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During the welding process, the weld heat-affected zone (HAZ) has properties different from those of the weld itself. The properties of welds can be met through adjustments and redistribution of chemical compositions as well as appropriate welding processes; however, the properties of the heat-affected zone cannot be altered by chemical composition, as it is a problem of uneven tissue distribution resulting from thermal cycles. For general welded structures, the main concerns are the hardening, embrittlement, toughening, and softening of the heat-affected zone, as well as overall mechanical properties, corrosion resistance, and fatigue resistance. The specific aspects to consider depend on the actual operational requirements of the welded structure. Next, we will take a closer look at these characteristics of the welding heat-affected zone. Hardening of the weld heat-affected zone: The hardness of the weld heat-affected zone is primarily determined by the chemical composition of the steel being welded and the cooling conditions; it essentially reflects the properties of different microstructural phases. Due to the simplicity of conducting hardness tests, the highest hardness Hmax in the heat-affected zone (usually in the fusion zone) is commonly used to assess the properties of this zone; it allows for an indirect estimation of the zone’s toughness, brittleness, and crack resistance. In recent years, Hmax of HAZ has become a key indicator for evaluating weldability. It should be noted that even within the same organization, hardness can vary due to differences in carbon content, alloy composition, and cooling conditions of the steel. The embrittlement of the weld heat-affected zone is often the main factor causing cracks and brittle failure in welded joints. Currently, its forms of embrittlement include coarse-grain embrittlement, precipitation embrittlement, microstructural transformation embrittlement, thermal strain aging embrittlement, hydrogen embrittlement, and graphite embrittlement, among others. Coarse-grain embrittlement: Under the action of thermal cycling, grain coarsening occurs near the weld seam and in the overheated areas of the welded joint. Excessively large grains have a significant impact on the brittleness of the material; generally, the larger the grains, the higher the temperature at which brittleness occurs. Precipitation embrittlement: During aging or tempering, carbides, nitrides, intermetallic compounds, and other metastable intermediate phases precipitate from the supersaturated solid solution. The precipitation of these new phases increases the strength and hardness of the metal or alloy, while simultaneously increasing its brittleness; this phenomenon is known as precipitation hardening. Microstructural embrittlement: It refers to the embrittlement that occurs in the welded HAZ due to the presence of brittle and hard microstructures. For commonly used low-carbon, low-alloy high-strength steels, the microstructural embrittlement of the welded HAZ is mainly caused by M-A phases, upper bainite, and coarse Widmanstatten structure. However, for steels with a higher carbon content (generally ≥0.2%), the embrittlement of the microstructure is mainly caused by high-carbon martensite. Thermal strain aging embrittlement of the HAZ: During the manufacturing of welded structures, operations such as cutting, shearing, cold forming, gas cutting, welding, and other heat treatment processes generate local strains and plastic deformations that have a significant impact on the embrittlement of the welded HAZ. The embrittlement resulting from this is known as thermal strain aging embrittlement. Strain aging embrittlement can be roughly divided into static strain aging embrittlement and dynamic strain aging embrittlement. The common “blue brittleness” is an example of dynamic strain aging. Toughening of the weld HAZ: The weld HAZ exhibits non-uniform characteristics in terms of structure and properties; in particular, the fusion zone and the coarse-grained areas are prone to embrittlement, making them the weak points of the entire welded joint. Therefore, measures need to be taken to improve the toughness of the welded HAZ. According to research, the toughening of HAZ can be approached from the following two aspects. Controlled microstructure: For low-alloy steel, it is necessary to control the carbon content and establish a strengthening mechanism based on low carbon levels along with trace amounts of various alloying elements. As a result, under welding cooling conditions, the HAZ will contain dispersed strengthening particles, enabling the formation of microstructures such as low-carbon martensite, lower bainite, and acicular ferrite, which possess good toughness. Furthermore, grain boundary segregation should be controlled as much as possible. Tempering treatment: For some critical structures, post-weld heat treatment is often employed to improve joint performance. However, for large and complex structures, even local heat treatment poses difficulties. Formulating a proper welding process, selecting the appropriate welding heat input, as well as the preheating and post-heating temperatures, are effective ways to improve welding toughness. In addition, there are many methods to improve the toughness of HAZ. For example, fine-grained steel can improve the toughness of the material by further refining the ferrite grains through controlled processing. Metallurgical refining techniques can significantly reduce the levels of impurities in steel such as S, P, O, N, etc. These measures not only improve the properties of the steel but also enhance the toughness of the welded HAZ. Softening of the weld HAZ: In metals or alloys that have been cold-worked or heat-treated to enhance their strength prior to welding, a varying degree of strength reduction typically occurs in the weld heat-affected zone. Among them, high-strength steels that have undergone quenching and tempering treatment, as well as alloys with precipitation strengthening and dispersion strengthening, are the most typical; softening or a decrease in strength occurs in the heat-affected zone after welding. When welding quenched and tempered steel, the degree of softening in the HAZ is related to the heat treatment condition of the base metal prior to welding. The lower the tempering temperature used for the heat treatment of the base material before welding, the greater the degree of strengthening, and the more severe the softening that occurs after welding. Numerous experimental studies have shown that, under different welding methods and weld heat inputs, the area in the HAZ where softening is most pronounced is the section with temperatures between A1 and A3.