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10 Key Points on Welding Technology 1. What is arc starting? What are the methods for starting an arc? What should be noted when striking an arc? The process of igniting and generating a stable arc is called arc starting. There are two arc-starting methods: the scraping arc-starting method and the direct arc-starting method. The scratch arc-starting method involves aligning the tip of the welding rod with the arc-starting point, then, similar to lighting a match, gently sweeping the rod across the surface of the workpiece using wrist strength; the sweep distance should be 10–20 millimeters, and the welding rod should be lifted by 3–3 millimeters. In the direct arc starting method, first align the welding rod with the area of the workpiece to be welded and make light contact there; then lift the welding rod by 2-3 millimeters at the appropriate moment to initiate the arc. The movement of lifting the welding rod at the start of arc formation must be quick; otherwise, it is likely to stick to the workpiece. If sticking occurs, the welding rod can be moved back and forth before pulling it apart; if separation is not possible, release the welding torch to break the welding circuit, and wait until the welded parts have cooled slightly before attempting further action. The following points should be noted when starting the arc: (1) After starting the arc, ensure that the arc length does not exceed the diameter of the electrode. Do not apply too much force when starting the arc, to prevent the flux at the arc-starting end from cracking or even falling off, which could affect arc starting and welding. (2) When striking an arc, do not randomly strike sparks on the weldment, especially with high-strength steel, low-temperature steel, and stainless steel. This is because the area scratched by the arc is prone to hardening or micro-cracking, which reduces the corrosion resistance of stainless steel. (3) When starting the arc, if the welding rod becomes connected to the workpiece and cannot be removed by shaking, the welding torch should be taken away immediately; once the welding rod has cooled down, it can be removed easily. ) l( 2. What is spot welding before welding? To fix the relative position of the two welded parts, tack welding is performed at both ends of the workpiece before welding (commonly referred to as spot fixing). After point fixation, the slag must be cleaned thoroughly. If the weldment is relatively long, a tack weld can be applied every 200–300 mm or so. 3. What is positive connection? What is reverse connection? During the welding process, the two poles of the arc welder are connected to the electrode and the workpiece, thereby forming a complete welding circuit. For DC welders, one pole is the positive pole, and the other is the negative pole. When the workpiece is connected to the positive pole and the welding torch (electrode) to the negative pole, it is called direct polarity connection (or simply direct connection); conversely, it is called reverse polarity connection (or simply reverse connection). For AC welding machines, since the polarity of the power supply is alternating, there is no issue of straight or reverse polarity. 4. How to start welding with shielded metal arc welding? The start of a weld is its initial portion. Due to the low temperature of the workpiece, it is not possible to raise its temperature quickly after starting the arc. In general, the bead height in this area is slightly higher, the penetration depth is shallower, and issues such as poor fusion and slag inclusions may occur. Therefore, after starting the arc, it is necessary to lengthen the arc slightly to preheat the workpiece, before proceeding with normal welding using a steady arc. For flat welding and welding with alkaline electrodes, the back-welding method is commonly used: an arc is initiated 10 millimeters from the starting point, and then the arc is moved back to that starting point. The voltage of the arc is gradually reduced, while the electrode is moved slightly back and forth, thereby achieving the desired weld width; after that, normal welding proceeds. 5. What is the welding arc voltage? What’s its purpose? The voltage drop between the two ends of the arc (the two electrodes) is called the arc voltage. It includes the cathode voltage drop, anode voltage drop, and arc column voltage drop; numerically, it is equal to the sum of these three voltage drops. The arc voltage mainly depends on the arc length; the longer the arc, the higher the voltage, and the shorter the arc, the lower the voltage. A higher arc voltage results in a wider weld bead, while a lower arc voltage leads to a narrower weld bead. During welding, in order to achieve an appropriate weld width and depth, the arc voltage should be increased accordingly as the welding current increases. 6. What is the welding speed? What’s its purpose? The length of a single weld seam completed per unit time is called the welding speed. The welding speed depends on the magnitude of the welding current. A high welding current can increase the welding speed, but it should not be assumed that the efficiency of welding production depends on the welding speed ； It is not the higher the welding speed that results in higher welding production efficiency; rather, it is the magnitude of the welding current that determines the level of welding production efficiency. The greater the welding current, the more metal is melted per unit of time, and thus the higher the welding production efficiency. At a certain welding current, the welding speed affects the thickness of the weld bead. A higher welding speed results in a thinner weld bead, while a lower welding speed results in a thicker weld bead. 6 _/ g8 o. C* H3 H; L+ E%N% _$ i7. Why does the welding arc drift? What are the consequences of this? There are three main factors that cause deviation of the welding arc: (1) Deviation of the arc is caused by an excessive degree of eccentricity in the electrode coating: During shielded metal arc welding, the melting rate of the electrode coating is slower than that of the core; during welding, the electrode coating forms a kind of sleeve that helps to protect the arc. When the thickness of the welding rod coating is uneven, the melting speed of the thicker coating is slower than that of the thinner coating; as a result, a deviation occurs in the length direction of the coating sleeve during welding, which inevitably causes the arc to shift toward the side where the coating is thinner. (2) Arc deflection caused by uneven magnetic field strength in the arc area: Due to the asymmetric magnetic forces acting on both sides of the arc axis, the arc is deflected in the direction where the magnetic field strength is weaker. When a good conductor (such as a gusset plate) is present on one side of the arc zone, the arc is drawn toward that conductor. If the cable connecting the workpiece is attached to the left side of the arc axis, the arc will blow to the right. (3) Gas flow disturbances around the arc cause the arc to deflect: the arc deflects in the direction of the gas flow, that is, from areas with strong gas flow to areas with weak gas flow. The consequence of arc deflection is that it disrupts the stability of the arc, resulting in poor protection of the arc zone and severe spattering; this can lead to welding defects such as pores and lack of penetration. 8. What are the measures to prevent arc deflection? Measures to prevent arc deflection include: (1) selecting electrodes with a non-eccentric coating and drying them as specified. (2) Short-arc operation is employed to enhance the \"stiffness\" of the arc in order to counteract the effect of arc deflection. (3) Adjust the welding angle of the electrode so that the tilt angle of the electrode is in the direction opposite to the arc deflection, thereby improving the arc deflection phenomenon by changing the welding tilt angle. (4) When welding the first layer of a butt weld with a gap, a backing plate or filler wire can be placed at the back of the gap to prevent arc deflection caused by thermal convection. The reverse segmentation method can also be used, or arc starting plates and arc extinguishing plates can be added at both ends of the weld to mitigate the arc deviation caused by thermal convection. (5) For welding steel structures (such as reinforced beam and column structures), an AC power source should be used whenever possible. (6) By changing the location of the cable (ground wire) connecting the welded parts, the magnetic field force around the arc should be distributed as evenly as possible. 9. What are the requirements for electrodes in shielded metal arc welding? The following principles should be considered when selecting welding electrodes: (1) Choose the appropriate type of welding electrode (general category) based on the type of metal material to be welded. For example, when welding carbon steel or ordinary low-alloy steel, structural steel electrodes should be used. (2) The weld properties must be the same as those of the base material, or the chemical composition of the weld must be the same as that of the base material to ensure identical properties. When selecting structural steel electrodes, first, in accordance with the tensile strength of the base material and following the \"equal strength\" principle, choose structural steel electrodes of the same strength grade. Secondly, for important structures where high requirements are placed on weld properties (ductility, toughness), or when welding steel materials and structures prone to cracking (such as those with large thicknesses, high rigidity, or welding in low-temperature environments), alkaline electrodes should be used, or even ultra-low hydrogen electrodes or high-toughness electrodes. When using stainless steel electrodes as well as molybdenum and chromomolybdenum heat-resistant steel electrodes, it is necessary to select electrodes with the same chemical composition based on the type of chemical composition of the base metal. (3) The process properties of the welding electrode must meet the requirements for welding. When welding in a non-horizontal position, electrodes suitable for welding in various positions should be used. Similarly, for vertical down welding, pipeline welding, root welding, cover welding, and gravity welding, corresponding specialized electrodes can be used. Furthermore, while ensuring the required performance standards, electrodes with low prices and high deposition efficiency should be given priority. ! h) s10, What is tack welding? What precautions should be taken for tack welding? Welding performed before welding, to assemble and fix the joint positions on the weldments. Assembly and welding are the most important production stages in the manufacturing process of welded structures. Position welding is a method commonly used in the assembly process. Since the tack welds remain in the welded structure as actual welds, the electrodes, welding processes, and operational skills used for these tack welds must be exactly the same as those used for the actual welds. The requirements for tack welding are as follows: (1) In principle, the electrode used for tack welding should be the same as that used for the main weld of the product. When using a tension plate for tack welding of alloy structural steel, low-hydrogen flux-cored electrodes with a strength lower than that of the product weld are permitted. (2) The welding electrodes for tack welding shall be dried as specified. (3) The preheating temperature for tack welding should be set at the upper limit of the preheating temperature for the product’s welds. The preheating area for the tack weld is approximately 150 mm around the tack weld. (4) For tack welding, a slightly higher wire energy can be used; this ensures a strong weld while preventing undercutting, and multi-layer, multi-pass welding can be employed. (5) Avoid performing tack welding at welds or where there are sudden changes in the structural cross-section.