Normally hydrogen removal is carried out on carbon steel or low alloy that have been operated in hydrogen service, before welding repair is carried out. Is it necessary to perform hydrogen removal/degassing on austenitic stainless that have been in H2 service?

I am going to nit pick here ""Aus S.S. does not require any hydrogen removal (generally) since S.S being a single phase FCC structure is resistant to hydrogen permeation."" The statement above is not totally correct. Hydrogen can readily dissolve in FCC austenite, the real answer is that the diffusivity of hydrogen through FCC austenite is orders of magnitude lower in comparison to BCC CS and low alloy steels, and does not result in embrittlement.

>Josehua /gwg > >Hydrogen permeation in Carbon Steel or Low Alloy Steel or >S.S materials depends on the rate of permeation, rate of >diffusivity. This again depends on process design >temperature, partial pressure of hydrogen in the system and >the duration of exposure in such conditions. This is a well >known fact. > >It is also a well known fact that S.S may require longer >time than C.S in hydrogen service due to FCC structure (all >the same - Also FCC structures are not immune to hydrogen >diffusivity - also a known fact) > >We have to understand the question raised by Josehua in >proper perspective: Is hydrogen removal before repair >welding necessary for S.S material? For this answer should >relate whether it was under high pressure of hydrogen and at >which design temperature and how long the equipment was >operated in such conditions? > >Josehua had not amplified about the service temp and service >design pressure and service duties and exposure duration. If >he had amplified clearly, this doubt or clarification would >not be necessary. > >Assuming this question is related to a S.S vessel or >exchanger subjected to high hydrogen partial pressures + >high temperature service ( say pressures of 200- 340 Kg/Cm2g >and temperature around 320-480 deg cent), and also subjected >to longer duration of say 40,000 to 80,000 hours of >operation,repair welding of exchangers or tube sheet or >pressure vessel components in such service may be needed. >This would require a careful approach for repair welding - >due to possibility of hydrogen permeation (both rate of >diffusivity / rate of permeation taken into account due to >high temp / high partial pressures). This includes hydrogen >removal in the affected portions prior to repair welding. > >My reply and understanding of the question raised by Josehua >was based on this assumption. > >If the same vessel (S.S material used) is used at a lower >partial pressure and lower temperature than needed to meet >Nelson curve limits, S.S will not require hydrogen removal >approach for repair welding of the exchanger or pressure >vessel or piping components. For those dealing in S.S >materials used within Nelson curve limits would know about >this. > >Hope this helps and clarifies all doubts about use of S.S in >hydrogen service and the repair approach for such equipment > >C.V.Srinivasan Nishi Engineers Pvt Ltd India May 30, 2008 >E-mail: nishi@vsnl.com > >>I am going to nit pick here >> >>""Aus S.S. does not require any hydrogen removal (generally) >>since S.S being a single phase FCC structure is resistant to >>hydrogen permeation."" >> >> >>The statement above is not totally correct. Hydrogen can >>readily dissolve in FCC austenite, the real answer is that >>the diffusivity of hydrogen through FCC austenite is orders >>of magnitude lower in comparison to BCC CS and low alloy >>steels, and does not result in embrittlement. What is the procedure for repair welding of a equipment in this condition: Assuming a S.S vessel or exchanger subjected to high hydrogen partial pressures + high temperature service ( say pressures of 200- 340 Kg/Cm2g and temperature around 320-480 deg cent), and also subjected to longer duration of say 40,000 to 80,000 hours of operation, Masoud