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Precautions for operating welding equipment: It is a device that supplies electrical energy to the arc; it is simply referred to as a welding machine. 1) Check whether the wiring is correct; the equipment enclosure must be grounded. In the event of an electric shock to a welder, the power supply should be turned off first before attempting rescue. 2) Wear dry gloves when operating the push-pull power switch; do not face the switch to avoid burns to the face caused by arc sparks. 3) Welding cables must not be placed near the welding machine or on hot metal welds; they should also be protected from collisions and wear. 4) When stopping work, the power should be turned off promptly; when working outdoors, the equipment must be properly covered. Selection of welding materials: The principles for selecting welding electrodes are the principle of equal strength, the principle of equivalence, and the principle of equal conditions. Selection of welding current 1) In actual production, welders select the welding current based on the results of test welds and their own practical experience. 2) The current is too low, making it difficult to initiate an arc; the welding rod tends to stick to the workpiece, resulting in coarse fish-scale patterns and poor fusion on both sides. 3) The current is too high, resulting in excessive spatter and smoke during welding; the electrode turns red, and the surface of the weld pool becomes very bright, which makes it easy to cause burn-through and undercutting. 4) The current is appropriate; it’s easy to strike an arc, which remains stable. There is very little spatter. A uniform crackling sound can be heard. The areas on both sides of the weld smoothly transition into the base metal. The fish-scale pattern on the surface is fine, and the slag can be easily removed. Selection of arc voltage: The arc voltage primarily affects the width of the weld. In shielded metal arc welding, control is primarily achieved through the lateral movement of the electrode, so the influence of arc voltage is not significant. Once the welding current is adjusted, the longer the arc, the higher the voltage. However, when the arc is too long, combustion becomes unstable, there is excessive spatter, and defects such as undercutting and porosity are likely to occur ； If the arc is too short, it can easily stick to the welding rod; generally, it is advisable for the arc length to be either 1/2 or equal to the diameter of the welding rod. Selection of welding speed and number of weld layers: Welding speed refers to the length of weld completed per unit of time. Under the principle of ensuring the required dimensions and shape as well as good weld quality, the welding speed is flexibly controlled by the welder. When welding thick plates, multi-pass welding or multiple layers of welding must be employed. The previous weld pass preheats the subsequent one, while the subsequent weld pass provides heat treatment to the previous one. It helps to improve the ductility and toughness of the weld metal. The thickness of each weld pass shall not exceed 1.5 times the diameter of the electrode. Techniques for moving the welding rod, Arc starting: At the beginning of arc welding, the process of initiating the welding arc is called arc starting. The methods of arc initiation include the following two types: 1) Contactless arc initiation: This refers to using a high-frequency voltage to make the gas between the electrode tip and the workpiece conductive, thereby generating an arc. This method is rarely used in shielded metal arc welding. 2) Contact arc initiation: To start the arc, first short-circuit the electrode with the workpiece, then separate the electrodes to ignite the arc. Depending on the technique used, it can be further divided into the tapping method and the scraping method. Tap method: The electrode is brought into perpendicular contact with the surface of the workpiece. Upon lightly touching the surface, the electrode is quickly lifted while maintaining a certain distance from it; this instantly ignites the arc. During operation, the welder must control the timing and distance of the up-and-down wrist movements. Scraping method: First, align the tip of the welding rod with the workpiece, then scrape the rod across the surface of the workpiece. Once the arc is established, and just before the metal begins to melt in large quantities, maintain a distance of 2–4 mm between the tip of the welding rod and the surface being welded; this will allow the arc to burn steadily. If the electrode sticks to the workpiece, shaking it back and forth a few times will allow it to separate from the workpiece. If it still doesn’t separate at this point, the welding torch should be released immediately to break the welding circuit; the electrode can then be removed once it has cooled down a bit. 3) Application: Due to the low temperature at the arc-starting end, the penetration depth is shallow, making incomplete fusion likely to occur. When starting the arc with acidic welding electrodes, the arc can be slightly elongated to preheat the root of the groove, after which the arc is lowered to carry out normal welding. Alkaline electrodes are advantageous for achieving full penetration at the root due to the properties of their coating; thus, there is no need to use the arc-starting method employed with acidic electrodes. However, one should not start the arc directly – instead, the arc should be started at a certain distance from the beginning of the groove, after which the starting point should be quickly pulled back and the arc lowered in order to carry out welding. ▶▶Welding stroke refers to the collective term for all the movements made by the welding rod relative to the weld during the welding process. Wire feeding involves three actions: feeding along the axis of the welding wire, moving longitudinally in the direction of the weld axis, and swinging laterally. 1) Basic movements for electrode feeding: The welding electrode is fed along its axis toward the weld pool so that after it melts, the length of the arc remains constant. Therefore, the speed at which the electrode is fed toward the weld pool must be equal to the speed at which the electrode melts. If the feed rate of the electrode is slower than the rate at which it melts, the length of the arc will gradually increase, resulting in arc extinction ； If the welding rod is fed too quickly, the arc length shortens rapidly, and the tip of the rod comes into contact with the workpiece, causing a short circuit that also leads to the extinction of the arc. The electrode does not move longitudinally in the welding direction; this movement enables the metal deposited by the electrode to form a weld with the melted base metal. Lateral swing of the welding electrode. The purpose of the lateral movement of the welding electrode is to achieve a weld of a certain width and to ensure good fusion on both sides of the weld. Its swing amplitude should be determined by the width of the weld root and the diameter of the electrode. The lateral oscillation force must be kept uniform in order to achieve a weld with the desired width and welding speed. The normal weld width generally does not exceed 2–5 times the diameter of the electrode. 2) Welding rod movement method: There are many methods for moving the welding rod, and the choice should be determined based on factors such as the type of weld joint, the assembly gap, the spatial position of the weld, the diameter and properties of the welding rod, the welding current, and the skill level of the welder. The welding rod should be moved slightly side to side during operation; this is done to achieve a uniform weld shape, as well as to control the temperature of the molten pool and prevent weld burn-through caused by excessively high temperatures in the molten pool. Depending on the method of lateral movement of the welding rod, the common rod movement techniques used during welding include: straight back-and-forth movement, crescent-shaped movement, diagonal circular movement, triangular movement, and zigzag movement. Crescent-shaped welding motion: The tip of the welding rod moves in a crescent shape from side to side along the welding direction, with faster movement in the middle and a slight pause on the sides. This method can effectively control the pool temperature; since the molten pool is shallow, it is necessary to prevent undercutting on both the front and back sides. The crescent-shaped stroke is one of the main welding stroke methods for single-sided welding with double-sided formation in continuous arc welding. Zigzag welding technique: The end of the welding rod is moved forward in a zigzag pattern, with brief pauses on each side to prevent undercutting. This method is easy to operate and widely applied. Suitable for welding the various layers of weld beads in butt welds performed in the flat, vertical, and overhead welding positions. Straight-line reciprocating welding method: The tip of the welding rod moves in a straight line along the length of the weld. This welding method allows for fast welding speeds and produces narrow weld seams; it is suitable for single-sided welding with double-sided bead formation in flat welding positions where the gap is narrow. It is particularly appropriate for welding stainless steel, as it helps to control the temperature of the molten pool during welding and ensures proper weld formation. Triangle welding motion method: The tip of the electrode moves forward in a continuous, even triangular pattern. This welding technique is suitable for welding thick plates, and it helps to achieve a good joint in the weld metal at the root of the weld. The joint of the weld is a single-sided welding with double-sided formation; the root pass welding is a challenging aspect to master. The jointing method was appropriate; the weld surface is even and smooth on both sides, with no internal defects ； Improper methods can easily lead to defects such as weld beads, excessive burr height, depressions, and separation. The quality of the joint is related to the quality of arc initiation and weld finishing. Generally speaking, if the arc is started quickly and properly, and a high temperature is maintained at the end of the preheating or previous welding process, it is easier to form a good-quality weld joint. If the electrode replacement is slow or the arc is unstable during arc initiation, a good weld joint cannot be obtained. Slant circular weaving method: The tip of the welding rod moves in a slant circular pattern while continuing to move forward. This welding technique is suitable for root pass welding in the overhead welding position for saddle-type tube sheets, as well as for single-sided welding with double-sided formation in the 45-degree inclined position for plate and tube shapes, or in the transverse position for thick plates. 3) Start of the weld: The start of the weld refers to the part of the weld where welding begins. The excess height of this portion of the weld tends to be higher. This is because the temperature of the workpiece is low at the start of welding; after arc initiation, it is not possible to raise the temperature of this metal quickly, resulting in a shallower penetration depth and a greater excess height. To avoid this situation, after igniting the arc, it is possible to slightly lengthen the arc first, carry out the necessary preheating of the welded parts, and then lower the arc appropriately to proceed with normal welding. The application of continuous arc welding and intermittent arc welding: In shielded metal arc welding for single-sided welding with double-sided formation as a root pass process, it can be divided into two methods based on the technique used – continuous arc welding and intermittent arc welding. 1) Continuous arc welding: This method involves using a lower welding current and a weld rod of smaller diameter. During welding, the arc remains continuously burning; it moves evenly forward within a small groove gap, thereby forming a uniform weld on the back side of the workpiece. This method is simple to operate, requires few adjustments, and is easy to master. The weld on the back side turns out to be dense and well-organized, with good internal quality and excellent mechanical properties. It is widely used both internationally and domestically. Its drawback is that it is limited by the size of the groove gap. The problem of difficult joint formation is even more pronounced when using acidic welding electrodes. The continuous arc welding method is mainly used for welding in various positions with alkaline electrodes, as well as for vertical and overhead welding with acidic electrodes. 2) Interrupted arc welding: This welding method involves periodically starting and stopping the arc during the welding process, thereby controlling the temperature of the molten pool and achieving good weld shape and internal quality. Its advantages include the ability to use larger groove gaps and higher welding currents; for single-sided welding with double-sided completion on thinner workpieces, there are no restrictions on the welding current used. Interrupted arc welding is mainly employed for flat welding and horizontal welding using acidic electrodes, as well as for root welding in single-sided welding with double-sided completion of thin-walled components such as tube sheets. This method is quite practical in production and maintenance. However, compared to continuous arc welding, it is more difficult to master and requires higher standards of the welder’s skills. The joint of a weld: The connection between a weld that is welded later and a weld that is welded earlier is called the weld joint. Due to the limitations imposed by the length of the welding rod, joints at the beginning and end of the weld are inevitable; however, the stress at these weld joints should be kept uniform, and defects such as excessive thickness, separation, or uneven width at the weld joints should be avoided. There are four types of weld joint conditions as follows: 1) Intermediate joint: The subsequent weld is started from the end of the previously welded weld. It is necessary to initiate the arc about 10 mm in front of the root of the weld; the arc length should be slightly longer than that in normal welding. After that, the welder moves back to the root of the weld, lowers the arc and makes slight movements, before continuing with normal welding forward. This jointing method is the most commonly used, suitable for surface joints in single-layer welding as well as multi-layer welding. 2) Back-to-back joint: The starts of the two welds meet; the start of the first weld should be slightly lower. The second weld must begin welding slightly in front of the start of the previous weld, after which the arc is extended gradually to guide it toward the start of the previous weld, covering its end. Once that area is welded smoothly, the welding process can proceed in the direction of welding. 3) Opposite-direction joint: This involves the ends of two welds meeting each other. When the second weld reaches the end of the first weld, the welding speed should be reduced slightly; after filling in the hollow area left by the first weld, welding should continue at a faster speed for a short distance before stopping the arc. 4) Segmented back-welded joint: This involves first welding the beginning of the weld seam and then welding the end portion. When the subsequent weld seam is welded close to the starting point of the previous weld seam, it is necessary to change the angle of the welding rod so that it points toward the rear end of the previous weld seam; this elongates the arc. Once a molten pool is formed, the arc is lowered and the welding rod is moved backward, finally returning to the original location of the molten pool to complete the welding process. The flatness of the joint connection is related to the welder’s skill, as well as to the temperature at the joint. The higher the temperature, the smoother the connection. Therefore, intermediate joints require short arc interruption times and fast electrode change actions. During multi-layer welding, the joints between layers should be offset to improve the density of the weld. Except when joining welds at their midpoints where it is not necessary to remove slag, slag at the area to be joined must be removed first in all other cases; otherwise, a proper weld joint cannot be formed. If necessary, the area to be joined can be ground into an inclined surface before performing the join. Weld finish: The weld finish refers to how to end the arc after completing a weld. At the end of welding, it is necessary to properly finish the weld seam. At the end, it is also necessary to maintain a normal molten pool temperature to facilitate the joint of the weld. There are various ways to conclude the process, with common methods including repeated arc interruption for termination, circular motion for termination, re-soldering for termination, and transfer for termination. For single-sided welding with double-sided formation, the finishing of the weld is mainly achieved using the repeated arc interruption method and the back-welding method. This method of intermittent arc interruption is generally used for finishing welds made with acidic electrodes, while the back-welding method is more commonly used for finishing welds made with alkaline electrodes. If the arc is suddenly extinguished, a concave crater remains on the surface of the weld, which reduces the strength of that area and increases the risk of crater cracks. If the arc is extinguished too quickly at the end, the gas in the liquid metal does not have time to escape, which can easily lead to defects such as pores. To overcome the crater defect, the following methods can be used for finishing: 1) Repeated arc interruption method: When the welding rod reaches the end of the weld, the arc is repeatedly extinguished and reignited at the crater several times until the crater is filled. This method is suitable for finishing welds in thin plate welding and high-current welding, but not for finishing welds using alkaline electrodes. 2) Circular finishing method: When the welding rod reaches the end of the weld, make circular movements at the crater until it is filled, then extinguish the arc. This method is suitable for finishing thick plates. 3) Transfer completion method: When the welding rod reaches the end of the weld, pause briefly at the arc crater, slowly raise the arc, and then guide it into the root groove of the base material at the edge of the weld. At this point, the molten pool gradually shrinks, and defects generally do not appear after solidification. Suitable for replacing electrodes or finishing work when the arc is temporarily stopped. Prohibitions regarding wire feeding: 1) Using the striking method for wire feeding is difficult for beginners to master; it often leads to the arc going out or to short circuits, which is due to an improper control of the speed at which the welder moves away from the workpiece and of the distance that should be maintained. 2) It is relatively easy to master the wire movement using the scraping method; if the welding rod is pulled upward too quickly or too high during operation, the arc cannot be ignited, or the arc burns for only a moment before going out. On the contrary, if the movement is too fast, it may cause the welding rod to stick to the workpiece, resulting in a short circuit in the welding circuit. 3) If the electrode sticks to the workpiece during arc initiation, the welding torch should be released immediately. If the short-circuit duration is too long, the excessive short-circuit current can damage the welding machine. 4) The movement speed of the welding electrode has a significant impact on weld quality and welding productivity. If the welding rod moves too fast, the arc does not have enough time to melt sufficient amount of the welding rod and base metal, which can result in incomplete welding or narrow welds ； If the welding rod moves too slowly, the temperature of the molten pool will become too high, which can cause the workpiece to be burned through. It can also lead to issues such as weld beads, excessively wide weld seams, metal accumulation, overly high welds, and an irregular shape. It is easy to cause penetration when welding thinner workpieces. Therefore, the moving speed of the welding rod must be appropriate to ensure a uniform weld. 5) When ending the weld, it is necessary to ensure that the gases inside the molten pool are fully expelled, and to prevent air from entering due to too rapid termination of welding, which could lead to defects such as cold shrinkage pores and internal pores