Numerical Simulation of Post Weld Heat Treatment

In pressure equipment, welding-induced residual stresses can create manufacturing defects and increase low stress brittle fracture. Numerical simulation has become an invaluable way of predicting where this residual stress will accumulate over time.

Simulation results often vary from those obtained through in-situ testing, due to differences in time at peak temperature.

Effect of Temperature

An efficient method to evaluate the mechanical performance of pressure equipment is through Simulated Stress-Relieving Heat Treatment (SPWHT). This involves heating components to a specific temperature for an extended duration before slowly cooling them back off again. SPWHT can effectively alleviate heterogeneity and stress in welded components while increasing ductility and toughness.

However, PWHT processes may also cause detrimental microstructure changes in the coarse-grained heat-affected zone (CGHAZ) of welded components during processing, leading to decreased fracture toughness and fatigue damage. Therefore, it is crucial that we fully comprehend its effect on CGHAZ microstructure and mechanical properties before starting PWHT process.

SPWHT simulation offers an efficient and cost-effective means for assessing the effects of repeated PWHT cycles on base metal mechanical properties. Unfortunately, current simulations often employ simplified qualitative approaches and inaccurate creep constitutive models that do not take into account temperature and stress state influences on residual stresses.

Effect of Time

Time taken for PWHTs to reach stress relieve and tempering temperatures is often long enough to cause distortions or collapse of structural components, necessitating normalizing post weld heat treatments on only those structures which can sustain such high temperatures, such as those made up of large and complex shapes like pressure vessels with weld dished ends. To protect structural integrity during these high temperature treatments, only those structures which are capable of supporting themselves must undergo them, which may prove challenging when dealing with large structures like those with complex shapes such as pressure vessels needing such treatments.

X-ray diffraction and hole-drilling techniques can only measure surface residual stresses; they cannot document their dynamics during heating-holding-cooling cycles. A fundamental understanding of this behavior is essential in optimizing PWHT cycles for certain materials such as Zircaloy-4 where one PWHT cycle alone can lower transverse and longitudinal residual stresses by as much as 140 MPa; to achieve this effect requires studying strain relaxation behavior at various temperatures and holding times.

Effect of Pressure

Because of limited testing methods and cost considerations, post weld heat treatment in pressure equipment must primarily be studied using numerical simulation. It is therefore vitally important that an appropriate and reliable creep constitutive model be selected so as to achieve accurate simulation results.

As part of its welding thermal cycle, pressure vessel steel weld metal (HAZ) undergoes a complex heating-cooling cycle that leads to significant changes in microstructure and mechanical properties of its weld metal (HAZ). These changes may lead to reduced impact toughness and tensile strength as well as delayed cracking issues in its HAZ.

Researchers have developed PWHT as an approach to enhance the performance of welded structures by relaxing welding stresses and decreasing cracking risk [1, 2]. PWHT involves heating components up to an ideal temperature range before holding for an appropriate duration and cooling slowly – this treatment improves weld microstructure, reduces residual stresses and facilitates diffusible hydrogen release.

Effect of Environment

There are various simulations of post weld heat treatment (PWHT). Some involve altering temperature and duration; other research looks at environmental influences on PWHT; some studies even investigated creep during the heating stage – leading to better insight into residual stress developments during PWHT process.

Another key consideration in heat treating base metals is how it affects their mechanical properties, with chemical composition having a major influence on strength and ductility following PWHT processing. Simulated PWHT treatments can assist with identifying optimal heat treatment conditions for any given plate material.

Post weld heat treatment is an integral step in welding corrosive components such as oil rigs and LNG plants, which must endure harsh environments. It involves heating the weld area to a specified temperature range for an extended period of time before slowly cooling it to reduce residual stresses caused by welding as well as susceptibility to delayed cracking.