What are the technological characteristics of low-frequency pulsed argon tungsten arc welding? Low-frequency pulse tungsten argon arc welding has the following technological characteristics. 1. During the duration of the pulse current, a spot-shaped molten pool is formed on the weldment; during the period of the pulse current stop, the base current can only maintain the stable combustion of the arc, the heat input to the weldment is significantly reduced, and the molten pool metal solidifies to form a welding spot. Therefore, the weld is composed of a series of weld points. 2. The arc is stable and the stiffness is good. When the current is small, general TIG welding is easy to arc, but pulsed TIG welding has good arc stiffness and stability, so this welding method is especially suitable for thin plate welding. 3. Low arc line energy. The pulse arc heats the workpiece centrally and has high thermal efficiency. Therefore, the average current required to penetrate the workpiece with the same thickness is about 20% lower than that of general tungsten arc welding, thereby reducing the heat input, which is beneficial to reduce the heat-affected zone and Reduce welding distortion. 4. Easy to control the weld formation. The welding pool has fast solidification speed and short high temperature residence time, so it can ensure a certain penetration depth, and is not easy to cause overheating, flow or burn-through, which is beneficial to realize single-sided welding without backing and double-sided welding and all-position welding. 5. The weld quality is good. Pulsed tungsten argon arc welding is formed by overlapping welding points, and the thermal cycle of subsequent welding points has a heat treatment effect on the previous welding point. At the same time, because the pulse current has a strong stirring effect on the spot molten pool, the cooling rate of the molten pool is fast, and the high temperature residence time is short, so the weld metal structure is fine and the dendrites are not obvious. These all make the weld performance improved. What are the technological characteristics of high-frequency pulse argon tungsten arc welding? The frequency range of the current is 10~20kHz. The process characteristics of this method are as follows. ①Suitable for high-speed welding The high-frequency pulse arc has a large electromagnetic shrinkage effect and a good arc rigidity. It can avoid the bend or discontinuity of the weld bead caused by the adhesion of the anode spot during high-speed welding; avoid defects such as undercut and poor back forming. Therefore, it is especially suitable for high-speed automatic welding of thin plates. ②Big penetration The arc pressure is large and the energy density is large, so the arc penetration capability is significantly increased. ③Good weld quality The molten pool is subjected to ultrasonic vibration, its fluidity increases, and the physical and metallurgical properties of the welding are improved, which is conducive to the improvement of the quality of the weld. ④Suitable for large groove weld In DC tungsten argon arc welding, if there are too many filler wires, the weld pool and the side of the groove will be poorly fused, and the weld bead will be raised and deflected to one side. When welding the next weld bead, poor melting on both sides of the weld bead easily leads to poor fusion or incomplete penetration. High-frequency pulse tungsten argon arc welding can well overcome this defect. Many characteristics of high frequency argon tungsten arc welding are between general argon tungsten arc welding and plasma arc welding, see table What are the technological characteristics of intermediate frequency pulsed argon tungsten arc welding? The frequency range of the current is 10~500Hz. Its characteristic is that the arc is very stable under small current, and the arc force is not as high as high-frequency argon tungsten arc welding, so it is an ideal method for manual welding of thin plates below 0.5mm. Development trend of argon arc welding tungsten electrode Because of its high melting point and good high temperature performance, tungsten is the best material for electron emission and the best choice for argon arc welding starting electrodes. The earliest used tungsten argon arc welding electrode is a pure tungsten electrode, but the electron work function of the pure tungsten electrode is relatively high, about 4.52eV, and it is not easy to start arcing under the condition of small and medium currents. Therefore, all subsequent researches on electron emission materials are centered on reducing the work function of electrons. The earliest doped tungsten electrode material is thorium tungsten, which is doped with about 1-2% thorium oxide in the tungsten base. (This kind of thorium oxide was not prepared for argon arc welding at the beginning, but a material invented by an American named Pintsch to improve the brittleness of tungsten wires for light bulbs.)

Welding heat affected zone The welded joint is a welding consisting of three parts: the weld, the fusion zone and the heat-affected zone. Welding heat-affected zone: referred to as HAZ (Heat Affect Zone), under the action of welding heat cycle, the area where the solid base metal on both sides of the weld undergoes significant changes in structure and performance is called the welding heat-affected zone. Microstructure distribution of hardened steel Features: Martensite is not easy to form under welding air cooling conditions. Such as low carbon steel, 16Mn, 15MnV and 15MnTi, etc. According to heating temperature and organizational characteristics, it can be divided into four areas: overheating zone, normalizing zone, partial normalizing zone and recrystallization zone. as the picture shows. Overheating zone (coarse crystal zone) The temperature is between the solidus to 1100°C, and the width is about 1 to 3 mm. During welding, the austenite grains in this area grow up severely. After cooling, a superheated structure with coarse grains is obtained, and the plasticity and toughness are obviously reduced. Phase change recrystallization zone Phase change recrystallization area (normalizing area or fine-grained area) The temperature is between 1100℃～Ac3, and the width is about 1.2～4.0mm. Air cooling after welding makes the metal in this area equivalent to normalizing treatment, so its structure is uniform and fine ferrite and pearlite, and its mechanical properties are better than the base material. Incomplete recrystallization zone Incomplete recrystallization zone (also called partial normalizing zone) The heating temperature is between Ac3 and Ac1. When welding, only part of the structure is transformed into austenite; after cooling, fine ferrite and pearlite are obtained, and the rest is still the original structure, so the grain size is uneven and the mechanical properties are also poor. Recrystallization zone If the base material undergoes cold working deformation before welding, the temperature is between Ac1～450℃, and there is a recrystallization zone. The mechanical properties of the metal in this area have not changed much, but the plasticity has increased. If there is no cold plastic deformation before welding, there is no recrystallization zone in the heat-affected zone. Weld quality standards and the first, second and third level classification of welds 1. Guaranteed items 1. Welding materials should meet the design requirements and relevant standards, and the quality certificate and baking records should be checked. 2. The welder must pass the examination and check the qualification certificate and assessment date of the welder's corresponding welding conditions. 3. Welds of class I and II must be inspected by flaw detection, and should meet the design requirements, construction and acceptance specifications, and check the weld flaw detection report. 4. The surface of the welded seam should not have cracks, weld bead, burn through, arc crater and other defects. Class Ⅱ welds must not have defects such as surface pores, slag inclusions, arc craters, cracks, arc scratches, etc., and class Ⅰ welds must not have defects such as undercuts and insufficient welds. 2. Basic items 1. Appearance of weld seam: uniform weld seam appearance, smooth transition between weld bead and weld bead, weld bead and base metal, and clean welding slag and spatter. 2. Surface pores: Class Ⅰ and Ⅱ welds are not allowed; for class Ⅲ welds, the allowable diameter of each 50mm length of the weld is ≤0.4t; and there are 2 pores ≤3mm; the spacing of pores is ≤6 times the diameter. 3. Undercut: Class I welds are not allowed. Class Ⅱ weld: depth of undercut ≤0.05t, and ≤0.5mm, continuous length ≤100mm, and total length of undercut on both sides ≤10% of weld length. Grade III weld: undercut depth ≤0.lt, and ≤lmm. Note: t is the thinner plate thickness at the joint. 4. The allowable deviation items are shown in Table 5-1. 3. Finished product protection 1. Don't hit the joint after welding, and don't water the steel that has just been welded. Slow cooling measures should be taken at low temperatures. 2. It is not allowed to ignite the arc on the base metal outside the weld at will. 3. Welding can only be performed after the various components are calibrated, and the shim and fixtures should not be moved at will to prevent the size deviation of the components. The welds of concealed parts must go through the concealed acceptance procedures before proceeding to the next concealed process. 4. It is not allowed to clean the slag immediately after low-temperature welding, and it should be done after the welding seam cools down. Fourth, the quality issues that should be paid attention to 1. The size exceeds the allowable deviation: For the deviation of the weld length, width, thickness, center line offset, bending, etc., the relative position and size of the welding part should be strictly controll

Manual arc welding is a manual welding method that uses the heat generated by the arc to melt the metal to be welded. Because it requires simple equipment and flexible operation, it can easily weld the welds formed by different positions and different joints in space. Therefore, it is still widely used at present. Manual arc welding is shown in the figure: Before welding, connect the welded workpiece and the welding tongs to the two poles of the electric welding machine and clamp the welding rod with the welding tongs. During welding, the welding rod and the workpiece are in instantaneous contact, forming a short circuit, and then they are separated by a certain distance (about 2-4mm), and the arc is ignited. The workpiece under the arc immediately melts to form a semi-oval molten pool. After the electrode coating is melted, part of it becomes a gas that surrounds the arc to isolate it from the air, thereby protecting the liquid metal from oxygen and nitrogen; part of it becomes molten slag, or sprayed into the molten pool alone, or melted with the core The molten droplets of liquid metal are sprayed to the molten pool together. In the arc and the molten pool, the liquid metal, slag and arc gas will undergo certain physical and chemical changes with each other, such as the dissolution of the gas into the liquid metal and the oxidation-reduction reaction. The gas and slag in the molten pool float up due to its light weight. When the arc is removed, the temperature drops and the metal and slag will solidify one after another. In this way, two pieces of metal are joined by melting and crystallizing weld metal. Because the shrinkage of the slag is different from that of the metal, it will slip on the slag shell and the metal boundary, and the slag shell may fall off automatically, or fall off after being knocked, and the metal weld seam with fish scales can be exposed. The main equipment of manual arc welding is electric welding machine. Electric welding machine is a power source that generates welding arc, and there are two kinds of AC and DC. At present, there are many kinds of electric welding machines produced in China, which can be divided into AC electric welding machines and DC electric welding machines according to their structure. There are two different connection methods for DC welding machines. When the electrode is connected to the negative electrode and the workpiece is connected to the positive electrode, it is the positive connection method; the opposite is the reverse connection method. Generally, when welding with alkaline low-hydrogen electrode (such as J507), in order to make the arc burn stably, it is stipulated to use the DC reverse connection method; when using the acid electrode (such as J422) to weld thick steel plates, the forward connection method is used, because the anode part The temperature is higher than the cathode part, and the forward connection method can get a larger penetration depth; when welding thin steel plates and non-ferrous metals, the reverse connection method is used. When welding with alternating current, since the polarity changes alternately, there is no need to choose the polarity connection. The welding material for manual welding is an electric welding rod, which consists of a steel core and a coating on the outside of the steel core. Steel core The role of the steel core (welding core) is mainly to conduct electricity and form a deposited metal with a certain composition at the end of the electrode. The welding core can be made of various steels. The composition of the welding core directly affects the composition and performance of the deposited metal. Therefore, the welding core is required to minimize the content of harmful elements. In addition to limiting S and P, some welding rods have required the welding core to control As, Sb, Sn and other elements. Medicated skin Electrode coating can also be called paint. The main purpose of coating it on the core is to facilitate the welding operation and to ensure that the deposited metal has a certain composition and performance. Electrode coatings can be mixed with hundreds of raw material powders such as oxides, carbonates, silicates, organics, fluorides, ferroalloys, and chemical products in a certain formula ratio. Various raw materials can be divided into the following categories according to their role in the electrode coating: 1. Stabilizer makes the electrode easy to start the arc and can keep the arc burning stably during the welding process. Any substance that is easy to ionize can stabilize the arc. Generally, compounds of alkali metals and alkaline earth metals, such as potassium carbonate, sodium carbonate, marble, etc., are used. 2. Slag-forming agent can form molten slag with certain physical and chemical properties during welding, covering the surface of the molten metal, protecting the welding pool and improving the shape of the weld. 3. Deoxidizer through the metallurgical