Regarding post-weld heat treatment

Section 8 Heat Treatment in GB150.4-2011 is explained very clearly.

In pressure vessel manufacturing, heat treatment is generally divided into two main categories: one is post-welding heat treatment； Another category is heat treatment to improve mechanical properties. The purposes of post-weld heat treatment can be summarized as follows: (1) Eliminate and reduce welding stresses ； 　　(2) Prevent cracks from forming in welded structures (such as hot cracks, cold cracks, etc.) ； 　　(3) Improve the plasticity and toughness of the welded joint area ； 　　(4) Restore the mechanical properties lost due to cold working and aging. 　　Based on the above circumstances, Clause 10.4 of GB150 specifies the situations in which post-weld heat treatment is required and the methods for such heat treatment. To ensure the achievement of the desired heat treatment results, it is necessary to properly formulate the heat treatment process, establishing clear and strict limits on all the key process parameters throughout the entire heat treatment procedure (such as the temperature upon entering the furnace, the rate of temperature increase, the temperature differences across different parts of the workpiece during heating, the atmosphere inside the furnace, the holding time, the temperature differences across different parts of the workpiece during holding, the rate of temperature decrease, and the temperature upon exiting the furnace). Strictly comply with the requirements specified in the heat treatment process specifications. Keep proper records of the actual measured heat treatment temperatures, times, etc., with complete signatures, and store them in the product quality archive. Thermal treatment instruments are regularly calibrated. 　　It is worth noting that post-weld heat treatment should also be carried out in the following cases: (1) The butt joints of flange rings made from plates or profiles shall undergo post-weld heat treatment. 　　(2) Carbon steel or low-alloy steel flanges with a flange cross-sectional thickness greater than 76 mm ； Welded integral flange ； Forged flanges shall be heat-treated by normalizing or full annealing. 　　(3) When stress corrosion resistance is required, the cold-formed sections of carbon steel and low-alloy steel heat exchange tubes, as well as the straight sections thereof covering at least 150 mm, shall undergo stress-relief heat treatment. 　　(4) Except for stainless steel, the spliced tube sheet shall undergo stress-relief heat treatment. 　　(5) Stress-relief heat treatment must be carried out on cold-formed ferritic steel expansion joints. 　　(6) For tube boxes and floating heads made of carbon steel or low-alloy steel, as well as those with lateral openings exceeding 1/3 of the cylinder’s inner diameter, stress-relief heat treatment shall be carried out after welding; the sealing surfaces of the equipment flanges shall be processed after heat treatment.

If it’s a container, does it depend on the medium inside? For example, if the medium contains hydrogen sulfide, thank you.

It’s not necessary. If it’s hydrogen sulfide, then you need to consider corrosion prevention issues.

Pressure vessels also have heat treatment to restore mechanical properties and hydrogen removal heat treatment: lol

The heat exchanger tube boxes are made of carbon steel or low-alloy steel; situations such as large openings and the presence of partitions require heat treatment

Based on your materials, there will be hydrogen sulfide stress corrosion, so heat treatment is required

You’re right too; it depends on the condition of your equipment. Under normal circumstances, you can choose to carry out treatment or not – heat treatment isn’t necessarily required

Answer it yourself: the key factors to consider are the thickness of the sheet material, the toxicity of the medium, and whether there is a tendency for stress corrosion.