Is medium carbon steel difficult to weld? How should it be welded?

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Medium carbon steel refers to carbon steel with a carbon content of 0.25% to 0.60%, including 30, 35, 45, 50, 55, etc. in high-quality carbon structures and ZG230-450, ZG270-500, ZG310-570, ZG340-640 and other steel types in cast carbon steel. 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 mass fraction of carbon is close to 0.30% and the manganese content is not high, the weldability is still good, but as the carbon content increases, the weldability gradually becomes worse. When the mass fraction of carbon reaches about 0.50%, the weldability becomes significantly worse. 1. Common problems and process measures when welding medium carbon steel. Common problems when welding medium carbon steel are as follows:: 1. Cold crack problem: Due to the high carbon content of steel, hard and brittle martensite structure is easily generated in the heat-affected zone during welding, so cold cracks are prone to occur. If the welding materials are improperly selected or the welding process is improperly formulated, the weld is also prone to cold cracks. 2. Hot crack problem During welding, after the base metal with higher carbon content melts, carbon will also be brought into the weld, increasing the carbon content in the welding. Carbon can intensify the effect of sulfur, phosphorus, etc. in metals that cause hot cracks, so when welding medium carbon steel, the weld is prone to hot cracks. Especially when the sulfur and phosphorus content in the base metal or welding materials is not strictly controlled, hot cracks are more likely to occur. In addition, the high carbon content in steel also increases the tendency of CO pores in the weld seam. Process Measures Since medium carbon steel is prone to defects such as cold cracks and hot cracks during welding, some special process measures are required during welding to achieve normal 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 and is not a large-scale welded structure, the electrode arc welding method is most commonly used in production. 2. Welding materials In order to prevent cold cracks and hot cracks in the welded joints, low-hydrogen electrodes are usually used during arc welding. Low-hydrogen welding rods not only reduce the hydrogen content in the weld, but also have certain desulfurization and phosphorus effects, which can improve the plasticity and toughness of the weld. When the carbon content in the steel is low and the joints are not constrained, ilmenite or titanium-calcium electrodes can also be used for welding, but strict process measures must be taken, such as minimizing the fusion ratio, strictly preheating the weldment, and controlling the interlayer temperature. If the weldment cannot be preheated, chromium-nickel austenitic stainless steel electrodes can also be used for welding, 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 interpass temperature Preheating before welding is the most effective process measure to prevent cracks when welding medium carbon steel. Preheating can not only reduce the cooling rate of the joint to prevent the generation of martensite, but also reduce welding stress and accelerate the diffusion and escape of hydrogen. Preheating and maintaining the interpass 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 welding rod, etc. The preheating temperature can be determined through a weldability test or through the empirical formula T0 = 550 (C-0.12) + 0.4δ, where T0 represents the preheating temperature (°C), C represents the mass fraction of carbon in the welded base metal (%), and δ represents the thickness of the steel plate (mm). The preheating temperature and interpass temperature during welding of 30, 35, and 45 steel can be found in Table 1. 4. It is best to have a U-shaped or V-shaped groove on the groove type weldment, which can reduce the proportion of the base metal melting into the weld. If the casting defect is repaired by welding, the shape of the groove dug by the shovel should be rounded to reduce the amount of base metal melting into the weld. 5. Welding parameters: DC reverse polarity power supply should be used for welding. During multi-layer welding, since the proportion (mass fraction) of the base metal melted into the first layer of weld can reach up to about 30%, it is advisable to use small-diameter electrodes, low current, and slow welding speeds for welding. 6. Post-weld heat treatment: After welding, it is best to perform stress-relieving heat treatment on the weldment immediately. Especially for large thickness weldments, large rigid structural parts and weldments working under dynamic load or impact load. The temperature for stress relief tempering is generally 600~650℃. If stress relief heat treatment cannot be performed immediately after welding, post-heating should be performed, that is, heating slightly higher than the preheating temperature, and the holding time is approximately 1 hour per 10mm thickness. 2. Compilation of typical medium carbon steel welding processes (1) 35 steel and ZG270-500 cast carbon steel The mass fraction of 35 carbon steel is 0.32% to 0.39%, and the mass fraction of carbon in ZG270-500 cast carbon steel is 0.31% to 0.40%. The carbon equivalent is about 0.45%, so the weldability of this type of steel is acceptable. However, during welding, the heat-affected zone may still produce a hard and brittle martensite structure with a tendency to produce cracks, so certain process measures should be taken when welding this type of steel. 1. Selection of welding materials When arc welding, when the strength of the weld and the base metal is required to be equal, E5016 (J506) or E5015 (J507) electrodes can be used for welding. ； When the strength of the weld and the base metal is not required, E4316 (J426), E4315 (J427), E4303 (J422), E4310 (J423) and other electrodes can be used for welding. When submerged arc welding, the flux can be HJ430 or HJ431. ； Welding wires can be selected from H08MnA, H10Mn2, etc. When electroslag welding, the flux can be HJ430, HJ431, HJ360 ； Welding wires can be selected from H10Mn2, H08Mn2Si, H08Mn2SiA, etc. 2. Preheating temperature and interlayer temperature When welding 35 steel and ZG270-500 cast steel, the preheating temperature and interlayer temperature of the weldment are usually about 150℃. When the weldment is relatively rigid, the preheating temperature and interlayer temperature should be increased to 200~250℃. The heating range of local preheating is 150~200mm on both sides of the groove. 3. Post-weld heat treatment For large-thickness weldments, large rigid structural parts and weldments with working conditions of dynamic load or impact load, stress relief tempering should be carried out immediately after welding. The tempering temperature is generally 600~650℃. For weldments of normal thickness, post-heating can be used to allow the diffused hydrogen to escape. The post-heating temperature is generally 200~350℃, and the holding time is 2~6h, depending on the thickness of the weldment. (2) 45 steel and ZG310-570 cast carbon steel. The mass fraction of carbon in 45 steel is 0.42% to 0.5%, and the mass fraction of carbon in ZG310-570 cast steel is 0.41% to 0.50%. The carbon equivalent is about 0.56%, the hardening tendency is relatively large, cracks are prone to occur, and the weldability is relatively poor. 1. Selection of welding materials When arc welding, try to use low-hydrogen welding rods. When the strength of the weld and the base metal is required, E5516-G (J556) and E5515-G (J557) welding rods can be used. ； When the strength of the weld and the base metal is not required, welding rods such as E4316 (J426), E4315 (J427), E5016 (J506), E5015 (J507), E4303 (J422), and E4301 (J423) can be used. During submerged arc welding, the flux can be HJ350 or SJ101, and the welding wire can be H08MnMoA. 2. Selection of welding parameters When welding 45 steel and ZG310-570 cast carbon steel, a smaller welding current should be selected to reduce the fusion ratio of the weld and reduce the amount of carbon in the base metal transitioning into the weld. 3. Preheating temperature and interlayer temperature When welding this type of steel, it is best to use overall preheating before welding, and the preheating temperature is above 200°C. For T-shaped joints, since there are more heat dissipation directions than flat butt joints, the cooling of the welded joint will be accelerated and the tendency of cold cracks will increase. The preheating temperature should be appropriately increased. The preheating temperature is 250~400℃, depending on the thickness of the weldment. The interpass temperature should not be lower than the preheating temperature. 4. Post-weld heat treatment: The weldment should be stress-relieved and tempered immediately after welding. The tempering temperature is 600~650℃.