Medium carbon steel is carbon steel with a carbon content of 0.25-0.6%. Its strength and hardness are higher than low carbon steel, and its toughness and toughness are lower than low carbon steel. It includes most high quality carbon structural steel and some common carbon structural steel.
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Hot rolled and cold drawn materials may be applied directly without heat treatment or may be applied after heat treatment. Medium carbon steel after quenching and tempering has good comprehensive properties. It is most widely used in a wide range of applications in medium strength and is used in large quantities to produce various mechanical parts in addition to building materials. Compared to low carbon steel, medium carbon steel has high carbon content and high strength. But the welding ability is poor. The liquid-solid phase of the weld metal is large, the separation is serious, and the tendency of hot cracking is large. As medium carbon steel is welded, a hardened micro-martensite structure is produced in the heat-affected zone with increased carbon content and increased tendency to harden. Especially when the material thickness and 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 a possibility of cold cracking.
Medium carbon steels generally have poor weldability and are commonly used as machine components. Welding is often repairable, so the most suitable method of welding medium carbon steel is electrode arc welding. This is followed by other welding methods such as double welding. For the selection of welding consumables, low hydrogen welding rods with strong S-strong capacity, low diffusion hydrogen content, and good plasticity and ductility are often used. When the strength of the weld metal and base metal is required, select the appropriate level of low hydrogen welding wire. When equal strength is not required, low hydrogen electrodes with lower strength levels than base metals are selected. If it is bonded to heat-treated parts, low hydrogen electrodes should be selected, and relevant measures should be taken to prevent cracking and softening of the heat-affected zone. When using carbon dioxide shielding welding, No. 30 and No. 35 steel wires are generally selected from H08Mn2SiA, H04Mn2SiTiA and H04MnSiAlA.
When welding medium carbon steel, the welding wire must be dried and stored in advance according to the regulations. The resulting defect should be cleaned before welding, and the V-shaped or U-shaped groove should be used according to the actual situation. In medium carbon steel welding, it is necessary to preheat and control the temperature between the weld beads to reduce the cooling rate of the weld metal and the heat affected zone, and to improve the plasticity of the road to reduce the stress. left The preheating temperature depends on the carbon equivalent, base metal thickness, structural stiffness and electrode type. Medium carbon steels should be treated with stress relief treatment after welding. If heat treatment cannot be performed immediately, it should be performed at least prior to cooling to the preheating or interpass temperature. Specific application 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, multi-layer welding should use small welding wire, small current welding to reduce wood penetration. But must go through the penetration. The middle layer can be welded with higher line energy. One or more times the past is welded as much as possible in the weld metal of the previous layer which acts as a tempering action on the weld bead face and the heat affected network to reduce it. hard and brittle and anti-welding Cracks occur before post heat treatment.
Medium carbon steel 45 steel is a common steel grade with a high temperature of 150-250 ° C; If the carbon content is too high or the thickness and hardness are large, the crack will be large, and the preheating can be increased to 250-400 °C. If the welding is too large and the overall preheating is difficult, the preheating temperature in Local can be carried out, and the heat range of local preheating is 150 to 200mm on both sides of the weld. Welding wires are made from basic electrodes. The fracture opens as much of a U-shaped groove as possible. If it is a casting defect, the shape of the fracture after excavation should be smooth, and the purpose is to reduce the proportion of the molten base metal into the weld metal. Reduced carbon content in the weld to prevent cracking. When welding the first layer of weld metal, a small current should be used, and slow welding should be used to reduce penetration. Stress relief treatment should be carried out immediately after welding, and the stress tempering temperature is 600 to 650 °C.
