Welding heat process and its characteristics

Generally, the welding part needs to go through the process of heating melting metallurgical reaction solidification crystallization solid phase transformation joint formation, which can also be summarized as the following three interrelated processes.

1.Welding thermal process

Under the action of welding heat source, the metal is heated and melted locally, and the phenomenon of heat transmission and distribution occurs at the same time. This phenomenon runs through the whole welding process, that is, the welding thermal process and the welding physical and chemical process occur and develop in this process, which directly affects the welding quality and productivity.

2.Welding metallurgy process

At high temperature, a series of Chemical Metallurgical reactions take place among molten metal, slag and gas phase, which directly affect the composition, microstructure and properties of weld metal.

3.Metal crystallization and phase transformation during welding

Under the welding condition, the molten metal will be cooled continuously and rapidly after the heat source leaves, and the process of crystallization and intersection will take place to form the weld. In this process, the defects such as segregation, inclusion, porosity, hot crack, hardening, embrittlement and cold crack may appear in the weld metal. Controlling and adjusting the crystallization and phase transformation of the weld metal is another key to ensure the welding quality, The base metal on both sides of the weld is affected by heat due to heat conduction, so the microstructure changes, forming the welding heat affected zone (HAZ), which may lead to defects or performance deterioration in this area.

Characteristics of welding thermal process

1.Localization of thermal action

The localization of heat action refers to the welding heat concentration in the joint part of the welding part, rather than uniformly heating the whole welding part. Unlike the metal heat treatment, uneven heating is the basic feature of welding process.

2.Instantaneity of thermal action

In the welding process, the heat source moves at a certain speed, and the heating effect of any point of the weldment is instantaneous, that is, with the change of time, under the action of the concentrated heat source, the heating speed is very fast (the arc welding temperature is above 1500 ℃ · s-1), and the heat is transferred from the heat source to the weldment in a very short time.

As the heat source moves forward, the metal heated to high temperature will rapidly export heat and cool down. The heating temperature of each point on the weldment changes continuously, which indicates that the heat transfer process is unstable.

The internal quality of weld metal, the change of microstructure and properties of heat affected zone, the stress state of welded joint and the welding productivity are directly affected by the thermal process. The welding thermal process determines the temperature and existence time of the weld pool. The temperature and time directly affect the physical and chemical reaction of the weld pool metal. If the reaction is not complete, there will be segregation in the weld metal In the process of welding heating, due to heat conduction, the structure and properties of the base metal near the seam will change, which is related to the nature of welding heat source, heating time and cooling rate, and may produce hardening, embrittlement or softening phenomenon in this region.

Welding is a process of non-uniform heating and cooling. There are different degrees of thermal elastoplastic changes in the joint area, resulting in non-uniform stress state and deformation after welding. When the welding stress and metallurgical factors work together, cracks may appear. Improving the melting rate of base metal and filler material is an important way to improve the welding productivity, and the melting rate depends on the effect of heat, The welding thermal process has an impact on the welding production rate. In order to study the welding thermal process, attention should be paid to the characteristics of the heat source and the heat transfer characteristics of the welded metal (base metal), including the welding heat source, welding temperature field, basic law of welding heat transfer, heating and melting of base metal and welding material, welding thermal cycle, etc.

Basic knowledge about welded pipe