This type of steel as a high-strength welded structural steel, the carbon content is limited to low, usually less than 0.18% carbon mass fraction, and in the design of the alloy composition are also considered weldability requirements, so low carbon tempered steel welding is basically similar to normalized steel. The following problems mainly occur during welding.
① thermal cracking in the weld and liquefaction cracking in the heat-affected zone. Low carbon tempered steel generally low carbon content, and high manganese content, S, P control is also tighter, so the tendency of thermal cracking is smaller, but high nickel and low manganese type of low-alloy high-strength steel, it will increase the tendency of thermal cracking and liquefaction cracking.
② Cold cracking. Because this type of steel contains more alloying elements that can improve hardenability, there is a great tendency to cold cracking. However, due to the high Ms point of this type of steel, if the joint can be made to cool more slowly at that temperature, so that the generated martensite has time to carry out a "self-tempering" treatment, to a certain extent to reduce the tendency of cold cracking, so the actual tendency of cold cracking is not necessarily large.
③ Reheat cracking. Low-carbon tempered steel contains V, Mo, Nb, Cr and other strong carbide forming elements, so it has a certain tendency to reheat cracking.
④ heat-affected zone softening. Softening occurs in the area between the original tempering temperature of the base material when heating the welding temperature has been to Ac1. The lower the original tempering temperature, the greater the range of softening zone, the more severe the degree of softening.
⑤ heat-affected zone embrittlement. If the superheated zone produces low carbon martensite and volume fraction of 10%-30% of the lower bainite, high toughness can be obtained. But when the cooling rate is too fast, the formation of a volume fraction of 100% low carbon martensite, toughness will decline; when the cooling rate is too slow, on the one hand, so that the grain coarsening, on the other hand, in the superheated zone will produce low carbon martensite plus bainite plus M-A elements of the mixed organization, will make the superheated zone produce more serious embrittlement.
In welding σs ≥ 980MPa tempered steel, must be used tungsten arc welding or electron beam welding and other welding methods. For σs < 980MPa low carbon tempered steel, electrode arc welding, submerged arc automatic welding, molten gas shielded welding and tungsten arc welding can be used. But for σs ≥ 686MPa steel, molten gas shielded welding is the most appropriate automatic welding process method. In addition, if you must use multi-wire submerged arc welding and electroslag welding and other welding methods with high heat input and very low cooling rate, it is necessary to carry out post-weld tempering treatment.
When the heat input is increased to the maximum allowable value when cracking can not be avoided, preheating measures must be taken. For low-carbon tempered steel, the purpose of preheating is mainly to prevent cold cracking, and preheating may have a detrimental effect on toughness, so generally used in welding low-carbon tempered steel with a lower preheating temperature (≤ 200 ℃). Preheating mainly hopes to reduce the cooling rate of the martensite transformation, through the self-tempering effect of martensite to improve the crack resistance. When the preheating temperature is too high, not only to prevent cold and cold is not necessary, but will make 800-500 ℃ cooling rate is lower than the emergence of brittle mixed tissue critical cooling rate, so that the heat-affected zone appears obvious embrittlement, so to avoid blindly increase the preheating temperature, which also includes the interlayer temperature.
Low-carbon tempered steel after welding is generally no longer heat treatment, so in the selection of welding materials, the resulting weld metal should be close to the mechanical properties of the parent material in the weld state. In special cases, such as the stiffness of the structure is very large, cold cracking is difficult to avoid, you must choose a slightly lower strength than the base material as the filler metal.