Hydrogen bakeout

[Hang Suin]

Dear all,
Please let me ask for clarifications with respect to hydrogen bakeout:
1. Why is hydrogen bakeout considered to be a critical and important process?
2. Could hydrogen bakeout be a stand alone process or it always goes with PWHT?
3. Is hydrogen bakeout connected to the base material thickness?
4. Is hydrogen bakeout performed after welding and prior to PWHT and if so how many time after welding and prior to PWHT?
Thanks
Hang

[arweldcon]

Dear Hang,
Please have a look here below:
1. Hydrogen bakeout (or postheating) is considered to be a critical and important process under certain conditions with certain materials to release hydrogen and it is to avoid delayed hydrogen cracking.
2. Hydrogen bakeout is a standalone process. It’s not necessary to do a PWHT after the hydrogen bakeout. If you do immediately after welding a stress relieving PWHT, then hydrogen bakeout is not necessary.
3. Yes, hydrogen bakeout is related to material thickness because of the diffusion speed of hydrogen in the steel.
4. Hydrogen bakeout has to be done immediately after welding to minimize the risk of hydrogen cracking.
regards
arweldcon

[nomibucha]

I would like to have a better understanding of the term "Hydrogen Bakeout". I work in the petrochemical industry and have heard this term quite often. I understand how it is done but not why. Can any one shed some light on this for me.

[techupon]

Nomibucha,

Hydrogen bakeout is done so as to release the diffused hydrogen from the metal because it chemically react with the alloyed carbon forming methane that builds up between the grain boundaries with increasing pressure that after a limit leads to cracks. Please have a look at the attached photo.

Hope this helps
regards,
techupon

[mohd]

Dear Mr Techupon,

Nice explanation.

Is there any code/std/spec that specifies the post heating/hydrogen bakeout temperature and time?

Is it hydrogen bakeout applicable to alloy steel only?

regards,
mohd

[Logosweld]

mainly it is done to remove the diffused hydrogen from the metal because it reacts with metal and it may leds to crack in it after sometime

[Logosweld]

when the steel contains hydrogen as a result of service exposure (or corrosion, or high temperature - high pressure hydrogen processing) then a hydrogen bake out may be needed to avoid cracking problems during or after welding. However, not every attempt in outgassing meets with success . In theory, you simply heat the steel to an elevated temperature and allow time for the hydrogen to diffuse out of the steel, leaving it hydrogen free and weldable.