Titanium Thick Plate

Our company's titanium thick plate is made of pure sponge titanium, the international top production process, titanium thick plate has excellent corrosion resistance, widely used in power generation condenser tube plate, a variety of chemical equipment, copper electrolysis drum, etc., is a very important material in the aviation field, ship, and other fields.

According to the process requirements and the good performance of titanium itself, titanium pipeline is widely used in many domestic chemical fiber engineering, especially in PTA plants. However, titanium absorbs hydrogen at 250 ° C, oxygen at 400 ° C, and nitrogen at 600 ° C in the solid melting state, and its absorption capacity increases with the increase in temperature. When titanium absorbs a certain amount of oxygen, nitrogen, hydrogen, and other gaseous impurities, its mechanical properties such as yield limit and elongation will decrease sharply. In this paper, the common defects of titanium welding are analyzed and a reasonable welding process is developed to improve the welding quality of titanium pipe and ensure the quality and progress of the project.

Defect analysis of titanium pipe welding

1. Pollution tendency analysis of gas and other impurities

Industrial pure titanium is relatively stable at room temperature, forming dense oxide film with oxygen and having high corrosion resistance. However, its chemical activity increases sharply at high temperatures, and it has a strong ability to absorb oxygen, hydrogen, nitrogen, and other impurities in the air, reducing the plasticity and toughness of the welding joint. It is especially severe in the melting state. Therefore, when welding, the temperature of more than 250 degrees of weld and heat-affected metal (including the back of the molten pool) should be properly protected.

2. Coarse-grain tendency analysis

Due to the high melting point of industrial pure titanium, poor thermal conductivity, low thermal conductivity, small heat capacity, and large resistivity, in the welding process, the liquid molten pool metal has the characteristics of large size, and long residence time at high temperature and slow cooling speed. In this situation is easy to make the welding joint superheated structure, the grain becomes coarse, and the brittle tendency is obvious.

3. Welding crack tendency analysis

Due to the few impurities such as sulfur, phosphorus, and carbon in pure titanium, the eutectic with a low melting point is difficult to appear at the grain boundary, the crystallization temperature zone is narrow and the shrinkage of weld solidification is small, so the thermal crack rarely occurs. When welding pure titanium, it is easy to be polluted by oxygen, hydrogen, nitrogen, and other impurities. When the content of these impurities is high, the welding seam and the heat affect the performance become brittle, and it is easy to produce cold cracks under the action of welding stress. Hydrogen is the main cause of the cold crack. Hydrogen diffuses from the high-temperature molten pool to the heat-affected zone at a lower temperature, and TiH is precipitated from the solid solution when hydrogen is enriched to a certain extent in the zone. Embrittlement; With TiH. Precipitation will produce a large volume change and cause large internal stress. These factors promote the formation of cold cracks and have a delayed nature.

Two, titanium pipe welding

1. According to the analysis of the above defects, argon tungsten arc welding (TIG) is generally selected at the site, and ce tungsten electrode is used. The welding environment must be clean and clean. Temporary shelter is required when welding fixed ports on site. Protection gas selection of purity in > >; More than 99.999% high purity liquid argon. During welding, a special protective device is adopted: a nozzle supporting cover, argon gas is sprayed from the drag cover to protect the welding high-temperature area, the size of the drag cover can be determined according to the shape of the weld, the size of the welding piece and the operation method. The back of the weldment shall also be protected by argon. Due to the difficulty in achieving good protection for complex welds, it is advisable to weld them in a box filled with an argon-helium mixture.

2. Cleaning before welding

Before welding, the grease and oxide on the surface of the welding wire and welding parts should be cleaned up to prevent carbon, oxygen, and hydrogen in the welding seam, so that the welding seam has pores and cracks, and reduce the mechanical properties of the welding metal. The groove must be treated with a reamer and diamond file to remove oxidation film, burr (burr will cause porosity), and cleaned with acetone together with welding wire. After treating the groove of the oxide film, welding is required within 4h. If no welding is performed within 4h, the oxide film must retreat before welding.

3. Key points of titanium pipe welding operation

3.1 During titanium pipe welding, the weld gap contraction is relatively large, and the groove gap should be slightly increased. The groove gap should be 4mm without a blunt edge. The cover wire should be 2mm, the diameter of the cover wire is too thick to grasp. The wire must be pickled before it can be used, and can also be sanded and scrubbed with acetone.

3.2 Before group pair, welders must wear clean gloves, and it is forbidden to beat with iron, because iron pollution will reduce mechanical properties and corrosion resistance, rubber hammer is not suitable, and a wood hammer can be used when necessary. Weld group pair, spot solid welding can not be as point solid stainless steel weld groove bridge on both sides. The requirements for point-solid welding are the same as those for formal welds. Argon gas is required to be filled in the weld before point-solid welding. Large-caliber pipelines can be protected by a backside protective drag cover.

3.3 Arc attenuation should be used to prevent arc pit shrinkage and crack during welding. Argon arc welding is used when welding. If short arc welding is used, the labor is large, so long arc welding should be used.

3.4 Before welding the weld back cover, the air in the tube should be cleaned before welding. Test whether the argon gas filled in the pipe is qualified. Can be used to use the following method, the welding wire head on the weld with argon arc welding burn (pay attention to not burn to the weld groove), quickly into the pipeline, and so on after cooling out, the silver wire head is qualified, can be welded normally, blue is not qualified, must be cut off the end can be used normally.

3.5 When welding seam back cover, due to the protective gas cover, welding gun swing is not convenient and flexible, prone to internal weld sag, edge bite on both sides, failure to fuse, and other defects. Backcover welding is generally not prone to collapse. When paying attention to the joint, the joint must be twisted out of the slope with a reamer to facilitate the joint. To hit the bottom layer, the inner wire filling method is generally used in the welding position of the inverted face, and the outer wire filling method is used in the vertical and horizontal welding parts. The inverted face welding joint is not well mastered and is prone to concave defects. When welding the joint, feed the welding wire after the hole appears. The first two drops can be given more welding wire to avoid the joint concave, and then weld as normal. The gas flow rate of the drag hood should be 30-40L /min.

3.6 No matter where the welding is performed, the tungsten pole should be perpendicular to the pipe axis, so that the size of the molten pool can be better controlled, and the nozzle and drag cover protection gas can uniformly protect the molten pool from oxidation. According to the length of the protective drag cover, the welding length is generally 100mm, it should be stopped, and continue welding after the welding seam recedes red and becomes silver-white. When observing the weld color, do not remove the drag cover, nor should it be observed from the front of the drag cover. It is better to observe from the rear of the drag cover. The temperature between the layers should be less than 50 degrees, and it is advisable to touch the weld with your hand. It is best to weld the second time after the weld cools. When urgent, cold air can be used to blow the weld, or wipe the two sides of the weld with a wet cloth, pay attention to not wipe the weld.

3.7 When welding stops arc closing, arc pits should be filled, welding gun and drag cover gas delay protection, to avoid air pollution of welding seam at high temperature so that the welding seam becomes qualified color. The end of the welding wire can not be removed from the protection area immediately, otherwise, the end of the welding wire will immediately change color and oxidize, and can not continue to use. If used again, to use pliers to cut off the oxidation end can be used.

3.8 Grinding the end into a sloped shape, when welding to the slope, suspend wire feeding, first melt the slope into a hole with arc, quickly add wire, so that the weld is closed, pay attention to the second half of the circle remaining a half, should reduce the flow of internal protection gas, to prevent too much pressure to make the weld concave.

3.9 Poor gas protection, prone to porosity, brittle hard tissue, brittle fracture. Weld and heat-affected zone silver is the best, yellow is poor, and blue is not qualified.

The above method has been used to weld titanium pipes in several PTA installations in China, and good results have been achieved. The appearance of IZl welding is mostly silver white, with a small amount of light yellow, and the pass rate of one take is above 97%.

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