[Special sticker for equipment container manufacturing] What is simulated post-weld heat treatment?

Of common concern, simulated post-weld heat treatment is the heat treatment of the test plate.

This post was last edited by ztr1118 on 2012-12-27 10:49. Let me say something first, which is intended to inspire others. :lol Proposal of the concept of simulated post-weld heat treatment: In engineering practice, it has been found that after a long period of post-weld heat treatment of metal materials, the mechanical properties (mainly tensile strength) are reduced to a certain extent. In order to ensure that the mechanical properties of the material can still meet the design requirements after a long period of heat treatment, people simulate the sum of all post-weld heat treatment times that the equipment has gone through during the manufacturing process in advance, heat treat the material samples, and conduct mechanical performance tests on the samples to ensure that the material properties can still meet the design requirements. It has a history of more than 30 years to put forward the requirement of simulated post-weld heat treatment for materials in advance and use it as a material ordering requirement (actually a requirement for steel mills). Currently, simulated post-weld heat treatment issues need to be considered mainly for thick steel plates and forgings that require long-term post-weld heat treatment. GB150-2011 has not yet covered simulated post-weld heat treatment, but the HG20580-20585 series of standards has covered this part. However, in a large number of engineering standards, especially those for hydrogen-facing equipment, simulated post-weld heat treatment requirements have become one of the most important contents of the standards.

1. Materials, in addition to thick plates, especially chromium-molybdenum steel, require simulated PWHT of MAX and MIN. 2. Simulated PWHT not only simulates heat treatment in the production process, but also includes hot forming such as hot pressing. 3. For different inspection items of the sample, MAX or MIN simulated PWHT should be used respectively. For example, the bending of the sample does not need to experience the MAX PWHT. 4. In addition to the raw materials that need to be considered for simulated PWHT, the welds should also be considered. Both the welding evaluation test plate and the product welding test plate should be simulated. 5. Generally, the MAX pwht time is four times the MIN PWHT time. Some engineering companies have regulations in this regard.

Here are several standards, which all involve the concept and requirements of "simulated post-weld heat treatment". In 2000, the requirement for "simulated post-weld heat treatment of mechanical property specimen blanks" was proposed in JB/T4726-2000 "Carbon Steel and Low Alloy Steel Forgings for Pressure Vessels". In 1994, JB/T7215-1994 "Technical Conditions for Forged Welded Structure Hot Wall Hydrogenation Reactors" and JB/T7556-1994 "Technical Conditions for 2.25Cr-1Mo Steel Forgings for Hot Wall Hydrogenation Reactors" have systematically stipulated the requirements and regulations for simulated post-weld heat treatment.

This post was last edited by ztr1118 on 2012-12-27 14:35 Simulating post-weld heat treatment should simulate all post-weld heat treatment processes that the equipment goes through during the manufacturing process. :lol The post-weld heat treatment here is broad and generally refers to the Ac1 transformation temperature above 480°C (the temperature at which pearlite transforms into austenite when heated. Or all thermal processes below the lower transition temperature). It includes thermal processes such as medium temperature forming, medium temperature rounding, intermediate heat treatment, segmented heat treatment, local heat treatment and final heat treatment. However, it does not include thermal processes below 480°C such as thermal cutting, preheating before welding and hydrogen elimination heat treatment after welding, nor does it include thermal processes exceeding the Ac1 phase transition temperature such as head thermal processing, normalizing heat treatment, quenching and tempering heat treatment, etc. :lol

Generally, when calculating the heat treatment time, the time for equipment repair and re-entering the furnace for heat treatment is also considered. It is common to define a cycle of 2 to 3 hours, and then determine it according to the heat treatment procedures that may be required in the production process. Several heat treatments require several cycles. The Min heat treatment time is generally one cycle, and the MAX depends on how many heat treatments may be involved in production, and up to 1 to 2 more rework cycles may be added. When doing simulation experiments, the MAX heat treatment time can be combined according to the number of cycles. For example, if one cycle requires 3 hours, then if the MAX requires 3 times, it can be done directly for 9 hours.

The purpose of simulating post-weld heat treatment is to examine the effect of heat on material properties (mechanical properties? ), from this perspective, thermal processes above AC1, such as hot press forming, normalizing, etc., should also be included in the simulated post-weld heat treatment! Why not consider it?

Personally, I think that simulated post-weld heat treatment should mainly focus on post-weld heat treatment and hot pressing. Normalizing generally requires a base metal test plate, and normalizing has caused the material to re-phase change, which is different from the simulation purpose.

When the metal is heated, it exceeds the Ac1 transformation point and pearlite begins to transform into austenite. After cooling, a new microstructure will be obtained, and the properties of the material will also be different from the original properties. This performance change of the material is beyond the scope of discussion of simulated post-weld heat treatment.

It is the total post-weld reaction time required for the entire container to be formed and repaired. It is only for the main material test block. The test block does not need to be welded. The main purpose is to see whether the various properties of the material can meet the requirements after such a long time of heat treatment. Because no more sampling can be done after the container is formed, it can only be replaced by simulation. Generally, thick steel plates and CrMo steel require simulated post-weld heat treatment. Although simulated heat treatment such as high-temperature hot forming, normalizing and tempering of the head steel plate test block does not belong to simulated post-weld heat treatment, it must be done before simulated post-weld heat treatment (some manufacturers may only perform simulated post-weld heat treatment on the head steel plate test block, depending on its technical requirements). MIN.PWHT is the post-weld reaction time required for container molding. ； MAX.PWHT is the total post-weld reaction time required after the container is formed + reworked (usually two reworks are considered). Therefore, the technical conditions of the design institute are generally 3 times that of MIN.PWHT. There is no specific specification for this. Generally, the temperature is set by the design institute (Q345R is generally 620°C, CrMo steel is generally 675°C or 690°C). The container manufacturer determines the specific time based on the specific thickness of the material and its own process.