Patent Number: 042082064
Section: description

DETAILED DESCRIPTION It was expected that utilization of pneumatic refining for the treatment of steel melts for castings would produce most of the chemical benefits obtained by refining molten steel for the production of wrought steel products. In particular, it was expected that some improved internal quality would be obtained by better deoxidation of the melt, by better separation of deoxidation products, and by the attainment of lower sulfur levels and lower hydrogen content. However, it was unexpectedly discovered that pneumatic refining in accordance with this invention produces improvements in the surface quality of the castings beyond any expectations, that it produces castings with greatly improved strength, ductility and toughness, and that it makes possible the production of castings of far superior quality than previously possible from low alloy steels and carbon steels. As a result of the present invention, foundries are now able to cast with significantly increased assurance of obtaining satisfactory castings, as well as of obtaining castings of higher quality. More specifically, the surface quality of the resultant castings have fewer cracks and reduced hot tears. In addition, it has been found that use of the present invention produces a smoother casting surface, believed to result from reduced interaction of the sand mold with the melt. It has also been found that the physical properties of the castings have been unexpectedly improved. The improvements are believed to be related to the lower levels of inclusions, lower hydrogen flaking, as well as lower porosity found in castings made in accordance with this invention. Molten steel treated in accordance with the present invention has a higher flowability or fluidity at the same temperature than untreated metal, resulting in superior castings, since the metal will flow into smaller and more intricate crevices than unrefined melt. Alternatively, the same fluidity may be achieved at a lower casting temperature. This again contributes to improved casting surface quality. The pneumatic refining treatment of the present invention may be advantageously employed on any type of iron or steel melt, and also on cobalt and nickel alloys, normally used for the manufacture of metal castings. It has, however, been found to be particularly beneficial in the treatment of ferritic and austenitic stainless steels, low alloy steels and carbon steels. Special benefits are obtained in castings made steels such as WC6 and HY80 which are sensitive to hydrogen flaking as well as hot tearing. High strength steels such as HY130 which normally require extensive chipping, grinding and welding in order to repair as-cast defects, are significantly improved by the present invention, resulting in considerable finishing cost savings. Austenitic stainless grades such as CN7M, CH20, CK20, 310L, and 347L, which, prior to the present invention, were extremely difficult to cast without cracking or microfissuring, can now by means of the present invention, be readily cast without fear of cracking. The advantages of the present invention while applicable to small, simple castings as well as to complex or large ones, are of particular significance when producing high quality castings such as required, for example, for pumps and turbines used in the aircraft, shipbuilding and nuclear power industries. In addition to the unexpected results of the present invention described above, other benefits resulting from use of the present invention include raw material savings due to minimized oxidation of molten metal and the ability to use lower grade charge materials. Increased production also results from greater accuracy in achieving desired aim melt chemistries and fewer rejects due to improved casting quality. In practicing the present invention, melting of the charge materials may be accomplished by any means known in the art. The most common foundry melting furnaces include fuel fired furnaces of the hearth or crucible type, as well as electric furnaces of the resistance, induction or arc type. The last two are preferred. Following melting of the charge materials, the melt is transferred by a ladle or otherwise poured into the pneumatic converter shown in FIG. 1. FIG. 1 is a cross-sectional view of a preferred refining vessel 1 for use in practicing the present invention. Vessel 1 comprises an outer steel shell 2, removably attached to a trunion ring 3. The trunion ring and consequently the vessel is tiltable by being fixedly attached by drive means (not shown), in order to facilitate charging, sampling, slag removal and tapping. Shell 2 is lined with basic refractory bricks 4. A removable shell arrangement is preferred, since several shells are necessary to maintain uninterrupted operations. While one shell is in use, the spare or spares are being relined. A horizontally disposed concentric tube tuyere 5 is located in the side-wall of the vessel near the bottom of the vessel for injection of the fluids. If desired, the tuyeres can be located in the bottom of the vessel in place of or in addition to the sides. Preferably, however, at least two tuyeres are used, and positioned in the side-wall of the vessel, near the bottom and horizontally disposed in such manner as to be asymmetric. That is, no two tuyeres should be positioned so that their axes, and consequently the fluid streams are injected diametrically opposed to each other. Asymmetric positioning of the tuyeres improves mixing of the melt by the injected gases. The tuyere 5 consists of an inner tube 6 and a concentric outer tube 7. Oxygen alone or admixed with a dilution gas is injected through the inner tube 6, and the protective gas is injected through the outer tube 7 of the tuyere. The latter forms a protective annular shround around the oxygen stream which protects the refractory lining from rapid deterioration. The pressure of the fluids must be sufficiently great to penetrate into the melt. Preferably, the absolute pressures of the fluids at the tuyere inlets, of both the central and annular passages, are at least two times greater than the absolute pressures of the fluids at the outlets. A detailed description of a suitable vessel and tuyeres for carrying out the present invention is shown by Saccomano and Ellis in U.S. Pat. No. 3,703,279. The sparging gas may be injected into the melt either through the same tuyere or tuyeres as used for the oxygen stream or through separate tuyeres; the former is preferred. Preferably, after the oxygen blow is completed, the sparging gas is injected through the center passage of the tuyere as well as through the annular passage in order to prevent molten metal from flowing back into the tuyere where it would freeze. In general, the molten metal refining step of the present process is carried out by injecting oxygen and a dilution gas, as well as protective fluid (both of which may be argon) into the melt through the submerged tuyeres. The decarburization, i.e., the reaction of the injected oxygen with carbon in the melt, produces controlled oxidation of the bath components, as well as heat which maintains bath temperature. The melt is initially blown with a high ratio of oxygen to dilution and protective gases. Depending on the steel composition being refined, as the carbon content of the melt decreases, the ratio of oxygen to dilution gas and protective fluid may be lowered, generally in several steps, in order to maintain favorable thermodynamic conditions throughout the blow. Since the oxygen and other gases are introduced below the level of the melt and at high velocity, excellent mixing takes place within the melt and intimate gas-metal and slag-metal contact occurs. As a result, the reaction kinetics of all chemical processes which take place within the vessel are greatly improved. This permits desulfurization to very low levels (under 0.005%) generally, in less than ten minutes of blowing and without addition of expensive desulfurizing agents such as calcium, magnesium or rare earths. Dephosphorization of alloys containing less than approximately one percent chromium can readily be achieved by decarburizing the bath to below 0.1% carbon by using a gas mixture containing at least 75% oxygen. The phosphorous bearing slag so formed must then be decanted prior to blowing with a sparging gas or adding any reducing agents, deoxidants, or desulfurizing agents. Other major benefits of the invention are very close control of the end point carbon and very low residual values of oxygen, nitrogen and hydrogen. Typical residual values for these three elements obtained by practicing the invention are shown in Table I. TABLE I ______________________________________ Stainless Steel Low Alloy Steel ______________________________________ Oxygen 40-70 ppm 20-50 ppm Hydrogen 2-4 ppm 1-3 ppm Nitrogen 150-200 ppm 20-50 ppm ______________________________________ In addition, lead, and zinc in the melt are reduced to levels that are metallurgically harmless. The synergistic results obtained by the present invention, i.e., low gas content (oxygen, nitrogen and hydrogen) together with low sulfur and increased fluidity of the melt have combined to produce castings of unprecedented surface quality, internal cleanliness and improved mechanical properties. Table II below compares the chemical and physical properties of two castings of stainless steel grade CA6NM, one made by conventional practice and the other by the present invention with ASTM specification A296. TABLE II ______________________________________ Chemistry ASTM Spec. (%) A296 Conventional Invention ______________________________________ C 0.06 max .05 0.026 Mn 1.00 max .60 0.47 Si 1.00 max .55 0.96 Cr 11.5-14.0 12.70 12.81 Ni 3.5-4.5 3.80 4.00 Mo 0.40-1.00 0.50 0.57 S 0.03 max 0.025 0.022 P 0.04 max 0.020 0.025 Mechanical Tensile (ksi) 110 min. 115 122.8 Yield (ksi) 80 min. 100 108.3 Elongation (%) 15 min. 20 21 Red. of area (%) 35 min. 60 67 Impact Strength none 65 77-80 Charpy V-notch (at R.T.) ______________________________________ It can be seen from Table II that the casting made in accordance with the present invention is superior in all respects, and particularly in impact resistance. The difference in toughness is even more impressive when one recognizes that in this particular casting the sulfur level was 0.22% rather than the customary value of less than 0.01% obtainable with pneumatic refining. In this case no special desulfurizing treatment was employed. With high strength alloys such as HY-130 and 85% improvement in impact strength has been obtained on a casting made from HY-130 in accordance with this invention when compared to a casting of the same alloy made from vacuum degassed metal. Such high impact strength far exceeds any previously obtained impact strength on castings made from this alloy. EXAMPLE 1 An electric arc furnace was charged with 6290 lbs. of HY-80 scrap, 5869 lbs. of mild steel scrap and 300 lbs. of lime. Power was applied to the electrodes and the charge was melted in approximately one hour. Following melt down, the composition was adjusted, in accordance with conventional practice, to have the furnace tap composition shown below, and a temperature of about 3100.degree. F. The above melt was tapped from arc furnace into a transfer ladle, and then charged into the refining vessel. 500 lbs. of lime, 100 lbs. of MgO and 60 lbs. of aluminum were added to the charge. At the start of the pneumatic refining period the temperature of the melt was 2900.degree. F. The melt was blown through two submerged, horizontal, concentric-tube tuyeres, asymmetrically positioned in the lower side-wall of a refractory-lined refining vessel such as shown in FIG. 1. The blowing gas, consisting of oxygen diluted with argon, was injected through the center tube of the tuyeres. Argon was used as the protective fluid, and injected through the annular passage of the tuyeres. The ratio of the oxygen flow rate to that of the combined argon flows was 3 to 1. A total of 2150 ft..sup.3 of oxygen was injected. The combined gas flow rate of the injected gases was about 6000 SCFH. About 9 minutes after the flow began, 11 lbs. of charge chrome and 18 lbs. of standard manganese were added to the melt. At the end of the blow the temperature of the melt was 3080.degree. F. and the carbon content was 0.10%. Following the addition of 100 lbs. of 50% FeSi, the melt was sparged and stirred by injecting argon at a rate of about 4000 SCFH for 4 minutes through both passages of both tuyeres. The melt temperature at this time was 3000.degree. F. The melt was then conventionally deoxidized and sparged with argon for 2 more minutes before being tapped into a bottom pouring ladle for subsequent teeming into molds. The furnace tap composition and the final composition of the refined melt at tap are tabulated below. ______________________________________ % % % % % % % % Analysis C Mn Si Cr Ni Mo P S ______________________________________ Furnace Tap 0.32 0.54 0.55 1.29 2.85 0.43 0.014 0.004 Refined Melt 0.10 0.61 0.35 1.49 2.97 0.42 0.017 0.001 ______________________________________ EXAMPLE 2 For purposes of comparison, a conventionally processed heat of HY-80 was prepared as follows. An electric arc furnace was charged with 15,000 lbs. of HY-80 scrap, 55 lbs. of charge chrome, 14,082 lbs. of mild steel scrap and 600 lbs. of lime. Power was applied to the electrodes and the charge was melted and heated to 2790.degree. F. in approximately 75 minutes. About 4000 SCF of oxygen was then injected into the bath by means of a hand-held consumable lance. The slag formed thereby was skimmed off, and the bath temperature was measured to be 2850.degree. F. The following additions were then made to the melt: 200 lbs. carbon, 500 lbs. 50% FeSi, 500 lbs. lime, 220 lbs. charge chrome, 285 lbs. Ni, and 66 lbs. Mo O.sub.3. Power was again applied to the electrodes and the bath temperature was increased during a period of 45 minutes to 3020.degree. F. At this point, a preliminary sample was taken which had the analysis shown below. Thereafter, additions of 500 lbs. lime, 200 lbs. charge chrome, 135 lbs. Ni and 28 lbs. FeMo were made, and the melt was further decarburized by injecting 6700 SCF of oxygen into the bath by means of a handheld consumable lance. After about 20 minutes of blowing, the carbon was measured to be 0.07%. 275 lbs. of SiMn and 131 lbs. of 75% FeSi were added, and the heat was immediately tapped and sampled. The final tap composition is also shown below. ______________________________________ % % % % % % % % Analysis C Mn Si Cr Ni Mo P S ______________________________________ Preliminary 0.63 0.26 1.06 0.93 2.32 0.34 0.016 0.006 Furnace Tap 0.10 0.63 0.47 1.40 2.79 0.40 0.015 0.007 ______________________________________ Table III below compares the physical properties of the castings produced from the melts prepared in Examples 1 and 2 above, both of which were heat treated in substantially the same manner in accordance with conventional techniques TABLE III ______________________________________ Example 1 Example 2 ______________________________________ Tensile Strength (psi) 102,750 102,325 Yield Strength (psi) 87,200 87,900 Elongation (%) 22 21 Reduction of Area (%) 55 53 Impact Strength (ft. lbs) 58,100,108 44,45,37 at-100.degree. F. (Charpy "V"-notch) ______________________________________ It can be seen from Table III that all of the properties of the castings, other than greatly improved impact strength of the castings made by the present invention, are substantially the same. One would expect, to obtain similar properties since both the chemical composition and heat treatment of the castings were substantially the same. The improved impact strength is believed to reflect the improved internal cleanliness of the melt produced in accordance with the present invention. While this increase in toughness is, in itself, a considerable improvement in the quality of the casting, an additional improvement of great significance was observed in the cleaning and finishing of the castings. The castings made from the melt of Example 1, required substantially less cleaning, grinding, welding and other repair than the prior art casting made from the melt of Example 2. This improvement was unexpected and not predictable from past experience, and is of great importance to the foundry industry since the labor savings involved represent a significant portion of the value of the casting. In addition to the unexpected improvements described above, other improvements on HY-80 castings made in accordance with this invention have also been found. For example, the welds required to repair an experimental casting made by the present invention numbered only 5, as compared to 95 repair welds required on the same casting made by conventional practice. Further, castings made by the present invention displayed no hydrogen flaking even in 13" sections. EXAMPLE 3 An electric arc furnace was charged with 8947 lbs. of 18-8 stainless steel scrap, 40 lbs of carbon and 500 lbs. of lime. Power was applied to the electrodes and the charge was melted. Following melt down, the composition was conventionally adjusted to have a furnace tape composition shown below and a temperature of about 3100.degree. F. The above melt was tapped from the arc furnace into a transfer ladle and then charged into the refining vessel. 500 lbs. of lime was added to the charge. At the start of the pneumatic refining period the temperature of the melt was 2910.degree. F. The melt was blown through two submerged, horizontal, concentric-tube tuyeres, asymmetrically positioned in the lower side-wall of a refining vessel as shown in FIG. 1. The blowing gas consisted of oxygen diluted with argon injected through the center tubes. Argon was injected as the protective fluid through the annular passage of the tuyeres. The ratio of oxygen to the combined argon flow rates was 3 to 1. A total of 1800 ft..sup.3 of oxygen was injected. The combined flow rate of the injected gases (i.e., oxygen plus argon) was about 7000 SCFH. After 21 minutes of blowing at the 3:1 ratio, the melt temperature was 3120.degree. F. and the carbon content was 0.15%. The ratio of the oxygen flow rate to that of the combined argon flows was then changed to 1:1. At this ratio the injection was continued for about 15 minutes during which time 1000 ft..sup.3 of total oxygen was injected. Thereafter, the ratio of oxygen to combined argon flows was again changed to 1:3, and 100 ft..sup.3 of oxygen was injected over about 4 minutes time. 400 lbs. of FeCrSi, 100 lbs. lime and 215 lbs. of 50% FeSi was then added, and the melt stirred and sparged for 17 minutes with argon alone injected through both passages of both tuyeres. The tap temperature was 2920.degree. F. The heat was then tapped into a bottom pouring ladle for subsequent teeming into molds. __________________________________________________________________________ Analysis % C % Mn % Si % Cr % Ni % Cu % Mo % P % S __________________________________________________________________________ Furnace Tap 0.35 0.75 0.34 19.29 8.95 0.34 0.65 0.029 0.00 Refined Melt 0.02 0.70 1.47 20.09 9.54 0.33 0.63 0.028 0.00 __________________________________________________________________________ EXAMPLE 4 For purposes of comparison, a conventionally processed heat of 18-8 stainless steel was prepared as follows. An electric arc furnace was charged with 18,702 lbs. of 18-8 scrap, 374 lbs. FeNi, 150 lbs. carbon and 2500 lbs. of lime. Power was applied to the electrodes and the charge was melted and heated to 2850.degree. F. in approximately 118 minutes. A preliminary sample taken at this time had the composition shown below. About 12,000 SCF of oxygen was then injected into the bath via a hand-held consumable lance. The slag formed thereby was skimmed off, and the following additions were made to the melt: 2278 lbs. FeCrSi, 300 lbs. low CFeCr, 800 lbs. lime, 80 lbs. Ni. Power was again applied to the electrodes and the heat was tapped into a ladle for subsequent teeming into molds. The preliminary sample composition and the final tap composition are shown below. ______________________________________ % % % % % % % % Analysis C Mn Si Cr Ni Mo P S ______________________________________ Preliminary 0.45 0.58 0.42 17.65 8.78 0.83 0.028 0.010 Tap 0.05 0.63 1.21 19.84 8.85 0.78 0.033 0.005 ______________________________________ The mechanical properties of the castings made from the melts of Examples 3 and 4, i.e., the invention and the prior art respectively, were substantially the same. However, the average time required for cleaning and repair, based on 6 castings, made by the invention was approximately 30% less than the average time required for cleaning and repair of 7 like castings made by the prior art.