Dehydrogenation treatment, also called dehydrogenation heat treatment, or post-welding heat treatment.

The purpose of post-heat treatment of the weld area immediately after welding is to reduce the hardness of the weld area, or to eliminate harmful substances such as hydrogen in the weld area, at this point, the role of post-heat and post-weld heat treatment is partly the same.

The decrease of cooling rate of welding seam and welding joint by heat after welding promotes hydrogen escape and avoids hardness appreciation.

(1) The post heating for the purpose of improving the performance of the welded joint and reducing its hardness can be effective only when the welding zone is still at a higher temperature after welding.

(2) The post heating to prevent low temperature cracking is mainly to promote the full exclusion of hydrogen energy in the welding zone.

The exclusion of hydrogen depends on the temperature and holding time of the post-heat, and the temperature for the main purpose of hydrogen elimination is generally 200 to 300 degrees, and the post-heat time is 0.5 to 1 hour.

For welds under the following conditions, post-heat dehydrogenation treatment should be carried out immediately after welding (4 points) :

(1) thickness greater than 32mm, and the standard tensile strength of the material σb > 540MPa;

(2) low alloy steel material with thickness greater than 38mm;

(3) Butt weld between the embedded nozzle and the pressure vessel;

(4) Welding process evaluation to determine the need to eliminate hydrogen treatment.

The value of the afterheat temperature is usually expressed by the following formula:

Tp = 455.5[Ceq] p-111.4

Where Tp -- after 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 welding zone is one of the important effects of post-heat treatment. It is reported that at 298K, the process of hydrogen diffusing outward from the low-carbon steel weld is 1.5 to 2 months.

Increasing the temperature to 320K can shorten the process to 2 to 3 days and nights, and heating to 470K, 10 to 15h.

The main function of post-thermal and dehydrogenation treatment is to prevent the formation of cold cracks in the weld metal or in the heat-affected zone.

When the pre-welding preheating is not enough to prevent the formation of cold cracks, such as in the welding of high-restraint joints and hard-to-weld steel, the post-heating process must be used to reliably prevent the formation of cold cracks.