More on Titanium.... Titanium Welding Processes The following fusion-welding processes are used for joining titanium and titanium alloys: • Gas-tungsten arc welding (GTAW) • Gas-metal arc welding (GMAW) • Plasma Arc Welding (PAW) • Electro-Beam Welding (EBW) • Laser Beam Welding(LBW) • Friction Resistance Welding (FRW) • Resistance Welding (RW) • Gas-tungsten arc welding is the most widely used process for joining titanium and titanium alloys except for parts with thick sections. Square-groove butt joints can be welded without filler metal in base metals up to 2.5 mm thick. For thicker base metals, the joint should be grooved, and filler metal is required. The heated weld metal in the weld zone must be shielded from the atmosphere to prevent contamination with oxygen, nitrogen, and carbon, which will degrade the weldment ductility. • Gas-metal arc welding is used to join titanium and titanium alloys more than 3 mm thick. It is applied using pulsed current or the spray mode and is less costly than GTAW, especially when the base metal thickness is greater than 13 mm. Titanium Welding Mistakes There are 3 big mistake people make when tig welding titanium: • Using the wrong filler metal – Trying to weld titanium with anything other than titanium turns the weld into a hard as glass material. You can hear it cracking before it even cools off. • Not shielding the weld puddle adequately – When Titanium gets red hot, it loves to suck in all kind of impurities like oxygen and hydrogen. Once this happens, you are screwed, glued, and tattooed. Game over. The weld has to be removed. • Not cleaning the metal – porosity is a problem when tig welding titanium. Anything on the surface like oil, or dust, will cause porosity. Strict welding codes like American Welding Society D17.1 have very strict limitations on porosity. The Importance of Shielding Gas Coverage Pure argon is recommended for welding titanium because of its high purity and low moisture content. A 75/25 mixture of argon/helium may be used to improve stability and increase penetration only when specified. The American Welding Society (AWS) recommends measuring welding gas purity to make sure it meets the standards set for each application. Typical specifications suggest that shielding gas be at least 99.995 percent pure with no more than 20 parts per million (PPM) of oxygen and a dew point greater than -76 degrees F. Other applications require a 99.999 percent pure flow of argon. Outfitting your welding torch with a trailing shield is critical—otherwise the risk of oxygen contamination rises, and with it the potential for cracking. Some welders fabricate their own trailing shields, although many styles are available for purchase. Trailing shields conform to the shape of the tube and follow the GTAW torch around the pipe. The shields provide an extra protection of argon over the weld after the torch and its argon flow have passed. Setting the torch and trailing shield gas flow at 20 cubic feet per hour (CFH) provides the best coverage. Purging, a process that eliminates the oxygen contained within the pipe, also is required when welding titanium tubing. This process can be completed with any kind of purge dam: water-soluble dams, rubber gaskets, specialty tape, or inflatable bladders. Argon flows into the dammed area to replace the oxygen contained within the tubing. Allow the argon to flow long enough to replace the oxygen 10 times over to ensure the purest welding environment. Always use a clean, nonporous plastic hose to transport the shielding gas to the torch, trailing shield, and purge. Do not use rubber hose; rubber is porous and absorbs oxygen that could contaminate the weld. Tid Bits on Titanium: 2010 SECTION VIII, DIVISION 2 6-15 6.2.8 Special Requirements for Welding Test Plates for Titanium Materials 6.2.8.1 If a vessel of welded titanium contains Category A or B weld joints, then a production test plate of the same specification, grade, and thickness shall be made of sufficient size to provide at least one face and one root bend specimen or two-side bend specimens dependent upon plate thickness. Where longitudinal joints are involved, the test plate shall be attached to one end of the longitudinal joint and welded continuously with the joint. Where circumferential joints only are involved, the test plate need not be attached but shall be welded along with the joint, and each welder or welding operator shall deposit weld metal in the test plate at the location and proportional to that deposited in the production weld. 6.2.8.2 Test plates shall represent each welding process or combination of processes or a change from machine to manual or vice versa. At least one test plate is required for each vessel, provided not over 30 m (100 ft.) of Category A or B joints are involved. An additional test plate, meeting the same requirements as outlined above, shall be made for each additional 30 m (100 ft.) of Category A or B joints involved. The bend specimens shall be prepared and tested in accordance with Section IX, QW-160. Failure of either bend specimen constitutes rejection of the weld. 2010 SECTION VIII, DIVISION 1 UNF 19 WELDED JOINTS (a) For vessels constructed of titanium or zirconium and their alloys, all joints of Categories A and B shall be of Type No. (1) or No. (2) of TableUW-12. (b)Titanium or zirconium and their alloys shall not be welded to other materials. UNF-57 RADIOGRAPHIC EXAMINATION (a) Vessels or parts of vessels constructed of non ferrous materials shall be radiographed in accordance with the requirements of UW-11. (b) In addition, for vessels constructed of titanium or zirconium and their alloys, all joints of Categories A and B shall be fully radiographed in accordance with UW-51 FORGING UNF-78 WELDING Welding of titanium or zirconium and their alloys is to be by the gas-shielded tungsten arc process, the gas-shielded metal arc (consumable-electrode) process, the plasma arc welding process, the electron beam process, the laser be am process, or the resistance welding process, meeting the requirements of SectionIX or Appendix 17 of this Division, whichever is applicable. UNF-95 WELDING TEST PLATES If a vessel of welded titanium or zirconium and their alloys construction incorporates joints of Category A or B as described in UW-3, a production test plate of the same specification, grade, and thickness shall be made of sufficient size to provide at least one face and one root bend specimen or two side bend specimens dependent upon plate thickness. Where longitudinal joints are involved, the test plate shall be attached to one end of the longitudinal joint and welded continuously with the joint. Where circumferential joints only are involved, the test plate need not be attached but shall be welded along with the joint and each welder or welding operator shall deposit weld metal in the test plate at the location and proportional to that deposited in the production weld. Test plates shall represent each welding process or combination of processes or a change from machine to manual or vice versa. At least one test plate is required for each vessel provided not over 100ft of Category A o rB joints are involved. An additional test plate, meeting the same requirements as outlined above, shall be made for each additional 100ft of Category A or B joints involved. The bend specimens shall be prepared and tested in accordance with SectionIX, QW-160. Failure of either bend specimen constitutes rejection of the weld. MANDATORY APPENDIX 23 EXTERNAL PRESSURE DESIGN OF COPPER, COPPER ALLOY, AND TITANIUM ALLOY CONDENSER AND HEAT EXCHANGER TUBES WITH INTEGRAL FINS
Posted on: Thu, 23 Oct 2014 06:40:54 +0000
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