Welding of medium-carbon steel and repair welding of high-carbon steel

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Welding of medium carbon steel: The weldability of medium carbon steel is poor, as it contains a higher amount of carbon, which makes it less weldable than low carbon steel. When w(C) is close to the lower limit (0.25%), weldability is good; as the carbon content increases, the tendency for hardening also increases, and low-plasticity martensite structures tend to form in the heat-affected zone. Cold cracks are likely to occur when the weldment has high rigidity or when the welding materials and process parameters are improperly selected. When welding the first layer in multi-layer welding, a large proportion of the base metal melts into the weld, which increases its carbon content as well as the levels of sulfur and phosphorus, thereby increasing the risk of thermal cracking. Furthermore, higher carbon content also increases pore sensitivity. Welding process of medium carbon steel: preheating before welding and interpass temperature. Preheating before welding is an effective technique for preventing cracks when welding or repairing medium carbon steel, as it reduces the cooling rate of the weld metal and the heat-affected zone, thereby inhibiting the formation of martensite. The preheating temperature depends on factors such as carbon content, base metal thickness, structural stiffness, electrode type, and welding process. It is best to preheat the entire area; if partial preheating is used, the heating zone should cover 150–200 mm on each side of the weld joint. During multi-layer welding, it is necessary to control the interlayer temperature, which should generally be no lower than the preheating temperature. To minimize the proportion of base metal melting into the weld, the weld joint can be prepared with a U-shaped or V-shaped groove. When welding to repair casting defects, the shape of the gouged groove should be smooth. For multi-pass welding, a small-diameter electrode and low welding current should be used to minimize the penetration depth. Post-weld treatment should preferably involve stress relief heat treatment before the weldment is cooled to the preheating temperature; this is especially true for thick-section workpieces or structures with high rigidity. The stress-relief heat treatment temperature is generally between 600 and 650°C. If stress-relief heat treatment cannot be carried out immediately after welding, post-heating should be performed first to allow diffused hydrogen to escape. The post-heating temperature is about 150°C, with a holding time of 2 hours. When there are no conditions for heat treatment to relieve welding stresses in hammer-peened weld metal, the method of hammer-peening the hot weld metal during welding can be used to reduce such stresses; efforts should also be made to ensure that the weld cools down slowly. Welding of high-carbon steel: High-carbon steel with a carbon content of w(C) > 0.6% has high hardenability, and it is prone to the formation of hard and brittle high-carbon martensite. Cracks tend to form in the weld and heat-affected zone, making welding difficult. Therefore, this type of steel is generally not used for fabricating welded structures; instead, it is used to make components or parts requiring high hardness or wear resistance. In most cases, welding is performed on such parts for repairing damaged items. The high hardness or high wear resistance of high-carbon steel parts is achieved through heat treatment; therefore, these parts should be annealed before welding in order to reduce welding cracks, followed by heat treatment again after welding. Welding process for high-carbon steel: High-carbon steel has poor weldability, and the welding process is as follows: 1. Groove preparation should be determined based on factors such as the thickness of the workpiece, the type of joint, and the location of the weld. 2. Select high-strength low-alloy steel electrodes with a carbon content lower than that of high-carbon steel, so as to reduce the carbon content in the weld metal. E7015(J707) or E6015(J607) welding rods can be used; when lower requirements apply, E5016(J506) or E5015(J507) welding rods can be chosen. Chromium and nickel austenitic stainless steel welding electrodes can also be used, such as E309-16 (A302), E309-15 (A307), etc.; in this case, the preheating temperature can be reduced or preheating may not be necessary at all. Before welding, the electrodes are baked at 350–400°C for 1 hour to remove moisture and crystalline water from the coating. They are then kept at 100°C and used as needed, thereby reducing the content of oxygen and hydrogen in the weld metal and preventing cracks and pores. 3. Before welding, the rust and other contaminants that could interfere with the welding process at the weld area should be removed. 4. The welding methods are as follows: a. Shielded metal arc welding: It is a convenient and flexible welding method, and it is also the most commonly used one. Low-hydrogen electrodes are used for welding; therefore, the power supply should be connected in reverse DC mode. The welding current is about 10% lower than that used for welding low-carbon steel, and the diameter of the electrodes is also reduced accordingly. b. Gas welding: For small workpieces, gas welding is a convenient method for welding. A larger welding torch and nozzles are used during welding. At the start of welding, the area around the joint is first heated (preheated) with the flame before welding begins. Due to the dispersed heat source energy in gas welding, the cooling rate of the weld can be significantly reduced, making it difficult for hardened microstructures to form. However, during operation, welding must be carried out while preheating with a flame. In gas welding, when high performance is required, a welding wire with a composition similar to that of the base material can be used ； When requirements are not high, low-carbon steel welding wire can be used. c. Flame brazing: When repairing cracks in high-carbon steel workpieces, if the strength requirements for the weld are not stringent and the operating temperature does not exceed 200°C, “run copper” can be used, that is, copper brazing. To prevent the base material from softening, when brazing high-carbon steel, the brazing temperature should be kept as low as possible. Silver-based braze materials with low melting points can be used; for example, the BAg40CuZnCdNi braze material has a crystallization temperature range of 605°C to 595°C, and a brazing temperature of 650°C is sufficient. The fluxes used are QJ101, QJ102, QJ104, FB101, or FB102; the joint gap is 0.025–0.130 mm. The heating speed during welding should be fast, and the flame should be removed from the welding area immediately after welding. Points to note during welding: 1. When welding workpieces with a thickness of less than 5 mm, weld from both sides without creating a groove; the welding electrode should be moved back and forth in a straight line. 2. When welding multi-layer U-shaped or X-shaped groove welds, the first layer should be welded using a weld rod of smaller diameter along the root of the groove, with the weld rod moved in a straight line only. For subsequent layers of welding, the circular welding motion method can be employed depending on the width of the weld. Each layer of weld should be 3–4 mm thick; after each layer is welded, the slag and spatter metal must be removed before proceeding to the next layer. When welding an X-shaped groove, welding should be carried out alternately on both sides to prevent the welded piece from bending to one side. 3. To prevent hardening of the metal structure in the heat-affected zone, the welding speed should be reduced during welding to allow the molten pool to cool slowly. Furthermore, multi-layer welding can also be employed, allowing the subsequently welded seam to pass through the hardened zone of the previous seam, thereby tempering the earlier weld. In this way, the hardened area can be reduced, but it is not possible to temper the entire hardened area; the final weld may still cause a hardened layer and cracks in the area near the weld. To overcome this phenomenon, a “annealing” weld can be applied; that is, when welding the final weld bead, the welding torch should be moved along the center of the weld to avoid contact with the base metal, thereby preventing the formation of a hardened layer on its surface. 4. For tack welding, use a small-diameter electrode to ensure full penetration. Due to the higher tendency of cracking in high-carbon steel, the tack welds should be applied for a longer time compared to welding low-carbon steel; moreover, the distance between the tack points should be reduced appropriately, and they should not rise above the weld surface. During intermittent welding, do not strike an arc on the surface of the base metal; instead, strike the arc at the front end of the weld metal. After striking the arc, return to the original molten pool and continue welding forward. At the end of welding, the crater must be filled. The deposited metal may be higher than that of a normal weld to reduce porosity and cracks at the termination point. 5. Post-weld treatment. The welded parts should be tempered at 600–700°C after welding to relieve stress and prevent cracks from forming.