Ultra-high vacuum vessel weld requirements
In the case of the vacuum unit configured to determine, the device can achieve the ultimate pressure depends on the flow inside the system from outside the system of total leakage rate Qo and the formation of a virtual total leakage “leakage” rate ΣQ1 sum of the size and pumping system The effective pumping speed.
Leak in vacuum technology means that when talking about the closure of the inner container and the external gas pressure or the concentration is not the same, you can make the gas leakage from the side wall to the other side to the holes, defects, or apertures and penetration element or leakage devices. Due to the small size of the leak, complex shape, can not be represented by the geometry of its size, it is generally by the equivalent conductance or leak rate (referred to as “leakage rate”) to represent its size, the unit is Pa · L / s.
Weld is one of the regions most vacuum manufacturing process prone to leak, the common weld defects and other defects in the appearance, if not fusion, incomplete penetration, slag, pores, cracks and internal stress are ultra-high vacuum furnace system The drain-source leak or imaginary risks. To minimize weld defects, to facilitate the detection, ultra-high vacuum requirements as far as possible so that the weld parts welded in a vacuum-side, the atmosphere side only as intermittent weld to strengthen the weld, while the maximum amount of energy in the manufacture of the weld stage were tested before final assembly to give correction. Due to the small cross-sectional dimensions of some parts, the inner wall welding is limited, some parts can only use the atmospheric side weld, the weld should be a weld, in order to avoid harmful space twice welding, it is essential that the double-sided welding surface forming effect of the fine.
Should also be noted that the stress and strain due to the pressure, vibration and thermal cycling and other reasons resulting in the joint may cause defects in the weld contain cracks so that the weld damaged, or in the course of the weld corrosion This may form the leak.
Thus welding method used must allow the leak rate of the weld meet the technical requirements under these conditions.
At present, the commonly used ultra-high vacuum vessel of austenitic stainless steel. It should be said, austenitic stainless steel weldability and welding technology is good, why should propose ultrahigh vacuum vessel weld demands? The reason is that the requirements of ultra-high vacuum vessel leak rate is very low, it also brings to demanding sealing seam, tiny leak elusive, hard fill, difficult to eliminate. Practice has proved that the non-ultrahigh vacuum vessel pressure leak detection method usually does not meet the requirements of ultra-high vacuum vessel leak rate, that is, in the case of a pressure leak undetectable leak, using helium leak may still be found in drain hole, reach the leak rate requirements. So in terms of the ultra-high vacuum vessel, shall be carried out with high sensitivity helium leak.
To ensure the welds meet the requirements of ultra-high vacuum vessel leak rate for the austenitic stainless steel, the main two issues should be resolved on the welding process,
One is to prevent hot cracking, which produces a crystallization mechanism is in the weld, the low melting point eutectic composition accumulate in the weld centerline, formed under the action of heat stress. And it should prevent the formation of carbides, resulting in brittle crack hard tissue.
The second is to minimize the generation of welding defects.