vacuum chamber welding manufacturing process

Weld is one of the most easy to leak areas in vacuum manufacturing process. Common weld appearance defects and other defects, such as incomplete fusion, incomplete penetration, slag inclusion, pores, cracks and internal stress, are the hidden dangers of leakage or virtual leakage source in ultra-high vacuum system. In order to minimize weld defects and facilitate inspection, parts welded under ultra-high vacuum shall be placed on the vacuum side as far as possible, while the atmospheric side welds are only used as intermittent strengthening welds. At the same time, the maximum number of welds can be tested separately in the manufacturing stage and corrected before final assembly. Due to the small section size of some parts, the welding of inner wall is limited. Some parts can only use atmospheric side welding seam. The welding seam should be welded once to avoid harmful space caused by twice welding. Then the vacuum chamber must ensure the good effect of single side welding and double side forming.

In addition, it should be pointed out that the stress and strain generated in the joint due to pressure difference, vibration and thermal cycle may cause cracks in the weld with defects, which may damage the weld, or cause corrosion of the weld during use, which may form leakage points.

Therefore, the welding method used must be able to make the leakage rate of weld meet the technical requirements under these conditions.

At present, stainless steel is widely used in ultra-high vacuum vessels. It should be said that the weldability and welding process of stainless steel are good. Why put forward the requirements of ultra-high vacuum vessel for welding seam? The reason is that the leakage rate of ultra-high vacuum vessel is very low, and it also brings severe requirements for weld sealing. Tiny leakage is difficult to detect, repair and eliminate. It has been proved that the pressure leak detection method usually used in non ultra-high vacuum vessels can not meet the requirements of leakage rate of ultra-high vacuum vessels. In other words, when the pressure leak detection fails to detect the leakage point, the helium mass spectrometry leak detection may still find the leakage hole, which can not meet the leakage rate requirements. Therefore, for ultra-high vacuum vessels, it is necessary to use high sensitivity helium mass spectrometry for leak detection.

In order to ensure that the welding seam can meet the requirements of leakage rate of ultra-high vacuum vessel, two main problems should be solved in welding process for stainless steel,

The first is to prevent the generation of hot cracks. The mechanism is that eutectic with low melting point accumulates at the center line of weld and forms under the action of thermal stress. At the same time, the vacuum cavity should prevent the formation of carbides, which will cause brittle hard structure cracks.

The second is to reduce welding defects as much as possible.