1. Gas pollution during welding of titanium plates
(1) Prepare for welding. Before welding the titanium plate, it is necessary to strictly clean the surface of the weld to prevent the entry of hydrogen, oxygen, and nitrogen.
(2) Before welding, an accurate argon gas flow meter needs to be selected to control the gas flow. How to choose the gas flow rate? Mainly need to achieve a good protection effect. The flow rate of argon gas has a considerable impact on the protection. Excessive flow rate is not easy to form a stable gas flow layer. Instead, turbulent flow is formed in the protection zone, so that harmful gases are immersed in the molten pool, so that welding Micro-cracks are prone to appear on the seam surface. Too little air flow makes the protection in place and fails to achieve the protection effect. When the flow of argon gas in the drag hood is insufficient, the welding seam shows a different oxidation color.
(3) Welding protection should be strengthened during the welding process. During welding, the end of the welding wire must not be moved out of the argon gas protection zone; when the arc is broken and the welding seam ends, the argon gas protection should be continued until the welding seam and the metal in the heat affected zone have cooled to below 100℃ before removing the welding gun. Refers to the protection of argon gas during the entire welding process.
2. Welding joint cracks when welding titanium plates
When titanium is welded, the possibility of thermal cracks in the welded joint is very small. This is because the content of impurities such as S, P, C in titanium and titanium alloys is very small, and the shrinkage of titanium and titanium alloys during solidification is small, so the weld metal will not Thermal cracking occurs.
During the welding of titanium plates, cold cracks may appear in the heat-affected areas. These cracks are characterized by a few hours or more after welding. How did this crack occur? Mainly caused by the influence of carbon and hydrogen and the excessively fast cooling rate. The way to prevent this delayed cracking is mainly to reduce the source of hydrogen and carbon in the welded joint. The main method is to avoid gas pollution before welding, refer to the information in 1;
On the other hand, we need to strictly control the interlayer temperature during welding. Under the premise of ensuring good fusion, the welding should be performed with low heat input as much as possible, that is, the fusion ratio should be reduced.
Using small diameter welding wire, low welding current, narrow bead technology, fast welding. It is best to control the cooling rate at around 100°C/s.
3. Porosity in the welding seam when welding titanium plates
Porosity is a defect that is easy to produce when welding titanium plates. The main reason is the result of the influence of hydrogen. The surface of the plate and welding material is not clean. The moisture and grease on the operator’s gloves, the sand particles and the flying dust grinded by the angle grinder are all sources of hydrogen. The main method is to avoid many gas pollution problems before welding according to the information in 1.
Technical measures to prevent the formation of pores are:
(1) The neon gas should be pure and the purity should not be less than 99.99%. The air guide tube should be reinforced plastic tube, and rubber tube should not be used.
(2) Thoroughly remove the scale and other organic substances on the surface of the weldment and the surface of the welding wire.
(3) Apply good gas protection to the molten pool, control the flow rate and flow rate of argon gas, prevent turbulence, and affect the protection effect.
(4) Correctly select welding process parameters, increase the residence time of the weld metal in the weld pool, make the air bubbles escape, and effectively reduce the porosity.
(5) Welding with a small heat input, preferably pulsed argon arc welding, can improve the joint plasticity, reduce overheating and coarse grains, reduce deformation, but also increase the penetration depth and reduce the occurrence of pores.
4. Preparation before welding titanium plate
It is best to separate an area at the welding site of titanium. Non-professionals are not allowed to enter to protect the cleanliness of the area. All welding personnel should wear clean work clothes, knitted gloves or thin sheepskin gloves, and not cotton gloves. Welding in the tank, the shoes should be covered with shoe covers. Ensure that no possible sources of pollution can be brought in from the outside.
The groove of the titanium plate and the inner and outer surfaces within 50mm on both sides and the surface of the welding wire should be cleaned of oil, and the oxide film, burrs and surface defects should be removed by mechanical methods such as fine file austenitic stainless steel wire brush and milling cutter. The cleaning tool should be dedicated , And keep clean; the surface after mechanical cleaning should be degreased with sulfur-free acetone or ethanol before welding. The use of chloride solvents such as trichloroethylene and carbon tetrachloride is strictly prohibited, and cotton fibers should not be attached to the slope Mouth surface. According to the technical requirements, a “phenanthrene” test was carried out on the welding environment before welding, and an “iron ion” pollution test was carried out around the weld, and the welding was carried out after passing the test.
The difference in cleaning quality will directly affect the generation of welding cracks. When the cleaning quality is not good, a gray-white getter layer will be formed on the surface of the base metal and the welding wire, resulting in the generation of welding cracks and pores. Therefore, pay attention to the following when cleaning A few points:
① First pickle. The pickling 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.
②The welding wire generally needs to be dehydrogenated after pickling, and then degrease with acetone before welding.
③ Oxide degreasing is strictly prohibited.
④Do not wear rubber gloves when cleaning, so as to avoid the rubber and degreasing solvent react to make the welding seam produce air holes.
⑤ welded joint after cleaning
must be welded immediately, and should not be placed for a long time (generally not more than 4h), otherwise it needs to be re-cleaned according to the previous steps.
The protective cover used for welding titanium is very important. A skilled welder does not have a good protective cover. Even if the weld is beautifully welded, it will be oxidized and discolored after welding. This weld is still unqualified and must be scrapped.
5. Basis for selection of welding process parameters
1) If the welding current is large, the welding is very smooth, but the grains of the weld are thick and the performance is poor, and large residual stress is likely to occur around the weld. Therefore, the upper limit of the welding current should be controlled. When determining the current of the fillet weld of the cover plate, the first pass is self-melting and the second pass is adding wire. The welding current is 130-140A.
2) Arc voltage selection The arc voltage is actually to choose the length of the arc. The commonly used argon arc welding machine does not have a voltage selection button. The arc should be as short as possible to control the arc voltage between 14-16V.
3) Welding speed The thermal conductivity of titanium material during welding is very poor, and the current and speed must be matched well, otherwise the fusion is not good. After testing, when the welding current is 130-140A, the welding speed is 160-170mm/min is more appropriate.
4) Determination of argon flow
Titanium should be cooled in the argon protective atmosphere within 100mm of the weld area behind the weld pool (including the front and back of the weld and heat affected zone), otherwise the weld will be oxidized and discolored, which will degrade performance. After field test adjustment, the final determined argon flow rate is: nozzle 10-11L/min, holding cover 12-13L/min, backside 3-5L/min.
5) Titanium plate argon arc welding uses a DC argon arc welding power source with reduced external characteristics and high-frequency arc starting. The tungsten pole tip should be ground into a conical shape and horizontal rotation flat welding is used. The welding torch should be supplied with gas in advance, and the current attenuation device and gas delay protection device should be used when extinguishing the arc. The delayed gas delivery time should be no less than 15 seconds to prevent the weld from being oxidized or polluted.
In order to reduce welding deformation, tack welding is required before welding. The tack welding seam should use 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 of the welding joint. Before welding, remove the oxide layer, oil stain, moisture, rust, etc. on the surface of the welding wire and the welding groove and on both sides, and should be welded by a qualified welder. Position welding is welded on the base weld, and the length of the weld should be 10-15mm, the spacing is 100-150mm, and the height should not exceed 1/3 of the wall thickness. The gap is 0-1mm and the blunt edge is 0-1.0mm.
Welds must not have defects such as cracks, pores, slag inclusions and discoloration due to oxidation, and should be eliminated in time when defects are found.
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