High-carbon steel has poor weldability and tends to form hardened microstructures; so what is the best way to work with it?

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High-carbon steel refers to carbon steel with a W(C) content higher than 0.6%. It has a greater tendency to harden compared to medium-carbon steel, forms high-carbon martensite, and is more susceptible to the formation of cold cracks. Meanwhile, the martensitic structure formed in the heat-affected zone during welding is hard and brittle, resulting in a **decrease in the plasticity and toughness of the joint**. Therefore, the weldability of high-carbon steel is quite poor; special welding procedures must be employed to ensure the desired properties of the joint. Therefore, it is rarely used in welded structures. High-carbon steel is mainly used for machine parts that require high hardness and wear resistance, such as shafts, large gears, and couplings. To save steel and simplify the manufacturing process, these machine components are also often assembled using welded structures. In heavy machinery manufacturing, welding issues with high-carbon steel components also arise. When formulating the welding process for high-carbon steel weldments, it is necessary to comprehensively analyze various potential welding defects and take corresponding welding process measures. 1 Weldability of high-carbon steel 1.1 Welding methods High-carbon steel is primarily used in structures that require high hardness and wear resistance; therefore, the main welding methods are shielded metal arc welding, brazing, and submerged arc welding. 1.2 Welding materials: For welding high-carbon steel, it is generally not required that the joint have the same strength as the base metal. In shielded metal arc welding, low-hydrogen electrodes are generally used, as they have strong desulfurization capabilities, a low content of diffused hydrogen in the deposited metal, and good toughness. When equal strength between the weld metal and the base material is required, low-hydrogen electrodes of the appropriate grade should be used; when equal strength is not required, low-hydrogen electrodes with a strength grade lower than that of the base material should be chosen. It is important to remember that electrodes with a strength grade higher than that of the base material must not be used. If preheating of the base material is not allowed during welding, in order to prevent cold cracks in the heat-affected zone, austenitic stainless steel electrodes can be used to obtain an austenitic structure with good plasticity and strong crack resistance. 1.3 Groove preparation: To limit the mass fraction of carbon in the weld metal, the fusion ratio should be reduced. Therefore, U-shaped or V-shaped grooves are generally used during welding. It is also important to thoroughly remove oil, rust, and other contaminants from the groove and within a 20-mm range on both sides of it. 1.4 Preheating: When welding with structural steel electrodes, preheating is necessary before welding, with the preheating temperature controlled between 250°C and 350°C. 1.5 Interpass treatment: When welding in multiple layers and passes, a weld rod with a small diameter is used for the first pass, along with a low welding current. Generally, the workpiece is placed in a semi-upward welding position or the welding rod is moved laterally, so that the entire heat-affected zone of the base metal is heated within a short time, thereby achieving preheating and heat retention effects. 1.6 Post-weld heat treatment: Immediately after welding, the workpiece is placed in a heating furnace and held at 650°C to carry out stress-relief annealing. 2 Welding defects of high-carbon steel and preventive measures: Due to the strong hardening tendency of high-carbon steel, hot cracks and cold cracks are likely to occur during welding. 2.1 Preventive measures against hot cracking 1) Control the chemical composition of the weld; strictly limit the contents of sulfur and phosphorus, and appropriately increase the manganese content to improve the weld structure and reduce segregation. 2) Control the shape of the weld cross-section; the width-to-depth ratio should be slightly larger to avoid segregation at the center of the weld. 3) For weldments with high rigidity, appropriate welding parameters, as well as the right welding sequence and direction, should be selected. 4) Preheating and slow cooling measures should be taken when necessary to prevent the formation of hot cracks. 5) Increase the alkalinity of the electrode or flux in order to reduce the impurity content in the weld and improve the degree of segregation. 2.2 Measures to prevent cold cracks 1) Preheating before welding and slow cooling after welding can not only reduce the hardness and brittleness of the heat-affected zone, but also accelerate the outward diffusion of hydrogen in the weld. 2) Select appropriate welding measures. 3) Adopt an appropriate assembly and welding sequence to reduce the restraint stress in the welded joints and improve the stress state of the welded parts. 4) Select appropriate welding materials; dry the electrodes and flux before welding, and ensure they are used immediately after being taken out. 5) Before welding, water, rust, and other contaminants on the surface of the base metal surrounding the groove should be carefully removed in order to reduce the amount of diffused hydrogen in the weld. 6) Dehydrogenation treatment should be carried out immediately before welding to allow hydrogen to escape fully from the welded joint. 7) Stress-relief annealing should be carried out immediately after welding to facilitate the outward diffusion of hydrogen in the weld. 3 Conclusion: Due to its high carbon content, high-carbon steel has good hardenability but poor weldability. During welding, it tends to form high-carbon martensite structures, which can lead to welding cracks. Therefore, when welding high-carbon steel, it is necessary to choose an appropriate welding process and take timely measures to reduce the occurrence of welding cracks and improve the performance of the welded joints.