According to the process requirements and the good performance of titanium itself, titanium pipes are widely used in many domestic chemical fiber projects, especially in PTA plants. However, titanium starts to absorb hydrogen from 250 degrees, oxygen from 400 degrees, and nitrogen from 600 degrees in the solid state. With the increase of temperature, its absorption capacity also increases. When titanium metal absorbs a certain amount of gas impurities such as oxygen, nitrogen and hydrogen, its mechanical properties such as yield limit and elongation will drop sharply. Therefore, the welding requirements of titanium are very high. This paper analyzes the common defects in titanium welding and formulates a reasonable welding process, so as to improve the welding quality of titanium pipes and ensure the project quality and progress.
Defect analysis of titanium pipe welding
(1) analysis on pollution tendency of gas and other impurities
Industrial pure titanium is relatively stable at room temperature, and forms a dense oxide film with oxygen, which has high corrosion resistance. However, at high temperature, its chemical activity increases rapidly, and it has a strong ability to absorb oxygen, hydrogen, nitrogen and other impurities in the air, reducing the plasticity and toughness of welded joints. Especially in the molten state. Therefore, the weld and heat affected metal (including the back of the weld pool) with a temperature exceeding 250 ° C shall be properly protected during welding.
(2) coarse crystal tendency analysis
Due to the high melting point, poor thermal conductivity, low thermal conductivity, small heat capacity and large resistance coefficient of industrial pure titanium, the liquid molten pool metal has the characteristics of large size, long high temperature residence time and slow cooling speed during welding. In this case, the welded joint is easy to produce overheated structure, the grain becomes coarse, and the embrittlement tendency is obvious.
Welding of titanium pipe
1 according to the analysis of the above defects, tungsten argon arc welding is generally selected on site, and cerium tungsten electrode is used. The welding environment must be clean. Temporary wind shelter is required to be erected during welding of fixed port on site. High purity liquid argon with purity above 99.999% is selected as the shielding gas. During welding, a special protective device, a nozzle with a supporting cover, is used. Argon is ejected from the trailing cover to protect the high-temperature welding area. The size of the trailing cover can be determined according to the shape of the weld, the size of the weldment and the operation method. Argon filled protective devices shall also be used on the back of weldments. The weldment with complex structure should be welded in the box filled with argon helium mixture because it is difficult to achieve good protection.
(2) cleaning before welding
Before welding, the oil stain and oxide on the surface of welding wire and weldment shall be cleaned to prevent carburization, oxygen and hydrogen in the weld, resulting in pores and cracks in the weld and reducing the mechanical properties of weld metal. The groove must be treated with reamer and diamond file to remove oxide film and burr (burr will cause air hole), and cleaned with acetone together with welding wire. The groove with oxide film shall be welded within 4H. If no welding is carried out within 4h, the oxide film must be re processed before welding.
(3) welding operation points of titanium pipe
1 when titanium pipe is welded, the shrinkage of weld gap is relatively large, and the groove gap should be slightly increased. The groove gap should be 4mm without leaving blunt edges. The back-end welding wire should be 2mm, and the diameter of the back-end welding wire is too thick to grasp. The welding wire can only be used after pickling, or it can be polished with sandpaper and scrubbed with acetone before application.
2. Before assembly, welders must wear clean gloves. It is forbidden to knock with iron. Iron pollution will reduce mechanical properties and corrosion resistance. Rubber hammers are not recommended. Wooden hammers can be used if necessary. When the weld is assembled, the spot welding shall not be conducted on both sides of the groove in the same way as the spot welding of stainless steel. The requirements for spot welding are the same as those for formal welds. Argon gas is required to be filled in the weld before spot welding. Large diameter pipes can be protected with back protective towing covers. After spot welding, use a cutter to make slopes on both sides of the spot welding point (or the spot welding point should be as small as possible) to facilitate the joint, and then paste it with high-temperature fusible paper.
3 in order to prevent crater shrinkage and crack during welding, the welding arc should be attenuated by arc. Argon arc welding shall be used for welding. If short arc welding is used, it will require a lot of labor, so long arc welding shall be used.
Amanda
Company: Baoji Dynamic Trading Co., Ltd
Country:China
Add:Baoti road,Jintai,Baoji city,Shaanxi,China
Cel/Whatsapp :+86 18309262795
Website:www.jm-titanium.com
