The most commonly used metal for pinch-off is OFHC Copper (ASTM B68-83, B75-84, B133-83 and B170-82). This has the specifications, chemistry and state of ductility for billet-certified 99.99% pinch off grade copper. It is important that the material be bright annealed at 650? C to 850? C for 30 minutes in a dry hydrogen atmosphere. This is required because the material will undergo a deformation of approximately 350% during pinch-off. High-Purity nickel ( A Nickel, NI270, NI200 or 99.4% nickel ASTM-B161) is another commonly used material. High-purity nickel offers several advantages: minimal out gassing during bake out and pinch-off, minimal oxidation and higher temperature bake ability.
Nickel tubulation must be fully annealed at 1150? C for 30 minutes before pinch-off. Good results can also be obtained using aluminum (annealed 3003 H14, 98% classified non-heat treatable), pure iron, gold, platinum, silver and columbium.
The cold welded area will be work hardened during the pinch-off. The size and length of the cold welded area will depend on material, type of annealing, cleanliness, wall thickness and radius of the pinch-off anvil inserts.
|PINCH -OFF PREPARATION
The tubular material being used must be totally free of contamination at the weld point. Mechanical or sonic cleaning rather than chemical cleaning just prior to pinch-off yields the best cold welds. The O.D. of the tubing should be polished with 320-grit emery cloth to remove oxide crystals.
The tungsten carbide inserts of the tools jaws must be cleaned before each pinch-off. Any contamination pressed into the metal at the weld point can injure the cold-welded seal. The pinch-off is a cold extrusion and a lubricant can be used to aid the material flow. Clean #10 machine oil works well on most metals.
During the pinch-off, pressure must be applied evenly until the tubing severs suddenly. Any interruption of this process while the material is in a plastic state will result in an incomplete cold weld. Leaks are apt to occur if the pinch-off phase is incomplete and the tubing needs to be wiggled apart. The HY-187 Hydraulic outfit provides reliable service and assures the best results.
There are a few methods for checking the finished crimp for leakage.
Vacuum insulation time rates and electric resistance measurements can be avoided if a sample tube is carried through the entire process, then subjected to a helium test or microscopic examination, using the sample for comparative analysis. Process procedures should be duplicated precisely, changes as subtle as bending a piece of copper tubing will change its grain structure and work-harden the piece considerably. There could be significant changes in grain size, crystal structure and ductility during any thermal process such as brazing, bake-out, soldiering or welding.
The cold-welded stub should be protected with a plastic cap after pinchoff, as it is a delicate seal and very sharp.