What are the welding problems of vacuum system -
when determining the configuration of the vacuum system, the equipment can reach the limit of pressure depends on the total system external to the internal leakage rate Qo and formed by virtual leakage rate of total 'leak' ∑ Q1 and effective pumping speed of the pumping system combined.
the leakage hole in vacuum technology refers to the hole, defect or gap, and permeable elements or gas leak device, when the concentration of gas pressure inside and outside airtight container or not at the same time, these components or gas leakage devices allow gas from the container wall on one side of the leak to the other side. Due to the leakage hole size, complicated shape, cannot use geometry size, so its size generally use equivalent conductance or leakage rate ( Known as 'leakage rate') To represent, the unit for pa l/s.
weld is a vacuum in the manufacturing process of one of the most prone to leak hole area.
common appearance of welding defects and other defects, such as incomplete fusion, incomplete fusion, slag inclusion, porosity, cracks and internal stress, etc. , are in ultra high vacuum system leakage hole or virtual source leakage hidden trouble. In order to minimize weld defects and easy to check, ultra high vacuum requirements side of weld seam welding parts as far as possible be placed in a vacuum, atmospheric side only used for intermittent reinforcement of weld seam, and the largest number of weld can separate testing in the manufacturing stage, and corrected before final assembly. Due to some parts of the small size of cross section, the lining of the welding is restricted, and some parts can only use the atmospheric side seam. Should be a welding seam, to avoid harmful space in the two welding. Therefore, it is necessary to ensure one-side welding and double forming good effect.
in addition, it should be noted that due to the differential pressure, vibration, thermal cycling and other reasons in the joint of stress and strain may lead to crack of include defects in the weld, to cause damage to weld, or weld in use process may be corrosion, which may form a leak.
as a result, the welding method used must be able to make the weld under the condition of the leakage rate to meet the technical requirements.
at present, the stainless steel is widely used in ultra high vacuum container. It should be said that the weldability of stainless steel and solderability is very good. Why do you want to ask for ultra high vacuum container to weld? Reason is ultra high vacuum container requires very low leakage rate, it also puts forward strict requirements on welding seam sealing performance. Tiny leakage for inspection, repair and eliminate. Practice has proved that the ultra high vacuum container pressure leak detection methods can not meet the requirement of the ultra high vacuum vessel leakage rate, namely under the condition of pressure detection can't find funnelled, helium mass spectrometer leak detection may still find leak hole, can not meet the requirement of the leakage rate. Therefore, for ultra high vacuum container, must use the high sensitivity helium mass spectrometer leak detection.
in order to ensure that meet the requirements of ultra high vacuum container leakage rate, welding seam welding of stainless steel in the process of two major problems should be paid attention to:
one is to prevent to produce hot crack. Its mechanism is a crystal in the weld process, low melting eutectic in the weld centerline gathered, and formed under the action of thermal stress. At the same time, should prevent the formation of carbides causes the crack in the brittle and hard tissue.
second, reducing the occurrence of the welding defects as soon as possible.
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