Abstract:
A metallic positive expulsion fuel tank with stress free weld seams may include a first hemispherical shell with a first edge; a pressurized gas inlet attached to the first hemispherical shell; and a metallic cylinder with first and second edges attached to the first hemispherical shell along matching first edges by a first weld seam. The tank may also include a second hemispherical shell with a first edge attached to a fuel outlet fixture. An elastomeric diaphragm may be attached to the fuel outlet fixture on the second hemispherical shell. The second hemispherical shell may be attached to the second edge of the metallic cylinder along matching edges by a second weld seam thereby forming a positive expulsion fuel tank with two interior chambers separated by the elastomeric diaphragm. The first and second weld seams may be subjected to a localized post-weld stress relief heat treatment in which heating of the tank is confined to a distance of 2 inches (5.08 cm) of the first weld seam and a distance of 2 inches (5.08 cm) of the second weld seam such that the stresses in the first and second weld seams are relieved and the elastomeric diaphragm is unaffected by the heat treatment.

Description:
BACKGROUND 
       [0001]    This invention relates to the post-weld heat treatment of thin wall metal structures. In particular the invention relates to a method of locally heat treating a weld seam without thermally affecting material adjacent to the weld. 
         [0002]    The shells of typical positive expulsion propellant fuel tanks for spacecraft that incorporate elastomeric diaphragms are fabricated by welding metal domes or domes and cylinders together. Additionally, the diaphragm and its supporting structure are typically welded into the tank shell components prior to final shell assembly. Of the many important design criteria associated with a spacecraft, an overriding design driver is the need for low mass. The need for reduced mass drives the material choices and the wall thickness of the tank design. In the area of the welds that hold the domes and/or cylinders together, the wall thickness is often greater than the rest of the tank wall because of the reduced strength and toughness available in the welds and the adjacent heat affected zone. Much of the available tensile strength can be restored through the use of post-weld heat treatment (PWHT) for recovery of ductility and/or stress relief. 
         [0003]    While the PWHT of the tank shell can be readily accomplished using industry practices such as vacuum heat treatment and retort heat treatment with the part sealed in a container of inert gas, many tanks utilize elastomeric diaphragms to separate the propellants from pressurants to enable positive expulsion of propellants in microgravity. The elastomeric materials may be damaged by exposure to the high temperatures required for thermal stress relief during PWHT, which prevents the use of conventional furnaces to raise the temperature of the entire tank to the stress relief temperatures. Without stress relief the reduced material properties and residual tensile stresses remain in the weld areas of the tank shell. As a result, the tank must be made thicker to provide sufficient safety margin. The increased thickness and resultant increased mass is detrimental to the utility of the tank. 
       SUMMARY 
       [0004]    A metallic positive expulsion fuel tank with stress relieved weld seams may include a first hemispherical shell with a first edge. The tank may also include a pressurized propellant gas inlet fixture attached to the first hemispherical shell. The tank may also include a metallic cylinder with first and second edges attached to the first hemispherical shell along matching first edges by a first weld seam. The tank may also include a second hemispherical shell with a first edge and an attached fuel outlet fixture. An elastomeric diaphragm may be attached to the fuel outlet fixture on the second hemispherical shell. The second hemispherical shell may be attached to the cylinder along matching first and second edges by a second weld seam thereby forming a positive expulsion fuel tank with two interior chambers separated by the elastomeric diaphragm. The first and second weld seams may be subjected to a localized post-weld stress relief heat treatment in which heating of the tank is confined to a distance of 2 inches (5.08 cm) from the first weld seam and a distance of 2 inches (5.08 cm) from the second weld seam such that stresses in the first and second weld seams are relieved and the elastomeric diaphragm is unaffected by the heat treatment. 
         [0005]    A method of forming a metallic positive expulsion fuel tank may include forming a first hemispherical shell with a first edge and attaching a pressurized gas inlet fixture to the first hemispherical shell. The method may also include forming a metallic cylinder with first and second edges and attaching the cylinder to the first hemispherical shell along matching first edges by a first weld seam. The method may further include forming a second hemispherical shell with a first edge and attaching a fuel outlet fixture to the second hemispherical shell. The method may also include forming an elastomeric diaphragm and attaching the elastomeric diaphragm to the fuel outlet fixture on the second hemispherical shell. Additional steps may include welding the second hemispherical shell to the metallic cylinder along matching first and second edges to form a second weld seam and a positive expulsion fuel tank with two interior chambers separated by the elastomeric diaphragm. In a final step, a localized post-weld stress relief heat treatment may be performed on the first and second weld seams during which heating of the tank is confined to a distance of 2 inches (5.08 cm) from the first weld seam and a distance of 2 inches (5.08 cm) from the second weld seam such that stresses in the first and second weld seams are relieved and the elastomeric diaphragm is unaffected by the heat treatment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a schematic view of the exterior of a positive expulsion fuel tank. 
           [0007]      FIG. 2  is a schematic illustration of the positive expulsion fuel tank of  FIG. 1  illustrating one form of an elastomeric diaphragm. 
           [0008]      FIG. 3  is a schematic illustration of the positive expulsion fuel tank of  FIG. 1  illustrating another form of an elastomeric diaphragm. 
           [0009]      FIG. 4  is a schematic representation of a post-weld heat treatment apparatus. 
           [0010]      FIG. 5  is a diagram of a post-weld heat treatment process. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    An accepted method of providing fuel to spacecraft engines in gravity free environments is by positive expulsion propellant fuel tanks. These tanks are structures that include hemispherical shells or hemispherical shells and cylinders welded together to form a tank shell. A flexible diaphragm may be positioned inside the tank to form two chambers separated by the diaphragm. One chamber may contain fuel and the other chamber may contain compressed gas. Increased pressure in the gas filled chamber forces fuel out of the fuel chamber through a fuel supply fitting to an engine during operation of the system. 
         [0012]    In prior art practice, weld seams in completed spacecraft fuel tanks can only be heat treated by subjecting the entire tank to a heat treat schedule wherein the high temperatures may degrade the mechanical properties of sensitive propulsion management devices and other structures already in place in the welded tank structure. The ability to perform post-weld heat treatments in a welded structure wherein the heat treat temperatures are confined to the immediate vicinity of the welds may alleviate many of the problems associated with overheating. 
         [0013]      FIG. 1  is a schematic illustration showing the external surface of positive expulsion fuel tank  10 . In the embodiment shown in  FIG. 1 , positive expulsion fuel tank  10  may comprise hemispherical metal shells  12  and  14  joined to cylinder  16  along weld seams  18  and  20 . Positive expulsion fuel tank  10  may also have pressurized gas inlet  22  and fuel outlet  24 . Positive expulsion fuel tank  10  may also contain an elastomeric diaphragm (not shown) separating a pressurized gas chamber (not shown) connected to pressurized gas inlet  22  and a fuel chamber (not shown) containing propellant connected to fuel outlet  24 . 
         [0014]    Metal shells  12 ,  14 , and  16  of fuel tank  10  may be a titanium alloy, an aluminum alloy, a corrosion resistant steel, a nickel alloy, or others known in the art. A preferred tank material for some embodiments is a titanium alloy. The thickness of tank shell  12  depends on the material and may be from about 20 mils (508 microns) to about 125 mils (3175 microns) in typical embodiments. 
         [0015]      FIG. 2  is a schematic illustration of an embodiment of fuel tank  10  containing internal elastomeric diaphragm  30  shown in dashed lines in  FIG. 2 . Interior chamber  32  of hemispherical elastomeric diaphragm  30  may be connected to fuel outlet  24  through connection  36 . Diaphragm  30  separates positive expulsion fuel tank  10  into two chambers. Chamber  32  may contain propellant and chamber  38  may contain pressurized gas that enters chamber  38  through connection  40  connected to pressurized gas inlet  22  as indicated by arrow  42 . 
         [0016]    In another embodiment, the configuration of elastomeric diaphragm  30  and positive expulsion fuel tank  10  may take the form shown in  FIG. 3 . In this embodiment, diaphragm  30  may be supported by central perforated pipe  46  and may be attached at both ends of pipe  46 . When compressed gas chamber  38  is pressurized by gas entering chamber  38  from connection  40  connected to the pressurized gas inlet  22  as shown by arrows  42 , diaphragm  30  may be compressed towards central perforated pipe  46 . Fuel in fuel chamber  32  may then pass into pipe  46  and exit pipe  46  through connection  36  into fuel outlet  42 . 
         [0017]    Elastomeric diaphragm  30  may be formed from a number of elastomeric materials including, but not limited to silicon, ethylene propylene diene rubber, fluoroelastomers and perfluororelastomers. 
         [0018]    In the welded condition, welds  18  and  20  may contain at least internal tensile stresses that may need to be relieved by a post-weld heat treatment (PWHT). In addition, for many alloys, a PWHT may be required to restore lost tempers by, for instance, a solution heat treatment followed by quench and aging heat treatments. This may be accomplished by a PWHT method and PWHT apparatus described in co-pending patent application Ser. No. 14/287,975 which is hereby incorporated by reference in its entirety. 
         [0019]    The PWHT apparatus may include an enclosure covering a weld seam that may contain an inert flowing gas, water cooled cooling bands mounted on each side of the weld seam, a thermal insulating blanket covering the weld seam, and an induction coil or coils in close proximity to the weld seam to locally heat the weld seam. A schematic cross section of PWHT apparatus  50  is shown in  FIG. 4 . PWHT apparatus  50  is shown positioned on external surfaces of cylinder  16  and shells  12  or  14  of welded thin wall positive expulsion fuel tank  10  surrounding weld seams  18  and  20 . 
         [0020]    Apparatus  50  may further comprise induction coil  56  proximate weld seams  18  or  20 . Induction coil  56  may comprise multiple induction coils as needed. Induction coil  56  may be energized by induction power supply and control system  58 , as shown schematically by dashed line  60 . Apparatus  50  may further comprise cooling bands  62 , thermal insulating blanket  64 , and thermocouple  68 . Cooling bands  62  may be fluid cooled, thermoelectrically cooled, or cooled by other means known in the art. An exemplary cooling medium is water. Cooling bands  62  may be attached to positive expulsion fuel tank  10  with thermally conductive adhesive  65  to ensure maximum thermal conductivity between cooling bands  62  and positive expulsion fuel tank  10  to prevent regions of positive expulsion fuel tank  10  external to cooling bands  62  from overheating. Preferably, cooling bands  62  are placed at least about 2 inches (5.08 cm) from weld seams  18  and  20 . 
         [0021]    Thermocouple  68  supplies temperature data to induction power supply and control system  58  as schematically indicated by dashed line  70 . In some embodiments, thermocouple  68  may be replaced with an infrared pyrometer, thermistor or other temperature sensing devices known in the art. Weld seams  18  and  20 , cooling bands  62 , thermal insulating blanket  64 , and thermocouple  68  may be covered with inert atmosphere enclosure  72 . Inert atmosphere enclosure  72  may include inlet port  76  attached to a source of inert gas schematically indicated by arrow  78  and exhaust port  80  containing exhaust gas schematically indicated by arrow  82 . Inert atmosphere enclosure  72  may be an electrically non-conducting material enclosure that is transparent to an inductive field. Suitable non-conducting materials for inert atmosphere enclosure  72  may include flexible heat resistant materials such as silicone or rigid composites. 
         [0022]    Inert gas (at arrow  78 ) may be argon, nitrogen, helium, or others known in the art. Exhaust gas (at arrow  82 ) may be passed through oxygen analyzer  83  to determine oxygen levels of the inert atmosphere leaving inert atmosphere enclosure  38 . Oxygen levels of less than 50 ppm are preferred to prevent oxidation during the post-weld heat treatment process. 
         [0023]    External surface  84  of weld seams  18  and  20  and adjacent regions may be under an inert atmosphere during a post-weld heat treatment. Interior  38  of positive expulsion fuel tank  10  may be filled with inert gas to prevent oxidation during the PWHT process. 
         [0024]    In positive expulsion fuel tanks with elastomeric diaphragms, prior to a post-weld heat treatment of the present invention, the diaphragms are positioned such that the diaphragm material is safely removed from any proximity of the weld seams being treated. In the embodiment shown in  FIG. 2  for instance, where diaphragm  30  is shown partially emptied, diaphragm  30  would be totally collapsed inside tank  10  such that there would be no diaphragm material near weld seam  20  during a PWHT. In the embodiment shown in  FIG. 3 , diaphragm  30  would be collapsed toward central post  46  such that no diaphragm material would be in a radial position where it would be near weld seams  18  and  20  during a PWHT. 
         [0025]      FIG. 5  is a flow diagram illustrating post-weld heat treatment method  90  according to an embodiment of the present invention. In the first step, thermocouple  68  may be attached to weld seams  18  and  20  to indicate temperature during the heat treatment (step  92 ). Weld seams  18  and  20  may then be covered with thermal insulating blanket  64  (step  94 ). Blanket  64  may be fabricated from any material with appropriate physical characteristics coupled with thermal and electrical insulating properties. Preferred insulating materials include woven fiberglass cloth or woven ceramic or refractory fiber cloth. 
         [0026]    In the next step, cooling bands  62  may be attached to positive expulsion fuel tank  10  on each side of weld seams  18  and  20  (step  96 ). Cooling bands  62  may be shaped to closely follow the contour of the external surface of tank shell  12  and cylinder  16  and tank shell  14  and cylinder  16  of positive expulsion fuel tank  10 . In an embodiment, interfaces between cooling bands  62  and positive expulsion fuel tank  10  may be filled with a thermally conducting adhesive  65  to ensure maximum thermal conductivity between cooling bands  62  and external surfaces of shell  12  and cylinder  16  and shell  14  and cylinder  16  of positive expulsion fuel tank  10  as shown in  FIG. 4 . Examples of thermally conducting adhesive material forms include tapes, greases, pastes, and sheets. Preferably cooling bands  62  may be placed at a distance of at least about 2 inches (5.08 cm) from the weld seams 
         [0027]    In the next step, weld seams  18  and  20 , thermocouples  68 , and cooling bands  62  may be covered with inert atmosphere enclosure  72  (step  98 ). Inert atmosphere enclosure  72  may contain inlet port  76  connected to an inert gas source as indicated by arrow  78  ( FIG. 4 ). Inert atmosphere enclosure  62  may also include exhaust port  80  wherein the exhaust gases, indicated by arrow  82 , may be analyzed by oxygen analyzer  83 . As noted earlier, inert atmosphere enclosure  72  may be composed of an electrical non-conductor such that it is transparent to the induction field from induction coil  56 . 
         [0028]    Induction coil  56  (or multiple induction coils) may then be positioned proximate weld seams  18  and  20  outside inert atmosphere enclosure  72  (step  100 ). The induction system may include power supply and control system  58  connected to thermocouple  68  on weld seams  18  and  20  to provide a controllable time-temperature profile during the heat treatment. In some embodiments a larger enclosure may place the induction coil or coils within the inert atmosphere. 
         [0029]    In the next step, inert gas may be inserted into inert atmosphere enclosure  72  at arrow  78  (step  102 ). At this point, the internal surface of positive expulsion fuel tank  10  may also be protected by filling interior  38  of positive expulsion fuel tank  10  with an inert atmosphere. A preferred oxygen level surrounding weld seams  18  and  20  is less than 50 ppm to prevent oxidation. Tank interior  38  may also be filled with flowing inert gas (step  104 ). 
         [0030]    Power supply and control system  58  may then be activated to perform a post-weld heat treatment of weld seams  18  and  20  (step  106 ). During the post-weld heat treatment, the oxygen content of inert gas exiting inert atmosphere enclosure  72  at arrow  82  and the tank interior may be monitored by oxygen analyzer  83  to ensure the absence of oxidation during the heat treatment process (step  108 ). 
         [0031]    A benefit of apparatus  50  is that weldments in thin wall metallic structures may be subjected to heat treatment profiles at temperatures exceeding 2000° F. (1098° C.) on site during which the heat affected zone is restricted to a minimum distance of at least about 2 inches (5.08 cm) from the weld seam. As a result, the microstructure and properties of material directly outside this distance from the weld seam are unaffected. In addition, the low thermal mass of the method allows rapid cool down following solution treatment, limiting unwanted precipitation and grain growth in the thin wall structure. Expensive and elaborate furnaces, retorts, and other equipment associated with heat treatment of complete structures are unnecessary. 
         [0032]    In some embodiments, positive expulsion fuel tanks may be titanium or titanium alloys. Non-limiting examples may include CPTi, Ti-6Al-4V, and Ti-2.5V-4Al-1.5Fe (Ti38). Welds in these materials may be given a PWHT at about 1025° F. (552° C.) for about 3 hours. 
         [0033]    While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.