Welded joint

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During the welding process, the weld metal and the base metal are heated from room temperature to higher temperatures (even to melting point), and then cooled back to room temperature at different rates. Depending on the heating conditions during welding and the resulting microstructure after cooling, weld joints are divided into the weld zone, fusion zone, and heat-affected zone. Among them, the heat-affected zone is classified into the overheated zone, the normalized zone, the partially transformed zone, and the recrystallized zone, depending on the different tissue transformations that occur as a result of the exposure to heat. (1) Weld zone. Weld metal is a cast structure that results directly from the crystallization of liquid metal. The solidification and cooling of the weld metal rely primarily on heat conduction from the base metal; as a result, crystallization begins at the edge of the molten pool, with columnar crystals (having relatively large grains) growing in a direction perpendicular to the walls of the molten pool, eventually converging in the center of the pool to form eight-shaped columnar dendrites. Columnar crystals are directional and have large grain sizes. This microstructure not only has a significant impact on crack formation, but also exerts a decisive influence on the service properties of the weld. Therefore, during welding, grain refinement and improvement of chemical composition uniformity are often achieved by controlling the nucleation and grain growth during solidification, as well as by disrupting the directionality of columnar grains. The main methods include optimizing the welding process, modification treatment, and post-weld heat treatment. (2) Fusion zone. In a welded joint, the region where the weld metal transitions into the heat-affected zone is called the fusion zone, also known as the partially melted zone. Under fusion welding conditions, the range of this region is narrow, and the temperature lies between the solid and liquid phases; therefore, the microstructure includes some cast structure. Furthermore, since the austenite in this area is heated above the superheated temperature, the grain size becomes large, and the chemical composition and microstructure are highly uneven; as a result, the structure obtained after cooling is a superheated one. Therefore, the plasticity and impact toughness of this area are very poor, especially when welding dissimilar metals, where the situation becomes even more complex. In many cases, the fusion zone is the origin of cracks and local brittle failure. (3) Heat-Affected Zone. Under the action of the welding heat source, the region on both sides of the weld where changes in microstructure and properties occur is called the heat-affected zone, also known as the near-weld zone. The points in the heat-affected zone are at different distances from the heat source; therefore, they experience different heat cycles, resulting in varying changes in their structure and properties. Carbon steel and low-alloy steel, which are the most commonly used materials for power transmission and transformation equipment, generally belong to steels that are not easy to quench; that is, they have little tendency to harden when welded. Based on their similar microstructures and comparable properties, they can be divided into the following four categories. 1 Overheating zone. The overheating zone, also known as the coarse-grain zone, is located right next to the fusion zone. Its maximum temperature ranges from below the solidus line to 1100–1200°C, and its width varies depending on the welding method, being approximately between 1 and 4 mm. The metal in the overheated zone is heated to the austenite overheating temperature, resulting in a coarse-grained austenite overheated structure; upon cooling, a coarse overheated structure is obtained. The product of low-carbon steel is mainly wurtzite, while the product of low-alloy steel is wurtzite plus bainite. The plasticity and impact toughness of such overheated tissue are much lower than those of the base material; typically, the impact toughness is 25%~30% lower than that of the base material. Cracks also tend to form more easily, especially when the steel contains high levels of carbon and alloying elements, as the mechanical properties of the overheated area become even worse in such cases. Therefore, the overheated zone is also the main area that weakens the performance of the welded joint. The formation of Wei’ s structure is related to the degree of overheating, that is, the time the metal remains at high temperatures. Among welding methods such as manual arc welding, submerged arc welding, and electroslag welding, manual arc welding results in the shortest time of high-temperature exposure, so grain growth is relatively minimal. In contrast, in electroslag welding, the high-temperature exposure time is the longest; as a result, coarse Widmanstätten structure tends to form during welding. To improve the properties of the joint and eliminate severe overheated structures, normalizing heat treatment after welding is generally required. 2 Normalizing zone. The normalizing zone is also known as the fine-grain zone or recrystallization zone; in this zone, the metal is heated to a temperature slightly above the Ac₃ line, ranging from about 900 to 1200°C for low-carbon steel. Although the metal in the normalizing zone is heated to high temperatures and both ferrite and pearlite are converted into austenite, the rapid heating rate and short time at these high temperatures prevent the austenite grains from growing large. Upon cooling, uniform and fine ferrite and pearlite structures are formed, which corresponds to the normalized structure obtained through heat treatment. Therefore, the mechanical properties of this region are good, even better than those of the base material. 3 Zone of incomplete recrystallization. The zone of incomplete recrystallization is also known as the zone of partial phase transformation. The maximum heating temperature in this zone ranges from … to …; for low-carbon steel, it is approximately … For low-carbon steel and certain low-alloy steels, when the heating temperature is slightly above …, pearlite first transforms into austenite. As the temperature continues to rise, part of the ferrite gradually dissolves into austenite, and the more the temperature increases, the more ferrite dissolves, until at … all of the ferrite has dissolved in austenite. During cooling, fine ferrite precipitates again within the austenite; as cooling continues to point A₁, the residual austenite transforms into pearlite. Between..., only a portion of the tissue underwent phase transformation and recrystallization, while the remaining part consisted of the original ferrite that never dissolved into austenite. Therefore, the incomplete recrystallization zone is a mixed area of coarse and fine grains, with inconsistent grain sizes, uneven microstructure, and poor mechanical properties. 4 Recrystallization zone. The metal in this area is heated to a temperature range of 450°C down to below the Ac₁ line. Recrystallization is different from recrystallization; during recrystallization, the lattice structure within the metal changes, that is, during an allotropic transformation the metal shifts from one lattice structure to another ； During recrystallization, only the shape of the grains changes, while there is no change in the internal crystal structure. For metals that have been work-hardened prior to welding, due to lattice distortion and grain fragmentation, when they are in the recrystallization zone, heating induces recrystallization, which eliminates the lattice distortion and allows the fragmented grains to nucleate and grow again, enabling the metal to regain its mechanical properties prior to work hardening. Typical welded parts are made from steel plates in the hot-rolled or annealed state, and generally do not exhibit work hardening; therefore, there is no recrystallization zone in the heat-affected zone after welding. Furthermore, since the cooling rate of the welded joint is higher than that during the hot rolling of steel, the strength of the welded joint is greater than that of the base metal of the hot-rolled steel; however, its plasticity and impact toughness are lower in both the semi-melted zone and the overheated zone. For easily quenchable steels, such as medium-carbon steels (steels grades 35, 40, 45, 50), low-carbon quenched and tempered high-strength steels (C≤0.25%), and medium-carbon quenched and tempered high-strength steels (C=0.25%~0.45%), the microstructural distribution in the weld heat-affected zone is related to the heat treatment condition of the base metal prior to welding. If the base metal is in a normalized or annealed state prior to welding, the heat-affected zone will consist of fully quenched areas and partially quenched areas. In the fully quenched zone, a quenched microstructure appears after welding; it has high hardness and strength, but reduced plasticity and toughness, making it prone to cold cracking. The performance in the partially quenched area is uneven, with a decrease in plasticity and toughness. If the base material is in a quenched and tempered state prior to welding, a tempering softened zone will also appear in the heat-affected zone; the hardness and strength of this zone are lower than those of the base material, thereby complicating the properties of the welded joint. Therefore, for easily hardenable steels, preheating before welding and slow cooling after welding are often employed to prevent or reduce the formation of hardened structures in the weld metal or heat-affected zone. As can be seen from the above, the heat-affected zone of the welded joint suffers from coarse grains and localized embrittlement, which deteriorates the mechanical properties of the welded joint. Production experience shows that the quality of welded joints depends not only on the weld itself but also on the heat-affected zone; sometimes the problems in the heat-affected zone are even more complex than those in the weld, which is particularly evident when welding alloy steels. Therefore, it is very necessary to study the variation laws of the microstructure and properties in the heat-affected zone. At the same time, as the heat-affected zone of the welded joint widens, areas with uneven microstructure increase, leading to greater deformation of the welded part; therefore, it is better for the heat-affected zone to be as narrow as possible during welding. The size of the heat-affected zone is primarily related to the heat input per unit length during welding, that is, it is mainly determined by the welding method and welding parameters used. The more concentrated the welding heat source energy and the faster the welding speed, the smaller the heat-affected zone. Among various welding methods, electron beam welding has the smallest heat-affected zone, followed by plasma welding, TIG welding, arc welding, and gas welding. Using inappropriate welding parameters, such as too high a welding current or too slow a welding speed, will increase the width of the heat-affected zone. For important welded parts, appropriate heat treatment can be applied after welding if necessary to improve the microstructure and properties of the heat-affected zone.