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 plant. However, titanium in the solid state of 250 degrees began to absorb hydrogen, from 400 degrees began to absorb oxygen, from 600 degrees began to absorb nitrogen, and with the increase in temperature, its absorption capacity is also enhanced. When the titanium metal in the absorption of a certain amount of oxygen, nitrogen, hydrogen and other gaseous impurities, its yield limit and elongation and other mechanical properties will drop dramatically. Therefore, the welding of titanium has high requirements, this paper analyzes the common defects in titanium welding, and formulates a reasonable welding process, thus improving the welding quality of titanium pipe and ensuring the quality and progress of the project.
1, titanium pipe welding defect analysis
1.1 Analysis of impurities such as gas and other contamination tendency
Industrial pure titanium at room temperature is relatively stable, and oxygen generated dense oxide film has high corrosion resistance. But at high temperatures its chemical activity increases dramatically, with a strong ability to absorb oxygen, hydrogen, nitrogen and other impurities in the air, reducing the plasticity and toughness of welded joints. In the melting state is particularly serious. Therefore, the welding temperature exceeds 250 degrees of the weld and heat-affected metals (including the back of the melt pool) should be properly protected.
1.2 Analysis of coarse crystal tendency
Due to the high melting point of industrial pure titanium, poor thermal conductivity, low thermal conductivity, small heat capacity, high resistance coefficient, in the welding process, the liquid molten pool metal has a large size, high temperature stay a long time and slow cooling speed and other characteristics. This situation is easy to make the welded joints produce superheated organization, the grain becomes coarse, embrittlement tendency is obvious.
1.3 Welding crack tendency analysis
Due to the pure titanium in sulfur, phosphorus, carbon and other impurities are very few, low melting point eutectic difficult to appear at the grain boundary, and the crystallization temperature zone is narrow and the weld solidification shrinkage is small, so it will rarely produce thermal cracks. Welding of pure titanium is very susceptible to oxygen, hydrogen, nitrogen and other impurities contamination, when the content of these impurities is high, the weld and heat-affected properties become brittle, easy to produce cold cracks under the action of welding stress. Among them, hydrogen is the main cause of cold cracking. Hydrogen from the high-temperature molten pool to the lower temperature of the heat-affected zone diffusion, when the region of hydrogen enriched to a certain extent from the solid solution will precipitate TiH. embrittlement; with the precipitation of TiH. will produce a large volume change caused by a large internal stress. These factors, contribute to the generation of cold cracks, and has a delayed nature.
2, titanium pipe welding
2.1 According to the analysis of the above defects, the general choice of tungsten argon arc welding in the field, the use of electrodes for cerium tungsten electrode, the welding environment must be clean and clean. Site fixed mouth welding, the requirement to set up a temporary wind shed. Protective gas selection purity in > 99.999% above high purity liquid argon. In the welding of a special protective device a nozzle plus bracket, argon gas from the drag cover, to protect the welding of high-temperature areas, the size of the drag cover can be based on the shape of the weld, the size of the weldment and the method of operation to determine. The back of the weldment also need to use argon protection device. Complex structure of the weldments due to the difficulty of realizing good protection, it is appropriate to fill a helium-argon gas mixture in the box welding.
2.2 Cleaning before welding
Welding should be cleaned up before welding wire and weldment surface oil and oxides, in order to prevent the weld from increasing carbon, oxygen, hydrogen, so that the weld produces pores and cracks, and reduce the mechanical properties of the weld metal. The bevel must be processed with a reamer and diamond file to remove the oxide film, burr (because the burr will lead to porosity), with acetone together with the welding wire a scrub clean. After processing the oxide film of the bevel required within 4h welding, if not within 4h welding, and then before welding must be re-treatment of the oxide film can be welded.
2.3 Titanium pipeline welding operation points
2.3.1 Titanium pipe welding weld gap shrinkage is relatively large, the bevel gap should be slightly increased, the bevel gap should be 4mm, do not leave a blunt edge. Sealing wire should be 2mm, sealing wire diameter is too thick and not easy to grasp. Welding wire must be acid washed before use, can also be sandpaper grinding and then acetone scrubbing after application.
2.3.2 grouping before, the welder must wear clean gloves, it is strictly prohibited to hammer with iron, iron pollution will reduce the mechanical properties and corrosion resistance, and should not be used with rubber hammers, if necessary, can be used wooden hammer. Weld pairing, point-fixed welding can not be point-fixed stainless steel weld like bevel bridging on both sides. Point solid point requirements and the same formal weld, the weld required to be filled with argon gas in order to point solid welding, large-diameter pipeline can be used to protect the back of the protective mop protection, point solid welding, point solid point with a reamer on both sides of the point solid point to hit the slope (or point solid point as small as possible), in order to facilitate the joint, and then with high-temperature fusible paper pasted up.
2.3.3 Welding in order to prevent arc crater shrinkage, cracks, welding arc is appropriate to use arc attenuation. Welding all the use of argon arc welding, if the short arc welding, labor is larger, so it is appropriate to use long arc welding.
2.3.4 Welding seam bottoming before welding, the pipe should be replaced with clean air before welding. Test pipe filled with argon gas is qualified. Can be used under the approach, the wire head on the weld with argon arc welding burn (pay attention not to burn to the weld bevel), quickly put inside the pipe, and so cooled out, the wire head silver-white qualified, you can weld normally, turn blue for unqualified, have to cut off the end can be used normally.
2.3.5 Welding seam bottom welding, due to the protective gas bracket, the welding torch swing is not convenient, inflexible, easy to appear inside the weld depression, both sides of the bite, not fused and other defects. Bottom sealing welding is generally not prone to collapse. Note that the joints must be strung out with a reamer at the joints with a slope for the joints. The bottoming layer is generally applied by filling the wire inside the elevated face welding position, and filling the wire outside the vertical and flat welding parts. Elevation face welding head is not mastered well, it is easy to appear in the concave defects. Welding joints to appear after the melt hole and then give the wire feed, the first two drops can give more wire to avoid concave joints, after the normal welding. Mopping gas flow rate should be 30~40L/min.
2.3.6 No matter what position of the welding, the tungsten electrode should be perpendicular to the axis of the pipe, so that better control of the size of the molten pool, and can make the nozzle, towing mask protective gas uniformly protect the molten pool from oxidation. According to the length of the protective mop, the welding length of 100mm should generally stop, and so the weld recedes red to silver-white, and then continue to weld. Observe the color of the weld do not take away from the towing cover, and should not be observed from the front of the towing cover, observation from the back of the towing cover is preferred. The temperature between layers should be lower than 50 degrees Celsius, it is appropriate to touch the weld seam with your hand without burning, and it is better to wait for the weld to cool down before welding for the second time. In case of emergency, cold air can be used to blow the weld, or wipe the sides of the weld with a wet cloth, pay attention to not wipe the weld.
2.3.7 Welding stop closing the arc, to fill the arc pit, welding torch and drag shield gas delay protection, to avoid the weld in the high temperature by the atmosphere pollution, so that the weld becomes qualified color. The end of the welding wire can not be immediately withdrawn from the protected area, otherwise, the end of the welding wire immediately discoloration oxidation, can not continue to use. If re-use, use pliers to cut off the oxidized end before use.
2.3.8 Finish polished into a slope, weld to the slope, pause to give wire, first use the arc to melt the slope into a molten hole, quickly add wire, so that the weld is closed, pay attention to the weld to the second half of the circle left a small half, you should reduce the flow of the internal protective gas, in order to prevent the gas pressure is too large and make the weld concave.
2.3.9 Gas protection is not good, prone to porosity, and brittle hard tissue, brittle fracture. Weld and heat-affected zone silver-white color is the best, yellow is poor, blue is unqualified.
3, using the above methods in a number of domestic PTA plant for titanium pipe welding, achieved good results, weld IZl appearance is mostly silver-white, a small amount of yellowish, a film pass rate of 97% or more.
