The Mechanics of Consumption of Graphite Electrodes in Electric Steel Furnaces

Transcription

1 140 Electric Furnaco Proceedings, 1971 The Mechanics of Consumption of Graphite Electrodes in Electric Steel Furnaces In the economics of electric steelmaking, two cost items can be determined with ease and accuracy: the specific energy consumption (kw-hr per ton) from meter readings and specific electrode consumption (Ibs of electrode per ton of steel produced) from number (or weight) of electrodes consumed relative to steel produced over a given period of time. I As we shall see later, it is important to realize that these items depend on many other process variables; although it is desirable to keep their values low, the minimum overall cost of producing a ton of electric steel is not necessarily reached when kw-hr/ton or lbs electrode/ton are at a minimum rate. Electrode consumption can be classified in two main categories, as illustrated in Figure 1. At the tip of the electrode column, the rapidly traveling arc spot, and the slag and metal, remove graphite in an incremental fashion without abruptly shortening the length of the column. Similarly, oxidation of the sidewall causes progressive tapering of the electrode. Arc tip, or linear (vertical), consumption occurs mostly when the furnace is under power, whereas electrode sidewall consumption in the horizontal direction occurs at all times when the electrode is hot. This combination constitutes technological electrode consumption and is controllable to some degree by furnace practice. In contrast to this gradual type of consumption occurring on the surface of the electrode, electrode column length can change abruptly and drastically by breakage, stubbage, and splitting. The causes for this type of consumption can be found mainly in the areas of furnace practice, type of scrap, regulator malfunctions, incorrect electrical phase sequence, and electrode handling and joint making. Human error, of course, plays a very important role. The underlying principles of both types of consumption will be discussed in this paper. ELECTRODE CONSUMPTION AT THE TIP OF THE ELECTRODE In order to understand the mechanism of consumption at the arc tip, we must study the flow pattern of electric current in the column. The distribution of the current over the cross section is nonuniform due to a variety of electromagnetic effects (skin effect, proximity effect), and by the temperature-dependent resistivity and thermal conductivity at various locations throughout the column and its joints. Figure 2 shows the current densities measured on the surface of 24" diameter electrodes near the roof portholes of a typical large ultrahigh powered furnace. The crowding of the current toward the axis of the furnace is obvious and leads to a substantial raise of the electrode temperature in these areas. (Needless to say, the thermal stresses resulting from this situation are eccentric, and the rate of sidewall oxidation is strongly affected by temperature.) Further down, about a foot (250 mm) above the arc tip, the current begins to concentrate toward the hot spot of the arc, which is roughly Y4" to Yz" in diameter (as schematically shown in Figure 3). The temperature of this coin-size area is 3600"-4000 C (6500" F), and the rest of the surrounding graphite body is at C (3600"-4200"F). The major portion of linear incremental consumption of the graphite electrode takes place in this arc spot. A detailed analysis of the physics of the furnace arc with respect to electrode consumption is beyond the scope of this paper. For practical purposes, it is sufficient to identify the effects of the arc on electrode consumption as "vaporization". ELECTRODE SIDEWALL ELECTRODE CONSUMPTION I! INCREMENTAL CONSUMPTION OF ELECTRODE COLUMN! ARC AClION SLAG EROSION OXIDATION I SUDDEN SHORTENING OF ELECTRODE COLUMN Fig. 1--Categories of electrode consumption. THERM~L SHOCK JOINT MAKEUP HANDLING Electrode Di~rneler 24" = 610 rnrn C&4 hose Sequence 1-1, L12Ll, 3 W. E. SCHWABE is Associate Director of Research and Engineering, Union Carbide Corp., Carbon Products Div., Parma Technical Center, Parma, Ohio. Fig. 2-Current densities measured on surface of 24" diameter electrodes near roof portholes.

2 Interrelation of Materials and Equipment 141 Fig. 3-Equipotentiol and current flow lines in an electrode tip section. Original diameter of electrode24" (610mm). Tip diameter- 17" (430mm). Diameter of arc spot-o.5" (12mm). Current-70,000 amp rms. From observations of small arcs and test work conducted in experimental arc furnaces, as well as large production units, it may be concluded that the rate of linear consumption increases when operating currents and/or power is increased. As increased power and current levels usually result in higher productivity of the furnace, electrode consumption in lbs/ton of product sometimes remains unchanged. In addition to electrode consumption by arc effects, analyses of high speed arc motion pictures have shown that graphite particles are removed from the arc spot and its vicinity. As schematically shown in Figure 4, thermal expansion of the arc spot yields radial and axial forces, f. and f., which may pry graphite particles loose. The electric current flowing through the arc tip and the hot spot into the arc column generates forces of its own. The pinch effect force, f,, opposes the radial extension force, f,. Furthermore, a force, f,, in the axial direction Fundamentals of Tip Consumption of an Electrode Al = Primary Zone of Arcing A2 = Secondary Zones of Arcing E = Zone af Erosion Fig. 5-Schematic arc tip. presentation of consumption of an electrode at the opposing the expansion force, fa, is present because of the streamlined shape of the current flow lines through the lower part of the electrode. To what extent a cancellation of these opposing forces of different origin may take place is not known. In addition, erosion of the electrode end face by metal and slag also adds to linear consumption. Finally, oxidation of the electrode end face contributes in a minor way. As a result of arc flare forces in furnaces with electrodes arranged in an equilateral triangle, electrodes are consumed in a pattern illustrated in Figure 5. Three zones of consumption on the end face can be distinguished, however their borderlines may overlap considerably. Long arcs and/or strong flare cause the arc tip to bevel at angles of 20"-30, whereas the "heel" of short arcs is distinctly eroded by slag and metal, resulting in more or less horizontal arc tips (see Figure 6). Short high current arcs tend to form a slightly concave tip (see Figure 6c). CONSUMPTION OF THE ELECTRODE SIDEWALL In contrast with "linear" consumption, sidewall consumption of the electrode column acts more or less in the horizontal direction. It causes the column to taper DIRECTION OF FORCES CAUSED BY EXPANSION fa AXIAL DIRECTION OF FORCES 3 + fp CAUSED BY FLOW OF CURRENT f FORCE IN CONDUCTORS HAVING A VARIABLE CROSS SECTION Fig. &Direction electrode. P'x{LFECT of forces existing in an arc spot on the tip of an Long Arc Medium Arc Short Arc Fig. &Electrode tip configurations for three different arc- lengths.

3 142 Electric Furnace Proceedings, 1971 and affects mostly the portions of the columns which are below the furnace roof. The most important factor of sidewall consumption is oxidation. The oxygen content of the furnace atmosphere during a heat undergoes great changes. An oxygen content in the furnace atmosphere equivalent to that existing in air is seldom reached unless excess oxygen from oxygen lancing flows over the electrode surface or, in cases in which the furnace is equipped with a fume extraction system, excessive amounts of air are drawn into the furnace. Of minor, although not negligible, importance is condensation of metal and slag vapors on the electrode, a phenomenon which causes droplets to form in the middle section of the electrode. The droplets, by way of gravity, then run downwards toward the tip, from where they fall back into the bath. On their way down, they pick up small amounts of graphite. During flat bath operation with short arcs, the outer edge of the electrode tip is continuously rounded off as a result of abrasion from contact with slag and metal. This effect is pronounced when continuously fed prereduced materials (metallized pellets) are used. Although the oxidation rate of electrode graphite depends, to some extent, on the grade of the graphite, it depends strongly on the surface temperature of the electrode, velocity, and the turbulence of the passing gas and its oxygen content. Effect of Temperature and Velocity 00 Surrounding Atmosphere on Oxidation Experiments to compare the effect of temperature and velocity of typical furnace atmospheres have been conducted under controlled conditions in wind tunnels at different temperatures and velocity values. Figure 7 presents the results from a test series in which air is used, indicating that increasing air velocity, and the surface temperature of the graphite markedly increase the rate of oxidation. Surface temperatures of the graphite electrode under 600 C (-1100 F) are, for practical purposes, noncritical, since appreciable oxidation does not occur. The oxidation rates illustrated in Figure 7 are higher than those experienced in normal electric furnace practice, because the oxygen content of the furnace atmosphere is considerably lower than that which prevails in the wind tunnel. Test results covering Grade AGR and Grade AGX electrodes are shown in Figures 8 and 9. The lower rate of oxidation of Grade AGX, particularly in range of 1100"-1600 F (600"-90O0C), is quite obvious. Aside from these influences, sidewall consumption is in direct proportion to time of exposure and, therefore, Fig. 7-Rate wind tunnel Su) lcn, 1100 lax, 13Ul Temperature, O C of oxidation of electrode graphite vs, temperature in Temperature AGR AGX Fig. 8-Oxidation rates of grade AGR and grade AGX electrodes in air vs. surface temperature (wind tunnel). - Temperature IOft/sec =3.05 m/sec 5 fl/rec = 1.53 m/sec Air Velocities - - Fig. 9-Oxidation of graphite in wind tunnel-air ratios of oxidation rates AGX/AGR vs. temperature for different air velocities. increases with the time of residence of a particular surface unit in the furnace system, counting from the first exposure to oxidation in the upper part of the column to the time when this portion arrives at the edge of the end face before final consumption in the arc zone. Large residence times can lead to pronounced penciling of the electrodes, a condition which is sometimes erroneously interpreted as that which results from insufficient resistance against oxidation of the electrode involved. On the other hand, a short residence time of the electrode in the furnace due to high linear consumption results in less electrode taper. The oxidation rate around the periphery of the electrode column is not equal at all points. Effects of draft through open doors, glands, and/or fume control systems may generate flow patterns in the furnace atmosphere, enhancing oxidation in certain areas on the electrode surface. This condition results in eccentric deformation of the electrode columns. In some furnaces, the surface portions lying within the electrode triangle oxidize somewhat more than the rest of the surface. Mutual heat radiation from these portions raises the surface temperature in these sections of the columns and causes higher rates of oxidation. This eccentric temperature pattern of the electrodes is often indicated during furnace operation on the section of the electrode columns between holder and roofline where the border lines between visible red and black of the electrode surface show a downward slope from the inside of the electrode triangle to the outside.

4 Interrelation of Materials and Equipment 143 During the process of oxidation, which takes place not only on the sidewall but also on the arc end face of the electrode, heat is released due to combustion. The theoretical value for the heat is approximately 3.4 kwhr/lb (7.5 kw-hr/kg) of graphite. Based on these figures, one may speculate that the combustion heat amounts to kw-hr per ton of steel. Whether or not this energy can be classified as useful to the melting process is unknown; however, the heat of combustion does perform a useful purpose when the hot electrode columns are withdrawn from the furnace during recharging. The heat of combustion counteracts to some extent the cooling of the surface caused by radiation and thereby reduces tangential stress near the surface of the electrode during this state of thermal shock. COMBINATION OF LINEAR CONSUMPTION AT ARC END AND OXIDATION OF SIDEWALL Linear consumption at the arc tip and oxidation on the sidewall occur, for practical purposes, simultaneously if one considers furnace operation over longer periods, for example, days, weeks, or months. The geometry of the electrode column permits interesting conclusions as to the relative significance of both contributing factors, as shown in Figure 10. In most furnace operations, one finds that sidewall oxidation comprises approximately 50% of the "net consumption" (or consumption excluding breakage, stubbage, etc.). Linear rate of consumption per phase, defined in inch or centimeter per operating hour, can be readily calculated from operating data. One example is graphically shown in Figure 11. For given electrical operating conditions, the rate of linear consumption is a function of area of the electrode tip. Generally, it increases as the tip area decreases. Linear electrode consumption, then, depends on the following factors: 1. the height of the electrode column exposed to sidewall oxidation, usually from roofline to arc tip; 2. the rate of oxidation, given by furnace conditions (draft, air flow) and grade of graphite; and 3. residence time of electrode column between roofline and arc tip. One example of the results of mathematical analysis of these conditions is shown in Figure 12. Here, linear consumption is plotted against height of the tapered column for various rates of oxidation. The point, P, on the solid line in the center is based on observation. (In this case, the electrodes were red hot almost up to the holder, covering a column height of 173" in a 22' diameter furnace.) The curves were calculated on the basis Fig. 10-Percent taper ratio, d/d. Net Consumption, C = Ti I si Tip Consumption, Ti. = C. (d/d)2 Sidewol l Consumption, Si = c[i-(~/d)~] of Tapered Electrodes d/d net consumption of sidewall and tip as function of I O- CONSUMPTION (eo). Kg/ Ton Metr~c , I I 5, CONSUMPTION (eo). Ibs/ ton Fig. 11-Linear consumption per column of 24" diameter AGX electrodes as function of consumption rate (Ib/ton) and furnace productivity (ton/h). I I h = 0. IBIO" ( 25%) 10 - h =0.1455" 9 (0%) f j 8 h =0.1091" (- 25%) -.- 'x & 5 0 U b 4 c h =O Height (H-inches) of Tapered Section of Column Hourly Rote. of Oxidation (Reduction of Electrode Rodiur) Fig. 12-Linear consumption vs.. electrode height for 24" diameter electrodes at various sidewall oxidation rates, h. of rate of linear consumption being inversely proportional to the electrode tip area. Also shown are the results referring to oxidation rates 25% higher and lower. The following conclusions can be drawn: 1. When the rate of sidewall oxidation increases, for example, by 25 percent over the existing value, linear consumption increases from 7.2"/hr to 8"/hr or approximately 11%; 2. Lowering oxidation rate by 25% lowers linear consumption to 6.35"/hr or by approximately 12%; and 3. Reducing the height of the tapered portion of the column by 25%, or 43", would also reduce electrode consumption by 12%. (This fact illustrates the significance of the height of a furnace; a tall furnace is automatically penalized with higher electrode consumption.) OTHER FACTORS OF CONSUMPTION Electrode breakage high in the column is the major contributing factor in this category. The principal cause for such breakage is falling scrap resulting in cantilever action often fracturing the electrode joint closest to the clamp (top joint). The direction of this impact force is generally aimed from a point outside of the electrode triangle more or less toward the center of the furnace. It is interesting to note that the three electrode columns are also affected by electrodynamic forces in the opposite direction, forces which tend to separate the columns in the same direction as arc flare (see Figure 5). These forces increase with the square of the current amperage and decrease with the distance between the electrodes.

5 144 Electric Furnace Proceedings, 1971 Table 1. 17' Diameter Furnace, 20" Diameter Electrodes, 66 Tons Hot Metal Per Heat Average Current Productivity Power Level Meltdown ka Tap-Tap h:min tous/hr Elapsed - - Electrode Relative Values Consumption Produc- Electrode Pr Ibs/tou tivity Consumption - Liquid Pr E r Er Low High UHP Table II. K-Values 24' Diameter ( ) ' Diameter ( ) ' Diameter ( ) In a large furnace in which a current of ka is used, these forces can amount to several thousand pounds. The presence of forces of this magnitude demands good electrode practice with respect to handling and joint making, as well as nonnal functioning of the regulator mechanism, and absence of excessive mechanical movements of the arms. Stub losses may be caused by a variety of conditions, such as loose joints (which may be due to improper joining practice) and/or wrong phase rotation (right l l,, W regr.- Electrode Consumption vs. Top-Top Time - 20" Dio. AGR 43 Ko Meltdown Single slog lb/ton/hr,,,,,,,, I f Top/Top Time, hrs. Fig. 13-Electrode Electrode Diameter consumption vs. tap-to-tap time. c - $2 7.: E * - Em g- & - a E, 2-56 u w 2 handed threads require counterclockwise phase rotation) of the furnace. Excessive thermal shock, developing when a column is raised out of the furnace chamber, may establish tangential stress on the surface and, under extreme conditions, even split an entire electrode section lengthwise. When, during meltdown of heavy scrap, the arc remains stationary, e.g., on the outer edge of the tip, severe temperature gradients can develop generating so-called V-cracks which may propagate to the nearest joint. Depending on the depth of the crack, the stub may split open and drop off. Whatever the reasons for a sudden shortening of an electrode may be, if it affects a well-tapered column, the arc finds an 'enlarged area represented by the fractured surface. Many hours of furnace operation are then required to reestablish the normal column taper typical for this furnace. ELECTRODE CONSUMPTION AND FACTORS OF FURNACE OPERATION Effect of Tap-to-Tap Time Electrode consumption at the arc tip, caused mainly by arc action and erosion by slag and melt, occurs during power-on periods. In contrast, sidewall oxidation occurs practically at all times, both during power-on and power-off periods, as long as the electrodes are hot. Furthermore, during these periods, such effects as spalling and thermal shock exposure may occur. Obviously, when furnace operations are slow, specific electrode consumption must be high. The effect of furnace productivity (or tap-to-tap time) upon electrode consumption is shown in Figure 13, where the results of a 17-ft diameter furnace (65 t) producing single slag heats are shown. Shortening the Electrode Diameter 9 A 24" = 600 rnm Heat Time (Tap-Tap),.,,.. h Fig. 14-Specific electrode consumption vs. heat time, single slag and double slag operation combined. Heat Time (Tap-Tap), h Fig. 1jSpecific electrode consumption vs. heat time.

6 Interrelation of Materials and Equipment 145 t 20" AGR Single Slag 3 hr. 4 hr. 30min.3 hr. I Roductivitv 1517f /4,2hr.30min.,2hr.20rnin. / 2 hr. Tap/ Top Time Short ton, Metric Tons Kg/Ton Chorged Metric Ib/ton Charged Short - 2 MD Current Fig. IbTwenty-inch diameter electrode consumption (Ibs/short ton) as function of meltdown current, J, for various tap-to-tap times, T. tap-to-tap time by one hour reduced electrode consumption by 3.9 lb/ton. Figures 14 and 15 show the results of computer studies covering numerous furnaces in which 20" diameter and 24" diameter electrodes were employed and where single and double slag heats were produced. Here one hour tap-to-tap increment changes electrode consumption by 1, respectively, 1.3 lb/ton. Effect of Current Levels Used During Meltdown Frequently, tap-to-tap times are shortened by increasing furnace power and operating currents. An interesting example is shown in Figure 16, which shows the history of a 17-ft furnace in which 20" diameter electrodes were employed before and after conversion to ultrahigh power. Principally, when higher operating currents are used, the rate of "burn-off" during power application increases; however, scrap is melted faster and production rate is increased. This result is also demonstrated in Table I and Figure 17. This 19-ft, 77-ton UHP furnace, equipped with a 65- mva transformer and 20" diameter electrodes, was operated at various meltdown current levels ranging from 40 to 62 ka. Electrode consumption and productivity increase with current; however, if one relates both variables to each other, one finds that an increase of electrode consumption from 10.2 lb/ton at 40 ka to 11.2 lb/ ton at 50/55 ka or 10% is accompanied by a productivity increase from 27.5 t/hr to 34 t/hr or 23.5%. The ratio P./E, is also Similar relationships between furnace productivity, current levels, and electrode consumption have also ~ Arc Current (kaj '50 '55 '58 '58 ' s";b'i: & :; Fig. 17-Electrode consumption and productivity of a 19-ft diame+er UHP furnace-tapping weight 77 tons, transformer rating 45/65 mva. been established at larger and smaller furnaces. Statistically, electrode consumption, C, can be expressed as where C = Electrode consumption (lb/ton), K = Numerical factor, related to operating characteristics,, J = Current during meltdown (ka), and T = Tap-to-tap time (h). Typical K-value ranges are listed in Table 11. A variety of statistical relationships have been developed to correlate electrode consumption with operating factors. Although useful as guides, their weakness is that, in many cases, no precise information co,ncerning magnitude of the operating current is available. I CONCLUSIONS The mechanics of electrode consumption in an electric arc furnace are quite complex. Some factors are well understood, while others require still more indepth study. The objective, of course, is to keep electrode consumption as low as possible. This benefits the steelmaker in terms of low cost per ton of product, and the electrode manufacturer by enhancing the competitive position of the arc furnace in the steel industry. I DISCUSSION by J. S. Davis and P. Schroth ELECTRODE BREAKAGE PATTERNS AS A FUNCTION OF ELECTRODE/NIPPLE STRENGTH RELATIONSHIP INTRODUCTION Total electrode consumption results from longitudinal and transverse loss as well as breakage. The wear rates in the longitudinal and transverse directions are gen- J. 5. DAVIS and P. SCHROTH are affiliated with Research and Technology, Armco Steel Corp., Middletown, Ohio. erally referred to as net consumption. When loss due to breakage is added to the net consumption the total is designated as gross consumption. Net consumption is responsible for roughly 70 to 90% of gross consumption. Breakage accounts for the remaining 10 to 30%. In spite of the fact that net consumption constitutes the largest part of electrode wear and electrode material cost, at this point in time very little can be done to significantly reduce net consumption short of drastically altering the electric arc furnace process. To reduce costs, emphasis must be placed on those causes responsible for electrode breakage. Although breakage represents primarily a material cost, it also will show

7 146 Electric Furnace Proceedings, 1971 up as cost related to non-productive furnace time and interrupted smooth operations. ECONOMICS OF BREAKAGE LOSSES It is obvious that reduced graphite electrode breakage will lower gross electrode consumption. But, still just as important, to increase the salvageability of electrodes will also decrease gross electrode consumption. Figure 1 defines the socket area and indicates the three important planes of reference, namely (1) top of socket, (2) center of pin, and (3) bottom of socket. Figure 2 illustrates the types of breaks most frequently experienced. Important here is that when either a top or bottom of socket break occurs a portion of the electrode is destroyed. From the standpoint of salvageability of broken electrodes, the pin break and top of socket break offer the greatest potential. When a nipple break is experienced, the portion of the nipple remaining in the fallen section is merely removed. The salvaged electrode can at least be reused as a starter section. If the fallen piece contains two electrode sections, it can be disassembled and the upper section can be used again as a regular addition to the furnace. A top of socket break offers the same salvage potential as the center of pin break because only the collar of the uppermost section need be Socket Area Fig. 1-Electrode column. Top of Socket Center of Pin Bottom of Socket Top of Socket Center of Pin Bottom of Socket removed and discarded. Therefore, of the three most frequently experienced types of breaks, two are less costly. However, an electrode joining system can be designed only in two ways: (1) the nipple is weak relative to the electrode so a high percentage of center of pin breaks ensue or (2) the nipple is strong relative to the electrode so there is a predominance of collar breaks. If there is a tendency for collar breaks to occur, only chance will decide if there will be a top or bottom of socket break. Since the bottom of socket break offers the least potential for salvage, the electrode joining system must be designed in such a manner that the nipple is weaker than the electrode. Only such a design will preclude the frequent occurrence of bottom of socket breaks and insure the highest salvage rate. OPTIMUM JOINT SYSTEM DESIGN The proper relationship between electrode strength, nipple strength and nipple size can help to determine (1) the breakage rate, and (2) the breakage pattern. (1). The breakage rate is the total number of breaks experienced as a percentage of the total number of electrodes added. (2). The breakage pattern is the number of collar breaks and nipple breaks as a percentage of the total number of breaks experienced. It must be made clear that the joining system design considerations regarding the relative strength of electrodes and nipples are intended to alter breakage patterns without increasing breakage rates. Only if the breakage rate remains unchanged or is decreased will changes in the breakage pattern favorably affect the economics of the process. In order to develop this approach further, calculations of the optimum assemblage have been conducted. It is understood that other variables might also affect breakage rates and patterns. In spite of all these factors, experience has shown that the maximum load bearing ability of a joint does control overall breakage pattern and breakage rate. ELECTRODE AND NIPPLE LOAD BEARING ABILITY To calculate the load bearing ability of an electrode joint, based on the flexural strength of graphite, the cross-sectional area of the nipple at its maximum diameter is first calculated. The area is multiplied by the flexural strength to yield the maximum load that can be sustained. Table I shows the calculated strengths of nipples from 9%"~ through 12%"~ with flexural strengths ranging between 1950 and 3000 psi. Next, the strength of the electrode at the plane corresponding to the normal top or bottom of socket break is calculated. The calculation accounts for the taper of the socket. The cross-sectional area of the socket breakage plane is affected by both the nipple diameter and the overall length of the nipple. In any event, decreasing the area of the socket breakage plane lowers the maximum strength of the electrode. Tables I1 and I11 illustrate the maximum load that an electrode can sustain at the socket plane for 20"~ and 24"~ electrodes for different nipple diameters and electrode flexural strengths. The dimensions of the most frequently used commercially available standard nipples are noted in Table I. Table I. Nipple Load ~ea'rin~ Ability Maximum Load Bearing Abillty (Ih) Area at with Nominal Flexural Strengths of: Maximum Nipple Diameter Size (in.) (in.2) Psi psi psi PSI Fig. 2-Types of electrode breaks.

8 Interrelation of Materials and Equipment 147 Table 11.' Electrode Load Bearing Ability 20"+ Electrodes FLEX. STR. FLEX. STR. APPLIED P.S.1. P. S.I. Maximum Load Bearlng Electrode Ablllty (lb) wlth Nomlnal Cross- Area at Area of Flexural Strengths of: Nlpple Sectional Bottom of Breakage Size Area Socket Plane (In.) (Ln.2) (111.2) (111.2) PSI PSI PSI 600,000 Table Ill. Electrode Load Bearing Ability Electrode Elec- Maximum Load Bearlng Abillty trode Area at Area of (Ib) wlth Nomlnal Flexural Cross- Bottom Break- Strengths of: Sectional of ase Nlpple Area Socket Plane Size (In.?) (in.2) (In?) psl psi psl psl Plotting of the numerical values from Tables I through 111 result in Figures 3 and 4. The solid lines indicate the maximum load bearing ability of the electrodes. The dotted lines show the strengths of nipples. The graphs denote the maximum load that can be sustained by an electrode joint area once the following items are determined: (1) nipple diameter, (2) nipple flexural strength, (3) electrode diameter, and (4) electrode flexural strength. For a given set of diameters and flexural strengths the graphs also predict whether the nipple or electrode will fail under the maximum load. Domestically, there are two grades of electrodes available to the steel industry, namely high density or premium electrodes and low density or regular electrodes. The premium electrodes generally have higher strength than the regular electrodes. Typically, premium electrodes will have flexural strengths of about psi ( kg/cm" whereas regular electrodes show flexural strengths of about psi (56-70 kg/cm2). It is frequently stated that the stronger the electrode, the better. This axiom is at best misleading. The following example will serve to prove that a stronger electrode alone is not what is needed in many cases. In the case of a 24"+ premium electrode with a 1600 psi (112 kg/cm2) flexural strength and an 11-%"+ nipple with a NIPPLE DIAMETER (IN.) Fig. &Load bearing relationship between electrodes and nipples- 24-inch diameter electrodes. O/oNIPPLE 0BREAKS loo APPLIED LOAD (POUNDS) P s I. FLEX. STR. FLEX. STR. P. S.I. I I I O ELECTRODE /NIPPLE LOAD BEARING ASILITY RATIO Fig. ftelectrode/nipple load bearing ability vs % nipple breaks. NIPPLE DIAMETER (IN.) Fig. 3-Load bearing relationships-20-inch diameter electrodes. flexural strength of 3000 psi (210 kg/cm2) the maximum load bearing ability of the nipple is 330,000 pounds (150,000 kg) and that of the electrode 615,000 pounds (280,000 kg). Obviously, when a load of greater than 330,000 pounds (150,000 kg) is applied to the electrode column the nipple will fracture. Even if the electrode were twice as strong, the break would not have been prevented. In an electrode socket area the "weakest link" will be the most likely to fracture under load. If the illustration is changed to use a nipple of 13'hW+ with a 3000 psi flexural strength, the maximum load that could be sustained would be about 430,000 pounds (195,000 kg). The probability of a break occurring would be lessened considerably. Still, a stronger electrode alone would not lower the frequency of breaks. The foregoing example illustrates that the weakest link will cause failure and that breakage rates as well as patterns can be altered. In current electrode/nipple

9 148 Electric Furnace Proceedings, 1971 Table IV. Breakage Rates and Patterns of Armco Melt Shops (September 1971) ZI"$-Premium Shop A Shoo B Breakage Pattern (I) % Break- Top of Center Bottom age Rate Socket of Pin of Socket Other r~ hop B Shop C Shop D joint designs a difference exists between the maximum load carrying ability of the electrode and the nipple. The magnitude of the difference is what gives rise to a particular breakage pattern. BREAKAGE PATTERN At the various Armco shops, electrode breakage has been monitored for several years. Table IV illustrates both the breakage rates and patterns for each melt shop. In the shops utilizing 24"@ premium density electrodes the average nipple strengths are about 3000 psi (210 kg/ cm') ; the average electrode strength about 1600 psi (112 kg/cm2). Houston No. 2 also uses 24"@ regular electrodes with nipple strengths of 3000 psi (210 kg/cm2) and electrode strengths of about 1000 psi (70 kg/cm2). In shops utilizing 20"@ regular electrodes average nipple strengths run 2200 psi (155 kg/cm2) ; the average electrode strengths about 1000 psi (70 kg/cm2). The shops utilizing 24"@ electrodes use 113/4"@ nipples; the shops using 20"@ electrodes use lo3/4"@ nipples. With nipple and electrode diameters and their flexural strengths known, the load bearing abilities can be calculated. A ratio of the electrode to nipple load bearing ability is determined. For Armco Melt Shops using 24"@ premium electrodes the ratio is The shop using 24"@ regular electrodes has a ratio of Shops using 20"@ regulars have a ratio of Figure 5 is a plot of the electrode/nipple load bearing ability ratio vs. the % nipple breaks as shown in Table IV. The linear regression of the ratio on the % nipple breaks has an explained variance of 61.9%. In summary, the electrode/nipple load bearing ability ratio can reasonably predict the breakage pattern for an electric furnace operation.