I was asking because I saw that many companies are doing the preheat with the electrical resistance, and I was wondering if using propane it's a problem due hydrogen content.
I think that if you are using only GTAW, with low hydrogen filler metal and you respect the preheat & interpass temperature, there would be no concern of the preheating method.
Another question is if the heat treatment it's mandatory immediately after welding, or it's possible to postpone it with the condition to make a dehydrogenation (min 1h at 300deg.)?
Also, dehydrogenation it's mandatory if you use a filler metal with low hydrogen content?
georgebogdan
You can postpone or delay, for a reasonable period, PWHT with a post weld bake (aka dehydrogenation). Before PWHT, I would perform a surface NDT to ensure no cracks in the weld region before the application of PWHT.
If you use low hydrogen electrodes with supplement H4 designation and you follow good preheat maintenance and surface cleaning before welding, you can omit the post weld bake before PWHT.
If you are using torch preheating in the field in cold and windy conditions you will most likely need the preheat to be maintained by a welder's helper. When the welder performs the preheat, it may not be effectively maintained. We had this issue a couple of times in the past with hydrogen related cracking the result. PWHT was not done immediately after weld completion. Thickness and weld restraint should also dictate when torch or heating pad preheat should be applied and whether you should provide a dehydrogenation treatment prior to final PWHT.
