Fluid Mold Casting of Forging Ingots

Transcription

1 Fluid Mold Casting of Forging Ingots THE Torrance plant ovcrates as a jobbing shop in carbon and alloy steel forgings and castings. This type of operation ncccssitates the acccvtance of oneheat orders to almost any chemical specification. As would be expected, the company is asked to make steels upon which production shops are hesitant to quote. Such items as high-tcmperature alloys, stainless steels, tool stcels, and UHS steels, in sizes up to 56,000 Ib are examples of the ingot production. Obviously, producing ingots of this character on a "make it right the first time" basis, has its problems. One of these problems is to get a good ingot surface, since even small surface imperfections can develop into large tears under the press. This is of particular importancc on critical grades and all large ingots. A process known as Fluid Mold Casting appcared to be a possible aid in insuring an ingot surface good enough to help the forge shop to omit excessive conditioning and reheating on the more difficultly forgeable ingots. The fluid mold casting process is simple in principle. A slag with a very narrow mclting range is melted and poured into the bottom of the ingot mold. The metal is then introduced either by tundish for small ingots or directly by a bottom-pour ladle for large ingots. As the metal rises, the floating molten slag is coated out on the ingot mold wall as a thin membrane, as shown in Fig 1. Imperfections in the mold wall are filled with the slag, so that the ingot surface is in contact with a smooth, continuous envelope of the molten material (Fig 2). The fluid mold slag is a premelted matcrial that is broken into about oneinch pieces and shipped in 100-lb bags. The slag melts over a fairly narrow range in the neighborhood of 2200 F. The vis-

2 372 Proceedings of Electric Furnace Cor )ference, 1960 cosity approaches a minimum at about 2300 F. The density of the slag is just over 2.5, which allows for good separation from the body of thc ingot. directly from the bottom-pour ladle and has not caused any entrapment of the slag. Some users believe it necessary to use a tundish to control pouring rate but FROZEN F M C SLAG FIRE CHECK MOLTEN F M C SLAG WALL FROZEN METAL CHILL SKIN F M C SLAG Fig 1-Floating molten slag coating mold walls. Fig 2-Smooth, continuous envelope of molten slag. The application of this technique in the melt shop will vary with the equipment available, the size of ingot, and thc analysis of the mctal being poured. The slag is melted at the Torrance plant in a conventional 3-ton direct-arc melting furnace. The bottom of the furnace and the tapping spout are made of a graphite ramming mix because of the rapid solution rates of the usual refractories in the fluid molding slag. A graphite tee is used to start the heating, since the slag is a nonconductor when cold. After a pool is formed in the delta zone, the graphite tee is removed, the electrodes are immersed and melting is continued, using a submerged arc to prcvent excessive effluent and a possible change in chcmical balance. After melting and superheating to 3200 F, the slag is tapped into a transfer ladle and taken to the pouring pit. The molten slag is poured into the mold in a quantity of 50 Ib per ton of gross ingot weight and the steel is immediately teemed. The steel is poured this techniquc is unnccessary for the ingot sizes poured in the Torrance shop. Thc use of fluid mold casting produces ingots with superior surface properties. Even with badly fire-checked molds, the surface of the ingot is smooth. Fig 3 shows an ingot mold with fire checks up to 36 in. wide and 36 in. deep and an ingot that was produced with it using fluid mold casting. Temperature control of alloy heats can be tailored to the metallurgical requirements of the particular analysis without consideration for the ingot mold. Experimental ingots have been poured into stecl molds using fluid mold casting where the heat was in excess of 3100 F, with no damage to thc mold. Pouring rates can be varied within a wide range with no noticeable effect on the ingot surface quality. Conditioning costs from ingot to billet are reduced to lcss than one third of those of standard ingots. It has been possible to

3 Stainless Steel 373 go directly from the ingot to the billet on several high-alloy heats where intermcdiate conditioning was almost always previously required. it is being poured. This effect was first noted on a two-ingot heat of 12 pct chromium steel. The heat was being usecl to check the difference in surface proper- Fig 3-Ingot mold with flre checks (a) and ingot produced with it (b) using fluid mold casting. Sufficient data have been collected to date to indicate that the fluid mold cast slag is fairly efficient in stripping refractory-type inclusions from the metal while ties between fluid mold cast and standard cast ingots. The etches showed so great a difference that a microscopic study was made. The fluid mold cast ingot showed no

4 374 Proceedings of Electric Furnace Conference, 1960 a Fig 4-Surfaces of fluid mold cast ingot (a) and of standard cast ingot (b). X 100. b evidence of chromium nitride inclusions while the standard cast ingot showed the usual number that would be expected from the analysis. This effect is shown in Fig 4. The stripping technique has been developed and is now used on certain UHS grades for missile applications where extreme cleanliness is required. It is felt that the fluid mold casting process has a definite place in the oper- ation at Torrance. When it is superimposed upon good practice, it will supply excellent ingot surface, low refractory inclusion counts, increased mold life, and low ingot to billet conditioning costs. Further development of reagent steelmaking additions should lead to a method of removing undesirable reaction products and allow for much greater flexibility in melting practices for specialty steels. Discussion S. J. MYFORD, CHAIRMAN-Are there J. T. EVANS, JR.-NO. any questions? TO date we have not found a case of slag entrapment. If it was there, the metallurgical group would T. J. WAYNE-Did YOU find slag en- have found it. It is amazing; we really trapment in the surface? expected to have some. However, we have

5 Stainless Steel 375 poured ingots from 14,000 to 70,000 lb first one will be presented by Mr. M. F. with this process and have not had one Hoffman, Project Engineer, Linde Comcase of entrapped slag. pany, Newark, New Jersey. His coauthors are from the same company, Mr. R. P. P. G. Bailey, and Mr. R. L. W. Holmes, Hill presiding Development Engineer, Linde Gases Divi- R. P. HILL, CHAIRMAN-The next two sion, union carbide Canada, Ltd., Topapers have to do with argon casting. The ronto, Ont., Canada.