Welding Consequent Crystallization Transformation Treatment Method
In welding processes, the crystallization structure of the welding seams is affected by various factors, including current, welding speed, electrode pressure, electrode power, welding gap, arc duration, electrode slide range, current type, heat input and joint gap fit, etc. In addition, it is also affected by the type of welding material and the pre-welding heat treatment condition. It is necessary to understand the process of crystallization transformation of the welding joint to ensure the quality of the welding joint.
Crystallization transformation is one of the most important aspects of welding processes. Crystallization transformation can be divided into welding crystallization and welding consistency. Welding consistency means adjusting the physical, chemical, mechanical and metallurgical properties of the welding joint by controlling the welding process parameters and the pre-and post-welding conditions. The crystallization transformation of the welding joint mainly refers to the change of the grain size and the composition structure of the welding joint after welding.
When the welding joint is subjected to welding process, the residual stress of the welding joint is used as the crystallization base material, and the superheating of the welding joint is carried out under the influence of various external factors. Under the action of the heat source and other external factors, the original grain structure of the welding joint gradually changes. The microstructure of the welding joint gradually becomes homogeneous and refined, the grain size gradually decreases, and the welding joint structure gradually approaches the pre-welding state, which is called welding crystallization.
The pre-welding state of the welding joint before welding has been destroyed by the welding process. The pre-existing dendritic grain boundaries have been destroyed, and the heat input makes the dendrite boundaries reconstruct and change. The refined grain boundaries of the welding joint become uniform, and the precipitation of intermetallic compounds and other substances further refine the microstructure of the welding joint. The microstructure of the welding joint gradually becomes homogeneous and refined, and the grain size gradually decreases. This process is called welding consistency.
In order to ensure the quality of welded joints, it is necessary to ensure that the crystallization transformation and welding consistency occur properly during the welding process. When the heat input is insufficient, the grain size of the welding joint becomes coarser and the grain boundary becomes coarser. In order to reduce the brittleness of the welding joint and improve the strength of the welding joint, it is necessary to carry out post-welding heat treatment to change the microstructure of the welding joint and improve its toughness and ductility.
1. Pre-welding Heat Treatment
Pre-welding heat treatment is a method of controlling welding parameters and pre-and post-welding conditions to achieve consistent transformation of the welding joints. The purpose of pre-welding heat treatment is to improve the quality of the welding joint and to eliminate the residual stress caused by welding. When pre-welding heat treatment is performed on the welding joint, the heat is conducted through the weldment and distributed on both sides of the weldment. On the one hand, the temperature at the welding joint area is the same as the temperature at the pre-welding temperature. On the other hand, the redistributed temperature gradient on both sides of the welding joint is heated according to the thermal conduction law, thereby changing the structure of the welding joint. Pre-welding heat treatment can refine the grain size of the welding joint and reduce the brittleness and residual stress of the welding joint.
2. Post-welding Heat Treatment
After welding, the welding joint is often subjected to a post-welding heat treatment to improve the mechanical properties and fatigue strength of the welding joint. The post-welding heat treatment is generally performed in order to reduce the residual stress of the welding joint and the brittleness caused by the large grain size. In general, the post-welding heat treatment technology includes recrystallization annealing and hardening. In order to achieve the uniform deposition of intermetallic compounds and other substances, after recrystallization annealing, the grain size of the welding joint becomes uniform, and the intermetallic compound particles evenly distributed in the welding joint.
The welding joint also has higher fatigue strength. In addition, hardening and tempering can also be used to further improve the strength, toughness and fatigue resistance of the welding joint. After hardening or tempering treatment, the microstructure of the welding joint can be changed, and the hardness, strength and wear resistance of the welding joint can be improved, and the brittleness at the welding joint can be reduced.
3. Conclusion
Crystallization transformation is one of the important aspects of welding processes. The quality of the welded joint is affected by various factors such as the pre-welding heat treatment and the welding process parameters. In order to ensure the quality of the welding joint, it is necessary to control the welding process and pre-and post-welding conditions. Pre-welding heat treatment can refine the grain size of the welding joint, reduce the residual stress of the welding joint, and eliminate the brittleness caused by coarse grain size. Post-welding heat treatment is mainly to perform recrystallization annealing to improve the mechanical properties of the welding joint and hardening and tempering treatment to further improve the strength, toughness and fatigue resistance.
