Can post-welding full stress-relief heat treatment replace the heat treatment after cold forming of the head?

It is feasible in terms of performance and standards.

The heat treatment of cold-formed elliptical heads shall comply with the provisions of Clause 6.4.5.1 of standard GB25198.

Whether heat treatment is required can be determined in accordance with Clause 6.4.5.1 of Standard GB25198. However, if the equipment in question undergoes overall stress-relief heat treatment after welding, then the stress-relief heat treatment process specified in Clause 6.4.5.1 of GB25198 can be replaced by post-welding overall heat treatment. Of course, such overall heat treatment must meet the requirements for stress relief of the head.

The nominal thickness of the head of this container is 20 mm; the actual thickness after material feeding is usually 22 mm. In this case, the degree of deformation during cold forming exceeds 5%, necessitating heat treatment to restore its properties. According to GB150, post-weld heat treatment cannot replace performance-restoring heat treatment. In comparison, normalizing the head after hot forming or performing a recovery treatment after cold forming has little impact on costs.

Many manufacturing plants do this, using overall heat treatment in place of normal heat treatment. In fact, these two heat treatment methods have different functions and achieve different objectives. The overall heat treatment is a annealing process, aimed primarily at relieving the residual stresses in the weld; it involves cooling in a furnace, with varying heating temperatures ； Normalizing is a process to restore the mechanical properties of the material, and it involves air cooling.

For cold-formed heads, the deformation at the opening is significant; there are internal stresses present, as well as work hardening! When the strength of the base material or the formed head is high and stress levels are significant, welding it to the cylinder can lead to problems such as deformation and cracks; therefore, it is recommended to perform annealing treatment after the head is formed. For this Q245R material, the problems mentioned earlier are much less severe. If there is a tight timeline between head forming and equipment manufacturing, heat treatment should be combined in order to save costs.

1. If cold forming is used, a final overall heat treatment can be employed as a substitute for the heat treatment of the head； 2. If hot forming is used, it is necessary to determine the temperature at which hot forming takes place; if this temperature is higher than the normalizing temperature, the end cap must undergo normalizing heat treatment to restore its material properties ； If the thermoforming temperature is below the normalizing temperature, it may be sufficient to perform only a final overall heat treatment on the equipment.

This post was last edited by thinker21 on 2016-12-28 at 15:53: Cold spinning press

After carefully examining the answers provided by my friends, I found them all to be very reasonable. I then reviewed the standards again to gain a deeper understanding of this issue: 1. According to GB/T25198-2010 \"Pressure Vessel Heads\", (6.4.5.1) states that steel spherical, elliptical, disc-shaped, and flat-bottomed heads that are formed by whole-plate molding or by first assembling plates before molding should undergo heat treatment after cold forming. ) Here, the standard does not specify which heat treatment to use; our previous understanding was stress-relief heat treatment. II. For compressed components made of steel plates as specified in 8.1.1 of GB150-2011, when any one of the conditions a) to e) below is met and the degree of deformation exceeds the range given in Table 4 (5% or more for carbon steel, low-alloy steel, and other materials), appropriate treatment shall be carried out after forming to restore the properties of the material. a) Containers holding extremely or highly hazardous toxic substances ； b) Containers marked in the diagram as being subject to stress corrosion ； c) for carbon steel, low-alloy steel, with a thickness greater than 16 mm before shaping ； d) For carbon steel and low-alloy steel, the thinning rate after forming is greater than 10%. e) (Cancelled in Corrigendum No. 1.) III. Interpretation of Standard GB150-2011 regarding Article 8.1.1: 1. For cold-formed compression components, the degree of cold work hardening is quantified by the degree of deformation; when compression components made of different materials reach the corresponding deformation levels after forming, heat treatment to restore their properties should be carried out after forming. Note: “Carrying out appropriate heat treatment to restore the material’s properties” means: a) For carbon steel and low-alloy steel, this involves performing annealing at the recrystallization temperature in order to eliminate residual stresses, strain hardening, and other adverse effects. Recrystallization occurs when the annealing temperature is high enough and the time is long enough, resulting in the formation of strain-free new crystals—recrystallization nuclei—in the microstructure of deformed metals or alloys. New crystal grains continue to grow until the original deformed structure is completely eliminated, and the properties of the metal or alloy also change significantly; this process is known as recrystallization. Among them, the temperature at which new grains begin to form is called the onset recrystallization temperature, while the temperature at which the entire microstructure is occupied by new grains is called the end recrystallization temperature or complete recrystallization temperature. The temperature range over which the recrystallization process takes place is influenced by factors such as the alloy composition, degree of deformation, initial grain size, and annealing temperature. In practical applications, the arithmetic average of the starting recrystallization temperature and the ending recrystallization temperature is commonly used as a parameter to measure the thermal stability of metals or alloys; this value is referred to as the recrystallization temperature. b) For high-alloy steels such as austenitic, ferritic, and duplex stainless steels, since their microstructural properties change to varying degrees during cold forming depending on the degree of deformation, ordinary stress-relief heat treatment alone is insufficient to restore the material’s properties (including mechanical properties and corrosion resistance). Therefore, heat treatment in the condition in which the raw material was supplied is usually required in order to achieve the desired restoration of the material’s properties. In summary: 1. If the deformation rate of the head does not exceed 5% (for carbon steel and low-alloy steel), according to GB150, heat treatment to restore its properties is not required; however, per GB/T25198, such heat treatment is necessary. In my understanding, stress-relief heat treatment would suffice. If the equipment requires overall stress-relief heat treatment after welding, then that treatment can be used in place of the heat treatment required for cold-formed heads. 2. If the deformation rate of the head exceeds 5% (for carbon steel and low-alloy steel), the thickness of the material used exceeds 16 mm, or the wall thickness decreases by more than 10%, then, in accordance with GB150 requirements, recrystallization temperature annealing is necessary. The main purpose of this treatment is to address the work hardening that occurs after cold forming, as well as to relieve stresses. In such cases, there should be test plates made from the base material of the head. At this point, the post-weld stress-relief heat treatment of the equipment cannot replace the recrystallization temperature degradation treatment after cold forming of the head. 3. At this point, another question arises: why, despite the standard having been in effect for such a long time, people have not paid attention to nor understood recrystallization temperature annealing?

This post was last edited by zxhslm on 2016-12-30 08:54. The heat treatment aimed at restoring the material’s properties due to cold working hardening is discussed in Teacher Li Shiyu’s training course for pressure vessel design engineers; stress-relief annealing can be used, as well as normalizing or a combination of normalizing and tempering. In China, stress-relief annealing is the method most commonly employed. I think what the original poster said is feasible.