WO2005017233A2

WO2005017233A2 - Insulated cold hearth for refinning metals having improved thermal efficiency

Info

Publication number: WO2005017233A2
Application number: PCT/US2004/017766
Authority: WO
Inventors: Kuang-O Yu; Frank P. Spadafora
Original assignee: Rmi Titanium Company
Priority date: 2003-06-06
Filing date: 2004-06-04
Publication date: 2005-02-24
Also published as: WO2005017233A3

Abstract

An insulated cold retort or hearth for producing refined metal from raw metal contaminated with high density inclusions and/or hard alpha particles. The hearth has a hollowed body with a rectangular bottom wall and respective upstanding rectangular side and end walls which have imbedded water pipes for cooling water to circulate. An upwardly open chamber is defined within the body which terminates at an upper rim for holding a skull of metal formed on the cooled walls of the hearth. A plurality of insulating panels are closely received in the chamber covering those areas of the inner surface where heat retention is desired. Alternatively, an insulating inner liner may be utilized which is closely received in the chamber covering part or all of the inner surface to prevent heat transfer between the skull and the cooled retort/hearth interior surface and retain heat within the skull. The liner has an interior surface which defines a skull-receiving sub-chamber sized to closely receive the skull therewithin. A sub-liner may be used which fits within the liner to further prevent heat transfer between the skull and the cooled interior surface and retain heat within the skull.

Description

INSULATED COLD HEARTH FOR REFINING METALS HAVING IMPROVED THERMAL EFFICIENCY

BACKGROUND OF THE INVENTION

1. TECHNICAL FIELD Generally, the invention relates to the melting and refining of metals using electron beam guns or plasma torches in a water-cooled cold retort or hearth. Particularly, the invention relates to a cold retort or hearth for melting or refining metals which retains heat where needed to provide improved thermal efficiency and which produces ingots of improved properties. Specifically, the invention relates to an insulated cold retort or hearth which utilizes insulation to retain heat in the solid skull to increase raw material melting rate and liquid metal superheat.

2. BACKGROUND INFORMATION Titanium is increasingly used in many design applications where material having high strength and low weight is required. One obvious example is the aircraft industry where every pound of weight reduction to an aircraft means increased range or more payload capacity. Turbine aircraft engines in particular use substantial amounts of titanium alloys in the blades and disks of the fan and compressor sections where high strength and durability are required. These blades and disks must hold together under high stresses and heat for thousands of heating and cooling cycles as the engine is started and stopped. Failures are typically catastrophic, resulting in the engine self-destructing. One major cause of blade and disk failure is defects such as unrefined inclusions present in the titanium alloy which act as stress risers from which cracks propagate. Even the smallest amounts of unrefined inclusions in these rotating engine parts can cause catastrophic failure due to the high centrifugal force the rotating parts are subjected to during high rotational speeds of the rotating components of the fan and compressor sections. Typical defects present include high-density inclusions and hard alpha particles. High-density inclusions are contaminates introduced during ingot production which are of much higher density than the titanium. These defects include tungsten, tungsten carbide, tantalum, and molybdenum. Conversely, hard alpha defects are titanium particles and regions with high concentrations of interstitial alpha stabilizers such as nitrogen, oxygen, and carbon. Cold hearth refining has been used to produce commercially pure titanium, titanium alloys and superalloys. Quality improvements result from the removal of the high-density inclusions and hard alpha particles in titanium alloys. For superalloys, removal of oxide and nitride inclusions and other volatile tramp elements are the major factors of quality improvement. One version of cold hearth refining utilizes Electron Beam Melting (EBM) wherein a plurality of electron beam guns are used to melt the titanium, called Electron Beam Cold Hearth Refining (EBCHR). Raw metal in the form of titanium sponge compacts, chips, scraps, and machine turnings from the machining of components made of titanium alloy parts is loaded into a cold retort or hearth and melted by electron beams from the electron beam guns disposed over the retort or hearth. When the molten metal contacts cooled bottom and side surfaces of the retort or hearth, the titanium alloy solidifies at the walls of the retort or hearth such that only a relatively small melt pool of molten metal remains on the skull. As additional raw metal is supplied to a retort or hearth the electron beams melt the raw metal into the melt pool. The high density inclusions settle out of the melt pool to an interface with the skull and the now refined molten metal flows to an outlet end of the melt pool as additional raw metal is added, flowing out through a pouring spout at a front end of the hearth. Hard alpha defects are removed or dissolved in the molten titanium. The refined molten metal flows from the pouring spout into an ingot mold and solidifies into an ingot of refined metal, which is subsequently removed from the mold. The method is particularly effective in removing high-density inclusions and inhibiting the formation of hard alpha defects due to the fact that the molten metal continuously travels through the retort and hearth before flowing into the ingot mold. Such separation of the melting, refining and casting areas produces a more controlled residence time of the molten metal which better eliminates inclusions by dissolution and density separation processes in the melt pool. Another version of cold hearth refining utilizes Plasma Arc Melting (PAM) wherein a plurality of plasma torches are used to melt the titanium, called Plasma Arc Cold Hearth Refining (PACHR). The process is the same as electron beam cold hearth refining except for using the plasma torches which produce plasma plumes utilizing an inert gas. EBM and PAM are primarily used to melt titanium and titanium alloys. EBM is currently being used extensively for melting and recycling commercially pure (CP) titanium. For jet engine rotating component applications, the so-called Hearth + VAR (EBM + VAR or PAM + VAR) process is used to melt titanium alloys. In this case, EBM or PAM ingots are used as electrodes for the subsequent VAR (Vacuum Arc Remelting) processing. Recently, the Hearth +

VAR materials are also being considered for airframe structural component applications. Recent developments for both EBM and PAM have focused more on cost reduction considerations. In this case, the Hearth Only (PAM Only or EBM Only) process is used to replace currently used 2 X VAR, 3 X VAR, EBM + VAR and

PAM + VAR processes. Hearth Only process provides advantages of flexible use of various forms of raw materials, reducing the conventional titanium melting processes from the typical two or three melting steps required to a single cold hearth melting (EBM or PAM) step, and increasing product yield from the processes. Before the Hearth Only process can be successfully applied in production, however, certain technical difficulties associated with PAM and EBM processes need to be overcome. For EBM, the high degree of vacuum and high power electron beam can cause difficulty in controlling the chemistry of molten metal. Because the small electron beam size (typical 0.25 inch diameter) and high energy density in the beam direct heating area, high vapor pressure elements such as aluminum and chromium tend to evaporate relatively quickly and result in the depletion of the aluminum and chromium in the molten metal. The higher the power of the electron beam, the higher the evaporation rate of aluminum and chromium, producing greater difficulty in controlling the chemistry of molten metal and the resulting ingot. For PAM, helium/argon gas may be entrapped in the molten metal and result in helium/argon porosities in the resultant ingot. For both

EBM and PAM, the quality of the ingot surface is not as good as that of the VAR ingot and can result in some material yield loss during subsequent hot working operations. The surface quality of the EBM ingot is, in general, better than that of the PAM ingot. An important factor which has significant impacts on the formation of above mentioned defects in PAM and EBM ingots is the low thermal efficiency for melting and refining of metals in the cold hearth typically used for EBM and PAM. A significant amount of the heat input to the skull is lost from the top surface of the skull and molten metal pool due to radiation (both EBM and PAM) and helium/argon gas convection (PAM only). The contact between respective bottom, side, and end surfaces of the skull with the water-cooled copper retort or hearth results in a large quantity of heat lost to the cooling water (both EBM and PAM). As a result, high power input from plasma torch (PAM) or electron beam gun (EBM) is required to maintain a desired raw metal melting rate, liquid metal superheat, and molten metal pool volume. Although the prior art cold hearths are adequate for the purpose for which they were intended, they do not provide optimal thermal efficiency to produce quality PAM or EBM processed titanium ingots. Therefore, the need exists for an improved cold retort or hearth having improved thermal efficiency and which produces PAM or EBM processed titanium ingots with acceptable quality.

BRIEF SUMMARY OF THE INVENTION Objectives of the invention include providing a cold retort or hearth, which provides improved thermal efficiency during melting and refining. Another objective is to provide a cold retort or hearth in which the insulation may be tailored to melt the skull where inclusions are present. A still further objective of the invention is to provide such a cold retort or hearth which is producible by retrofitting existing retort or hearth and which solves problems and satisfies needs existing in the art. These objectives and advantages are obtained by the improved insulated cold retort or hearth of the present invention for producing refined metal from raw metal contaminated with high density inclusions, and/or hard alpha particles, the general nature of which may be stated as including: a cold retort or hearth and at least one insulating panel or unitary insulating liner. The retort or hearth has a hollowed body with a cooled interior surface, which defines an upwardly open skull-receiving chamber terminating at an upper rim. The chamber is adapted to hold a skull of metal. The retort or hearth is preferably of conventional rectangular configuration although the retort may be cylindrical in cross-section, having a bottom wall, a pair of side opposing walls, and respective input and output end walls, and water-cooled using water pipes disposed in the walls. The insulating panel or panels are closely received in the chamber covering at least a part of the inner surface to retain heat within a desired part of the skull.

The interior surface and the insulating panels define a skull-receiving sub-chamber in which the skull may be disposed. Each insulating panel is preferably rectangular in shape to fit conventional cold hearths which has an interior chamber that is rectangular in configuration. Such insulating panels may include, individually or in combination, an insulating bottom panel, an insulating side panel, and an insulating end panel. The unitary insulating inner liner is closely received in the chamber covering part or all of the inner surface retaining heat within the skull. The liner is of a shape to closely fit within the retort or hearth, having an interior surface which defines a skull-receiving sub-chamber in which the skull may be disposed. A unitary insulating sub-liner may be closely received in the interior sub-chamber of the liner. The sub-liner covers part or all of an interior surface of the inner liner to further retain heat within the skull, defining a skull-receiving sub-sub-chamber in which the skull may be disposed. The liner and sub-liner are preferably rectangular in configuration to fit conventional cold hearths having sub and sub- sub chambers that are rectangular in configuration. The liner and sub-liner preferably include respective rectangular insulating bottom panels and respective insulating peripheral walls comprising a pair of opposing rectangular insulating side walls and a pair of opposing rectangular insulating end walls. The insulated cold retort and/or hearth is utilized as part of a cold hearth metal refining system for producing refined metal from raw metal contaminated with high density inclusions and/or hard alpha particles. A preferred metal refining system includes the insulated cold retort or hearth, at least one heating electron beam gun (EBM) or plasma torch (PAM), an input feed device, and an ingot casting device. The insulated cold retort or hearth holds a skull of metal. The heating electron beam guns (EBM) or plasma torches (PAM) are disposed above the retort or hearth and produce heating electron beams (EBM) or plasma plumes

(PAM) directed onto the skull. The input feed device feeds raw metals to be refined onto the skull disposed in the retort or hearth. The ingot casting device to receive refined molten metal from the hearth and form solid ingots thereof. The insulated cold retort and hearth are utilized in a method for improving the thermal efficiency of melting and refining metals contaminated with high density inclusions and/or hard alpha particles which includes the steps of: 1) providing an insulated cold retort or hearth; 2) supplying raw metal which includes high density inclusions and/or hard alpha particles to be refined therefrom into the retort or hearth; 3) melting the raw metal to form a melt pool of molten metal having an input end and an outlet end using at least one heating electron beam gun (EBM) or plasma torch (PAM) which produces a heating electron beam (EBM) or plasma plume (PAM) directed onto a skull formed in the retort or hearth; 4) maintaining the molten metal in a molten state a sufficient amount of time to permit impurities of a higher density than the metal to settle out in the melt pool to produce a refined molten metal; 5) transferring the refined molten metal in a molten state into an ingot mold and allowing to cool to form a solid ingot; 6) removing the ingot from the ingot mold; and 7) wherein necessary heat input of the heating electron beam gun (EBM) or plasma torch (PAM) from the heating electron beam (EBM) or plasma plume (PAM) to the melt pool is reduced due to the insulation in the retort or hearth.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS The preferred embodiments of the invention, illustrative of the best mode in which applicant has contemplated applying the principles, are set forth in the following description and are shown in the drawings and are particularly and distinctly pointed out and set forth in the appended claims. FIG. 1 is a perspective view of a conventional uninsulated cold hearth and ingot casting mold; FIG. 2 is a sectional view of the hearth and ingot casting mold taken along line 2-2, FIG. 1 ; FIG. 3 is a lateral vertical sectional view of a first embodiment insulated cold hearth of the present invention shown in section and having a first insulation system comprised of a pair of insulating bottom panels; FIG. 4 is a lateral vertical sectional view of a second embodiment insulated cold hearth of the present invention having a second insulation system comprised of a trio of insulating bottom panels and respective insulating side panels; and FIG. 5 is a longitudinal vertical sectional view of a third embodiment insulated cold hearth of the present invention having a third insulation system comprised of respective insulating inner and a sub-liners. Similar numerals refer to similar parts throughout the drawings. DETAILED DESCRIPTION OF THE INVENTION FIGS. 1 and 2 depict a conventional cold hearth metal refining system, designated generally at 20, for producing refined metal from raw metals such as sponge and machine turnings of the metal, typically titanium based, contaminated with high density inclusions and/or hard alpha particles. The metal refining system 20 includes a conventional cold hearth 23, typically of the water-cooled type, and an ingot casting device preferably comprising an ingot casting mold 26. The hearth 23 is a hollowed body of rectangular configuration, comprising a rectangular bottom wall 32 (FIG. 2) and an upstanding peripheral wall which includes a rectangular input end wall 35, a rectangular output end wall 38, and a pair of rectangular side walls 41 and 44. A cooled interior surface 45 of hearth 23 defines an upwardly open, skull-receiving chamber 47 of rectangular configuration terminating at an upper rim 50. The walls 32, 35, 38, 41 , and 44 are made of copper or copper alloy with a plurality of cooling pipes 53 contained therewithin through which cooling water may be circulated from a water source (not shown) to cool the interior surface 45. The hearth 23 holds a skull 56 of the metal being refined formed in the hearth 23 by contact and solidification on the cooled interior surface 45 in the present or in previous melting operations. The skull 56 includes an upper portion 59 and a bottom portion 62 both of which include generally horizontally-disposed areas containing contaminates 65 that are heavier than the metal and which settle out when the metal is in a molten state. One or more heating devices 68, typically of the plasma torch (PAM) or the electron beam gun (EBM) type, are disposed above the hearth 23 each of which produce a heating electron beam (EBM) or plasma plume (PAM) 71 directed onto the skull 56 to form a melt pool 74 of molten metal on the skull 56. The heating devices 68 are fixed to direct the heating electron beam (EBM) or plasma plume (PAM) 71 onto the skull 56 to form the melt pool 74. Raw metal 77 to be refined, such as titanium scraps, titanium sponge, or machine turnings, is fed from a raw metal source (not shown) over the input wall 35 at a back end 78 of the hearth 23 into an input end 80 of the melt pool 74 by an input feed device (such as a conveyor) 83. The raw metal 77 is melted in the melt pool 74 by the heating electron beam (EBM) or plasma plume (PAM) 71 such that the heavier contaminates 65 fall to a bottom interface 86 of the melt pool 74 with the skull 56 forming refined molten metal at the top of melt pool 74. The vertical level of a surface 89 of the melt pool 74 rises as the raw metal 77 enters the melt pool 74 and refined molten metal overflows from an output end 92 of melt pool 74 disposed at a front end 93 of the hearth 23 through a pouring lip 95 of the output end wall 38 into the ingot casting mold 26. The ingot casting mold or sleeve 26 is cylindrical in cross-section, though it may be rectangular, polygonal, or other such shape, comprising an upright side wall 98 of circular cross-section. Side wall 98 has an open upper end 101 and an open lower end 104 which defines an interior bore 107. The wall 98 is made of copper or copper alloy with a plurality of cooling pipes 110 contained therewithin through which cooling water may be circulated. A vertically movable piston 113 closely fits abutting mold 26 to initially close the lower end 104 for containing the refined molten metal which overflows through pouring lip 95, forming an overflow pool 116 of the refined molten metal within bore 107. Piston 113 is vertically movable by an attached piston rod 119 of a withdrawal device, such as a hydraulic cylinder (not shown). A solid cylindrical ingot 122 of refined metal is formed within bore 107 as cooling of the overflow pool 116 progresses, and the piston 113 is gradually lowered below the lower end 104 of ingot casting mold 26 to permit forming of the ingot 122 to a desired length. As should also be apparent, ingot casting mold 26 may take a variety of forms, such as a batch casting ingot mold, without departing from the spirit of the present invention. A heating device 125 comprising a plasma torch (PAM) or electron beam gun (EBM) similar to heating device 68 is disposed above the ingot casting mold 26 and produces a heating electron beam (EBM) or plasma plume (PAM) 128 directed into the overflow pool 116 to control the rate of cooling thereof to harden into ingot 122. When the ingot 122 is of the desired length, the addition of raw metal 77 into hearth 23 is temporarily ceased, stopping the overflow of the refined molten metal into ingot casting mold 26. The overflow pool 116 is permitted to harden at a controlled rate using heating device 125 and the piston 113 is lowered to allow ingot 122 to be removed from ingot casting mold 26. A problem with the prior art arrangement is that the thermal efficiency of melting and refining of metals in cold retorts and hearths such as hearth 23 is low. Retorts and hearths refer to different styles of cold metal refining cavities and the cards will be used interchangeably to read both types of cavities. The temperature of raw metal 77 added to the hearth 23 by the feed device 83 is low, typically at room temperature. This requires significantly more heating energy to heat the input end 80 of melt pool 74 than the output end 92 to maintain a desired melting rate and molten metal pool superheat. FIG. 3 depicts an insulated cold hearth of the present invention, designated generally at 131 , designed to improve the thermal efficiency in producing refined metal from raw metal 77 contaminated with high density inclusions and/or hard alpha particles 65. The hearth 131 comprises the conventional cold hearth 23 and a first insulation system 134. The hearth 131 is used as part of the cold hearth metal refining system 20. The hearth 23 is rectangular in configuration, comprising the rectangular bottom wall 32, the narrow input end wall 35, the narrow output end wall 38, and the pair of elongate side walls 41 and 44, which define the interior chamber 47 beginning at the upper lip 50. The walls 32, 35, 38, 41 , and 44 are preferably made of copper with the plurality of cooling pipes 53 contained therewithin through which cooling water may be circulated. However, hearth 131 could take a variety of shapes without departing from the spirit of the present invention. The insulation system 134 includes a pair of insulating bottom panels 137 and 140 which are closely received in the chamber 47 covering at least a part of a rectangular bottom surface 141 of the interior surface 45 to prevent heat transfer between the skull 149 and the interior surface 45 and retain heat within a desired part of the skull 149. The interior surface 45 and bottom panels 137 and 140 define a skull-receiving sub-chamber 142. The bottom panels 137 and 140 are preferably of a mating rectangular shape to fit juxtaposed closely disposed on the bottom surface 141 of the bottom wall 32 of the hearth 23 with respective outer perimeters 143 and 146 thereof closely abutting the walls 35, 38, 41 , and 44 of hearth 23. The bottom panels 137 and 140 are preferably Kaowool Wrap™ manufactured by the Thermal Ceramics Corporation of Augusta, GA. Other suitable insulating materials include ceramics, asbestos, and similar heat-resistant materials, or even an insulating air gap. The bottom panels 137 and 140 may also cover only part of the bottom surface 141 of the hearth 23 and be of other than rectangular shape to provide the desired heat distribution within the skull 149. A skull 149 of the metal being refined is shown disposed within the sub- chamber 142 of the interior chamber 47 above the bottom panels 137 and 140.

The skull 149 is formed in the hearth 131 by contact and solidification on the cooled interior surface 45 of hearth 23 and on bottom panels 137 and 140 in the present or in previous melting operations. The skull 149 is slightly shorter in height than skull 56 due to the presence of bottom panels 137 and 140 and includes generally horizontally-disposed areas containing contaminates 65 that are heavier than the metal and which settle out when the metal is in a molten state. FIG. 4 depicts a second embodiment of the insulated melting hearth, designated generally at 161 , designed to improve the thermal efficiency in producing refined metal from raw metal 77 contaminated with high density inclusions and/or hard alpha particles 65. The hearth 161 comprises the uninsulated hearth 23 and a second insulation system 164. The hearth 161 is used as part of the cold hearth metal refining system 20. The hearth 23 is rectangular in configuration, comprising the rectangular bottom wall 32, the narrow input end wall 35, the narrow output end wall 38, and the pair of elongate side walls 41 and 44, which define the interior chamber 47 beginning at the upper lip 50. The walls 32, 35, 38, 41 , and 44 are preferably made of copper with the plurality of cooling pipes 53 contained therewithin through which cooling water may be circulated. The insulation system 164 includes a trio of rectangular insulating bottom panels 137, 140, and 165 which are closely received in the chamber 47 covering at least a part of the rectangular bottom surface 141 of the interior surface 45 to prevent heat transfer between the skull 188 and the interior surface 45 and retain heat within a desired part of the skull 188. The bottom panels 137, 140, and 165 are preferably of a mating rectangular shape to fit juxtaposed closely disposed on the bottom surface 141 of the bottom wall 32 of the hearth 23 with respective outer perimeters 143, 146, and 167 thereof closely abutting the walls 35, 38, 41 , and 44 of hearth 23. A pair of upstanding insulating side panels 170 and 173 are closely received in the chamber 47 disposed on respective side surfaces 174 and 175 of the side walls 41 and 44 covering at least part thereof. The side panels 170 and 173 are preferably of a mating rectangular shape to fit the side surfaces 174 and 175 of side walls 41 and 44 with respective outer perimeters 176 and 179 closely abutting the end walls 35 and 38 of hearth 23 and the bottom insulating panel 164. A pair of upstanding insulating end panels, only an end panel 182 being shown, are closely received in the chamber 47 disposed on respective rectangular end surfaces 183 and 184 of the end walls 35 and 38 covering at least part thereof.

The end panels including end panel 182 are preferably of a mating rectangular shape to fit the end surfaces 183 and 184 of end walls 35 and 38 with respective outer perimeters, including an outer perimeter185 of end panel 182, closely abutting the bottom panel 164 and the side panels 170 and 173. The panels 137, 140, 165, 170, 173, and 182 are made of the materials described above, and may also cover only part of the bottom surface 141 , the side surfaces 174 and 175, and the end surfaces 183 and 184 of the hearth 23 and be of other than rectangular shape to provide the desired heat distribution within the skull 188. A skull 188 of the metal being refined is shown disposed within an interior sub-chamber 191 of the interior chamber 47 above the bottom panels 137, 140, and 164, and between the side panels 170 and 173, and the end panels including end panel 182. The skull 188 is formed in the hearth 161 by contact and solidification on any part of the cooled interior surface 45 left exposed and uninsulated, on the bottom panels 137, 140, and 165, the side panels 170 and

173, and the end panels including end panel 182 in the present or in previous melting operations. The skull 188 is slightly smaller in height, width, and length than skull 56 due to the presence of the bottom panels 137, 140, and 167, the side panels 170 and 173, and the end panels including end panel 182 and includes generally horizontally-disposed areas containing contaminates 65 that are heavier than the metal and which settle out when the metal is in a molten state. FIG. 5 depicts a third embodiment of the insulated melting hearth, designated generally at 200, designed to improve the thermal efficiency in producing refined metal from raw metal 77 contaminated with high density inclusions and/or hard alpha particles 65. The hearth 200 comprises the uninsulated hearth 23 and a third insulation system 203. The hearth 200 is used as part of the cold hearth metal refining system 20. The hearth 23 is rectangular in configuration, comprising the rectangular bottom wall 32, the narrow input end wall 35, the narrow output end wall 38, and the pair of elongate side walls 41 and 44, which define the interior chamber 47 beginning at the upper lip 50. The walls 32,

35, 38, 41 , and 44 are preferably made of copper with the plurality of cooling pipes 53 contained therewithin through which cooling water may be circulated. The insulation system 203 includes an insulating inner liner 206 and sub- liner 209, each preferably unitary or one-piece in design. Inner liner 206 is of a mating shape to fit in the hearth 23 closely received in the chamber 47, preferably substantially covering the interior surface 45 to prevent heat transfer between the skull 236 and the interior surface 45 and to retain heat within the skull 236. Inner liner 206 includes an insulating bottom panel 212 of rectangular shape to closely fit to and cover the bottom surface 141 of bottom wall 32, and an upstanding insulating peripheral wall comprised of a pair of opposing rectangular insulating side panels 215 and 218 to cover the side surfaces 174 and 175 of side walls 41 and 44, and a pair of opposing rectangular insulating end panels, only an end panel 221 being shown, to cover the end surfaces 183 and 184 of the end walls 35 and 38. The inner liner has an interior surface 222 defining a skull-receiving sub- chamber 223 of rectangular configuration sized to closely receive the inner liner 206. The panels 212, 215, 218, and 221 are preferably integrally formed or joined together to form an integral unit. Sub-liner 209 is closely received in the interior sub-chamber 223 of inner liner 206, preferably substantially covering the inner liner 206 to further prevent heat transfer between the skull 236 and the interior surface 45 and retain heat within the skull 236. Sub-liner 209 includes a rectangular insulating bottom panel 224 disposed to cover the bottom panel 212 of the inner liner 206, and an upstanding insulating wall comprised of a pair of opposing rectangular insulating side panels 227 and 230 disposed to cover the side panels 215 and 218, and a pair of opposing rectangular insulating end panels, only an end panel 233 being shown, disposed to cover the end panels including end panel 221. An interior surface 234 of sub-liner 209 defines a skull-receiving sub-sub-chamber 235 sized to closely receive a skull 236 therewithin. The panels 224, 227, 230, and 233 are preferably integrally formed or joined together to form an integral unit. The inner liner 206 and sub-liner 209 are made of the materials described above, and may also cover only part of the bottom surface 141 , the side surfaces 174 and 175, and the end surfaces 183 and 184 of the hearth 23 and be of other than rectangular shape to provide the desired heat distribution within the skull 236. The skull 236 of the metal being refined is shown disposed within an interior sub-sub-chamber 235 of the interior chamber 47. The skull 236 is formed in the hearth 200 by contact and solidification on any part of the cooled interior surface 45 left exposed and uninsulated, any exposed part of the interior surface 222 of inner liner 206, and the interior surface 234 of sub-liner 209 in the present or in previous melting operations. The skull 236 is slightly smaller in height, width, and length than skull 56 due to the presence of the inner liner 206 and sub-liner 209, and includes generally horizontally-disposed areas containing contaminates 65 that are heavier than the metal and which settle out when the metal is in a molten state. As should be apparent from a review of the first, second, and third embodiments above, these embodiments have been provided to show and exemplary apparatus and method for providing insulation and control heat loss from a combination of a hearth and skull during the creation of a molten pool. As should be apparent from this review, a variety of configurations of insulation on both the bottom, side, and top wall could be provided without departing from the spirit of the present invention. Similarly, and as set forth above, the materials within individual insulation layers, and areas of insulation, may be varied one from another to provide custom insulation characteristics for a particular melt strategy. A method for improving the thermal efficiency of melting and refining metals contaminated with high density inclusions and/or hard alpha particles includes first providing an insulated cold retort or hearth of the type described above. An upper portion of the skull is melted to form a melt pool of molten metal on the skull using one or more heating devices which produces a heating electron beam (EBM) or plasma plume (PAM) directed onto the skull. The melt pool has an input end where raw metal to be refined is added and melted and an outlet end where refined metal exits the hearth. Raw metal is supplied which includes high-density inclusions and/or hard alpha particles to be refined therefrom to the input end of the melt pool and melted into the melt pool using the heating device. The molten metal is maintained in a molten state in the melt pool on the skull using the heating device a sufficient amount of time to permit the high density inclusions and/or hard alpha particles to settle out in the melt pool. The inclusions sink to the bottom interface of the melt pool with the skull to produce a refined molten metal thereabove. The refined molten metal is transferred from the hearth while being maintained in a molten state into an ingot mold. The refined molten metal is allowed to cool to form a solid ingot of refined metal in the ingot mold. The solid ingot is removed from the ingot mold. The method can use reduced heat input of the heating devices into the melt pool due to the insulation in the retort and hearth.

The methods and apparatus of the present invention provide improved melting and refining efficiency resulting in the following benefits for both PAM and

EBM: 1 ) increased melting rate of the raw metal, higher liquid metal superheat temperature, and deeper melt pool of increased volume; 2) lower total processing time for producing ingots from raw metal and associated higher productivity of the melt shop; 3) higher dissolution rate for the hard alpha particles in the melt pool; 4) better surface quality of the ingot; and 5) increased probability for high density inclusions (HDI) and solid hard alpha particles to sink to the bottom of the molten pool and reduce the chances for the HDIs and other unmelted solid particles being flushed out of the hearth and fall into the ingots. The methods of the present invention provide improved melting and refining efficiency resulting in the following benefits for PAM: 1 ) decreased viscosity of the molten metal of the melt pool, resulting increased probability for entrapped helium/argon gas bubbles to escape from the molten pool of the resultant PAM ingot. The methods of the present invention provide improved melting and refining efficiency resulting in the following benefits for EBM: 1 ) reduced degree of chemical composition variation due to evaporation of high vapor pressure elements such as aluminum and chromium, and reduced overall loss of titanium base material due to evaporation, both due to a higher melting rate that reduces the total processing time and/or the required power level of the electron beam. Accordingly, the cold hearth provides improved thermal efficiency, produces ingots which have fewer inclusions, has insulation to prevent heat transfer between the skull and the cold retort/hearth interior surface and retain heat in the skull where inclusions may be present, and which is producible by retrofitting existing cold retort and hearths which achieves all the enumerated objectives, provides for eliminating difficulties encountered with prior art devices, and solves problems and obtains new results in the art. References for the present invention include the following documents with the most pertinent pages cited, which documents are herein incorporated by reference in their entirety: 1 ) C.E. Shamblen, G.B. Hunter, "Titanium Base Alloys Clean Melt Process Development", Proceedings of the 1989 Vacuum Metallurgy Conference on the Melting and Processing of Specialty Materials, Iron and Steel Society, Inc., Warrendale, PA, pp. 3-11 , 1989; 2) D.J. Tilly, C.E. Shamblen, W.H.

Buttrill, "Premium Quality Ti Alloy Production: HM+VAR Status", Proceedings of the 1997 International Symposium on Liquid Metal Processing and Casting, A.

Mitchell, P. Auburtin, eds., Vacuum Metallurgy Division, American Vacuum Society, Santa Fe, NM, pp. 85-96, 1997; 3) C.E. Shamblen, DJ Tilly, "Inclusion Free Titanium Material Efforts", Proceedings of the Electron Beam Melting and Refining Conference - State of the Art 1997, R. Bakish, ed., Bakish Materials Corporation, Reno, NV, pp. 39-47, 1997; 4) K.O. Yu, "Plasma Arc Melting for

Titanium Alloys", Proceedings of the Technical Program from the 1998 International Conference, International Titanium Association, Monte Carlo, Monaco, pp. 371 -385, 1998; 5) K.O. Yu, J.G. Ferrero, CM. Bugle, "Evaluation of the TMP Behavior of PAM Cast Ti6AI-4V and Its Effect on Microstructure and Mechanical Properties", 12^th Advanced Aerospace Materials and Processes

Conference & Exposition, Long Beach, CA, 2001 ; and 6) K.O. Yu, F.P. Spadafora, J.M. Hjelm, B. Martin, S. Fellows, and M. Jacques, "Plasma Arc Melting of Titanium Alloys for Non-Rotating Component Applications", Proceedings of the 2001 International Symposium on Liquid Metal Processing and Casting, A. Mitchell, J. Van Den Avyle, eds., Vacuum Metallurgy Division, American Vacuum

Society, Santa Fe, NM, pp. 1-17, 2001. In the foregoing description, certain terms have been used for brevity, clearness and understanding; but no unnecessary limitations are to be implied therefrom beyond the requirements of the prior art, because such terms are used for descriptive purposes and are intended to be broadly construed. Moreover, the description and illustration of the invention is by way of example, and the scope of the invention is not limited to the exact details shown or described. Having now described the features, discoveries and principles of the invention, the manner in which the improved cold retort and hearth and method for melting and refining metals is constructed and used, the characteristics of the construction, and the advantageous, new and useful results obtained; the new and useful structures, devices, elements, arrangements, parts and combinations, are set forth in the appended claims.

Claims

1 . An insulated cold hearth for producing refined metal from raw metal contaminated with higher density impurities, the hearth being adapted to hold a skull of metal therein, comprising: a cold hearth having a hollowed body which defines an upwardly open skull- receiving chamber terminating at an upper rim, said chamber being adapted to hold the skull; and at least one insulating panel adapted to be received in said chamber covering at least a part of said inner surface to retain heat within a desired part of said chamber, said interior surface and insulating panel defining a skull-receiving sub-chamber in which the skull may be disposed.

2. The insulated cold hearth defined in Claim 1 in which the hearth comprises a bottom wall and an upstanding peripheral wall which define the skull-receiving interior chamber.

3. The insulated cold hearth defined in Claim 2 in which there is at least one insulating bottom panel positioned above the bottom wall of the hearth.

4. The insulated cold hearth defined in Claim 3 in which there are at least a pair of insulating bottom panels which are juxtaposed on the bottom wall of the hearth.

5. The insulated cold hearth defined in Claim 4 in which one of the insulating panels is made of titanium.

6. The insulated cold hearth defined in Claim 2 in which there is at least one upstanding insulating side panel which closely fits disposed at the side wall of the hearth.

7. The insulated cold hearth defined in Claim 6 further comprising an insulating bottom panel which closely fits to the bottom wall of the hearth, the insulating side panel having an outer perimeter which closely abut the bottom insulating panel to provide a substantially leak-proof seal therewith.

8. The insulated cold hearth defined in Claim 1 in which the interior chamber of the hearth is rectangular in configuration, having a rectangular bottom surface, a pair of rectangular input and output end surfaces, and a pair of rectangular side surfaces which define the interior chamber, each insulating panel being of a mating rectangular shape to fit respective of said surfaces of said hearth.

9. The insulated cold hearth defined in Claim 8 in which there is at least one rectangular insulating bottom panel which closely fits disposed covering at least part of the bottom surface of the hearth.

10. The insulated cold hearth defined in Claim 8 in which there is at least one upstanding rectangular insulating side panel which closely fits disposed covering at least one side surface of the hearth.

1 1. The insulated cold hearth defined in Claim 8 in which there is at least one rectangular insulating end panel which closely fits disposed covering at least part of one end surface of the hearth.

12. The insulated cold hearth defined in Claim 1 in which the hearth is water- cooled.

13. The insulated cold hearth defined in Claim 12 in which the hearth is made of copper with a plurality of cooling pipes contained therewithin through which cooling water may be circulated.

14. The insulated cold hearth defined in Claim 12 in which the hearth is rectangular in configuration, comprising a rectangular bottom wall, a pair of rectangular input and output end walls, and a pair of rectangular side walls which define the interior chamber, each insulating panel being of a mating rectangular shape to fit said hearth.

15. The insulated cold hearth defined in Claim 14 in which there is at least one rectangular insulating bottom panel which is disposed on the bottom wall of the hearth.

16. The insulated cold hearth defined in Claim 15 in which there is at least one upstanding rectangular insulating side panel which closely fits disposed at one side wall of the hearth.

17. The insulated cold hearth defined in Claim 16 in which there is at least one insulating end panel of rectangular shape which closely fits disposed covering at least part of one end surface of the hearth.

18. The insulated cold hearth defined in Claim 14 in which at least one insulating panel is made of a material selected from the group consisting of Kaowool Wrap™, ceramics, asbestos, and titanium.

19. An insulated cold hearth for producing refined metal from raw metal contaminated with higher density impurities, the hearth being adapted to hold a skull of metal therein, comprising: a cold hearth having an interior surface which defines an upwardly open skull-receiving chamber; and an insulating inner liner adapted to be closely received in said chamber covering at least part of said inner surface to retain heat within said chamber, said inner liner having an interior surface defining a skull-receiving sub-chamber in which the skull may be disposed.

20. The insulated cold hearth defined in Claim 19 further comprising an insulating sub-liner adapted to be closely received in the interior sub-chambers the inner liner, said sub-liner adapted to be closely received in said sub-chamber covering at least part of said inner liner to further retain heat within said chamber, said interior surface and insulating panel defining a skull-receiving sub-chamber in which the skull is disposed.

21. The insulated cold hearth defined in Claim 19 in which the hearth is rectangular in configuration, comprising a rectangular bottom wall, a pair of rectangular input and output end walls, and a pair of rectangular side walls which define the interior chamber, the insulating inner liner being of a mating shape to fit said hearth, and in which the primary inner liner is rectangular in configuration defining an interior sub-chamber of rectangular configuration, said primary inner liner including a rectangular insulating bottom panel to cover said bottom wall and an insulating peripheral wall comprising a pair of opposing rectangular insulating side walls and a pair of opposing rectangular insulating end walls to cover respective rectangular side and end wails of said hearth.

22. The insulated cold hearth defined in Claim 21 further comprising an insulating sub-liner adapted to be closely received in the interior sub-chambers the inner liner, said sub-liner including a rectangular insulating bottom panel to cover the bottom panel of said inner liner and an insulating peripheral wall comprising a pair of opposing rectangular insulating side walls and a pair of opposing rectangular insulating end walls to cover respective rectangular insulating side and end panels of said inner liner.

23. The insulation cold hearth defined in Claim 19 in which the insulating liner is rectangular in shape to fit a hearth which has an interior chamber that is rectangular in configuration, and in which the insulating liner includes at least one panel being chosen from the group consisting of an insulating bottom panel, an insulating side panel, and an insulating end panel.

24. An insulation system for a cold hearth which produces refined metal from raw metal contaminated with higher density impurities, the cold hearth having an interior surface which defines an upwardly open skull-receiving chamber to hold a skull of metal, comprising: an insulating inner liner adapted to be closely received in said chamber covering at least part of said inner surface to retain heat within said chamber, said inner liner having an interior surface defining a skull-receiving sub-chamber in which the skull may be disposed.

25. A cold hearth metal refining system for producing refined metal from raw metal contaminated with higher density impurities, comprising: an insulated cold hearth having a hollowed body with a cooled interior surface which defines an upwardly open skull-receiving chamber terminating at an upper rim, and at least one insulating panel adapted to be closely received in said chamber covering at least a part of said inner surface to retain heat within a desired part of said chamber, said interior surface and insulating panel defining a skull-receiving sub-chamber in which a skull may be disposed; an input feed device adapted to feed raw metal to be refined into said hearth; at least one heating gun disposed above said hearth which produces a heating beam directed onto metal disposed in said chamber to form a melt pool of metal on a skull; and an ingot casting device adapted to receive refined molten metal from said hearth and form solid ingots thereof.

26. A method for improving the thermal efficiency of refining metals contaminated with higher density impurities, comprising the steps of: providing an insulated cold hearth having a hollowed body with a cooled interior surface which defines an upwardly open skull-receiving chamber terminating at an upper rim, the hearth having disposed within the chamber insulation covering at least a part of the inner surface to retain heat within a desired part of the chamber, the interior surface and insulation defining a skull- receiving sub-chamber in which a skull may be disposed; supplying raw metal which includes higher density impurities to be refined therefrom into the hearth; melting the raw metal to form a melt pool of molten metal having an input end and an outlet end using at least one heating gun which produces a heating beam directed onto a skull formed in the hearth; maintaining the molten metal in a molten state in the melt pool on the skull using the heating gun a sufficient amount of time to permit impurities of a higher density than the metal to settle out in the melt pool onto a bottom interface of the melt pool with the skull to produce a refined molten metal; transferring the refined molten metal while maintaining the metal in a molten state into an ingot mold and allowing the metal to cool to form a solid ingot of refined metal in the ingot mold; removing the solid ingot from the ingot moid; and wherein necessary heat input of the heating gun from the heating beam to the melt pool is reduced due to the insulation in the hearth.