The purpose of DHT is to drive out hydrogen to minimize the risk of hydrogen cracking, and to minimize problems due to low as-welded toughness. This is a suggested practice in API-934C. The DHT should be performed at a minimum metal temperature of 570 °F (300 °C) for duration of one hour minimum(934C).
ASME Sec-IX addresses DHT as post-heating. The prime function of this practice is to dissipate the diffusible hydrogen in the weld metal. Many a time if the section thickness is less (typically below 2 inch or below) and if welding consumables are properly conditioned as per manufacturer's recommendations this practice of DHT/Post Heating may be avoided.
However if the section thickness of the equipment is large (typically 3" and above) and intended service would hydrogen related services (which is to be confirmed by the client), DHT would a good and recommended practice.
Note:-This is a not an essential variable.Addition or deletion would not call for requalification of welding procedures. However if DHT was followed during PQR qualification and if project technical requirement incorporates API 934C specification, DHT needs to be performed in the production welds also.
Thanks.
Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist & Consultant
Ontario, Canada.
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