Acid-Electric Arc Melting

Transcription

1 I Refractories 157 Acid-Electric Arc Melting by D.E. Dutcher Direct arc melting using acid linings has found wide application in the steel foundry industry. Furnaces from 1 to 10 tons capacity are the most popular sizes for this type of production. This relatively small heat size has the advantage of producing a variety of alloys in a short melting time. All types of steels can be produced with acid practice including carbon, low alloy, and high alloy steels. The acid furnace is capable of producing metal at high temperatures which allows the production of thin section castings. The cheapness of the refractory material and its durability in intermittent operation also makes it attractive. MELTING EQUIPMENT Most of the furnaces in use today are of the top charge variety. These furnaces have a definite advantage over door charge furnaces in the ease and speed of charging. With a top charge model a clam shell or orange peel type charge bucket can be used to place the charge in the furnace. Fig. la shows the charging of a furnace with an orange peel charge bucket. Recharging, if necessary, can be done in a similar manner. (Fig. lb) Transformers for this type of furnace will vary with furnace capacity and to some degree with foundry practice. Ultra high power transformers are not very common, probably due to small furnace sizes involved. Since the furnaces are generally run at 1.4 to 2.2 times the rated capacity, transformer sizing is somewhat dependent on the operation. Graphite electrodes of 4 to 12 in. diam are most commonly used. REFRACTORY PRACTICE The sidewalls of the furnace are built of high quality silica brick The bricks are layed in courses with an expansion space left between the brick and the shell. This space is sometimes filled with dry sand, but at KO Steel the following mixture is used: 227 kg (500 lb) silica sand 18 kg ( 40 1b) Western bentonite 38 1 ( 10 gal) sodium silicate Special shaped brick are often used to form the door arch, tap hole, and spout areas, however, it is this author's experience that a 90% alumina plastic material rammed in these areas is more durable and cuts down the brick inventory which must be kept on hand. Every foundry has their own special ram mix for the working botton of the furnace. K 0 Steel uses the following mix: 907 kg (2000 lb) zircon sand (100 AFS grain fineness) 9 kg ( 20 1b) fireclay 191 ( 5 gal) sodium silicate 191 ( 5 gal) water Fig. 1-Charging operation, (a) Orange-peel charging bucket. (b) Recharging the furnace. Other foundries use ganister mixes, silica sand mixes, or other materials for this purpose. The above mix has proved best in one operation. The same mix is used for patching between heats. Many furnace operations include a repair schedule of gunning the furnace. Many proprietary gun mixes are available, but most operations use a mined product from Alabama consisting of 85 to 90% SiOn and 6 to 8% A1303. This will vary, but every 1 to 3 days is the most common procedure. Gunning the furnace daily has increased refractory life from 100 heats to approximately 1500 heats at K 0 Steel. Other authors have stated that lining life can be extended to as long as eight - years. using this procedure.' Furnace roofs are constructed of silica brick. Several manufacturers have so-called "K W A brick which allows construction of a roof with only two shapes of brick, pather than a multitude of -special &apes. Rammed or cast alumina is often used in the delta section of the roof. Electrode ports may be bricked or of the same material as the rest of the center section, if it is rammed or cast alumina. This author has found success using alumina brick with a cast alumina delta section for roof materials. The cost of these materials (2 to 3 times that of silica) may make them unattractive in many operations. Roof life varies but 300 to 500 heats is D. E. DUTCHER is Manager of Quality Assurance, K. 0. Steel Castings, Inc., San Antonio, Texas.

2 158 Electric Furnace Proceedings, 1975 Fig. 2-Roof construction, (a) Bricked portion of roof under construction, (b) Completed furnace roof showing rammed alumina delta section. Table I-Heat Log for Alloy Steel Heat Number: AA871 Alloy: SAE 4140 Chemistry Limits Heat Size: 2087 kg (4600 lb) C Mn Si P S Cr N1 Mo.3M max.60 max.040 max.045 max max J5.25 Charge - kg - lb Foundry returns Purchased scrap Charcoal Asbury graphite 4% 10 Time, Min Event Power on No. 1 tap Recharge Power on No. 3 tap Meltdown sample Add FeMo Begin 0 2 blow Finish 03 blow, preliminary sample Block heat FeCr addition Temperature FeMn, SiMn addition Tap heat Alloys 60% Ferromolybdenum 50% Ferrosilicon Sorelmetal Charge chrome (70% Cr) Standard ferromanganese (78% Mn) Silicomanganese Aluminum kg lb Remarks Initial charge approximately 1360, kg. (3000 lb) Recharge remaining 680 kg (1500 lb) of plate Meltdown chemistry C Cr Mo % C (3050 F) Preliminary chemistry - C - Cr Mo Vz 8 19% 43 3 points carbon 17% 39 4 points carbon 1717" (3120 F) points carbon 3 M 8 3 V2 8 Ladle addition 3 % 8 Plunged in ladle - Final Chemistry Total Charge Foundry returns Purchased scrap Charcoal Asbury graphlte Ferromolybdenum Ferrosilicon Sorelmetal Charge chrome Standard ferromanganese Silicomanganese Aluminum

3 Retractories 158 Fig. LOxygen lance, (a) Coated oxygen lance, (b) Oxygen lance in use. generally considered good in most foundry operations. Fig. 2 shows a furnace roof being constructed. MELTING PROCEDURE-CARBON AND LOW ALLOY STEELS Because of the inability in acid practice to remove sulfur and phosphorus during the melting operation it is necessary to purchase only high quality scrap which is low in these elements. The increased price of this scrap is offset by the cheap refractory cost and ease of maintenance in acid furnaces as opposed to basic melting. In addition, oxidizable elements such as manganese and chromium cannot be fully recovered from the charge because the acid slag is always oxidizing, i.e., no reducing slag can be produced in the furnace. A typical heat log for a low alloy steel is shown in Table I. The charge consists of approximately 50% foundry returns (gates and risers) and 50% purchased plate. Many varieties of carbon raisers are used including Sorelmetal, coke, coai and graphite. These additions insure a high meltdown carbon in the bath. Nickel and molybdenum if required, can be added to the initial charge or thrown through the door, but at any rate should be added prior to the oxygen boil. Since nearly all foundries use a complete oxidation method of producing steel this will insure a low gas content in the finished product. Nickel is particularly high in hydrogen which can lead to pinholes and gas in castings if not removed by a vigorous boil. Some foundries use a lower tap setting on the initial charge to cut down on arc flare on the roof and subsequent loss in refractory life. In the interest of increased production most foundries forego this procedure and start out on No. 1 tap. During the first few minutes of melting this creates a strain on the roof and upper sidewalls, but decreases meltdown time. When the initial charge is approximately 2/3 melted down the recharge, if any, is made. The furnace continues on No. 1 tap until almost completely molten. At this point foundry procedure varies, but most operations change to a lower tap setting. When the heat is completely molten and is brought to the proper temperature for the oxygen blow, oxygen should be injected. Many foundries use the oxygen reaction to increase metal temperatures and therefore will inject oxygen at lower temperatures than others. It is this author's experience that a vigorous oxygen boil cannot be sustained until a temperature of at least 3000 F is obtained. If a direct reading spectrometer is available a meltdown sample is taken and chemistry adjustment, except for oxidizable elements, should be made prior to the oxygen blow. When an adequate temperature is reached, oxygen is injected using a consumable lance. The oxygen is generally at 80 to 100 psi, but the lance size will vary with the size of heat being produced. Fig. 3 shows the lance used at K 0 Steel. It consists of a 3/8 in. steel pipe coated with a refractory mud to increase the life of the lance. A minimum of 20 points (0.20%) carbon should be removed to insure adequate degassing of the metal. Many foundries also use some iron ore or mill scale in the charge to create a strongly -. oxidizing bath and to begin the ;xygen boil. After the boil lime should be added to thin the slag, which is auite heavy at this point. If a spectrograph is available,-a sample should be taken fir preliminary analysis. If the carbon is too high, additional oxygen should be injected. If the carbon is too low, Sorelmetal or other carbon raisers should be added to reach the desired carbon level. Prior to any addition of alloys the heat should be blocked with ferrosilicon to prevent Fig. 4--Melter preparing to plunge aluminum into the ladlc during the tap.

4 160 Electric Furnace Proceec further boiling. After the silicon biock the metal is brought to tapping temperature on No. 3 tap. The silicomanganese and ferromanganese addition is made and the heat tapped. When the ladle is approximateiy 1/3 to 1/2 full proprietary deoxidizers such as calsibar are added and aluminum is plunged into the ladle (Fig. 4). This deoxidation procedure removes the excess oxygen from the steel which is present after the boil. When the ladle is removed from the pit the furnace should be tilted completely forward to remove any excess metal and slag. If no patching is required, the furnace can immediately be charged for the next heat. MELTING PROCEDURE HIGH ALLOY STEELS Table I1 shows a heat log for high alloy steel. The loss of oxidizable elements is more dramatic in heats of stainless steel than in low alloy steels. Each operation is different, but K 0's experience has been that approximately 60 to 70% of the chromium in the original charge (as foundry returns), and about 70 to 80% of the chromium added after the oxygen blow (as low carbon ferrochromium) is recovered. The charge consists primarily of foundry returns but also includes purchased scrap, nickel and a carbon raiser. After a meltdown sample is taken, adjustments are made to the nickel content to bring it to the desired level. Once the proper temperature is reached, the elec- trodes are raised and oxygen is injected until the desired carbon level is reached. At this point another spectrographic sample is taken and the ferrochrornium addition is calculated. Addition of the ferrochromium can be made in several ways, but generally it is shoveled through the door. Approximately half the addition is made with the power off, and the other half made with the furnace on No. 2 tap. Excess use of the electrodes at this point will cause silicon reversion from the slag and correspondingly high silicon in the finished steel. The use of No. 2 tap causes the electrodes to draw a fairly long arc and thus minimize dipping. After the oxygen blow the slag cover in the furnace is very heavy and must be thinned with copious amounts of lime. The lime addition also seems to increase chromium recovery. The furnace is left on No. 2 tap until the desired temperature is reached. Deoxidizers, including low carbon ferromanganese because of its small size, are added to the ladle. Some melters use aluminum as a deoxidizer on these heats but this is generally regarded as unnecessary in stainless steels. Because there is appreciable alloy pickup (primarily chromium) in the heat following a stainless steel, it is necessary to schedule a non-critical steel as a wash heat. Because the pickup of ailoy elements is not predictable, it is very nice to have a customer who is not fussy about the analysis. Table Il-Heat Log for High Alloy Steel Heat Number: AA631 Alloy: SAE 304 (A296 GRADE CF8) C Mn.08 max 1.50 max Chemistry Limits P.04 max Heat Size 2087 kg (4600 Ib) 8 Cr Ni -04 max Charge Foundry returns (304) Purchased scrap Electrolytic nickel Charcoal 2 Ya 5 Time, Min. Power on No. I tap Power on No. 3 tap Meltdown sample Meltdown chemistry C Cr N1 39 Add nickel Electrolytic nickel 43 Begin 02 blow 53 Fln~sh On blow 54 Preliminary sample Preliminary chemistry C Cr NI Begin FeCr addition Power on No. 2 tap Finish FeCr addit~on FeCr melted in, temperature Tap Low carbon ferrochromium Low carbon Ferromanganese Zirconium silicon : 1 Ladle additions Final Chemistry Total Charge Foundry returns Purchased scrap Electrolytic nickel 88 '/a 195 Charcoal 2 'h 5 Low ca~.oon ferrochromium Low carbon ferromanganese % 8 Zirconium silicon - 3 %

5 SUMMARY The acid-electric arc furnace can be a very useful tool in foundry applications. High quality steel can be produced quickly and efficiently by this method. The relative cheapness and ease of repair make the refractories very attractive. Metal with good fluidity and lowgas content can be produced with a good melting practice. Of primary importance in achieving this end is a vigorous oxygen boil removing at least 20 points of carbon. Contrasted to basic melting practice an acid furnace has the disadvantage of not being ab!e to remove sulfur and phosphorus. This can be overcome by careful selection of purchased scrap to be low in these elements. The other disadvantage of the acid process is the inability to reduce oxidizable elements, primarily chromium an manganese, from the slag. The oxidation loss of thez elements must be taken into account, but a consister melting practice will produce excellent final results. Although only touched on briefly, a spectrometer is a,, extremely helpful tool in the production of high quality steel. Wet chemical methods are too slow to compete with the high production rates possible in these furnaces. Without a rapid means of chemical analysis, the melter is always guessing. REFERENCES IHlavacek, C. O., '%Electric Arc Melting of Carbon and LOW Alloy Steel-Acid Practice." Steel Foundry Melting Practice. R. W. Zillrnan, ed., Steel Founders' Society of America, 1973.