Regarding \"post-weld hydrogen removal treatment\"”

The process measure of heating or insulating the entire (or part of) the welded joint immediately after welding to allow it to cool slowly is called post-heating. The purpose of post-heating is to reduce the content of diffused hydrogen in the welded joint, especially in the heat-affected zone; hence it is also known as degassing treatment. It is an important process measure for welding certain low-alloy structural steels in order to prevent the formation of delayed cracks.

Post-weld stress-relief heat treatment can replace hydrogen-removal heat treatment (post-heating). For base materials with a strong tendency to harden, delayed cracks (a type of cold crack) are likely to form after welding; therefore, hydrogen removal treatment must be carried out after welding to allow as much hydrogen as possible to escape from the weld, thereby preventing the formation of delayed cracks. If stress-relief heat treatment is carried out immediately after welding, the dehydrogenation treatment can be omitted. Without post-heating, no cracks were detected; however, this does not mean that no cracks will form. Delayed cracks can occur several hours, dozens of hours, or even longer after welding, and some may only be discovered during operation. Therefore, one should not rely on luck when using this material; post-weld heat treatment or post-weld stress relief treatment must be carried out strictly in accordance with the requirements. Typically, for materials prone to delayed cracking, non-destructive testing is required to be carried out 24 hours after welding is completed.

This post was last edited by lhm217561 on 2009-10-19 10:49. During welding, hydrogen from the electrode, flux, and air is decomposed into atomic form at high temperatures and dissolves in the liquid metal. As the weld cools, the solubility of hydrogen in steel drops sharply; due to the rapid cooling of the weld, hydrogen does not have time to escape and remains in the weld metal. After some time, it accumulates at the welds and fusion lines; once it has accumulated to a certain extent, the welding stress causes cracks to form in the heat-affected zone of the weld. Cracks caused by hydrogen usually occur within a few hours to days after welding, which is why they are known as delayed cracks. The hydrogen absorbed in the welds and metal materials can be diffused out through post-heat treatment. The purpose of post-heating is to accelerate the escape of hydrogen from the welded joint; in other words, it is used to remove hydrogen, and it is an effective measure to prevent cold cracks. The post-heating temperature varies depending on the type of steel; it is generally between 200 and 350°C. It should be carried out immediately after welding. If the post-heating temperature is too low, the dehydrogenation effect is inadequate, while if it is too high and exceeds the temperature at which martensitic transformation ends, residual martensite structure may remain in the welded joint. It should be noted that not all metal materials develop delayed cracking during welding; such cracking only occurs in low-alloy steels with high strength levels, such as chromium-molybdenum steels.

I would like to ask again about the concept of \"post-heating time\". The standard states that \"post-heating time of 0.5~1 hour\" refers to the hydrogen removal being carried out within 0.5~1 hour after welding, or does it mean that the hydrogen removal process itself lasts 0.5~1 hour?

Heat treatment time. You can think of it as the holding time for hydrogen removal heat treatment.

The post-heating time of 0.5–1 hour represents the holding time for hydrogen removal during heat treatment; then, within how much time after welding is hydrogen removal effective? Within 12 hours? If the steel plate is thick and there is a pause in the process, such as going to eat, is it necessary to carry out post-heating immediately? Is post-heating only necessary when the welding thickness reaches a certain level?