First, gas pollution
1. Prepare for welding. Strictly clean the weld surface to eliminate the intrusion of hydrogen, oxygen and nitrogen.
2. Precise argon flowmeter is selected to control the gas flow. The gas flow rate is selected to achieve a good protection effect. Argon gas flow rate has a considerable influence on the protection. Too large flow rate is not easy to form a stable airflow layer, but turbulent flow in the protection area, which makes harmful gas enter the molten pool and makes the weld surface prone to micro-cracks. Too small air flow makes the protection not in place and cannot reach the protection effect. When the argon gas flow in the trailing cover is insufficient, the weld will show different oxidation color.
3. Strengthen weld protection. When welding, do not move the end of the wire out of the argon gas protection zone; At the end of arc breaking and welding seams, argon gas should continue to protect the welding torch until the weld and heat-affected zone metal cooling below 100℃.
Two, welding joint crack problem
During the welding of Titanium Tubing, there is little possibility of hot cracks in the welded joints. This is because the impurities such as S, P, and C in titanium and titanium alloys are low in content, and the low melting point eutectic formed by S and P is not easy to occur at the grain boundaries. In addition, the effective crystallization temperature range is narrow, and the shrinkage of titanium and titanium alloys during solidification is small, so the weld metal will not generate hot cracks.
During titanium welding, cold cracks can appear in the heat affected zone, which is characterized by the crack generation hours or even longer after welding. The research shows that the crack is mainly caused by the influence of carbon and hydrogen and the rapid cooling rate. The main way to prevent the generation of such delayed cracks is to reduce the sources of hydrogen and carbon in the welded joint, and to protect and clean the weld area before welding to prevent the contamination of harmful impurities.
Secondly, the temperature between layers should be strictly controlled. On the premise of ensuring good fusion, welding with low heat input is adopted as far as possible, that is, to reduce the fusion ratio.
Adopt small diameter wire, low welding current, narrow pass technology, fast welding. The cooling speed is controlled at about 100℃/s.
Three, the porosity in the weld
Porosity is a relatively easy defect in titanium plate welding, mainly due to the effect of hydrogen. The surface of the plate and welding material is not clean, the water and grease on the operator's gloves, the sand and flying dust under the Angle grinder are all sources of hydrogen. The porosity of weld metal mainly affects the fatigue strength of joint.
The main technological measures to prevent the formation of pores are:
1. Neon gas shall be pure, the purity shall not be less than 99.99%. Reinforced plastic tube shall be used for the pilot pipe, not rubber tube.
2, thoroughly remove the surface of the weldment, the surface of the welding wire oxidation skin oil and other organic matter.
3. Apply good gas protection to the molten pool and control the flow and velocity of argon gas to prevent turbulence and affect the protection effect.
4, the correct selection of welding process parameters, increase the welding pool weld metal residence time, so that bubbles overflow, can effectively reduce the porosity.
5, when welding with small heat input, the best use of pulsed argon arc welding, can improve the joint plasticity, reduce overheating and coarse crystal, reduce deformation, but also can increase the depth of hot melt, reduce the generation of pores.
Four, before welding preparation
In the titanium welding site is best divided into a separate area, non-professional personnel shall not enter, in order to protect the cleanliness of the area, all welding personnel should wear clean work clothes, wear knitted gloves or thin leather gloves, can not wear cotton gloves, into the tank when welding, shoes to put on the shoe cover.
The internal and external surface and the welding wire surface within 50mm on both sides of the groove should be cleared of oil, and the oxide film, burr and surface defects should be removed by mechanical methods such as fine file austenitic stainless steel wire brush and milling cutter. Cleaning tools should be dedicated and kept clean; After mechanical cleaning, the surface should be degreased with acetone or ethanol without sulfur before welding. It is forbidden to use trichloroethylene, carbon tetrachloride and other chloride solvents. Cotton fibers should not be attached to the groove surface. According to the technical requirements, the welding environment was tested by "Phillorine" before welding, and the pollution test of "iron ion" was carried out around the weld. The welding was carried out only after the test was qualified.
The cleaning quality will directly affect the generation of welding cracks. When the cleaning quality is not good, a layer of grayish-white suction layer will be generated on the surface of the base metal and welding wire, leading to the generation of welding cracks and pores. Therefore, the following points should be paid attention to when cleaning:
1. Use pickling first. The acid washing solution is 2%~4%HF+30%~40%HN03+H20(residual). Finally, rinse with clean water and blow dry. Scrub with acetone or alcohol before welding.
2. The welding wire generally needs to be dehydrogenated after pickling, and then degreased with acetone before welding.
3. It is strictly prohibited to use oxide degreasing.
4. Do not wear rubber gloves when cleaning, so as not to react with the rubber and degreasing solvent to make the weld produce pores.
5. Welding joint after cleaning
It must be welded immediately, not for a long time (generally not more than 4h), otherwise it needs to be cleaned again according to the previous steps.
The protective cover used in titanium welding is very important. A skilled welder, without a good protective cover, even if the weld is beautiful, after welding all oxidation discoloration, the weld is still unqualified, and to scrap.
Five, titanium manual tungsten argon arc welding operation essentials
1. During manual argon arc welding, the Angle between the welding wire and the weldment should be kept at the minimum (10~15°). The welding wire is smoothly and evenly fed into the molten pool along the front end of the molten pool, and the end of the welding wire shall not be moved out of the argon gas protection zone.
2. When welding, the welding torch does not swing laterally. When it needs to swing, the frequency should be low and the swing amplitude should not be too large to prevent affecting the protection of argon gas.
3. When arc breaking and weld ending, argon gas should continue to protect the welding torch until the metal of weld and heat affected zone cools down to below 100℃.
4, the quality of cleaning will directly affect the generation of welding cracks, cleaning quality is not good, will generate a layer of gray suction layer on the surface of the base metal and welding wire, leading to the generation of welding cracks and pores.
Vi. Basis for selection of process parameters
1, welding current such as the use of large current, welding is very smooth, but the weld grain is coarse, poor performance, easy to produce large residual stress around the weld. Therefore, the upper limit of welding current should be controlled. When determining the current of the fillet weld of the cover plate, the first pass of self-melting and the second pass of wire adding, the welding current is 130-140A.
2, arc voltage selection arc voltage is actually to choose the length of the arc, commonly used argon arc welding machine has no voltage selection button, should try to make the arc shorter, in order to control the arc voltage between 14-16V.
3. Welding speed Titanium has poor thermal conductivity in welding, and the current and speed should be matched well, otherwise the fusion is not good. The test shows that when the welding current is 130-140A, the welding speed is 160-170mm/min.
4. Determination of argon flow rate
When welding titanium, within 100mm of the weld zone behind the weld pool (including both sides of the weld and the heat affected zone) should be cooled in an argon atmosphere, otherwise the weld will be oxidized and discolored, which will reduce the performance. After field test adjustment, the argon gas flow rate is finally determined as follows: nozzle 10-11L/min, supporting cover 12-13L/min, back 3-5L/min.
5. Dc argon arc welding power supply with descending external characteristics and high frequency arc induction should be selected for titanium plate argon arc welding. The extreme part of tungsten should be ground into a cone shape and horizontal rotation flat welding should be adopted. The welding torch should be pumped in advance, and the current attenuating device and gas delay protection device should be used when extinguishing the arc. The delay time of gas transmission should be no less than 15 seconds to avoid oxidation and pollution of the weld.
Vii. Assembly
In order to reduce welding deformation, positioning welding should be carried out before welding. The positioning weld should adopt the same welding materials and welding process as the formal welding, that is, the welding wire, welding process parameters and gas protection conditions should be the same as those used in welding joints. Before welding, remove the oxide layer, oil, moisture and corrosion on the surface of the welding wire, welding groove and its sides within the range of 20mm, and welding should be performed by qualified welders. The positioning welding should be welded on the base weld. The length of the weld should be 10-15mm, the spacing should be 100-150mm, and the height should not exceed 1/3 of the wall thickness. Clearance 0-1mm, blunt edge 0-1.0mm. Weld shall not have cracks, pores, slag inclusion, oxidation discoloration and other defects, when found defects should be eliminated in time.