Code for Post-Weld Heat Treatment Process of Pressure Vessels

1 Subject Content and Scope of Application

1.1 Subject Content

This Code stipulates the carbon Process requirements for post-weld heat treatment in the manufacture of steel and low-alloy steel pressure vessels.

1.2 Scope of application

This code applies to post-weld heat treatment in the manufacture of carbon steel and low alloy steel pressure vessels.

2 Use of heat treatment methods

According to the company’s equipment conditions, the following three methods can be used for post-weld heat treatment of steel pressure vessels:

2.1 Overall furnace welding Post-heat treatment, when the container is small in size and can be put into the heat treatment furnace, the entire welded container can be put into the heating furnace for post-weld heat treatment.

2.2 Post-weld heat treatment outside the overall furnace, insulate the outer shell of the container, and use various heat sources to heat the interior according to local conditions for post-weld heat treatment, such as "internal oil method" and "internal electric heating method".

2.3 Local post-weld heat treatment. When the outer dimensions of the container are large and the design drawing requires local heat treatment, the welded joints can be heat treated by the "electric heating method".

3 Preparations before post-weld heat treatment

3.1 Carefully familiarize yourself with the heat treatment process documents, master the technical requirements and processes specified in the process documents, and report any problems to the relevant technical personnel in a timely manner. Heat treatment can only be carried out after the problem is solved.

3.2 Check the tooling equipment and calibration instruments used for post-weld heat treatment. Only after confirming that the tooling and electrical equipment can be used normally, the instruments and meter indications are accurate and are within the measurement and inspection cycle. Perform post-weld heat treatment.

3.3 All welding work on the container must be completed, all process inspections before heat treatment have passed, relevant inspection records and reports have been obtained, and the quality inspection department personnel have checked and confirmed.

3.4 The product welding test plates that are heat treated at the same time as the furnace have been placed in the appropriate position in the furnace according to regulations. 4 Key points of the post-heat treatment process for the entire container in the furnace

4.1 For containers with larger diameters and thinner walls that do not have internal support rings, appropriate internal supports should be added according to process requirements to prevent deformation during heating. Saddle-type movable supports should be placed at the lower part of the vessel. The distance between the supports should not be greater than 4.5m. The supports should be placed on the same horizontal plane. Those that are not in good contact with the pressure vessel shell should be adjusted to ensure that the supports are in good contact with the vessel and are not affected by the pressure. Uniform force.

4.2 The parts of the container with sealing surfaces or high-precision screw holes should be wrapped with asbestos rope or asbestos cloth, or other

insulation materials can be used to protect them.

4.3 Post-weld heat treatment process

4.3.1 Furnace temperature: When the container is put into the furnace, the temperature in the furnace should not be higher than 400°C.

4.3.2 Heating rate: After heating to 400°C, the heating rate should not exceed 5000/δ°C/h (δ—the thickness of the steel at the weldment joint in mm) and shall not exceed 200°C/h. The minimum can be 50℃/h.

4.3.3 Heat treatment temperature difference: In order to reduce the thermal stress of the container during heating, the furnace temperature should be uniform. During heating, the furnace temperature difference should not be greater than 120°C along any length of the container within 5000mm. During heat preservation , the difference between the highest and lowest temperature in the heating zone shall not be greater than 65°C, and shall not exceed the provisions of the process documents.

4.3.4 Heat treatment temperature: The temperature of the post-weld heat treatment of the vessel depends on the steel grade used to manufacture the pressure vessel (the same applies to the steel grade with R in the tail), see Table 1

Post-weld heat treatment of commonly used materials steel pressure vessels °C scale 1

Note: If the material is used in a quenched and tempered or normalized + tempered state, the post-weld stress relief annealing temperature should be lower than the previous tempering temperature.

4.4 Heating time: In order to ensure the effect of post-weld heat treatment of the vessel, the post-weld heat treatment temperature must be determined at the specified temperature according to the type of steel used in the pressure vessel and the maximum thickness of the pressure vessel shell manufactured. The specific regulations are as follows:

1) For containers made of carbon steel, when the post-weld heat treatment temperature is within the specified range, the minimum holding time is δ/25 hours, and not less than 1/4 hour, δ is the maximum weld thickness in the heat treatment part, where the butt penetration weld does not include reinforcement. When the weld is the fillet waist height, the combined fillet weld should be the sum of both the fillet waist height and the groove depth. Larger value.

2) When the post-weld heat treatment temperature of a container made of carbon steel cannot reach the specified minimum temperature for some reason, it is allowed to perform heat treatment with a longer holding time according to Table 2.

Insulation schedule 2 for post-weld heat treatment of pressure vessels made of carbon steel at different temperatures

Note 1: This value is the minimum insulation time when the thickness is less than 25mm. When the thickness is greater than After 25mm, for every 25mm increase in thickness, the holding time should be increased by at least 15 minutes.

4.5 Cooling rate and furnace temperature

4.5.1 When performing post-weld heat treatment on containers made of steel that is not sensitive to temper brittleness, the cooling rate of the furnace should be as slow as possible. , when the temperature of the weldment is higher than 400℃, the cooling rate shall not exceed 6500/δ℃/h, and shall not exceed 260℃/h, and the minimum can be 50℃/h. It can be naturally cooled below 300°C. The temperature of the container should be lower than 400°C. It should be cooled in still air after being released from the oven.

4.5.2 For containers made of steel that is sensitive to temper brittleness, the cooling rate of the furnace and the outlet temperature of the container during post-weld heat treatment shall be implemented in accordance with the process requirements.

5 Key points of the overall post-furnace post-weld heat treatment using "internal electric heating method" after welding

5.1 Determination of temperature measurement points and arrangement of thermocouples: after the container is welded outside the furnace During heat treatment, a sufficient number of temperature measurement points must be installed on the surface of the container shell, and the temperature measurement points must be installed in representative locations. The location and number of temperature measurement points for the "internal electric heating method" are related to the number and distribution of resistance heaters arranged inside the container during annealing. Usually, several groups of resistance heaters are used to divide the interior of the container into several heating zones. In the middle of each heating zone, a temperature measurement point is determined on the top, middle, and bottom of the surface of the container shell, and a thermocouple is installed on each. , in addition, determine a temperature measurement point at the center of both ends of the container, and install a thermocouple on each. When the entire container undergoes post-weld heat treatment outside the furnace, the total number of temperature measurement points (thermocouples installed) is determined by the process technician.

5.2 Insulation materials and insulation used for post-weld heat treatment outside the overall furnace

5.2.1 Rock wool or non-alkali silicate glass wool 40~50mm thick, two layers*** 80~100mm, externally tied with 10# galvanized iron wire. In order to ensure the thermal insulation effect, the joints of single-layer insulation materials should be strict, and the joints of double-layer insulation materials should be staggered to avoid heat flow loss.

5.2.2 The quality of the insulation must be inspected after insulation. During the post-weld heat treatment process, if there is a problem with the insulation quality that may affect the effect of the post-weld heat treatment, efforts should be made immediately to repair it to ensure Heat treatment quality.

5.3 The heat treatment process is the same as 4.1~4.6.

6 Key points of the local post-weld heat treatment process using the "electric heating method"

6.1 Determination of temperature measurement points and installation of thermocouples: When performing local post-weld heat treatment on the vessel, it must be installed on the vessel shell A sufficient number of temperature measurement points are installed on the surface of the body and thermocouples are installed to accurately control the temperature changes during the heat treatment process.

6.2 Put the heating resistance wire on the insulating porcelain ring, then wrap it around the area to be annealed, and wrap it with rock wool or non-alkali silicate glass wool with a thickness of 80~100mm.

6.3 Other process requirements are the same as 4.1~4.6.

7 No matter which method is used for post-weld heat treatment of pressure vessels, it must be carried out after all welding work has been completed, all processes have been inspected, relevant information has been obtained, and it has been inspected and confirmed by the engineer responsible for quality inspection.

8 If welding repair occurs after post-weld heat treatment, post-weld heat treatment must be performed again according to the original post-weld heat treatment process requirements after repair.

9 The following work should be carried out promptly after heat treatment

9.1 Organize the heat treatment temperature, time curve and heat treatment temperature records.

9.2 Conduct mechanical performance tests on heat-treated product test panels and obtain test reports.

10 The technical measures or process documents for pressure vessel heat treatment prepared by construction technicians are specific supplements to this code. In the event of a conflict, the technical measures or process documents shall prevail.