Dehydrogenation treatment, also known as dehydrogenation heat treatment, or post-weld heat treatment.
The purpose of post-heat treatment of the weld area immediately after welding is to reduce the hardness of the weld zone, or to remove harmful substances such as hydrogen in the weld zone. In this regard, the post-heat treatment and post-weld heat treatment have the same partial effect.
After welding, the heat reduces the cooling rate of the weld seam and the welded joint to promote the escape of hydrogen and avoid the increase in hardness.
(1) After-heating for the purpose of improving the performance of the welded joint and reducing its hardness can only be effective when the welding zone is still at a relatively high temperature after welding.
(2) Afterheating to prevent low-temperature cracks is mainly to promote the sufficient removal of hydrogen energy in the welding zone.
The removal of hydrogen depends on the temperature and holding time of post-heating. The temperature for the main purpose of hydrogen elimination is generally 200-300 degrees, and the post-heating time is 0.5-1 hour.
For welds in the following situations, post-thermal hydrogen elimination treatment should be carried out immediately after welding (4 points):
(1) Thickness greater than 32mm, and material standard tensile strength σb>540MPa;
(2) Low-alloy steel materials with a thickness greater than 38mm;
(3) The butt weld between the embedded nozzle and the pressure vessel;
(4) Welding procedure assessment determines that hydrogen elimination treatment is required.
The value of post-heat temperature is usually expressed by the following formula:
Tp＝455.5[Ceq]p－111.4
In the formula, Tp——post-heating temperature ℃;
[Ceq]p——Carbon equivalent formula.
[Ceq]p=C+0.2033Mn+0.0473Cr+0.1228Mo+0.0292Ni+0.0359Cu+0.0792Si－1.595P+1.692S+0.844V
To reduce the hydrogen content in the weld zone is one of the important effects of post heat treatment. According to reports, at 298K, the process of hydrogen diffusion from low carbon steel welds is 1.5 to 2 months.
When the temperature is increased to 320K, this process can be shortened to 2 to 3 days and nights, and after heating to 470K, it takes 10 to 15 hours.
The main function of post-heat and dehydrogenation treatment is to prevent the formation of cold cracks in the weld metal or in the heat-affected zone.
When the preheating of the weldment before welding is not enough to prevent the formation of cold cracks, such as in the welding of high-constraint joints and difficult-to-weld steels, the post-heating process must be used to reliably prevent the formation of cold cracks.
Post time: Mar-29-2023
