All or part of the weldment is heated Immediately after welding or the process of  keeping it warm and slow is called post-heating. The purpose is to accelerate the escape of diffusible hydrogen in the welding zone to prevent “hydrogen embrittlement”, especially delayed cracking, so the post-heat is also called “dehydrogenation heat treatment“.

In the process of welding, hydrogen from electrode, flux and air is decomposed into atoms at high temperature and dissolved in liquid metal. When the weld is cooled, the solubility of hydrogen in the steel drops sharply. Because the weld is cooled fast, the hydrogen has no time to escape and remains in the weld metal. After a period of time, it will gather at the weld and the weld line. When it gathers to a certain extent, it will cause cracks in the heat affected zone of the weld under the welding stress. The cracks caused by hydrogen often occurs within hours to days after welding, so it is called delayed cracks.

Prefabrication pipeline

Hydrogen absorbed in weld and metal materials can be diffused through post heat treatment. The purpose of the so-called post-heating is to accelerate the escape of hydrogen from the welded joints. In short, it is dehydrogenation, which is an effective measure to prevent cold cracking. The post heat temperature varies according to different steel types, generally 200-350℃, which should be carried out immediately after welding. The post heat temperature is too low, and the effect of hydrogen elimination is not good. If the temperature is too high, it exceeds the end temperature of martensite transformation. It is easy to retain the residual martensite structure in the welded joint.

Post heating is not required for all metal materials during welding. It’s usually requested in cladding or overlay,like flange cladding or pipe cladding.The heating temperature and holding time of post heating depend on the material, thickness, joint form of base metal and initial hydrogen content of weld. Only low alloy steel with high strength level can produce delayed cracks, and post heating, such as chromium molybdenum steel, can be required.

As professional supplier of prefabrication pipeline or weldment, Hebei HaiHao Group accept all kinds welding works.Welcome your inquiry.Email:sales@haihaogroup.com