1. Welding protection measures
Due to the poor thermal conductivity of titanium material, the weld pool diameter is larger, so in order to increase the weld pool protection area, welding torch nozzle should be appropriately increased, generally more than 16mm nozzle should be appropriate. Welding protection is the biggest problem of titanium tube welding. In the welding process, in order to protect the high temperature zone above 400 degrees, usually add a drag cover on the welding torch to achieve, some data also provides a lot of methods.
For the protection of pipeline welding cover design is the technical specification of pipe making complete way, obviously, this is not much in the specification of pipe can be used, but when the specifications, high cost, and complicated installation is inconvenient in the construction of intermediate frequency. After several experiments, the author has designed a low cost and easy to operate local protective cover, which effectively solves the difficult problem of field welding protection of titanium pipe. The size of the cover is determined according to the temperature range of the high temperature zone (above 400 degrees Celsius) when the titanium is welded (usually 80 mm * 60 mm). The cover is made of copper skin, with an oval argon filter ball wrapped with copper mesh (wrapped with copper mesh, stainless steel or copper mesh). The purpose is to ensure that argon fills the hood cavity and forms a steady laminar flow (which is very important, otherwise it will greatly affect the protection effect). The upper part of the elliptical sphere is connected with the copper tube and welded with high pure argon. The protection on the back adopts the integral and local protection methods according to the diameter of the pipe. The DN is greater than or equal to 450 mm titanium tube, using local protection method, and make the local protective cover (front cover and similar to DN); less than 450 mm titanium tube take whole argon protection, the same method with the conventional methods, but should pay attention to in the back sealing two layers of sealing, to ensure the protection of the effect of.
2. Prevention of welding porosity
Because the welding porosity is mainly caused by the contamination of the groove surface, so as to prevent the welding holes, in addition to the use of the clean gas with low Ar content protection, the key is to press the impurity cleaning procedure strictly on the groove surface of the groove, after medical paste white tape sealing protection, avoid the groove again polluted.
3. Control of welding line energy
The relevant foreign standards are required to ensure the fusion of the premise, as far as possible to use small line energy welding, information is generally in the 7 ~ 35kJ / cm. Considering that the heat dissipation rate is slow, it needs argon to cool down rapidly after welding, so the welding line energy should be selected at lower value. It is advisable to adopt small current and slow welding in construction.
4. Temperature control between floors
The higher the interlayer temperature is, the higher the residence time of the weld and HAZ zone is, the slower the cooling rate is, thus increasing the grain size at room temperature. The interlayer temperature should be controlled below 200 centigrade.
5. Argon flow rate and ventilation time
The argon flow rate of the nozzle shall be 12~15 L/ min according to the conventional conditions. The flow rate of the protective hood and the protective gas on the back shall be no less than 25 L/ min. Before welding shall be in the back by argon gas (general in 15 ~ 60 s), the volume should not be less than 6 times more than the volume of reserve back cavity, so that the air full replacement of AR and back, at the same time to protect the surface after arcing at the arc welding temperature, should be in 300 C (time in 15 ~ 60 s, according to the diameter of ascending gradually extended) to stop after the air protection.
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