During the welding process, the metal to be welded undergoes heating, melting (or reaching a thermoplastic state) and subsequent solidification and continuous cooling due to heat input and transmission, which is called the welding heat process.
The welding heat process runs through the entire welding process, and becomes one of the main factors affecting and determining welding quality and welding productivity through the following aspects:
1) The size and distribution of heat applied to the weldment metal determine the shape and size of the molten pool.
2) The degree of metallurgical reaction in the welding pool is closely related to the effect of heat and the length of time the pool exists.
3) The change of welding heating and cooling parameters affects the solidification and phase transformation process of molten pool metal, and affects the transformation of metal microstructure in the heat-affected zone, so the structure and properties of the weld and the welding heat-affected zone are also related to the heat function related.
4) Since each part of the welding is subjected to uneven heating and cooling, resulting in uneven stress state, resulting in different degrees of stress deformation and strain.
5) Under the action of welding heat, due to the joint influence of metallurgy, stress factors and the structure of the metal to be welded, various forms of cracks and other metallurgical defects may occur.
6) The welding input heat and its efficiency determine the melting speed of the base metal and the welding rod (welding wire), thus affecting the welding productivity.
The welding heat process is much more complicated than that under general heat treatment conditions, and it has the following four main characteristics:
a. Local concentration of welding heat process
The weldment is not heated as a whole during welding, but the heat source only heats the area near the direct action point, and the heating and cooling are extremely uneven.
b. Mobility of welding heat source
During the welding process, the heat source is moving relative to the weldment, and the heated area of the weldment is constantly changing. When the welding heat source is close to a certain point of the weldment, the temperature of the point rises rapidly, and when the heat source gradually moves away, the point cools down again.
c. Transientity of welding heat process
Under the action of a highly concentrated heat source, the heating speed is extremely fast (in the case of arc welding, it can reach more than 1500°C/s), that is, a large amount of heat energy is transferred from the heat source to the weldment in a very short time, and due to the heating The cooling rate is also high due to the localization and movement of the heat source.
d. Combination of weldment heat transfer process
The liquid metal in the weld pool is in a state of intense motion. Inside the molten pool, the heat transfer process is dominated by fluid convection, while outside the molten pool, solid heat transfer is dominant, and there are also convective heat transfer and radiation heat transfer. Therefore, the welding heat process involves various heat transfer methods, which is a compound heat transfer problem.
The characteristics of the above aspects make the problem of welding heat transfer very complicated. However, because it has an important impact on the control of welding quality and the improvement of productivity, XINFA suggests that welding workers must master its basic laws and changing trends under various process parameters.
Post time: Apr-07-2023
