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
From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of Vijay Katkar
Sent: Saturday, February 04, 2012 12:06 PM
To: materials-welding@googlegroups.com
Subject: [MW:13609] Post Heating(DHT) requirement for SA 387 GR 12 CL2 LOW ALLOY MATERIAL
Regarding post heat treatment(DHT) for SA 387 GR 12 CL2 Low alloy steel material,
I have PQR'S with post heating a temperature of 300 Degree C for 2 Hrs,
My question are,
1) Is it possible to make WPS (production weld) without post heating(DHT)?
2) Is it any code(ASME SEC VIII or IX) call for DHT after welding?
3) Why we have to do DHT, if it is required?
4) Is it any where mentioned that the SA 387 GR 12 CL2 material is required post heating(DHT)?
we are doing DHT because avoid cold cracking(Hydrogen cracking) in weld and HAZ . any other major reason to carry out DHT for low alloy steel.
Kindly provide your valuable guidance/ advice/ experiance on this subject.
Regards
Vijay Katkar
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