Sealing anything at cryogenic temperatures requires extremely tight tolerances. If tight tolerances are not considered, holes may open at the source of the seal, allowing cold leaks to occur as referenced in this past post. In today’s How To, we’re going to discuss how to butt weld VCR fittings utilizing orbital TIG welding. Orbital welding has given the lab an advantage in that all our welds are now rid of human error and the whole operation is computer operated. The system being used is Swagelok’s M200 orbital welding system. The procedure is as follows:
Begin by flipping on the machine and selecting the “Program” key from the main menu.
Next, once the program is entered with specific parameters the following operating screen will appear.
Since the tube is 0.25 in., the Arc Gap must be set as to allow the tungsten rod to make accurate contact with the tubing. To do this, the height of the arc gauge must be set to a specific height. In this case, the height is 0.777 inches.
Next, place the arc gauge into the weld head where the tungsten rod is located. From there, loosen the top two bolts so that the tungsten rod can move freely. After, allow the rod to fall into the divot on the arc gauge. This will properly set the rod for welding. Finally, re-tighten the bolts and remove the gauge.
Since, the rod is now properly calibrated, the weld pressure must be set. First, review to the welders display and notice the “Normal Purge” table. The values are listed for correct ID flows and pressures. To test that the system is correctly measuring these values, the following system must be set-up as to accomplish this.
The system consists of a “T” with inlets from the weld pressure gauge and low-flow outlet. The tube at the junction is directed from the ID Weld outlet and the other is from the low-flow outlet. The crimped tubing is there to allow adequate back pressure so pressure measurements may be taken. To conduct the test, vent argon gas from the low-flow outlet and refer to the pressure and ID weld pressure gauge. If they correspond approximately with the values needed for your weld then the system is ready.
Next, the weld fixture must be set with the correct parts being welded. In this case, a piece of quarter tubing and a corresponding VCR fitting. Place one side of the tubing into the collet as shown with the stopper butting up against it to center the tubing within the fixture.
Next, tighten down the tubing fitted into the fixture and place the next half into its respective collet. Tighten down the tubing and the fixture is set and your piece is ready for welding.
Notice that the VCR side of the tubing has a male fitting with a crimped end attached. This ensures that during welding back pressure is kept consistent.
Finally, insert the weld held into the fixture and tighten it down. From there, attach the low-gas piping to the open side of the fixture. Once everything is tightened down and ready to go your system will look like this.
Start the weld by hitting the “Start” button on the control panel. The system will conduct a gas purge before welding as well as a post-purging. This ensures that there is no oxygen in the system before and after welding.
If the weld goes correctly and all pieces were set correctly, then you should have a beautiful weld as shown below. Congrats!