In the welding process, the welded metal undergoes heating, melting (or reaching thermoplastic state) and subsequent solidification and continuous cooling due to heat input and transmission, which is called welding thermal process.
The welding thermal process runs through the whole welding process and becomes one of the main factors affecting and determining the 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 welding pool is closely related to the action of heat and the length of existence time of weld pool.
3) The changes of welding heating and cooling parameters affect the solidification and phase transformation process of molten pool metal and the transformation of metal microstructure in heat affected zone.
Therefore, the microstructure and properties of weld and welding heat affected zone are also related to the effect of heat.
4) Due to uneven heating and cooling of welding parts, uneven stress state is caused, resulting in different degrees of stress, deformation and strain.
5) Under the action of welding heat, various forms of cracks and other metallurgical defects may occur due to the joint influence of metallurgy, stress factors and the structure of the welded metal.
6) The welding input heat and its efficiency determine the melting speed of base metal and electrode (welding wire), thus affecting the welding productivity.
Characteristics of welding thermal process
The welding thermal process is much more complex than that under general heat treatment conditions. It has the following four main characteristics:
a. Local concentration of welding thermal process
The weldment is not heated as a whole during welding, but the heat source is only the area near the direct action point of heating, and the heating and cooling are extremely uneven.
b. Mobility of welding heat source
In the welding process, the heat source moves relative to the weldment, and the heated area of the weldment changes constantly.
When the welding heat source approaches a certain point of the weldment, the temperature at that point increases rapidly, and when the heat source gradually moves away, the point cools down.
c. Instantaneity of welding thermal process
Under the action of highly concentrated heat source, the heating speed is very fast (more than 1500 ℃ / s in the case of arc welding), that is, a large amount of heat energy is transferred from the heat source to the weldment in a very short time, and the cooling speed is also very high due to the localization of heating and the movement of heat source.
d. Heat transfer process of weldment
The liquid metal in the welding pool is in a strong motion state.
Inside the molten pool, the heat transfer process is mainly fluid convection, while outside the molten pool, it is mainly solid heat conduction, and there are convective heat transfer and radiation heat transfer.
Therefore, the welding thermal process involves various heat transfer modes, which is a composite heat transfer problem.
Wrap it up
The above characteristics make the problem of welding heat transfer very complex.
However, because it has an important impact on the control of welding quality and the improvement of productivity, welding workers must master its basic law and the change trend under various process parameters.