When welding titanium pressure vessels, because the argon gas carried by the welding nozzle can only protect the welding pool, while the solidified weld and its heat affected zone have no protective gas effect, whose part of the metal is still at a high temperature. When the titanium metal is higher than 400 ℃, it begins to absorb oxygen and when it is higher than 600 ℃, it begins to absorb nitrogen. With the increase of oxidation degree, the color of the weld will change, and its mechanical properties also will deteriorate gradually. If the weld is silver white, it indicates no oxidation, golden yellow indicates slight oxidation, blue indicates slightly serious oxidation, and gray indicates serious oxidation.
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The color inspection shall be carried out on the surface of all welds and heat-affected zone: when the surface is silver white and golden yellow, it can not be treated; Blue welded joints can be used for non-important parts after blue removal treatment; Welded joints of other colors shall be repaired, and attention shall be paid to the repair times. Therefore, titanium pressure vessels must pay attention to the linear energy in the welding process, control the welding speed and strictly follow the welding process specification.
According to the design, the weld and heat-affected zone shall be 100% inspected with a 10x magnifying glass without undercut. 100% radiographic non-destructive testing is required for class A and B welds, and grade II is qualified. In case of over standard defects, the defects shall be removed before repair welding, and re-tested with rays until they are qualified. The surface of all welds (butt joint and fillet joint) shall be subject to penetrant testing before and after the hydrostatic test. If any impermissible defect is found, it can be polished. If necessary, it shall be repaired and retested by penetrant method until it is qualified. The non-destructive testing requirements of titanium pressure vessels for welds are significantly higher than the construction standard, which should be paid attention to in the manufacturing process.
