Changes in metal structure and properties of welded joints

The metal structure and properties of the welded joint will change, mainly affected by the thermal cycle during the welding process, resulting in different structures and properties in each area.

The welded joint is composed of weld metal, fusion zone, heat-affected zone and parent metal. The weld zone metal is solidified from the liquid metal of the molten pool, and its structure is a cast structure crystallized from the liquid metal, with coarse grains, component segregation, and loose structure. However, due to the small welding pool, fast cooling, and strict control of chemical composition, the properties of the weld metal can meet the performance requirements, especially the strength is easy to achieve. The fusion zone is the transition part between the melting zone and the non-melting zone, with uneven chemical composition and coarse structure, often coarse overheated structure or coarse hardened structure, and its performance is often the worst among welded joints. The heat affected zone is the area where the high temperature heating of the weld zone causes changes in structure and performance. According to its organizational characteristics, the heat affected zone of low carbon steel can be divided into the following six temperature zones: semi-melting zone (fusion zone), overheating zone, partial phase transformation zone (incomplete recrystallization zone), recrystallization zone, and blue brittle zone. The structure and performance of these areas are different from those of the parent material. Among them, the mechanical properties of the overheating zone and partial phase transformation zone are poor, while the performance of the recrystallization zone is improved.

During the welding process, the structural changes of steel include that most of the steel used for welding is hypoeutectoid steel. During welding, the molten pool temperature is around 1700℃. Since the molten pool temperature is in an overheated state, the temperature distribution is very uneven. When cooling slightly below the Ar3 temperature, a fully diffused high-product block (polygonal) or network-like ferrite is formed at the austenite grain boundary, which is called grain boundary free ferrite. When the degree of supercooling is very small, it is mainly formed by spontaneous nucleation. Due to the small free energy difference between the new and old phases, the incubation period of transformation is long, the nucleation rate is low, and the crystal grows coarsely under specific growth conditions. With the increase of cooling, a large block of grain boundary free ferrite changes into a grain boundary network with uneven thickness. Further increase the degree of supercooling (or reduce the formation temperature), the grain boundary ferrite network becomes thinner, but more uniform. It disappears at a certain degree of supercooling (the transformation is inhibited). Grain boundary free ferrite in carbon-manganese low alloy steel welds begins to form at 1000℃. At high temperature, ferrite is blocky, and at a low transformation temperature, it is distributed in a network at the austenite grain boundary.

In general, the changes in the organization and properties of the welded joint are caused by different thermal cycles experienced by different regions during the welding process. These changes include the cast structure of the weld zone, the transition structure of the fusion zone, and the changes in the structure and properties of the heat-affected zone. The heat-affected zone has undergone different heating and cooling processes, and its structure and properties are different from those of the parent material. It is the weakest part of the welded joint with the worst mechanical properties.