Characteristics of the welding heat process

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During the welding process, the metal to be welded undergoes heating, melting (or reaching a thermoplastic state), followed by solidification and continuous cooling due to the input and propagation of heat; this is known as the welding thermal process. The welding heat process is present throughout the entire welding process, and through its effects in the following areas, it becomes one of the main factors that influence and determine welding quality and productivity: 1) The amount of heat applied to the weld metal and its distribution determine the shape and size of the molten pool. 2) The extent of metallurgical reactions in the welding pool is closely related to the effect of heat and the duration that the pool exists. 3) Changes in welding heating and cooling parameters affect the solidification and phase transformation processes of the molten pool metal, as well as the changes in the microstructure of the metal in the heat-affected zone; therefore, the structure and properties of the weld and the heat-affected zone are also related to the effects of heat. 4) Since various parts of the weld are subjected to uneven heating and cooling, this results in an uneven stress state, leading to stress deformation and strain of varying degrees. 5) Under the influence of welding heat, and due to the combined effects of metallurgical factors, stress, and the structure of the metals being welded, various types of cracks and other metallurgical defects can occur. 6) The heat input during welding and its efficiency determine the melting rate of the base material and the electrode (wire), thereby affecting welding productivity. The thermal process during welding is much more complex than that under normal heat treatment conditions. It has the following four main characteristics: a. Localized nature of the welding thermal process – During welding, the workpiece is not heated as a whole; instead, the heat source only heats the area surrounding the point of direct contact, resulting in extremely uneven heating and cooling. b. Mobility of the welding heat source: During welding, the heat source moves relative to the workpiece, causing the area of the workpiece that is heated to change continuously. When the welding heat source approaches a certain point on the workpiece, the temperature at that point rises rapidly; as the heat source moves away, the temperature at that point drops again. c. The instantaneous nature of the welding heat process: Under the action of a highly concentrated heat source, the heating rate is extremely fast (in the case of arc welding, it can exceed 1500°C/s). A large amount of thermal energy is transferred from the heat source to the workpiece in an extremely short time. Moreover, due to the localized nature of heating and the movement of the heat source, the cooling rate is also very high. d. Complexity of the heat transfer process in welded joints: The liquid metal in the welding pool is in a state of intense motion. Inside the molten pool, heat transfer is primarily driven by fluid convection, while outside the molten pool, it is mainly due to solid heat conduction; in addition, convective and radiative heat transfer also occur. Therefore, the welding heat process involves various heat transfer mechanisms and is a composite heat transfer problem. The characteristics mentioned above make the problem of heat transfer in welding highly complex. However, since it has a significant impact on weld quality control and productivity improvement, welders must master its basic principles as well as the trends in its behavior under various process parameters.