Properties of the heat-affected zone of welding

Welding heat affected zone (HAZ) is different from weld. The performance of weld can be guaranteed by adjusting and redistributing chemical composition and proper welding process. However, the performance of HAZ can not be adjusted by chemical composition. This is due to the uneven distribution of microstructure under the effect of thermal cycle. For general welding structure, the main consideration is the hardening of HAZ Embrittlement, toughening and softening, as well as comprehensive mechanical properties, corrosion resistance and fatigue properties should be determined according to the specific application requirements of welded structure.

The hardening of welding heat affected zone mainly depends on the chemical composition and cooling conditions of the steel to be welded, and its essence is to reflect the properties of different metallographic structures. Due to the convenience of hardness test, the higher hardness Hmax of heat affected zone (generally in the fusion zone) is commonly used to judge the properties of heat affected zone, which can indirectly judge the toughness, brittleness and crack resistance of heat affected zone.

At present, the embrittlement forms include coarse-grained embrittlement, precipitation embrittlement, microstructure transformation embrittlement, thermal strain aging embrittlement, hydrogen embrittlement and graphite embrittlement.

Toughening of welding heat affected zone (HAZ). HAZ is a non-uniform body in structure and properties, especially the fusion zone and coarse grain zone, which is the weak zone of the whole welding joint. Therefore, measures should be taken to increase the toughness of HAZ. According to the research, the following two measures can be adopted for the toughening of HAZ.

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 micro alloying elements. In this way, under the condition of welding cooling, there are dispersive strengthening particles in the heat affected zone of welding, and the microstructure with good toughness such as low-carbon martensite, lower bainite and acicular ferrite can be obtained, Grain boundary segregation should be controlled as much as possible.

2.Toughening treatment. For some important structures, post weld heat treatment is usually used to improve the performance of joints. However, for some large and complex structures, it is difficult to use local heat treatment. Reasonable welding process, correct selection of welding line energy, preheating and post heating temperature are the measures to increase welding toughness.

In addition, there are many ways to increase the toughness of HAZ. For example, fine grain steel adopts control process to further refine ferrite grains, which will also increase the toughness of material. Metallurgical refining technology can reduce the content of impurities (s, P, O, N, etc.) and increase the toughness of HAZ.

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