Digital modelling: a tool for optimizing heat treatments

Abstract

During the manufacture, complex or large-sized pressure devices, constructed in self-hardening steels such as those of the CrMo type, can be subjected to several intermediate heat treatments, designed to prevent cold cracking, post-heating and dehydrogenation heat treatment, and to soften hardened structures in the heat-affected zone and de-stress welds (intermediate stress relieving), before the execution of the final post-weld heat treatment. In the majority of construction codes, the conditions for executing the intermediate treatments are not defined or precise enough. The present study, carried out on CrMo and CrMoV steels, made it possible to determine the effects produced by these various heat treatments, in different conditions of heat and duration, on dehydrogenation and the relaxing of residual stresses, through digital modelling. The predictions were tested with the help of experimental measurements on representative scale models (butt and tap welding of 150 mm thick in 75 passes). Optimized treatment conditions are proposed.