Post weld heat treatment (PWHT) process refers to a welding heat treatment process in which the welds are heated to a certain temperature and held for a certain period of time after the completion of the welding work, so that the welds cool down slowly to improve the microstructure and properties of the welded joints or eliminate residual stress.Post-welding heat treatment process generally includes three processes: heating, heat preservation and cooling. These processes are interconnected and uninterruptible.
Selection of heat treatment methods Post-welding heat treatment generally uses a single high temperature tempering or normalizing and high temperature tempering treatment.For the gas welding joint using normalizing and high temperature tempering heat treatment.This is because the grain size of the welding seam and heat affected zone of gas welding is coarse, and the grain size needs to be refined, so normalizing treatment is adopted.However, single normalizing can not eliminate residual stress, so it is necessary to add high temperature tempering to eliminate stress.The single medium temperature tempering is only suitable for the assembly and welding of large ordinary low carbon steel vessels assembled on site, which aims to partially eliminate residual stress and dehydrogenation.The vast majority of occasions are to choose a single high temperature tempering.The heating and cooling of heat treatment should not be too fast, and the inner and outer walls should be uniform.
The heating method of post welding heat treatment
(1) induction heating.Steel produces induction potential in the alternating magnetic field, and the effect of eddy current and hysteresis makes steel heat, that is, induction heating.Now the project mostly uses simple power frequency induction heating equipment.
⑵ Radiative heating.Radiative heating is the radiating of heat from a heat source to a metal surface, and then the metal surface conducts the heat in other directions.Therefore, the temperature difference between the inner and outer walls of the metal is great during the radiation heating, and the heating effect is worse than that of the induction heating.Radiative heating commonly used flame heating, resistance furnace heating, infrared heating method.
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