Post weld heat treatments, or PWHT, are used to alleviate residual stresses in a weld, or the Heat Affected Zone (HAZ). Residual stresses combine with load stress and can lead to structural failure through cracking and brittle fracture. Post weld heat treatments, or PWHT, can alleviate these stresses by reheating and slowing down the rate of cooling when a weld is completed.
What is the most common post weld heat treatment?
The correct time and temperature range for a post weld heat treatment is crucial to avoid excessive losses of strength. Using temperatures and times that are too high can cause the metal to exceed its lower critical transformation temperature, which will start to transform the martensite into austenite. This will reduce the ductility of the material. This is particularly noticeable in 9%Cr steels, which are more susceptible to hydrogen cracking. This is because the martensite in the weld toe becomes coarsened and loses its pinning effect.
Locally Applied External Heating
Another method of post weld heat treatment is to apply the heating only to the area that needs it, rather than to the entire component. This is commonly used for circumferential welds on piping or closure welds on long pressure vessels. This is done by a process called induction heating, which uses an element that has a high frequency alternating current passing through it, which creates eddy currents in the metal. This in turn results in resistive heating.
The key to a successful induction heating process is the proper set-up of the equipment and facilities, the correct heating temperature, and the soaking time at the peak temperature. In addition, the heat treatment must be performed in a controlled environment that is free of contaminants and other factors that could affect the performance of the PWHT.