Welding residual stress

For CRMO steel, dehydrogenation is often carried out immediately after welding, but for ISR, it can often be delayed; how long can this delay last and how is it determined? In what other situations can the ISR be omitted and only the final heat treatment be carried out?

Generally, materials that are sensitive to H need to undergo dehydrogenation treatment as soon as possible after welding. For cases where hydrogen removal cannot be carried out immediately and it is necessary to control the weld temperature, this is because as the temperature rises, the solubility of H in the crystal increases gradually. If the temperature is reduced directly, it will cause H to escape and accumulate, leading to cracks or bubbles forming inside. For other non-H-sensitive materials, heat treatment after welding is primarily aimed at eliminating the residual stress remaining after welding, thereby gradually stabilizing the properties of the welded area through changes in temperature T. The situation in which residual stress causes cracks does not become apparent in the short term. “The premise that \"under the absence of external forces, welding stresses are balanced within the welded joint\" is based on the assumption of static space and time. Whenever the environment changes, forces are generated. It is true that the forces within the welded joint are in balance; however, the presence of adverse stresses with a strong directional component leads to a decrease in the available stress level, thereby reducing the strength of the weld under operational conditions.

Once a weld is completed, in the case of CrMo material, after dehydrogenation is carried out, until when can the intermediate heat treatment be postponed? Sometimes intermediate heat treatment is not necessary; one can simply wait for the final heat treatment. In what situations can intermediate heat treatment be skipped and it’s sufficient to wait directly for the final heat treatment?

This situation also occurs: there is a fear that cracks will form upon release. But on the other hand, \"under conditions with no external forces, the welding stresses are balanced within the welded joint.\" ”Cracks may not necessarily occur either. Direct final heat treatment often results in cracks after the process, and repairing such cracks can be quite difficult; it is therefore recommended to use ISR instead – it’s even better if no issues arise. Note that, in the absence of external forces, the welding stresses are balanced within the welded joint. ”There is nothing wrong with this statement; as a pressure vessel, it is subjected to pressure during use, which causes changes in the internal stresses. Flow, release, deformation, or even cracking are all possible. If there are problems during use, the trouble will become serious.

Whether CrMo is subjected to stress relief treatment is determined based on specific circumstances, and it is related to the material, thickness, structure, and the way the processing is carried out. For 15CrMo, we usually carry out post-weld dehydrogenation treatment, while for 12Cr2Mo1, ISR is typically used ； Ring seams are usually subjected to dehydrogenation treatment, while longitudinal seams and joint welds undergo ISR immediately after welding, without the need for DHT ； For example, in the case of thin 15CrMo steel plates, after welding we carry out DHT treatment; if the inspection 48 hours later is satisfactory, we proceed directly with PWHT. For 12Cr2Mo1, we first perform ISR, and only after that, if the results are satisfactory, do we carry out PWHT. If the preheating and DHT are carried out well according to the process, ISR may not occur sometimes.