The Dual Process Importance Of Preheating Before Welding And Heat Treatment After Welding

In the welding process, pre-welding preheating and post-welding heat treatment two links play a vital role, they not only affect the quality and performance of welded joints, but also directly related to the stability and safety of the entire welded structure.
First, the importance of preheating before welding
Pre-welding preheating refers to the welding of the workpiece to be welded before a certain temperature under the heating process. The importance of this step is mainly reflected in the following aspects:
1. slow down the cooling rate: preheating can slow down the cooling rate after welding, is conducive to the escape of diffusion hydrogen in the weld metal, thus avoiding hydrogen cracking. At the same time, it reduces the degree of hardening of the weld and the heat-affected zone and improves the crack resistance of the welded joint.
2. Reducing welding stress: Through uniform local preheating or overall preheating, the temperature difference (temperature gradient) between the welded workpieces in the welding area can be reduced, thus reducing the welding stress and welding strain rate, and helping to avoid welding cracks.
3. Reduce the constraint of the welded structure: preheating to reduce the constraint of the angle joint is particularly obvious, with the increase in preheating temperature, the incidence of cracks will be reduced accordingly.

Preheating temperature and interlayer temperature selection needs to consider the chemical composition of steel and electrode, the rigidity of the welded structure, welding method and ambient temperature and other factors to ensure that the quality and performance of welded joints.
Second, the importance of post-weld heat treatment
Post-weld heat treatment refers to the completion of welding, the welding joints at a certain temperature under the heating and cooling treatment. Its main purpose includes hydrogen elimination, elimination of welding stress and improve the organization and overall performance of the weld.
1. Hydrogen elimination: After welding, the weld has not yet cooled to below 100 ℃ when the low-temperature heat treatment can accelerate the weld and heat-affected zone in the escape of hydrogen, to prevent low alloy steel welding weld cracks.
2. Elimination of welding stress: Welding stress generated during the welding process will reduce the actual bearing capacity of the welded joint area, resulting in plastic deformation, and even lead to the destruction of the member. Through high-temperature tempering and other heat treatment methods, you can make the weldment yield strength decline at high temperatures, to achieve the purpose of relaxation of welding stress.
3. Improve the organization and overall performance of the weld: some alloy steel materials will appear after welding hardened organization, affecting the mechanical properties of the material. After heat treatment, the metallurgical organization of the joint is improved, improving the plasticity, toughness and other comprehensive mechanical properties of welded joints.
In short, pre-welding preheating and post-welding heat treatment are two indispensable links in the welding process. They ensure that the quality and performance of welded joints meet the design requirements by controlling the temperature and cooling rate and other factors in the welding process. In practice, the appropriate preheating and heat treatment parameters and methods should be selected according to the specific circumstances.