This type of steel, used for high-strength welded structures, has a relatively low carbon content—typically below 0.18% by mass—and its alloy composition is specifically designed to meet welding requirements. Consequently, the welding process for low-carbon quenched and tempered steel is essentially similar to that of normalized steel. The following issues primarily arise during welding:

① Thermal cracks in welds and liquefaction cracks in the heat-affected zone. Low-carbon quenched and tempered steels generally have lower carbon content and higher manganese content, with stricter control over sulfur (S) and phosphorus (P), resulting in a lower tendency for thermal cracking. In contrast, high-nickel, low-manganese low-alloy high-strength steels exhibit an increased propensity for both thermal and liquefaction cracks.

② Cold cracking. Since this type of steel contains a relatively high amount of alloying elements that enhance its hardenability, it exhibits a significant tendency toward cold cracking. However, due to its high Ms temperature, if the joint cools slowly enough at this temperature, allowing the formed martensite to undergo a "self-tempering" process, the cold cracking tendency is reduced to some extent; consequently, the actual cold cracking tendency is not necessarily severe.

③ Reheat cracking. Low-carbon quenched and tempered steels contain elements such as V, Mo, Nb, and Cr that promote carbide formation, thereby exhibiting a certain tendency for reheat cracking.

④ Heat-affected zone softening. Softening occurs during welding at temperatures ranging from the original tempering temperature of the base material to Ac1. The lower the original tempering temperature, the wider the extent of the softening zone and the more severe the softening degree.

⑤ Brittleness in the heat-affected zone. The formation of low-carbon martensite and a lower bainite phase with a volume fraction of 10%-30% in the overheated zone yields high toughness. However, excessively rapid cooling leads to the formation of 100% low-carbon martensite, resulting in reduced toughness; conversely, slow cooling causes grain coarsening and the development of a mixed microstructure comprising low-carbon martensite, bainite, and M-A phase elements in the overheated zone, exacerbating brittleness.

When welding quenched and tempered steels with σs ≥ 980 MPa, welding methods such as tungsten-electrod arc welding or electron beam welding must be employed. For low-carbon quenched and tempered steels with σs <980 MPa, techniques including electrode arc welding, submerged arc automatic welding, submerged arc welding with gas shielded arc welding (SAW), and tungsten-electrod arc welding are applicable. However, for steels with σs ≥ 686 MPa, SAW is the most suitable automatic welding process. Additionally, if high-energy input and low cooling-rate welding methods such as multi-wire submerged arc welding or electroslag welding are required, post-weld quenching and tempering treatment is mandatory.

When the heat input reaches the maximum allowable value and crack formation remains unavoidable, preheating measures must be implemented. For low-carbon quenched and tempered steel, the primary purpose of preheating is to prevent cold cracking; however, preheating may adversely affect toughness. Therefore, a lower preheating temperature (≤200°C) is generally adopted during welding of such steel. Preheating aims to reduce the cooling rate during martensitic transformation and enhance crack resistance through the self-tempering effect of martensite. Excessively high preheating temperatures not only fail to prevent cold cracking but also reduce the cooling rate between 800–500°C below the critical cooling rate required for the formation of a brittle microstructure, leading to significant embrittlement in the heat-affected zone. Thus, arbitrary increases in preheating temperature—including interlayer temperature—are to be avoided.

Low-carbon conditioning steel generally does not require additional heat treatment after welding. Therefore, when selecting welding materials, the resulting weld metal must possess mechanical properties close to those of the base material in the welded state. In special cases—such as structures with high stiffness where cold cracking is difficult to avoid—it is essential to use a filler metal with slightly lower strength than the base material.