Max. hydrogen content to limit hydrogen cracking ? 1

[jonesey]

We have to do some welding on the inside of an existing pressure vessel that has been in service where there has been hydrogen sulfide in the product. Knowing the problems that can arise through hydrogen cracking after welding, are there guidelines for how long the vessel needs to be warmed up at a particular temeprature dependant on hydrogen concentration to drive the diffused hydrogen from the steel so the risk of hydrogen cracking is minimised ?

 

[metengr]

Since you are going to be performing weld repairs in the field, the best advice I could give is to use a generous preheat of 400 deg F in the repair areas. I would use 30 minutes at preheat temperature, prior to welding. At about this temperature, dissolved hydrogen in steels becomes mobile and will lessen the chances of hydrogen cracking in the base material. Also, use NDT (wet MT) prior to and after weld repairs.

 

[jonesey]

Thanks for the reply. We have done some previous repairs on vessels that have had high hydrogen levels where we have preheated the vessel at 250-300degC (390-570F) for approx 16hrs. I was wondering that with lower concentration levels of hydrogen the preheating time could be less. The whole 10m long bottom section of the vesel needs to be preheated as we need to weld a large number of anchor pins in the bottom of the vessel ready for application of a gunite lining.

 

[metengr]

Yes, if you remain at or above 450 deg F prior to and during welding. The higher the preheat temperature, the greater the diffusivity of hydrogen atoms. The 16 hours seems a bit long, I would expect a four hour hold would suffice for base material 1" or less in thickness.

 

[yates]

Four hour soak at 190 deg C is the standard embrittlement relief used for steels that have undergone electrolytic processes (eg. cad plate)

 

[banaeian]

Hi Jonesey,
Are you going to use Stud Welding process for welding anchors? If yes preheat is also important to have low hardness in HAZ area. If you keep eye on preheat to be maintained within the specified limit, it is the great help.
If you are going to use SMAW process, you have to control the electrode baking too.