Patent Number: 042082064
Section: summary

This application relates in general to the manufacture of metal castings, and more particularly to a method for improving the quality of castings by pneumatically refining the melt prior to casting. Metal articles are generally divided into two product classifications depending on their method of manufacture, wrought products and cast products. Wrought products are made by first teeming molten metal into a mold, and then mechanically working or deforming the intermediate product by rolling, drawing, extruding or forging. In contrast, cast products are made without the second step. i.e., without the mechanical deformation of the solidified product. While cast products are generally heat treated, and may also be mechanically cleaned, machined or repaired subsequent to casting, they are not subject to plastic deformation. This difference between a wrought and a cast product, i.e., the presence of absence of mechanical deformation, is extremely important because it offers the manufacturer of wrought products opportunities to correct or eliminate various defects which may have occurred during solidification. For example, it is well known that while solidified ingots of rimmed steel have very good surface characteristics, they contain many small blow holes beneath the surface. Similarly, in most continously cast steel shapes, there is a center region containing shrinkage porosity. Nonetheless, these blow holes and regions of porosity are almost entirely eliminated during subsequent rolling, and the final wrought product contains virtually no evidence of the original porosity. Similarly, surface defects in ingots, slabs and billets are not a problem to the producer of wrought products, because these are intermediate products which undergo considerable mechanical reworking and plastic deformation prior to shipment. Furthermore, when surface defects occur, they can readily be removed by grinding or scarfing before further mechanical processing. In contrast, the surface quality of castings is very important because castings are a final product and any defect must be removed by costly and time consuming manual grinding, gauging or chipping. Then the cavity so caused must be rebuilt by welding or overlaying of metal. In addition, surface repair may diminish the dimensional accuracy and mechanical properties of the casting. It is evident, therefore, that since ingots, slabs and billets are intermediate products, certain surface and internal defects can be tolerated in them, while in castings such defects cannot, because castings are poured directly into their final shape. The metal founding industry has long been plagued with a number of difficult problems caused by unsatisfactory castings. These problems are due both to surface defects and to internal defects. While many surface defects can be remedied by the costly finishing operations mentioned above, internally defective castings frequently have to be scrapped, remelted and cast over. Some of the common surface flaws in casting include: hot tears, surface cracks, rough surface, and holes ranging in size from pinholes to gross blow holes. In general, the ultimate causes of these defects are not well understood. Consequently, melting and casting practices to produce satisfactory castings require a large amount of experience and empirical evaluation. Internal defects are due mainly to porosity and inclusions which adversely effect the mechanical properties of castings, i.e., its strength, ductility, toughness and impact resistance. The above-mentioned defects, as well as others such as embrittlement, age-hardening and the presence of fish-eyes or white spots, are believed to be related to the presence of uncontrolled amounts of oxygen, nitrogen, hydrogen, phosphorous and sulfur in the melt. Consequently, it has long been an objective of the foundry industry to produce sound castings with low or controlled levels of these five elements. In the production of stainless castings, where corrosion resistance is of paramount importance, it is often an additional objective to produce sound castings with low carbon levels. Casting defects are conventionally remedied during the so-called finishing operations. Most of these operations are highly labor intensive and consequently very costly. In addition, much of the finishing consists of grinding which causes dust that can be harmful to health. Some castings, however, cannot be repaired because the critical application for the part does not allow it. In such case, the defective casting must be scrapped. Consequently, the foundry art has long sought a method which would improve castings both in terms of their surface quality and physical properties. Various techniques have been used in the foundry art to refine melts prior to casting in order to improve the quality of the resultant castings. The final stage of melting often includes some form of purification or refining treatment intended to influence the microstructure and cleanliness of the casting. Such treatments usually involve the blowing of gases or the addition of certain reagents to the furnace or transfer ladle. These treatments may include decarburization, dephosphorization, deoxidation, desulfurization and degassing. Prior to the present invention, decarburization of molten steel for castings, was generally accomplished by blowing oxygen into the melt through a consumable lance inserted through an opening in the furnace. This technique of decarburization is, in the first place, dangerous to the operator because it exposes him to hot metal and sparks, and because the operator usually holds the lance manually, which is in itself hazardous. Secondly, this technique of decarburization is frequently inaccurate because all the oxygen does not always react with the bath. Hence, it is often necessary to reblow the molten steel because insufficient carbon was removed initially. Lastly, such prior art methods of decarburization tend to generate a great deal of fume and smoke which is hazardous to health and damaging to the environment. Because of the presence of oxygen is known to be detrimental to the properties of the castings, foundries generally deoxidize the molten metal prior to pouring. In addition, deoxidation is generally required to prevent the formation of blow holes during solidification. This is normally accomplished by the addition of well-known deoxidants such as silicon or aluminum, and also by the addition of special deoxidants, such as "Calcibar" and "Hypercal." The attainment of a well deoxidized melt prior to casting is essential for the production of sound, tough castings. Desulfurization of molten steel for castings, prior to this invention, has generally been accomplished by the formation of basic slags in the furnace, i.e., slags containing a high ratio of lime to silica or lime to alumina, and by subsequently mixing the slags with well deoxidized metal. Equilibrium between the slag and the metal causes the sulfur to be transferred from the metal to the slag. This process is very slow, often requiring several hours, particularly when very low (i.e., under 0.005%) sulfur is desired. Indeed, it is often necessary to remove the slag and to produce a new one. Sometimes this step has to be repeated several times in order to reach the desired low level of sulfur. This process is very laborious and time consuming, and unnecessarily exposes the furnace operators to molten metal and to unhealthy fumes. An alternative, and much more costly desulfurization technique is to add expensive sulfur scavenging elements, such as calcium, magnesium or the rare earth elements, to the furnace immediately prior to tapping or to the transfer ladle. The expense of this technique, as well as its non-reproducibility, militates against its general use. Known degassing treatments include vacuum melting, vacuum degassing, as well as degassing by bubbling scavenging gases, such as argon, through the melt. While argon degassing in the ladle, prior to casting, can improve the quality of castings by lowering the hydrogen and oxygen content of the melt, it does not remove all impurities or achieve low hydrogen levels in the limited time available. Because the time available for degassing is strictly limited by heat loss from the degassing vessel, it has been found that it is not possible to lower the dissolved gas content sufficiently for many applications. Furthermore, degassing by itself does not remove sulfur and may necessitate reheating the melt in order to obtain sufficient fluidity for casting. Prior to the present invention, therefore, the foundry art utilized the above-described techniques in an effort to produce defect-free castings. However, these prior art techniques are expensive, often inaccurate or non-reproducible, time-consuming, generally hazardous to the health of the operators, and by-and-large inadequate to the needs of the industry. Consequently, extensive post-solidification repair of castings is usually still required. In fact, in castings, for example, destined for nuclear applications, the cost of inspection and repair often exceeds the material value of the castings themselves. During the past twenty-five years, the manufacturers of wrought steel products have made large gains in upgrading their molten metal processing techniques through the adoption of one of several now well known refining processes such as the BOF, AOD, OBM or Q-BOP and LWS processes. U.S. Pat. Nos. illustrative of these processes, respectively, are: 2,800,631; 3,252,790; 3,706,549; 3,930,843 and 3,844,768. The production of wrought steels containing controlled levels of carbon, phosphorous, sulfur, oxygen, nitrogen and hydrogen is now readily and economically achievable through judicious selection of one, or a combination of more than one, of the above processes. In the foundry or cast metal industry, however, comparable advances have been absent. While the industry has, at various times, produced products with low or controlled levels of one or perhaps two of the above six elements, the manufacture of castings with low or controlled levels of all six elements has hitherto not been possible, and consequently, the value or advantages of being able to control all six elements have hitherto not been known. The pneumatic treatment of molten stainless steel for the production of wrought steel by the simultaneous injection of argon and oxygen into the melt, commonly referred to as the AOD process, has achieved wide commercial acceptance in stainless steel mills for the manufacture of wrought products. The basic AOD refining process is disclosed by Krivsky in U.S. Pat. No. 3,752,790. An improvement on Krivsky relating to the programmed blowing of the gases is disclosed in Nelson et al, U.S. Pat. No. 3,046,107. The use of nitrogen in combination with argon and oxygen to achieve predetermined nitrogen contents is disclosed in Saccomano et al in U.S. Pat. No. 3,754,894. A modification of the AOD process is also shown by Johnsson et al in U.S. Pat. No. 3,867,135 which utilizes steam or ammonia in combination with oxygen to refine molten metal. It is worthy of note that none of the above-mentioned pneumatic melt refining techniques have, prior to this invention, been used by the foundry art for the production of castings. OBJECTS It is an object of the present invention to improve the surface quality, internal quality and physical properties of castings. It is another object of this present invention to improve the method of producing castings by pneumatically refining the melt prior to casting. It is still another object of this invention to increase the yield of acceptable castings. SUMMARY It has now been discovered that by pneumatically refining the melt in a separate vessel prior to casting, castings of a quality superior to that heretofore obtainable can be produced. Such castings have unexpectedly superior surface quality and internal quality. The above, and other objects which will be apparent to those skilled in the art are achieved by the present invention which comprises: a process for producing metal castings having improved surface quality and internal quality by: melting selected charge materials in a furnace, teeming the melt into a mold, permitting the melt to solidify in the mold, and removing the casting from the mold, wherein the improvement comprises: (1) transferring the melt from the melting furnace into a refining vessel provided with at least one submerged tuyere, and (2) refining said melt by (a) injecting into the melt through said tuyere(s) an oxygen-containing gas containing up to 90% of a dilution gas, and (b) thereafter injecting a sparging gas into the melt through said tuyere(s). Preferably, the oxygen-containing gas stream is surrounded by an annular stream of protective fluid. The term "refining" as used in the present specification and claims is meant to include any one or more of the following effects: decarburization, dephosphorization, desulfurization, degassing, deoxidation, gaseous alloying, impurity oxidation, impurity volatilization, slag reduction and flotation and homogenization of non-metallic impurities. The present invention is applicable to refining of any iron, cobalt or nickle based alloy, and the term "metal" is used in that sense. The term "dilution gas" as used herein is intended to mean one or more gases that are added to the oxygen stream for the purpose of reducing the partial pressure of the carbon monoxide in the gas bubbles formed during decarburization of the melt, and/or for the purpose of altering the feed rate of oxygen to the melt without substantially altering the total injected gas flow rate. Suitable dilution gases include: argon, helium, hydrogen, nitrogen, carbon monoxide, carbon dioxide, steam and hydrocarbon gases, for example, methane, ethane, propane and natural gas. Argon is the most preferred dilution gas. The term "protective fluid" as used herein is meant to include one or more fluids which surround the oxygen containing gas and protect the tuyere and surrounding refractory lining from excessive wear. Suitable protective fluids include: argon, helium, nitrogen, hydrogen, carbon monoxide, carbon dioxide, hydrocarbon fluids (gas or liquid) and steam. Methane, ethane, propane or natural gas are suitable hydrocarbon gases. No. 2, diesel oil is a suitable hydrocarbon liquid. Argon is the most preferred protective fluid. The term "sparging gas" as used herein is intended to mean one or more gases which remove impurities from the melt by volatilization or transfer to the slag by entrapment or reaction with the slag. Suitable sparging gases include: argon, helium, nitrogen and steam. Argon is also the preferred sparging gas. Castings having improved surface quality are defined as castings which when compared to the prior art require reduced cleaning, grinding, chipping, welding or other repair. Such improved surface quality can be evidenced by a reduced level of defects determined during dye penetrant or magnaflux testing. Castings having improved internal quality are defined as castings which when compared to the prior art display one or more of the following characteristics: a lower level of inclusions, finer as-cast grain size, reduced internal porosity, reduced tendency for hydrogen flaking during machining, reduced evidence of defects when inspected by X-ray techniques or better physical properties such as toughness.