Why cannot welding be performed again after heat treatment?

Welding is possible, but heat treatment is still required after welding. The heat treatment process must be the same as that used previously; of course, local heat treatment can also be applied, with the aim of ensuring that the entire weld meets the same heat treatment requirements and thus satisfies the mechanical property specifications

Because the purpose of post-weld heat treatment is to eliminate welding stresses.

It can be said that the equipment can be welded after heat treatment, but it is necessary to undergo heat treatment again in accordance with the requirements specified in the original design; depending on the circumstances, this may involve whole-body heat treatment or partial heat treatment. The purposes of heat treatment include: post-welding heat treatment, heat treatment to restore mechanical properties, heat treatment to improve mechanical properties, and hydrogen removal treatment. If welding is performed on the equipment after heat treatment, it will alter the structure and properties of the welded joint, generate welding stresses, change its mechanical properties, or lead to the formation of delayed cracks.

After product welding, there are often various types of heat treatment available, such as normalizing, tempering, or quenching and tempering. If the weld is repaired after heat treatment, the heat-treated state of the weld will be compromised, and it will be necessary to carry out the heat treatment again according to the original requirements; this increases production costs while reducing production efficiency.

This topic was covered in Kunming’s continuing education courses. Both American and Japanese standards specify that, under certain conditions, heat treatment may be omitted. China’s JB/T4708 also follows ASME; except for vessels subject to stress corrosion and those holding lethal media, heat treatment may not be required if certain conditions are met, as detailed in the standard. The most important condition among them is the thickness of the welding repair. This thickness refers to the thickness required for welding repairs, not the thickness of the steel plate or the shell

It is best not to weld parts after heat treatment, as heat treatment is carried out to ensure that the parts meet various performance requirements. If a part that has already been heat-treated is welded, its excellent properties will be compromised, because welding generates heat, which is equivalent to subjecting the part to another round of heat treatment – one of inferior quality. Moreover, performing heat treatment again after welding will damage the weld quality and reduce its strength. Therefore, welding should be avoided after heat treatment.

I agree with the person upstairs; the purpose of heat treatment is mainly to eliminate welding stresses and carry out dehydrogenation treatments, etc. If welding is carried out after heat treatment, another heat treatment must be performed according to the original requirements.

The people upstairs have already provided very comprehensive answers to the original poster’s questions. There is another situation in which welding can be carried out without the need for further heat treatment. This applies to the heat treatment required for tube boxes as specified in GB151, due to issues related to the diameter of partitions and openings. In such cases, the heat treatment is not necessary for the safety of the equipment as stipulated in GB150 and related standards; it is intended merely to prevent deformation of the flanges or partitions caused by stress release during processing or operation, which could lead to poor sealing. On the other hand, even if no heat treatment is carried out and leakage occurs or the heat exchange efficiency is low, it does not pose significant risks (in terms of human life or serious environmental pollution). This is just my personal opinion for reference only.

Post-weld heat treatment can degrade weldability and reduce weld strength. Therefore, welding should be avoided after heat treatment.

Welding after heat treatment alters the microstructure and mechanical properties of the weld joint, leading to the formation of delayed cracks. At this point, overall or partial heat treatment should be performed again.

Heat treatment involves heating a material in a certain medium, maintaining that temperature, and then cooling it, thereby altering the structural organization of the material’s surface or interior to achieve the desired mechanical or physical properties. The purpose of post-weld heat treatment is to eliminate welding stresses.

Generally, there is a misconception that parts cannot be repaired after heat treatment. However, according to Senior Ge Zhaowen’s explanation during the lecture, heat treatment is not required only if the thickness of the repaired area is equal to the standard thickness, or if that repaired thickness is no greater than the specified standard.

"Generally, there is a misconception that parts cannot be repaired after heat treatment. However, according to Senior Ge Zhaowen’s explanation during the lecture, heat treatment is not required only if the thickness of the repaired area is equal to the standard thickness, or if that repaired thickness is no greater than the specified standard. " Question: What does “thickness specified by standards” mean?