HYDROGEN PICK UP AND DIFFUSION IN TIG WELDING OF SUPERMARTENSITIC 13 CR STAINLESS STEEL WITH SUPERDUPLEX WIRE

摘要

Supermartensitic 13% Cr stainless steels have been in use in offshore satellite flowlines for several years. Since they contain microstructure that is susceptible to hydrogen cracking, the pick up of hydrogen in welding with subsequent transport to critical areas may be very important, also with respect to hydrogen embrittlement when hydrogen is coming from other sources than welding (e.g., cathodic protection). In the present investigation the pick up of hydrogen has been assessed using mechanized TIG welding with superduplex 25% Cr wire. The WM and HAZ hydrogen levels were analyzed. With addition of hydrogen in the shielding gas in multipass welding, the mean WM hydrogen contents were found to be approximately 10 and 6 ppm in the cap layer and root pass, respectively. The corresponding mean HAZ concentration was 3.1 ppm (scatter between 1.3 and 4.8) immediately after welding. Post weld hydrogen diffusion heat treatment showed that hydrogen diffusion was retarded at room temperature, even for 1 month storage. Limited diffusion took place at 90℃, particularly for the cap region. The results indicate that superduplex weld metal with high hydrogen content (6-10ppm) will act as a hydrogen reservoir supplying H to the 13% Cr HAZ as long as 2-3 years after welding. Fitting the data by using the uniaxial diffusion model gave diffusion coefficients in the range of ～3-5x10~(-13) m~2/s at room temperature for the superduplex WM. At 90℃ a diffusivity of 5.5x10~(-12) m~2/s for the cap area and 2.5x10~(-11) m~2/s for the root area were found. For a holding temperature of 150℃, diffusion from the WM was much more significant. The hydrogen WM cap content was reduced from an initial level of 10 ppm down to 2 ppm within 3 months giving a diffusion coefficient of 1.0x10~(-11)m~2/s. The Supermartensitic HAZ samples contained up to 5 ppm hydrogen a short time after welding. This is an important observation, since it may provide sufficient amount of hydrogen in the HAZ to cause cold cracking in the as welded condition. The uniaxial model indicated diffusivities of D=8.0x10~(-11) m~2/s at 20℃ and D=2.0x10~(-10) m~2/s at 90℃ in the HAZ.