1. Purpose of preheating of steel pipe
- Reduce the cooling rate after welding.
- Reduce the hardening tendency and prevent cracks.
- Reduce the temperature difference in the heat-affected zone, which is conducive to reducing welding stress.
2. Scope of application of preheating of steel pipe
When welding steel with a large hardening tendency, preheating is required before welding; preheating cannot be performed when welding chromium-nickel austenitic stainless steel.
The choice of preheating should be based on the composition, thickness, structural rigidity, joint form, welding materials, welding methods and environmental factors of the steel, and determined by weldability experiments.
Heating range: Generally, a uniform heating area should be maintained within 75 to 100 mm on both sides of the groove. The temperature measurement point should be taken at the edge of the hot area. For butt joints, the heating width on each side shall not be less than 5 times the plate thickness.
3. Post-heating of steel pipe
(After welding, the steel pipe is kept warm and slowly cooled, which can slow down the cooling rate of the weld and the heat-affected zone, and play the same role as preheating).
(1) H removal treatment: Heat the steel pipe to 250-350℃ immediately after welding, keep it warm for 2-6 hours, and then air cool it.
(2) Purpose: Accelerate the escape of H in the weld metal, greatly reduce the H content in the weld and heat-affected zone, and prevent the occurrence of cold cracks.
(3) Scope of application: If the steel pipe cannot be heat treated immediately and the steel pipe must be de-H in time, it must be de-H treated immediately.
4. Post-weld heat treatment of steel pipe
(1) Meaning of post-weld heat treatment: A treatment method in which the steel pipe is heated and kept warm as a whole or locally, and then furnace-cooled or air-cooled.
(2) Functions of post-weld heat treatment:
- Reduce welding residual stress.
- Soften the hardened area.
- Improve the structure and properties of the weld and heat-affected areas.
- Improve the plasticity and toughness of the joint.
- Stabilize the size of the structure.
5. Overall heating treatment of steel pipe welding: Placing the steel pipe in a heating furnace for overall heating treatment can obtain satisfactory treatment results. The temperature of the steel pipe when entering and exiting the furnace should be below 300℃. The heating and cooling rates below 300℃ are related to the plate thickness.
For thick-walled containers, the heating and cooling rates are 50-150℃/h, and the maximum temperature difference in the furnace during overall treatment shall not exceed 50℃. If the steel pipe is too long and needs to be divided into secondary treatments, the overlapping heating part should be more than 1.5m.
Local heat treatment: For simple cylindrical containers and pipe fittings that are too long to be processed as a whole but have relatively regular shapes, local treatment can be performed. For local treatment, sufficient heating width should be ensured on both sides of the weld. The heating width of the cylinder is related to the cylinder radius and wall thickness.
Generally, post-weld heat treatment should be considered in the following cases: ordinary low-alloy steel with a high metal strength grade of steel pipes and a greater tendency to produce delayed cracks. Pressure vessels and other welded structures work at low temperatures, especially pressure vessels used below the brittle transition temperature. Ordinary low alloy steel with a higher strength grade of the base metal and a greater tendency to produce delayed cracks. Pressure vessels and other welded structures work at low temperatures, especially pressure vessels used below the brittle transition temperature. Components that withstand alternating loads and require fatigue strength. Large pressure vessels.
Post time: Apr-09-2025