to it by users.
Electrode Surface Oxidization Caused Spalling and Cracking
In normal smelting production, if graphite electrode surface appears saga and crests, or spalling phenomenon appears, there must have carbon increase in steel liquid. This phenomenon in one hand reflects electrode weak resistance to oxidization and thermal shock, in the other hand indicates that horizontal oxygen blowing time is over long or oxygen blowing quantity is overlarge, furnace inner part and furnace top is oxygen rich, and therefore electrode oxidization loss is accelerated. If severe shelling phenomenon appears, electrode quality must be considered.
Graphite Electrode Rupture Caused Loss
Graphite electrode rupture is very usual during EAF smelting, and it is the most important factor affecting electrode consumption. Electrode rupture is mainly caused by man-made rupture and mechanical rupture. Man-made rupture mainly include: collision and scratch during hoisting, improper connection or wrong method, improper sliding of clamper, rigid impact or insensitive transmission control system. Mechanical rupture mainly includes machine malfunction, electrode quality problem and improper operation.
Electrode Body Rupture
One reason is structural defect or low strength of electrode, the second reason is short arc operation and accompanied with lateral impact of material collapse, the third reason is three-phase electrodes are significantly not perpendicular and slag is adhered to them or has furnace cover scraping phenomenon.
Electrode main body hole bottom rupture phenomenon
The first reason is electrode end part loose structure or dark fringe, or unmatched nipples and holes, or unmatched linear expansion coefficient due to material differences. The second reason is eccentric electrodes caused overlong electrode travel or insensitive ascending and descending control. The third reason is furnace charge is not properly loaded, nonconductive material is loaded below electrode.
Ruptured Nipple Appears Irregular Shape
One reason is nipple processing conicity is different or nipple ovality is overlarge, another reason is there is dust in connection hole, contact resistance is over large and causes over quick nipple threads local oxidization. The third reason is electrodes are not well connected and loose connection causes rupture. The forth reason is clamper is tilted, electrodes and furnace cover holes are not concentric.
Ruptured Nipple Appears Regular Shape
One reason is nipples quality difference, i.e., nipple strength can’t meet EAF furnace smelting requirement. Another reason is electrode holes and nipples tolerance are not matched or connection torque is lower than requirement and so caused loosening. The third reason is overlarge secondary current fluctuation or rapid increase, max. inrush current exceeds 1.2 times that of rated value. The forth is over large thermal vibration caused by overlarge input power, electrodes connection part appears red and resistance is overlarge.
