Medium carbon steel is a carbon steel with a carbon content of 0.25-0.6%. Its strength and hardness are higher than that of low-carbon steel, and its ductility and toughness are lower than that of low-carbon steel. It includes most of the high quality carbon structural steel and some ordinary carbon structural steel. The hot rolled material and the cold drawn material may be directly used without heat treatment, or may be used after heat treatment. The medium carbon steel after quenching and tempering has good comprehensive properties. It is the most widely used in various applications at medium strength levels, and is used in large quantities to manufacture various mechanical parts in addition to building materials. Compared with low carbon steel, medium carbon steel has high carbon content and high strength, but its weldability is poor. The liquid-solid phase of the weld metal is large, the segregation is serious, and the tendency of hot cracking is large; since the medium carbon steel is welded, the hardened microstructure-martensite is produced in the heat-affected zone, with The carbon content increases and the tendency to harden increases. Especially when the material is thick and the rigidity is large, cold cracks are likely to occur in the heat-affected zone; when the carbon content of the weld metal is high, there is also the possibility of cold cracking.
Medium carbon steel is generally poor in weldability and is generally used as a component of a machine. The welding is usually repairable, so the most suitable method for welding medium carbon steel is electrode arc welding. Followed by other welding methods such as two welding. For the selection of welding consumables, low-hydrogen welding rods with strong S-capacitance, low diffusion hydrogen content, and good plasticity and toughness are generally used. When the strength of the weld metal and the base metal is required, select the corresponding level of low-hydrogen welding rod. When no equal strength is required, a low-hydrogen electrode with a lower strength level than the base metal is selected. If it is welded on the heat-treated part, a low-hydrogen electrode should be selected and corresponding measures should be taken to prevent cracking and softening of the heat-affected zone. When carbon dioxide gas shielded welding is used, steel wires No. 30 and No. 35 are generally selected from H08Mn2SiA, H04Mn2SiTiA and H04MnSiAlA.
When welding medium carbon steel, the welding rod must be dried and kept in advance according to the regulations; the defects to be welded before welding repair should be cleaned, and V-shaped or U-shaped groove should be used according to the actual situation. In the medium carbon steel welding, it is necessary to preheat and control the temperature between the weld bead to reduce the cooling rate of the weld metal and the heat affected zone, and improve the plasticity of the street to reduce the residual stress. The preheating temperature depends on the carbon equivalent, base metal thickness, structural stiffness and electrode type. The medium carbon steel should be immediately subjected to stress relief treatment after welding. If the heat treatment cannot be performed immediately, it should be carried out at least before cooling to the preheating temperature or the interpass temperature. The specific operation specifications are shown in Figure 1.
When welding medium carbon steel, try to use narrow bead, short arc welding; if it is multi-layer welding, several layers of welding should use small diameter welding rod, small current welding to reduce the penetration of wood, but must be penetrated . The middle layer can be welded with a higher line energy. The last one or several covers are welded as much as possible on the weld metal of the previous layer, which acts as a tempering action on the front weld bead and the heat affected zone to reduce the hardness and brittleness and prevent welding. Cracks occur before the post heat treatment.
Medium carbon steel 45 steel is a typical steel grade with a preheating temperature of 150-250 ° C; if the carbon content is too high or the thickness and stiffness are large, the crack tends to be large, and the preheating can be increased to 250-400 ° C. . If the weldment is too large and the overall preheating is difficult, the local preheating temperature can be performed, and the heating range of the local preheating is 150 to 200 mm on both sides of the welded joint. The welding rod is preferably made of a basic electrode; the fracture is opened as a U-shaped groove as much as possible. If it is a casting defect, the shape of the fracture after the excavation should be smooth, and the purpose is to reduce the proportion of the base metal melted into the weld metal. Reduce the carbon content in the weld to prevent cracks. When welding the first layer of weld metal, a small current should be used, and slow welding should be used to reduce the penetration. The stress relief treatment should be carried out immediately after welding, and the temperature of the tempering of the stress relief is 600 to 650 °C.