The welding heat affected zone (HAZ) is different from the weld. The weld can be adjusted by chemical composition, redistribution and appropriate welding process to ensure the performance requirements, while the performance of the heat affected zone cannot be adjusted by chemical composition. It is a problem of uneven tissue distribution caused by thermal cycling. For general welded structures, the hardening, embrittlement, toughening, softening, and comprehensive mechanical properties, corrosion resistance and fatigue properties of the heat affected zone are mainly considered, which should be determined according to the specific use requirements of the welded structure.
01
Hardening of welding heat affected zone
The hardness of the welding heat affected zone is mainly determined by the chemical composition and cooling conditions of the welded steel. Its essence is to reflect the performance of different metallographic structures. Since the hardness test is more convenient, the highest hardness Hmax of the heat affected zone (generally in the fusion zone) is often used to judge the performance of the heat affected zone. It can indirectly predict the toughness, brittleness and crack resistance of the heat affected zone. In recent years, the Hmax of the tail HAZ has been used as an important indicator for evaluating weldability. It should be pointed out that even the same structure has different hardness. This is related to the carbon content, alloy composition and cooling conditions of the steel.
02
Embrittlement of welding heat affected zone
Embrittlement of welding heat affected zone 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.
① Coarse grain embrittlement. Under the action of thermal cycle, grain coarsening will occur near the fusion line and overheating zone of the welded joint. Coarse grains seriously affect the brittleness of the structure. 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 intermediate phases will precipitate in the supersaturated solid solution. Due to the precipitation of these new phases, the strength, hardness and brittleness of the metal or alloy are improved. This phenomenon is called precipitation embrittlement.
③ Structural embrittlement. The embrittlement caused by the appearance of brittle and hard structures in the welding HAZ is called structural embrittlement. For commonly used low-carbon, low-alloy, high-strength steels, the structural embrittlement of the welded HAZ is mainly caused by M-A components, upper bainite, coarse Widmanstatten structure, etc. However, for steels with a high carbon content (generally ≥0.2%), the structural embrittlement is mainly caused by high-carbon martensite.
④ Thermal strain aging embrittlement of HAZ. During the manufacturing process, the welded structure must be processed, such as blanking, shearing, cold forming, gas cutting, welding and other hot processing. The local strain and plastic deformation caused by these processes have a great influence on the embrittlement of the welded HAZ, and the embrittlement caused by this is called thermal strain aging embrittlement. Strain aging embrittlement can be roughly divided into two categories: static strain aging embrittlement and dynamic strain aging embrittlement. The so-called “blue brittleness” belongs to the dynamic strain aging phenomenon.
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Toughening of welding HAZ
The welding HAZ is a non-uniform body in terms of structure and performance, especially the fusion zone and coarse grain zone are prone to embrittlement, which is the weak zone of the entire welded joint. Therefore, measures should be taken to improve the toughness of the welding HAZ. According to research, the toughening of HAZ can be achieved by the following two measures.
① Control the structure. For low alloy steel, the carbon content should be controlled so that the alloy element system is a strengthening system of low carbon trace multiple alloy elements. In this way, under the cooling conditions of welding, the HAZ is distributed with dispersed strengthening particles, and the structure can obtain low carbon martensite, lower bainite and acicular ferrite with good toughness. In addition, the grain boundary segregation should be controlled as much as possible.
② Toughening treatment. For some important structures, post-weld heat treatment is often used to improve the performance of the joint. However, for some large and complex structures, it is difficult to use local heat treatment. Reasonable formulation of welding process and correct selection of welding line energy and preheating and post-heating temperature are effective measures to improve welding toughness.
In addition, there are many ways to improve the toughness of HAZ. For example, fine-grained steel adopts controlled processes to further refine the ferrite grains, which will also improve the toughness of the material. Metallurgical refining technology can make the impurity content (S, P, O, N, etc.) in steel extremely low. These measures greatly improve the steel’s pedestrian quality, thereby also improving the toughness of the welded HAZ.
04
Softening of welding HAZ
For metals or alloys that are hardened by cold work or strengthened by heat treatment before welding, different degrees of vector strength phenomenon will generally occur in the heat affected zone of welding. The most typical example is the softening or vector strength produced in the heat affected zone after welding for high-strength steel that has been modulated and alloys with precipitation strengthening and dispersion strengthening.
When welding quenched and tempered steel, the softening degree of HAZ is related to the heat treatment state of the parent material before welding. The lower the tempering temperature of the parent material before welding, that is, the greater the degree of strengthening, the more severe the softening degree after welding. A large number of experimental studies have shown that when different welding methods and different welding line energy are used, the most obvious softening part in the HAZ is the section with a temperature between A1 and A3.
Post time: Jan-14-2025
