Generally, manual arc welding is the basic project in welder training, which mainly teaches welders to master the selection of welding current, welding speed, wire feeding method, and control of welding arc. In the welding process, defects such as weld nodules, burn-through, incomplete welding, concave, slag inclusion, and poor forming often occur. The reasons for these defects are analyzed. For welders who have just entered the industry, the main reason is that they are not good at observing the changes in the temperature of the molten pool during the welding operation and fail to effectively control the temperature of the molten pool, which leads to the above defects. By simplifying the complex theory, trainees can quickly understand and master the welding operation technology.
During fusion welding, under the action of welding heat source, the liquid metal part with a certain geometric shape formed by the molten electrode metal and the partially melted parent material on the weldment is called the molten pool. After cooling, it becomes a weld, so the molten pool temperature directly affects the welding quality. When the molten pool temperature is high, the molten pool is large, the molten iron has good fluidity, and the fusion zone is easy to fuse; but when it is too high, the molten iron is easy to flow down, the back of the single-sided welding and double-sided forming is easy to burn through, weld nodules are formed, and the forming is difficult to control, and the plasticity of the joint decreases, and the bending is easy to crack; when the molten pool temperature is low, the molten pool is small, the molten iron is darker, the fluidity is poor, and it is easy to produce defects such as incomplete welding, incomplete fusion, and slag inclusion. Therefore, effectively controlling the temperature of the molten pool, especially for a beginner, is a necessary way to successfully enter the industry and even become an excellent welding technician. The molten pool temperature is closely related to the welding current, electrode diameter, rod feeding method, electrode angle, arc burning time, etc. The following measures are taken to control the molten pool temperature according to the relevant factors.
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01
Welding current and electrode diameter are important factors in welding, and the two are inseparable. During fusion welding, the current flowing back through the weldment is called welding current. The electrode diameter refers to the cross-sectional size of the filler metal rod. In simple terms, whether the electrode can be properly melted is determined by the current passing through. If the current is too small, it is difficult to strike an arc, the electrode is easy to stick to the weldment, the fish scale pattern is coarse, and the two sides are not well fused; if the current is too large, there will be a lot of spatter and smoke during welding, the electrode will be red, the surface of the molten pool will be very bright, and it is easy to burn through and bite the edge; if the current is appropriate, it is easy to ignite and the arc is stable, the spatter is small, and a uniform crackling sound can be heard. The two sides of the weld are smoothly transitioned to the parent material, the surface fish scale pattern is very fine, and the welding slag is easy to knock off. In terms of its application, there is a complex relationship.
1.1 Select welding current and electrode diameter according to the spatial position of the weld. In vertical, horizontal, and supine positions, the current is correspondingly smaller than that of flat welding, and the current should usually be about 10% smaller than that of flat welding. Similarly, in vertical, horizontal and overhead positions, the diameter of the welding rod is usually smaller than that in flat welding. For example, in flat welding filling and capping welding of flat plates larger than 12mm, it is common to use 5.0mm welding rods for thick plates, while in vertical, horizontal and overhead positions, 5.0mm diameter welding rods are rarely used.
1.2 Select welding current and electrode diameter according to the welding layer of the weld. For example, for 12mm flat plate butt joint, a 3.2mm electrode is used for the bottom layer of flat welding, and the welding current is 90-110A. A 4.0mm electrode can be used for the filling cover layer, and the welding current is 160-175A. Therefore, a reasonable selection of welding current and electrode diameter can easily control the molten pool temperature, which is the basis for good weld formation. If the welding current is too small and the weld pool temperature is too low, the arc will be unstable and the workpiece may not be welded through. If the welding current is too large and the molten pool temperature is too high, it will cause serious splashing or flow of molten metal, and even burn through the workpiece to form a weld nodule. The following lists the relationship between welding current and electrode diameter. You can make a reasonable choice based on your own experience or habits. You don’t have to determine the same parameters as others. As long as you feel it is appropriate and ensure good weld formation, it will be fine.
02
Rod feeding After the electrode is fed along the axis toward the molten pool to melt the electrode, the length of the arc can be kept constant. Therefore, the feeding speed of the electrode toward the molten pool is required to be equal to the melting speed of the electrode. If the feeding speed of the electrode is less than the melting speed of the electrode, the length of the arc will gradually increase, resulting in arc breaking; if the feeding speed of the electrode is too fast, the arc length will be shortened rapidly, and the end of the electrode will short-circuit with the weldment, which will also extinguish the arc.
Crescent-shaped electrode feeding method: The end of the electrode swings left and right in a crescent shape along the welding direction, and the middle movement should be fast, and the two sides should stay for a while. This method can effectively control the temperature of the molten pool. The molten pool is shallow, and the front and back sides should be prevented from biting. Crescent-shaped electrode feeding is one of the main electrode feeding methods for single-sided welding and double-sided forming continuous arc welding. Sawtooth-shaped electrode feeding method: The end of the electrode swings forward in a serrated shape and stays for a while on both sides to prevent biting. This method is easy to operate and widely used. It is suitable for welding of each layer of welds in flat, vertical and overhead welding positions. The temperature of the crescent-shaped feeding rod is higher than the molten pool temperature of the sawtooth-shaped feeding rod. In the 12mm flat welding sealing bottom layer, the sawtooth-shaped feeding rod is used, and the swing amplitude and the pause on both sides of the groove are used to effectively control the molten pool temperature, so that the size of the molten hole is basically the same, the probability of forming weld nodules and burn-through at the root of the groove is reduced, and the incomplete penetration is improved, so that the single-sided welding and double-sided forming of the flat plate butt welding are no longer difficult.
03
The feeding position and feeding position of the welding rod should change with the change of the welding position during welding, and the molten pool temperature on both sides of the blunt edge should always be kept appropriate. Too high temperature will cause burn-through, and too low temperature will cause incomplete penetration and non-fusion. When the angle between the welding rod and the welding direction is 90 degrees, the arc is concentrated and the molten pool temperature is high; if the angle becomes smaller, the arc is dispersed and the molten pool temperature is lower. For example, if the angle of the welding rod is 50-70 degrees for the 12mm flat welding sealing bottom layer, the molten pool temperature will be reduced at this time, avoiding the phenomenon of weld nodules or rise on the back. For example, after changing the welding rod for the bottom layer of the 12mm plate vertical welding seal, we use a 90-95 degree welding rod angle when transporting the welding rod, so that the temperature of the molten pool can be quickly increased, the molten hole can be opened smoothly, the back side is relatively flat, and the concave phenomenon of the joint point is effectively controlled. If the welding rod is not fed into the position enough, it will cause incomplete penetration or groove. Because the arc is more dispersed at this time, the melting temperature of the blunt edge of the parent material is not enough, resulting in the unfusion of the bottom parent material; if you want to fully melt the metal, you must increase the melting time. The molten pool cannot support the molten iron, which will cause it to flow down, forming grooves or weld nodules. The multi-layer superposition of the molten pool will also cause slag inclusion. The correct method is to extend the welding rod into the blunt edge groove at an angle of 75 degrees, melt the groove parent material and swing it on both sides. Each action takes about 1 second. At this point, the first molten pool is formed, and then the next molten pool is formed. At this time, the melting time of each molten pool is short and the weight is light, so it is not easy to cause falling, weld nodules will not be formed, and the shallow groove is also conducive to cover welding. The latter molten pool covers 2/3 of the former one in a superimposed manner. Each molten pool is thinner, and the latter one plays a role of post-heat melting on the former one, ensuring that the gas in the molten pool has sufficient time to overflow and does not produce pores.
04
Arc burning time 57×3.5 In the internship teaching of horizontal fixed and vertical fixed welding of pipes, the arc breaking method is used for welding. When welding, the temperature of the base material is low. If the welding rod is not swung to the edge of the groove, the molten iron will shrink back quickly, resulting in undercut, and the weld formation will be high and narrow, which will not achieve the effect of smooth transition, and it is easy to produce surface unfusion. From the analysis of the shape of the molten pool, if it is in the shape of a falling droplet, the welding shape will definitely be bad, and weld nodules may be produced. Therefore, the welding point of the overhead welding should be fully preheated, the angle between the welding rod and the pipe should be 75 degrees, and the arc should be elongated after the arc is ignited for preheating. After the first drop of molten iron on the head of the welding rod falls, the welding rod should be fed in. At this time, the temperature of the molten pool should ensure that the size of the molten pool is the groove width plus about 1 mm, so that the parent material can be fully melted into the molten droplet to form a weld.
05
In summary, in the actual welding operation, it is necessary to learn to observe the changes in the molten pool temperature and master the method of effectively controlling the molten pool temperature, which is the basis for learning welding technology. It is necessary to be able to judge the welding rod angle, feeding position and melting time according to the molten pool conditions of various parts, and quickly master the operation process of several key parts. After a period of practical training, the technical level will improve rapidly, and the occurrence rate of various welding defects will be significantly reduced, which will improve the adaptability in complex construction welding, which is conducive to the future learning of welding technology. Only by laying this solid foundation can we make breakthroughs and become an excellent welding technician.
Post time: Nov-18-2024
