Why do we need post-weld heat treatment

Because steel pipes that transport gas or liquid need to withstand a certain pressure, the internal stress generated during the cold rolling process must be eliminated. During the welding process, due to local overheating will cause pipeline deformation and shrinkage stress, the steel pipe must be heat treated after welding to eliminate internal stress.

PWHT is based on the hardness of the steel, the thickness of the welded parts, the structural stiffness, the welding method and the conditions of use. Generally, the welding performance of carbon steel is good, and preheating and heat treatment are not required during welding. However, due to the high carbon content and high rigidity of welded parts, pre-heat treatment and heat treatment are required, such as the wall thickness of 30mm carbon steel pipe.

PWHT method
The PWHT method is electric heating. The crawler electric heater and linear electric heater should be fixed on the parts required by PWHT; they are connected to the automatic controller through cables. In addition, the thermocouple should be fixed to the parts required by the PWHT through the compensation conductor. It is connected with the computer automatic controller. Then, the controller will automatically work according to the process parameters entered into the controller.

General rules of PWHT
For stress corrosion of welds, heat treatment should be carried out after welding.
Cracks are prone to occur in steel welding, and heat treatment should be carried out in time after welding. When the heat treatment is not carried out in time, it should be heated to 200°C~300°C immediately after welding, and kept warm and slowly cooled. The heating range should be the same as that of PWHT.
After welding heat treatment and before pressure test, visual inspection and non-destructive testing shall be carried out.
Qualified PWHT parts should not be welded, otherwise they should be heat treated again.
Welding inspectors should receive professional training, and Heat Treatment Equipment personnel should perform heat treatment in accordance with the specifications and welding work instructions.
During heat treatment, temperature measurement and recording should be carried out, the location and number of temperature measuring points should be reasonable, and temperature measuring instruments should be qualified and effective.