How to carry out heat treatment when welding heat-resistant steel for pressure vessels

The welding heat treatment process for commonly used heat-resistant steel in boiler pressure vessels mainly involves heating treatment before, during, and after welding.
1、 Control of preheating temperature and interlayer temperature
But in the process of equipment manufacturing, it is also necessary to combine practical selection. For areas with thick walls and high restraint (such as large plug-in joint welds), preheating and layer temperature should be above the upper limit; If welding joints are required to have high toughness, lower preheating and layer temperatures should be taken while ensuring that no cracks occur. In addition, preheating and interlayer temperature must be lower than the Mf point (the end point of martensitic transformation) of the steel. Otherwise, after SR treatment of the welded part, residual austenite may undergo martensitic transformation, and the escape of supersaturated hydrogen will promote the cracking of the steel. Therefore, the preheating and high-level temperature of 12Cr2Mo1R should be below 300 ℃.
When steel is cut during hot cutting, similar to the heat affected zone of welding, the quenched layer at the cutting edge may become a crack source during subsequent steel rolling or stamping, so it should also be preheated appropriately. The components of the hydrogenation reactor are subjected to thermal cutting, and the heat affected zone of the incision is machined to remove the hardened layer. All welding grooves need to be machined and prepared, and pass magnetic particle inspection.
2、 Post heat and post weld heat treatment
This type of steel has a high tendency for cold cracking, and among the factors that cause cracking, hydrogen is the most important. Therefore, hydrogen must be removed immediately after welding (or during intermediate welding). Generally speaking, this type of steel container has a thick wall, high rigidity, and long manufacturing cycle. After welding, SR treatment cannot be carried out quickly. In order to prevent cracking and stabilize the size of the component, intermediate heat treatment at a lower temperature than the final post weld heat treatment is carried out after the completion of the main weld (or the main weld and the shell connection weld). The post heating of this type of steel is generally 300-350 ℃, with a few units taking 350-400 ℃.
The specifications for intermediate and final heat treatment vary depending on the steel grade, structure, and manufacturer's experience. Generally, the intermediate heat treatment temperature is (620-640 ℃) ± 15 ℃; The final post weld heat treatment temperature is (670-690 ℃) ± 15 ℃. The time for heat treatment varies with the thickness of the vessel wall.
SR treatment is mainly used to relieve stress in carbon steel or steel with low quenching tendency. Through heat treatment at 600-650 ℃, tempering structure can be obtained and residual stress can be eliminated. The main purpose of intermediate heat treatment for heat-resistant steel is to eliminate stress; The main purpose of final heat treatment is to obtain the desired microstructure through high-temperature tempering (to decompose the quenched structure, precipitate carbides, aggregate and transform into tempered martensite), while also eliminating residual stresses.