Welding of low carbon tempered steel

Welding of low carbon tempered steel
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 the requirements of weldability, so low carbon tempered steel welding is basically similar to normalized steel. The following problems mainly occur when welding.
① thermal cracking in the weld and liquefaction cracking in the heat affected zone. Low-carbon tempered steel generally contains low carbon and high manganese content, and the control of S, P is also more stringent, 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. As these steels contain more alloying elements that improve hardenability, they have a high tendency to cold crack. 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 very large.
③ Reheat cracking. Low carbon tempered steels contain V, Mo, Nb, Cr and other strong carbide forming elements, and therefore have a certain tendency to reheat cracking.
④ Softening of the heat-affected zone. Softening occurs when welding the heating temperature for the original tempering temperature of the base material has been to the region between 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 in the superheated zone to produce low carbon martensite and volume fraction of 10%-30% of the lower bainite, you can get high toughness. But when the cooling rate is too fast, the formation of volume fraction of 100% low-carbon martensite, toughness will decline; when the cooling rate is too slow, on the one hand, the grain coarsening, on the other hand, in the superheated area will produce low-carbon martensite plus bainite plus M-A group elements of mixed organization, will make the superheated area produce more serious embrittlement.
In the welding of 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 suitable 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 permissible value when cracking cannot be avoided, preheating measures must be taken. For low-carbon tempered steels, the purpose of preheating is mainly to prevent cold cracking, and preheating may have a detrimental effect on toughness, so a lower preheating temperature (≤200°C) is generally used when welding low-carbon tempered steels. Preheating is mainly desired to reduce the cooling rate during martensite transformation and to improve crack resistance through the self-tempering effect of martensite. When the preheating temperature is too high, not only to prevent cold and cold is not necessary, but will make 800-500 ℃ cooling rate below the emergence of brittle mixed organization of the 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 is generally no longer heat treatment after welding, 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 when the stiffness of the structure is great and cold cracking is difficult to avoid, a material with a slightly lower strength than the base material must be chosen as the filler metal.
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