1. The temperature of the overheated zone is between the solidus and 1100 ℃, with a width of approximately 1-3mm. During welding, the austenite grains in this area grow severely, and after cooling, a coarse-grained overheated structure is obtained, resulting in a significant decrease in plasticity and toughness. 2. The temperature of the phase change recrystallization zone is between 1100 ℃ and Ac3, with a width of approximately 1.2-4.0mm. After welding, air cooling causes the metal in this area to undergo normalizing treatment, resulting in a uniform and fine microstructure of ferrite and pearlite, with better mechanical properties than the base metal. 3. The heating temperature of the incomplete recrystallization zone is between Ac3 and Ac1. During welding, only a portion of the structure transforms into austenite; After cooling, fine ferrite and pearlite are obtained, while the remaining parts remain the original structure, resulting in uneven grain size and poor mechanical properties. 4. If the base material undergoes cold working deformation before welding in the recrystallization zone, with a temperature between Ac1 and 450 ℃, there is also a recrystallization zone. The mechanical properties of the metal in this area have not changed much, but the plasticity has increased. If there is no cold plastic deformation before welding, there is no recrystallization zone in the heat affected zone. The structural distribution characteristics of easily quenched steel: martensite is easily formed by quenching under air cooling. For example, 18MnMoNb, 30CrMnSi, etc. 1. When welding in the fully quenched zone, the heat affected zone is above AC3. Due to the greater tendency of this type of steel to harden, the quenched structure (martensite) is obtained after welding. Near the weld seam (equivalent to the overheated zone of low-carbon steel), due to severe grain growth, coarse martensite is obtained, while small martensite is obtained in the area equivalent to the normalized zone. Depending on the cooling rate and line energy, bainite may also occur, resulting in the formation of a mixed structure coexisting with martensite. This zone belongs to the same type (martensite) in terms of organizational characteristics, but with different thicknesses, therefore it is collectively referred to as the fully quenched zone. 2. The base material in the incomplete quenching zone is heated to the heat affected zone between AC1 and AC3 temperatures. Under rapid heating conditions, ferrite rarely dissolves into austenite, while pearlite, bainite, and sorbite transform into austenite. During subsequent rapid cooling, austenite transforms into martensite. The original ferrite remains unchanged and grows to varying degrees, ultimately forming a martensite ferrite structure, hence the name incomplete quenching zone. If the carbon content and alloy element content are not high or the cooling rate is low, sorbite and pearlite may also appear. If the base material is in a quenched and tempered state before welding, the microstructure of the welding heat affected zone may undergo varying degrees of tempering treatment in addition to the fully quenched and partially quenched zones mentioned above, known as the tempering zone (areas below AC1). Under the conditions of rapid heating and continuous cooling in welding, phase transformation belongs to non equilibrium transformation. The common structures in the welding heat affected zone include ferrite, pearlite, Weinstein structure, upper bainite, lower bainite, granular bainite, low-carbon martensite, high carbon martensite, and M-A components. Under certain conditions, the types of structures that appear in the heat affected zone are mainly related to the chemical composition of the base metal and welding process conditions, and the chemical composition of the base metal is the main factor determining the structure of the heat affected zone.