Do you know the welding heat affected zone?

Unlike welds, HAZ can be guaranteed by chemical composition adjustment, redistribution and proper welding process. However, HAZ performance can not be adjusted by chemical composition. It is a problem of uneven distribution of structure caused by thermal cycle. For general welded structures, the hardening, embrittlement, toughening and softening of HAZ, as well as the comprehensive mechanical properties, corrosion resistance and fatigue properties are mainly considered, which should be determined according to the specific application requirements of welded structures. 01 Hardening of HAZ The hardness of HAZ is mainly determined by the chemical composition and cooling conditions of the welded steel. The essence of HAZ is to reflect the properties of different metallographic structures. Because hardness test is convenient, Hmax, the highest hardness of heat-affected zone (usually in fusion zone), is often used to judge the performance of heat-affected zone. It can indirectly predict the toughness, brittleness and crack resistance of heat-affected zone. In recent years, HAZ’s Hmax has been used as an important mark to evaluate weldability. It should be pointed out that even the same structure has different hardness. This is related to the carbon content of steel, alloy composition and cooling conditions. 02 Embrittlement of welding heat affected zone The embrittlement of HAZ is often the main cause of cracking and brittle failure of welded joints. At present, the forms of embrittlement include coarse grain embrittlement, precipitation embrittlement, structural transformation embrittlement, thermal strain aging embrittlement, hydrogen embrittlement and graphite embrittlement. Coarse grain embrittlement. Under the action of thermal cycle, grain coarsening will occur near the fusion line and superheated zone of welded joint. The coarse grain size seriously affects the brittleness of the structure. Generally speaking, the coarser the grain size, the higher the brittle transition temperature. (2) precipitation embrittlement. During aging or tempering, carbides, nitrides, intermetallics and other metastable intermediates will be precipitated from the supersaturated solid solution. Because of the precipitation of these new phases, the strength, hardness and brittleness of metals or alloys are improved. This phenomenon is called precipitation embrittlement. Tissue embrittlement. The embrittlement of welded HAZ due to the appearance of brittle and hard structure is called structure embrittlement. For commonly used low carbon and low alloy high strength steel, the embrittlement of welded HAZ is mainly caused by M-A component, upper bainite and coarse Widmanstatten structure. However, for steels with high carbon content (generally (> 0.2%), the embrittlement is mainly caused by high carbon martensite. Thermal strain aging embrittlement of HAZ. In the manufacturing process, the welding structure should be processed, such as material, shearing, cold forming, gas cutting, welding and other thermal processing. The local strain and plastic deformation caused by these processes have a great influence on the embrittlement of HAZ. The embrittlement caused by these processes is called thermal strain aging embrittlement. Strain aging embrittlement can be divided into static strain aging embrittlement and dynamic strain aging embrittlement. The blue brittleness is a dynamic strain aging phenomenon. 03 Toughening of Welded HAZ Welding HAZ is a non-homogeneous body in structure and properties, especially in fusion zone and coarse grain zone, it is easy to embrittle, and it is the weak zone of the whole welding joint. Therefore, measures should be taken to improve the toughness of welded HAZ. According to the research, the toughening of HAZ can adopt the following two measures. Control organization. For low alloy steels, the carbon content should be controlled so that the alloying element system is a strengthening system of low carbon and trace alloying elements. In this way, under the cooling condition of welding, HAZ can be distributed with dispersive strengthening particles, and low carbon martensite, lower bainite and acicular ferrite can be obtained. In addition, grain boundary segregation should be controlled as far as possible. (2) Toughening treatment. For some important structures, post-weld heat treatment is often used to improve the performance of joints. But for some large and complex structures, even local heat treatment is difficult. Reasonable formulation of welding process, correct selection of welding line energy, preheating and post-heating temperature are effective measures to improve welding toughness. In addition, there are many ways to improve HAZ toughness. If fine grain steels adopt control technology to further refine ferrite grains, the toughness of materials will also be improved. Metallurgical refining technology can make the content of impurities (S, P, O, N, etc.) in steel very low. These measures improve the steel pavement and the toughness of welded HAZ. 04 Softening of Welding HAZ For metals or alloys strengthened by cold work hardening or heat treatment before welding, the sagittal strength phenomena of varying degrees usually occur in HAZ. The most typical ones are high strength steels treated by modulation and alloys with precipitation strengthening and dispersion strengthening, and softening or sagittal strength occurs in HAZ after welding. When welding tempered steel, the softening degree of HAZ is related to the heat treatment state of base metal before welding. The lower tempering temperature of quenching and tempering treatment before base metal welding, that is, the greater the degree of strengthening, the more serious the degree of softening after welding. A large number of experimental studies have shown that the most obvious softening position in HAZ is the zone between A1 and A3 when different welding methods and different welding line energies are used.铜焊丝,Copper Welding Rods,铝焊丝,aluminium welding wire,镍焊条,Nickel electrode,药皮焊条,Flux Coated Brazing Welding Rods