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Medium carbon steel refers to carbon steel with a carbon content of 0.25% to 0.60%, including grades such as 30, 35, 45, 50, 55 in high-quality carbon structural steels, as well as grades such as ZG230-450, ZG270-500, ZG310-570, ZG340-640 in cast carbon steels. Since medium-carbon steel has a higher carbon content than low-carbon steel, its weldability is worse than that of low-carbon steel. When the carbon mass fraction is close to 0.30% and the manganese content is low, weldability remains good; however, as the carbon content increases, weldability gradually deteriorates significantly. When the carbon mass fraction reaches around 0.50%, weldability deteriorates significantly. I. Problems that occur easily when welding carbon steel and corresponding process measures. The problems that arise frequently when welding carbon steel are as follows: 1. Cold crack problem. Due to the high carbon content in the steel, a hard and brittle martensitic structure is likely to form in the heat-affected zone during welding, which facilitates the occurrence of cold cracks. If the welding material is not selected properly or the welding process is not formulated correctly, cold cracks can also occur in the weld. 2. Thermal crack issue: During welding, when the base material with a high carbon content melts, carbon is also introduced into the weld, thereby increasing the carbon content in the weld. Carbon can exacerbate the effect of elements such as sulfur and phosphorus in metals on inducing hot cracks; therefore, hot cracks tend to occur in welds when welding carbon steel. Heat cracks are more likely to occur, especially when the sulfur and phosphorus contents in the base material or welding materials are not strictly controlled. Furthermore, a high carbon content in steel also increases the tendency for CO porosity to occur in the weld seam. Process measures: Since medium-carbon steel is prone to defects such as cold cracks and hot cracks during welding, certain special process measures must be taken to ensure proper welding. 1. Welding methods: Various arc welding methods can be used to weld medium-carbon steel. Since medium-carbon steel is mostly used to manufacture machine parts rather than large welded structures, the shielded metal arc welding method is the most widely used in production. 2. Welding materials: To prevent the formation of cold cracks and hot cracks in weld joints, low-hydrogen electrodes are generally used in shielded metal arc welding. Low-hydrogen electrodes not only result in a lower hydrogen content in the weld, but also have a desulfurizing and dephosphorizing effect, thereby improving the plasticity and toughness of the weld. When the carbon content in the steel is low and the constraint on the joint is not high, ilmenite-type or titancalcium-type electrodes can also be used for welding; however, strict process controls must be implemented, such as minimizing the fusion ratio, properly preheating the welded parts, and controlling the interpass temperature. If the weldment cannot be preheated, it is also possible to use chromium-nickel austenitic stainless steel electrodes, such as E308L-16 (A102), E308L-15 (A107), E309-16 (A302), E309-15 (A307), E310-16 (A402), E310-15 (A407), etc. 3. Preheating temperature and interlayer temperature: Preheating before welding is the most effective process measure for preventing cracks when welding carbon steel. Preheating not only reduces the cooling rate of the joint, thereby preventing the formation of martensite, but it also decreases welding stress and accelerates the diffusion and escape of hydrogen. Preheating and maintaining the interlayer temperature are required in most cases. The selection of preheating temperature and interpass temperature depends on the carbon equivalent of the steel, the thickness of the base metal, the rigidity of the structure, the type of electrode, etc. The preheating temperature can be determined through weldability tests, or it can be calculated using the empirical formula T0=550 (C-0.12)+0.4δ, where T0 represents the preheating temperature in °C, C is the mass fraction of carbon in the base material to be welded (%), and δ is the thickness of the steel plate in mm. The preheating temperature and interpass temperature for welding 30, 35, and 45 steels can be referred to in Table 1. 4. Groove type: It is preferable to use U-shaped or V-shaped grooves in the weldments, as this reduces the proportion of base metal that melts into the weld. When welding repairs are performed on castings, the shape of the gouged groove should be smooth, so as to minimize the amount of base metal that melts into the weld. 5. Welding parameters: A DC reverse polarity power supply is recommended for welding. During multi-layer welding, since the proportion (mass fraction) of the base material that melts into the first layer of weld can reach up to about 30%, it is advisable to use electrodes with a small diameter, a low current, and a slow welding speed. 6. Post-weld heat treatment: After welding, it is advisable to carry out stress-relief heat treatment on the welded part as soon as possible. This is especially true for thick-walled weldments, highly rigid structural components, and weldments that operate under dynamic or impact loads. The temperature for stress relief annealing is generally 600–650°C. If stress-relief heat treatment cannot be carried out immediately after welding, post-heating should be performed, that is, heating to a temperature slightly higher than the preheating temperature, with a holding time of approximately 1 hour per 10 mm of thickness. II. Compilation of Welding Processes for Typical Medium Carbon Steels (I) 35 Steel and ZG270-500 Cast Carbon Steel: The carbon content in 35 steel ranges from 0.32% to 0.39%, while that in ZG270-500 cast carbon steel ranges from 0.31% to 0.40%. The carbon equivalent is about 0.45%, so the weldability of this type of steel is fairly good. However, during welding, a hard and brittle martensitic structure may still form in the heat-affected zone, which tends to cause cracks; therefore, certain welding procedures must be adopted when working with this type of steel. 1. Selection of welding materials: In shielded metal arc welding, when it is required that the weld have the same strength as the base material, E5016 (J506) or E5015 (J507) electrodes can be used for welding ； When equal strength between the weld and the base metal is not required, electrodes such as E4316 (J426), E4315 (J427), E4303 (J422), and E4310 (J423) can be used for welding. For submerged arc welding, fluxes such as HJ430 and HJ431 can be used ； Welding wires such as H08MnA and H10Mn2 can be selected. For electroslag welding, fluxes such as HJ430, HJ431, and HJ360 can be used ； Welding wires such as H10Mn2, H08Mn2Si, and H08Mn2SiA can be used. 2. Preheating temperature and interpass temperature: When welding 35 steel and ZG270-500 cast steel, the preheating temperature and interpass temperature for the welded parts are usually around 150°C. When the rigidity of the welded parts is high, these temperatures should be increased to 200–250°C. The heating range for local preheating is 150–200 mm on both sides of the groove. 3. Post-weld heat treatment: For thick-walled welded parts, highly rigid structural components, and those subjected to dynamic or impact loads, stress-relief tempering should be carried out immediately after welding. The tempering temperature is generally between 600 and 650°C. For weldments of normal thickness, post-heating can be used to allow diffused hydrogen to escape. The post-heating temperature is generally 200–350°C, with a holding time of 2–6 hours, depending on the thickness of the welded parts. (II) 45 steel and ZG310-570 cast carbon steel: The carbon content in 45 steel ranges from 0.42% to 0.5%, while that in ZG310-570 cast steel ranges from 0.41% to 0.50%. The carbon equivalent is approximately 0.56%. It has a relatively high hardenability, is prone to cracking, and has poor weldability. 1. Selection of welding materials: For shielded metal arc welding, low-hydrogen electrodes should be preferred. When it is necessary for the weld to have the same strength as the base material, electrodes such as E5516-G (J556) and E5515-G (J557) can be used ； When equal strength between the weld and the base metal is not required, electrodes such as E4316 (J426), E4315 (J427), E5016 (J506), E5015 (J507), E4303 (J422), and E4301 (J423) can be used. For submerged arc welding, HJ350 or SJ101 can be used as flux, while H08MnMoA can be used as wire. 2. Selection of welding parameters: When welding 45 steel and ZG310-570 cast carbon steel, a lower welding current should be used to reduce the fusion ratio of the weld and minimize the amount of carbon from the base material that transfers into the weld. 3. Preheating temperature and interlayer temperature: When welding such steels, it is advisable to carry out overall preheating before welding, with a preheating temperature of over 200°C. For T-joints, since there are more heat dissipation directions compared to butt joints, the welding joint cools more rapidly, increasing the tendency to develop cold cracks; therefore, the preheating temperature should be increased appropriately, ranging from 250 to 400°C, depending on the thickness of the welded parts. The interlayer temperature should not be lower than the preheating temperature. 4. Post-weld heat treatment: Stress-relief tempering should be carried out immediately after welding the joint, with a tempering temperature of 600–650°C.