Properties of the weld heat-affected zone

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The weld heat-affected zone (HAZ) is different from the weld itself; the properties of the weld can be ensured by adjusting and redistributing its chemical composition as well as by using appropriate welding techniques, whereas the properties of the heat-affected zone cannot be adjusted through chemical composition – it is a problem of uneven microstructural distribution that arises as a result of thermal cycles. For general welded structures, the main considerations are the hardening, embrittlement, toughening, and softening in the heat-affected zone, as well as the overall mechanical properties, corrosion resistance, and fatigue performance. These factors must be determined based on the specific service requirements of the welded structure. 1. Hardening of the welding heat-affected zone: The hardness of the welding heat-affected zone is primarily determined by the chemical composition of the steel being welded and the cooling conditions; essentially, it relates to the properties of different microstructural phases. Since hardness testing is relatively simple, 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 can indirectly predict the toughness, brittleness, and crack resistance of the heat-affected zone. In recent years, Hmax of the tail HAZ has become an important indicator for assessing weldability. It should be noted that even within the same organization, there are different levels of hardness. This is related to the carbon content of the steel, its alloy composition, and the cooling conditions. 2. Embrittlement of the welding heat-affected zone: The embrittlement of the welding heat-affected zone is often the main cause of cracking and brittle failure in welded joints. Currently, the forms of its embrittlement include coarse-grain embrittlement, precipitation embrittlement, microstructural transformation embrittlement, thermal strain aging embrittlement, hydrogen embrittlement, and graphite embrittlement. ① Coarse-grain embrittlement. Under the action of thermal cycling, grain coarsening occurs near the weld joint’s fusion line and in the overheated zone. Coarse grains severely affect the brittleness of the material. Generally speaking, the coarser the grains, the higher the brittle transition temperature. ② Precipitation embrittlement. During aging or tempering, carbides, nitrides, intermetallic compounds, and other metastable intermediates will precipitate from the supersaturated solid solution. Due to the precipitation of these new phases, the strength, hardness, and brittleness of metals or alloys increase; this phenomenon is known as precipitation embrittlement. ③ Tissue fragility. The embrittlement that occurs in the welded HAZ due to the formation of brittle and hard tissues is known as structural embrittlement. For commonly used low-carbon, low-alloy high-strength steels, the microstructural embrittlement of the weld HAZ is mainly caused by M-A components, 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. ④ Thermal strain aging embrittlement of HAZ. During the manufacturing process, welded structures must be processed, such as cutting, shearing, cold forming, gas cutting, welding, and other heat treatment processes. The local strains and plastic deformations caused by these processes have a significant effect on the embrittlement of the welded HAZ; this type of embrittlement is known as thermal strain aging embrittlement. Strain aging embrittlement can generally be divided into two categories: static strain aging embrittlement and dynamic strain aging embrittlement. The so-called “blue brittleness” falls under the category of dynamic strain aging phenomena. 3. Toughening of the welded HAZ: The welded HAZ is a heterogeneous region in terms of structure and properties; in particular, the fusion zone and the areas with coarse grains are prone to embrittlement, making them the weak points of the entire welded joint. Therefore, measures should be taken to improve the toughness of the welded HAZ. According to research, the toughening of HAZ can be achieved through the following two approaches. ① Control tissue. For low-alloy steel, the carbon content should be controlled so that the alloying element system consists of a low-carbon configuration with multiple trace alloying elements to achieve strengthening. In this way, under the cooling conditions of welding, the HAZ is enriched with 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. ② Tempering treatment. For some important structures, post-weld heat treatment is often used to improve the properties of the joints. However, for some large and complex structures, it is difficult to apply localized heat treatment even then. Formulating a reasonable welding process and selecting the appropriate welding heat input as well as preheating and post-heating temperatures are effective measures to improve welding toughness. Furthermore, there are many ways to improve the toughness of HAZ. In fine-grained steels, by using controlled processes to further refine the ferrite grains, the toughness of the material is also improved. Metallurgical refining techniques can reduce the content of impurities in steel (such as S, P, O, N, etc.) to very low levels. These measures improve the quality of steel walkways, thereby enhancing the toughness of the welded HAZ as well. 4. Softening of the weld HAZ: In metals or alloys that have been cold-worked or heat-treated to enhance their strength prior to welding, varying degrees of softening occur in the weld heat-affected zone. This phenomenon is most typical in highly strength-enhanced steels that have undergone modification treatments, as well as in alloys that benefit from precipitation strengthening and dispersion strengthening; softening or strength reduction 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 quenching and tempering treatment of the base material before welding, that is, the greater the degree of strengthening, the more severe the softening that occurs after welding. Numerous experimental studies have shown that, for different welding methods and different welding heat inputs, the area in the HAZ where softening is most pronounced is the zone with temperatures between A1 and A3.