Welding of low carbon quenched and tempered steel

This kind of steel is used for high strength welding structure steel, the carbon content is limited to a low level, usually the mass fraction of carbon is less than 0.18%, and the design of alloy composition has also considered the requirements of weldability, so the welding of low carbon quenched and tempered steel is basically similar to that of normalizing steel. The following problems mainly occur during welding: ① Thermal cracks in welds and liquefaction cracks in heat-affected zones. Low-carbon quenching and tempering steels generally have lower carbon content, higher manganese content and more strict control over S and P, so the tendency of hot cracking is small. However, the high-Ni and low-Mn type low-alloy high-strength steels will increase the tendency of hot cracking and liquefaction cracking. ② cold crack. Because this type of steel contains more alloying elements which can improve hardenability, it has a great tendency to cold crack. However, due to the high MS point of this kind of steel, if the joint can be cooled slowly at this temperature, so that the generated martensite can carry out a “self-tempering” treatment in time, the cold cracking tendency will be reduced to a certain extent, so in fact the cold cracking tendency is not necessarily large. ③ reheat crack. The low carbon quenched and tempered steels contain strong carbides such as V, Mo, Nb, Cr and so on, so they have a certain tendency of reheating crack. The heat affected zone is softened. Softening occurs in the area where the welding heating temperature is between the original tempering temperature of the base metal and AC1. The lower the tempering temperature, the larger the softening area, the more serious the softening degree. ⑤ Embrittlement in the heat-affected zone. High toughness can be obtained if low carbon martensite and lower bainite with a volume fraction of 10%-30% are produced in the overheated area. However, when the cooling rate is too high, the low carbon martensite with a volume fraction of 100% will be formed, and the toughness will decrease. When the cooling rate is too slow, on the one hand, the grain will be coarsened, and on the other hand, the mixed structure of low carbon martensite, bainite and M-A component will be generated in the overheated area, which will cause more serious embrittlement in the overheated area. When welding quenched and tempered steel with σs≥980MPa, welding methods such as argon tungsten arc welding or electron beam welding must be adopted. For low carbon tempered and tempered steels with σs<980MPa, electrode arc welding, submerged arc automatic welding, MGW and TGW can be used. But for the steel σs≥686MPa, MGW is the most suitable automatic welding process. In addition, if it is necessary to use multi-wire submerged arc welding and electroslag welding and other welding methods with large heat input and low cooling rate, it is necessary to conduct post-welding tempering and tempering treatment. When the heat input is increased to the maximum allowable value, the crack cannot be avoided, preheating measures must be taken. For low carbon quenched and tempered steel, the purpose of preheating is mainly to prevent cold cracks, and preheating may have adverse effects on toughness, so generally in the welding of low carbon quenched and tempered steel are used at a lower preheating temperature (≤200℃). The main hope of preheating is to reduce the cooling rate of martensite transformation and improve the crack resistance through the self-tempering effect of martensite. When the preheating temperature is too high, it is not only unnecessary to prevent cold, but will make the cooling rate of 800-500℃ lower than the critical cooling rate of brittle mixed tissue, so that the heat affected zone appears obvious embrittlement, so we should avoid blindly increasing the preheating temperature, which also includes the interlayer temperature. Low-carbon quenched and tempered steel is generally not subjected to heat treatment after welding, so in the selection of welding materials, it is required that the weld metal obtained in the welding state should have mechanical properties close to that of the base metal. In special cases, such as the stiffness of the structure is very large, cold crack is difficult to avoid, must choose a material with a slightly lower strength than the base metal as the filling metal.铜焊丝,Copper Welding Rods,铝焊丝,aluminium welding wire,镍焊条,Nickel electrode,药皮焊条,Flux Coated Brazing Welding Rods